Slabs In Fire Research Articles

Behaviour of full-scale two-way simply supported concrete slabs in fire

This paper presents two tests of full-scale simply supported two-way concrete slabs in a high-temperature environment, based on the standard temperature–time curve (ISO 834). Details of the support conditions, the reinforcement arrangements and the positions of displacement transducers and thermocouple trees are given. Crack development, the temperature distribution, central vertical deflection and edge horizontal deflection of the concrete slabs were recorded during the tests. It was observed that, despite large deflection, neither slab collapsed; this was due to the significant influence of tensile membrane effects on maintaining the structural stability of the floor slabs under fire conditions. Based on the experimental results, an energy-based model to determine the limit load-carrying capacity of concrete slabs at elevated temperatures is verified and discussed. A comparison of the energy-based model results and the test results showed good agreement, while conventional yield theory provides conservative critical reinforcement temperatures in the slabs.

Review of the Punching Shear Behavior of Concrete Flat Slabs in Ambient and Elevated Temperature

There are a number of benefits associated with two-way concrete flat slab construction for office buildings, parking garages and apartments - for example, reduced formwork, prompt erection, flexibility of partitions, and minimal increase in story heights. However, concrete flat slabs could be quite vulnerable to punching shear failure in the event of a fire. The objective of the present article is to provide a state of the art review of the existing research and the issues associated with concrete flat slabs in fire and elevated temperature. There are a number of experimental and analytical studies on the punching shear behavior of concrete flat slabs in ambient conditions, available in the literature. Based on these studies, it is found that punching shear capacity in ambient condition is affected by many factors, which may not remain constant during a fire exposure. Only a limited number of studies on concrete flat slabs for punching shear failure in fire are available. This paper reviews the available experimental and analytical studies, standards and codes to address the research gap in estimating of punching shear strength of concrete flat slab-column connections without shear reinforcement.

The effect of reinforcement ratios on composite slabs in fire

Public-domain software currently allows designers to incorporate the enhancement of fire resistance due to tensile membrane action of composite slabs into their analysis of building behaviour in fire. Based on the Bailey–BRE Method, it enhances slab capacity by optimising reinforcement size in order to carry its fire limit state loading at the required fire resistance time. The method assumes that protected edge beams maintain vertical support of the slab at its boundaries, only allowing for failure of compartmentation integrity by tensile fracture of mid-panel reinforcement or by concrete crushing at the corners. However, these beams deflect under their increased loading in fire, which can cause premature structural failure of the panel. It is imperative to determine the real contribution of reinforcement to tensile membrane action, considering the loss of edge support. This paper presents a series of finite element studies conducted with Vulcan, compared with the generic simplified method and the public-domain software, to determine the influence of reinforcement on the failure of slab panels at elevated temperatures.-strength bolts after fire and that the effects of natural cooling are very different from those of water cooling.

Behaviour of fibre reinforced polymer-strengthened T-beams and slabs in fire

This paper presents experimental and numerical results of intermediate-scale and full-scale fire tests conducted on flexurally strengthened reinforced concrete members. Two full-scale reinforced concrete T-beams and two intermediate-scale slabs were strengthened in flexure with carbon fibre reinforced polymer sheets or plates and insulated with a layer of spray-on material. T-beams and slabs were then exposed to a standard fire. T-beams were loaded to their service load. Deflections and temperatures were measured during the tests. A numerical finite-difference/volume heat transfer model was used to simulate the temperature field inside the section. The validity of the numerical model was verified by comparing the predicted temperatures with those recorded during the fire test. Fire test results show that fire endurances of more than 4 h can be achieved using an appropriate insulation system.

