Corus Tubes library publication

The contents of this publication are current, when republished it will be in the new Corus housestyle.

SHSDesigntoBS 5950Part 1

DESIGN TO BS 5950 : PART 1: 1990LIMIT STATE DESIGN OF TUBULAR STRUCTURES

USING HOT FINISHED STRUCTURAL HOLLOW SECTIONS.

Preface

This brochure has been produced to assist Structural Engineers and Designers who use Hot finishedStructural Hollow Sections whose section properties comply with BS 4848 : Part 2 (Ref 1) andwhose Steel qualities comply with EN 102 10-l (Ref 2) to design using the Limit State basis ascovered by BS 5950 : Part 1 (Ref 3).

It is important for designers to recognise that hollow sections, unlike their rolled open sectioncounterparts, can be produced by a hot or cold forming process. The process of cold formingproduces different mechanical properties in the section to that of hot forming and a reduction in thesectional properties used in design.

Designers wishing to use cold formed hollow sections should design in accordance with theappropriate cold formed standard and with the sectional properties as given in BS 6363 (Ref 4).Further guidance is given in British Steels Tubes & Pipes publication TD369 - ‘Cold Formed Hollow Sections’.

In addition to the information contained in this publication, further assistance and design guidance isgiven in companion British Steel Tubes & Pipes publications (references 16 to 21) and the SteelConstruction Institute (references 8 to 15) publications as given in Appendix A.

BRITISH STANDARDS INSTITUTION

Extracts from British Standards are reproduced by permission of the British Standards Institution,2 Park Street, London, W1 A 2BS from whom complete copies can be obtained.

The information given is not intended in any way to replace that given in the Standards themselvesto which reference should always be made. Where reference is made to specific clauses and tablesthroughout the text, these refer to BS 5950 : Part 1 : 1990 plus amendment No. 1.

DisclaimerCare has been taken to ensure that the contents of this publication are accurate, but British Steel plc and its subsidiarycompanies do not accept responsibility for errors or for information which is found to be misleading. Suggestions for ordescriptions of the end use or application of products or methods of working are for information only and British Steelplc and its subsidiaries accept no liability in respect thereof. Before using products supplied or manufactured by BritishSteel plc the customer should satisfy himself of their suitability. If further assistance is required, British Steel plc withinthe operational limits of its research facilities may often be able to help.

Contents

1.0 INTRODUCTION 2

2.02.12.2

L 2.32.42.5.2.6

PROPERTIES OF MATERIALS AND SECTION PROPERTIES 2Structural Steel 2Physical properties of structural steel 2Hollow Section range 2Design Grade 3Design Strength py 4Section Classification 5

3.0 MEMBER DESIGN 83.1 Members in tension 83.2 Members in compression 103.3 Members in bending 14

4.0 AXIALLY LOADED MEMBERS WITH MOMENTS 174.1 Tension members 174.2 Compression members 184.3 Web bearing and buckling 19

5.0 MEMBERS IN TORSION 20

6.0 PURLINS AND SIDE RAILS 21

7.0 COLUMN BASES 227.1 Empirical rules 227.2 Effective area 22

8.0

9.0

10.0

DESIGN OF WELDED JOINTS 24

COMPOSITION CONSTRUCTION 24

FIRE RESISTANCE 24

APPENDIX A REFERENCES

APPENDIX B FORMULAE AND CONVERSION FACTORS

APPENDIX C DESIGN EXAMPLES

Page

25

26

29

1.0 INTRODUCTION

Limit State Design

The Limit State design approach for buildings and structures is based on BS 5950 : Part 1 codeof practice for design in simple and continuous construction : hot rolled sections, and relates tothe use of hot rolled steel sections and plates and hot finished Structural hollow sections.

Whilst the standard requires that all relevant limit states of strength, stability and serviceabilityshould be considered this publication relates primarily to the limit state of strength. Whenconsidering the limit states of stability and serviceability due regard must be taken of the differingload factors and combinations that may apply.

2.0 PROPERTIES OF MATERIALS AND SECTIONAL PROPERTIES

2.1 Structural Steel

BS 5950 : Part 1 covers the design of structures fabricated from weldable structural steels indesignated design grades to the appropriate product grade which for hot finished structuralhollow sections are found in EN 10210-l (Ref 2). Other steels may be used provided dueallowance is made for variation in properties, including ductility.

EN 10210-l : 1993 contains the technical delivery requirements for Hot Finished StructuralHollow Sections, including British Steel Tubes dz Pipes two standard production grades ofS275J2H and S355J2H. Tables in annex A and B detail the required chemical analysis,mechanical properties, testing and certification for hot finished SHS. For further details seeTD 364 (Ref 17).

Designers are recommended to consider the use of Grade S355 steels in structures. Designstrengths for grade S355 are significantly higher than grade S275 steels at only a small extracost, often resulting in more economical designs.

2.2 Physical Properties of Structural Steel

The values of tensile strength and minimum yield strength specified in EN 102 lo- 1 for the twostandard production grades of Hot Finished SHS are;Product Grade Tensile Strength Yield Strength Yield Strength

N/mm2 N/mm2 N/mm2(3mm 5 t 5 65) (t 5 16mm) (16 <t s 40)

S275J2H 410/560 275 265S355J2H 490/6 30 355 345

Note: Other grades and sub-grades including steel for off-shore applications are available.

2.3 Hollow Section Range

Structural Hollow Sections (SHS) comprise a range of circular hollow sections (HFCHS) andrectangular hollow sections - including squares (HFRHS).

