BRITISH STANDARD BS 5950-8: 1990 Incorporating Amendment No Structural use of steelwork in building — Part 8: Code of practice for fire resistant design UDC 693.814:669.14.018.29:614.84:62-11:[006.76 (083.75)] BS 5950-8:1990 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Policy Committee (CSB/-) to Technical Committee CSB/27, upon which the following bodies were represented: British Constructional Steelwork Association Ltd British Railways Board British Steel Industry Department of the Environment (Building Research Establishment) Department of the Environment (Housing and Construction Industries) Department of the Environment (Property Services Agency) Health and Safety Executive Institution of Civil Engineers Institution of Structural Engineers Royal Institute of British Architects Steel Construction Institute Welding Institute The following bodies were also represented in the drafting of the standard, through subcommittees and panels: Association of Structural Fire Protection Contractors and Manufacturers Department of the Environment (Fire Research station) This British Standard, having been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee, was published under the authority of the Board of BSI and comes into effect on 29 June 1990 © BSI 12-1998 The following BSI references relate to the work on this standard: Committee reference CSB/27 Draft for comment 85/12865 DC ISBN 580 18344 Amendments issued since publication Amd No Date of issue Comments 8858 November 1995 Indicated by a sideline in the margin BS 5950-8:1990 Contents Committees responsible Foreword Page Inside front cover iv Section General 1.0 Introduction 1.0.1 Aims of fire precautions 1.0.2 Steel in fire 1.1 Scope 1.2 Definitions 1.3 Major symbols 1 1 Section Steel in fire 2.1 Properties at elevated temperature 2.2 Strength reduction factors 2.3 Strain levels 3 Section Fire limit states 3.1 General 3.2 Material strength factors 3.3 Performance criteria 3.4 Bracing members 3.5 Re-use of steel after a fire 5 5 Section Evaluation of fire resistance 4.1 General 4.2 Section factor 4.2.1 General 4.2.2 Rolled fabricated and hollow sections excluding castellated sections 4.2.3 Castellated sections 4.2.4 Tapered sections 4.3 Fire resistance derived from testing 4.3.1 General 4.3.2 Unprotected members 4.3.3 Protected members 4.4 Fire resistance derived from calculation 4.4.1 General 4.4.2 Limiting temperature method 4.4.3 Design temperature 4.4.4 Moment capacity method 4.5 Portal frames 4.6 Concrete-filled hollow section columns 4.6.1 General 4.6.2 Concrete-filled rectangular hollow section 4.6.3 Externally applied fire protection to concrete-filled circular or rectangular hollow sections 4.7 Water-filled structures 4.8 External bare steel 4.9 Floor and roof slabs 4.9.1 General 4.9.2 Unprotected composite slabs with profiled steel sheeting 4.9.3 Protected composite slabs with profiled steel sheeting 4.9.4 Composite beams © BSI 12-1998 6 6 6 6 6 7 11 11 11 11 11 12 13 13 13 13 13 14 14 i BS 5950-8:1990 4.10 4.10.1 4.10.2 4.10.3 4.10.4 4.11 4.12 4.12.1 4.12.2 Page 15 15 16 16 16 16 16 16 16 Walls General Walls connected to steel members Walls under beams Independent fire-resisting walls Roofs Ceilings General Dry suspended ceiling systems Appendix A Fire design flow chart Appendix B Strength reduction factors for cold formed steels complying with BS 2989 Appendix C Re-use of steel after a fire Appendix D Calculation of thickness of fire protection material Appendix E Simplified method of calculation for beams with shelf angles Appendix F Simple method of calculation for portal frame buildings Appendix G Bibliography 17 Figure — Measurement of depth into concrete slab Figure — Insulation thickness for trapezoidal profiled steel sheets Figure — Insulation thickness for re-entrant profiled steel sheets Figure — Effect of beam deflection on a fire-resisting wall Figure — Fire design procedures Figure — Temperature blocks for beams with shelf angles Figure — Definition of dimension x Figure — Definition of blocks 4, and 14 15 15 15 17 21 22 22 18 18 19 20 22 26 Table — Strength reduction factors for steel complying with grades 43 and 50 of BS 4360 Table — Load factors for fire limit state Table — Calculation of Hp/A values Table — Maximum section factor for unprotected members Table — Limiting temperatures for design of protected and unprotected hot finished members Table — Design temperature for columns and tension members Table — Design temperature for beams Table — Design temperature reductions Table — Time dependent load ratio h Table 10 — Concrete core buckling factor K Table 11 — Fire protection thickness modification factor Table 12 — Temperature distribution through a composite floor with profiled steel sheeting Table 13 — Minimum thickness of concrete for trapezoidal profiled steel sheets Table 14 — Minimum thickness of concrete for re-entrant profiled steel sheets Table 15 — Strength reduction