BS 5950-1:2000 Changes from 1990 revision

Introduction

The following is a list of changes spotted in the revised code. Only significant changes have been noted. The clause number refers to the numbers in the revised code. Section 1 General

Clause Number Change Comment

1.1 Scope Note added that the code now covers cold formed hollow sections to BS EN 10219. Note added that design for seismic resistance is not covered in BS5950 Specifically mentioned that although detailed recommendations are not given for “second order” analysis it’s use is not precluded.

Section 2 Limit states design

Clause No Change Comment

2.1 General principles and design methods

2.1.2 Methods of design

Methods now simple, continuous and Semi-continuous. Simple rules for Semi-rigid design removed

Reference to SCI publications for Semi-continuous design

2.2 Loading

2.2.2 Dead imposed and wind loading

Wind load can be from either CP3 or BS6399

Later clauses 4.12 on purlins and side rails, and 5.1.2.4 on continuous structures refer only to 6399

2.2.4 Earth and Ground water

1.2 load factor to be used with max credible loads from BS8002.

2.4 Ultimate limit states

Table 2 Load factor of 1.05 for drifted snow

As recommended in BS 6399-3

2.4.2.3 Resistance to horizontal forces

Factored wind load to be not less than 1%of factored dead load.

2.4.2.4 Notional horizontal forces

Now just one value i.e. 0.5% factored dead plus live. Noted that NHLs need not be considered with pattern loads.

2.4.2.5 to 8 Sway stiffness

All frames to be considered for sway. Second order analysis required if λcr is less than 4. Amplification factor slightly different.

1990 code implied only relevant for moment frames.

2.4.4 Brittle fracture Instead of K either 1 or 2 it can now vary between 0.5 and 4. Slight changes in some of thicknesses for various grades. JR grades (old grade B) not allowed for External conditions.

Note that baseplates with nominal welds “for purpose of location in use and security in transit” can be considered to be plain steel.

2.4.5 Structural integrity

Clarification of difference between “column ties” and general tying. No special precautions required for transfer beams. Limitations on collapse area for alternative method clarified.

2.4.5.3 Avoidance of disproportionate collapse

Tie force requirements deemed to be satisfied by a beam tie force equal to the largest end reaction. Column tie requirement increased from 2/3 to 1 times floor load.

This is a major relaxation in the tie force for primary beams but it is still best to avoid designing the connections for tie forces.

Section 3 Properties of materials and section properties

Clause No Change Comment

3.1 Structural Steel

Table 3.1 Design Strength py (Table 6)

Grade 43, 50 and 55 replaced by new designations S275, S355 and S460.

S460: Design strengths greater than for Grade 55. Strengths given for thicker material.

3.2 Fasteners and Welds

3.2.1 Bolts, Nuts and Washers (3.2.2)

Reference to part 2 for matching nuts and washers.

See Structures note 1998NST/2

3.2.2 Friction Grip Fasteners (3.2.3)

Fasteners other than those to BS4395 can be used provided they can be reliably tightened to the minimum shank tension in BS4604.

States friction grip fasteners should generally be preloaded HSFG bolts.

3.2.3 Welding Consumables (3.2.1)

Details on the yield strength, tensile strength and minimum elongation of welds. Table 3.3 is new.

3.3 Steel Castings and Forgings (3.1.3)

New Clause Reference to BS 3100 and BS EN 10250-2. Reference to SCI guide on steel castings. Design strength corresponding to S275 recommended to be used.

3.4 Section Properties (3.3)

-

3.4.3 Effective Net Area

For other steels ke = (Us/1.2)/py ≤ 1.2 when it was ke = 0,75Us/ys ≤ 1.2

ke larger

3.5 Classification of Cross Sections

Tables 11 and 12 (Table 7)

Table 11 (sections other than CHS and RHS) Limits altered Table 12 Limits for SHS added. Distinction between hot finished and cold formed. Distinction between CHS in compression and bending.