Unbonded Post Tensioned Concrete Slabs in Fire - Part II - Modelling Tendon Response and the Consequences of Localized Heating

This is Part II of a two part paper dealing with the current state of knowledge of the fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures. Part I provided detailed results of nineteen transient high temperature stress relaxation tests on restrained UPT tendons of realistic length and parabolic longitudinal profiles. Experimentation identified several credible concerns for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating, which may result from a number of possible causes in a real structure. The real world response of continuous UPT tendons both during and after heating is largely unknown, and is dependent on factors which are not currently accounted for either in standard fire tests or by available prescriptive design guidance. This second part of the paper presents and applies a numerical model to predict the time-temperaturestress-strength interdependencies of stressed UPT tendons under localized transient heating, as may be experienced by tendons in a real concrete building in a real fire. The model is used, along with previously developed and validated computational models for heat transfer and prestress relaxation in UPT tendons, to assess existing prescriptive concrete cover requirements for UPT slabs. It is shown that localized heating of UPT tendons is likely to induce premature tendon rupture during fire, and that current prescriptive code procedures based on concrete cover alone are, in general, insufficient to prevent this. Based on the data presented it appears that minimum code prescribed concrete covers for UPT structures require revision if premature tendon rupture during fire is to be avoided.

Unbonded Post Tensioned Concrete Slabs in Fire - Part I - Experimental Response of Unbonded Tendons under Transient Localized Heating

The fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures has recently come under debate in the UK, and questions are being raised regarding the response to fire of post-tensioned concrete slabs. Related to these concerns is the real world response of continuous UPT tendons inside such structures both during and after a fire, which is largely unknown and depends on many potentially important factors which are not currently accounted for in standard fire tests. Several credible concerns exist for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating which may result from a number of possible causes (discussed herein). The research presented in this paper deals specifically with the time-temperature-stress-strength interdependencies of stressed UPT tendons under localized transient heating, as may be experienced by tendons in a real UPT building in a real fire. Nineteen high temperature stress relaxation tests on UPT tendons of realistic length and parabolic longitudinal profile are reported. It is shown that localized heating of UPT tendons is likely to induce premature tendon rupture during fire, even in structures which meet the prescriptive concrete cover requirements imposed by available design codes.

Tensile membrane effects of concrete slabs in fire

This paper presents an energy-based model to determine the limit load-carrying capacity of concrete slab at elevated temperatures. This model considers the plastic energy dissipation both due to the resultant moment rotation and the extension of reinforcement along the yield line after the formation of the mechanism of the plastic hinge line in fire. The model could predict the load-carrying capacity of either rectangular or square concrete slabs with both orthotropic and isotropic reinforcement, and could interpret why failure of fracture of reinforcement is along the short span and occurred at both ambient and elevated temperatures. Comparison between the developed model and test results shows good correlation.

The behaviour of reinforced concrete slabs in fire

In this paper a robust model is presented based on the previous layer procedure developed by the author to also take into account the effects of concrete spalling on the behaviour of concrete slabs under fire conditions. In this study, a detailed analysis of a uniformly loaded reinforced concrete slab subject to different degrees of concrete spalling under a standard fire regime is first carried out. Further, a series of analysis of floor slabs with different degrees of concrete spalling is also performed on a generic reinforced concrete building. A total of 16 cases have been analysed using different degrees of spalling on the slabs, with different extents and positions of localised fire compartments. It is clear that adjacent cool structures provide considerable thermal restraint to the floor slabs within the fire compartment. And it is evident that the compressive membrane force within the slabs is a major player in reducing the impact of concrete spalling on the structural behaviour of floor slabs in fire.

Prediction of the moment capacity of reinforced concrete slabs in fire using artificial neural networks

In this study, the application of artificial neural networks (ANN) to predict the ultimate moment capacity of reinforced concrete (RC) slabs in fire is investigated. An ANN model is built, trained and tested using 294 data for slabs exposed to fire. The data used in the ANN model consists of seven input parameters, which are the distance from the extreme fiber in tension to the centroid of the steel on the tension side of the slab (d′), the effective depth (d), the ratio of previous parameters (d′/d), the area of reinforcement on the tension face of the slab (As), the fire exposure time (t), the compressive strength of the concrete (fcd), and the yield strength of the reinforcement (fyd). It is shown that ANN model predicts the ultimate moment capacity (Mu) of RC slabs in fire with high degree of accuracy within the range of input parameters considered. The moment capacities predicted by ANN are in line with the results provided by the ultimate moment capacity equation. These results are important as ANN model alleviates the problem of computational complexity in determining Mu.

