Practice Periodical on Structural Design and Construction

Investigative reports on the 1994 Northridge earthquake in California indicated significant damage experienced by skewed highway bridges. Specifically, the damage sustained by the Pico-Lyons Bridge near Newhall, California, was noted to have been triggered by the skewed geometry of the bridge. In an effort to portray the behavior of this bridge analytically, a series of simulation studies was conducted using nonlinear finite-element analyses. The objective was to demonstrate that certain damage potentials in skewed bridges during earthquakes can be captured analytically. Dynamic time history and pushover analyses were used to capture the behavior of the superstructure of the skewed bridge using the Northridge ground motion record as the input seismic force. Results from the simulation studies showed that potential damage areas, comparable to those reported in the field investigation of the Pico-Lyons Bridge, can be portrayed through analytical modeling. The study also provided the percentage increase in critical stresses in the superstructure of skewed bridges as the angle of skew increases compared with a comparable nonskewed bridge. The study showed that cases in which the angle of skew is approximately 40°, the percentage increase in stress due to the skewness effect at the end girders can be as high as 50–60%.

Although Dhaka city has experienced any moderate to large earthquakes in the past, some recent ground shakings are certainly indications of its earthquake source and vulnerability. In addition, microseismicity data also supports the existence of at least four earthquake source points in and around Dhaka. However, it is important to appropriately consider the seismic lateral load effect in structural design. A newly adopted technology of seismic design is to isolate the superstructure from the substructure with the use of a base isolator. This paper covers the design of base isolators for a building located in Dhaka, Bangladesh, along with its structural and economic feasibility. A time history is generated for Dhaka, adjusting peak ground acceleration as per seismic region from a nearby recorded earthquake. The response spectrum curve based on the site geology of Dhaka is also generated from this time history. Linear static as well as dynamic (time history and response spectrum) analyses have been carried out for both isolated and nonisolated buildings. Similar analyses have also been repeated for buildings with different heights but similar plan areas. The study reveals that for low-to medium-rise buildings, isolation can reduce seismic force along with some savings in structural cost of the building, though incorporating base isolators increase the overall price and installation cost. A meticulous review indicates that savings may be in the order of 5–10% of the total structural cost of the respective building.

Reinforced concrete plates are the most widely used structural element in civil engineering structures. In the available literature, design methods have been suggested for proportioning the plate systems on the basis of the results of an extensive series of tests and the well-established performance record of various plate systems. An analytical model useful for the analysis of rectangular reinforced concrete plates stiffened by internal stiffening-beams is presented in this paper. This model can be used to predict the collapse load and the corresponding failure mode of the plate system for the given flexural-capacity of the stiffening-beams. The use of the stiffening-beams becomes mandatory in the plate systems to satisfy the serviceability requirements of the design codes, when it was intended to use the thin plates for supporting the external loading, or if there are some architectural constraints of low ceiling height and/or very long beam spans. To validate the analytical results from the proposed model, these are compared with the results from well-established literature on the plate analysis and are found to be in good agreement.

This paper presents a new method for direct time integration of nonlinear structural dynamic problems. In the proposed method, the order of time integration scheme is higher than the conventional Newmark’s family of methods. This method assumes second-order variation of the acceleration at each time step. Two variable parameters are used to increase the stability and accuracy of the method. The result obtained from this new higher-order method is compared with two implicit methods, namely the Wilson-$\theta$ and the Newmark’s average acceleration methods. The proposed method is an alternative quick method that can be written using matrix laboratory (MATLAB) for checking results from a more refined analysis or for a quick calculation with adequate accuracy.