2

BS 5950Part 1

1.1

2.4

2.5

3.1

2.4.42.4.4.3

BS 5950Part 1

The range of Structural Hollow Sections manufactured by British Steel is generally inaccordance with BS 4848, Hot Rolled Structural Steel Sections, Part 2 Hollow Sections. Thisstandard gives dimensional and sectional properties and relates generally to the internationalstandard IS0 657 Hot Rolled Steel Sections, Part 14 Hot Finished Structural Hollow Sections.

The manufacturing range normally produced by British Steel is given in the leaflet TD 167(Ref 16). Larger sizes of HFCHS, not detailed in that leaflet, may be obtained on application.

2.4 Design Grade

To provide a transition path from the single U.K. standard of BS 4360 to the four EuropeanStandards for Structural Steel, BS 5950 : Part 1 Amendment 1 has introduced the term designgrade.

Reference to Table 2 of BS 5950 : Part 2 will show the relationship between the design gradeand the appropriate product standard and grade which following the implementation of theEuropean Standard for hot finished structural hollow sections is EN 10 2 10-l.Note: BS4360 is now withdrawn.

The following table shows the appropriate product grade in EN 102 10-l for the BS 5950 designgrade plus, for information purposes only, the previous product grades as given in BS4360 :1990.

BS 5950Design Grade

Current PreviousEN 10 210-l BS 4360 : 1990

Product Grade Product Grade

43c S 275 JOH 43c43D S 275 J2H 43D43EE S 275 NLH 43EE

5oc S 355 JOH 5oc50D S 355 J2H 50D50EE S 355 NLH 50EE

55c - 55c55EE - 55EE55F S 460 NLH 55F

Note: The Standard production grades for Tubes & Pipes SHS is S275J2H and S355J2H,previously BS 4360 Grade 43D and 50D respectively.

3

2.5 Design Strength py

Throughout the design procedures the two factors of Design Strength (py) and SectionClassification repeatedly occur. One or both will be required to be known in order to completethe design procedure.

The design strength pY is taken as equal to the specified minimum yield strength but not morethan 0.84 of the minimum ultimate tensile strength from EN 10210-l.

Based upon EN 10210-l the design strengths for different design grades of steel are given inBS 5950 Table 6 which for the range of SHS thicknesses are:

Design strength py

Design Grade Thickness (mm) Design strengthPy Nmm2)

43 5 16 275> 16 540 265

50 1.16 355> 16 1 4 0 345

55 * 5 16 460> 16 1 2 5 440

* Values have been shown for steel in grade 55 but the designer isadvised to check availability before proceeding with its use.

Note that currently rectangular hollow sections are limited to 20mm thick and circular hollowsections to 50mm thick.

Thus in practice for sections up to and including 16mm thick, py = 275 N/mm2 (Design Grade 43)or 355 N/mm2 (Design Grade 50). For sections over 16mm and up to 40mm thick py = 265 and345 respectively for Design Grades 43 and 50. However, this design strength will be reducedwhen, due to their size and thickness, sections are classified as slender (see section classification).

BS 5950Part 1

3.1.1

Table 6(part)

4

2.6 Section Classification

The classification of a section gives an indication of its expected performance withparticular reference to the elements of the cross section that are in compression due tobending or axial load.

The majority of SHS members will tend to fail at ultimate load by compressive yieldingin one or more elements of the cross section. However it is recognised that cross sectionswith slender elements may fail in compression by local buckling before reaching the fullyield strength thus limiting the ultimate capacity of the section.

The onset of local buckling will also be influenced by the level of axial stress. Thus ifhigh yield steel is used (and the applied stress is greater) then the width to thickness ratioof elements must be within smaller limits to prevent local buckling.

The Code places sections into four classifications namely plastic, compact,semi-compact and slender as follows:(illustrated diagramatically in Figure 1).

CLASS 1 PLASTIC(Can form plastic hinge withrotation capacity requiredfor plastic design)

CLASS 2 COMPACT(Can develop full plasticmoment but with limitedrotation capacity)

CLASS 3 SEMI-COMPACT(Stress in extreme fibres canreach yield)

CLASS 4 SLENDER(Design strength pY must bereduced to prevent localbuckling)

Figure 1 Section classification

5

BS 5950Part 1

3.5

3.5.2

3.6Table 7

Class 1 Plastic:

Sections in which under bending a plastic hinge can form with sufficient rotation capacity toallow redistribution of moment within the structure. Stress will reach the full design strengthpY in a rectangular stress block configuration which will be retained during rotation with nosignificant deformation of the cross section.

Class 2 Compact:

Sections in which under bending the full plastic moment can be developed but local bucklingmay prevent the development of a plastic hinge with sufficient rotation capacity to permitredistribution of moment. In this case elastic analysis of the frame should be used.

Class 3 Semi-compact:

Sections in which under bending the stress at the extreme fibres can reach the design strengthpY in a triangular stress block configuration but local buckling will prevent the development ofthe full plastic moment, and the moment capacity is therefore based upon the elastic modulus.

Class 4 Slender:

Sections in which local buckling will prevent the stress in the section from reaching the designstrength pi and consequently a reduced value of design strength pyr must be used in calculatingthe bending capacity, which is based upon the elastic modulus, and the compression resistance.

The classification of a section is determined by comparing the width to thickness ratio of eachelement of the section, or diameter to thickness ratio for a HFCHS, with the appropriate limitingvalues given in BS 5950 Table 7. These limiting values are scheduled for each classificationand type of element and include a factor E (defined as (275/~y)O.~) which takes account of thesteel grade of the section.

