1.0.1 This code is formulated with a view to reasonably carrying out fire protection design of steel structures in buildings, ensuring construction quality, standardizing acceptance and maintenance management, reducing fire hazards, and protecting personal and property safety.
1.0.2 This code is applicable to the fire protection design and the construction and acceptance of fire protection of the steel structures and composite structures such as concrete-filled steel tubular columns, steel deck-concrete composite slabs and composite steel and concrete beams in industrial and civil buildings. It is not applicable to built-in steel reinforced concrete composite structures.
1.0.3 In addition to those specified in this code, the fire protection design and construction and acceptance of fire protection of steel structures in buildings shall also comply with those specified in the current relevant standards of the nation.
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2 Terms and symbols
2.1 Terms
2.1.1 fire-resistant steel
steel whose yield strength at 600℃ is not less than 2/3 of its yield strength at room temperature
2.1.2 concrete-filled steel tubular column
structural member formed by filling concrete into a steel tube and capable of bearing external load with the steel tube and its core concrete
2.1.3 composite steel and concrete beam
beam formed by the combination of concrete flange plate and steel beam via shear connector and capable of bearing force as a whole
2.1.4 steel deck-concrete composite slab
floor slab formed by pouring concrete onto the profiled steel deck and capable of bearing force with both
2.1.5 section factor
ratio of surface area exposed to fire of a steel member to its corresponding volume
2.1.6 standard fire temperature-time curve
curve of the average temperature of air in a fire resistance test furnace over time in a standard fire resistance test
2.1.7 standard fire
fire with the temperature of hot flue gas determined according to the standard fire temperature-time curve
2.1.8 equivalent time of fire exposure
time taken by a steel member in an actual fire to reach a temperature the same as that it reaches after being exposed to a standard fire
2.1.9 temperature effects on structural behavior
internal force and deformation of a structure (member) due to its temperature change
2.1.10 fire limit state
deformation state reached by a structure or member at the time it is no longer being able to bear external action or suitable for continuing to bear load due to fire
2.1.11 load ratio
ratio of the design load effect of a structure or member in fire to its design bearing capacity at room temperature
2.1.12 critical temperature
temperature at which a steel member reaches its fire limit state in fire
2.2 Symbols
2.2.1 Material properties
cc——the specific heat capacity of concrete;
ci——the specific heat capacity of fire protection layer;
cs——the specific heat capacity of steel;
Ec——the elastic modulus of concrete at room temperature;
EcT——the elastic modulus of concrete at high temperature;
Es——the elastic modulus of steel at room temperature;
EsT——the elastic modulus of steel at high temperature;
f——the design strength of steel at room temperature;
fc——the design axial compressive strength of concrete at room temperature;
fck——the standard axial compressive strength of concrete at room temperature;
ft——the design tensile strength of concrete at room temperature;
fT——the design strength of steel at high temperature;
Ri——the equivalent thermal resistance of fire protection layer;
αc——the thermal expansion coefficient of concrete;
αs——the thermal expansion coefficient of steel;
λc——the thermal conductivity coefficient of concrete;
λs——the thermal conductivity coefficient of steel;
ρi——the density of fire protection material;
ρs——the density of steel;
ρc——the density of concrete.
2.2.2 Action, effect and resistance
Mp——the plastic moment;
Mu——the design flexural capacity of concrete filled steel tube under pure bending at room temperature;
Nu——the design compressive bearing capacity of short concrete-filled steel tubular column subjected to axial compression at room temperature;
N*——the design compressive bearing capacity of concrete-filled steel tubular column at room temperature;
Rd——the design resistance of structural member;
SGK——the load effect value calculated according to the standard value of permanent load;
Sm——the design value of action (load) effect combination;
SQk——the load effect value calculated according to the standard value of floor or roof live load;
STk——the action effect value calculated according to the standard temperature value of the structure in fire;
SWk——the load effect value calculated according to the standard value of wind load.
2.2.3 Geometric parameters
Ac——the cross-sectional area of concrete in concrete-filled steel tubular column;
As——the cross-sectional area of steel tube in concrete-filled steel tubular column;
C——the perimeter of section;
D——the section height of concrete-filled steel tubular column;
di——the thickness of fire protection layer;
F——the surface area exposed to fire per unit length of a member;
Fi——the surface area exposed to fire per unit length of steel member with fire protection;
hc1——the thickness of concrete flange plate;
hc2——the height of pallet of the profiled steel sheet;
hcb——the equivalent thickness of concrete flange plate;
hs——the height of steel beam;
hw——the height of web of the steel beam;
l——the length or the span length;
l0——the calculation length;
ttf——the thickness of upper flange of steel beam;
tw——the thickness of web of the steel beam;
tbf——the thickness of lower flange of the steel beam;
V——the volume per unit length of steel member;
W——the gross section modulus;
Wn——the net section modulus;
Wp——the plastic modulus of section.