factors for cold formed steels complying with BS 2989 Table 16 — Insulation factor If Table 17 — Fire protection material density factor Table 18 — Block temperature ii 9 10 10 10 12 12 13 14 15 15 18 19 19 21 © BSI 12-1998 BS 5950-8:1990 Table 19 — Temperature gradient Table 20 — Factors A and C for various rafter pitch Table 21 — Modification factor K for multi-bay frames Table 22 — Percentage dead weight of roof cladding systems remaining at time of rafter collapse Publications referred to © BSI 12-1998 Page 21 23 23 25 Inside back cover iii BS 5950-8:1990 Foreword This Part of BS 5950 has been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee BS 5950 is a document combining codes of practice to cover the design, construction and fire resistance of steel structures and specifications for materials, workmanship and erection It comprises the following Parts: — Part 1: Code of practice for design in simple and continuous construction: hot rolled sections; — Part 2: Specification for materials, fabrication and erection: hot rolled sections; — Part 3: Design in composite construction; — Section 3.1: Code of practice for design of simple and continuous composite beams; — Section 3.21): Code of practice for design of composite columns and frames; — Part 4: Code of practice for design of floors with profiled steel sheeting; — Part 5: Code of practice for design in cold formed sections; — Part 61): Code of practice for design in light gauge sheeting, decking and cladding; — Part 71): Specification for materials and workmanship: cold formed sections; — Part 8: Code of practice for fire resistant design; — Part 91): Code of practice for stressed skin design This Part of BS 5950 gives recommendations for evaluating the fire resistance of steel structures Methods are given for determining the thermal response of the structure and evaluating the protection required, if any, to achieve the specified performance, although it is recognized that there are situations where other proven methods may be appropriate It has been assumed in the drafting of this British Standard that the execution of its provision will be entrusted to appropriately qualified and experienced people; also that construction, the application of any fire protection and supervision will be carried out by capable and experienced organizations This code of practice represents a standard of good practice and therefore takes the form of recommendations 1) iv In preparation © BSI 12-1998 BS 5950-8:1990 The full list of organizations who have taken part in the work of the Technical Committee is given on the inside front cover The Chairman of the Committee is Mr P R Brett and the following people have made a particular contribution in the drafting of the code Mr J T Robinson Dr G M E Cooke Mr J I Hardwick Dr R M Lawson Mr G M Newman Dr C I Smith Mr A D Weller Chairman of Drafting Panel NOTE The numbers in square brackets used throughout the text of this standard relate to the bibliographic references given in Appendix G A British Standard does not purport to include all the necessary provisions of a contract Users of British Standards are responsible for their correct application Compliance with a British Standard does not of itself confer immunity from legal obligations Summary of pages This document comprises a front cover, an inside front cover, pages i to vi, pages to 26, an inside back cover and a back cover This standard has been updated (see copyright date) and may have had amendments incorporated This will be indicated in the amendment table on the inside front cover © BSI 12-1998 v vi blank Section BS 5950-8:1990 Section General 1.0 Introduction 1.0.1 Aims of fire precautions The aims of fire precautions are to safeguard life and to minimize fire damage to property and financial loss These aims are principally achieved by: a) minimizing the risk of ignition; b) providing a safe exit for occupants; c) restricting the spread of fire; d) minimizing the risk of structural collapse This Part of BS 5950 is concerned with items c) and d) 1.0.2 Steel in fire Steel progressively weakens with increasing temperature and eventually failure occurs in a member as a result of its inability to sustain the applied load, e.g buckling in the case of a column or excessive deflection in the case of flexural members The limiting temperature at which failure occurs varies and is dependent on the loading which the member is carrying, its support conditions, the change in its properties as the temperature rises, and the temperature gradient through the cross section 1.1 Scope This Part of BS 5950 gives recommendations for the two following methods of achieving the specified fire resistance for steel building members and sub-assemblies (see Appendix A) a) fire resistance derived from tests in accordance with BS 476-20 and BS 476-21; b) fire resistance derived from calculations NOTE These methods may also be applied to