The limiting values that apply to structural hollow sections are given in Table 7, part of whichis reproduced on page 7. Elements that exceed the semi-compact limits of 39s for HFRHS or80~~ for HFCHS are classified as slender.

BS 5950Part 1

Class 1

Class 2

Class 3

Class 4

Table 7

Table 7

Limiting width to thickness ratio for SHS

Type of element Class of section

1. Plastic 2. Compact 3. Semi-compact

Internal element ofcompression flange

b I 26~ b I 32~ b I 39&i t t

Web, with neutral axis atmid-depth

d I 79& d I 98& dt t t

I 120&

Web, generally d< 79&t-

d<9&0.4 + 0.60~ t - OL

See Clause 3.5.4

Web, where whole sectionis subject to compression. d

I 39& d I 39& dt t t 5 39s

Circular tube subject to momentor axial compression

Dt I 40&2 D

T I 57E2 Dt I 80E2

0.5 and a = 2y,

d

Note that for HFRHS the element width is taken as the side wall length less 3 times thethickness as shown in Figure 2.

HFRHS

b=B-3td=D-3t

Figure 2 Dimensions of SHS

When members are subject to bending about one axis only the classification may be readdirectly from Steelwork Design Guide to BS 5950 : Part 1 (Ref 10);

7

BS 5950Part 1

Table 7

Notes toTable 7

Figure 3

3.0 MEMBER DESIGN

Design examples of members to BS 5950 : Part 1 can be found in reference 11 and Appendix C.

3.1

3.1.1

MEMBERS IN TENSION

Brittle Fracture

BS 5950: Part 1 imposes maximum thickness limits for members subject to tensile stresses inservice due to applied axial load or moment when brittle fracture is required to be considered.SHS supplied to EN 102 10-l grades have adequate notch toughness within the service temperaturelimits taken as -5 “C for internal conditions and -15 “C for external conditions except for sectionsgreater than 30mm thick in design grade 50C.

2.4.4

Table 4

When steel is subject to lower temperatures the charpy impact performance at the minimumservice temperature should not be less than:

2.4.4.3

y,t710 K

where K is determined from table 3 of BS 5950: Part 1. Table 3

3.1.2 Axially Loaded tension members 4.6

The tension capacity, Pt of a member should be taken from:

pt = Ae Py 4.6.1

where A, is the effective area of the section as determined from Clause 3.3.3.

Eccentric connections

When members are connected eccentric to the axis of the member the resulting momenthas to be allowed for in accordance with Clause 4.8.2 (See Section 4.0, axially loadedmembers with moment).

4.6.2

Effective area at connections.

The effective area, &, of each element of a member at a connection, where fastenerholes occur may be taken as K, times its net area, but not more than its gross area,where for steels complying with BS 5950 design grades:

3.3.3

K, = 1.2 for design grade 43

K, = 1.1 for design grade 50

K, = 1.0 for design grade 55

BS 5950Part 1

In hollow section members with bolted flattened end connections, as in Figure 3, it isnecessary to make allowance for fastener holes.

- 76.1 x 5 HFCHSwith 2 holes 18mm diameter

Figure 3

The 76.1 x 5 circular hollow section (design Grade 43) member in Figure 3 has a gross crosssection area of 11.2 cm (from Dimensions and Properties tables).

In accordance with clauses 3.3.2 and 3.3.3 the effective area at the connection may betaken as K, times the net area, but not more than the gross area, where K, = 1.2 for designgrade 43 steel.

The effective area, 4, is therefore as follows:-

& = (gross area - area of holes) x K,= (11.2 x 102) - (4 x 18 x 5) x 1.2= (1120 - 360) x 1.2= 760 x 1.2= 912 mm* 5 1120

Use effective area of 912 mm*

9

BS 5950Part 1

3.2 MEMBERS IN COMPRESSION

3.2.1 Compression resistance

The compression resistance P, of a member is obtained from P,the gross cross sectional area and pc is the compressive strength

=AgPC where A, is

The compressive strength pc is obtained from the relevant strut curve and depends uponthe slenderness l/r ( = h) and the design’strength py, or reduced design strength pyr inthe case of slender sections.

The design procedure as applied to hollow sections is given as follows:

3.2.2 Design procedure for hollow sections

(1) Select trial section and determine the value of the design strength py.

(2)

(3)

Determine whether or not the section is slender. For slender sections in order to preventlocal buckling reduce the design strength as follows.For HFCHS obtain the reduced design strength pyr from the semi-compact limit of the circulartube in table 7. Thus from d/t s = 80 &2 and E = (275/~,)O.~ pyr = 80 x (275/ (d/t))For HFRHS reduce the design strength py by the stress reduction factor for the internal element otcompression flange in table 8: 31

b -8TE

Determine the effective length b

(4) Calculate the slenderness ratio h = b/r

(5) Select the relevant strut curve according to the section-shape and the axis of buckling.For hot rolled structural hollow sections strut curve a is always used.

(6) Obtain the compressive strength pc from strut table 27a for the appropriate values ofdesign strength py (or pyr) and slenderness h (= L/r)

(7) Calculate the compression resistance P, = A, pc

Slender sections

Slender sections under compression will fail by local buckling before the stress in thesection reaches the full design strength py. Local buckling is avoided by reducing thedesign strength pyr as described above.

