International Concrete Abstracts Portal

The effect of excessive debonding of prestressed strands can present a marked impact on the shear performance of prestressed concrete girders. This effect is taken into account by the AASHTO LRFD Specifications and the maximum percent of strand debonding is limited to 25% of the total number of strands. This paper presents field and experimental results demonstrating the effect of strand anchorage on the shear behavior of prestressed concrete girder. Two case studies presented and discussed in this paper show the effect of excessive strand debonding and proper strand anchorage on the shear performance of prestressed concrete girders. The shear behavior of the bridge girders under different load cases was predicted up to failure using nonlinear finite element analysis. The analysis accounts for the influence of strand debonding at the ends of the girders on the shear capacity. The applicability of the AASHTO bridge design specifications for the calculation of the nominal shear sectional capacity of prestressed girders with shielded strands is demonstrated. The interaction between shear and bond, and the favorable effects of strand anchorage on the shear capacity of the girders is highlighted.

Solid slabs supported directly on columns offer elegant solutions for short span bridges. For buildings, flat slabs are becoming more and more popular. Solid slabs are easy and economical to construct, and for buildings they offer ease of mechanical installations and maximum story height. The slab thickness is primarily governed by deflection/vibration limits or punching shear. The latter may lead to a very brittle and sudden type of failure that can be avoided by increasing the slab thickness, using high-strength concrete, or providing shear reinforcement. The most effective of these is shear reinforcement because it deals directly with the localized problem of punching and it also prevents brittle failure. The main challenges of shear reinforcement are the installation and the anchorage of the shear elements. These problems are most adequately solved by using headed studs mounted on rails. To investigate the effect of layout and the extent of the shear reinforcement on the punching shear resistance, six slab-column connections were tested at the University of Calgary. The tests showed that the strength of the connections and their ductility were significantly enhanced by shear stud reinforcement. They also demonstrated that the radial layout of the studrails as required by Eurocode 2 exhibited no advantage in performance over an orthogonal layout where the studrails were aligned with the orthogonal reinforcing mesh. This is the standard arrangement in North America. The latter is preferable because of the minimal interference with the non-prestressed or prestressed flexural reinforcement. In the present test series, maximum ductility was achieved by extending the shear reinforcement to 4d from the face of the column. Where the shear reinforcement was only extended to 2d from the face of the column, an increase in strength was recorded, but the mode of failure still had to be classified as brittle because the failure surface occurred outside the shear reinforced zone. Providing shear studs spaced at d/2 to a distance 2d from the column and spaced at d between 2d and 4d significantly increased the ductility of the connection. Ductile behavior is especially important for the performance of slab-column connections of bridges and buildings in seismically active zones.

Five specific limitations to the existing shear design methodologies of the AASHTO LRFD Bridge Design Specifications and ACI 318-08 are discussed: (1) the issues resulting from the fact that what has been tested in the laboratory is not representative of what is built in the field for bridge structures and therefore where additional laboratory testing is needed particularly for bridge members; (2) the equivalency and non-equivalency of the treatment of axial load and prestress in shear provisions; (3) the basis for the minimum and maximum shear reinforcement requirements or limits for members and why those requirements differ in the AASHTO LRFD Specifications from those in ACI 318-08; (4) shear design considerations for the end regions of bridge girders and the need to design for the effects of the funneling of the shear force into the support and the balancing of the tension caused by shear at the face of the support; and (5) the relative variations in the components of the shear resistance with increasing load and changes in member behavior and the significance of those variations for the limitations to the existing shear design concepts of the AASHTO LRFD Specifications and ACI 318-08.

Reinforced concrete (RC) bridge columns are subjected to combined flexural, axial, shear, and torsional loading during earthquake excitations. This combination of loading can result in complex flexural and shear failure. The work presented herein included an experimental study conducted to understand the behavior of RC circular columns under combined loading. The main variables considered are the ratio of torsion to bending moment, and the level of detailing for high and moderate seismicity (low or high spiral reinforcement ratio). This paper presents the results of tests on eight reinforced concrete columns subjected to cyclic bending shear, cyclic pure torsion, and various levels of combined cyclic bending and torsional moments. It discusses the effects of combined loading on the hysteretic lateral load-deformation response, torsional moment-twist response, reinforcement strain variations, and plastic hinge characteristics. It also includes diagrams of interaction between bending and torsional moment. In addition, the results of this study highlight the significance of proper detailing of transverse reinforcement and its effect on torsional resistance under combined loading. Test results demonstrate that combined loading decreases both flexural and torsional capacity. Further, they show a significant improvement in the performance of columns with an increase in the spiral reinforcement ratio.

The two-dimensional design and behavior of typical reinforced concrete (RC) structures has been extensively studied in the past several decades. Such design requires knowledge of the constitutive behavior of reinforced concrete elements subjected to a biaxial state of stress. These constitutive models were accurately derived from experimental test data on representative reinforced concrete panel elements. The true behavior of many large complex structures, however, requires knowledge of the constitutive laws of RC elements subjected to a triaxial state of stress. The goal of the proposed work is to develop new constitutive relations for RC elements subjected to a triaxial state of stress. To accomplish this task, largescale tests on representative concrete panels need to be conducted. The University of Houston is equipped with a unique universal panel testing machine that was used for this purpose. This universal panel tester is the only one of its kind in the United States, and the only one in the world that allows for both displacement and forcecontrolled load application through its newly upgraded servo-control system. The panel tester enhanced the understanding of the in-plane shear behavior of reinforced concrete elements. Recently, 20 additional hydraulic cylinders were mounted in the out-of-plane direction of the universal panel tester to facilitate testing of concrete elements subjected to tridirectional shear stresses. The addition of these cylinders makes the panel tester the only one of its kind in the world that is capable of applying such combinations of stresses on full-scale reinforced concrete elements. This paper presents the details of the mounting and installation of the additional hydraulic cylinders on the universal panel tester, and preliminary results of large-scale tests of a series of RC panels subjected to three-dimensional shear loads.