Concrete Floor Slabs Research Articles

NON-LINEAR 3D MODELLING OF RC SLAB PUNCHING SHEAR FAILURE

Modern technologies of 3D computer modelling with the help of finite‐element methods of numerical analysis in non‐linear aproach enable a reliable simulation of real reinforced concrete slabs behaviour under the conditions of complicated stress‐strain state in all the stages of their behaviour under loading, including a stage of elasticity‐plasticity state in concrete and reinforcement as well as formation and development of destruction mechanism. Numerical methods of modelling of critical behaviour of structures go beyond the borders of scientific research, being increasingly applied for solving non‐trivial engineering tasks when it becomes evident that possibilities of analytical approach, acceptable for the existing design codes are not sufficient. In such a case an exceptional attention is to be paid for verification of calculated models against experimental research of similar numeric models. This article reviews a case of complex experimental‐theoretical and numerical calculation and analysis methods applied for a reinforced concrete floor slab and column connection. The object of research is of exceptional interest as for practical calculation methods for this type of constructions following design norms of different countries revision and additional processing are necessary. The results of the reinforced concrete floor slab and column junction computer model non‐linear numerical analysis are presented and compared with the results of an experimental study.

Realistic Modeling of Composite and Reinforced Concrete Floor Slabs under Extreme Loading. I: Analytical Method

This paper presents a new flat shell element for composite and reinforced concrete (RC) floor slabs subject to extreme loading conditions, accounting for the effects of geometric as well as material nonlinearities. A novel feature of the proposed element is its treatment of problems associated with the geometric orthotropy of composite floor slabs, achieved through a modification of the conventional Reissner–Mindlin hypothesis. The new element is formulated in a local corotational framework, enabling the use of linear strain-displacement relationships, with the influence of geometric nonlinearity addressed through transformations between the local and global systems. In addition, a robust nonlinear material model is proposed for concrete which captures the salient response characteristics under extreme loading conditions, including the effects of elevated temperature due to fire. The proposed element is implemented within the nonlinear structural analysis program \IADAPTIC\N, which is used in this paper to provide several verification examples, focusing principally on the significance of the assumptions made in the element formulation. Extensive verification against experiments on composite and RC floor slabs is undertaken in the companion paper, where favorable comparisons between the predictions of the proposed method and experimental results are generally achieved.

Realistic Modeling of Composite and Reinforced Concrete Floor Slabs under Extreme Loading. II: Verification and Application

This paper deals with the large displacement behavior of floor slab systems under extreme loading conditions. The analytical model, presented in the companion paper, is verified through comparisons against existing experimental results on both reinforced concrete and composite slabs, with flat or ribbed profiles. The new model incorporates a novel shell element and accounts for material and geometric nonlinearities under ambient as well as elevated temperatures. The verification studies examine the response of reinforced concrete slabs under unrestrained and restrained edge conditions. An assessment of the behavior of ribbed floor slabs is also undertaken, in addition to simulation of the structural response of a full-scale composite beam-slab floor system under a realistic compartment fire situation. The results show good correlation between the experimental findings and numerical predictions, and demonstrate the reliability and robustness of the proposed analytical model. Additionally, the studies and discussions presented in this investigation provide an insight into the key behavioral aspects of floor slabs under extreme conditions. In particular, the significance of compressive arching and tensile membrane actions, under various boundary conditions, is illustrated. Also, the importance of adopting a realistic representation of the composite slab geometry is highlighted. The proposed analytical model is of particular relevance to developments in performance-based structural fire design, where realistic assessment of the floor slab response is of paramount importance.

Experimental behaviour of concrete floor slabs at large displacements

This paper presents the results from 15 small-scale tests conducted on horizontally unrestrained slabs which were subjected to large vertical displacements. All the tested slabs showed a load-carrying capacity far greater than the design capacity using the well-established yield-line theory. The purpose of these tests was to investigate the influence of isotropic and orthotropic reinforcement, together with reinforcement bond strength, on the degree of mobilisation of tensile membrane action, and thus on the load capacity of the slab. The results of the tests have been compared to a previously developed design method which incorporates membrane action of composite floor slabs into the estimation of their load capacity in fire, where large vertical displacements are acceptable provided that compartmentation is maintained. This comparison shows that the design method compares well with experimental results and is generally conservative, although it is in need of development to account for the bond characteristics of reinforcement and changes of failure mode for some orthotropically reinforced slabs.