Effect of Aspect Ratio on Fire Resistance of Hollow Core Concrete Floors

Several factors can affect the behaviour of the hollowcore floor slabs in fire. This paper analytically investigates the relationship between the fire resistance of the overall floor system and the floor length to width ratio as well as the type of side supports. The study uses beam grillage and shell elements to model the hollowcore slabs and the topping concrete under the platform of the non-linear finite element program, SAFIR. Different methods to model precast, prestressed concrete hollowcore floor slabs subjected to fire are also investigated. The results show that side supports can enhance the fire performance of hollowcore floor slabs provided that the spacing of the side supports does not greatly exceed the span length.

Transient high-temperature stress relaxation of prestressing tendons in unbonded construction

Unbonded post-tensioned (UPT) flat plate concrete slabs are popular for modern continuous multiple bay floor assemblies due to economic and sustainability benefits (reductions in slab thickness and building self-weight) and structural advantages (decreased deflections over larger spans). Only limited research has been conducted on the performance of UPT flat plate slabs under fire conditions, yet the inherent fire endurance of these systems is sometimes quoted as a benefit of this type of construction. One concern for these structures in fire is that high-temperature stress relaxation of the unbonded prestressed reinforcement may cause considerable and irrecoverable prestress loss, with subsequent structural consequences. This paper uses a computational model which has been developed to predict the transient high-temperature stress relaxation (i.e., prestress loss) for typical UPT multiple span flat plate slabs in fire, to study the potential prestress relaxation behaviour under various plausible temperature conditions as might occur during exposure to a standard fire. The model is validated using experimental data from relaxation tests performed on locally heated unbonded seven-wire prestressing stand. The initial prestress level, concrete cover to the prestressed reinforcement, and ratio of heated length to overall tendon length are varied to investigate the potential implications for prestress loss, and subsequently for flexural and punching shear capacity. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate concrete cover for structural fire safety.

Modelling of bonded post-tensioned concrete slabs in fire

This paper presents a finite element model highlighting the behaviour of bonded post-tensioned one-way spanning concrete slabs in fire conditions. The model was verified against ten fire tests on bonded post-tensioned concrete slabs at ambient and elevated temperatures. The slabs were simply supported and post-tensioned with 15·7 mm nominal diameter seven-wire mono-strand tendons. The mechanical and thermal material non-linearities of the entire slab’s components, consisting of the concrete, plastic and galvanised steel ducts, prestressing tendon and the anchorages, have been carefully inserted into the model. The interface between the tendon and surrounding grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the slab. The temperature distributions throughout the slab, together with the slab’s development of displacement and stress, as it was heated, were predicted by the model and verified against test data. A parametric study was conducted to investigate the effects on the global structural behaviour owing to the change in the aggregate type, duct type, load ratio, boundary conditions and different fire scenarios. The study has shown that the bonded post-tensioned concrete slabs investigated in this study are capable of achieving the designed 90 min fire resistance. It is also shown that the fire resistance given by BS 8110-2 and BS EN 1992-1-2 are acceptable for the design of bonded post-tensioned one-way spanning concrete slabs under fire conditions.

An embedded FE model for modelling reinforced concrete slabs in fire

It is evident from a series of tests on simply supported reinforced concrete slabs that the failure of the slabs at large deflections is due to the formation of individual large cracks. This failure mode was also observed in the Cardington full-scale fire tests. Previous research indicates that the global behaviour of concrete slabs subject to large deflections can be well predicted by the smeared cracking model; however, the model cannot quantitatively predict the openings of individual cracks within the slabs at large deflections. For the discrete approach it is usually assumed that the cracks are formed along element edges, therefore continuous re-meshing is required during the analysis. Consequently, the results are mesh-dependent and the computing cost is high. In recent years, mesh independent finite element procedures, such as embedded (EFEM) and extended (XFEM) approaches, were widely used for modelling of the crack initiation and growth in structural members. However, most of the meshless models developed are either based on in-plane loading conditions or confined to thin shells with assumed full-depth cracks, which form apparent displacement jumps within an element. For a reinforced concrete slab, an out-of-plane load causes coupled stretching and bending of the slab, cracks are usually initiated at discrete positions and then propagated, until at last some individual full-depth cracks are formed. Pure stretching or assumed full-depth cracking is inadequate for modelling this kind of failure. Therefore, in this research, a non-linear layered procedure with embedded weak discontinuity is developed to quantitatively model the progressive tensile failure of reinforced concrete slabs subjected to large deflections. The current model inherits the advantage of the smeared approach, and at the same time, introduces the opening width of crack explicitly by taking the advantage of the better description of the kinematic characteristics of the EFEM approach. A series of validations have been conducted against test data at both ambient and elevated temperatures, and the research shows that the model developed in this paper is not sensitive to the FE mesh size and the aspect ratio of the slab. The results predicted by the model developed agreed well with the test data in terms of deflection and crack open width, also agreeing well with those modelled by the smeared model. Hence, this new approach provides a numerical method to predict the load capacity as well as identifying the occurrence and severity of crack failure in reinforced concrete slabs subjected to extreme loading conditions, such as fire.

