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Est im a t ing t he re sponse of fla t pla t e c onc re t e sla b syst e m s t o fire e x posure
N R C C - 5 3 5 5 1
G h o r e i s h i , M . ; B a g c h i , A . ; S u l t a n , M . A .
J u l y 2 0 1 0
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Estimating the Response of Flat Plate Concrete Slab Systems to Fire Exposure Mehrafarid Ghoreishi1, Ashutosh Bagchi2 and Mohamed A. Sultan3
ABSTRACT
Two-way flat plate slabs provide a number of benefits for office buildings and apartments – for example, reducing formwork, flexibility of partitions, minimal increase in story heights, and prompt erection. This paper shows that the fire resistance of flat plate slab structures is affected by many factors including concrete cover, cross section of the structural elements which is not essentially constant during fire exposure, and the material properties of concrete and reinforcing steel. Concrete has fairly excellent fire resistance properties. However, the strength of concrete reduces as temperature rises. There is a lack of studies, both analytical and experimental, on flat plate concrete slabs in the literature. The objective of this research is to investigate the effects of concrete and steel behavior of slab in fire and the tensile membrane action conditions.
INTRODUCTION
Performance of structures exposed to fire is usually studied based on the performance of single structural elements. In recent years, various studies have been undertaken on the overall behavior of structures when subjected to fire. Concrete is considered to be comparatively fire resistant, while steel is adversely affected by fire exposure. Now the question is: is fire resistance of a complete concrete structure different from that of a single member? Does a structural system as a whole, concrete or steel, enhance the fire safety of structure? Which parameters are influencing the fire resistance of reinforced concrete structure? This article presents a study on two-way flat plate concrete slabs to address some of the above questions. Although there are studies on fire resistance of reinforced concrete components at the element levels, including different materials such as high strength concrete, concrete columns and beams strengthened with fiber reinforced polymer, there is a lack of analytical and experimental studies on assemblies or whole structural systems with flat plate slabs. This paper presents the non-linear analysis of reinforced concrete flat slabs at high temperature and service loads. Predictions from this analysis compared with the available experimental results which resulted in a good agreement.
LITERATURE REVIEW
In recent years, a number of studies on numerical analysis of behavior of reinforced concrete structures in fire have been carried out. A brief summary of the state of the art is provided in this paper. Nwosu and Kodur in 1997 [1] presented an extensive review of available studies for steel structures. Ghoreishi et al. in 2009 [2] provided
a review of available experimental and numerical studies on whole structural systems under fire, and observed that there is a lack of experimental studies on whole structural systems, particularly for concrete. Nizamuddin [3] in 1976 developed a non-linear layered finite element approach based on the Kirchoff’s thin plate theory, while Huang et al. in 2003 [4] used Mindline-Reissner’s theory and Reynourd and Nechnech in 2002 [5] used elasto–plastic damage model to analyze R.C. slabs in fire. Effects of different aggregates reinforced concrete slabs was studied in two experimental ISO-834 fire tests by Wade in 1992 [6]. Cooper and Franssen studied on 1-D, 2-D and 3-D of concrete slabs in 1999 [7]. Issen et al. in 1970 [8] explained that axial restraint increases the fire resistance of reinforced concrete floor systems. However, analytical studies by Anderberg and Forsen in 1982 [9] have shown increase of axial restraint does not always increase the fire resistance of flat slabs. Experimental fire tests on restrained flat slabs by Cooke in 1993 [10] have shown that position of the restraint force at the supports is beneficial to the fire resistance and not always compressive axial restraint. In 2004, Lim et al. [11] described axially restrained one-way reinforced concrete slabs broadly in fire conditions.
While fire behavior of reinforced concrete slabs has been previously studied by various researchers, most studies focused on one-way reinforced concrete slabs. Also, research on the fire performance of reinforced concrete structures has been mainly focused on material properties rather than structural performance. Although there are some studies on the element level fire resistance, there is a lack of analytical and experimental studies on assemblies or whole structural systems with flat plate slabs.
EXISTING EXPERIMENTAL STUDY
An experimental test on whole structure under fire was carried out in 2001 for a seven storey in-situ reinforced concrete building constructed at the Building Research Establishment (BRE) laboratories in Cardington, UK [12]. The design of fire was based on the parametric approach provided in Annex A of the fire part of Eurocode [13]. The predicted time-temperature response from the Eurocode and ISO standard curve is shown in Figure 1. The test showed that spalling of the floor slab was extensive which resulted in exposure of the bottom reinforcement. Although concrete spalling considerably reduced the flexural strength of the slab, collapse did not occur. This could be attributed to the slab behaving in compressive membrane action, which is currently not considered in codified design methods.
