Underground structures with large openings (USLO), especially those that allow natural light and fresh air, have emerged as alternatives to mitigate the weaknesses of traditional underground frame-box structures. For the USLO, two ends of the upper-story beam are generally recognised as weakest regions during strong earthquakes; however, insufficient attention has been paid to improving their seismic safety. This study performed a detailed numerical comparison of the conventional USLO and beam-end horizontal haunch retrofitting USLO under different seismic intensities, and evaluated the effectiveness of the proposed retrofitting scheme. The finite element numerical modelling approach was validated against shaking table test results, where the numerical results were in good agreement with measured data. Based on the validated numerical methods, the two ends of the upper-story beam in the conventional USLO were strengthened with horizontal haunches. Both soil-structure systems were excited by equal earthquake loads. Various seismic responses were compared between the conventional and retrofitted USLO, including structural strain, tensile damage, and story drift. Numerical simulation results indicate that beam-end horizontal haunch retrofitting significantly reduces the tensile strain and maximum damage degree at the ends of the upper-story beam, as well as the upper-story drift, without changing the lower-story drift. Therefore, beam-end horizontal haunch retrofitting is a potentially effective measure for improving the seismic performance of the USLO.

Seismic retrofitting of existing bridges has been in practice for years to meet the stringent seismic requirements set forward by revised design codes. For retrofitting, however, bridge piers are often prioritized while less attention is given to the bridge foundations, which are equally prone to damage under seismic loadings. The current work presents a series of experimental studies in assessing the performance of 2 x 2 pile groups reinforced with micropiles in terms of head-level stiffness and damping under low-to-high levels of static and dynamic loadings, encompassing the influence of loading-induced soil nonlinearity. Practical micropiles inclinations of 0 degrees, 5 degrees, and 10 degrees with respect to the vertical are studied. Experimental results reveal that the head-level stiffnesses of pile groups reinforced with micropiles, contrary to the general expectations, become smaller than the pile group without micropiles at higher levels of applied loading. To elucidate the governing mechanism for such experimentally obtained results, three-dimensional nonlinear finite-element analyses were carried out. Results from the numerical analyses support the experimental results, suggesting that the presence of micropiles may not always increase the head-level stiffness of soil-foundation systems, particularly at higher levels of applied loading where the soil nonlinearity generated at the vicinity of piles and micropiles governs the overall head-level stiffnesses.

This paper presents an experimental and analytical study on a steel slit damper designed as an energy dissipative device for earthquake protection of structures considering soil-structure interaction. The steel slit damper is made of a steel plate with a number of slits cut out of it. The slit damper has an advantage as a seismic energy dissipation device in that the stiffness and the yield force of the damper can be easily controlled by changing the number and size of the vertical strips. Cyclic loading tests of the slit damper are carried out to verify its energy dissipation capability, and an analytical model is developed validated based on the test results. The seismic performance of a case study building is then assessed using nonlinear dynamic analysis with and without soil-structure interaction. The soil-structure system turns out to show larger seismic responses and thus seismic retrofit is required to satisfy a predefined performance limit state. The developed slit dampers are employed as a seismic energy dissipation device for retrofitting the case study structure taking into account the soil-structure interaction. The seismic performance evaluation of the model structure shows that the device works stably and dissipates significant amount of seismic energy during earthquake excitations, and is effective in lowering the seismic response of structures standing on soft soil.

Unpreventable constructional defects are the main issues in the case of steel Moment-Resisting Frames (MRFs) that mostly occur in the rigidities of beam-to-column connections. The present article aims to investigate the effects of different rigidities of structures and to propose Infill Masonry Walls (IMWs) as retrofitting strategy for the steel damaged buildings. A fault or failure to meet a certain consideration of the soil type beneath the building and the current rigidity of connections can cause mistake in determining the performance of building. Therefore, this study comprehensively explores different conditions of soil types, connection rigidities, and implementing IMWs on the 3-, 5-, 7-, and 9-story MRFs. Two nonlinear analyses, namely Nonlinear Dynamic Analysis (NDA) and Incremental Dynamic Analysis (IDA) were performed on 384 steel MRFs having different conditions of defects and the results of the analysis include 3456 performance curves assuming three ground motion subsets recommended by FEMA P695. The results confirm that the proposed retrofitting procedure can effectively improve the performance levels of MRFs, which the connections rigidity of 90 %, 80 %, 70 %, 60 %, and 50 % can reduce the collapse performance level by 2.86 %, 5.35 %, 9.31 %, 16.56 %, and 34.65 %, respectively.