Fire Safety Day Fire Behaviour of Steel and Composite Floor Systems Simple design method

Transcription

1 Fire Safety Day 011 Fire Behaviour of Steel and Composite Floor Systems method Prof. Colin Bailey 1

2 3 Content of presentation in a fire situation method of reinforced concrete slabs at 0 C Floor slab model Failure modes method of at Extension to fire behaviour Membrane effect at Contribution from unprotected beams Design of protected beams 4

3 Traditional design method Protected beams method of reinforced concrete slabs at 0 C method of at Column Beam Floor Existing design methods assume isolated members will perform in a similar way in actual buildings Fire compartment 5 Real composite floor with reinforcing steel mesh in concrete slab Temperature increase during fire (a) (b) (c) (d) Simple bending Membrane effect behaviour 6 3

4 method of reinforced concrete slabs at 0 C Method developed by Professor Colin Bailey University of Manchester formerly with Building Research Establishment (BRE) 7 Designing for membrane action in fire Yield line pattern Unprotected beams Protected beams 8 4

5 method of reinforced concrete slabs at 0 C Floor slab model with 4 vertically restrained sides (Plastic yield lines) horizontally unrestrained very conservative assumption Yield lines Simple support on 4 edges 9 method of reinforced concrete slabs at 0 C Floor slab model Membrane effect enhancing yield lines resistance Yield lines Region of tension Simply supported on 4 edges Compression across yield line Tension across yield line 10 5

6 method of reinforced concrete slabs at 0 C Membrane forces along yield lines (1) k b K T 0 C E D S C L Element 1 F B C φ nl S A T b K T 0 Element 1 T 1 T Element l Element 11 method of reinforced concrete slabs at 0 C Membrane forces along yield lines () k, b are parameters defining magnitude of membrane forces, n K T 0 T 1, T, C, S is a factor deduced from yield line theory, is the ratio of the reinforcement in the shorter span to the reinforcement in the longer span, is the reinforcement per unit width in the longer span, are resulting membrane forces along yield lines. 1 6

7 method of reinforced concrete slabs at 0 C Contribution of membrane action (1) Element 1 In-plane view of the resulting membrane forces Side-view of the resulting membrane forces under a deflection equal to w 13 method of reinforced concrete slabs at 0 C Contribution of membrane action () Element In-plane view of the resulting membrane forces Side-view of the resulting membrane forces under a deflection equal to w 14 7

8 method of reinforced concrete slabs at 0 C Contribution of membrane action (3) Enhancement factor for each element e i, i=1, = e im : enhancement due to membrane forces on element i + e ib : Enhancement due to the effect of in-plane forces on the bending capacity. Overall enhancement e = e 1 e 1 e 1+ µ a where: µ is the coefficient of orthotropy of the reinforcement a is the aspect ratio of the slab = L/l 15 method of reinforced concrete slabs at 0 C Contribution of membrane action (4) Resistance Load bearing capacity based membrane action Load capacity based on yield line theory w 1 Enhancement factor due to membrane forces for a given displacement (w 1 ) Displacement (w) 16 8

9 method of reinforced concrete slabs at 0 C Failure modes (tensile failure of reinforcement) Full depth crack Compression failure of concrete Reinforcement in longer span fractures Yield-line pattern Edge of slab moves towards centre of slab and 'relieves' the strains of the reinforcement in the short span 17 method of reinforced concrete slabs at 0 C Failure modes (compressive failure of concrete) More likely to occur in case of strong reinforcement mesh Concrete crushing due to in-plane stresses Yield-line pattern 18 9

10 method of reinforced concrete slabs at 0 C Failure modes (experimental evidence) Tensile failure of reinforcement Compressive failure of concrete 19 Floor slab model at (1) On the basis of the same model at room temperature Account taken of temperature effects on material properties. Account for thermal bowing of the slab due to temperature gradient in depth which equals to: w θ ( T T1 ) = α 19.h l where: h is the effective depth of the slab l is the shorter span of the slab 0 10