members for which the required fire resistance has been derived from the consideration of natural fires NOTE The titles of the publications referred to in this standard are listed on the inside back cover 1.2 Definitions For the purpose of this Part of BS 5950 the following definitions apply 1.2.1 critical element the element of a section that would reach the highest temperature in fire conditions NOTE The web of an I, H or channel section or the stalk of a T section, is not normally critical 1.2.2 design temperature the temperature that the critical element will reach at the end of the specified period of fire resistance in a test in accordance with BS 476-20 and BS 476-21 © BSI 12-1998 1.2.3 element an element may be taken as one of the following: a) a flange of a rolled or built-up I, H or channel section; b) the web of a rolled or built-up I, H or channel section; c) a leg of an angle; d) the flange or the stalk of a T section; e) a side of a rectangular hollow section 1.2.4 fire protection material a material, which has been shown by fire resistance tests in accordance with BS 476-20 and BS 476-21, to be capable of remaining in position and providing adequate thermal insulation for the fire resistance period under consideration 1.2.5 insulation the ability of a separating component to restrict the temperature rise of its unexposed face to below specified levels 1.2.6 integrity the ability of a separating component to contain a fire to specified criteria for collapse, freedom from holes, cracks and fissures and sustained flaming on its unexposed face 1.2.7 limiting temperature the temperature of the critical element of a member at failure under fire conditions 1.2.8 load capacity limit of force or moment which may be applied without causing failure due to yielding or rupture 1.2.9 structural member part of a structure designed to resist force or moment, such as a steel section formed by hot rolling, cold forming or welding sections and/or plates together 1.2.10 fire resistance the length of time for which the member or other component is required to withstand exposure to the fire regime given in BS 476-20 without the load capacity falling below the fire limit state factored load or loss of integrity and/or insulation BS 5950-8:1990 Section 1.2.11 thermal expansion increase in length, cross-sectional area or volume of a material per degree increase in temperature 1.3 Major symbols A Gross cross-sectional area of a section Ff Applied axial load at the fire limit state, using the factored loads given in 3.1 Hp Heated perimeter (see Table 3) Mf Applied moment at the fire limit state, using the factored loads given in 3.1 Mcf Moment capacity at the required period of fire resistance Mc Moment capacity at 20 °C uL Limiting temperature uD Design temperature gf Load factor gm Material strength factor © BSI 12-1998 Section BS 5950-8:1990 4.9.2.3 Integrity The integrity of a composite slab with profiled steel sheeting should be maintained by forming a continuous membrane with the side seams being locked into and sealed by the concrete 4.9.4 Composite beams 4.9.3 Protected composite slabs with profiled steel sheeting NOTE Composite beams should have their fire resistance assessed in the same way as non-composite beams, see 4.3 and 4.4 For further information see [9] The fire resistance of protected composite slabs with pro-filed steel sheeting may be assessed by tests in accordance with BS 476-21 Table 12 — Temperature distribution through a composite floor with profiled steel sheeting Temperature distribution for a fire resistance period of: Depth into slab (see note 2) 30 60 90 NW LW NW LW NW LW °C °C °C °C °C °C 10 470 460 650 620 790 20 340 330 530 480 30 250 260 420 40 180 200 50 140 60 120 mm NW 720 580 380 540 330 290 160 250 110 130 70 90 80 °C a 650 LW °C 180 NW 240 LW °C NW °C LW °C °C 770 a a a a 720 640 a 740 a a 460 610 530 700 630 770 700 430 360 510 430 600 520 670 600 220 370 280 440 340 520 430 600 510 200 170 310 230 370 280 460 380 540 440 80 170 130 260 170 320 220 410 320 480 380 80 60 140 80 220 130 270 180 360 270 430 320 90 70 40 120 70 180 100 240 150 320 230 380 280 100 60 40 100 60 160 80 210 140 280 190 360 270 NOTE NW is ordinary dense structural concrete and LW is lightweight concrete NOTE For any profile shape the depth into the concrete is measured normal to the surface of the profiled steel sheet (see Figure 1) a Indicates a temperature greater than 800 °C Figure — Measurement of depth into concrete slab 14 © BSI 12-1998 Section BS 5950-8:1990 Figure — Insulation thickness for re-entrant profiled steel sheets Figure — Insulation thickness for trapezoidal profiled steel sheets Table 13 — Minimum thickness of concrete for trapezoidal profiled steel sheets (see Figure 2) Minimum thickness of concrete for a fire resistance period of: Concrete type 30 60 90 120 180 240 mm mm mm mm mm mm Ordinary dense structural concrete 60 70 80 95 115 