Hollow‐core concrete slabs exposed to fire

AbstractThe aim of this paper is to provide recommendations to designers and to propose a simple method for designers to model the structural behaviour of hollow‐core concrete (HC) floor slabs in fire. The proposed finite element model incorporates a grillage system using beam elements to capture the thermal expansion of the precast units in both directions, with the topping concrete over several precast units modelled by shell elements. The research reported herein compares the proposed model with various fire test results of HC slabs. The simulation outcomes show good agreement with the experimental results.Several HC slab flooring systems tested previously at the University of Canterbury for seismic purposes were simulated using this modelling scheme. Various supporting schemes have been considered, and the results show that different arrangements of axial and rotational restraint at the supports can significantly influence the fire performance of the concrete slab floors. Copyright © 2008 John Wiley & Sons, Ltd.

Nonlinear analysis of orthotropic composite slabs in fire

In this study an orthotropic slab finite element is developed to model orthotropic slabs in fire, using a layered 9-noded isoparametric slab element and a 3-noded beam element. The element is assembled from a solid slab element which represents the continuous upper portion of the profile, and a special beam element which represents the ribbed lower portion. An equivalent width for the cross-section of this beam element is determined according to the dimensions of the solid slab element and the cross-section of the ribbed profile, and the beam shares the nodes of the solid slab element. The temperature within each layer of the slab element can vary between adjacent Gauss integration points so as to reflect temperature variations in the horizontal plane. Several fire tests on composite slabs have been modelled to validate the approach. Cases of orthotropic slabs with wide range of parameters defining the ribbed profile have been studied, which show that the orthotropic slab model is robust and effective in reflecting the influence of the shape of ribs on the thermal and structural performance of the slabs in fire. The study shows the influence of decking shape on the thermal and structural behaviours of orthotropic slabs. A simple evaluation method for profile selection is proposed.

Modeling of membrane action in floor slabs subjected to fire

Through observations from experiments and fire-caused damage in real buildings, it is found that membrane action plays an important role in enhancing the fire-resistant capacity of floor slabs. Considering the mechanism of membrane action in slabs subjected to fire, an explicit theoretical model is proposed. Based on the force and moment equilibrium requirement on slabs in fire, formulas for calculating the ultimate load- bearing capacity of floor slabs in fire conditions considering the effects of membrane action are obtained. The effectiveness of the theory for modeling membrane action of floor slabs subjected to fire is validated through satisfactory comparison with results from experiments and different theoretical simulations.

Numerical modelling of two-way reinforced concrete slabs in fire

This paper describes numerical modelling of the fire behaviour of two-way reinforced concrete slabs using a special purpose non-linear finite-element program, SAFIR. Several two-way reinforced concrete and composite steel–concrete slabs are tested under exposure to the ISO standard fire in order to validate the shell finite element in the SAFIR program. The modelling results showed agreement with the fire tests and demonstrate that the SAFIR shell element can be used to predict tensile membrane behaviour of two-way reinforced concrete slabs in fire conditions. The analyses show that two-way slabs have excellent fire resistance if they deform in double curvature and develop tensile membrane action.