Problematically, on occasion it will be found that pyr is less that the lowest value of py(= 225 N/mm2) tabulated in table 27 and in such cases the compressive strength pc mustbe calculated according to Appendix Cl and C2.

Note that since no HFCHS section in the current range is slender, the reduced value of pYwill only apply to a limited range of HFRHS.

Generally speaking it is the thinner wall sections of side length 200 and above that need to bechecked. Section clarification for members subject to bending is given in published form(see Appendix A reference 9).

10

3.2.3 Effective length of struts

The effective length L of a compression member should be determined from itsactual length L centre to centre of restraint in the relevant plane as given in table 24of BS 5950: Part 1. The effective length factors from Table 24 are given below andare shown diagrammatically in Figure 4 as follows:

Figure 4

Nominal effective length, L, for a strut.

2.0.!Conditions of restraint at ends (in planeunder consideration)

Effectively Restrained in directionheld in at both endsposition atboth ends Partially restrained in

direction at both ends

Restrained in directionat one end

NOT restrained in directionat either end

One end Other end

Effectively Not held Effectivelyheld in in restrainedposition and position in directionrestrainedin direction Partially

restrainedin direction

NOT restrainedin direction

Effectivelength, L

0.7L

0.85L

OML

l.OL

1.2L

1.5L

2.OL

11

BS 5950Part 1

7.2.4

Table 24

3.2.4 Lateral restraint

BS 5950Part 1

Lateral restraint to compression members should have sufficient strength and stiffnessto inhibit movement of the restrained point in position and direction as appropriate.

4.7.1.2

Compression chord members forming part of a lattice beam, girder or roof truss may requireone or more lateral restraint within the span, these intermediate lateral restraints should becapable of resisting a total force of not less than 2.5% of the maximum factored force in thecompression chord, divided between the intermediate lateral restraints in proportion to theirspacing.

All intermediate lateral restraints should be either connected to an appropriate system ofbracing capable of transferring the restraint forces to the beam, girder or trusses, effective pointsof support, or else connected to an independent robust part of the structure capable of fulfillinga similar function.

Where two or more parallel members require lateral restraint at intervals, it is not adequatemerely to connect the members together such that they become mutually dependent.

Where three or more intermediate lateral restraints are provided, each intermediate lateralrestraint should be capable of resisting a force of not less than 1% of the maximum factored forcein the compression chord.

4.3.2.2

In this case, the bracing system should be capable of resisting the greater of the effects of:

(a) the 1% restraint force considered as acting at only one point at a time;(b) the restraint forces described in 4.3.2.1

Where more than three parallel members share the same system of restraints, the combinedlateral restraint force should be taken as the sum of the three largest lateral restraint forcesrequired for each individual restrained member, as determined in accordance with 4.3.2:1 and4.3.2.2.

4.3.2.3

3.2.5 Maximum Slenderness

The value of the slenderness ratio h (= L&) should not exceed the following: 4.7.3.2

60 for members resisting loads other than wind loads 180I

09 for members resisting self weight and wind loads only 250I

@I for any member normally acting as a tie but subject toreversal of stress resulting from action of wind

Members whose slenderness exceeds 180 should be checked for self weight deflection. If thisexceeds length/1000 the effect of bending should be taken into account in design.

1 2

3.3 MEMBERS IN BENDING

3.3.1 General conditions

All members in bending should be checked to take account of the following: 4.2

a) At critical points the combination of maximum moment and co-existent shear andthe combination of maximum shear and co-existent moment.

W Deflection limits given in Clause 2.5.1.

d The resistance of a member toflange has full lateral restraint,

lateral torsional buckling unless the compression

4 Local buckling (of slender elements).

e) Web buckling and web bearing.

3.3.2 Shear

Shear force F, should not exceed the shear capacity P, which is given by: P, = 0.6 pyA,.

For rectangulararea A, is:

hollow sections with the applied load parallel to the web the shear

A, = (D/D+B)) x A

For circular hollow sections, A, = 0.6 A

A further check is given in the Code for shear buckling of thin webs (where d/t ratioexceeds 63&), but since the sizes in the current BS 4848: Part 2 range do not exceedthis limit the check is not applicable.

3.3.3 Lateral torsional buckling

Where a flexural member is not provided with sufficient lateral restraint to thecompression flange, then lateral torsional buckling will occur before the member candevelop its maximum bending capacity.

Hollow sections possess a highfrom lateral torsional buckling.

degree of torsional stiffness and do not generally suffer

It can be seen from Appendix B that for square and circular hollow sections(where I, = IY) the factory’ will always be equal to zero. It follows that the bucklingindex @t., and the equivalent slenderness h LT will also be equal to zero, indicating thatlateral torsional buckling does not occur with square or circular hollow sections.

BS 5950Part 1

4.2.1.3

4.2.3

4.3

4.2.2

B.2.5

14

The Code (Appendix B.2.6.1. Table 38) gives a limiting slenderness ratio below which boxsections of uniform wall thickness (including RHS) need not be checked for lateral torsionalbuckling effects. In practice, where normal deflection limits apply, this limit of slenderness isunlikely to be exceeded, and consequently hollow sections in bending may generally beconsidered as beams with full lateral restraint.

By applying the limits as given in Table 38 to the current range of Rectangular Hollow Sectionssizes in Design grades 43 and 50 the following limiting values of h are obtained:

D/B h for design grade 43 h for design grade 50

1 infinity infinity2 350 271

-_

D

01-_

i - iB !

D and B are overall depth and breadth of RHS respectively.