Table 11 Limits generally equal or higher. Limits for stems of T sections slightly reduced. Web, generally; dependant on r1 and r2. i.e. stress in web. Table 12 CHS-Limit for compact sections in bending is less. For semi compact is more RHS- Web with neutral axis at mid-depth - values less. Web generally; -dependant on r1 and r2. Web, whole section axially compressed: hot rolled value is greater. Cold formed value is less.

3.5.6 and 3.6 Effective Plastic Modulus and slender sections

New clauses Effective section properties introduced.

Section 4

Clause Number Change Comment

4.2 Members subject to bending

4.2.5. Moment Capacity

The previous factor was the ratio of factored to unfactored load. It has now been replaced by a ratio of 1.5 generally or 1.2 for simply supported beams and cantilevers.

4.2.2 Full lateral restraint

When full lateral restraint to the compression flange is provided the requirement for the ends to have torsional restraint has been added.

Clarification of what was always required.

4.2.3 Shear capacity The slenderness limits for shear buckling for webs are different. 70ε for rolled sections and 62ε for welded sections

The old limit was 63ε for both types. The new limit means that the two UB sections that were susceptible to shear buckling are no longer effected.

4.2.5 Moment capacity

Limit on moment capacity as a factor of elastic capacity changed.

The previous factor was the ratio of factored to unfactored load. It has now been replaced by a ratio of 1.5 generally or 1.2 for simply supported beams and cantilevers.

4.2.5.2 High shear The calculation for the reduction in Mc has been changed and also clarifies what to do with semi-compact and slender sections. There are also equations for notched ends of I sections. For these high shear is only when Fv≥ 0.75Pv

The equation for sections with two flanges is as part 3.1 EC3 although the limit for high shear in these codes is 0.5Pv. At 0.6Pv the reduction in Mc is small. The reduction in Mc for Fv>0.6Pv is less than in the current code.

4.3 Lateral-torsional buckling

4.3.1 General Guidance as to the position of intermediate lateral restraints is provided i.e. "as close as practicable to the top flange and in any case closer to the level of the shear centre of the top flange than the shear centre of the member" There is no longer mention of Torsional restraint.

It appears that torsional restraints do not by themselves reduce the effective length. Their only role appears to be to allow the lateral restraint to be located away from the compression flange.

4.3.2.2.3 Reduction factor for systems restraining multiple beams rather than worst three.

4.3.3 Torsional restraints

As well as the requirement for torsional restraints to take a couple from 1% of the flange force the set of restraints must take 2.5% divided between them in proportion to their spacing.

4.3.5 Effective length for lateral torsional buckling

4.3.5.3 Beams with double curvature bending

Additional guidance given Reference to annex G for parts where restraint is to tension flange plus guidance on effect of torsional restraint to tension flange.

4.3.5.4 Cantilevers without intermediate restraint (4.3.6.2)

Increased effective length required if there is a moment applied at the tip of the cantilever.

4.3.5.5 Cantilevers with intermediate restraint (4.3.6.1)

Additional requirements for destabilizing loads

For destabilising loads intermediate restraints only effective if to both flanges

Table 13 Efective length for beams without intermediate restraint (Table 9)

Category added for full and partial restraint of Compression flange

Table 14 Efective length for cantilevers without intermediate restraint (Table 10)

Category added for continuous cantilevers with partial torsional restraint at the support

4.3.6 Resistance to lateral torsional buckling (4.3.6)

The "n" method in the current code has been deleted, there is only a "m" method. Tees are now a special case and reference is made to annex B. Limiting slendersnesses for RHS are now included here rather than in the appendix.

4.3.6.4 Buckling resistance moment Mb (4.3.7.3)

This is no longer pbSx for all sections. For class 3 it is pbZx and for class 4 it is pbZeff

4.3.6.7 Equivalent This is uvλ√ β w where β w is the ratio

slenderness λ LT (4.3.7.5)

of the elastic or effective elastic modulus to the full plastic modulus for class 3 and 4 sections. As well as a table for v there are formulae which are similar to those which were in appendix B.

4.3.8 Buckling resistance moment for single angles

There is only a simple equation for Mb for equal angles, unequal angles must be designed for biaxial bending.