Numerical modelling of two-way reinforced concrete slabs in fire

This paper describes numerical modelling of the fire behaviour of two-way reinforced concrete slabs using a special purpose non-linear finite-element program, SAFIR. Several two-way reinforced concrete and composite steel–concrete slabs are tested under exposure to the ISO standard fire in order to validate the shell finite element in the SAFIR program. The modelling results showed agreement with the fire tests and demonstrate that the SAFIR shell element can be used to predict tensile membrane behaviour of two-way reinforced concrete slabs in fire conditions. The analyses show that two-way slabs have excellent fire resistance if they deform in double curvature and develop tensile membrane action.

Comparison of BRE simple design method for composite floor slabs in fire with non‐linear FE modelling

AbstractOn the basis of test evidence a new design method has been developed recently by one of the authors, for calculating the performance of composite flooring systems subject to fire. The method models simply the influence of tensile membrane action in the composite floor slabs. The objective of this paper is to show some detailed comparisons between the simple design method and finite element modelling using the computer program Vulcan, which has been developed at the University of Sheffield, in order to check the applicability and inherent conservatism of the method. Initially a 9 m × 9 m square ribbed concrete slab, for which all four edges are vertically supported, is analysed. Different temperature distribution patterns across the thickness of the slab are used to investigate the influence of thermal curvature on the structural behaviour. The effect of changing the edge support conditions is also analysed.As part of this study a large generic composite flooring system with a footprint of 36 m × 36 m has been designed. The frame is based on a regular 9 m × 9 m column grid. A series of analyses has been performed, based on different patterns of fire protection to the downstand steel beams. The influence of the proportion of steel reinforcement on the structural behaviour has been investigated, and it is evident that the presence or absence of tensile membrane action in the concrete slabs is a major influence on the ultimate integrity of the flooring system at high distortions. The ability of the slab reinforcement to sustain the tensile stresses caused at high temperatures and deflections is clearly a key factor in ensuring that fracture of slabs does not occur. From both the Vulcan modelling and the simplified design method it is shown that tensile membrane action can be important in carrying the loads applied to the slabs at high temperatures and deflections. However, it is apparent that the simple design method predicts a greater contribution to load‐carrying capacity due to tensile membrane action than does the Vulcan modelling, especially for high reinforcement ratios, and that further work needs to be done to resolve this discrepancy. Copyright © 2004 John Wiley & Sons, Ltd.

Limit capacity of laterally restrained reinforced concrete floor slabs in fire

This paper presents a new concept for determining the limit capacity of rectangular reinforced concrete slab panels subjected to fire. The results from extensive previous work on computational modelling of composite steel frame structures in fire at the University of Edinburgh has shown that the response of such structures is dominated by the thermal strains induced in the structure as a result of heating. Naturally, as the limit state of collapse is approached, the conventional influences of loading and structural strength and stiffness (reduced by heating) will once again begin to dominate the response. However, in tests carried out on such structures and in many instances of real fires no collapse or failure has been observed and the exact mechanism of failure is still a matter for speculation. Because of this apparently considerable robustness of composite framed structures in fire, tests (such as the full scale fire tests at Cardington) have been carried out (with fire protection applied to the steel columns only while most steel beams being left unprotected), but again without any structural failure. A number of investigations, including the authors own, have attributed this robustness to the tensile membrane mechanism in the composite deck slab. The research group at Edinburgh has discovered that the development of tensile membrane mechanism in fire is much more reliable relative to ambient conditions. This is because the large amount of thermal strain allows composite floor systems to assume highly deflected shapes while limiting the magnitude of damaging tensile mechanical strains, thereby retaining the ability to carry loads for much longer. In the context of performance based design this additional capacity of composite decks or reinforced concrete slabs can be exploited for providing structural stability in fire given that a quantitative estimate of this capacity can be reliably made. Some previous work exists in this regard but none of this work attempts to determine the correct deflected geometry of the floors and therefore the membrane capacity is not properly estimated. This paper introduces a new three-step method that analyses the limit capacity of laterally restrained RC slabs in fire. First the temperature distribution over the depth of the slab is estimated for a given fire scenario. Then the deflected shape of the RC slab and its membrane stress state is determined using a rigorous analytical method. Finally an energy based method is used to determine the maximum load that the slab could carry based on the geometric form and stress state determined in the previous steps.