Crushing and Flexural Strength of Slab-Column Joints

In multi-storey buildings, columns are usually not continuous through the slabs to enhance ease of construction. Consequently, in slab–column joints, slabs have to carry column loads in addition to the shear and bending moments due to loads applied to the slab. In most cases, when high strength concrete is used for the columns and normal strength concrete for the slabs, compression stresses at the support areas of the inner columns exceed the uniaxial compressive strength of the concrete of the slab. Due to this reason, most current details for such regions reinforce the concrete of the slab between columns to ensure load transfer. Typically, this is achieved by linking top and bottom columns with reinforcement. Sometimes, it is also needed to incorporate special load transfer devices. This latter solution is however relatively complicated and expensive. In this paper, the crushing and flexural strength of slab–column joints is investigated accounting for an increase of the compressive strength of the failure region (concrete between columns) due to confinement stresses provided by the flexural reinforcement of the slab. The results of an experimental programme on 6 full-scale slabs (250 mm thick) are presented showing that flexural reinforcement of a slab significantly increases the crushing strength of slab–column joints. This allows ensuring load transfer without incorporating special devices or even without linking top and bottom column reinforcement for a wide range of cases leading potentially to more economic designs. An analytical approach, grounded on the theory of plasticity, is also presented allowing one to determine a failure criterion for such regions. This approach, which can also be used for design purposes, leads to an excellent correlation with test results.

Structural behavior and durability of multifunctional fiber-reinforced polymer components

Florian Riebel

In the context of sustainable development, building insulation represents a major concern as a means of increasing energy efficiency. In order to comply with new norms such as the Passive House standard, locations where load-bearing components must penetrate the building's insulating envelope, such as when cantilevered slabs (e.g. balconies) are anchored to building walls, constitute a particular challenge. With conventional steel reinforcement, thermal bridges are inevitable and insulation is weakened. To overcome this deficiency, a multifunctional joint has been developed in which GFRP (glass-fiber reinforced polymer) elements performthe necessary structural functions without compromising the continuity of the building's insulating envelope. This is due to GFRP materials' low thermal conductivity, which is magnitudes smaller than that of concrete and steel, making them particularly suitable. The anchorage of balconies requires linkages that can resist shear as well as tensile and compression forces in the upper and lower parts of the slab. In a first step, a hybrid joint was created in which the lower compression steel reinforcement was replaced by a compression-shear (CS-)element made of GFRP. This consisted of a short pultruded profile with cap plates bonded to its cut ends. In addition to compression forces, the element was intended to bear parts of the shear load. The joint was investigated in various full-scale beam specimens each representing a section of the slab. Based on the results, the joint's structural behavior was modeled analytically and structural requirements for the CS-element were determined. It was seen that shear transfer through the element increased with increasing shear-to-moment ratios. In a second step, an all-GFRP joint was created in which, in addition to the element in the compression zone, a tension-shear (TS-)element replaced the remaining steel reinforcement. This element consisted of the same pultruded profile cut to longer sections and penetrating the concrete. To anchor the tensile forces in the concrete, ribs were bonded to its surface. This joint type was also investigated through full-scale beam experiments similar to the hybrid-joint beams. The load transfer through the joint and into the concrete was studied and modeled analytically. The behavior of the all-GFRP joint was as ductile as that of the hybrid joints. The TS-element bore the main portion of the shear load independently of the shear-to-moment ratio however. The CS-element used in both joint types was of special interest with regard to remaining strength and stiffness after long-term service life. Exposure to alkaline concrete-pore solution represents a particular threat to the polyester matrix and glass fibers inside the GFRP material. Therefore, CS-elements were immersed in alkaline liquids at different temperatures and their compression strength and stiffness were studied during a period of eighteen months. Material degradation was investigated by SEM-microscopic images and EDX-analysis. The observed loss of compressive strength was ascribed to moisture diffusion and chemical degradation of the fibers, matrix, and fiber-matrix interface. It was shown that the strength degradation rate at different temperatures followed the Arrhenius rate law and that remaining strength after long time spans could therefore be projected by extrapolating the measurements. Remaining strength after 70 years of service life was found to be sufficient for the element never to become the critical failure location and classic concrete theory can be applied to verify the joint. Since stiffness at high temperatures was observed to already remain constant after short exposure times, this value could directly be extrapolated to apply to a 70-year service lifespan.

