Economical Structural Steelwork

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economical structuralsteelwork

student edition - 2004

(ABN)/ACN (94) 000 973 839 AUSTRALIAN STEEL INSTITUTE

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ECONOMICAL STRUCTURAL STEELWORK

Student Edition 2004Limit States Edition to AS 4100 -- 1990 AUSTRALIAN STEEL INSTITUTE --1979, 1984, 1991, 1996, 2004

NATIONAL LIBRARY OF AUSTRALIA CARD NUMBER AND ISBN 0909945 77 2

FIRST EDITION 1979

SECOND EDITION 1984

THIRD EDITION 1991

REPRINTED 1992, 1995

FOURTH EDITION 1996

E Australian Institute of Steel Construction 1996

While every effort has been made and all reasonable care taken to ensurethe accuracy of the material contained herein the Authors, Editors andPublishers of this Publication shall not be held to be liable or responsible inany way whatsoever for any loss or damage costs or expenses howsoeverincurred by any person whether the purchaser of this work or otherwiseincluding but without in any way limiting any loss or damage costs orexpenses incurred as a result of or in connection with the reliance whetherwhole or partial by any personas aforesaid upon any part of the contents of this publication.

Should expert assistance be required, the services of a competentprofessional person should be sought.


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FOREWORD

Economical Structural Steelwork was first published in 1979 and quicklybecame the Institutes most popular publication among practisingdesigners and students.

This fourth edition has been up--dated in its references to Australian

Standards, practices and cost concepts, and has other amendments. Itcontinues to provide the useful practical advice given by its previouseditions towards the achievement of the optimum result in structuralsteelwork.

E Australian Institute of Steel Construction 1996

While every effort has been made and all reasonable care taken to ensurethe accuracy of the material contained herein the Authors, Editors andPublishers of this Publication shall not be held to be liable or responsible inany way whatsoever for any loss or damage costs or expenses howsoeverincurred by any person whether the purchaser of this work or otherwise

including but without in any way limiting any loss or damage costs orexpenses incurred as a result of or in connection with the reliance whetherwhole or partial by any personas aforesaid upon any part of the contents of this publication.

Should expert assistance be required, the services of a competentprofessional person should be sought.


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Preface

When considering steel structures it is not difficult to obtain information onengineering and technological aspects, butvery little is availableon howtochoose steelwork economically. Yet more and more the viability of abuilding project depends upon critical financial considerations. Thus it isimportant for designers to have a good general appreciation of thecomponents that make up thecost of fabricated steel, and of howdecisionsmade at the design stage can influence these costs.

This publication aims to supply some of this information. It is not a designmanual, but rather a publication that discusses from a cost point of view allof the matters that a structural steel designer should consider. It takes intoaccount current fabrication practices and material/labour relationships,both of which have changed markedly over the last few years.

Adherence to the principles outlined in this publication will do much to

assist designers in reaching decisions that will lead to effective andeconomic structures.

It should be noted that this edition has substantially adopted therationalised approach to the costing of fabricated steel by using a cost per

metre for sections and cost per square metre for plates, depending onthe size, in lieu of cost per tonne . The reasoning behind this is presentedin a paper entitled: A Rational Approach to Costing Steelwork by T Main,K B Watson and S Dallas. This paper was presented at the InternationalCost Engineering Council/The Australian Institute of Quantity SurveyorsInternational Symposium, Construction Economics -- The EssentialManagement Tool, Australia, May 1995.

We wish to thank all those who have contributed to this publication andspecial acknowledgment goes to all AISC Staff who submitted commentson the technical and editorial content of this publication.

edited by: Jose R ZaragozaBScCE,

MScCE, CPEng.

AISC StateManager--N

SW


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ECONOMICAL STRUCTURAL STEELWORK Contents

FOREWORD 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Preface 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Preliminary Considerations 1. . . . . . . . . . . . . . . . .

1.1. Introduction 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2. Factors influencing Framing Cost 1. . . . . . . . . . . .1.3. Integrated Design 2. . . . . . . . . . . . . . . . . . . . . . . . . .

2. General Factors Affecting Economy 3. . . . . . . . .

2.1. Steel Grades 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1.1. STRUCTURAL STEEL 3. . . . . . . . . . . . . . . . . . . . . . .2.1.2. WEATHERING STEEL 3. . . . . . . . . . . . . . . . . . . . . . .2.1.3. HOLLOW SECTIONS 3. . . . . . . . . . . . . . . . . . . . . . . .2.1.4. QUENCHED AND TEMPERED STEEL 4. . . . . . . . .2.1.5. CHOICE OF STEEL GRADE 4. . . . . . . . . . . . . . . . . .

2.2. Economy in use of Material 6. . . . . . . . . . . . . . . . .2.2.1. STEEL PRICING 6. . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2.2. PLATES 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2.3. SECTIONS 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2.4. SCRAP AND WASTE 7. . . . . . . . . . . . . . . . . . . . . . . .

2.3. Fabrication 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3.1. GENERAL 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3.2. BEAM AND COLUMN FABRICATION 8. . . . . . . . . .2.3.3. GIRDER AND TRUSS FABRICATION 8. . . . . . . . . .2.3.4. SUMMARY FOR ECONOMIC FABRICATION 9. . .

2.4. Erection 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.1. GENERAL CONSIDERATIONS 9. . . . . . . . . . . . . . .2.4.2. HANDLING AND TRANSPORT 10. . . . . . . . . . . . . . .2.4.3. CONNECTIONS 10. . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.4. FIELD BOLTING 11. . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4.5. FIELD WELDING 12. . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4.6. BRACING 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5. Surface Treatment 12. . . . . . . . . . . . . . . . . . . . . . . . .2.5.1. GENERAL CONSIDERATIONS 12. . . . . . . . . . . . . . .2.5.2. STEEL PERFORMANCE 13. . . . . . . . . . . . . . . . . . . . .2.5.3. SURFACE PREPARATION 13. . . . . . . . . . . . . . . . . . .2.5.4. PAINT SYSTEMS 13. . . . . . . . . . . . . . . . . . . . . . . . . . .2.5.5. HOT--DIP GALVANIZING 14. . . . . . . . . . . . . . . . . . . . .2.5.6. DESIGN AND DETAILS FOR CORROSION RESISTANCE .

142.5.7. SUMMARY CHECKLIST FOR SURFACE TREATMENT . . . .

142.6. Fire Resistance 15. . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.1. GENERAL CONSIDERATIONS 15. . . . . . . . . . . . . . .2.6.2. REGULATORY REQUIREMENTS 15. . . . . . . . . . . . .2.6.3. MATERIALS FOR FIRE PROTECTION 16. . . . . . . .

2.7. Specifications 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7.1. GENERAL CONSIDERATIONS 16. . . . . . . . . . . . . . .2.7.2. WORKMANSHIP STANDARDS 17. . . . . . . . . . . . . . .2.7.3. TOLERANCES 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7.4. CAMBERING 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7.5. TEMPORARY BRACING 19. . . . . . . . . . . . . . . . . . . . .

2.7.6. INSPECTION 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7.7. SUMMARY FOR SPECIFICATION WRITERS 20. . .


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6.5. Summary for Economic Bolting 61. . . . . . . . . . . . . .6.5.1. CHECKLIST 61. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.5.2. BOLT USAGE -- FLEXIBLE JOINTS 62. . . . . . . . . . .

6.5.3. BOLT USAGE--RIGID JOINTS 63. . . . . . . . . . . . . . . .7. Welding 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7.1. Introduction 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.1.1. PRINCIPLES FOR ECONOMY 64. . . . . . . . . . . . . . . .7.1.2. COST COMPONENTS 64. . . . . . . . . . . . . . . . . . . . . . .

7.2. Types of Welds 65. . . . . . . . . . . . . . . . . . . . . . . . . . . .7.2.1. FILLET WELDS (see Fig 7.1) 65. . . . . . . . . . . . . . . . .

7.2.2. BUTT WELDS (see Fig 7.2) 66. . . . . . . . . . . . . . . . . . .7.2.3. BUTT WELDS vs. FILLET WELDS 66. . . . . . . . . . . .

7.3. Welding Processes 67. . . . . . . . . . . . . . . . . . . . . . . .7.4. Other Cost Factors 68. . . . . . . . . . . . . . . . . . . . . . . .

7.4.1. WELD CATEGORIES 68. . . . . . . . . . . . . . . . . . . . . . . .7.4.2. WELDING SPECIFICATIONS 69. . . . . . . . . . . . . . . . .7.4.3. WELDING INSPECTION 69. . . . . . . . . . . . . . . . . . . . .