Bending and membrane action in concrete slabs

AbstractIt has become apparent from the series of fire tests carried out at Cardington, UK that the response of steel‐concrete composite structures in fire conditions is largely governed by the behaviour of the reinforced concrete floor slabs. This paper obtains a detailed understanding of the strength of heated concrete floor slabs when subject to the combined bending and membrane forces that typically result from restrained thermal expansion. To analyse such slabs the computer program FEAST is used. Initially the behaviour of the floor slab in the Cardington frame under pure bending and pure membrane forces is described and explained. The paper then explores the effect that interaction between bending and membrane behaviour has on the strength of the slab when it is uniformly heated, heated with a linear thermal gradient and heated with the non‐linear thermal gradient observed during the Cardington tests. Finally, the effects of varying the area and location of the reinforcement in the slab are analysed. Copyright © 2004 John Wiley & Sons, Ltd.

Airborne Sound Insulation of Beam and Block Floors: Direct and Flanking Transmission

Beam and block floors are commonly used as separating floors between dwellings. Measurements of the dynamic properties of these floors indicate that beam and block floors are highly orthotropic, and do not act as infinite plates in terms of their driving-point mobility. Transmission suite measurements of beam and block floors with or without a screed show that these floors do not achieve the same level of airborne sound insulation as a homogeneous isotropic concrete slab floor. Significant differences were found in the flanking transmission between masonry walls when coupled to a beam and block floor at a T-junction in comparison with homogeneous concrete slab floors. Vibration measurements indicate that at frequencies below 500Hz, flanking transmission along masonry flanking walls past beam and block floors is significantly higher than with homogeneous concrete slab floors of similar surface density.

Restraint of fire‐exposed concrete floor systems

AbstractThis paper describes the numerical analyses of restrained concrete floor slabs exposed to fire. The analyses of the slabs were carried out with the SAFIR finite element program considering a 200 mm thick slab, spanning 5 m between two end supports. The slabs were exposed to the ISO standard fire for up to 4 h and were analysed with pinned and rotationally restrained supports. Different heights of the line of thrust at the supports and different levels of axial restraint were also investigated. The analyses show that fully restrained pin‐supported slabs can survive the 4 h ISO fire without collapse if the position of the line of thrust is located near the soffit of the slab. If the position of the line of thrust is located much above the soffit of the slab, the slabs will rapidly undergo large deformations and sag into a catenary, imposing axial tensile forces at the supports. The analyses have shown that even if the line of thrust is located close to the soffit, the slab can still deform into a catenary if there is insufficient horizontal axial restraint. In this study, rotationally restrained slabs experience much smaller vertical deflections than pin‐supported slabs when exposed to fires. Rotationally restrained slabs with low levels of horizontal restraint do not collapse, due to the beneficial effects of moment redistribution. However, high levels of horizontal restraint can be detrimental, causing slabs to collapse at advanced stages of the fire. Copyright © 2004 John Wiley & Sons, Ltd.

Concrete from an agricultural waste-oil palm shell (OPS)

Wastes are produced in large quantities from agro-based industries and the use of these waste materials in construction industry would contribute towards a cleaner environment. It also results in a cost effective construction material. A concrete using oil palm shell (OPS) as coarse aggregate has been found useful as structural concrete. As the bulk density of OPS is much less than stone aggregate OPS concrete becomes a lightweight concrete with a density of about 1850 kg/ m 3 . The compressive strengths of OPS concrete range from 20 to 24 N/ mm 2 for 28 days; this satisfies the strength requirement of structural lightweight concrete. A test on concrete floor slab using OPS concrete is also discussed.

Modeling Membrane Action of Concrete Slabs in Composite Buildings in Fire. II: Validations

A companion paper has documented a geometrically nonlinear layered procedure for modeling of the membrane actions in concrete slabs subject to very high deflections and in fire. The model is based on a layered procedure developed previously by the writers in which only material nonlinearities were taken into account. In this paper two solid reinforced concrete slabs with simply supported edges, tested by other writers at ambient temperature under uniform loading, are modeled. This is followed by a simulation of a full-scale fire test on a solid reinforced concrete slab floor. Finally very detailed simulations of two full-scale fire tests on the composite frame at the Cardington Laboratory are conducted. It is evident that the proposed model can predict structural behavior of reinforced concrete slabs and their influence on composite steel-framed buildings in fire with good accuracy, although the multiplicity of parameters which can affect a test result cannot always be controlled or measured. In all cases...

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.

