Economics of Steelwork Design

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Paper to be read before a meeting of the Institu tion

f

Structural Engineers at Upper Belgrave Street, London

W l X

HB on Thursday

13 October 1977.

The economics

of

steelwork

design

F.

H NEEDHAM

A graduate of Impe rial College. Mr. M eedham spent the early

years of his career gaining experience in design of steelwork with

joini ng the steelindustry in 1962. From 1962 -70 he headed the

fabricators, consulting engineers and a local authority, before

structural engineering section of the British Iron Steel Research

Association, Battersea. and in 1970 was transferred to the

headquarters of Redpath Dorman Long Ltd., Bedford, as headof

Product Research and D evelopment Department. Since October,

1974 he has been Chief Engineer, Design D evelopment at

Constrado, Croydon.

He served on the Council of the Institu tion from 1963-66. and

during th at time was on the Science and Research Com mittee and

the Literature Committee. He wasa founder committee member

of the Bedfordshire

Aao inin g Counties Section of the

Institution. being Chairman in 1973-4. He currently serves on the

Institu tion Code Servicing Panel on Stability. He has been oin t

author of tw o revious papers presented o the Institution. for

one

of whichhe was awarded he Henry AdamsAward

Diploma in 1967-68.

He serves on a number of BSI committees, notably B.716 and

8.20. being chairman of the B.2012 General Princip les

sub-committee.

Synopsis

This paper sets ou t n qualitat ive terms guidance for engineers

n

ach ieving the economic

se

of structuralsteelwork.t

identif ies the major cost i temsn the total construction process

and emphasises the importance of their re lativealues. Then

the cost i tems of the steelwork i tselfre given and discuss edn

detai l and the paper indicateso w m o n e yan be saved or

wasted depend ing on the approach adopted by theesigner.

It ind ica tes ho

w

a knowled ge o f thearious steps involved

n

the steelwork contractor s function can be of great value

n

deriving the most economic structuralsolution.

The Structural Engineer/September 1977/No. 9/Volume

F.H. Needham B S ~

Eng FIStructE

FICE

Chie f Eng ineer, Design Deve lopment C onstrado

ntroduction

In considering the economics of steelwork esign, one is fre-

quently asked to give estimates on a costltonne basis for

various kinds of structural steelwork. Quite apart rom he

impossibility of quoting figures in money terms at a time of

rapid nflation and changing elative values, such nquirers

frequently overlook the fact thathe form of structure and he

manner in which

t

has been designed can have a profound

effect upon the overallost. Theaim of thisaper is to high light

some of the factors which need to be considered n order to

produce designs which satisfy the client andwhich the steel-

work contractor can produce and erect to schedule and at a

profit.

BeforeLproceeding

t is wor th defining 'design'. To he

academic or computer special ist

t

usually means analysis to

determine forces, moments and shears in apredetermined

structure under idealised oading. To the steelwork designer

or detailer i n a abricator's office t means the selection of

member sizes and the determination of beam reactions and

moments which are shown ona line diagram (frequently wi th

no thought eing given o overall behaviour y either of them).

To the workshop a good' design is one which gives a high

tonnage output bonus with the minimum of trouble. To the

erector a 'good' design is one in which allarts are of the ame

weight, and is one with the greatest proportionof nter-

changeable parts, the remainder being so different that they

cannot be wrongly transposed. Particularly good is one in

which only one bolt size is used. The meaning of 'design' as

used in this paper means thedevelopmentof hewhole

project from its inception, takingnto account all the influences

which have an effect upon he orm and cost of he inal

structure.Theeconomics of design must be considered

in

this ast context since he client is mainly concerned wi th

what he ays or, a complete buildingwhich meets his needs.

The Total

Project

In any economic tudy, the first stage is to identify the principal

cost items. In bu ilding work they are as follows:

Design calculations, preparation of contract drawings

and the writing ofdetailed specification

'Site clearance

Foundations andrainage

Construction of load bearingr skeletal tructure

Construction of floors and roofs

Fixing of ladding, and/orbuilding of alls and partitions

Fixing of services and lumbing

Finishes and painting

Site supervision

67



The relative costs of these items vary substantially depending

on the type of building and its location. Forexample, in a

modern office b lock services and plumbing (inc luding lifts, air

conditioning, etc.) can amount to more than

50

per cent of the

whole-excluding land costs. Conversely, in a farm building

or an open gantry structure, the framework, and cladding i f any,

account for 90 per cent of the cost. Similarly, in some cases

site clearance can dominate-in others foundation costs can

outweigh a l l other considerations. In a prestige public building,

finishes can be the largest single item.Clearly hen,

it

is

inappropriate to consider (iv) load bearing or skeletal structure,

in isolation from the other items. Therefore begin by

putting things in perspective by recognising the kind of

structure, its purpose, its location and the ground onwhich it s

to

be built.