0 200 400 600 800 1000 1200 0 20 40 60 80 1 Tem p erat ure (c °) Time (min.) 00 Design Fire ISO Standard
Figure 1. Design fire curve and ISO standard curve.
ANALYTICAL/NUMERICAL MODEL
At present, considerable attention has been given to simulate the behavior of structures in fire by analytical and numerical approaches. In this paper a 250 mm two-way flat plate slab is considered and the finite element (FE) method is used for modeling the slab. Details of the slab and the modeled area are shown in Figure 2a, 2b. Double layers (top and bottom) of 12 mm diameter reinforcing bars at 100 mm spacing around columns and 16 mm diameter bars at 300 mm spacing elsewhere in the slab have been provided. The concrete compressive strength is assumed to be 37 MPa. The reinforcing steel is modeled as hot-rolled reinforcing bars with yield strength of 400 MPa with 22 mm concrete cover. The thermal and mechanical properties of the concrete and reinforcing steel are assumed based on Eurocode 2. The slab is considered to be subjected to uniformly distributed load of 9.62 KPa which includes the self-weight, superimposed dead load and live load for an office building.
SAFIR, a finite element based computer program developed at the University of Liege in Belgium [14], has been used for thermal and structural analysis of fire resistance of slabs for performing nonlinear two and three dimensional analysis of steel, concrete, and composite structures. The behavior of a structure in fire is simulated in the program as a function of time using the temperature distributions in the structural elements evaluated from a thermal analysis.
A B
Slab exposed to fire
Slab not exposed to fire Study area slab exposed to fire 7500 mm 7500 mm 7500 mm 7500 mm 75 00 m m 75 00 m m 7 500 m m
Figure 2. a) Study area of concrete slab and b) boundary condition of proposed model. In this paper, other software, SAFE [15] and Direct Design Method (DDM) [16] have been used to compare the results with the base results obtained by SAFIR to check SAFIR model. A mesh size of 250 mm is found to be appropriate to gain a reasonable accuracy with acceptable computational time. Figure 2a shows the proposed model for a two-way flat plate concrete slab with a 400 mm rectangular column in the middle of the 7.5 m span and boundary conditions, for modeling tensile membrane action. Symmetrical boundary conditions are applied on the four edges of the meshed domain. Edges are restrained against move plan, rotation is allowed in out of plan direction and also slab centers (i.e., corners of the meshed area) are free in the transverse direction (Figure 2b).
Using the present FE model, the deflection at the center of the slab (point A) was found to be 0.551 mm and that at the midway between columns (point B) 0.426 mm under service loads condition. Slab design according to Eurocode requires 1200 mm2/m top steel around columns, 300 mm2/m bottom steel in mid span and 430 mm2/m for top, 330 mm2/m for bottom between columns is needed to resist on factor moment loads. Minimum reinforcement should be 340 mm2/m for all sections.
FE MODEL EXPOSED TO FIRE
Experimental results show that the vertical displacement at position A is 24 mm and that at B is 22 mm for fire duration of 21 minutes [12]. Displacement in this paper refers to the deflection under fire loads only (after filtering the deflection due to service loads). After 21 minutes, the instruments malfunction and no further data could be recorded. As observed in the test, spalling of concrete from soffit of slab occurred after 12 minutes of fire followed by large displacements in slab especially in areas where tensile membrane action could not prevent such deflections. Displacements at 10 minutes before spalling were reported to be between 1 to 2 mm at the center of the slab and between the columns [12]. In this period of time the proposed FE model under parametric fire shows 1.86 mm deflection in center and 1.57 mm between columns (Figure 3).
Figure 3. Displacement of the slab at A (slab center) and B (between columns) under design fire. Both UK and European codified design methods suggest that concrete spalling within the fire compartment could be ignored during the design. It is clear in the numerical results that a displacement of 20 mm is reached at 80 minutes when spalling is not considered, while the same displacements occur in just 17 minutes in the test where spalling occurs. Although SAFIR can be used to predict the fire response of slab, the model does not account for fire to include spalling of concrete.