EPFL2006

Post-punching behavior of reinforced concrete slabs

Yaser Mirzaei

Reinforced concrete flat slabs are extensively used in buildings and parking garages. Their design is governed by deflection at the serviceability limit state and punching shear at the ultimate limit state. When no punching shear reinforcement is provided, failure develops in a brittle manner. Punching shear failure occurs with almost no warning signs, because deflections are small and cracks at the top side of the slab are usually not visible. Over the past decades, several structural collapses occurred due to punching shear failure resulting in human casualties and large damages. These collapses revealed some shortcomings in codes of practice and the necessity of reconsidering punching provisions. The investigations of these collapses showed that the collapse initiated from a local punching failure and propagated throughout the structure, in a progressive collapse. The term progressive collapse refers to the spreading of an initial local failure triggered by the loss of one or more load carrying members and leading to partial or total collapse of the structure in a manner analogous to the chain reaction. As a local punching failure can trigger progressive collapse, the study of the post-punching behavior can help adopting constructive solutions to avoid progressive collapse. The post-punching behavior of flat slabs supported by columns has not yet been thoroughly investigated. Therefore, an extensive experimental campaign was performed in the framework of this dissertation to investigate the post-punching behavior of 24 slabs with various reinforcement layouts. The effects of bending reinforcement, integrity reinforcement, bent-up bars, steel type, and anchorage conditions on the post-punching behavior of slab-column connections were investigated. The performance and robustness of the various solutions was investigated to obtain physical explanations of the load-carrying mechanisms. Test results showed that the post-punching strength provided by the top reinforcement is small because the concrete cover is thin and spalling of the concrete cover occurs leaving the reinforcement ineffective. In addition, it was observed that integrity reinforcing bars passing through the column significantly improve the post-punching behavior in terms of strength and deformation capacity. The integrity bars behave as a tensile membrane inclined to the plane of the slab and are able to sustain damaged portions from the column. Thus, one possibility to enhance the robustness of the structure against progressive collapse is to provide well-anchored bottom reinforcing bars passing through the column. A mechanical model capable of predicting the post-punching behavior of slab-column connections without shear reinforcement was developed. The model predicts the contribution of the tensile reinforcement and of the integrity reinforcement to the post-punching strength. The model accounts for possible failure modes including the fracture of the bars and the destruction of the concrete over the integrity bars. The progressive destruction of the concrete within and outside the punching cone is treated by considering the pullout behavior of reinforcement embedded in the concrete. Finally, a parametric study was performed to evaluate the influence of various parameters and their relative importance in order to develop practical proposals for the estimation of the post-punching strength. It showed that the post-punching strength is not only a function of the cross sectional area and yield strength of the integrity reinforcement as it appears in provisions and codes of practices but also of the diameter of the bars, the effective depth, the ductility, and the type of reinforcement.

EPFL2010

Poinçonnement des planchers-dalles avec colonnes superposées fortement sollicitées

Roberto Guidotti

Flat slabs are commonly used in buildings due to their easiness of construction and economy. In order to keep these advantages, columns are usually not continuous through the slabs in multi-storey buildings. In these cases, the slabs are subjected to large compressive stresses at the support area of the columns, which can exceed the uniaxial compressive strength of the concrete of the slab. This critical zone is in addition subjected to large shear forces and bending moments due to the loads applied on the slab. This leads to a series of potential failure modes: crushing of the concrete of the slab between columns, flexural failures or punching shear failures. Most research has previously focused on the influence of bending of the slab on the column strength. However, no works have provided in-depth investigation of the strength of the slab when large column loads are applied. In this research, an extensive experimental program has shown that the stresses applied at the support area of the columns can be significantly larger than the uniaxial compressive strength of concrete. The test results have clearly shown that no special confinement or load transfer devices are required between columns for most cases (moderate column loads). In addition, two phenomena have been observed. The first one is a reduction on the flexural strength as column loads are applied. The second corresponds to a significant increase on the punching shear strength and deformation capacity with column loading. Existing theoretical approaches for flat slab behaviour and strength are shown not to be directly applicable for slabs subjected to large column loading. In this research, the principles of two general theories (the theory of plasticity and the critical shear crack theory) are thus used to investigate such cases. The theory of plasticity allows calculating a plastic failure envelope accounting for bending and column loading, whereas the critical shear crack theory, which in this work as been further investigated theoretically, is used to derive a failure criterion accounting for punching shear failure in presence of column loading. The results for both theories are finally presented in terms of a single interaction diagram between column loading and slab loading (bending and shear of the slab). The theoretical approaches require however the help of rather refined numerical tools for estimating the strength of a flat slab. In order to use the theoretical approaches for design, a simplified approach has been developed, allowing to calculate the strength as well as the deformation capacity of flat slabs. These tools were implemented in a design method for slab-column joints in multi-storey building. This design approach allows to derive simplified interaction diagrams that can be compared with the loading history of the structural element analysed.

EPFL2010

Poinçonnement des planchers-dalles avec colonnes superposées fortement sollicitées

Roberto Guidotti

EPFL2010

Post-punching behavior of reinforced concrete slabs

Yaser Mirzaei

EPFL2010

Structural behavior and durability of multifunctional fiber-reinforced polymer components

Florian Riebel

EPFL2006