It

is worth at this stage sounding a note of caution. Cost

savings are often attempted by reducing or eliminating (ix)

site supervision, on the supposition that if the contractor and

sub-contractors all know their jobs (and the building work is

subject to periodic inspection by the Local Authority) further

supervision is unnecessary.

It

is false economy. In nearly

everycase of failure either catastrophic collapse or simply

unserviceability leading to unacceptably heavy maintenance

charges,

it

is found that there were defects of supervision.

Failures make poor advertisements, apart. rom the distress,

delay and expense ncurred, and do not make for satisfied

clients.

The Steelwork

Having said al l this

it

falls to most structural engineers to

consider item (iv ) load bearing or skeletal structure, i n detail,

and again it is worth identifying the principal ost items, which

are

:

(a) Basic design-Is the building likely to be fully framed

or with load-bearing walls? If he former does one use

steel, concrete, or timber, or perhaps a combination? A

rational decision can only be taken by doing one or two

sketch designs and using approximate current cost

figures.

On the supposition that the outcome points to steel, or at

(b) Detailed design and workshop details

(c) Cost of steel from mills or stockholder

least shows steel to be feasible, the ensuing cost items are

:

N.B. Steel ex-stock costs some 10 per cent more than

direct from the rollingmills

(d) Fabrication cost

(e) Transport and shipment

(f) Erection

(g) Painting

(h) Cladding and finishing (in the case of industrial

All these facets interact wi th one another, as wil l be seen, bu t

buildings)

for clarity they w il l be considered individually.

(a) Basic design

The relative importance of various factors have to be weighed

up. They are

:

(i ) First cost

(i i) Ease of maintenance

(iii) Aesthetics

(iv ) Likelihood of future extension or change of use

Clearly in most cases, lowest first cost dominates, but some-

times one of the other factors is of greater importance. For

instance, in the case of a public building, such as an airport

concourse or

a

sports stadium, estheticsmay emore

important than first cost ormaintenance.Again, in highly

corrosive environments, such as a chemical plant, first cost

may be subordinated to the need to be able to repaint with the

minimum of trouble. Thus in this case one would opt for

welded portal frames ather than trusses,and rolled beams

rather than light lattice work, or alternatively, hollow sections

68

to minimise surface area of expensive coatings. Aesthetics do

not enter into consideration in that case. Again the need for

future extension, or adaptation to meet possible changes of

use may in other instances be al l important, and indeed may

dictate the choice of structural medium. Forexample, the

motor industry and manufacturers of consumer durables

nearly always use steel structures for this reason. A change of

model usually means a change of structural requirement in

terms of cranage needs and unway layout and capacity.

The answers to a l l these questions can only be obtained by

reference o the client, and t is surprising how rarely an original

inquiry gives any guidance on these points. Early discussions

with him are therefore vital in order to agree a design brief,

which will prevent a design being submitted which, although

it may be perfectly sound structurally, is not what he had in

mind. Such a discussion also serves to establish

a

good

working relationship.

Next structural layout,mustbe considered which wi ll be

influenced by the loads to be sustained, the requirements for

usablespace within the building, and in many cases the

constraints imposed on the structure by architectural matters.

It is

a t

this stage that overall structural behaviour mustbe

considered, particularly overturning due to wind.

LIFT STAIR

AND SERVICES

W E L L

’

‘OPEN PLANF F IC E

F L O O R

POSSIBLE LANESOF

>

BRACING SING

TWO

EXTRAOLUMNS

‘OPEN

PLAN OFFICE

‘,----

FLOOR

Fig 1 Example ofef fect o f brac ing on p lan layout

In high rise multi-storey buildings, sway due to wind s often

the design criterion. One then has to decide whether to use a

rigid-jointed frame,or rely on a stiff core, which can be a

braced steel tower or bu ilt of cast i n - s i t u reinforced concrete.

