Earthquake Engineering and Structural Dynamics

Taher, Sdiq Anwar; Li, Jian; Fang, Huazhen

doi: 10.1002/eqe.3382pmid: N/A

After earthquakes, structural response such as interstory drift is critical for accurate structural assessment for buildings. Typically, direct integration of absolute floor accelerations does not yield reliable floor displacements due to the long‐period drifts caused by noise, a widely acknowledged challenge. In this case, model‐based estimation strategies can be employed, which often require the ground input for better accuracy. However, in many cases the ground input may not be available for lack of instrumentation or even be unmeasurable due to soil‐structure interaction, hence needs to be estimated. Earthquake input estimation in this case is particularly challenging due to the lack of direct feedthrough term, leading to low observability of system input. As a result, input estimation is sensitive to modeling error, measurement noise, and incomplete measurements. To address this challenge, a hybrid strategy is proposed to estimate earthquake input, states, and acceleration response at unmeasured floors using limited absolute floor acceleration measurements. First, the earthquake input is estimated through a maximum a posteriori (MAP) estimation method, and then the estimated input is combined with Kalman filter to further estimate states and unmeasured responses. A comprehensive assessment was performed through a series of numerical and experimental tests including a comparative study with a popular online model‐based method. While the online method demonstrated certain sensitivity to modeling error and measurement noise due to weak observability, the proposed strategy showed robustness and accuracy under realistic and challenging conditions. Further verification is also performed using a real‐world building structure that experienced earthquake events.

Vassiliou, M. F.; Broccardo, M.; Cengiz, C.; Dietz, M.; Dihoru, L.; Gunay, S.; Mosalam, K. M.; Mylonakis, G.; Sextos, A.; Stojadinovic, B.

doi: 10.1002/eqe.3386pmid: N/A

Rocking motion is sensitive to the boundary and initial conditions of a rocking structure, making experiments nonrepeatable. Thus, the claims that numerical rocking motion models are not only inaccurate but that all rocking structures behave unpredictably. Hence, rocking is not used as a seismic design approach. This paper revisits the issue of rocking motion unpredictability. Seismic behavior of structures is inherently stochastic, because the loading is stochastic. Therefore, the question of interest is not whether models can predict the seismic response to a single ground motion, but if the statistical characteristics of the ensemble of responses to a set of ground motions that define the seismic hazard can be predicted. For this purpose, a rocking podium, which is a three‐dimensional structure comprising an aluminum slab supported by four tubular steel columns, was tested on a shake table excited by two sets of 100 consistently generated ground motions. It was found that the cumulative distribution function (CDF) of the experimentally obtained displacements is statistically stable. Next, a blind prediction contest was organized. The contestants were invited to predict the CDFs of the slab lateral displacement. They were able to predict the slab displacement CDF relatively well. Both finite element and discrete element modeling approaches were used, but no clear pattern emerged as it was found that the performance of either approach depends on the input parameters used and the assumptions made. It was also observed that the contestants who did not use Rayleigh damping in their models produced better predictions.

Buccella, Nathan; Wiebe, Lydell; Konstantinidis, Dimitrios; Steele, Taylor

doi: 10.1002/eqe.3385pmid: N/A

Controlled Rocking Braced Frames (CRBFs) have been developed as a high‐performance seismic force resisting system that can self‐center after an earthquake and avoid structural damage. A CRBF is designed to uplift and rock on its foundation, and this response is controlled using prestressing and energy dissipation devices that are engaged by uplift. Although CRBFs have been shown to have desirable structural performance, a comprehensive assessment of this system must also consider the performance of nonstructural components, which have a significant impact on the safety and economic viability of the system. The purpose of this paper is to evaluate the demands on nonstructural components in buildings with CRBFs in comparison to demands in a reference codified system, taken here as a buckling restrained braced frame (BRBF), as well as to identify which design parameters influence these demands. The responses of various types of nonstructural components, including anchored components, stocky unanchored components that slide, and slender unanchored components that rock, are determined using a cascading analysis approach, where absolute floor accelerations generated from nonlinear response history analyses of each system are used as input for computing the responses of nonstructural components. The results show that the downside of maintaining elastic behavior of the CRBF members is, in general, larger demands on nonstructural components compared to the BRBF system. These demands are not highly influenced by impact during rocking or by the supplemental energy dissipation provided, as the vibration of the CRBF in its higher modes is primarily responsible for the higher demands.

Inamasu, Hiroyuki; Castro e Sousa, Albano; Güell, Gerard; Lignos, Dimitrios G.

doi: 10.1002/eqe.3392pmid: N/A

This paper explores the concept of dissipative exposed column base connections by means of anchor rod yielding. This concept aims at enhancing the seismic performance of low‐rise steel moment‐resisting frames (MRFs). A mechanics‐based model is proposed that explicitly simulates a broad range of damage mechanisms observed in exposed column bases. The model is implemented in a frame finite element analysis program, and its hysteretic performance is validated with experimental data available in literature. Incorporating this modeling feature in standard nonlinear response history analyses offers new insights in steel MRF responses. It is shown that when low‐rise steel MRFs adopt a dissipative anchor‐yield column base concept, they are less likely to experience residual story drift ratios during low probability of occurrence seismic events. It is also found that low‐rise steel MRFs designed with nondissipative exposed column base connections are more prone to demolition than dissipative ones, due to their higher column residual axial shortening, particularly when ground motion duration is an important feature of the seismic hazard. Limitations of the present work are also discussed.