11 Floor slab model at () and: α is the coefficient of thermal expansion for concrete For LW concrete, EN value is taken α LWC = K -1 For NW concrete, a conservative value is taken α NWC = K -1 < K -1 (EN value) T is the temperature of the slab bottom side (fire-exposed side) T 1 is the temperature of the slab top side (unexposed side) 1 Floor slab model at (3) Assuming mechanical average strain at a stress equal to half the yield stress at room temperature Deflection of slab on the basis of a parabolic deflected shape of the slab due to transverse loading: w ε = 0.5fsy 3L E s 8 l 30 where: E s is the elastic modulus of the reinforcement at 0 C f sy is the yield strength of the reinforcement at 0 C L is the longer span of the slab 11

12 Floor slab model at (4) Hence, the maximum deflection of the floor slab is: w α ( T T1) l = 19. h 0.5fsy 3 E s 8 L + However, the maximum deflection of the floor slab is limited to: T T1 l w < α + l / 30 ( ) l w L h 3 Conservativeness of the floor slab model at Reinforcement over supports is assumed to fracture. The estimated vertical displacements due to thermal curvature are underestimated compared to theoretical values The thermal curvature is calculated based on the shorter span of the slab Any additional vertical displacements induced by the restrained thermal expansion when the slab is in a post buckled state are ignored Any contribution from the steel decking is ignored The increase of the mesh ductility with the temperature increase is ignored 4 1

13 Load bearing capacity of the floor slab model enhanced in presence of unprotected steel beams (1) Catenary action of unprotected beams is neglected The bending moment resistance of unprotected beams is taken into account with following assumptions: Simple support at both ends Heating of the steel cross-section calculated according to EN , considering shadow effect Thermal and mechanical properties for both steel and concrete given in EN Load bearing capacity of the floor slab model enhanced in presence of unprotected steel beams () Enhancement of load bearing capacity from unprotected beams is: 8M Rd, fi n 8M ub Rd, fi 1+ n = L B L l ub ub ub L ub where: n ub is the number of unprotected beams M Rd,fi is the moment resistance of each unprotected composite beam B ub 6 13

14 Basic Strength (Energy) Calculation. Load Capacity at the Fire Limit State = e Internal work done by the slab External work per unit load + Internal work done by the beam(s) External work per unit load 7 Temperature calculation of composite slab On the basis of advanced calculation models D finite difference method Material thermal properties from Eurocode 4 part 1- for both steel and concrete Shadow effect is taken into account for composite slabs h p Element i φ top φ side y x L φ =1.0 b 1 Element i 8 14

15 Load bearing capacity of protected perimeter beams Overall floor plastic mechanism based on beam resistance Load ratio in fire situation Additional load on protected beams Critical temperature simple calculation method (EN ) 9 Load bearing capacity of protected perimeter beams on the basis of global plastic mechanism o Axis of rotation o o Axis of rotation o M b,3 Edge beam Yield line Edge beam M fi,rd M b,1 M b, M fi,rd Axis of rotation o Axis of rotation o o M b,4 Yield line o 30 15

16 Validation against test data 7 Full-scale Cardington Tests 1 large-scale BRE test (cold but simulated for fire) 10 Cold tests carried out in the 1960/1970s 15 small scale tests conducted by Sheffield University in small-scale cold and fire tests carried out by the University of Manchester Full-scale test carried out by Ulster University 010. Plus more 31 Small Scale Experimental Behaviour and Design of Concrete Floor Slabs Cold Tests and Identical Hot tests (Both MS and SS mesh reinforcement) 3 16

17 BRE Digest 46 (001): Allowable vertical ( T T1 ) l υ= α displacement 19.h 0.5f E 3 8 y L + Reinf 0 C Slab ν mm T pred C T test C Ratio MF MF MF MF MF MF MF MF Slab ν mm T pred C T test C Ratio SF SF SF >840 - SF SF SF SF SF SF to 55% of beams can be left unprotected by placing protection where it is needed

18 35 References Bailey C.G. and Moore D.B. The structural steel frames with labs subject to fire: Part 1: Theory. The Structural Engineer Vol. 78 No. 11 June 000 pp Bailey, C. G, Membrane action of unrestrained lightly reinforced concrete slabs at large displacements, Engineering Structures, 3, pp , 001. Bailey C G, Toh W.S. "Behaviour of concrete floor slabs at ambient and ". Fire Safety Journal. Vol. 4. Issue pp EN : Eurocode 4 : Design of composite steel and concrete structures Part 1- : General rules Structural fire design, CEN. Fire Resistance Assessment of partially protected COmposite Floors (FRACOF): Engineering background. Technical Report, CTICM, SCI,