4.4 Plate girders

4.4.2 Design strength (4.4.3)

The capacity where the web has a lower strength than the flanges is explained.

4.4.4 Moment capacity The limiting thickness for web buckling is changed see 4.2.3 above plus concept of low shear applied to buckling.

If shear less than 60% of “simple” buckling capacity no effect on moment capacity.

4.4.5 Shear buckling resistance

Instead of design with or without tensile field action there is a “simplified” and a “more exact” method. The simplified method gives higher capacities than the old without tensile field method but it relies on tensile field action and end anchorage needs to be checked. There are some changes in the way the anchor forces etc are specified but the overall results do not change. You can now have a single end post which is not rigidly connected to the flange.

4.4.5.3 Tension field method. (4.4.5.4)

The equation for the flange dependent shear buckling resistance has been altered.

Direct calculation rather than via the flange dependent tension shear strength of the web.

4.4.6 Design of intermediate transverse web stiffeners

4.4.6.6 Buckling resistance

The moment to be used for checking the transverse stiffener must now include the effects of external lateral forces.

4.4.6.7 Connection to For intermediate transverse stiffeners

web of intermediate stiffeners

not subject to external load which do not connect to the tension flange the distance cut short of the weld is now a maximum of 4t rather than approximately 4t. It is only intermediate transverse stiffeners not subject to external loads which need not be connected to the compression flange.

4.5 Web bearing capacity, buckling resistance and stiffener design

4.5.1.3 Stiff bearing length

Note added that stiff bearing length can only include dispersion through packs if they are firmly fixed in place

4.5.1.5 Hollow sections(4.5.12)

Note about hollow sections and reference to the SCI "blue book"

4.5.2 Bearing capacity of web (4.5.3)

Spread through flange at end of member changed

Now spread is a min of 2T at end rather than 2.5T but method of adding any projection is given.

4.5.3.1 Buckling resistance of unstiffened web (4.5.2.1)

Direct calculation of buckling resistance rather than via strut curves.

Typical beam component of buckling is lower (up to 25%). Also the buckling capacity cannot be easily split up into beam, flange plate and stiff bearing components.

4.5.3.3 Buckling resistance of load carrying stiffeners(4.5.1.5)

The length of web to be taken as part of a stiffener is reduced from 20 to 15 times the web thickness each side of the centreline.

4.5.10 length of web stiffeners(4.5.9)

Specific guidance is given on the capacity of the remainder of the web where bearing or tension stiffeners are not full depth.

4.6 Tension members

4.6.3.1 Single angle, channel or T-section members

The effective area for use with single angles, channels or tees connected eccentrically is changed. There are different values for bolted and welded connections.

Ratio of new to old capacity varies between approx 0.94 to 1.13.

4.6.3.2 Double angle, The net area for double angles is also Capacity of angles connected

channel or T-section members

changed with different values for welded and bolted connections. The net area must also be used for double angles connected to both sides of a gusset plate.

each side of a gusset plate reduced.

4.7 Compression members

4.7.1.2 Restraint force for multiple members can be reduced using reduction factor as for beams.

4.7.2 Effective lengths

Advice on le for columns supporting internal platform floors of simple construction.

In annex D.

4.7.4 Compression resistance

The effective area is used to calculate the capacity of class 4 sections, the slenderness for these sections is reduced by the factor (Aeff/Ag)0.5 relative to that calculated for the gross section.

Table 4.13 Allocation of strut curve (Table 25)

Cold formed SHS use curve c.

4.7.7 Columns in simple construction

Reference to the "semi rigid " (now semi continuous) design method has been removed. How to use with SHS columns made clearer.

SHS simple columns not consistent with combined bending and axial force clauses.

4.8 Members with combined moment and axial force

4.8.1 General Effect of shear clarified. Section to be classified on combined forces.

4.8.2 Tension members with moments

Reference to annex I2 for the calculation of Mr.

4.8.3 Compression members with moments

4.8.3.1 General Reference is made to annex I 1 for an alternative approach for stocky doubly symmetric class 1 and class 2 sections.