7.5. Economical Design and Detailing 70. . . . . . . . . . . .

8. Detailing for Economy 75. . . . . . . . . . . . . . . . . . . . . .

8.1. Detailing on Design Engineer s Drawings 75. . . . .8.2. Beams 76. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.2.1. GENERAL 76. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.2.2. PLATED SECTIONS 76. . . . . . . . . . . . . . . . . . . . . . . . .8.2.3. WEB PENETRATIONS IN BEAMS 76. . . . . . . . . . . . .

8.2.4. CASTELLATED BEAMS 77. . . . . . . . . . . . . . . . . . . . . .8.2.5. THREE--PLATE GIRDERS 78. . . . . . . . . . . . . . . . . . . .8.3. Columns 80. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.3.1. GENERAL 80. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.3.2. COLUMN BASE PLATES 80. . . . . . . . . . . . . . . . . . . . .8.3.3. HOLDING--DOWN BOLTS 82. . . . . . . . . . . . . . . . . . . .8.3.4. COLUMN SPLICES 82. . . . . . . . . . . . . . . . . . . . . . . . . .8.3.5. COLUMN STIFFENERS 84. . . . . . . . . . . . . . . . . . . . . .

8.3.6. BUILT--UP COLUMNS 85. . . . . . . . . . . . . . . . . . . . . . .8.4. Trusses 87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.5. Portal Frames 90. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.5.1. CONNECTIONS 90. . . . . . . . . . . . . . . . . . . . . . . . . . . .8.5.2. PORTAL FRAME PRE--SET 91. . . . . . . . . . . . . . . . . .

8.6. Connection Detailing 92. . . . . . . . . . . . . . . . . . . . . . .8.6.1. GENERAL 92. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.6.2. SPECIFIC CONNECTIONS 93. . . . . . . . . . . . . . . . . . .

9. References & Further Reading 103. . . . . . . . . . . . . .

10. Standards 105. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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List of Tables

TABLE 2.1 -- AVAILABILITY OF PRODUCTS BY GRADE(check currency of information with steel suppliers) 4. . . .

TABLE 2.2 INDICATIVE COST RATIOS FOR DIFFERENTGRADES OF STRUCTURAL STEEL(per tonne, supply only) 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 2.3 PREFERRED STEEL PLATE THICKNESSES(in mm) 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 2.4 TRANSPORTATION COSTS 10. . . . . . . . . . . . . . . . .TABLE 2.5 SURFACE TREATMENT COSTS 13. . . . . . . . . . . . .

TABLE 2.6 PASSIVE FIRE PROTECTION COSTS 16. . . . . . .TABLE 5.1 SHEAR WALL vs LATTICE BRACING 52. . . . . . . . .TABLE 6.1 BOLT TYPES AND BOLTING CATEGORIES 58. . .TABLE 6.2 INDICATIVE COST RATIOS OF

DIFFERENT BOLT DIAMETERS 59. . . . . . . . . . . . . . . . . . . .

Table 6.3 INDICATIVE COST RATIOS OFDIFFERENT BOLTING CATEGORIES 60. . . . . . . . . . . . . . . .TABLE 7.1 FILLET WELD COMPARISON 65. . . . . . . . . . . . . . . .TABLE 8.1 WRENCH CLEARANCES 89. . . . . . . . . . . . . . . . . . .


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2. General Factors Affecting Economy

2.1. Steel Grades

2.1.1. STRUCTURAL STEEL

Throughout the world the least costly and most commonly used grades of steel for structural purposes are those generally referred to as normalstrength structural steel.

In Australia such steel is covered by AS 3678 or AS 3679 (Parts 1 & 2). Ithas a typical design yield strength of 250/300 MPa (varying above andbelow this figure depending on thickness), a tensile strength of at least410/430 MPa, a minimum elongation of 22% and a carbon equivalent of 0.43/0.44 so as to assure good weldability.

AS 3678 and AS 3679 (Parts 1 & 2) are omnibus standards covering afamily of structural steel grades including variants of the main gradeshaving superior low temperature toughness.

Plates, rolled sections, welded sections and bars are all produced to thesestandards, although not every product is available in every grade. This isexplained more fully in Table 2.1.

2.1.2. WEATHERING STEEL

AS 3678 and AS 3679 (Parts 1 & 2) also deal with so--called weatheringsteel. Weathering Steel contains alloying elements which cause it toweather to a uniform patinaafter whichno further corrosiontakes place.Bynature of the chemical composition the steel is high strength (Grade 350)steel. However in Australia it is available in only a limited number of products----see Table 2.1.

2.1.3. HOLLOW SECTIONS

In Australiastructural hollow sections areproducedto theproduct standard AS 1163. This standard covers a number of cold--formed (C) grades.Rectangular hollowsections areavailablein GradeC350 andGrade C450.Circular hollow sections (CHS) are available in Grade C250 and GradeC350.


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TABLE 2.1 -- AVAILABILITY OF PRODUCTS BY GRADE (check cur-rency of information with steel suppliers)

Steel Grade Plates (orFloor plates)

RolledSections

WeldedSections

StructuralHollow Sections

Grade AS3678

AS3679.1

AS3679.2

AS1163

200 Y -- --

250 Y Y (1) --

250L0 -- X --

250L15 + -- --

300 + Y (2) Y

300L15 + -- +

350 Y + --

350L0 -- X --

350L15 + -- --

400 + -- Y

400L15 X -- +WR350/1 + -- --

WR350/1 L0 + -- --

Grade C250 Y

C350 Y

C450 Y

Quenched &Tempered Structural

Steel

AS 3597

60 Y

70 Y

80 Y

Y = regular grade commonly produced, readily available from stockists+ = regular grade not commonly produced,availabilitysubject to time limitations and order sizeX = non--regular grade, availability subject to time limitations and order size

-- = not manufacturedNotes: 1. Applies to TFB, TFC and small angles.

2. Sections not considered in Note 1 above.

2.1.4. QUENCHED AND TEMPERED STEELSteel plates are produced in Australia in very high strength heat--treatedgrades known as quenched and tempered steel. These steel plates areuseful in special applicationswhere mass reduction is important (eg. cranebooms) or where their high wear resistance is needed (e.g. dump truckbodies). Australian Standard AS 3597 covers these steel plates for structural steel

applications and for use in pressure vessels.2.1.5. CHOICE OF STEEL GRADE

Table 2.1 lists the availability of various products by steel grade. Theindicative relative cost of grades is shown in Table 2.2. For most structuresthe greatest economy will be achieved by the selection of the least costlyand most readily available steel, i.e. Grade 300.In large structures with longer lead times the use of higher grades will oftenbe worth considering at least for parts of the frame.Heavy plate members such as bridge girders are one instance wherehigher grades may prove economical. Other applications include:

(a) Multi--storey structures, particularly with composite steel beams;also in maintaining the same column size down a building byvarying steel grades;

(b) Trusses and lattice girders.

Grade 350 steel costs around 10% more than Grade 300, and generallyabout 5% more to fabricate. To offset these cost extras, it provides greateryield strength but no increase in stiffness.In some frames, significant reduction in steel mass may overcome theincrease in material cost and fabrication cost by the use of higher grades.Each individual frame must be assessed on its merits, but there areundoubtedly applications where the use of higher grades is economical.

While the information presented in Table 2.1 is indicative of the generalsituation, it must be remembered that the steel suppliers are always willing


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to discuss special cases where, for example, the economics of a highstrength steel has been considered by the designer and the sectionsrequired are not normally manufactured in that grade. For a projectrequiring large tonnage of specific sections, it maybe possible to negotiatea special order with the supplier, provided that an arrangement has beenagreed at an early enough phase in the design.

Conversely, on average projects the designer should always be careful tokeep within the range of readily available products so as to ensure that noproblems of steel procurement occur at the fabrication stage.

TABLE 2.2 INDICATIVE COST RATIOS FOR DIFFERENT GRADESOF STRUCTURAL STEEL (per tonne, supply only)

Grade Plates Rolled

Sections

Welded

Sections AS 3678, AS3679.1 & AS3679.2

Grade 250 100 100 --

250L0 -- 105 --

250L15 105 105 --

300 100 110 130

300L15 105 -- 140

350 110 120 --

350L0 -- 130 --

350L15 120 -- --

400 115 -- 150

400L15 120 -- 155

WR350/1 125 -- --

WR350/1 L0 135 -- --

AS 1163

Grade C250 130

C350 130

C450 130

AS 3597 Quenched& Tempered Steel

Grade 60 150

70 160

80 160


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TABLE 2.3 PREFERRED STEEL PLATE THICKNESSES (in mm)

3 25 70

4 28 80

5 32 90

6 36 100

8 40 110

10 45 120

12 50 140

16 55 160

20 60 180

2.2.3. SECTIONS

Australia produces a range of welded products, universal sections,channels, angles, and hollow sections which provides the designer with areasonable choice without the proliferation which can lead to problems of availability.