130 Lightweight concrete 60 70 80 100 115 50 Table 14 — Minimum thickness of concrete for re-entrant profiled steel sheets (see Figure 3) Concrete type Minimum thickness of concrete for a fire resistance period of: 30 60 90 120 180 240 mm mm mm mm mm mm Ordinary dense structural concrete 90 90 110 125 150 170 Lightweight concrete 90 105 115 135 150 90 Figure — Effect of beam deflection on a fire-resisting wall 4.10 Walls 4.10.1 General The appropriate thickness of fire protection to be applied to steel members incorporated into fire-resisting walls should be determined in accordance with 4.3 or by using the calculation given in Appendix D If the wall itself provides © BSI 12-1998 protection to the steel member, this may be taken into account in assessing the section factor for the member NOTE To comply with statutory requirements, walls very close to a site boundary may also need to be checked for resistance to an external fire 15 BS 5950-8:1990 Section 4.10.2 Walls connected to steel members 4.12.2 Dry suspended ceiling systems Properly designed fire-resisting walls may be assumed to have sufficient inherent robustness to accommodate thermally induced differential movements between the wall and steel members incorporated into it or directly connected to it, except for walls directly under beams which support significant vertical loads, see 4.10.3 4.12.2.1 General For structural fire protection the complete ceiling and floor or roof construction should be considered Ceilings should be constructed in accordance with CP 290 4.12.2.2 Suspension systems The grid with its appropriate expansion cut-outs should be supported and restrained so as to ensure that the tiles or boards will remain in place and will not be dislodged in fire conditions 4.12.2.3 Fittings All fittings which penetrate the ceiling should have the same fire resistance as the ceiling, or be enclosed in a recess in the ceiling which is designed to provide the same level of fire protection as the ceiling Ventilation ducts and similar openings should be given special consideration to ensure that the integrity of the ceiling is not broken 4.12.2.4 Junctions Junctions with other elements of the building should be checked to ensure that there will be no breakdown in the integrity of the fire resistant barrier Care should be exercised, in particular, with the connection of internal partitions to ensure that they will not disrupt the ceiling in the event of a fire Fire barriers in the ceiling void should be so detailed and constructed as to ensure full continuity of protection 4.12.2.5 Installation and maintenance Particular care should be taken over the installation and maintenance of suspended ceilings to ensure that long term protection is given 4.10.3 Walls under beams Where a fire-resisting wall is liable to be subjected to significant additional vertical load due to the increased vertical deflection of a steel beam in a fire, see Figure 4, either: a) provision should be made to accommodate the anticipated vertical movement of the beam; or b) the wall should be designed to resist the additional vertical load in fire conditions For the purpose of this clause, the anticipated vertical movement at midspan of a vertically loaded steel beam in a fire should be taken as 1/100 of its span, unless a smaller value can be justified by an analytical assessment 4.10.4 Independent fire-resisting walls Where a steel member is very close to, or touching, a fire-resisting wall which obtains its resistance to horizontal forces independently of that steel member, the effects of horizontal thermal bowing of the wall and the steel member on the stability and integrity of the fire-resisting wall should be directly assessed Any fire protection applied to the steel member may be taken into account when determining its thermal bowing NOTE Further guidance is given in [10] 4.11 Roofs Where a roof spans across a fire resisting compartment wall and it is required that strips of the roof should be fire protected on the underside either side of the compartment wall, care should be taken to fire stop any gaps between the top of the wall and the underside of the roof cladding to allow for differential thermal movement in fire Where practicable, combustible insulation should also be fire stopped along the line of the wall 4.12 Ceilings 4.12.1 General The contribution of the protection provided by a ceiling may be considered as supplying all or part of the fire protection required by a floor or roof, subject to the requirements of 4.12.2 16 © BSI 12-1998 © BSI 12-1998 Appendix A Fire design flow chart Fire design procedures are illustrated in Figure Figure — Fire design procedures BS 17 BS 5950-8:1990 Appendix B Strength reduction factors for cold formed steels complying with BS 2989 The strength reduction factors for cold formed members made from steels complying with BS 2989 may be taken from Table 15 The appropriate value of strain