The experimental test conducted in [12] shows that the temperature is reduced initially for 12 to 13 minutes, and after that the average atmosphere temperature, at 300 mm below the soffit of the slab, remains between the design fire and ISO curve for 25 minutes until the instruments stop working. Figure 4 shows the displacements from the current numerical analysis at the midpoint of the slab (A) under standard and parametric fire curves. The full length of the slab in this paper is assumed to be exposed to parametric (design) fire. Only one type of slab is analyzed, while effects of different properties and conditions are studied by changing the values of those. The available experimental results that is described before is used to verify the numerical approach.
PARAMETERS CONSIDERED
Concrete generally provides relatively fire resistance properties. It does not burn and emit any toxic fumes, smoke or drip molten particles when exposed to fire.
The base FE model of the building include a 250 mm slab under parametric fire curve is used to allow the effects of variables to be assessed without having to perform a large number of expensive fire tests. The parametric fire scenario in this study predicted high maximum temperatures but had a short duration of approximately 30 minutes. A high temperature, short duration fire, may induce concrete spalling due to the thermal shock, whereas a lower temperature but longer duration will result in a greater average temperature in the concrete members. Compressive strength of concrete
Fire performance of concrete is controlled by type of aggregate and the cement paste. Concretes are conventionally classified as normal-weight concrete (NWC), lightweight concrete (LWC), depending on the density of the aggregates used. Concrete has a low thermal conductivity (50 times lower than steel) and therefore heats up very slowly during a fire [17]. The behavior of reinforced concrete slabs under high temperature is mainly affected by the strength of the concrete.
Figure 5. Vertical displacement of slab (point A) under various a) compression strength b) reinforcing slab scenarios.
Normal density concrete with a crushing strength of 37 MPa was used in the experimental test. In the current study, the compressive strength of concrete is varied. Figure 5a shows the vertical displacement at the center of the slab for various compressive strength. The displacement increases faster for low compressive strength of concrete. In Location “A” for 25 MPa concrete, the displacement is about 34 mm in 80 min and that for 37 MPa concrete is 20 mm.
There has been significant interest recently in high-strength concrete (HSC) as a high performance construction material due to its superior strength (with compressive strength at least 50 MPa), stiffness and durability. High strength concrete has a relatively higher strength loss when exposed to the same heating condition than normal strength concrete, since high strength concrete is prone to explosive spalling [17]. As seen in experimental test, high temperatures affect the strength of the HSC by explosive spalling which affects the integrity of the structure.
Area of reinforcing steels has significant effects on the performance of two-way flat plate slabs. The base FE model has a 0.452% reinforcing steel around columns and 0.266% in other areas. Figure 5b shows displacement of slab under various reinforcing scenarios. It is clear from the figure that the smallest displacement occurs when ratio of reinforcement is around the standard design. Higher reinforcement at the center of slab, and/or lower reinforcements around the columns results in larger displacement at center of the slab.
Imposed Load
The loads on the experimentally tested structure represented an office building with imposed live load of 2.5 KPa. The behavior of structures subjected to fire can be treated as an accidental limit state, with appropriate load factors. These factors are specified by the design codes (e.g., ENV 1991-1 [18] and BS5950 part 8 [19]). Various imposed load scenarios are shown in Figure 6a. As shown in this figure, an increase of 21.5% in imposed loads from 9.25 KPa to 11.25 KPa results in 46% increase in displacement in 30 minutes fire and 63% in 60 minutes. Also 43% increase in such loads causes 205% increase in displacements in 30 minutes fire and 286% increase in 60 minutes fire. Thus it is important for heavy live load areas such as cinema, and parking garages to give special attention for fire safety design.
Figure 6. Vertical displacement of slab (point A) under various a) service load b) concrete cover scenarios.
Concrete Cover
For reinforced concrete members such as slabs, the fire resistance is principally based on the amount of flexural reinforcements and the concrete cover to the reinforcing steel. As shown in Figure 6b, providing 62 mm instead of 22 mm cover of bottom reinforcement results in 56% decrease in the mid span displacement in 30 minutes and 69% in 60 minutes fire. As is described in Eurocode 2, the minimum thickness for fire rating of flat slab in 90 min is 200 mm with 25 mm distance for the axis of a reinforcing bar in concrete flat slabs [13].