Perdomo, Camilo; Monteiro, Ricardo

doi: 10.1002/eqe.3389pmid: N/A

Simplified seismic assessment procedures relying on displacement‐based formulations have recently been implemented for the evaluation of the seismic performance in buildings and bridges. For bridges, displacement‐based assessment procedures have been formulated to predict the displacement response at several performance levels defined based on the expected damage on the structural elements. These displacement profiles have been subsequently used to estimate the performance level achieved under a given seismic hazard intensity and to develop damage fragility curves. For buildings, these concepts have been extended to compute economic losses in agreement with current performance‐based earthquake engineering procedures. In this study, existing displacement‐based formulations for the approximate seismic assessment of single‐column multi‐span continuous RC bridges, with non‐sacrificial shear keys for the pier‐to‐deck connections, are extended for the computation of expected annual losses under transverse direction excitation. Additionally, an iterative response spectrum analysis procedure, which directly considers higher mode effects, is proposed for computing the displacement response, and these results are also used to estimate direct expected annual losses. Results show that both procedures tend to compute higher losses when compared with those obtained with a fully probabilistic loss assessment framework, using demand estimations computed with nonlinear response history analysis. Results also show that the simplified loss assessment procedure is highly sensitive to the definition of the collapse fragility curve.

Koutras, Andreas A.; Shing, P. Benson

doi: 10.1002/eqe.3388pmid: N/A

Modern design codes and performance‐based earthquake engineering rely heavily on computational tools to assess the seismic performance and collapse potential of structural systems. This paper presents a detailed finite‐element (FE) modeling scheme for the simulation of the seismic response of reinforced masonry (RM) wall structures. Smeared‐crack shell elements are combined with cohesive discrete‐crack interface elements to capture crushing and tensile fracture of masonry. Beam elements incorporating geometric as well as material nonlinearity are used to capture the yielding, buckling, and fracture of the reinforcing bars. The beam elements are connected to the shell elements through interface elements that simulate the bond‐slip and dowel‐action effects. An element removal scheme is introduced to enhance the robustness and accuracy of the numerical computation. The material models and interface elements have been implemented in a commercial FE analysis program. The modeling scheme is validated with data from quasi‐static cyclic tests on RM walls as well as with results from shake‐table tests on RM building systems.

Qi, Liangjie; Kurata, Masahiro; Ikeda, Yoshiki; Kunitomo, Keiichiro; Takaoka, Masashi

doi: 10.1002/eqe.3390pmid: N/A

Past earthquakes highlighted the vulnerability of the suspended ceiling, especially in earthquake‐prone countries like Japan; the post‐earthquake reconnaissance showed that damage to ceiling systems led to immeasurable economic loss and disturbance of the timely rescue of casualty. The existing studies have mostly focused on the seismic performance of regular square and leveled ceilings, whereas the inevitable requirements to accommodate air ducts, nonstructural piping, and electrical equipment resulted in a two‐elevation ceiling system. This paper reports a series of full‐scale shake table tests on a typical two‐elevation ceiling system, which includes electrical equipment, piping system, and commonly used suspended ceiling structure. Experimental observations showed that the two‐elevation ceiling system performed well under earthquake excitation. No fallen panels or overall system collapse occurred during shaking. This paper discusses the effects of a temporary‐positioning‐bracing bar (TPBB) between two ceiling elevations and peripheral constraints by surrounding wall on the acceleration and displacement response and torsional behavior in the ceiling system. Then, a simplified analytical model was established and verified with the experimental response of different ceiling configurations. The test results indicated that a sufficiently strong TPBB is essential to reduce the relative displacement between two ceiling elevations, and it ensures the integrity of two ceiling elevations in the ceiling system. In conclusion, the TPBB shall remain in the ceiling system after the construction stage.

Yu, Helu; Wang, Bin; Li, Yongle; Gao, Zongyu

doi: 10.1002/eqe.3394pmid: N/A

This paper presents a semi‐analytical method for stochastic response analysis of non‐classically damped linear structures subjected to non‐stationary seismic excitations modeled by arbitrary time‐frequency modulating functions. In this method, the inherent randomness of seismic excitation process is characterized by a set of orthogonal random variables obtained using the spectral representation method. Then, by adopting piecewise polynomials to interpolate the time‐frequency modulating function of seismic excitation, an explicit expression for the structural stochastic response in term of the orthogonal random variables is derived using the complex modal analysis. This expression can be used not only to efficiently predict the seismic response of structures subjected to an arbitrary excitation sample, but also to directly evaluate the structural response statistics in the time domain. Finally, a robust algorithm is proposed to determine the optimal locations of segmentation points for the piecewise polynomials, the computational efficiency can be further improved. In the numerical application, a typical non‐classically damped structural system subjected to two models of non‐stationary seismic excitations are studied. The classical evolutionary spectral method and the Monte Carlo simulation are used to verify the accuracy and efficiency of the proposed method. The effects of several parameters such as the order of polynomials on the performance of the proposed method are investigated.