should be determined from 2.3 Table 15 — Strength reduction factors for cold formed steels complying with BS 2989 Strain Strength reduction factors at a temperature (in °C) of: 200 250 300 350 400 450 500 550 600 % 0.5 0.945 0.890 0.834 0.758 0.680 0.575 0.471 0.370 0.269 1.5 1.000 0.985 0.949 0.883 0.815 0.685 0.556 0.453 0.349 2.0 1.000 1.000 1.000 0.935 0.867 0.730 0.590 0.490 0.390 NOTE Intermediate values may be obtained by linear interpolation Appendix C Re-use of steel after a fire C.1 General Structural steel may be re-used after a fire provided that its mechanical properties have not been significantly changed and that the members have not been distorted or damaged beyond the tolerances given in BS 5950-2 Members which have been distorted or damaged should be fully assessed to ensure their strength and suitability remain unimpaired Members outside the fire affected zone should be checked, whenever possible, to ensure that there has been no distortion or other damage due to thermal expansion C.2 Temperature effects on strength C.2.1 Hot finished steels Except for partially exposed members, hot finished steel members may be re-used provided that they remain within the specified tolerances for straightness and shape and that the loads are to be unchanged Partially exposed members may be subject to very high temperatures without distortion Such members should be tested by non-destructive methods to ensure compliance with appropriate British Standards C.2.2 Cast steel Cast steel members may be re-used providing they remain within the specified tolerances for straightness, shape and area C.2.3 Cold finished steel Cold finished steel up to grade Z35 of BS 2989 which remains within tolerance may be assumed to have 90 % of its original design strength after a fire and should be assessed accordingly Members should be checked for tolerance and coating integrity; remedial works or replacement should be carried out accordingly 18 C.3 Connections C.3.1 General Connections in the fire affected zone and adjoining areas, should be examined to ensure that there has been no distortion or damage due to heat or thermal expansion C.3.2 Bolted connections Bolts may be distorted due to thermal expansion or softening during a fire Where distortion is suspected then such bolts should be inspected and replaced as necessary C.3.3 Friction grip fasteners Pretensioned friction grip bolts may be seriously affected by the heating experienced in a fire All suspect bolts should be replaced by new bolts C.3.4 Welded connections Welded connections should be treated in the same way as the parent material They should also be checked to ensure that no cracking has occurred due to the effects of the fire C.4 Fire protection materials Many fire protection materials are rendered unsuitable for future use by fire All fire-protection materials should be examined and replaced as necessary C.5 On-site checks Inspection (and, where necessary, tests) should be carried out on site to verify the continued suitability of the structural members Members should be assessed on the basis of their compliance with the appropriate British Standard, see [11] © BSI 12-1998 BS 5950-8:1990 Appendix D Calculation of thickness of fire protection material ki is a function of the thermal properties of the fire protection material (W/m per degree centigrade) When the thermal and physical properties of a fire The function ki varies with temperature Values protection material other than an intumescent coating are known, the thickness t (in m) necessary should be derived from tests using representative to achieve the required period of fire resistance may values for the temperature and fire resistance period be calculated from: Values of the fire protection insulation factor If are t = ki If Fw (Hp/A)/106 given in Table 16 for the required period of fire where resistance and steel temperature If is the fire protection material insulation factor The physical and thermal properties should be derived from the results of fire resistance tests in (in m3/kW); accordance with BS 476-20 and BS 476-21 Thermal Fw is the fire protection material density factor; properties derived solely from small scale material –1 tests are not applicable Hp/A is the section factor (in m ); Tests should be carried out at an approved testing station and the properties derived from them should be approved by a suitably qualified person Table 16 — Insulation factor If Steel temperature Insulation factor for fire resistance period of: 30 °C 60 90 120 180 240 m3/kW m3/kW m3/kW m3/kW m3/kW m3/kW 400 500 230 100 000 100 400 450 400 980 650 400 100 900 500 325 800 360 980 350 900 550 275 680 150 670 850 100 600 240 590 990 440 450 550 650 210 510 870 260 150 100 700 185 450 770 120 890 750 750 165 405 690 000 690 450 800 150 365 620 900 530 200 The fire protection material density factor Fw may be obtained from Table 17, or calculated using: Table 17 — Fire protection material density factor µ Fw where ri is the fire protection material density (in kg/m3); c is the fire protection material moisture content (in % by mass); 0.0 1.0 0.05 0.95 0.1 The value of µ is given by: 0.92 0.5 0.73 1.0 0.62 1.5 0.55 2.0 0.50 rs is the steel density (in kg/m3) © BSI 12-1998 19 BS 5950-8:1990 Appendix E Simplified method of calculation for beams with shelf angles E.1 General The moment capacity of a beam with shelf angles supporting in situ or precast concrete slabs may be calculated using the method described in E.2 from the temperature profile in E.3, subject to the following conditions a) Precast concrete slabs should be made of normal weight concrete and should not have any deliberately designed voids in the end 75 mm of their length b) The void between the precast slab and the beam should be filled with grout c) Precast floor slabs should have at least 75 mm of bearing on the angles d) The steel angles should be of grade 50 steel, not less than 125 mm × 75 mm × 12 mm, fixed with the longer legs supporting the concrete slabs, and the vertical leg upwards, as shown in Figure to Figure e) The connections at either end of the beam should either be contained wholly within the depth of the floor slab or else fire protected to the same degree as the supporting member f) The moments due to the loads transmitted via the slab at the fire limit state, should not exceed the transverse moment capacity of the angles at the required period of fire resistance (Mcf) given by: Mcf = 1.2py ZkR where py is the design strength of steel; Z is the elastic modulus of angle leg, equal to t2/6 per unit length; t is the thickness of angle leg; kR is the strength reduction factor from Table for 1.5 % strain, for the temperature of the angle at the fire limit state 20 g) The angles may be welded or bolted to the beam In addition to resisting the applied vertical loads at the fire limit state, the connection of the angles to the beam should be capable of transmitting the longitudinal shear force necessary to develop the required axial forces in the angles at the point of maximum moment Any weld below the angle should be ignored In these calculations the strengths of welds and bolts should be taken as 80 % of the relevant design strength at elevated temperature derived using the appropriate strength reduction factor from Table for 0.5 % strain E.2 Calculation method In the calculations, a constant strain across the section as given in 2.3 should be assumed The deflection may be ignored The following procedure may be used a) Determine the temperature distribution across the section, at the fire limit state For beams with shelf angles the temperature distribution may be assumed to be as given in E.3 b) Divide the section into an appropriate number of blocks of constant width c) Calculate the elevated temperature load capacity of each block, assuming it to be entirely in tension or entirely in compression, as appropriate d) Determine the position of the horizontal plastic neutral axis of the section, which divides the total cross section into tension and compression zones subject to equal and opposite forces e) Take the moment capacity of the section at the fire limit state as the algebraic sum of the positive or negative moment contribution of each block, about any convenient horizontal axis E.3 Temperature profile E.3.1 General The dimensions and temperature blocks shown in Figure should be used to determine the moment capacity of a beam with shelf angles The temperature for each block should be taken at its mid-height position E.3.2 Exposed steelwork The temperature u1 of the lower flange (block 1) should be determined from 4.4.3 The temperatures of blocks and 3, and of the angle root (u2, u3 and uR respectively) should be determined from Table 18, in which Be and De are as defined in 4.4.3.2 (see also Figure 6) © BSI 12-1998 BS 5950-8:1990 E.3.3 Embedded steelwork Full steel strength is maintained at temperatures below 300 °C Thus the temperatures of blocks 4, and should be calculated using: ux = uR – Gx but ux > 300 °C where ux is the temperature (in °C) at location x; x is the distance (in mm) from angle root, measured as shown in Figure 7; G is the temperature gradient (in °C/mm), given in Table 19 Figure — Temperature blocks for beams with shelf angles Table 18 — Block temperature Block temperature for a fire resistance period of: 30 Aspect ratio 60 90 u2 u3 uR u2 u3 uR u2 u3 uR °C °C °C °C °C °C °C °C °C De/B < 0.6 u1 – 140 475 350 u1 – 90 725 600 u1 – 60 900 775 0.6 < De/B < 0.8 u1 – 90 510 385 u1 – 60 745 620 u1 – 30 910 785 0.8 < De/B < 1.1 u1 – 45 550 425 u1 – 30 765 640 u1 925 800 1.1 < De/B < 1.5 u1 – 25 550 425 u1 765 640 u1 925 800 1.5 < De/B u1 550 425 u1 765 640 u1 925 800 Table 19 — Temperature gradient Period of fire resistance G °C/mm 30 60 90 © BSI 12-1998 2.3 3.8 4.3 21 BS 5950-8:1990 Appendix F Simple method of calculation for