It follows that based upon these values of h (which is equal to L&y) for each RHS size thereis a corresponding limiting length L,, beyond which a check should be made for lateraltorsional buckling.

For example, consider a simply supported laterally unrestrained rectangular hollow sectionbeam 200 x 100 x 5 design grade 43 with D/B = 2.

200 x 100 x 5 (Design grade 43)----------m-------B--

------~~~~~~--_------AL IAL

I LC = 14.7m I

sl

The limiting value of h is 350

L = 350 x 4.2 x 1Omm= 14.7m

This length (14.7m) is termed the limiting length L, and is tabulated on pages 147, 148, 321 and322 of the Steelwork Design Guide Volume 1 (Reference 9).

Square and circular hollow sections have a limiting length of infinity and, as indicatedpreviously need not be checked for Lateral Torsional Buckling.

15

BS 5950Part 1

B.2.6.1

Table 38

3.3.4 Moment capacity M,

The Code gives two sets of expressions for determining M, moment capacity in the absence ofaxial load, the first for members with low shear load (where F, 5 = 0.6 Pv) and the second,giving reduced moment capacity, for members with high shear load (where Fv > 0.6 P,).

Since RHS beam members have two vertical webs it is unlikely that the shear load will exceed0.6 of the shear capacity and therefore the moment capacity can usually be taken as follows:

Class 1 + 2 Plastic or Compact sections M, = pYS 5 = 1.2 pYZ *Class 3 Semi-compact sections M, = pYZ.Class 4 Slender sections M, = PyrZ

where pyr < pY (Clause 3.6)

* This restriction is to ensure that plasticity does not occur at working load. For most I-sectionsthe shape factor S/Z is less than 1.2. Only for hollow sections is S/Z greater than 1.2 and insuch cases the constant 1.2 may be replaced by the average load factor (i.e. the ratio of thefactored load to the un-factored load).

Note that in the Steelwork Design Guide - Volume 1 (Reference 9) most of the tabulatedvalues of M, for hollow sections are governed by M, 5 = 1.2 pYZ. Since in most design casesthe average load factor is greater than 1.2 it generally follows that M, can be taken as thehigher value derived from M, = pYS.

3.3.5 Design procedure for hollow sections

The design procedure will normally be similar to that for a beam with full lateral restraint.

1)

2)

3)

4)

5)

6)

7)

8)

Select section and check that L&y < Limiting hOr check that span < L,

B.2.6.1

Determine the value of the design strength pY

Determine the section classification

For slender sections reduce the value of designstrength using the same procedure given section 3.2.2page 10 for members in compression.

Table 6

Table 7

3.6.4

Check the shear capacity 4.2.3

Check the moment capacityWith low shear loadWith high shear load

4.2.54.2.6

Check the buckling resistance of the webs 4.5.2.1

Check the bearing resistance of the webs 4.5.3

1 6

BS 5950Part 1

4.2.5

4.2.6

4.2.5

4.0

4.1

and

AXIALLY LOADED MEMBERS WITH MOMENTS

TENSION MEMBERS

Tension members with moments should be checked for the following two effects: 4.8.2

1) Lateral torsional buckling under the action of moment alone 4.3

2) check for capacity under the combined effects of axial load and moment at the points ofgreatest bending moments and axial loads, usually at the ends.

The following relationship should be satisfied:

F Mx MY+ - + - Il.0

Ae Py MC, MC,

where

F is the applied axial load in member;

PY

WC

MC, is the moment capacity about the major axis in the absence of axial load;

MY is the applied moment about the minor axis at critical region;

MC,

is the effective area; (3.3.3)

is the design strength;

is the applied moment about the major axis at critical region;

is the moment capacity about the minor axis in the absence of axial load

Alternatively for greater economyrelationship should be satisfied.

in plastic or compact cross sections only the following

where

Mr x* and Mr y * are the reduced moment capacities about the major and minoraxis respectively in the presence of axial load obtained from the published tables(see Ref 10).

andzl = Z2 are constants taken as:

2.0 for hollow circular sections;

513 for hollow rectangular sections;

)ie”rx and Mr Y are the reduced moment capacities which are calculated by using areduced plastic modulus. The published tables (ref. 9) are for the case where the plastic neutralaxis lies within the area bounded by the walls of a rectangular section or the internal radius of acircular section. When the plastic neutral axis lies within the wall thickness of rectangular sectionsthe modified reduced plastic modulus is given in Appendix B of this publication.

17

BS 5950Part 1

(4.2.5and 4.2.6)

(4.2.5and 4.2.6)

4.2 COMPRESSION MEMBERS

Check for 1) local capacity2) overall buckling

1) Local capacity

a) F Mx+ - +

Ai? PY MC,

or b) for plastic or compact sections only

(M,) Z 1 (My) Z2+ -

0% YJ (Mr y>

MY Il.0

MC,

Il.0

Note: for further reference to M rx and M ry see Page 17 of this publication. 4.8.3.3

2) Overall buckling

a> Simplified approach

F mMx+ - &Y Il.0

+ -A, PC M b * PY ZY

* Note: Mt., must not be taken as greater than M,,

b) Alternatively, more exact approach

+ -Max May

WhereMax is the maximum buckling moment about the major axis in the

presence of axial load, taken to be the lesser of:

MC,(I- $3(1+&q

Ma, is the maximum buckling moment about the minor axis in thepresence of axial load taken as:

M (l-e)CY

(1+y)where

M,x is the moment capacity about the major axis

MC, is the moment capacity about the minor axis but not subject tothe restriction M, I 1.2~~ Z;

Pcx is the compression resistance about the major axis;PCY is the compression resistance about the minor axis.

