Influence of a Prestressing Eccentricity on the Punching Shear Strength of Post-Tensioned Slab Bridges

A large number of medium and small span bridges in Switzerland and in other countries are slab bridges, often prestressed, multi-span and supported by columns. Punching in such structures is typically governing at failure. So far, the knowledge of the influence of prestressing on the punching shear strength is limited. Most results published in the litterature have been obtained on slabs prestressed with tendons. As a result the influence of prestressing is investigated globally, because all its effects (axial force, bending moment and deviation forces) have been investigated simultaneously. This paper presents a test campaign currently under way at the Ecole Polytechnique Fédérale de Lausanne. The aim of the tests is to investigate separately and then quantify the various effects of prestressing on the punching shear strength. The paper presents the first part of the test campaign (consideration of a moment), on reinforced concrete slabs. The first results will be presented and discussed on the basis of the critical shear crack theory.

Shear strength of RC slabs under concentrated loads near clamped linear supports

Miguel Fernández Ruiz, Francisco Manuel Maciel Natário, Aurelio Muttoni

Reinforced concrete slabs without shear reinforcement subjected to concentrated loads near linear sup- ports are usually designed or assessed in shear using design provisions that have been calibrated on the basis of tests on one-way slabs or beams with rectangular cross-section (slabs loaded over their full width). This approach may however be inconsistent, as the actual behavior of slabs under concentrated loads reflects a two-way slab response, with the shear forces not developing in a parallel manner in the failure region and with potential redistribution of the internal forces for increasing levels of load. In this research, a series of 12 tests on 6 full scale slabs (3.00 m 3.00 m 0.18 m) is presented. The slabs were centrally supported on an aluminum profile and were subjected to two concentrated symmet- rical loads. The support profile was equipped with vertical strain gauges, allowing to trace the distribu- tion of reactions at the supported line and its redistributions as the value of the concentrated loads increased. Significant redistributions of the reactions were actually measured prior to failure, which are presented and discussed within the paper. The aim of these measurements was to improve the under- standing of the mechanical behavior of such members and the role of shear cracking on the distribution of internal forces. A number of parameters was investigated, such as the location of the concentrated loads and the presence of ducts in the slab. This latter case (typical of prestressed concrete balanced cantilever bridges and residential buildings) has not been investigated in the past, despite its potential influence on the shear strength. The performance of these specimens is finally investigated and compared to the prediction of the fib - Model Code 2010 and the Critical Shear Crack Theory. This analysis shows that accurate predictions of the strength can be obtained if the influence of direct load strutting (for loads acting near supports) and redistribution of internal forces is accounted.

Engineering structures2014

Etude du comportement réel des ponts mixtes et modélisation pour le dimensionnement

Construction methods for steel-concrete structures have developed significantly over the last quarter of a century, and the number of steel-concrete bridges on road and rail networks is continually increasing. However, design methods for bridges have not evolved to the same extent as those for buildings and are not completely adapted to the limit state concept. Furthermore, the behaviour in service of steel-concrete bridges is raising questions about requirements for deck slabs (durability, cracking) and about design models applicable in this domain. The main aim of this research was to study the behaviour of steel-concrete composite bridges and to propose a model for the verification of structural safety. Objectives of the research were : measurement and numerical simulation of the influence of actions on the behaviour of steel-concrete composite bridges during construction and in service in order to analyse the effectiveness of methods for improving this behaviour, analysis of steel concrete composite bridge behaviour at the ultimate limit state and proposals for simplifying design. The research progressed in the following order : Serviceability detailed study of the behaviour of young concrete, analysis of different types of prestressing and prestressing losses over time, synthesis of the various causes of tensile stresses in deck slabs and proposa1 of design criteria as a function of structural properties and deck behaviour requirements. Structural safety critical evaluation of current design methods and possibilities for the plastification of sections, definition of limits for plastic design as a function of the type of bridge, and analysis of the influence of time-dependent effects in the concrete on the behaviour at the ultimate limit state. For the serviceability limit state, a study of the actions that create tensile stresses in deck slabs led to the determination of the principal parameters influencing the formation of transverse cracks. The restraint coefficient β, defined as the ratio of steel beam and concrete cross-sectional areas, was shown to be the key parameter for evaluating the probability of cracking with time. Methods for limiting the tensile stresses in the slab were analysed and compared as a function of the type of bridge. For the verification of structural safety, a study of the actual behaviour of steel-concrete composite sections led to the justification of using plastic design calculations in order to determine the resistance of midspan sections. Limits for the application of this approach were defined, and simplified methods for considering imposed deformations such as shrinkage and temperature effects were proposed.

EPFL1997

Shear strength of reinforced concrete bridge deck slabs

Rui Vaz Rodrigues

Reinforced concrete bridge deck slabs without shear reinforcement can be subjected to concentrated or distributed loads of important magnitude. Under these loads their structural response is not always ductile. In particular under concentrated loads their deformation capacity can be limited by shear or punching shear failures, which prevent them from reaching the ultimate load predicted by pure flexural analysis. This problem has been studied in this research by means of an important experimental program and theoretical modeling. The limited ductility of bridge decks was investigated by means of full scale tests on bridge deck cantilevers under groups of concentrated loads. Six large scale laboratory tests were performed on two bridge deck cantilevers with a span of 2.8 m and a length of 10.0 m. All slabs failed in a brittle manner, in shear or punching shear. The theoretical flexural failure load estimated using the yield-line method was never attained. Despite the brittle failures, the results of tests on cantilevers have shown that some amount of yielding can occur before the shear failure and therefore reduce the shear strength. This effect was quantified on eleven full scale tests on slab strips without shear reinforcement with a length of 8.4 m. The results clearly show that the increase of plastic strains in the flexural reinforcement leads to a reduction of the shear strength. The measured rotation capacity of the plastic hinge was thus limited by a shear failure. A particular problem of bridge deck slabs is the introduction of concentrated loads applied by wheels with pneumatic pressure. Punching shear with these loads is usually treated in a manner similar to punching by a column. A punching shear test was performed with a concentrated load simulating a vehicle wheel with pneumatic pressure to investigate the differences. It appears that punching shear with a wheel with pneumatic pressure is less critical because curvatures tend to be distributed over the surface of the applied load rather than concentrated near the edges of the column. In order to investigate the experimental results on slab strips without shear reinforcement, a mechanical model is proposed to predict the shear strength and rotation capacity of plastic hinges. The shear strength is formulated as a function of the opening of the shear crack and of the strength of the concrete compression zone. The results of the mechanical model are in good agreement with the measured values, both for the shear strength and for the shear carried across the shear crack. Based on the mechanical model, a simplified equation is proposed. The model can also be used to predict the shear capacity of yield-lines. A non linear finite element model was implemented during this work and used to correctly predict the measured rotations and load-displacements curves of the tested cantilevers and other full scale tests performed by other researchers. The measured failure loads are accurately estimated by using the results of the non-linear model and the one-way shear and punching shear criteria proposed by Prof. A. Muttoni (Muttoni 2003).

EPFL2007