portal frame buildings Figure — Definition of dimension x F.1 Symmetric ridged or flat roofed portal frames Portal frames, fabricated from universal sections, designed in accordance with BS 5950-1, may be checked for compliance with the recommendations given in 4.5 at the fire limit state using the following rules They are not applicable to frames with tapered rafters, for which reference should be made to specialist literature This method applies to steel portal frames which satisfy the following a) The frame should be constructed from hot rolled I or H sections The column may be tapered but the rafter may not b) The frame may or may not have haunches c) The frame may be single bay or multi-bay d) The rafter adjacent to the boundary should be symmetrical about the centre of its span e) The ratio of the span of the rafter adjacent to the boundary, to the height to the eaves should be not less than 1.6 f) The columns in the boundary wall should be adequately restrained in the plane of the wall, see F.4 g) The columns on the affected boundary should have the same fire resistance as the wall, up to the underside of the haunch, or up to the underside of the rafter if no haunch exists The bases and foundations of any column on an affected boundary should be designed, at the fire limit state, to resist the following forces and overturning moment 1) vertical reaction: 0.5 WfSL + dead weight of wall 2) horizontal reaction: K(WfSGA – MprC/G) but > Mpc/10Y Figure — Definition of blocks 4, and 22 3) overturning moment: K {WfSGY (A + B/Y) – Mpr(CY/G – 0.065)} but > Mpc/10 © BSI 12-1998 BS 5950-8:1990 where Wf is the factored load at time of collapse (in kN/m2); S is the spacing of frames, centre-to-centre (in m); G is the clear span between ends of haunches (in m); Y is the vertical height at end of haunches (in m); L is the overall span (in m); Mpc is the plastic moment of resistance of column at 20 °C (in kN·m); Mpr is the plastic moment of resistance of rafter at 20 °C (in kN·m); A and C are factors obtained from Table 20; K is a modification factor, taken as for a single bay frames or obtained from Table 21 for multi-bay frames; Table 21 — Modification factor K for multi-bay frames Pitch ≤ 3° ≤ 6° ≤ 9° ≤ 12° > 12° Range of span/height ratio ≥ 1.7 ≥ 1.6 ≥ 1.6 ≥ 1.6 A C degrees 1.01 1.05 0.99 1.02 0.93 0.96 0.85 0.88 12 0.76 0.79 15 0.68 0.70 18 0.61 0.62 21 0.54 0.56 24 0.49 0.50 27 0.44 0.45 30 0.40 0.41 © BSI 12-1998 ≥ 2.1 ≥ 1.8 ≥ 1.6 a) holding down bolts L –G B = -8G Rafter pitch ≥ 2.3 < 2.5 < 2.3 < 2.1 < 1.8 1.0 1.3 1.0 1.3 1.0 1.3 1.0 1.3 1.0 F.2 Columns and bases When designing columns and bases to resist overturning due to rafter collapse the following gm factors should be used: For frames without haunches, Y should be taken as the column height and G should be taken as equal to the span L Thus B is zero Table 20 — Factors A and c for various rafter pitches ≥ 2.5 Modification factor b) baseplate c) column 1.0 against yield strength; or 1.2 against ultimate tensile strength, whichever is more onerous; 1.2 against formation of a plastic hinge; 1.2 against formation of a plastic hinge F.3 Foundations Foundations should be designed to ensure that the ultimate bearing capacity of the soil is not exceeded F.4 Restraint The requirement for restraint in the plane of the wall (item f) of F.1) should be met by satisfying the following conditions: a) either: 1) by providing four equal diameter holding down bolts in each base plate, spaced symmetrically about the section in the longitudinal direction, at a minimum spacing equal to 70 % of the flange width, or 2) by the stanchion being set in a concrete base which is capable of resisting an overturning moment in the longitudinal direction equal to that resisted by the holding down bolts in item (1) above b) and either: 1) by providing a masonry wall with a height not less than 75 % of the height to the eaves, which is connected to the column and which restrains the column in the plane of the wall at normal temperatures, or 23 BS 5950-8:1990 2) in any other case by designing the horizontal members which restrain the column in the plane of the wall, as steel members to the appropriate Part of BS 5950 These members not require fire protection 24 © BSI 12-1998 BS 5950-8:1990 Table 22 — Percentage dead weight of roof cladding systems remaining at time of rafter collapse Systems Roof cladding Inner lining Material Insulation % Site Mineral insulation assembled constructions board Plaster board Plaster board Steel Material Percentage dead weight 100 Glass or mineral fibre Thermoplastic foams Thermosetting foams Bonded thermoplastic foams Glass or mineral fibre Unbonded foams 50 100 Bonded thermosetting foams Glass or mineral fibre Thermoplastic foams Thermosetting foams Fibre insulating board Thin linings, e.g foil, embossed papers and plastics Aluminium Fibre cement Factory assembled bonded systems Foil, paper or plastics facings