The simplified approach for overall buckling assumes that lateral / torsional buckling controls.This is not true for circular or square hollow sections and only true for rectangular hollowsections with long unrestrained lengths. It is therefore recommended that the more exactapproach be generally used.

18

BS 5950Part 1

4.8.3.24.8.3.3

4.8.3.2

4.8.3.3.1

4.8.3.3.2

( 4.2.5

4.;: )

4.3 WEB BEARING AND BUCKLING

The basic expression for web checks are given in Clause 4.5.2.1 (buckling) andClause 4.5.3. (bearing) as follows:

buckling resistance P, = (bl + nl) tpc

bearing resistance = (bl + n2) tP,,

In practice reference should be made to the Steelwork Design Guide - Volume 1(Ref 9) which contains tables for the beam factor Cl, stiff bearing factor C2 andthe flange plate factor C3. Details are shown in Figures 7 and 8.

ID

Figure 7 Web bearing

Where bl = length of stiff bearingt = thickness of webtp =Cl =c2 =

Bearing web capacity = Cl + (bl.C2) +(tp.C3) c3 =

Note

thickness of flange platebeam factorstiff bearing factorwelded flange plate factor

Where the flange plate is nonwelded factor C3 should bedivided by 2.5.

Figure 8 Web bearing

Buckling web capacity P, = Cl + (b,.C2) + ($.C3)

19

BS 5950Part 1

4.5

4.5.2.1

4.53

The beam buckling factor Cl allows for dispersion of load in two directions and appliesto a member which is continuous over bearing or an end bearing member with acontinuously welded sealing plate (see Figure 9).

Figure 9

2) There are two sets of values given for factor Cl, C2 and C3: the first (largervalues) are for welded flange plates, and the second (smaller values) fornon-welded flange plates.

When loads or reactions are applied through non-welded flange plates theadditional effects of moment in the web due to eccentric loading have to be takeninto account, resulting in lower buckling values (see Figure 10).

B

t-ltiJ07D

e e

P P

MYLTMY

Figure 10 Local moment in side walls of RHS

5.0 MEMBERS IN TORSION

The total resistance of a member to torsional loading is composed of the sum of twocomponents known as ‘uniform torsion’ and ‘warping torsion’.

As the torsional rigidity of a Structural hollow section is very large compared with itswarping rigidity, the section may be reasonably regarded as subject to pure torsion only.

In this case the total angle of twist is given by:

TsZIz) = GJ

WhereTg = the applied torquez = the length of member subject to T4G = Shear modulus of elasticity (79000 N/mm*)J = Torsional constant for cross section

BS 5950Part 1

and the pure torsional shear stress is given by:

TqTt =-

Cwhere C = Torsional Modulus Constant

Torsional constants for circular, square and rectangular hollow sections plus basic theoryand worked examples for members subject to torsion loading including combined bendingand torsion, are given in the SC1 publication (Ref 11).

6.0 PURLINS AND SIDE RAILS 4.12

Purlins and side rails may be designed on the assumption that the cladding provides lateralrestraint to the face against which it is connected. The type of cladding and its fixingsshould be such that it is capable of acting in this manner.

Deflection should be limited to suit the characteristics of the particular cladding system.

Wind loading, excluding local pressure effects, should be determined from CP3 Chapter V : part 2.

Alternatively, in the case of roof slopes not exceeding 30” from horizontal or wall claddingnot exceeding 15” from vertical, purlins and side rails may be designed using empirical rules.

For purlins a minimum substantially uniform loading of 0.75 N/mm2 should be used and themodulus (Z) and section depth (D) and width (B) for spans not exceeding 6.5m found by:

Purlins Section

CHS

RHS

Z (min) D B

WPL L/65

2000

WPL L/70 L/150

1800

Where Wp is the total unfactored load on one span of the purlin (in kN), L is the l.ength centre tocentre of the main supports (in mm).Side rails should generally be designed for wind loads and self weight of the cladding. Theminimum values of the elastic modulus (Zl) about the axis parallel to the plane of cladding and thesection depth (D) measured perpendicular to the cladding, and the corresponding elastic modulus(22) and section width (B) are found by:-

Side Rail

CHS

RHS

Zl (min) 22 (min) D B

WlL W2L L/65 -

2000 1350

WJ W2L L/70 L/l00

1800 1200

Where W1 and W2 are total unfactored loads on one span of the side rail acting perpendicular to andparallel to the plane of the cladding respectively, (in kN), L is the span of the rail (in mm) for a) ZlandDandb)Z2andB.

2 1

BS 5950Part 1

4.12.4

Table29

Table30

7.0 COLUMN BASES

BS5950 provides empirical rules for determining the thicknessbase plates but also allows other rational means to be used.

of concentrically loaded rectangular

7.1 Empirical Rules.The minimum thickness in mm for rectangular base plates carrying concentrically loadedRHS member is:

t =2.5 1 t

w (a* - 0.3b2) > RHS wall thickness

PYP

and for circular or square base plates for solid rounds or CHS members the minimumthickness in mm is given by the following expression which was based on requirements forsolid columns and may give unacceptably large thickness for CHS members. Reference to theeffective area method is recommended for CHS baseplates.