CONCLUSION AND RESEARCH NEEDS
While concrete structures are assumed to be very robust in fire exposure, there are many issues that need to be considered for safer design. Thermal exposure may
cause sudden failure of the concrete structural units. The behavior of two-way flat plate slab under combined elevated temperatures and gravity loading was studied in this paper. Combined fire and imposed loading can severely change structural behavior. FE model in SAFIR program has been utilized to study various scenarios such as the effects of compressive strength of concrete, amount of reinforcing steel, magnitude of imposed loads, and the thickness of concrete cover. It was found that the fire performance of a concrete flat plate slab is very sensitive to the the above mentioned parameters. Further investigation needs to be conducted to simulate more complex scenarios to include spalling and hygrothermal effects.
REFERENCES
1. Nwosu, D.I., Kodur, V.K.R. 1997. “Steel Structures Exposed to Fire – a state-of-art report” IRC, National Research Council of Canada, Ottawa, Ont., Internal Report No. 749, pp. 1-32. 2. Ghoreishi, M., Bagchi, A., Sultan, M.A. 2009. “Estimating the Response of Structural Systems
to Fire Exposure: State-of-the-Art Review”, 11th Int. Conf. on Fire and Materials, San Francisco, CA., pp. 475-483.
3. Nizamuddin, Z. T. 1976. ‘Thermal and structural analysis of reinforced concrete slabs in fire environments”. Ph.D. thesis, University of California, Berkeley, CA.
4. Huang, Z., Burgess, I. and Plank, R.J. 2003, “Modeling membrane action of concrete slabs in composite building in fire. I: Theoretical development”, ASCE J of Structural Eng, V 129, No.8. 5. Nechnech, W., Meftah, F., Reynouard, J.M. 2002. “Elasto - plastice damage model for plain
concrete subjected to high temperatures”, Engineering Structures 24, pp. 597-611.
6. Wade, C., 1992. “Fire resistance of New Zealand concretes (Study Report No. 40)”, Judgeford: BRANZ, The Resource Centre for Building Excellence.
7. Cooper, L. Y., Franssen, J-M. 1999. “A Basis for using Fire Modeling with 1-D Thermal Analyses of Partitions to Simulate 2-D and 3-D Structural Performance in Real Fires”. Fire Safety Journal. 33-2, pp.115-128.
8. Issen, L.A., Gustaferro, A. H., Carlson, C.C. 1970. “Fire tests of concrete members: An improved method for estimating thermal restraint forces.” Fire Test Performance, ASTM, STP 464, American Society for Testing and Materials, Philadelphia, pp. 153–185.
9. Anderberg, Y., Forsen, N. E. 1982. “Fire resistance of concrete structures.” Division of Structural Mechanics and Concrete Construction, Lund Institute of Technology, Lund, Sweden. 10. Cooke, G.M.E. 1993. “Results of tests on end-restrained reinforced concrete floor strips
exposed to standard fires.” Report prepared for the Construction Directorate of the Department of the Environment, Fire Research Station, Hertfordshire, U.K.
11. Lim, L., Buchanan, A., Moss, P., Franssen, J.M. 2004. “Computer Modeling of Restrained Reinforced Concrete Slabs in Fire Conditions” ASCE J of Structural Eng. 130, p.p. 1964-1971., 12. Bailey, C., 2002. “Holistic Behavior of Concrete Buildings in Fire”. Proceedings of the
Institution of Civil Engineers, Structures and Buildings. 152 Issue 3, pp.199-212.
13. Eurocode 2, 2002. Design of concrete structure, Part 1-2: General rule – structural fire design, prEN 1992-1-2
14. Franssen, J.M. 2003. “SAFIR: A thermal/structural program modeling structures under fire”. Proc., North American Steel Construction Conf., American Inst of Steel Constr, Baltimore. 15. SAFE, 2010, Computers and Structures Inc., Berkeley, CA., USA.
16. CAN/CSA A23.3-04, “Design of concrete structure.”, Can Standards Assoc, Rexdale, ON. 17. Moore, D., Lennon, T., Wang, Y. 2007. “Designers' Guide to En 1991-1-2, 1992-1-2, 1993-1-2
and 1994-1-2: Handbook for the Fire Design of Steel, Composite and Concrete Structures to the Eurocodes”, Thomas Telford Services Ltd.
18. DD ENV 1991-1: 1996. “Eurocode 1. Basis of design and actions on structures. Basis of design (together with U.K. national application document)”. British Research Institution, London. 19. BS5990 Part 8: 1990. “Structural use of steelwork in buildings: Part 8: Code of practice for fire