Steel Mineral or glass fibre Most foamed plastics 0 10 Outer covering Percentage dead weight % 100 70 0 Steel Aluminium 100 50 Fibre 50 cement 50 100 70 100 Steel Aluminium 100 50 Fibre cement Steel Aluminium 100 100 Fibre 100 cement 70 100 Steel Aluminium 10 Fibre cement 0 Steel Aluminium 100 50 Fibre 50 cement Unbonded glass or mineral fibre Thermosetting foams Thermoplastic foams 10 10 70 80 Fibre cement 10 100 Steel Aluminium 100 Thermosetting foams 80 Any 100 Thermosetting foams 100 Any 100 Single skin plastics rooflights Double skin plastics rooflights © BSI 12-1998 100 Aluminium Roof lights % Steel Aluminium 100 10 Fibre 10 cement 50 Urethane or isocyanurate foams Phenolics Percentage dead weight 100 Steel Aluminium 100 Fibre 100 cement Phenolic foams Material 50 25 BS 5950-8:1990 Appendix G Bibliography BUILDING RESEARCH ESTABLISHMENT BRE Digest 317 Fire resistant steel structures: Free standing blockwork-filled columns and stanchions HMSO December 1986 ASSOCIATION OF STRUCTURAL FIRE PROTECTION CONTRACTORS AND MANUFACTURERS/STEEL CONSTRUCTION INSTITUTE/FIRE TEST STUDY GROUP Fire protection of structural steel in buildings 1988 MORRIS, W.A., READ, R.E.H and COOKE, G.M.E Guidelines for the construction of fire resisting structural elements Building Research Establishment HMSO 1988 NEWMAN, G.M Fire and steel construction: The behaviour of steel portal frames in boundary conditions Steel Construction Institute 1990 BRITISH STEEL TUBES DIVISION Design manual for SHS concrete filled columns 1986 BOND, G.V.L Fire and steel construction: Water cooled hollow columns Steel Construction Institute 1975 LAW, M and O’BRIEN, T Fire and steel construction: Fire safety of bare external structural steel Steel Construction Institute 1989 CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION Fire resistance of composite slabs with steel decking CIRIA special publication 42 London 1986 NEWMAN, G.M Fire resistance of composite floors with steel decking Steel Construction Institute, 1989 10 COOKE, G.M.E Thermal bowing in fire and how it affects building design Building Research Establishment Information Paper 21/88 HMSO December 1988 11 BRITISH STEEL The reinstatement of fire damaged steel and iron framed structures British Steel, Swinden Laboratories 1986 26 © BSI 12-1998 BS 5950-8:1990 Publications referred to BS 476, Fire tests on building materials and structures BS 476-20, Method for determination of the fire resistance of elements of construction (general principles) BS 476-21, Methods for determination of the fire resistance of load bearing elements of construction BS 2989, Specification for continuously hot-dip zinc coated and iron-zinc alloy coated steel: wide strip, sheet/plate and slit wide strip BS 4360, Specification for weldable structural steels BS 4449, Specification for carbon steel bars for the reinforcement of concrete BS 4848, Specification for hot-rolled structural steel sections BS 4848-2, Hollow sections BS 5950, Structural use of steelwork in building BS 5950-1, Code of practice for design in simple and continuous construction: hot rolled sections BS 5950-2, Specification for materials, fabrication and erection: hot rolled sections BS 5950-4, Code of practice for design of floors with profiled steel sheeting BS 8110, Structural use of concrete BS 8110-2, Code of practice for special circumstances CP 290, Code of practice for suspended ceilings and linings of dry construction using metal fixing systems © BSI 12-1998 BS 5950-8: 1990 BSI — British Standards Institution BSI is the independent national body responsible for preparing British Standards It presents the UK view on standards in Europe and at the international level It is incorporated by Royal Charter Revisions British Standards are updated by amendment or revision Users of British Standards should make sure that they possess the latest amendments or editions It is the constant aim of BSI to improve the quality of our products and services We would be grateful if anyone finding an inaccuracy or ambiguity while using this British Standard 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permission is granted, the terms may include royalty payments or a licensing agreement Details and advice can be obtained from the Copyright Manager Tel: 0181 996 7070 BSI 389 Chiswick High Road London W4 4AL ... 3.1: Code of practice for design of simple and continuous composite beams; — Section 3.21): Code of practice for design of composite columns and frames; — Part 4: Code of practice for design of. .. Specification for materials and workmanship: cold formed sections; — Part 8: Code of practice for fire resistant design; — Part 91): Code of practice for stressed skin design This Part of BS 5950... 5950-4, Code of practice for design of floors with profiled steel sheeting BS 8110, Structural use of concrete BS 8110-2, Code of practice for special circumstances CP 290, Code of practice for suspended