This is a modification to the exact approach in 4.8.3.3.2.

4.8.3.2 Cross section There is a separate equation for class

capacity 4 slender sections which includes the effective area.

4.8.3.3 Member buckling resistance

4.8.3.3.2 Simplified approach

Lateral torsional buckling needs only to be considered with flexural buckling about the minor axis. Therefore there are now two checks. Also instead of "m" there is "mx" (major axis bending relative to major axis restraint) "my" (minor axis bending relative to minor axis restraint) and "mLT".(major axis bending related to minor axis restraint).

There are limits on the uniform moment factors mx, my, and myx to be used in continuous frames with sway mode effective lengths or where the amplified sway moments are calculated.

4.8.3.3.2 more exact approach for I or H sections with equal flanges

More expressions but clearer. “myx” factor introduced i.e. minor axis moments relative to major axis restraints.

4.8.3.3.3 More exact method for CHS, RHS or box sections with equal flanges

Similar to 4.8.3.3.2 but additional guidance if no LTB.

4.8.3.3.4Equivalent uniform moment factors.

See 4.8.3.3.1 and 2

4.9 Members with biaxial moments

Reference to annex.I4 for single angles.

4.10 Members in lattice frames and trusses.

The reference to secondary stresses being insignificant provided the members are sufficiently slender has been deleted. The moment for rafters is redefined as wL2/6.

Secondary moments which are due to joint fixity can be neglected in all cases. (consistent with BS5400).

4.11 Gantry Girders

4.11.3 lateral torsional buckling

Provided mLT is taken as 1 and no resilient pads used the crane loads need not be taken as destabilizing.

4.12 Purlins and side rails

4.12.3 Wind loading Wind load can be to BS6399-2 or CP3. The reference to being able to ignore local pressures is deleted. Instead there is "Where justified by sufficient general or particular evidence, the effects of load sharing with adjacent purlins and side rails, end fixity and end anchorage under wind loading, may be taken into account in determining the member capacity.".

4.12.4 Empirical design of purlins and side rails

4.12.3.2 Conditions Not applicable for spans exceeding 6.5m.

4.12.4.3 Purlins The reference to a minimum imposed load of 0.75kN/m² is deleted. There are now different equations for the Z required depending on whether the loading is from BS 6399 or from CP3. Downward load also has to be considered with the same formulae as CP3.

They obviously think BS6399 is an overestimate.

4.12.4.4 Side rails There are now different equations for the Z1 required depending on whether the loading is from BS 6399 or from CP3.

4.13 Column Bases

4.13.1 General The allowable bearing strength for concrete foundations has been increased from 0.4 to 0.6 fcu. The empirical method is replaced by an effective area method. The reference to grade 43A baseplates not being limited by brittle fracture has been deleted. See note on 2.4.4.

4.13.2.3 Eccentric forces or applied moments.

The limit on pyp of 270 N/mm² has been removed.

4.14 Cased Sections

4.14.1 General The reference to BS5950 Section 3.2 (under preparation) has been removed. The min concrete grade is increased from 20 to 25N/mm²

4.14.3 Cased members subject to bending (4.14.2)

The radius of gyration ry is as for cased columns i.e. with a limit of 0.2bc

4.14.4 Cased members subject to axial load and moment

Similar changes to 4.8.3.3.2 for buckling resistance.

4.15 Web openings

4.15.3 Members with isolated openings

There is some guidance for non circular rectangular openings and reference is made to the SCI publication 068.

4.15.4 Members with multiple openings

The previous rules for castellated beams are given more general application. Reference is made to the SCI publication P100 on cellular beams.

Shear stress on web post limited to 0.7py when SCI guide says 0.6py.

4.15.5 Castellated beams (4.15.3)

It may be assumed that. The web posts of castellated beams of standard proportions are stable provided d/t for the expanded cross section does not exceed 70ε .

It should be noted that the other parts of 4.15.4 apply.

Section 5 Continuous structures (previously Continuous construction)

Clause Number Change Comment

5.1 General

5.1.1 Scope Definition of the types of analysis covered. NB the classification of frames as sway/non-sway is now in 2.4.2.