The lowest weight in each nominal size of universal section is the moststructurally efficient and they account for over two--thirds of all UB sales.The designer shouldthereforemake every endeavour to keep to thelowestweights, although this will not always be possible.

Very long lengths of sections become difficult to keep straight and tohandle, and the mills impose a price extra for them. It should be especiallynoted that although universal sections are listed as being available up to18m long (and up to 27m by inquiry), the usual maximum length found instock is around 15m.

The available lengths of structural hollow sections are usually restricted to6.5m (circulars) or 12m (rectangulars and squares).

2.2.4. SCRAP AND WASTE

The real cost of material is affected by the quantity of scrap and waste, anddesigners should be receptive to suggestions for minimising andcontrolling the generation of waste. This may include greaterstandardisation of structural sizes, or of plate widths and thicknesses, inorder to take advantage of size andquantitydiscounts. It might also includea more liberal approach to thesplicing of beamsor other structural sectionsusing standard lengths.

Random splicing, which involves welded splices anywhere within thelength of a rolled structural member, can be particularly effective whenmaterial is sawn to length and fabricated on a conveyorised productionline. When carefully controlled, it can dramaticallyreduce theaccumulationof shorts and thus reduce the total cost. The only real restriction to randomsplicing applies to its use for beams subject to severe dynamic loads. Of course the savings in scrap have to be balanced against the welding costs,and the designer should be receptive to this technique where it isappropriate.

2.3. Fabrication

2.3.1. GENERAL

Fabrication costs are a function of complexity and are influenced by:

(a) Size of the component(b) Size and type of sections involved

(c) Amount of stiffening and reinforcing required

(d) Amount of repetition(e) Shop and field details

(f) Space requirements in the shop, and(g) Facilities available for handling, lifting and moving the structural

components.Fabrication costs are sensitive to simplicity or complexity of detail, and thedegree to which production line techniques can be applied. They arecontrolled by the quality of the shop detail drawings, which must reflect the


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In rigid frames, the following should be taken into consideration for thedesign of bolted connections:

(a) The end plate depth should be kept to a minimum to reduce the

tendency to jam during installation (Fig. 2.1).(b) The tolerance between the face of the end plate and the face of

the column should either be tightly controlled so that the buildingplumbs itself automatically, or allowance should be made forshimming in order to plumb the building. Shimming, however, canbe expensive.

(c) In end plate connections for portal frames careful considerationshould be given to access for installing and tensioning bolts, (seeTable 8.1).

If welded connections are preferred, the following should be taken intoconsideration:

(a) Welded connections are normally erected using a bolted erectionconnection. The same criteria should apply to the design of theseconnections as described above.

(b) Substantial erection clearance between the end of the girder andcolumn face should be provided where permitted by the design of the connection.

(c) Field welding should be kept to a minimum and overhead weld-ing should be avoided.

(d) Attention should be paid to access for welding and welding in-spection.

(e) Consideration should be given to plumbing the building.

The most significant time delays in the erection of a girder can be expectedto occur when it is installed with the end connection against a column web.The girder can normally only be manoeuvred in a vertical plane andfrequently jams. Gusset plates, stiffeners, and other members tend tointerfere with its installation. Access for bolting is usually difficult andsometimes impossible. Every effort should be made to get the connectionoutside the flanges of the column, or at least as far out from the web aspossible. This is especially important when the column section is compact.Consideration should always be given to excluding direct girder/web

connections evenif it involvesincreasing column weight, and/or fabricationcosts (see Fig. 2.2).

Fig. 2.1 Deep end plates can cause jamming

Fig. 2.2 Two ways of avoiding the problem of access to column

web connections

2.4.4. FIELD BOLTING

In projects with a predominance of large connections, threads may beexcluded from theshear plane forbearing type connections as this will helpto reduce the number of bolts. However with Australias ISO metriclong--thread bolts, care should be taken that the long stick--through thatoccurs does not cause fouling or access problems. In projects with smallconnections the saving in number of bolts is not so evident, and it is more


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demonstrated that bare steel will not reach a critical temperature should acar catch fire (Ref 2.5).

Another example is composite steel deck floor systems utilising fireemergency reinforcement (Refs 2.6, 5.4, 5.5).

2.6.3. MATERIALS FOR FIRE PROTECTION

Where steel has to be protected, the most practicable way is to cover it orencase it in a protective material. Such material should be:

(a) Non--combustible

(b) Unable to produce smoke or toxic gases at elevated temperature

(c) Able to be efficiently and uniformly applied

(d) Durable to prevent dislodgment, and

(e) Thermally protective.

Table 2.6compares passive fire protection costs and gives an approximateindication of their costs. These costs may not tell the whole story where aprotectedmember is exposed to view and will be given a decorative finish --some systems are less costly than others to decorate.

Another important factor to be borne in mind is that dry systems cause lessdisruption to other trades and the building schedule, and therefore canbring significant indirect cost savings in terms of shorter overallconstruction time.

Commercially available materials must be able to demonstrate theircapability of achieving a fire resistance level as part of building systems.

The various manufacturers can supply the necessary accreditation andtechnical data by reference to tests conducted at recognised fire testingstations. See also Ref 2.11.

TABLE 2.6 PASSIVE FIRE PROTECTION COSTS

Section Mass Intumescent Paint Vermiculite Spray Vermiculite Spray

Fire Rating Level Fire Rating Level Fire Rating Level

60 min 120 min 180 min

(kg/m) $/sq m $/sq m $/sq m

60.5 and less 108 27 37

60.6 to 160 106 23 35

160.1 to 455 104 17 33

Notes: 1. Rates include supply and paint/spray.2. Intumescent paint cost includes ZnSi and Class 2--1/2 blastcleaning costs.

2.7. Specifications

2.7.1. GENERAL CONSIDERATIONS

The specification is important because it forms part of the tenderdocuments and ultimately becomes part of the contract documents. Itspurpose is to cover aspects of the work that fall between the legal contractclauses and the technical data shown on drawings.Such aspects may include:

(a) Workmanship standards(b) Tolerances

(c) Inspection levels etc.In past years the specification was essential for the designer to convey tothe contractor exactly what was wanted. Nowadays so many of thesematters have been codified that a detailed specification has become lessnecessary.


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common experience, rather than hold up the work unnecessarilyon minor details.

2.7.7. SUMMARY FOR SPECIFICATION WRITERS

1. Specifications are not as important as in previous years becauseso much has now been codified.

2. Omit meaningless clauses, no matter how well--sounding. Theycan achieve nothing but may exacerbate disputes.

3. Do not include information in specifications that should be moreproperly shown on drawings.

4. Call up AS 4100 and associated documents.

5. Keep it brief.


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3.2.2. FLEXIBLE CONNECTIONS

Flexibleconnectionsare used in steel structures designedusing thesimpledesign method of AS 4100. These connections offer low restraint to beam

rotation, being close in behaviour to that of an ideal pin.Typical flexible connections are shown in Fig 3.1. The most commonflexible connections in use in Australia are the flexible end plate (Fig 3.1(c)), the angle cleat (Fig 3.1 (d)), and the web side plate (Fig 3.1 (e)).

Such connections are:

(a) Assumed to behave as a simple support

(b) Simple to fabricate

(c) Simple to erect, and

(d) Less costly of the two connection types.

Flexible connections shown in Fig 3.1 are totally standardised in the AISCStandardized Structural Connections publication (Ref 1).

(a) Angle Seat (b) Bearing Pad

(c) Flexible End Plate (d) Angle Cleat(single or double)

(e) Web Side PlateFig 3.1 Flexible connections


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Typical applications that may use this type of framing are Iow tomedium--rise rectangular frames (up to 50--storeys -- especially usingcores, either steel framed or slip--formed concrete).

Fig 3.5 Two--way braced framework

3.3.4. SUMMARY OF FRAMING SYSTEMS

Framing

System

Advantages Disadvantages

Two--way rigid No stabilizing elementsrequired for lateralforces in any plane.Freedom of layout plan-ning.Plastic design methodscan be used if desired --

economical in material.Continuous beam de-sign leads to reducedbeam size.

Requires the use of rig-id connections, whichare more costly thansimple connections.Columns ideally shouldhave near equal stiff-ness in both directions --

hence fabricated boxcolumns may be need-ed.Large column move-ments.

One--way rigid / One--waybraced

Simple connections(least costly type) usedin the braced plane.

Can use I columns --usually rolled sections.Can use plastic designmethods and continu-ous beam design inplane of rigid connec-tions -- saving in materi-al.

Rigid connections usedin unbraced plane.Some restriction on

planning layout; stabiliz-ing elements required inone plane.

Two--waybraced Simple connectionspossible -- least costlytype.Usually use I columns.Beams assumed simplysupported for design;columns designed foraxial load only at smalleccentricity.