From an economic point of view an ounce of bracing is worth

a

ton of stiff joints in controllingdeflections. For example, in a

recent design for a bank building n Hong Kong, subject to

typhoon wind forces, the cost of the structure could have been

reduced by some 30-40 per cent if bracing had been accept-

able. It is interesting to record that this alternative was offered

to the client, and rejected on the grounds that obstruction

would be caused to an otherwise open plan layout, although

that obstruction was small (Fig 1 .

In factory buildings, cranage needs dominate, and if heavy

lift ing capacity is needed one is designing not a building with

a crane, but a gantry structure with an envelope. n such cases,

each main column and its foundation has to be designed for

the situation when the loaded crane passes over it. Thus the

size and cost of the main columns are almost independent of

The Structural Engineer/September 1977/No. 9/Volume

55



Fig 2 Al ternat ive ayouts or heavy worksh op bui ld ing

their spacing. Viewed in isolation, long span gantry girders

appear to incur a cost penalty, but this may be more than

offset by the reduction in the number of columns and their

foundations, particularly if the ground conditions demand

piles.Clearly a balancehas to be struck and several trial

designsmay be needed (Fig

2).

The roof design in such

circumstances is an incidental, its layout having 'occurred',

and again, viewed in isolation

it

may not appear economic.

On the other hand, without substantial cranage, you are

designing only the envelope and here you have to weigh up

whether you use a number of small span bays or one or tw o

of large span. Theormer wi ll give the cheapest steel structure,

particularly if headroom requirements are small, but the latter

may be more convenient and indeed cheaperoverall,The

reduction of obstruction from internal columns, which could

interfere with the layout of machine tools, now and in the

future, coupled w ith the elimination of valley gutters, internal

drainage and foundations may swing the balance. In such

cases if the steel structure is designed in isolation an economic

solution in terms of total buildingcosts is unlikely to result.

In practice

it

is probable that the apparent economy of using

short spans has resulted in waste, and in most cases much

longer spans would havebeenappropriate, possibly using

lattice construction in high tensile steel.

The current trend is towards flatter pitches. This partly

arises from legislation making it necessary to heat and insulate

most factory buildings, and partly from the whims of archi-

tectural fashion. Nonetheless, a truly flat roof wi ll always be

more expensive to make and keep watertight than one w ith

a

moderate pitch. This saving wi ll more than compensate for the

cost of

a

parapet masking he shape of the roof from an observer

a t ground level, which is mostly al l that the architect is con-

cerned about. Further, ideal ly the depth of a lattice truss should

match the shape of the bending moment diagram, to keep the

main chord forces more or less constant throughout the span.

This cannot be achieved exactly, but t is a useful aim as it

avoids changes of section in the main chords. Thus for once

two factors act together and

a

moderate pitch results in

a

good truss design (Fig 3 .

The competition between trusses and portal frames

continues, and

it

is often worthwhile going to tender on

alternatives.Relative fabrication costs depend more than

anything else on shop layout, plant and labour skills. The

third alternative, which can often be cheaper than either, is

often not even considered, i.e. tied portals. The tie need only

be quite light, and can be at eaves height or part way up the

A

B

A . MOR

EFFICIENT

STRUCTURALLY

B MORE

PLEASING VISUALLY

Fig

3

Roof truss designs

rafter and in long spans may need to be slung a t intervals, but

the savings possible in the rafter and column sizes can be very

substantial. This principle is by no means new, having been

used to good effect by the Victorian railway engineers, and its

virtues are still valid. Whether one can se tied portals depends

on whether one can obstruct the roof void and also whether

it is required to sling runway beams. If so, trusses are usually

preferable as they present strong points on the tie, and can

easily be strengthened locally, sometimesyearsafter con-

struction.

However much is put in to the design of the roof principals

it is worth remembering that they only form part of the building

steelwork, sometimes less than half the otal weight. The

remainder consists of purlins, sheeting rails,bracing, door

lintels and posts, and gable framing. Where freedom exists,

it is worth designing the purlins and rails first, and let their

The Structural Engineer/September

1977/No.

S/Volume

55

69



design dictate the spacing of main frames. Admitted his

is not always possible, and in any case inept design of the

door arrangements and gable framing can negate any savings

made through careful design of the frames and rails.

It

is also

a t

the basic design stage that one must consider the

choice of steel grade, as this will frequently dictate the layout.