5.1.2 Pattern Loading

5.1.2.3 Imposed roof load

Clause added pointing out that "For load combination 1 (vertical loads only) asymmetrical loads, partial loads and local drifting of snow should be applied as recommended

in BS6399: Part 3.".

5.1.2.4 Wind load Clause added pointing out that "For load combination 2 (dead load, and wind load) , the asymmetric wind loads recommended in 2.1.3.7 of BS6399: Part 2 1997 should be applied.".

5.1.3 Base stiffness (5.1.2.4)

5.1.3.2 Nominally fixed bases

Although the stiffness of the base for ULS state is, as before, to be taken as the stiffness of the column, for deflection calcs under SLS loads the base may be considered as fixed.

5.1.3.3 Nominally pinned bases

A base stiffness of 10% of column may be used for calculating effective lengths and 20% of the column may be used to calculate deflections under SLS loads.

5.2 Global analysis

5.2.1 Methods Note that second order analysis is not precluded but no detailed recommendations are given for its use.

5.2.2 Elastic analysis (5.2 and 5.4.1)

The 10% redistribution of moments previously allowed for continuous class 1 or 2 beams is generally acceptable except for minor axis column moments.

5.2.3 Plastic analysis

5.2.3.3 Grades of steel (5.3.3)

It is made clear that all grades in BS5950-2 can be used. For other grades the "plateau" requirement is changed to a demand for the ultimate tensile strain to be 20 times the yield strain.

5.2.3.5 Cross section restrictions (5.3.4)

Requirements for cross sections that vary along their length are added.

5.3 Stability out-of–plane for plastic design

5.3.3 Adjacent segments (5.3.5)

An "Approximate method allowing for moment gradient" has been added.

5.3.4 Segments with one flange restrained.(5.5.3.5.2)

The simple method has rearranged and made more general.

5.5 Portal Frames

5.5.3 Plastic design (5.5.3)

5.5.4 In plane stability (5.5.3.2)

Either Sway-check, amplified moments or second order analysis is required. Tied portals are treated separately.

The sway check appears more complicated and has more limitations than the 1990 revision.

5.5.2.4 Eaves haunch

Restrictions applied to eaves haunches similar to 5.4.2.

5.6 Elastic design of multi-storey rigid frames

5.6.4 Non-sway frames

As before Non-sway effective lengths can be used

This is incorrect unless there is some other system providing the stability.

5.6.4 Sway sensitive frames (5.6.3 b)

If λ cr is less than 4 second order analysis should be used.

Section 6 Connections

Clause Number

Change Comment

6.1.9 Column web panel zone

New clause. Shear in the column web panel in a moment joint to be checked. Can reduce moment capacity of connection.

6.2.3 Effect of bolt holes on shear capacity

New clause If significant bolt holes on shear plane the capacity will be affected.

6.2.4 Block Shear

New clause Block shear failure of groups considered.

6.3 Non-preloaded bolts

Figures given for grade 10.9.

6.3.3.3 Reduction factors given for bolts in

Bearing capacity of connected parts

oversized or slotted holes.

6.3.4 Bolts subject to tension

Either simple approach with capacities as before but limits on connection geometry or more exact approach with increased capacity and prying to be taken into account

6.4 Preloaded Bolts

Clarification of capacity depending on whether slip can occur between SLS and ULS.

6.4.3 Slip factor

Table of values for various surface conditions

Typical value for blast cleaned steel now 0.5

6.5 Pin connections

Geometrical requirements for pin plates set out more rationally. Distinction between joints where rotation or pin removal is required and where not.

Capacity with rotation less than before. Bearing and bending capacity with no rotation greater.

6.7.5 Welded connections to unstiffened flanges

Capacity of welded connections to unstiffened flanges given and requirement for stiffeners.

6.8.7 Capacity of a fillet weld

Increased capacity in transverse direction can be used

Section 7 Loading tests

Clause Number

Change Comment

7.1 General

7.1.2 Types of loading tests

The separate check test has been removed. It is now covered by clause 7.1.3 on Quality control for strength or failure tests.