Restriction on planninglayout because of re-quirement for provisionof stabilizing elements.Little interaction be-tween elements.Heavier beam sizes.


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3.3.5. STABILISING ELEMENTS

Construction elements whose function is to provide a means of stabilisingthe framework in either one or two planes may be divided into the followingcategories:

(a) Triangulated steel bracing panels using the X, K, or diamond pat-tern of diagonal members -- Fig 3.6(a);

(b) Vertical Vierendeel cantilevers in steel -- Fig 3.6(b);

(c) Triangulated steel core -- Fig 3.6(c);

(d) Reinforced concrete or masonry shear walls -- Fig 3.7(a);

(e) Reinforced concrete or masonry cores or shear tubes -- Figs3.7(c) and (d);

(f) Brick in--fill panels and walls -- Fig 3.7(e);

(g) Light metal cladding used on the stressed skin principle.

(a) Triangulated bracing systems

(b) Vertical Vierendeel cantilever

(c) Triangulated core

Fig 3.6 Stabilising elements built in steel

When stabilising elements areconstructedof concrete or masonry, it is wellto remember that some means of temporary bracing may be requiredduring the early construction phase, since the steelwork may not havesufficient in--built resistance to withstand lateral forces prior to constructionof the stabilising elements. Rigid systems of wind girders or diaphragms(Fig 3.8) may also be required to distribute lateral forces to the stabilisingelements.

Openings can readily be incorporated in all types of stabilising elements,although there is some restriction on the maximum size of openings. It isimportant, however, to distinguish between the low--rise building whichdoes not require large stabilising elements, and tall building where thestabilising elements are required to carry very large forces and have arelatively high stiffness.

(a) Shear Wall (b) Opening may beaccommodated in shear wall


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(iv) Transverse wall

(v) Stairwell walls

(a) Vertical systems

(i) Lateral force transmitted to foundation at every column -- no horizontalbracing

(ii) Horizontal wind girder

(iii) Use of floor as diaphragm

(b) Horizontal bracing systems

Fig 3.9 Action of lateral force resisting systems (from Ref 5.2)

3.4. Cost and Framing System

The type of framing systemselected to satisfy all thedesignconstraints willhave a profound effect on the structural cost. The labour cost in thefabrication of a fully braced system employing simple flexible connectionsis much less than the labour cost in fabricating a fully rigid system usingmore complex moment connections. On average the rigid frameworkrequires about 2.5 times the labour cost input in the fabrication process.

To achieve the most economical final structure the designer has to find asolution which, within the various constraints, will provide for maximumcost effect in both material and fabrication labour input.


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3 4 2 SINGLE STOREY INDUSTRIAL BUILDING


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3.4.2. SINGLE--STOREY INDUSTRIAL BUILDING

Similarly in other types of structure the framing system will influence finalcost. In typical factory buildings, for instance, which were once framed by

column--and--truss systems, it is quite clear that the rigid portal frame is themost economical system. Fig 3.11 shows that truss systems are obviouslymore efficient on a mass/ unit area basis. However, on a cost basis, theinherent simplicity of the portal frame renders it less costly to fabricate andshows up as the economical solution within the range shown (see Fig3.12).

Fig 3.11 Relationship between mass/unit area and span

Fig 3.12 Relationship between cost/unit area and spanTheseexamples are intended to illustrate the importance of carrying out anexamination of framing system costs at the earliest design concept stage.The best end result will be obtained by selecting the framing system whichwill satisfy function and economy.

3.5. Framing DetailsHaving thus selected the framing system as previously discussed, it isimportant to consider framing details for that particular system so that thebest cost effect will be achieved.In general the following points must be considered.

3.5.1. SYMMETRYIn many cases symmetry is available in framing systems simply as a result

of functional requirement e.g. city building frames. However in other typesof structure it is often possible to arrange symmetrical layout withoutprejudice to function. Symmetry will invariably lead to the possibility of repetition and this will provide for the most economical fabrication anderection.


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connections are of several types and custom designed some using detailing (increased complexity required additional time) painting


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connections are of several types and custom designed, some usingfully--tensioned bolts.

Cost Index

Structure A Structure BMaterial 1.00 1.00Shop Labour 1.00 2.08Painting 1.00 1.22Shop Detailing 1.00 1.67Erection 1.00 1.25

Notes: 1. Cost indices are presented for the purpose of comparisononly.2. Some common items such as administrative overheads,profit and builders mark--up have been excluded from thiscomparison.

It can be seen therefore that for two structures performing similar functionthe final cost of structural steel is sensitive to the complexity of workrequired. For example, the introduction of truss work into the framingsystem together with more complex connections has more than doubledthe shop labour component for Structure B. Also costs are higher for shop

detailing (increased complexity required additional time), painting(increased surface area for truss work) anderection (complex connectionsand fully--tensioned bolts add to cost).

3.6. ConclusionThe selection of the system for a steel framework is the most fundamentaldeterminant of the final cost of the erected structure. Once the basicframing system is selected, the connection types which may be used arechosen. Thus, the basic cost of the erected framework is predetermined,recognising that this cost may vary within a certain range. Economicdetailing, fabrication and erection can only move the final cost towards theminimum possible within this range.

It is essential that at the preliminary design stage the full range of alternative framing systems are evaluated and compared before makingthe final selection. This comparison of alternatives must be done on thebasis of erected cost -- not on the basis of mass.

Good design i.e. economical design, should take into account all theinfluences which have an effect on the form and cost of the final structure.The economicsof design must beconsidered in this context since theclientis mainly concerned with what he/she pays for-- a complete building whichmeets his/her needs at least cost.


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be focused on cost components that can reduce the overall cost. Figure


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p g4.1 shows the various cost components in relation to a warehouse.

4.2.2. STANDARDIZED PORTAL FRAMES

Overseas, particularly in North America, the portal frame structure hasbeen developed to the stage where many companies offer a standardrange of buildings in spans up to as much as 50m. Economies of scale andproduction line manufacture have made these catalogue buildings acost--effective choice for many industrial as well as commercialapplications.The same manufacturing and marketing techniques have been attemptedin Australia, butwith limited success, probably dueto ourmuch smaller andmore widespread demand. As a consequence, practically all larger portalframe structures built in Australia today are custom designed andmanufactured. This is not as inefficient as it may sound, because there aremany standardized routines in both the design office and the fabricationshop.On the other hand, smaller buildings (sheds, garages, etc) are widelyavailable in Australia as standard catalogue items. Nowadays these areoften manufactured entirely from cold--formed sections rather than fromtraditional hot--rolled sections.

Roof & WallSheeting sup-ply & fix =37%

Purlins & GirtsSupply & Fix =24%

Steel Supply =20%

Fabrication = 15%

Surface treatment 2%

Steel erection 2%Fig 4.1 Steelwork Cost Components for Warehouses

(a) Column and Truss

(b) Portal Frame

Fig 4.2 Configuration of framing systems for a factory building

4.2.3. CUSTOM DESIGNED PORTAL FRAMES


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Fig 4.3 Details of bolted portal frame

In this case, a client engages an Architect and Consulting Engineer whoprepare design drawings and submit the project to tender. The contract is

usually awarded to a builder who then sub--contracts the structuralsteelwork to a steel fabricator on the basis of the Consulting Engineersdrawings.

The portal frames will usually consist of universal sections in order to beeconomic in fabrication -- see Fig 4.3. A variety of connection details areencountered,but only a limited numberaretruly economic for such frames.Fig 4.4 shows examples of economic details using bolted knee and apex joints, while Fig 4.5 shows examples of economic details for frames using

shop welded knee and apex joints and bolted rafter splices.For spans up to 20m a uniform column and rafter section is the mosteconomic but for greater spans haunching of the rafter may provide a moreeconomical system. Haunching is most economically achieved by using acut universal beamsection in the mannershown inFig 4.3,with the depth of the section at the haunchabout twice the rafter depth. The haunch length isusually of the order of 10%--15% of the span of the rafter.


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Temperature expansion of buildings 60--80m long is most commonly andi ll d f i h b ll i h i f


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economically catered for either by allowing the expansion force to act onthe end bay bracing or by the use of slotted holes (or oversize holes) in theconnections of the longitudinal struts to the columns.

In longer buildings (over 60--80m), corner bracing can be a disadvantagesince the expansion involved is too much to be accommodated by theabove methods. In such cases, a central expansion joint can be provided(thus effectively making two buildings -- Fig 4.7(a)) or alternatively, thebracing can be provided near the central interior bays -- Fig 4.7(b). For thelatter alternative, substantial longitudinal strutsmaybe required to transmitwind forces from the end walls through to the braced bays. Whether thissolution is economic depends on the increase in size of the longitudinalstruts required for the latter solution compared to the additional cost of theextra column in the expansion joint solution.