Further, one should also consider whether to use mainly open

rolled sections (UB's,UC's channels and angles) or closed

sections (CHS or RHS) and it may be that the use of tubes in

high tensile steel can produce the best structure for the

particular purpose. (Tubes not being as sensitive to local

buckling as open sections). Again, only by trial designs can

the optimum solution e evolved.

In multi-storey work, particularly when composite action

is used in the beams and slabs, Grade 50

is frequently the most

economic solution. Further, if one can maintain the same basic

column size down the height of thebuilding, by using different

grades of steel as well as different weights of he column

section, money wi ll be saved on the etailing of curtain walling,

glazing, etc., apart from reducing substantially the cost of

stanchion splices. One must beware, though, of the possibili ty

of cross substitution and possibly deliberately introduce de-

tailed differences.

In single storey work, Grade50, or even55, may be economic

for trusses and lattice girders. Deflection and local buckling

may however preclude its use in portal frames. Strictly speaking

then, one should compare lattice work in HT steel with portal

frames or tied portals in mild steel. One wonders how often

this is done.

(b)

Deta i led design and workshop detai ls

The actual cost of design, relative to total building costs, s

really verysmall. It follows therefore that greatsavings in

design costs by the use of the computer, for instance, are

largely illusory, unless it can be made to

opt imise

on some of

the conflicting actors already considered. The cost of detailing,

however, can be significant and can amount to twenty times

the cost of he design process. Great potential cost savings

can therefore be made by computer detailing, which is being

developed in

a

number of quarters.

Whilst

it

may not be intellectually satisfying or philo-

sophically elegant, the aim of detailed design should be to get

the job done fast and efficiently and therefore at lowest cost.

To do this one should make the ensuing activities easy, for

by so doing mistakes are reduced and highproductivity

achieved. I t is fortunate that if a job is easy t o detail,

it

tends

to foll ow that fabrication is easy also. Detailed design there-

fore should aim at simplicity and repetition.

Therefore avoid individual design of everyeam

and every column length, i n an attempt to achieve leastweight.

nly rarely does this achieve lowest cost. Therefore

aim

a t

'grouping' of members to achieve which, it maybe

necessary to adjust the layout, if this is possible.

It

is in this

grouping hat heskill and judgement of the experienced

designer reveals itself. If too much grouping is done, material

is wasted-if too little, draughtsmen,plater's and erector's

time is wasted. Only by having done it or supervised

it

can the

skill be acquired.

By way ofexample, the standardisation of cleats and fittings

can achieve great savings; similarly stanchion basesand

splices should only vary with the column section. Of course

standard details which suit one shop may not suit another,

depending partly on he machine tools available and partly

on shop layout, wh ich may have been inherited. Conversely,

thehip-ended trussed roof which entailed a special half -

depth truss and hip rafters, necessitating setting out ful l size

on

a

template floor for purlin cleats is now

so

expensive that

it is never done. If templating has to be resorted to,

it

should

only be for members which are required in great numbers.

Detailed design then should recognise hat in spite of greatly

increased steel costs, labour costs account for some half to

tw o thirds of ex-works rices, and areikely to continue to do

so

The aim therefore should be to reduce labour costs to the

minimum. Again for example, web cleats for a beam can be

fixed with the beam standing on the work bench, the holes

in the web having possibly been drilled on an automatic

sawing and drilling machine. A welded end plate to perform

the same function requires the beam to be turned over, at least

once. If

it

can be manhandled, well and good, but if the crane

isneededdelay inevitably occurs,sinceone cannot plan a

total workshop activity so well as to ensure that the crane is

always available exactly when i t

is

needed (There ishe further

advantage of web cleats that should the erector discover that

the column flange to which the beam is to be fixed should be

out of square, some adjustment s possible).

Continuous consultation and collaboration between the

designer the estimator, the chief draughtsman,and works

manager and foremen is essential. Theyshould agree on basic

principles of detailed design and review their findings fre-

quently to adjust to varying conditions, including the personal

skills their menpossessor donot possess. In particular,

changes in labour rates for different classes of labour should

be fed back to the designers and estimators, as they are likely

to affect relative costs of different details which may call for

changes in standard procedures.

(c)

Cost

o

steel

It

is usual or the designer to assume that all steel of

a

particular

grade costs he same per ton. At the time of writing the cost of

plate is broadly comparable with that of sections, whereas at

one time plate cost somewhat more. Both plate and section

prices change not infrequently, independently of each other.

