International Concrete Abstracts Portal

The seismic design of building diaphragms is one of the most vexing tasks today. Diaphragms are the structural elements primarily designed to transfer in-plane forces to the lateral force-resisting system. Design challenges increase when modeling diaphragms in nonlinear response-history analyses. The main complexity lies in choosing a computationally efficient model and establishing the demands and force distribution throughout the diaphragm. This paper describes two commonly used methods and compares the results in the design forces. A reinforced concrete core-wall building with a flat-slab transfer diaphragm is presented as a case study. Diaphragms were modeled with linear shell elements and the stringer-panel model. Differences in the magnitude of the forces are not significant, although visible differences are observed in the presentation of the results. The stringer-panel model shows a clear and unambiguous load path for the in-plane forces, making it an attractive alternative for the analysis of diaphragms.

High-strength concrete (HSC) with a low water-cement ratio (w/c) may experience large autogenous shrinkage (AS). When shrinkage of concrete is restrained by the subgrade, foundation, or other part of the structure, HSC is more prone to crack. However, studies devoted to the early-age cracking resistance of reinforced HSC under uniaxial restrained conditions and adiabatic conditions are still lacking. In the current research, the effect of reinforcement percentage and reinforcement configuration on the temperature history, shrinkage, stress, and creep behavior of reinforced HSC at early age was analyzed using the temperature-stress test machine. Test results showed that reinforcement could effectively restrain the development of concrete shrinkage and creep. The cracking resistance of HSC increased with increasing reinforcement percentage, evaluated by the integrated criterion. With the same reinforcement percentage, reinforced HSC with distributed reinforcement along with a proper thickness of concrete cover exhibited higher cracking resistance compared with that of central reinforcement.

This paper documents an experimental study on load-transfer mechanisms of six precast concrete (PC) frames with different emulative connections to resist progressive collapse. Load-transfer mechanisms, such as compressive arch action (CAA) and catenary action (CA), were observed during the loading history, while the CA dominated the ultimate load capacity. The robustness of PC frames assembled by mechanical couplers or U-shaped bars was evaluated experimentally and analytically. To improve the robustness of PC frames assembled by U-shaped bars, two refined strategies were introduced: 1) adding additional straight bars in the trough connection; and 2) replacing U-shaped deformed bars with plain bars. It was found that, with the additional straight bars in the beam troughs, the CAA capacity, CA capacity, and deformation capacity can be increased. Replacing U-shaped deformed bars with plain bars can improve the CA capacity and deformation capacity effectively, while it may decrease the CAA capacity slightly. To further understand the load-transfer mechanisms of PC frames with different connections, an analytical elaboration was conducted. It was demonstrated that, at the CAA stage, shear force (related to flexural action) dominated the load-transfer mechanisms. At the CA stage, shear force still dominated the load-transfer mechanisms of the beam-side column interface, while tensile axial force dominated the load-transfer mechanisms of the beam-middle column interface.

An accurate estimation of the seismic demands on bridges depends on, among others, the correct assumption of the column’s force-displacement hysteresis. However, there is little guidance in seismic codes regarding the choice of a hysteretic model for nonlinear analysis of bridges containing reinforced concrete (RC) circular columns. This paper addresses this shortcoming by proposing the unloading (α) and reloading (β) factors for the Takeda degrading stiffness hysteretic model that may be employed for the nonlinear time-history analysis of bridges containing circular RC columns. These recommendations were obtained through analysis of the reloading/unloading stiffnesses and the energy dissipation of 24 previously conducted cyclic quasi-static tests of circular RC columns. Moreover, evaluation of the hysteretic damping of the experiments allows an equation to estimate the equivalent viscous damping as a function of displacement ductility level to be introduced. A sensitivity analysis on the mean values of analysis results shows an impact of up to 20% in terms of peak deformation demands.

For tall buildings, wind demand under extreme wind loads is so large that design based on conventional elastic behavior can be difficult. A practical solution is to permit inelastic behavior to introduce hysteretic damping and reduce design wind force. Therefore, the inelastic behavior of structures subject to wind loads should be thoroughly investigated. The presumption is that applications currently applied in inelastic seismic design can be employed in inelastic wind design of concrete buildings. In this research, behaviors of elastic, bilinear, and self-centering single-degree-of freedom systems under along-wind load were studied for various design wind speeds using nonlinear time-history analysis. Self centering systems—in particular, unbonded post-tensioned concrete systems—given their wide use and high potential and flexibility for self-centering behavior, were found to have smaller maximum displacement than bilinear systems, with differences being considerable at higher wind speeds. Results found highlight the role of post-yield stiffness in reduction of maximum displacement. Self-centering systems were also found to be highly influenced by reverse-yielding values. Systems with reverse yield strength equal to the standard deviation of the elastic force (approximately 20% of [mean + background + resonant components]) demonstrated minimum displacement. In addition, design parameters such as ductility, overstrength, and displacement factor are reported for inelastic design based on the concept of a response modification factor.