7.3 Test procedures

7.3.3 Coupon tests

A description of required coupon tests is given

7.4 Relative strength coefficient

A concept of a relative strength coefficient is introduced for strength and failure tests. It includes the effects of differences in material and dimensions between the test specimen and the nominal values.

7.7 Failure test (7.3.5)

7.7.3 Determination of design capacity.

The Kt value for single tests is reduced from 0.9 to 0.8 and that for two or three tests is reduced from 1.0 to 0.9. For four test or more statistical methods should be used. If the design is to be based on the results of the tests, at least four tests should be carried out.

Annex B lateral-torsion al buckling of members subject to bending

Clause Number

Change Comment

B.2 Buckling resistance

B.2.2 Perry factor and Robertson factor (B.2.3)

The expressions for welded sections have been made more understandable.

B.2.4 Uniform I and H sections with unequal flanges

The theoretical equation for the monosymmetry index ψ has been included.

B.2.4.2 Double –curvature bending

This has been added

B.2.8 T-sections

This section has been added. There may be some inconsistencies here

B.2.9 Angle sections

This section has been added.

B.3 Internal moments

This section has been added giving values for the second order moments.

This is for the design of splices

Annex C Compressive strength

Clause Number

Change Comment

C.3 Strut action

Formula for internal second order moment due to strut action is simplified.

Annex D Effective lengths of columns in simple construction

Clause Number

Change Comment

D.2 Columns supporting internal platform floors

This section has been added with a table of effective lengths.

Annex E Effective lengths of compression members in continuous structures

Clause Number

Change Comment

E.1 General For moment frames which provide stability to simple columns the effective length must be increased (as described in E5) or the effective length calculated from the elastic critical load factor (as described in E6).

E.2 Columns in multi-storey buildings

E.2.1 Limited frame method

This is limited to frames with "concrete or composite floor and roof slabs"

E.4 Other compression members

E.4.1 Other rectilinear frames

More guidance in how to apply the method in E.2 to other frames is given.

E.4.2 Effect of axial loads in restraining members

Conservative approximations on the effect of compression on the bending stiffness of members is given

Annex F Frame stability

Clause Number

Change Comment

F.1 General “The method is not to beused for single storey frames.” "The possibility of localised (storey height) sway modes should also be taken into account."

Annex G Restrained members with an unrestrained compression flange

Clause Number

Change Comment

G.1 General

G.1.1 Application

A note that moments and forces "should be related to the axis of the minimum depth section" is made.

G.2 Lateral buckling resistance

G.2.2 Tapered or haunched sections (G.2)

The check for haunched sections has been changed. Both a compression resistance and a moment resistance must be calculated.

G.2.4.2 Equivalent slenderness λTB . Haunched and tapered members

There is a different equation for three flange sections, i.e. where a T has been welded on to an I section. The equation for the taper factor is different.

G.3 Lateral restraint adjacent to plastic hinges

Requirements which were in G2 now moved and expanded.

G.4 Non-uniform moments

G.4.1 Methods

Either an equivalent uniform moment factor or a slenderness correction factor can be used.

G.4.2 Equivalent uniform moment factor (G.3.4)

This is now allowed where loads are applied between effective torsional restraints.

Annex H Web buckling resistance

Clause Number

Change Comment

H.2 Shear buckling resistance utilising tension field action

H.1Shear buckling strength

This has been added. Formula for values given in the tables in section 4. Includes some tensile field action.

H.2 Critical shear buckling resistance

Was H1

H3 Resistance of a web to combined effects

Completely rewritten. No longer an equation for compression, bending shear and edge loads. The previous expression was too conservative so BS5400 should be used instead.

H4 End anchorage

Requirements moved from section 4 to this annex.

Annex J Combined axial compression and bending This new annex gives additional advice on , the capacity of stocky members, the calculation of the reduced moment capacity, Unsymmetric members, single angles and internal moments. Annex K Bibliography This is an informative annex