Also the rolling mills charge extra for quantities smaller than

20

tonnes of any one size on a sliding scale and prefer not to

accept orders for less than five tonnes. Having regard to the

likely lapse in time between design and steel supply, in such

inflationary times the designer can be forgiven if he takes the

easy way out and ignores price variations.

It is only in the design offices of the leading fabricators that

steel cost variations are taken into account in design, and not

always then. Clearly though, in small to medium sized obs,

if such variations are ignored, considerable cost penalties can

be incurred quite fortuitously. The pursuit of minimum weight

by the use of a wide variety of beamand column sizes,

necessitating the purchase of most steel from stock, must be

avoided. As a rule of thumb, if the increase in weight through

grouping does not exceed say 10 per cent on small jobs and

say

5

per cent on medium sizedobs, it

is

worth doing,

particularly if by so doing individual section quantities exceed

10

onnes.

It

is obvious that a rolled section should always be used in

preference to

a

plate girder, if possible, because of the con-

siderableextra fabrication cost of the latter. It is not un-

common to find n

a

consultant's design a plate girder used,

incorporating such refinements as curtailed flange plates, and

thin stiffened web.Thedesigner wi ll havespent hours in

achieving the optimum design and is no doubt well satisfied

as

a

result. A much cheaper solution, albeit weighing more,

can often be calculated in minutes. Where for any reason, a

rolled section by itself wil l not serve, consideration should be

given to a compound section with welded reinforcing plates.

This solution often offers the best compromise, although it is

not very popular,perhapsbecause

it

is old fashioned and

reminiscent of riveted construction. It can be valuable none-

theless.

It

is also worth remembering that the constraints

inherent in the roll ing process necessitate, n big beams,

a

web

thickness-much greater than the comparable plate girder. The

effect of this is that rolled beams rarely need web sti ffeners

except at points of support or under heavyoint loads.

In the case of long span lightly loaded beams, where

deflection tends to govern the design of

a

rolled member

castellated beams should beconsideredL:aszthey often offer

370 The Structural Engineer/September 1977/No. 9/Vdum e 55



the best olution.Whenexposed they are often visually

more acceptable and when masked by false ceilings offer the

facility forservices to be passed hrough theopenings. Stric tly

speaking though the cost of both material and fabrication for

such a beam ystem shouldbe compared wi th minimum

plain beams capable of fulfi lling thedesign requirements.

As indicated earlier, another variable entershe picture when

one considers steel grade. Broadlyne can say that if buckling

or deflection are not design criteria, then Grade

50

should be

considered. Fabrication costs wi ll increase, particularly when

thick sectionshave to bewelded,necessitating different

welding processes, but fabrication estimates on

a

rate per ton

basis are misleading, since different quantities are nvolved.

To draw a t rue comparison between Grade 43 and Grade

50

one should be dealing in money terms for the supply of the

total structure, including transport and erection. If this were

done moreoften,and on

a

rational ather than an intuitive

basis, much more Grade 50 would be specified than is the

case. I n turn this wou ld bring about an mprovement in the

availability of high ensile steel, the supposed supply difficu lty

often being givenas the reason for not adopting

t

(d)

Fabrication cost

On this aspect whole books could be written. The cost areas

are manpower and time. You are familiar wi th the assessment

of man hours for

a

job, but

it

is less often appreciated that

it

costsmoney for a part to have to wait in the shops with

nothing being done to

it

Apart from interest on capital, it is

occupying productive floor space and obstructing other work

from going through. This is particularly true of box girders

which occupy much loor space, to the detriment of other

smaller orders. Part of the rue cost of a box irder therefore s

the oss of profit of such other orders. The aim therefore must

be to minimise abour hours and speed the job through (the

one does ot necessarily ollow from the ther).

It is axiomatic that everyone

a t

heart wants to do is o b well,

including the shop floor plater or welder provided he

is

paid

appropriately. In most abricating shops men enjoy a bonus

based on output onnage, as

a

financial inducement. The great

benefit of repetition is now evident. Each time a plater has to

read a new drawing, each time a welder has to set up some-

thing new, involves a period of 'learning time'. If this'learning

time' can be spread over IO,

20

r maybe

50

ident ical items,

the less significant it becomes. The output bonus is herefore

good, and within limits themen are pleased. If the job consists

of dozens of different sections, wi th a variety of details, all

fussy and difficult to weld, 'learning time' lasts the whole ob

through; productivity s lo w and men become frustrated and

careless; mistakes are made, delay occurs and profits are lost.

