Journal of Structural Engineering

In this study, the response of a multihinged articulated offshore tower subjected to different seismic excitations in the presence of random waves was investigated. The study includes near-fault as well as far-fault earthquake ground motions. The contribution of the vertical component to the overall seismic behavior of the articulated tower is examined. The nonlinearities associated with the system owing to variable submergence, drag force, variable buoyancy, and added mass, along with the geometry, are also considered. The nonlinear dynamic equation of motion is formulated considering the Lagrangian approach, which is solved in time domain by the Newmark-beta integration scheme. The results are expressed in the form of time histories and spectral densities of the dynamic responses. Dynamic response quantities such as rotational angle, hinge shear, axial force at the articulation, and bending moment at peak ground acceleration in different seismic sea environment are discussed. The spectral responses under different seismic sea excitation have different contributions to the response evaluation of the tower. Of these, the occurrence of peaks nearer to the average loading frequencies is a clear indication of the influence of seismic excitation on the structural response. Moreover, with the inclusion of the vertical ground motions, the demand in the axial thrust at the base articulation increases, which in turn seems to have an effect on moment and shear demands at their respective critical sections. Therefore, the integrity of the articulating system may be at high risk if subjected to stresses exceeding the design limits.

This study applies nonlinear dynamic analyses to assess the risk of collapse of RC special moment-frame (SMF) buildings to quantify the seismic safety implied by modern building codes. Thirty archetypical RC SMF buildings, ranging in height from 1 to 20 stories, are designed according to ASCE 7-02 and ACI 318-05 for a high-seismic region. The results of performance-based seismic assessments show that, on average, these buildings have an 11% probability of collapse under ground motion intensities with a 2% probability of exceedance in 50 years. The average mean annual rate of collapse of $3.1\times {10}^{-4}$ collapses per year corresponds to an average of 1.5% probability of collapse in 50 years. The study further examines the influence of specific design provisions on collapse safety. In particular, changes to the minimum seismic base shear requirement between 2002 and 2005 editions of ASCE 7 and variations in ACI 318 strong-column weak-beam (SCWB) design requirements are investigated. The study finds that the reduction in the minimum base shear, introduced in ASCE 7-05 and subsequently rescinded, dramatically increases the collapse risk of tall (long-period) frame buildings in high-seismic regions. An investigation of the SCWB requirements shows that the current ACI 318 provisions delay, but do not prevent, column yielding and the formation of story collapse mechanisms. An increase in the SCWB ratio above $6/5$ (1.2) does not significantly improve performance of low-rise frame buildings but may reduce collapse risk for midrise and taller buildings. This study of modern RC buildings is contrasted with the collapse safety of older (nonductile) RC moment-frame buildings in the companion paper.

This study is the second of two companion papers to examine the seismic collapse safety of reinforced concrete frame buildings, and examines nonductile moment frames that are representative of those built before the mid-1970s in California. The probabilistic assessment relies on nonlinear dynamic simulation of structural response to calculate the collapse risk, accounting for uncertainties in ground-motion characteristics and structural modeling. The evaluation considers a set of archetypical nonductile RC frame structures of varying height that are designed according to the seismic provisions of the 1967 Uniform Building Code. The results indicate that nonductile RC frame structures have a mean annual frequency of collapse ranging from 5 to $14\times {10}^{-3}$ at a typical high-seismic California site, which is approximately 40 times higher than corresponding results for modern code-conforming special RC moment frames. These metrics demonstrate the effectiveness of ductile detailing and capacity design requirements, which have been introduced over the past 30 years to improve the safety of RC buildings. Data on comparative safety between nonductile and ductile frames may also inform the development of policies for appraising and mitigating seismic collapse risk of existing RC frame buildings.

The existing designs for deep reinforced-concrete (RC) coupling beams with low shear-span ratios and conventionally reinforced shear stirrups tend to fail in a brittle way with limited ductility and deformability under reversed cyclic loading. This paper aims to develop a new retrofitting method for existing deep RC coupling beams that can enhance the deformability and energy dissipation while maintaining flexural stiffness, improving the beams’ performance during an earthquake. Experiments were conducted to test four half-scale deep RC coupling beams with the same geometry and reinforcement layout but with different retrofitting schemes. One beam was tested without retrofitting to act as a control, whereas the others were retrofitted by a bolted steel plate with or without adding a buckling restraining device. This study reveals that the deformation and energy dissipation of the deep RC coupling beams retrofitted with restrained steel plates improved while the flexural stiffness did not increase. Moreover, by using laterally restrained steel plates, the specimens had better postpeak behavior, a more ductile failure mode, and better rotation deformability.

This paper presents the results of a Monte Carlo simulation study on accidental eccentricity of low-rise office buildings. The study incorporates results of a live-load survey in several office buildings in Mexico City. Probability density functions (PDF) for both intensity and position of live load are initially obtained from this survey. These PDFs are used in the simulation procedure. Additionally, the position and intensity of dead load as well as the stiffness of lateral resisting elements are assumed to be random. Stiffness among columns is assumed to be uncorrelated. Five- and 10-story building models with square and rectangular plans are used. For each case, three types of slabs are included in order to account for different live-load to dead-load ratios. This study shows the effect of the following variables on the estimation of accidental eccentricities: vertical location of the story in the building, dead-load to live-load ratio, number of columns in a story, and the direction of analysis in rectangular plans.

This paper presents a capacity design procedure that aims to achieve good seismic performance for steel-plate shear walls and improve the overall economy of the system. The proposed method uses indirect capacity design principles to identify the infill plates most likely to yield in the design earthquake. Linear models of the boundary columns are used to determine the column design forces. The proposed method is used for the design of two 4-story and one 8-story steel-plate shear walls. Design axial forces and moments in the boundary columns for the three different steel-plate shear walls are compared with nonlinear seismic analysis results, and the results of the proposed procedure are shown to be in good agreement with those of the nonlinear analyses. For comparison, the paper also evaluates other capacity design approaches that have been proposed in the literature.

A model using lifetime functions is used to evaluate the probability of survival of bridge components. The possible outcomes associated with nondestructive inspections (NDIs) are incorporated in an event-tree model. Each time a bridge component is inspected, different decisions can be made. The use of a lifetime function for each component of the structural system enables one to express the probability that the component survives. In theory (i.e., perfect inspection), each NDI should be associated with two possible outcomes: survival or failure. In the first case, no damage is detected and the probability density function of time to failure is updated knowing that the component has survived until the inspection. In the second case, damage is detected and maintenance action is planned. In practice, NDIs are subjected to uncertainties (i.e., imperfect inspections) and detecting or not detecting damage depends on the inspection quality (i.e., probability of detection). For poor-quality inspections, there is a significant risk to overestimate the probability of safe performance. The aim of this paper is to provide a practical methodology for determining optimal NDI strategies for different components of steel bridges. The different types of inspections considered in this paper are visual, magnetic particle, and ultrasonic. An economic analysis is performed and NDI strategies are optimized by simultaneously minimizing both the expected inspection/maintenance cost (i.e., the sum of inspection and maintenance costs) and the expected failure cost. The proposed approach is applied to an existing steel bridge.