Elevation

Plan

Fig 4.6 Bracing panels

(a) Use of central expansion joint for buildings over 60--80m long

(b) Alternative bracing system for buildings over 60--80m longFig 4.7 Bracing for long buildings

Bracing Details

For sheds and small buildings rod bracing, tensioned by turnbuckle or bydeliberately detailing short, is the most economic solution, although thereis an alternative school of thought which uses angle bracing. With rodbracing, the ability to plumb frames and square the buildings by using theturnbuckle adjustment makes for easier erection.

For wide frame spacing, rod bracing will tend to sag over the longer spaninvolved andmay present some problemsin effectivelybracing theroof. Aswell, rod bracing in the walls may become subject to physical damageduring occupancy. Angle bracing can overcome these difficulties.

Tubular sections are efficient members for bracing in larger structures.Their inherent properties provide high load carrying capacities for lowmass of material and make circular and rectangular hollow sections (CHSand RHS) very attractive from a design point of view. However, for these

advantages to be reflected in the overall economy of the fabricatedstructure attention should be paid to the end connections since theirpreparation involves the largest part of the fabrication cost, (see Ref 4.7).

Economic connection details for bracing members are shown in Figs 4.8,4.9 and 4.10.


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(a) End connection(b) Simple crossover intersection

(c) Intersection using a pipe piece(no turnbuckles needed)

Fig 4.8 Details for rod bracing

(a) End connection (b) Typicalintersection

Fig 4.9 Details for angle bracing

(a) Flattened end (CHS only)

(b) Welded tee end

heavier structure providing lower maintenance cost in the future operationof the crane


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(c) Slotted end plate

(d) Typical intersection

Fig 4.10 Details for tubular bracing

4.2.5. CRANES IN PORTAL FRAME BUILDINGS

The most common crane type used in portal frame industrial buildings isthe electric overhead travelling crane. The crane bridge travels on twolongitudinal girders which are supported at each portal frame of thebuilding structure. Thedesignof a crane runwaygirdermust be consideredas an integral part of the whole building. At the same time, it must berecognised that because of the dynamic forces imposed on the runwaygirder, extreme economy in member and connection design is not

recommended and is considered unwise. The best solution may be a

of the crane.

Fig 4.11 Types of supporting columns

Fig 4.12 Crane runway brackets


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continuity may be off--set by extra cost and complication in the purlinsthemselves


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themselves.

C section purlins arenormallyused simply supportedat theends (Fig 4.16)

or continuous over two spans.

For shorter bay lengths purlins can be obtained long enough to be usedcontinuously over two spans. This reduces deflection compared withsimple spans but does not give the same structural performance as a fullylapped system.

The performance of purlin systems requires in most cases the provision of

adequate lateral stabilityby means of ties or bridging. Purlinmanufacturerssupply such systems, and some also offer accessory items such as rakinggirts, fascias, etc.

Details of proprietary purlin systems, design information and load tablescan be obtained from manufacturers literature.

Fig 4.14 Standard purlin cleats

Fig 4.15 Zed section purlins with lap

Fig 4.16 C section purlins with butt joint

4.2.7. FLY BRACING

In a portal frame building either flange of both the rafters and the columnscanbe a compression flange dependingupon theassumed magnitudeanddirection of wind loading. The exterior flanges are normally adequatelylaterally braced by the purlins and girts, but sometimes the design mayrequire the provision of bracing to the otherwise unrestrained interiorflanges.

This is most conveniently accomplished by the inclusion of so--called flybracing at purlin intersections (see Fig 4.17). This can easily become a

It can be seen that the choice of cladding determines the purlin spacingwhich in turn can influence some of the basic design parameters such as


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g p ( g ) yvery costly detail, and unnecessary expense can be avoided by the use of the simple flat bar arrangement as shown.

Fig 4.17 Method of fixing fly bracing to standard punching

4.2.8. SHEETINGCoated steel sheeting are the most popular and economic claddingmaterial for both the roof and walls of industrial buildings. (There may insome circumstances be regulatory constraints on its use in walling).

A variety of profiles is available, ranging from traditional corrugatedsheeting to sophisticated concealed fix products. All of these sheets aremanufactured from continuous strip and therefore can be supplied in most

cases so as to eliminate end laps. It is usual practice for sheeting to becustom cut by the manufacturer in the precise quantities and lengthsneeded for each particular project.

Except in cyclonic areas, steel roofing is capable of spanning about 1200mm in the case of corrugated sheeting up to as much as 2700 mm forstronger and deeper profiles. These figures relate to interior spans. Endspans for screw--fixed products should normally be limited to aboutthree--quarters of these figures. For walling, spans can be 25% to 50%

greater.

g ppurlin size and bay length.Steel sheeting is readily fixed to cold--formed purlins by means of

self--tapping screws. Special heavy duty self--drilling self--tapping screwswith in--built neoprene seals are normally used.Concealed--fix profiles are secured by separate clips or straps which arenormally attached to the purlins. On the finished job these straps arehidden and there is no piercing of the cladding surface.Where sheeting is to be painted for decorative purposes or to provideadded protection, considerable economy can be gained by the use of pre--painted cladding. The factory--applied finish avoids costly sitepaintingand provides far superior paint adhesion and quality.Full details of steel sheet cladding profiles, accessories, design and fixingdata etc., are obtained from manufacturers literature.

4.3. Large Span Storage Buildings

4.3.1. SPANS OF 45--70 METRES

When buildings of over 45m clear span are required for such purposes ascontainer storage, etc., consideration should be given to the use of portal--truss systems for economy. Spans of 45 to 70m are economicallysatisfied with such systems (Fig 4.18).

Fig 4.18 Three--pinned portal truss

The factors affecting the economy of the fabricated structure in such truss

systems are those common to truss--work in general and these are

discussed in Clause 8.4. Other considerations such as bracing, sheeting,etc., are as discussed in Clause 4.2.


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4.3.2. SPANS IN EXCESS OF 70 METRES

Spans greater than 70m are required for structures such as aircrafthangars, large stadia or storage buildings. Several buildings have beenbuilt in recent years using a space frame system of the flat double layertype (Fig 4.19), although other types are also available.

The success of space structures, as in all structures, greatly depends ontheuseof anefficient jointing method(orconnection). In Australia there areseveral proprietary joints readily available and a full discussion of spaceframe systems may be found in Refs 4.5 and 4.6.

The inherent economy of space structures lies in the fact that the frame ismade up of a large number of similar elements which can be fabricated in amass production operation. The erection of the frame can be oftenaccomplished by assembling the frame on site at ground level and jackingit into position on the column supports.

From an overall economy point of view, however, space frames should beconsidered only for applications where extremely large clear spans are

required to satisfy building function. They may be selected for otherapplications purely for architectural reasons.Fig 4.19 The basic square grid double layered space frame

4.4. Heavy Industrial Structures

These structures can be considered as almost entirely custom designed tofulfil the function demanded of the engineering or manufacturing processinvolved. It is therefore most important that the designer adopt a

rationalised approach to member selection and standardized connectiondetails in order to achieve the most economic frame within the functionalconstraints.In structures such as steel--mill buildings or power stations, the membersare often massive in comparison with normal building structures andcertain considerations assume greater importance.


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5. Commercial Buildings


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5.1. Introduction

In contrast to the industrial structures discussed in Section 4, where thecriterion controlling the framing arrangement was often building function,the commercial or office type building is usually of a more regular layout. It

is this characteristic which allows the greatest economy to be obtainedthrough standardization and repetition of structural elements andconnections.

This category of steel building comprises a grid of steel beams connectedto steel columns (or composite columns or concrete shear walls) usingeither simple or rigid connections. Resistance to lateral loads may beprovided by using some form of bracing with steel elements or other typessuch as in--fill walls or shear walls, or by frame action using rigid

connections.This type of building can be divided into two categories:

(a) Low--Rise Commercial -- e.g. suburban office blocks of up to 4storeys, schools, shopping centres, etc.

(b) High--Rise Commercial -- e.g. city office buildings, hospitals.

5.2. Low--Rise Commercial Buildings

This category can be further sub--divided into:

1. Fully steel--framed structures.

2. Composite frames (steel frames connected to concrete cores or

utilising masonry in--fill panels).

5.2.1. FULLY STEEL--FRAMED

Low--rise buildings fully framed in steel offer advantages in building speedandtherefore in theoverall economy of thefinal building. Because low--risebuildings do not require large stabilising elements, a steel frame using onlysimple connections can be used, offering economy in both fabrication and

erection. The stabilising element is usually provided in the form of a steelcross--bracing systemin oneor twodirections which canbe incorporated ina facade treatment so as not to intrude into window openings. Another framing system which has been used successfully for low--risebuildings is the one--way--rigid, one--way--braced system (see Fig 5.1).