Most big fabricating hops today possess automatic sawing

and drilling plants. These can saw members exactly to length

and dril l flange andweb holes wi thout the necessity of marking

out on the bar. I f members do not require cleats and fittings

to be fitted in he shops, the bars cango straight through to the

painting or despatch bays. Economy flows therefore if

al l

the

fittings can be fixed either to the beams or to the columns.

It

is for this reason that the old method ofseating bracket on

the column and a top cleat on the beamhas gone out of

favour.Holes only in the columns,and web cleats on he

beams are now preferred. There is the added advantage too,

of the brackets being less liable to damage in handling and

transport.

Regardingwelding,modernautomaticorsemi-automatic

processes give

a

deposition rate up to twenty times that of

manualmetal arc (M MA or stick welding). A continuous

automatic run is preferred to intermittentwelding which

should only be specified if some positive advantage accrues

thereby. Web/flange welds in plate girders should therefore

always be welded automatically. Stiffeners and other fittings

are fixed later andwelded manually.

A

significant part of the cost of a welded oin t is the cost

of

n

Fig

.

Weld detai ls

the weld preparation and the attendant setting up (Fig 4 . If

fil let welds can be used and preparation avoided altogether,

money wi ll be saved. f buttwelds are vital, it is best if only ne

edge has to be prepared, and that on the smallest piece, to

facilitate handling Fig 5).

MA1 FLA NGE TAPERED IN WI D TH

AND THICKNESS IN SECTION GROUND

/ / / / / / / / / / / / / / / / / / / / / / /

CHANGE

FLUSH

f i g

6.

Weldedsplice in rai l way brid ge girder

In plate girders, because of the cost of weld preparations,

it

is usually cheaper o run the ame flange plate hrough rather

than to curtail

it

at the nds, if by curtailing butt weld as to be

introduced. Only i f the plate has to be joined anyway, due to

availableplate engths, is

it

worthwhile curtailing.Then it

should be done y reducing the wid th rather than thehickness,

by flame cutting and grinding. The weld preparation can then

be confined to the short end piece. Only in quite exceptional

circumstances as for instance in a railway bridge girder where

fatigue governs the design, should one taper the thickness o f

the larger plate by machining (Fig 6 . n general it can be said

that he ight amount of preparationand weldings he

minimum possible consistantwi th structural requirements.

It

is often assumed that allshop joints should be welded, as

member sizes do not ave to allow fordeductions for holes. In

general this is true, but there are exceptions, notably in l ight

latticework where the savings throughwelding are more

apparent than real. Angle memberscanbecropped to any

length (square ends) and have holes punched on a Vernet or

SEG machine. Truss and lattice girder members thus prepared

accurately are self-jigging and cannot be wrongly assembled,

when bolted or riveted together. Welded trusses of the same

form require either accurate manual setting up and clamping,

or a separateassembly ji g to ensure geometrical accuracy.

The Structural Engineer/September 1977/No. S/Volume 55

371



The cost of such a jig can only be justified if identical frames

are needed n great numbers.

(e) Transpor t and s h ipment

Transport costs are high and rising and

it

is therefore important

to the contractor that vehicles travel fully-laden. The shipment

of tender light weight assemblies should be avoided for this

reason. If great distances are involved

it

can be economic to

despatch small lattice work pieces for assembly on site (as in

the case of electricity transmission towers)-another argument

in favour of bolted attice work. On the other hand oversize

loads involving police escorts should be very carefully

considered.

It

can be economic to do welded splices on the

site prior to final erection, rather than transport 1OOft long

girders in one piece.

Even greater attention needs to be given if the work has to

be shipped overseas.Early consultation with the shipping

agents to establish the basis of freight rates may well have an

influence on the form of the structure. Frequently, the ikeli-

hood of accidental damage is underestimated, and long slender

members are particularly susceptible. If such are essential they

should be firmly bundled at the works. Remembering that the

steelwork may be piled

2 ft

deep in the hold, it is sensible to

desptach the robust items from he works first. Small loose

items should be avoided if possible, but if ssential they should

be wired together in bundles or crated, to avoid loss Pressure

for steelwork to go deck cargo should be resisted. Apart from

likely damage to any corrosion protection, there is a real

possibility of items being lost overboard in heavy seas, as

has happened o an expensivebox girder crane bridge.

