Enhanced safety with post-installed shear reinforcement

The safety of existing concrete slabs against shear may be insufficient for several reasons such as design and construction errors, structural modifications or increased loads. Moreover, the knowledge about shear design has significantly increased over the last few decades and therefore, even structures correctly built according to older codes may not comply with newer codes based on the latest state of the art. This paper presents a method to enhance the one way shear strength of RC slabs or beams by postinstalled shear reinforcement. The reinforcement is bonded into inclined drilled holes from the bottom of the slab and anchored at the end by anchor heads. Compared to other methods, this has the advantages that the strengthening work can be carried out from one side of the structure and that the dimensions of slab are not modified. A series of beam shear tests has been performed to assess the influence of the geometric layout of the reinforcement as well as the anchorage. The tests show a clear strength increase due to the postinstalled reinforcement.

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

Post-tensioned girders with low amounts of shear reinforcement: Shear strength and influence of flanges

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

Assessing the strength of existing structures has become a major issue for structural engineers. Such analyses are often performed after changes of use of the structure or due to new design codes requirements. This is particularly relevant with respect to the shear strength of post-tensioned concrete bridges. Such structures were often designed in the past with fairly low amounts of shear reinforcement and do not comply with current code requirements in terms of amount of transverse reinforcement or shear strength. However, it should be noted that codes of practice cover the design of a wide range of cases and sometimes neglect some load-carrying actions or may be too conservative for assessing others. Therefore, the use of more refined models may potentially increase the predicted shear resistance and avoid unnecessary strengthening of existing structures. In this paper, an investigation on the behaviour of post-tensioned beams with low amounts of shear reinforcement and flanges is presented. First, the results of an experimental programme on twelve reinforced concrete beams (10.0 m long, 0.78 m high) failing in shear are described. The test series is used to analyse the most significant parameters influencing the shear strength and the failure modes. Its results are compared to a number of design codes showing different levels of accuracy. The test results are finally compared to the results of analyses based on elastic–plastic stress fields. This technique shows excellent results when compared to the test results and allows investigating on the role of the various shear-carrying actions, of the prestressing level and on the transverse reinforcement amount with respect to the various potential failure modes.

Engineering structures2013

Seismic behavior of lightly reinforced concrete squat shear walls

Christian Greifenhagen

This thesis addresses the seismic evaluation of existing buildings. In particular, it focuses on the seismic behavior of lightly reinforced shear walls that are not designed to withstand earthquake actions. A shear strength envelope for the assessment of deformation capacity of these non-ductile walls is presented. The approach is the result of experimental investigations and analytical modeling. Existing models for plastic hinges in beams are enhanced in order to determine drift capacity of lightly reinforced concrete shear walls. The static-cyclic behavior of non-ductile, reinforced concrete shear walls is investigated by testing four small-scale specimens of shear span ratio equal to 0.8. The design of the specimens includes reinforcement ratios, and axial force levels in existing shear wall buildings. Although the specimens were expected to fail in brittle shear, low to moderate ductile response is obtained. The deformation capacity, not the shear strength, is found to be restricted by shear failure. It is observed that inherent shear strength of concrete and the concrete compression zone are the principal contributors to the shear capacity of lightly reinforced shear walls. It is also observed that low reinforcement ratios and moderate levels of axial force can efficiently prevent brittle response in shear. The analytical model consists of a plastic hinge over the entire height of the low-rise shear wall. Proposals are made for the strain distribution inside the plastic hinge. Explicit relationships between drift and base shear are established and it is found that the model accurately predicts the envelope curve of static-cyclic loading. The shear strength envelope is formulated by using the analytical model. Criteria for the failure modes of diagonal tension, of concrete crushing, and of sliding enclose the shear strength envelope. In addition, inherent shear strength forms the lower bound of this envelope. The contributions of reinforcement and concrete to shear capacity are formulated in terms of initial strength and strength decay. Accurate prediction of both the ductility supply and the drift capacity obtained in static-cyclic tests is observed. Validation of the shear strength envelope on full-size walls prevalent in existing buildings shows potential for further application. The proposal contributes to more realistic evaluation of shear strength in selected situations where available methods are too conservative. Hence, it allows for both avoiding costly seismic strengthening in such situations and better allocation of resources where they are really needed.

EPFL2006

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

Seismic behavior of lightly reinforced concrete squat shear walls

Christian Greifenhagen

EPFL2006