Efficient arrangement of reinforcement for membrane behaviour of composite floor slabs in fire conditions

Abstract At large displacements concrete floor slabs can support, by membrane action, a vertical applied load which is significantly larger than that calculated assuming simple flexural behaviour. In an accidental limit state, such as a building fire, membrane action at large displacements can be beneficial to the survival of composite floor slabs used in steel-framed buildings. By utilising membrane action significant cost savings can be achieved by identifying a large number of steel beams, which would have required applied fire protection using previous design methods, to be left exposed to the high temperatures experienced during a fire. This paper extends a previous derivation of a design method, which predicted the membrane load-carrying capacity of composite floor slabs in fire, but was limited to isotropic reinforcement. The extension to the method allows the specification of orthotropic reinforcement, presenting the designer with the tools to specify the most economical arrangement of reinforcement in the floor slab. It is shown, in the fire design example presented in this paper, that for a given area of reinforcement the membrane load-carrying capacity of a rectangular floor slab with an aspect ratio of 2 can be increased by 23% by placing more reinforcement in the longer span.

A theoretical study of the thermal performance of the TermoDeck hollow core slab system

The TermoDeck hollow core slab system is a versatile energy storage technique for controlling the environment within large and medium sized buildings. It utilises the hollow cores within pre-cast concrete floor slabs as ventilation ducts to produce an environment which is thermally stable. Although many TermoDeck systems have successfully been installed in Scandinavia, the United Kingdom and in other northern European countries, the thermal performance of the system is not fully understood. This paper presents the results of a theoretical study, using a numerical model, into the thermal performance of the TermoDeck system. In particular, the role of the bends in the system is investigated and the conclusion reached that their impact on overall heat transfer is minimal. It is also concluded that greater thermal attenuation is achieved by using a five-core pass system in comparison with a three-core system.

The Behaviour of Multi-Storey Steel Frames in Fires

In order to investigate the effect of fire on a multi-storey steel frame building, a particular 17 storey building was studied to determine the behaviour as a fire spread throughout one floor of the building. Resistance to wind or seismic lateral forces acting along the major axis of the building is provided by moment resisting frames on two opposite sides while at right angles the lateral resistance is provided by eccentrically braced frames. A gravity load carrying frame runs along the major axis of the building. The columns and braces were fully fire protected, while the beams (acting compositely with the concrete floor slab) were unprotected. For the analyses carried out, the fire occurred on only the second level and spread from one quarter of the building.

A structural analysis of the Cardington British Steel Corner Test

This paper presents a structural analysis of the Cardington British Steel Corner Test. The test is analysed using ABAQUS, the commercial finite element program. The results of the analysis indicate that the response of the structure is dominated by the effects of thermal expansion and that material degradation and gravity loading are of secondary importance until very late in the test. It is noted however that at extreme temperatures a significant load carrying mechanism is tensile action in the reinforcement mesh and that gravity loading can effect the magnitude of the tensile forces produced. The results suggest that one method of helping maintain structural integrity in composite structures during extreme fires is to ensure that a sufficient amount of ductile reinforcement is present in the concrete floor slabs.

Non-linear structural modelling of a fire test subject to high restraint

The computer code VULCAN has been developed for the three-dimensional structural analysis of composite and steel-framed buildings in fire. In this paper, the main features of the program are outlined, with particular emphasis on the most recent development to the layered procedure for modelling of concrete floor slabs. This development has introduced geometric non-linearity into the modelling of slabs, whose layer structure already allowed temperature distributions and change of material properties through the thickness, as well as modelling the effect of the ribs at the bottom of composite decking slabs. The capabilities of the model are firstly tested at ambient temperature for a uniformly loaded ribbed reinforced concrete slab with simply supported edges, and this is followed by a very detailed modelling of the Cardington restrained beam fire test. In both cases the development of membrane action is demonstrated and the structural behaviour is compared with the geometrically linear case. A number of studies are carried out to demonstrate the influence of the major floor slab details on the behaviour of the structure in fire conditions. These studies provide evidence that when exposed steel temperatures are less than 400°C the concrete slab has little influence, other than to play a part in generating thermal curvature to composite beams. For temperatures higher than about 500°C the effect of the slab progressively becomes much greater, and it is very important to model concrete slabs correctly. The influence of membrane action cannot be ignored, particularly when the fire compartment is subject to high restraint because it is surrounded by cool, stiff structure. At very high temperatures the floor slab becomes the main load-bearing element and the floor loads above the fire compartment are carried by the membrane forces developed in the slab, with tension being carried mainly by the steel anti-cracking mesh or reinforcing bars. However, the effect of the very high in-plane restraint to thermal expansion in the particular Cardington test considered is to enhance the peripheral zone of compressive membrane force and to reduce the extent of the central area of tensile force compared with more usual cases.