2.2.4 Time and temperature
t——the duration of the time of fire;
te——the equivalent time of fire exposure;
Tc——the temperature of concrete;
Td, T′d, T″d——the critical temperature of a member;
Tg——the average temperature of hot flue gas at the time of fire development at t;
Tg0——the indoor ambient temperature before fire;
Tm——the highest temperature of a member within the time of design fire resistance rating;
Ts——the temperature of steel or a steel member;
Δt——the time step;
ΔTs——the temperature rise of a steel member within Δt.
2.2.5 Other parameters related to fire resistance calculation
F/V——the section factor of a member without fire protection;
Fi/V——the section factor of a member with fire protection;
kT——the load capacity factor of concrete-filled tubular column in fire;
R, R′——the load ratio;
α——the comprehensive heat transfer coefficient;
αb——the stability checking calculation parameter of flexural steel member at high temperature;
αc——the convective heat transfer coefficient, or the stability checking calculation parameter of axial compression steel member at high temperature;
αr——the heat transfer coefficient of thermal radiation;
βmx, βmy——the equivalent bending moment coefficient in the action plane of bending moment;
βtx, βty——the equivalent bending moment coefficient out the action plane of the bending moment;
γ, γm——the plastic adaption coefficient of section;
γ0T——the coefficient for importance of structure;
γG——the partial coefficient of permanent load;
εr—the comprehensive radiance;
η——the section influence coefficient;
ηcT——the reduction factor of axial compressive strength of concrete at high temperature;
ηsT——the reduction factor of yield strength of steel at high temperature;
λ——the slenderness ratio of a member;
λ0——the critical slenderness ratio for elastoplastic instability;
λp——the critical slenderness ratio for elastic instability;
σ——the Stefan-Boltzmann constant;
φ——the stability coefficient of axial compression steel member at room temperature;
φb——the stability coefficient of flexural steel member at room temperature;
φT——the stability coefficient of axial compression steel member at high temperature;
φbT——the stability coefficient of flexural steel member at high temperature;
φf——the frequent value coefficient of floor or roof live load;
φq——the quasi-permanent value coefficient of floor or roof live load;
φw——the frequent value coefficient of wind load;
χcT——the reduction factor of elastic modulus of concrete at high temperature;
χsT——the reduction factor of elastic modulus of steel at high temperature.
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3 Basic requirements
3.1 Fire protection requirements
3.1.1 The design fire resistance rating of steel structural members shall be determined based on the fire resistance class in accordance with the current national standard GB 50016 Code for fire protection design of buildings. The design fire resistance rating of column bracing shall be the same as that of the columns, the design fire resistance rating of floor bracing shall be the same as that of beams, and the design fire resistance rating of roof bracing and tie bars shall be the same as that of roof load-bearing members.
3.1.2 Fire protection measures shall be taken when the fire resistance rating of steel structural members is less than the design fire resistance rating upon checking calculation.
3.1.3 The fire protection of steel structure joints shall be the same as the highest fire protection requirements among the members connected.
3.1.4 The fire protection design documents of steel structures shall indicate the fire resistance class of the building, the design fire resistance rating of the members, the fire protection measures for the members, and the performance requirements and design indicators of the fire protection materials.
3.1.5 When the equivalent thermal conductivity coefficient of the fire protection materials used in construction is inconsistent with the requirements of the design documents, the application thickness of the protection layer shall be determined according to the principle that the equivalent thermal resistance of the fire protection layer is equal and shall also be approved by the design unit. For the non-intumescent fire retardant coating for steel structure and the fire board, the application thickness of the fire protection layer may be determined according to Annex A herein; for intumescent fire retardant coating, the aforesaid application thickness may be directly determined according to the equivalent thermal resistance of the coating.
3.2 Fire protection design
3.2.1 The steel structures shall be subjected to fire resistance checking and fire protection design according to their fire limit state.