W

1

3t = Dp (Dp - 0.9D)

2.4 Pyp

where a is the greater projection of the plate beyond the columnb is the lesser projection of the plate beyond the columnw is the uniform pressure on the underside of the platepYp is the design strength of the plate with a maximum of 270 N/mm*Dp is the length of side or diameter of base plate, not less than 1.5 (D + 75)nu-nD is the CHS diameter

If the bearing pressure beneath the base plate is not uniform, calculations should be made todetermine the bending moments in the plate which should not exceed 1.2 pypZ wherepyp I 270 N/mm* and Z is the elastic modulus of the base plate.

Caution must be exercised in use of the above formulae as they assume uniform loading is present atthe underside of the base plate and the maximum moment occurs at the external corner of the RHS.Resultant base sizes may not produce this condition particularly when dimension D is large inrelation to dimension B. As an alternative the effective area method may be used.

7.2 Effective Area MethodThe basis and method are taken from the publication entitled ‘Joints in Simple Construction,Volume 1 Design Methods’ published by the BCSA and Steel Construction Institute.

1) Baseplates for RHS ColumnsThe shaded area in figure 11 represents the area of base plate assumed to be effective intransmitting the factored column load onto the foundations by imposing a uniformpressure on the concrete not exceeding the limiting bearing strength. The bearing strengthis taken as 0.6 fcU in accordance with BS 8110: Part 1: 1985.

Required effective area Areq = ( mean wall perimeter length ) x ( width of wallthickness plus two outstands )

= (2D+2B-4t)(t+2K)

Note that when K is greater than ( B - 2t) /2 then the internal outstands overlap and theeffective area becomes Areq = ( D + 2K ) ( B + 2K )

BS 5950Part 1

4.13

4.13.2.2

4.13.2.3

22

8.0

9.0

10.0

DESIGN OF WELDED JOINTS

Whilst the Code gives guidance for bolted connections no specific detail is included for overalldesign of welded joints. The following reference is therefore included to assist the designer.Lattice structures are usually designed on the basis of pin jointed frames with members in tensionor compression and the forces noding at the joint. Bracing and chord members are determined inaccordance with the normal laws of statics. Research work has shown that the strength of suchjoints is dependent on a number of factors:

bracing width to chord width ratiochord width/thickness ratiochord compressive loadinggap or overlap of bracings

Work conducted with the International Institute of Welding and the CIDECT organisationhas led to the publications (Ref 18) of design rules for welded joints for circular, squareand rectangular chords, which will be included in Eurocode No. 3. This work has highlightedthat the final joint strength is substantially influenced by the relative bracing-to-chord sizes and thegeneral joint geometry which is the province of the design engineer as it involves his memberselection for both analysis and design. Resulting from this, it becomes necessary for the designerto fully consider the joint design as part of his work. Final sizing of welds can be left to thefabricator provided one important point is specified. The weld at the toe of a bracing member(highest stressed point) should, if the bracing angle is less than 60” be bevelled and a butt weldused. Further information on welding can be found in BS 5 135 (Ref 5) and British Steel WeldedTubes publication SHS Welding (Ref 19).

COMPOSITE CONSTRUCTION

BS 5950 : Part 1 gives guidance for the design of cased beam or column sections but noreference is made to hollow sections as this will be covered in Part 3, Code of practicefor design in composite construction. A separate design manual for limit state design ofconcrete filled hollow sections based on BS 5400 : Part 5 (Ref 6), is availablefrom British Steel Welded Tubes (Ref 20).

FIRE RESISTANCE

The means to evaluate and define the fire resistance of Structural elements is given inBS 5950 : part 8 (Ref 7).

The code recognises that Structural Steelwork can in certain conditions have the requiredfire resistance even when unprotected. Alternatively, adequate fire resistance can be achievedby use of either externally applied systems such as boards or spray materials or internalsystems such as concrete or water filling.

The thickness of externally applied systems is determinedand fire protection manufacturers data (Ref 15).

using the section factor Hp/A

The evaluation of concrete filling is contained in the standard (Ref 7) and further informationis contained in Welded Tubes publication for concrete filled columns (Ref 20) and, in thecase of CHS columns, in ECCS Technical Note 55 *.

The evaluation of water filling and the use of bare external structural steel is referenced bythe standard to publications issued by the Steel Construction Institute (Ref 12 and 13) whoalso publish a handbook to BS 5950 part 8 (Ref 15).

* ECCS publications are available through The British Constructional Steelwork AssociationLimited, 4 Whitehall Court, Westminster, London, SWlA 2ES.

24

BS 5950Part 1

4.14

Appendix A

REFERENCES

1.

2.

BS 4848 : Part 2 - Specification for hot-rolled Structural Steel Sections. Part 2 - Hollow Sections.

EN 10210-l : Hot finished structural Hollow Sections in non alloy and fine grain Structural Steels - Part 1 :Technical delivery conditions.

3. BS 5950 : Part 1 - Code of practice for design in simple and continuous construction. Hot rolled sections.

4. BS 6363 - Specification for Welded Cold formed Steel Structural Hollow sections.

5. BS 5 135 - Specification for Arc welding of carbon and carbon manganese steels.

6. BS 5400 : Part 5 - Code of practice for design of composite bridges.

7. BS 5950 : Part 8 - Code of practice for fire resistant design.

References 1 to 7 are available from the British Standards Institution, 2 Park Street, London, WlA 2BS

8.

9.

10.

11.

12.

13.

14.

15.

Introduction to Steelwork design to BS 5950 : Part 1

Steelwork Design Guide to BS 5950 : Part 1 : 1990, Vol 1. Section properties member capacities, 3rd Edition.