The cost of replacement parts, or repair of damaged parts

can be considerable, apart from the delay incurred. The intro-

duction of a few extra splices in, for example, rafters can do

much to ensure that the works arrive on the site in good order,

and can thus be erected immediately

a t

minimum cost and to

time.

(f) Erect ion

Again,

it

is only possible to touch on

a

few aspects. The range

of cranage available today is much wider than used to be the

case. The Scotch derrick and the Guy derrick are still with us,

but are now supplemented by

a

whole range of mobile and

tower cranes al l of which are available for hire, if need be for

short periods. Nonetheless certain basic principles are still

valid.

The capacity of crane required is dictated by the weight of

the heaviestpiece.Therefore a design which incorporates

one heavy girder of say 1 0 tons, with the remainder of the

work not exceeding

2

tons presents difficulties. If the heavy

plate girder can be replaced by one of lattice construction,

the cost of keeping a heavy craneon thesite, or hiring one ust

for one l ift can be avoided.

The virtues of repetitive parts have already been stressed.

The gains to be made can be partly last if interchangeable

parts are given individual marknumbers.

It

maymake the

marking diagram look consistant to do so, but

it

may entail

a

careful search by the erectors for a particular member when

one readily to hand would fit. After the completion of the

detailed design then i t is worth looking at the marking diagram

with this point inmind.

The erector's task is twofold, namely hoisting into position

and effecting connections.

It is in the design of the onnections

that the erector's work needs thought. Much has been written

on the undoubted virtues of HSFG bolts, unfortunately to the

extent where they are currently being specified when they are

quite unnecessary. Not only areHSFG bolts expensive in

themselves, but the on-site orquing process costs as much

again, regardless of diameter. This is because staging is nearly

always necessary, whereas

4.6

and

8.8

bolts can be fixed wi th

a hand spanner, mostly without even the use of a ladder.

HSFG bolts should only be specified if a large number are

needed for one connection, as for example a splice in a big

girder, or i f some other positive benef it accrues. They are uite

inappropriate for beam/column connections in ightmulti-

storey work. In such cases economy derives if the service bolts

used to tack the structure in position initially can become the

final fixing with nother pull on spanner.

The design of connections should recognise that no joint

wil l ever give an exact fi t. Reference should be made to the

tolerances on dimensions of sections given in B S 4 and allow-

able lack of straightness of members. The situation should not

arise that members within those tolerances give rise to trouble

in effecting connections.

It is only common sense that to avoid errors site bolts should

be either 4.6 ar

8 8

throughout any one job, and that the tota l

number of different diameters should be kept to

a

minimum.

Grip lengths however can scarcelye controlled, they

merely occur.

Different views are expressed regarding site bolt ing or site

welding. As in the case of shop lattice work, a correctly

fabricated bolted ob is self aligning. On the other hand no one

can weld

a

member which is hanging from a crane hook, and

it

is not uncommon in a job n which site welding has been

specified for the fabricator to weld on lugs with holes in the

shops for the purpose of tack-bolting prior to site welding.

That said, in certain circumstances site welding can be

economic; box girder bridges for example and also tubular

structures where bolted oints can be very diff icul t and look

unsightly. What is not economic is to have

a

mainly bolted

job in which, because the designer finds himself in a corner,

site welding has been specified only in one or two connections.

If the ntroduction of on-sitewelding equipment is really

necessary,

it

might as well be used in any connections in

which benefit wi ll be shown. The difficult corner though often

arises through last minute changes of requirement or loading.

If this occurs and time permits

it

is worth reviewing the whole

job for the reason given.

Thedesigner should give early thought to the need for

bracing. No great harm is done if too much bracing is incor-

porated. Too litt le can result in disaster Frequently a good

fabricator will add bracing in the detailing stage, butwill

content himself with temporary rope guys if he has already

been screwed into the ground on price.

To the inexperienced designer, much bracing appears to be

superfluous, particularly once the structure has been clad and

the floors cast. This is probably true, but bracing is cheap as

an insurance premium for stabiltiy during erection. Also it

costs money to take it out later and throw it away. Since the

client is paying for it, if it is not causing obstruction

it

can be

left in. (Incidentally, i t can prove invaluable during subsequent

alterations) (Fig 7).

(g)

Paint ing

Awhole range of different treatments is now available.