3.2.2 The design value of the most unfavorable load (action) effect combination for the fire limit state of steel structures shall be determined according to the most unfavorable value of the following combination values by taking the loads (actions) that may occur simultaneously on the structure during fire into account:
Sm=γ0T(γGSGk+STk+φfSQk) (3.2.2-1)
Sm=γ0T(γGSGk+STk+φqSQk+φwSWk) (3.2.2-2)
where,
Sm——the design value of action (load) effect combination;
SGk——the load effect value calculated according to the standard value of permanent load;
STk——the action effect value calculated according to the standard temperature value of the structure in fire;
SQk——the load effect value calculated according to the standard value of floor or roof live load;
SWk——the load effect calculated according to the standard value of wind load;
γ0T——the coefficient for importance of structure; for buildings with fire resistance class I, γ0T=1.1; for other buildings, γ0T=1.0;
γG——the partial coefficient of permanent load, which generally is 1.0; when the permanent load is favorable, γG=0.9;
φw——the frequent value coefficient of wind load, φw=0.4;
φf——the frequent value coefficient of floor or roof live load, which shall be taken in accordance with those specified in the current national standard GB 50009 Load code for the design of building structures;
φq——the quasi-permanent value coefficient of floor or roof live load, which shall be taken in accordance with those specified in the current national standard GB 50009 Load code for the design of building structures.
3.2.3 The fire protection design of steel structures shall be conducted with the fire protection design method which is based on fire resistance checking of integral structure or that of members according to the importance of the structure, structure type and load characteristics, etc., and shall also meet the following requirements:
1 for large-span steel structures with a span of not less than 60m, the fire protection design method based on fire resistance checking of integral structure should be adopted;
2 for prestressed steel structures and steel structures in large-span buildings with a span of not less than 120m, the fire protection design method based on fire resistance checking of integral structure shall be adopted.
3.2.4 The fire protection design method of steel structures based on the fire resistance checking of integral structure shall meet the following requirements:
1 each fire compartment shall be considered as a fire condition and shall be subjected to fire resistance checking according to the most unfavorable fire scenario;
2 the thermal expansion effect of the structure, the influence of high temperature on the material properties of structures, and, if necessary, the influence of geometric nonlinearity of structures shall be taken into account.
3.2.5 The fire protection design method of steel structures based on the fire resistance checking of members shall meet the following requirements:
1 in the calculation of the combined effect of members in fire, for the members mainly subjected to flexural deformation such as flexural members, tension-flexure members and compression-flexure members, the thermal expansion effect may not be taken into account, the boundary constraints of the members in fire and the internal forces generated by them under external loads may adopt boundary constraints and internal forces at room temperature, and thus the combined effect of members in fire will be calculated; for the members mainly subjected to axial deformation such as axial tension members and axial compression members, the influence of thermal expansion effect on internal force shall be taken into account.
2 in the calculation of the bearing capacity of a member in fire, the highest average temperature of its section shall be taken as the member temperature, and the strength and elastic modulus of structural materials at corresponding temperature shall be adopted.
3.2.6 The fire resistance checking and fire protection design of steel structural members may be conducted with fire resistance rating method, bearing capacity method or critical temperature method, and shall meet the following requirements:
1 fire resistance rating method. Under the action of design load, the actual fire resistance rating of steel structural members in fire shall not be less than their design fire resistance rating, and shall be checked using the following formula. Thereinto, the actual fire resistance rating of the members may be determined through test according to the current national standards GB/T 9978.1 Fire-resistance tests - Elements of building construction - Part 1: General requirements, GB/T 9978.5 Fire-resistance tests - Elements of building construction - Part 5: Specific requirements for loadbearing horizontal separating elements, GB/T 9978.6 Fire-resistance tests - Elements of building construction - Part 6: Specific requirements for beams and GB/T 9978.7 Fire-resistance tests - Elements of building construction - Part 7: Specific requirements for columns, or calculated according to the relevant requirements of this code.
tm≥td (3.2.6-1)
2 bearing capacity method. Within the time of design fire resistance rating, the design bearing capacity of steel structural members in fire shall not be less than the design combined effect value of the most unfavorable loads (actions), and shall be checked using the following formula.
Rd≥Sm (3.2.6-2)
3 critical temperature method. Within the time of design fire resistance rating, the highest temperature of steel structural members in fire shall not be higher than their critical temperature, and shall be checked using the following formula.
Td≥Tm (3.2.6-3)
where,
tm——the actual fire resistance rating of steel structural members in fire;
td——the design fire resistance rating of steel structural members, which shall be determined in accordance with those specified in 3.1.1 hereof;
Sm——the design value of action (load) effect combination, which shall be determined in accordance with those specified in 3.2.2 hereof;
Rd——the design resistance of structural member, which shall be determined in accordance with those specified in Clauses 7 and 8 hereof;
Tm——the highest temperature of a member within the time of design fire resistance rating, which shall be determined in accordance with those specified in Clause 6 hereof;
Td——the critical temperature of a member, which shall be determined in accordance with those specified in Clauses 7 and 8 hereof.