Fig 5.1 Framing system for low--rise commercial buildingThis is essentially an extension of the industrial portal frame structure andresults in an economic solution for small commercial buildings wherefreedom of layout and planning can be provided across the building widthsince no internal columns or bracing elements are necessary.In the design of such a building, it should be recognised that bays of equalsize will assist in gaining maximum economy by allowing the repetitive use

of similar sized beam and column sections. The economic detailing of

beams and columns is most important in achieving overall economy andaspects of this are contained in Section 8.

For the case shown in Fig 5.2 it should be remembered that the steel framemust be effectively temporarily braced during erection and properly

l b d b f th b i k k bl k k b l id If th t


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Undoubtedly the greatest advantage of a fully steel framed structure lies inthe ability to erect the entire structural framework on prepared footings, as

a self sustaining system before any other building trades are required onsite. With proper planning, this feature can lead to faster building speedandtheelimination of many of theproblems associated with diverse tradeson site simultaneously.

5.2.2. COMPOSITE FRAMES

Currently a favoured type of construction for steel low--rise commercial

buildings is the provision of a stabilising element comprising a masonry orreinforced concrete core, with the steel floor beams connected with simpleconnections between periphery steel columns and the concrete core. Forthe low--rise commercial building, it is also common to use in--fill masonrypanels to provide lateral stability. Examples of these systems are shown inFig 3.7.

Typical details of such a framing arrangement are shown in Fig 5.2 for thecase where masonry panelsareused to provide thestabilising element inabuilding frame.

Fig 5.2 Stability by masonry

plumbed before the brickwork or blockwork can be laid. If the temporarybracing has to be removed after stability is provided by the infill panels itcould be placed on the inner flange of the columns in order to facilitate laterremoval and in order not to interfere unduly with the masonry work.

Fig 5.3 Stability by concrete panels

Fig 5.3 shows an alternative method of providing a stabilising element inthe form of a concrete panel cast between two adjacent steel columns andtied into each. In this case, the wall thus produced would normally beconsidered as load--bearing and would support stair--landings etc.,throughout the height of the building.

In addition to the concept of composite frames, the use of compositebeam--slab systems will provide best economy in these buildings. This isdiscussed in Clause 5.5.

5.3. High--Rise Commercial Buildings

5.3.1. GENERAL

In Australiaat present a high--rise commercial building will usually be a cityoffice block of up to 50 floors. In these buildings, a regular column grid canbe established resulting in repetitive bays in one or both directions. Aspreviously mentioned, regularity of bays is important since it leads tomaximum economy due to repetition.The architectural and aesthetic requirements usually control the exteriorcolumn spacing and therefore the bay sizes. A panel wall design withcolumns contained within the wall thickness allows maximum freedom in

bay size selection, whereas when columns are exposed externally as an

architectural feature this results in the least flexibility in bay size selection.Bay sizes should be selected to produce minimum storey height. It isnoteworthy that a saving of 75 mm per floor in a 20 storey building will save

(b) Steel frames connected to reinforced concrete cores.

In the selection of the best framing system the most important


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noteworthy that a saving of 75 mm per floor in a 20 storey building will save1500 mm of exterior andinterior wall,partitioning, columns, lifts, etc. Ontheother hand, columns cannot be spaced so closely as to detract from theusefulness of the space through which they pass. Selection of bay sizes isalways a compromise between these two considerations.

In a way similar to low--rise commercial buildings, high--rise commercialbuildings can be sub--divided into:

(a) Fully steel--framed structures, and

In the selection of the best framing system the most importantconsideration is to find a structural form which is highly efficient underlateral loadings and which does not require an unreasonable premium inframe cost to resist those forces.

A vast number of alternative steel framing systems have been successfullyused in the past, but not all of these are economic under todays conditions.Fig 5.4 shows some of the frame types suitable for buildings of variousheights.


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Fig 5.4 -- Optimum steel framing systems for buildings of various heights

5.3.2. FULLY RIGID FRAME In view of the relative costs of shop and field welding, the stub girder shopwelded to the column will generally prove a more economic solution forrigid framework


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From a planning and layout point of view this system obviously createsmaximum freedom since no stabilising elements arerequired in thevertical

planes of the building framework.The system is suitable for buildings up to 30 storeys in height but should beconsidered only when constraints of planning and layout are unavoidable.

It has the advantage of allowing efficient use of material because of theconsiderable interaction between beams and columns due to the use of rigid connections with resultant continuity in beams. However, in todayssituation, rigid connections are more costly to fabricate and this will often

offset any savings in material. In addition columns will generally be moreexpensive because equal stiffness about both axes is required.

In the USA where frames of this type have been in use for many years, thebasic method was to erect columns and field--weld beams at floor levels --see Fig 5.5.

Fig 5.5. Field welded connection details

However, since this method required the field welding of the most critical joints in the structure, where both high quality welds and high constructionspeed was required (both being subject to weather and operator skill), thismethod has been refined by transferring the welding operation from thefield back into theshop. This is accomplished by using the Christmas Tree

concept as shown in Figs 5.6 and 7.9.

rigid framework.

5.3.3. FULLY BRACED FRAMES

Fully braced frames of the type mentioned below are braced tubes wherestability against lateral forces is provided by the braced action of theexternal building wall framing.

Fig 5.6 Shop welded connection details

Bracing across full building width

If the total facadewidth of the buildingcan beconsideredasa vertical truss,the resulting frame offers maximum stability against lateral forces and thissystem can be used for almost unlimited storey height.

The advantages of braced frames lie in the use of simple flexibleconnections throughout and these are the most economical to fabricate. Inaddition, smaller columns can be used, often merely rolled sections. Thefloor beams on the other hand will tend to be heavier because no beamcontinuity is available but this mass addition will almost always be more

than compensated by the less costly fabrication required.

Bracing by shear truss in external walls

For buildings upto50storeys a shear truss in the plane of the externalwalls

5.3.4. STABILITY BY MEANS OF SERVICE CORES

Si b ildi f h d di i i i bl i


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For buildings upto50storeys a shear truss in the plane of the externalwallsprovides good stability characteristics and has the advantage of notintruding into facade treatment as much as the full width bracingmentioned.

Architecturally, cross bracing has never been readily accepted. Someexceptions to this do exist overseas and in Australia, but in generalengineers are expected by their architect to conceal bracing in buildingfacades, and this can often be done by accepting a compromise betweenthe space possible in a bay opening and the bending induced in floorbeams-- see Fig 5.7.

Fig 5.7 Forms of bracing

Fig 5.8 Bracing should connect to column

Since building structures of the type under discussion invariably require aCore in which are contained lifts, stairs, service ducts etc., it is convenient

to consider the core as a major stabilising element to resist lateral forces.The floor beams are simply connected between steel periphery columnsand the core structure, with resultant economies in fabrication anderection.

Steel framed service core

A fully braced core structure using steel elements can be erected veryquickly as a free standing structure and provides convenient access to alllevels of the building throughout the construction phase.Bracing can normally be placed to accommodate the necessary openingsand provide adequate stabilising function for buildings up to 50 storeys.

Slip--formed concrete core

Development of efficient slip--forming techniques has resulted in theconstruction of concrete cores becoming a fast, economic buildingprocess. Because such a central core is essential to house buildingservices such as lifts, stairs, ducting, etc., it is logical to consider using thestrong core as themajor stabilising element for a multi--storey building(seeFig 5.9). This system has been successfully used in many recent buildingsconstructed in Australia and overseas.

Using this methodof stabilising the frame, the lateral forceson theexternalwalls of thebuildingaretransmitted to thecore through thefloors. Thefloor,which usually consists of a concrete slab acting compositely with its steelsupporting beams (see Clause 5.4), is considered as a deep horizontaldiaphragm, and is extremely effective in transmitting lateral forces to thecentral core.

The position of the concrete core within the building has a significant effecton its structural behaviour under lateral loads. If the core is asymmetrical,rotation in addition to translation will be generated under lateral loads. Thisis an important consideration when the core is situated at the extreme end

of a rectangular shaped building -- see Fig 5.10.


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Hybrid girders are plate girders using a stronger grade of steel on thetension flange of the beam and possibly part of the web. One economicalway of fabrication is to cut two universal sections of different grades


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symmetrically and reweld them with a central web butt weld. The beamsmay be made castellated or can have a solid web. These girders areparticularly suitedwhere thebeam is to be made compositewith a concretefloor slab, but have been rarely used in Australia.

The profiled cutting and rewelding of a universal section to form acastellated girder containing web openings results in a girder which isdeeper, stronger and stiffer than the original section. The web openingscan be used for ducts and piping. Consequently, castellated girders canpermit a reduction in the overall mass of the floor system, leading tosavings in total building cost. The savings in material must, however, beconsidered against the increasedcost of fabrication with this type of girder.Computer numerically controlled (CNC) cutting and welding equipmenthas improved the economic viability of castellated beams.