Steelwork Design Guide to BS 5950 : Part 1, Vol 2. Worked Examples.

Design of members subject to combined bending and torsion.

Water cooled hollow columns.

Fire Safety of bare external Structural Steel.

Fire protection for Structural Steel in Buildings.

Fire Resistant Design of Steel Structures - A Handbook to BS 5950 : Part 8

16. TD 167 RI-IS & CHS Sizes, Properties

17. TD 364 RHS & CHS Technical Data

18. TD 338 Design of SHS Welded Joints, to BS 5950

19. TD 328 SHS Welding

20. TD 296 Design manual for SHS concrete filled columns

21. TD 325 SHS Jointing

References 16 to 21 are available from British Steel Tubes & Pipes, SHS Technical Sales.

References 8 to 15 are available from The Steel Construction Institute, Silwood Park, Ascot, Berks SL5 7QN.

2 5

Appendix CC l 1 Design Examples

The publication of B.S. 5950 Part 1. Code of Practice forDesign in Simple and Continuous Construction: Hot RolledSections, presents the engineer with a change in designphilosophy with the use of limit state principles and, partly as adirect result of this, a change in the detail procedures of design.

It is anticipated that the D.O.E. will withdraw approval forB.S. 449 by the end of 1987 and in this event, familiarisation of thenew standard is important.

To assist engineers a number of design examples have beenprepared using Structural Hollow Sections. Whilst it has not beenpossible to cover every aspect of the code, it is hoped that theexamples will be of practical use in the design of elements.

In each case reference should be made to the appropriateclauses in B.S. 5950 which have been given in the marginthroughout the examples. In addition reference is made to tablesgiven in the “Steelwork Design Guide - Volume 1, SectionProperties and Member Capacities” published by the SteelConstruction Institute, and are shown in the margin thus 1x1.

Appendix C2 contains relevant extracts from the ‘SteelworkDesign Guide’.

EXAMPLE 2CONTINUOUS MULTI-STOREY COLUMN(SIMPLE CONSTRUCTION)

Factored Loading

3.0m

3.5m

f’

I Pinned Base

40 170 kN

100

140+I#-85

508040

100140

85

325 kN

Self Weight 5

TOTAL FACTORED LOAD F, 500 kN

Generally, for axially loaded compression members with moments, cl. 4.8.3,separate checks are required for local capacity cl. 4.8.3.2., and overall bucklingcl. 4.8.3.3. The requirements for overall buckling may be satisfied by either the‘Simplified approach’ cl. 4.8.3.3.1, or the ‘More exact approach’ usingcl. 4.8.3.3.2.

However for columns of the type in this example a further option is available byreference to cl. 4.7.7, ‘Columns in simple multi-storey construction’ subject tocompliance with the qualifying conditions. This clause dispenses with the needfor a separate local capacity check, and it is only necessary to apply the rulesdefined in the ‘Simplified approach’.

Simplified ApproachF mM, mM

+-+---ql.OA,p, Mb Pyz,

Where F = Applied axial loadpc = Compressive strengthA, = Gross cross - sectional aream = Equivalent uniform moment factorMb= Buckling resistance moment Capacity (major axis)Z, = Elastic section modulus (minor axis)pY = Design strength

2.1Ref.

4.7.7

4.8.3.3.1

4.5Ref.

Using ‘STEELWORK DESIGN GUIDE to BS5950Volume 1 (3rd Edition), Section properties, Member capacities’

RECTANGULAR HOLLOW SECTION 250 x 150 x 8 Design grade 50Section is PLASTIC for bending about X-X

Web ShearShear force F, (max) = 82.5 kNShear capacity P, = 814 kNCheck shear F, (at max. bending) = 60 kN < O-6 P, = 488 kN:. Shear force is ‘low’, and M, need not be reduced.

Moment CapacityApplied moment M, = 178 kNmMoment capacity M, = 176 kNm**governedbyM,=1.2pYZ=176kNmHigher value may be used since S, = 1.22 Z, > 1.2 Z,and constant 1 - 2 may be replaced by 1 - 5 (average load factor).

M,=p,S,<+5p,Z=355x505x10~3~1~5x355x413x10-3= 179 kNm < 220 kNm

:. M,= 179 kNm > M, = 178 kNm O.K.Limiting length L, = 16 - 7 mSince span 5.0 m < L, = 16.7 m LATERAL TORSIONALBUCKLING need not be checked.

Web BearingBearing load = 82 - 5 kNBeam factor (end) Cl = 114 kNStiff bearing factor (end) = 5 - 68 kN/mm = C2Bearing capacity = Cl + (b, x C2) + (tP x C3)

=114+(75x5~68)+(Ox14~2)= 540 kN O.K.

Web BucklingBuckling load = 82.5 kNBeam factor (end) Cl = 798 kNStiff bearing factor (end) C2 = 3 - 19 kN/mmBuckling resistance = Cl + (b, x C2) + (tP x C3)

=798+(75x3*19)+(0x3.19)= 1037 kN O.K.

El322

I330]

4.2.5

I322Note

047

I322

I330

cl330

APPENDIX C=2

Extracts reproduced from:-

STEELWORK DESIGN GUIDE TO BS5950 : PART 1 : 1990

Volume 1 - Section Properties and Member Capacities

@THE STEEL CONSTRUCTION INSTITUTE

PO Box 101, Weldon Road, Corby, Northants. NN17 5UATel: 01536 404120/1 Fax: 01536 404005

TD 365/5E/94