It

is not uncommon for a fabricator to stock forty different paints.

Early consideration should be given t o the corrosion environ-

ment to arrive at the appropriate total treatment, and the pro-

portion of shop and site painting. It is no longer sufficient

to

decide on the treatment after detailed design has been com-

pleted, since as indicated earlier, the corrosion environment

may affect the structural form. The ife and effectiveness of any

treatment materially depends upon the surface preparation.

Many fabricating shops have shot blasting plants in their

input bay and can carry out blasting and prefabrication

priming quite cheaply as routine. Such treatment is

a

good

investment in the long term whatever subsequent coating is

adopted.

Shop painting is always cheaper and more effective than site

painting, but no steel can be handled, transported and erected

without damage to the coating from crane slings, etc. Touch-

ing up of the base coats and the final top coat must therefore

be done on site.

Regarding the choice of system to adopt in any particular

372

The Structural Engineer/September 1977/No. S/Volume 55



Fig

7 .

Layout for por tal shed

case useful guidance can be found i n Constrado publication

No 3/74, Protection of s tructural steelwork from atmosp heric

corrosion

prepared by the Corrosion Advice Bureau of the BSc.

In a difficult case this unitcan be approached or more detailed

advice.

If

a

general comment can bepassed

it

is that he recent

developments n the field of corrosion protection have evolved

protective systems greatly superior to those available some

years’ago.Thesesystemsareexpensive but are invaluable

when appropriate, as in exposed structures in severe industrial

or

marine environments. However, this has led to some waste

of money by the specification of such sohpisticated treatments

in circumstances where they are not necessary. The ate of

corrosion of steel inside a normal heated building is really

very low, and the treatment used should be appropriate to the

situation.

(h)

Cladding and f in ishin g indust ria l bui ld ing s)

The weight of claddings for industrial buildings usually does

not greatly affect the structural design, being much less than

the design superimposed load. Overseas, where no snow load

is experienced his may no t be the case. The spacing of purlins

and sheeting rails is determined by the load capacity of the

sheeting, which can vary. As the purlins and rails also provide

lateral restraint to their supporting members, last minute

changes of mind as to the form of sheeting can result in major

redesign of the rincipals; clearly o be avoided.

Before finalising the design of a shed or mill building, the

available lengths and widths of the esired cladding should be

obtained.

It

should then be determined whether any non-

standard lengths or widths w il l be needed, and if so whether

they can be cut on site or need to be specially ordered.

It

is

often he case that slight adjustments in salient dimensions

or eaves and ridge details can eliminate the need for ‘specials’

entirely. Alternatively, or perhaps in addition, the specified

laps can be increased, either of which expedient wi ll save cost,

and particularly time.

The Structural Engineer/September 1977/No S/Volume

55

It

is usual for roofing and sheeting to be effected by a

separate sub-contractor, and savings in time can be made by

proper phasing of the different activities, which is a manage-

ment problem. The designer can assist, though, by giving an

indication on his drawings where the roofing contractor can

placeheavy loads (such as ‘blocks’ of sheets) ather than

eaving him to findut the ard way. Failures haveccurred asa

result of overloading from this cause, and cold-formed purlins

need consideration in this regard..

The use of stressed skin design should be considered as an

aid to economy. When metal cladding is used, its considerable

shear strength when adequately fixed, can be used to absorb

wind forces, obviating the need to provide separate wind

members.

Conclusions

This paperhas tried to give an insight into the factors

governing the economy of steeltructures.ecognise

the relative proportions of the major cost areas for any project,

and not ry to consider the steelwork in isolation. Ideally

a

number of trial designs should be carried out to explore

alternative arrangements.

Cost and time should be minimised by making the various

activities carried out by the contractor, viz, design, detailing,

fabrication and erection, slick and easy to perform efficiently.

This is most likely to achieve the aim of satisfying the client

and

a t

the same time make the job profitable to the contractor.

Conversely, the economic value of highly sophisticated

design techniques, whether computerised or not, will be

largely illusory if not complemented by a sound working

knowledge of the various processes involved in structural

engineering. Great nicety of calculation is therefore misplaced

if needless expense is incurred by lack of attention t o basic

principles of design. Most of the information given here is

really only common sense but it is not claimed as exhaustive.

Further enquiry from appropriate sources early in the develop-

ment of

a

project can yield substantial economic benefit.

373

Economics of Steelwork Design

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