Further discussion of these beam types is contained in Section 8.

One other type of floor support system deserves some mention -- the stubgirder system. This is a novel system reported from its use in the UnitedStates to increase the economic span limit for steel beams while, at the

same time, providing space for mechanical ducts without any increase infloor height. The system consists of short lengths (stubs) of rolled sectionwelded at intervals to the top flanges of the girder, the stub beams beingspaced at intervals which allow secondary floor beams and ducts to passbetween the floor slab and the girder.

Fig 5.11 Floor support members

5.5. Composite Construction

The current trend to steel framing forcommercial buildingshasbeen dueto

connection between the beam and slab at the interface. These elementsare known as shear connectors, of which the most economic type is thewelded stud (see Fig 5.13).


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g ga large extent to the development of composite construction techniques.This concept is based on designing a structure to rely on some degree of interaction between elements of different materials. The economical useof materials should be the keynote in all modern building design. Compositesteel--concrete construction in slabs,beams andcolumns, using both steeland concrete to maximum advantage, isone of the most effective means of achieving this objective.

5.5.1. FLOOR SYSTEMS

In composite structural framing the term composite steel beam refers to afloor system comprising a steel beam acting with a concrete slabcomponent on its top flange, interconnected to the slab such that both forman integral unit. The principal advantage of this lies in the fact that theconcrete slab not only spans between and distributes the loads to the mainbeams but also forms part of the beams themselves (Fig 5.12).

Fig 5.12 Composite floor beam system

In the types of composite beam--slab systems in this discussion, theconcrete slab can be constructed in several ways. One of the bestnowadays is to cast it on profiled steel sheeting -- the sheeting serving aspermanent formwork when the slab is poured. The method of achievingcomposite beam action involves the provision of some form of mechanical

Fig 5.13 Welded stud shear connector

A conventionally formed slab system could be used as an alternative, butrising costs of the removable formwork material and the associated labourare making steel decking systems more attractive. In addition, theprovision of extensive propping to the underside of formwork and the timedelay in its removal mean that following trades arehindered in proceeding,thus negating the advantage of steels fast construction.

If steel decking is to beusedit isprobablybetter to use a typewhich will alsoact compositely with the slab by becoming the positive reinforcement.Several forms of composite steel decking are currently available in Australia and are made from high--strength zinc--coated steel. Typicalprofiles are shown in Fig 5.14.

Fig. 5.14 Profiles of composite galvanized steel decking

The use of composite steel decking provides for double economy. Firstly, itprovides a low cost and efficient floor slab by eliminating the need for all ormost of the lower reinforcement. Secondly, it has thebenefits of permanentf k h d i t ll ti th d f t d


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formwork such as speedy installation, a weather and safety cover, and animmediate working platform for other trades.

Steel decking is used to its optimum advantage in steel framed buildingsbecause full advantage can be taken of sheet continuity to increase slabload capacity and because the resultant slab can also be made compositewith the steel beams. This means that composite action is achieved in twoways:

(a) Within the slab, and

(b) Between beam and slab.

Design methods for composite floors are readily available (see Refs 5.3,5.4, and 5.5).

5.5.2. COLUMNS

The concrete encased steel column is a further example of compositeaction. Encasing of columns is often required to satisfy the architecturalfeatures of building facades and to provide fire protection to the steel

column. The opportunity exists to consider a relatively small steel columnsection, designed to carry construction loadings, which can besubsequently encased and, as a compositesection, designed to carry totalvertical loading. The steel column can be used as reinforcement in the finalcomposite column, or, where a larger final section is required additionalreinforcement can be introduced (see Fig 5.15).

Fig 5.15 Composite columns incorporating a steel erection column

By proceeding in this way the erection of the structural frame is not

controlled by the time taken for the forming, pouring and curing the finalshape of a wholly concrete column. The steel column can be designed tosupport say 6 to 10 floors of structure, and the building program is plannedso that the encasement of the lower columns becomes a relativelynon--critical item in the construction sequence.

The converse of a concrete encased steel column is a steel tubular columnfilled with concrete, which also provides composite action.

Small or medium sized columns might be RHS or CHS; larger columns arebox or tubular sections fabricated from steel plates -- see Fig 5.16.

These tubular composite columns make for quick and easy erection and of course they eliminate the need for concrete formwork. In the larger sizestheir overall economy depends upon the ability of the fabricator tomanufacture the tubular sections efficiently.

remember is that such buildings, in order to be viable business ventures,require to be constructed with maximum economy of time, materials andlabour.


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Fig 5.16 Composite column comprising a concrete--filled tubularsection

5.6. Summary

From a technological point of view, the design of commercial buildings isrelatively well understood. However, in todays scene the important point to

Many city buildings in Australia in recent years have been constructed

using the steel frame to concrete core method and it is apparent that thissystem is proving economic in the current situation. High on--site labourcosts are causing a return to the principle of prefabricating buildingelements off--site and then simply assembling them to form a buildingstructure. As tall buildings, by virtueof their large number of identical floors,require a vast number of repetitive structural members, it is in thesestructures that economy can be achieved by the adoption of rationalisedmember design and standardization of connections. Steel beams whichconnect periphery columns to a central core and carry the slab on steelsheet decking (composite with the steel beams) will usually prove a mosteconomic solution in commercial buildings.

When assessing different structural systems, designers should becognizant of the relative cost components (see Fig 5.17) to enable a morerational approach to the framing system.

Steel Deck Supply& Fix = 21%

Slab = 23%Steel Supply= 31%

Fabrication= 8%

Surface Treatment = 13% Steel Erection = 4 %

Fig 5.17 Cost Components for a Multi--Storey Building

6. Bolting


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6.1. Introduction

The selection of a bolt for use in a structural steelwork connection will needto have regard to a variety of factors including:

(a) Load capacity of available bolt types

(b) Cost of the installed fastener

(c) Amount of joint slippage

(d) Nature of the forces to be resisted

(e) Degree of flexibility/rigidity desired in the joint;

in order to obtain, at least cost, a safe bolted connection.

Bolting of Steel Structures (Ref 6.1) contains a detailed discussion of allof the above factors and provides a state--of--the--art summary of mattersrelated to the use of bolts in steel structures.This section concentrates on aspects which affect the economic use of bolts. Ref 6.1 should be consulted for more details of all aspects of the useof bolts in steel structures.

The cost of a bolted connection includes:

1. Cost of obtaining, cutting and holing components

2. Cost of the bolts3. Cost of installing the bolts

4. Cost of inspection.

Every bolt specified should be a bolt that is needed -- bolt numbers shouldbe kept to the minimum needed from strength considerations.

The cost of installing bolts can vary considerably, depending on the boltingcategory.

6.2. Bolt Types

The two basic metric bolt types in use in structural engineering in Australiaare:

(a) the commercial (Strength Grade 4.6) bolt ;(b) the high--strength structural (Strength Grade 8.8) bolt.

The identification of high--strength structural bolt and nut assemblies canbe readily made from the bolt head and nut markings (see Ref 6.1). Inaddition, a distinguishing feature is the larger bolt head and nut of thehigh--strength structural bolt compared to the commercial bolt.Only a limited range of sizes of these bolts is of interest to structuralengineers.

6.2.1. COMMERCIAL BOLTS

The commercial bolt is commonly used in the following diameters (theprefix M is used to designate ISO metric bolts):M12 -- purlin and girt applications

M16 -- cleats, brackets (relatively lightly loaded)

M20, M24 -- general structural connections, holding down bolts

M30, M36 -- holding down bolts.

6.2.2. HIGH--STRENGTH STRUCTURAL BOLTS

The high--strength structural bolt is most commonly used in diameters:M16 -- designed connections in small members:

M20, M24, M30, M36 -- flexible connections, rigid connections.Larger sizes (M30, M36) ofthe high--strength structural boltshould be avoided when full tensioning is required, since


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The following flow chart is for bolt usage with flexible joints:

6.5.2. BOLT USAGE -- FLEXIBLE JOINTS

SIMPLE (FLEXIBLE) JOINTS


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SIMPLE (FLEXIBLE) JOINTS

Not calculated orvery low stress levels

Structural Joints

Commercial BoltsProperty Class 4.6 to AS1111 -- Snug tightened

Commercial BoltsProperty Class 4.6 to AS1111 -- Snug tightened

High Strength Structural BoltsProperty Class 8.8 to AS1252 -- Snug Tightened.