Foundation heat loss from heated concrete slab-on-grade floors

In recent years, there has been a renewed interest in heated concrete slab floors to provide for space heating in both residential and commercial buildings. The existing design procedures for these heating systems are based on simplified thermal models with several assumptions. In particular, the simplified models assume that both the upper and the lower concrete slab surface are isothermal and that the heat transfer through the bottom of the slab surfaces is uniformly distributed over the entire surface. In this paper, a more realistic and flexible model for heated or cooled concrete slab floors is considered to determine the heat transfer between the concrete slab and the ground. In particular, steady-state and steady-periodic semi-analytical solutions are developed to determine the temperature field within the ground medium and within the concrete slab-on-grade floor where hot or chilled water pipes are embedded. The solution presented in this paper is applied to determine the ground heat loss/gain for a heated or cooled floor under various design conditions including the level of floor insulation, and the temperature of the water pipes. These solutions are obtained using the interzone temperature profile estimation (ITPE) procedure. Detailed analysis is presented to determine the effect of the slab insulation configuration on soil and slab temperature field and on the monthly variation of the total slab heat loss.

A structural analysis of the first Cardington test

This paper presents a finite element analysis of the first Cardington test using shell elements to model the concrete floor slab. The behaviour of these elements is defined using FEAST, a program that allows the behaviour of the shell elements to be defined within the commercial finite element package ABAQUS, using stress-resultants. The model of the test is described and the assumptions that were made are noted and justified. The results of the analysis indicate that the response of the structure is overwhelmingly dominated by the effects of thermal expansion and that material degradation and gravity loading are of secondary importance. It is noted that as a consequence of the rectangular nature of the fire compartment, tensile membrane action occurs from the beginning of the test.

Behaviour of precast concrete floor slabs exposed to standardised fires

This paper quantifies the thermal movements of 14 simply supported precast reinforced concrete floor slabs of 4.5 m span and 900 mm width exposed to two standardised heating regimes used in fire resistance furnace tests. The tests were designed to show the effect of varying the slab thickness, type of concrete, imposed load, soffit protection and nature of fire exposure on the mid-span flexural deflection and axial movements of the slab ends. Measured deflections showed that during the 90 min design period of fire resistance thermal bowing was dominant and the effect of the 1.5 kN/m 2 design imposed load was small. The NPD hydrocarbon fire exposure caused a doubling of the flexural deflections achieved using the standard BS 476: Part 8 (now Part 20) fire exposure in the first 20 min of exposure.

Curling of Concrete Floor Slabs on Grade — Causes and Repairs

Many industrial floors are required to have high-quality flat surfaces for the operation of specialized equipment, particularly high-reach stackers operating from the surfaces of warehouse floors. For new floors, the essence of floor flatness lies in the manner of finishing and the systematic monitoring of the flatness achieved immediately following construction. Achieving such surfaces in floor slabs however, is quite difficult because of the moisture and temperature gradients that cause them to curl at the joints. Such curling seriously affects the operation of an industrial facility. Floors subjected to heavily loaded forklift traffic may rapidly deteriorate, causing safety problems. Curling is also exacerbated in industrial floors by the use of power-troweled surface hardeners to produce the dense high-strength top surface required for high wear resistance. Repair of curled floors in industrial locations involves grinding, patching, installation of dowels, and grouting underneath the curled slab. The timing and appropriateness of the method used are of vital importance to the durability of the repair. Aspects of design and construction to minimize curling of new industrial floors, the factors that contribute to cracking and curling, measures to minimize curling, and the repair of curled floors are discussed in this paper.

Experiments on composite steel beams with precast concrete hollow core floor slabs

Precast concrete hollow core floor units are frequently used in multistorey steel-framed buildings where they bear on to the top flanges of universal beams. The steel beam is normally designed in bending in isolation from the concrete slab and no account is taken of the composite beam action available with the precast units. Although some commercial data are available, there is no general design guidance to cover the wide range of material and geometric variables found in this form of construction. This paper summarizes research carried out at the University of Nottingham on this form of construction and presents the results of three full-scale bending tests of steel beams acting compositely with proprietary precast hollow core slabs. The 150 mm deep units were attached to the universal beams through 19 mm diameter headed shear studs, and tested in four-point bending over a span of 6 m. For typical geometry and serial sizes the composite beams were found to be twice as strong and nearly three times as stiff as the equivalent isolated steel beams. The failure mode was ductile, and may be controlled by the correct use of small quantities of tie steel and in situ infill concrete placed between the precast units. To generalize the findings, isolated push-off tests and eccentric compression tests were used to study the horizontal interface shear resistance of the headed studs and the strength of the slab, respectively.