Static and fatigue strength of RC slabs under concentrated loads near linear supports

RC slabs without shear reinforcement subjected to c oncentrated loads near lin ear supports are typical cases of deck slabs of bridges, transfer slabs or pile caps. Such elements are often designed or as sessed in shear with code provisions calibrated on the basis of tests on one-way slabs or beams with rectangular cross section, even t hough these tests are not representative of the actual behavior of two way slabs (non-parallel direction of shear forces and potential shear redistributions). Also the pres ence of prestressing ducts and slab inserts may infl uence the shear strength. In addition, the concen trated loads of the deck slabs are applied a number of cycles during the service life of the structure and may potentially lead to fatigue problems. In this investigation, two experi mental campaigns are presented. The first one consists of 12 static tests on 6 full-scale slabs subjected to a concentrat ed load with a central line support that allows evaluating the linear reaction. Parameters such as the location of the concentrated loads (3 locations) and presence of ducts (4 types) were varied. Th e second campaign has a sim ilar test setup and con sists of 4 static tests on 2 full-scale slabs (reference tests, failure load Q static ) and fatigue testing on 8 other slabs, varying the maximum applied load and the load location (2 locations). So far two slabs failed in shear ( Q max =90% and 80% Q static ) and two others due to bar fracture (fatigue in bending, Q max =70% and 60% Q static ) that ultimately led to shear failures. All slabs that statically failed in shear showed significant shear redist ributions prior to failure. This research aims to study the observed phenome na within the framework of the Critical Shear Crack Theory (CSCT) and to provide more data to the poor existing datasets . Comparisons with the fib- Model Code 2010 are presented. The ultimate goals of the research are to reduce the number of existing structures that need to be strengthened and to provide consistent design methods for new structures and assessment of existing ones.

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

Design of concrete girder bridges has significantly evolved during the last decades. This has been particularly relevant with respect to shear design, motivated by changes in actions and design models. As a consequence, assessing the shear strength of existing bridges leads in many cases to unsatisfactory safety levels. Furthermore, many existing bridges do not comply with current code regulations with respect to minimum amounts of shear reinforcement. This situation can lead to expensive retrofitting and strengthening of a significant number of existing bridges. The assessment of the shear strength of these bridges has thus become a significant task for structural engineers. Design codes are not always appropriate for assessing the strength of existing bridges. They propose safe models providing sufficient accuracy for design of a wide number of structures. However, these models do not account for some particularities of prestresses bridges and neglect a number of shear-transfer actions that can be relevant for their strength. This is typically the case of shear carried by the inclination of the compression chord, the increase of the stress in the tendons or the effective strength of concrete in the web of cracked prestressed girders. Accounting for more realistic approaches for these parameters may significantly increase the estimated strength of a structure with respect to code provisions, avoiding in many cases unnecessary retrofitting or strengthening. In this paper, the results of a tests campaign carried out at Ecole Polytechnique Fédérale de Lausanne on 10 prestressed concrete girders (10 meters long, 0.78 m high) are outlined. The specimens were provided with very low amounts of shear reinforcement and some of them present defective stirrup anchorage to simulate realistic conditions of existing structures. The experimental results are discussed with reference to the stress field method where the role of the different shear-transfer actions is investigated and compared to current code provisions.

fib Symposium Tel-Aviv 20132013

Static and Fatigue Shear Strength of Reinforced Concrete Slabs Under Concentrated Loads Near Linear Supports

Francisco Manuel Maciel Natário

Reinforced concrete slabs without shear reinforcement under concentrated loads near linear supports are typical cases of bridge deck slabs, transfer slabs or pile caps. Such members are often designed or assessed in shear/punching shear with code provisions calibrated on the basis of tests on beams or one-way slabs loaded over the full width, as well as tests on isolated flat slab elements supported on columns in axis-symmetric conditions. However, these tests are not representative of the actual behavior of slabs under concentrated loads near linear supports. Moreover, concentrated loads of heavy vehicles have a repetitive nature, causing loss of stiffness and potential strength reductions due to fatigue phenomena. In this thesis, two experimental campaigns are presented. The first one consists of twelve static tests on six full-scale cantilever slabs subjected to a concentrated load with a central line support, that allows tracing the linear reaction evolution. Parameters such as the location of the concentrated load, and the presence, material and injection of ducts were varied. All slabs failed in shear and significant redistributions of the linear reactions were observed prior to failure. The second campaign has a similar test setup and consists of four static tests on two full-scale cantilever slabs (reference tests) and other eleven fatigue tests on eight identical slabs. The results show that cantilever slabs are significantly less sensitive to shear-fatigue failures than beams without shear reinforcement. The static reference tests presented shear failures. Some of the fatigue tested slabs failed due to rebar fractures. They presented significant remaining life after the first rebar failure occurred and eventually failed due to shear. The shear failures exhibited by the static tests on cantilever slabs from this thesis and others from the literature can be reasonably predicted with the Critical Shear Crack Theory (CSCT), provided that the influence of direct load strutting and redistribution of internal forces is accounted for, and that no contribution due to the inclination of tapered members to shear transferring is considered. Simply supported slabs under concentrated loads near linear supports may exhibit shear or punching shear failures. Factors like the ratio between the dimension of the load parallel to the support and the slab width, or the type of loading (monolithic or not) seem to be crucial to determine the failure mode. In this thesis it is proposed to use the CSCT for both shear and non-axis-symmetric punching shear combined with certain proposals on how to determine the internal forces and punching perimeter lengths to assess tests from the literature. The proposed approaches are not fully capable of predicting the correct failure mode, but allow a safe design. A reasonable accuracy is obtained, either knowing or not a priori the correct failure mode. A consistent design approach for shear-fatigue failures of reinforced concrete members without shear reinforcement is also presented, based on FM applied to quasi-brittle materials in combination with the CSCT. This leads to simple, yet sound and rational design equation incorporating the different influences of fatigue actions (minimum and maximum load levels) and shear strength (size and strain effects, material and geometrical properties). The accuracy of the design expression is checked against available test data in terms of Wöhler (S-N) and Goodman diagram

EPFL2015

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

fib Symposium Tel-Aviv 20132013

Static and Fatigue Shear Strength of Reinforced Concrete Slabs Under Concentrated Loads Near Linear Supports

Francisco Manuel Maciel Natário

EPFL2015