Category 4.6/S

Category 4.6/S Category 8.8/S

Threads in shear orbearing plane is most

common situation

Low capacity Twice capacity of 4.6/S

Threads included in shear planeThreads excluded from shear plane

No stick through problem Possible stick--through problem

Most realistic from erection viewpoint Difficult to inspect

Lower capacity (30% less)than threads excluded

Greater capacity than threads includedGENERALLY PREFERRED(See Clauses 6.4.4)

The following flow chart is for bolt usage with rigid joints:

6.5.3. BOLT USAGE--RIGID JOINTS

RIGID JOINTS


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RIGID JOINTS

Friction Type

High Strength Structural Bolts Property Class 8.8 to AS 1252Fully tensioned to AS 4100 (Category 8.8/T)

Category 8.8 / TF

Threads permitted in shear plane

GENERALLY PREFERRED(See Clauses 6.4.4)

Category 8.8 / TB

Bearing Type

No slip

Lower Capacity than 8.8 / TB

Design for no slip in the

serviceability limit state but alsocheck for strength limit state.

Slip occurs

Higher Capacity than 8.8 / TF

Threads includedin shear plane

Threads excludedfrom shear plane

No stick--through problem Possible stick--throughproblem

Most realistic from erection viewpoint

Difficult to inspect

Lower capacity (30% less)than threads excluded Maximum Capacity

7. Welding


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7.1. Introduction

7.1.1. PRINCIPLES FOR ECONOMY

The aim of weld design should be to provide the necessary structuralperformance throughout the lifetime of the structure for the lowest

completed cost. To achieve this attention must be given to:(a) Eonomical design and detailing

(b) Good welding procedure and correct process selection, and

(c) Responsible inspection.

The design and detailing will greatly dictate whether or not an economicalwelded connection can be produced and consequently is one area wheregreat attention should be paid. Whereas the selection of the weldingprocedure and process to be used is the province of the fabricator, thedetailing of the welded connection can often influence or limit the range of options available. Consequently, the design and detailing of the weldedconnection must have some regard to the processes and proceduresavailable if an economical welded connection is to result. Responsibleinspection is also a vital item in keeping the final cost to a minimum.

The design engineer can best approach the objective of obtaining, at leastcost, a safe welded steel structure or connection by considering thefollowing influences during the design:

1. Available welding processes that might be used

2. Welding consumable selection

3. Code requirements (AS 4100, AS 1554)

4. Joint details and type of weld

5. Size of weld

6. Whether to use shop or field welds7. Accessibility8. Responsible specification9. Inspection.

7.1.2. COST COMPONENTS

The cost of welding can be considered as follows:

Cost of actualwelding =

Time to weldper unit length

Length ofweld

Cost perhour

OperatingFactor

where:

Time to weldper unit length=

Weld VolumeDeposition Rate

Operating Factor= Actual Arc Time

Total Time

Cost per Hour= labour rate plus oncost

These relationships indicate that a designer or detailer can minimise thecost of welding by attention to the following items:(a) Minimising weld volume(b) Allowing for the use of high deposition rate processes; in some

connections, the detailing can restrict the use of a particular pro-cess thus forcing the fabricator to use a less efficient process

(c) Considering other factors which influence the deposition rate. Forexample, downhand welding is far more productive than over-

head or vertical welding, so that details should be oriented fordownhand welding wherever practicable

(d) Using clean and simple detailing to assist in maintaining as highan operating factor as possible

TABLE 7.1 FILLET WELD COMPARISON

Fillet size (mm) Weld strengthrelative to 4mmsize

Weld area rela-tive to 4mmsize

Increase inweld strengthfor next size

Increase inweld area fornext size (%)


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p g p

(e) Aiming to permit as much welding in the shop as possible, be-cause the cost per hour and the operating factor are both morefavourable in the shop than in the field

(f) Selecting the material grade to assist in eliminating or minimisingthe costs of preheating or post weld treatment.

7.2. Types of Welds

7.2.1. FILLET WELDS (see Fig 7.1)

The features of fillet welds are:

(a) Economically attractive up to 12--16 mm leg size

(b) Minimum edge preparation

(c) Easy fit--up without tight tolerances(d) Poorer load carrying capacity than equivalent complete penetra-

tion butt weld and poorer fatigue characteristics

(e) Intermittent fillet welds are permitted but these are usually onlyeconomical for limited applications involving the use of manual orsemi--automatic processes; in many applications, a full lengthfillet weld of one size may be placed more economically using a

fully or semi--automatic process(f) In the horizontal--vee (HV) fillet position, up to 8 mm fillet sizes

may be placed in a single pass using manual metal arc pro-cesses; with other processes (semi--automatic or automatic) alarger single pass fillet weld is possible. Such processes are nowcommonly used.

(g) If more than a single pass fillet weld is used, the cost of the weldcan increase significantly.

size size for next size

(%)

next size (%)

4 1.00 1.00 25 565 1.25 1.56 20 446 1.50 2.25 33 788 2.00 4.00 25 5610 2.50 6.25 20 4412 3.00 9.00 33 78

16 4.00 16.00

The cross--sectional area of a fillet weld varies as the square of the leg sizewhile the strength of a fillet weld (which is based on the effective throat)varies only linearly with the leg size. As indicated in Table 7.1, there is aheavy cost penalty in over--welding.

Automatic processescan reduce the cost of a fillet weld since, in addition toimprovingproductivity, the increasedpenetration allows a reduced legsizefor the same throat thickness.

than complete penetration, although attention is needed to en-sure that the specified depth of penetration is achieved in prac-tice. These welds are permitted to carry only shear and compres-sion loads and have low ratings for fatigue conditions.

Typical details of both types are shown in Fig 7 2


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Cruciform

Fig 7.1 Types of fillet welds

(a) Complete penetration butt welds

(b) Partial penetration butt welds

Fig 7.2 Types of butt welds

7.2.2. BUTT WELDS (see Fig 7.2)Two forms of butt weld are permitted in AS 1554 and AS 4100:

(a) Complete penetration -- used where the full strength of the con-nected parts is required. Such a joint is given the full strength of the joined components.

(b) Partial penetration -- used where less than full strength is accept-able, such as in low stress areas. These welds are less costly

Typical details of both types are shown in Fig 7.2.Butt welds usually require special edge preparation which (depending onthe preparation type and the cutting practice) can add to the cost. Types of edge preparation normally in use are:

1. Square (no special preparation)2. Single or double bevel3. Single or double V4. Single or double J5. Single or double U.

When selecting joint preparations for butt welds, prequalified preparationsshould be used wherever possible to obviate the need for qualificationtesting of the weld geometry.In selecting the included angle in a butt weld preparation, it has beendemonstrated that, in general terms, the smaller the included angle in thepreparation the less is the weldvolume (Ref 7.2). There isa need to temperthis provision with a consideration for leaving sufficient angle for electrodeaccess -- the requirements will vary between processes.It is therefore probably better for the design engineer to specify therequirements (e.g. complete penetration butt weld or partial penetrationbutt weld, depthof penetration 12mm)andallow thefabricator to select thebest weld geometry/welding process combination to achieve the desiredresult. All such proposals can be submitted to the designer for approval if necessary.

7.2.3. BUTT WELDS vs. FILLET WELDSIt is important to note that the volume of weld metal in a butt weld (partialpenetration or complete penetration) depends on the type of preparationused as well as the depth of penetration. In contrast, the fillet weldincreases in weld volume as the square of the leg size.In comparing the relative costs of butt weldsandfillet welds, these differingrelationships should be borne in mind, in addition to the fact that the buttweld usually requires edge preparation while the fillet weld does not.


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7.4. Other Cost Factors

7.4.1. WELD CATEGORIES

category those welds which could otherwise, according to the guidelinesgiven in the Standard, be categorised as GP.

Mixing Weld Categories

Weld categories can be mixed on a project but should not be mixed along a


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The Structural Steel Welding Code, AS 1554 specifies two categories of weld, these being:

GP -- General Purpose

SP -- Structural Purpose

The difference between the two arises from the more stringent quality andinspection requirements of the SP category over the GP category.

The Steel Structures Design Code AS 4100 has been used as thereference standard from which the permissible levels of imperfections forGPand SPwelds havebeenset. In other words, AS 1554and AS 4100arecompatible.

Category GP

The GP weld is the less stringent of the categories. It is intended for use in joints which are statically loaded, and where the design load on the weld is

significantly below its full design capacity. It should be noted that for GPCategory, the capacity factor is 0.6 as compared to a range of 0.70 to 0.90for the SP Category (see Table 3.4 of AS 4100 -- 1990).

Category SP

The SP category is the full--strength structural weld for use in staticapplications where the higher range of capacity factors is used. SPcategory is also mandatory for dynamic (fatigue) applications -- see AS4100 and AS 1554.

Choice of Weld Category

GP category welds will occur quite frequently in certain types of application. The designer should always endeavour to specify

Economical Structural Steelwork

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