Behaviour and design of slab-column joints

This paper presents an investigation on the behaviour and strength of slab-column joints in multystorey buildings. Such joints may develop different potential failure modes such as crushing of the concrete of the slab (due to column loading), bending or punching shear failures (due to slab loading). The paper presents the results of a test series on 9 full-scale slabs (250 mm thick) loaded to simulate simultaneously column and slab loading. The ratio between the loads was varied from moderate to fairly large column loading. The experimental results confirm a strong interaction between column and slab loads with unexpected increases on the punching shear strength as column loads are larger than the uniaxial compressive strength of the slab. The observed behaviours can be explained on the basis of the theory of plasticity and by using the Critical Shear Crack Theory, leading to a physically-grounded design method for these regions.

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

Punching of flat slabs supported on rectangular columns

Miguel Fernández Ruiz, Aurelio Muttoni, Juan Sagaseta Albajar, Luca Tassinari

This paper investigates the structural behaviour of RC flat slabs supported on rectangular interior columns and the influence of the loading conditions (one or two-way bending) on their punching shear strength. The punching shear strength of slabs at rectangular columns can be lower than at equivalent square col- umns with a similar length of the control perimeter. This is due to a potential concentration of shear forces alongthecontrol perimeter. Some, but notall designformulas for punchingdesign, consider thisreduction on strength using empirical factors, which are written in terms of the column geometry only. However, in reality, the concentration of shear forces depends also on the deflected shape of the slab. It is shown in this paper that this can be consistently considered by means of the shear-resisting control perimeter. A sound approach is presented to estimate the shear-resisting control perimeter based on the shear fields of the slab accounting for the loading and boundary conditions. An alternative approach is presented based on the contact pressure in the support region which gives comparable predictions of the shear-resisting con- trol perimeter. Both approaches give a physical explanation of the phenomenon. It is also shown that the model previously developed by the authors for non-axis-symmetrical punching of square columns based onthe critical shear crack theory can also be applied to rectangular columns. Four punching shear tests are presented of slabs with one-way & two-way bending to validate the theoretical models presented. Accurate strength and deformation capacity predictions were obtained for the tests investigated.

Engineering Structures2014

Poinçonnement des planchers-dalles avec colonnes superposées fortement sollicitées

Roberto Guidotti

EPFL2010