Case studies on seismic assessment of historical buildings using advanced analysis

Seismic risk mitigation in existing buildings requires an engineering assessment of the current condition and expected seismic performance and an identification of possible deficiencies that should be addressed. For heritage and historical buildings in particular, there is significant benefit in using the most detailed analysis methods available to avoid the conservatism inherent in simpler methods and thereby minimise unnecessary interventions and more precisely pinpoint where strengthening is required. On recent heritage projects, Arup has used the analysis software LS-DYNA and a new material model, calibrated against experimental tests on unreinforced masonry components and buildings to carry out (or supplement) seismic assessments. The analysis method (non-linear response history analysis) is not new, but its application on detailed finite-element models of complex historic structures has previously been computationally prohibitive and requires significant analyst experience to deliver reliable results. This paper summarises three of these recent Arup projects: Woltersum Church (Netherlands), Procuratie Vecchie (Venice) and a building cluster in the historical centre of Appingedam (Netherlands). The case studies show that these analyses allow complex features of seismic performance to be considered, such as damage or modifications to the building over time, pounding (separate buildings colliding into one another due to seismic movements) and load sharing between adjacent structures.

Hysteretic Energy Demand under Superposition of Bidirectional Ground Motions

To address the irrationality of making a structure subjected to bidirectional ground motions equivalent to an SDOF system, a new approach method is presented in this paper. The ratio between modal participation factors of the two components of the structure is expressed as γ, and the superposition of bidirectional ground motions is regarded as one-directional earthquake excitation for the equivalent SDOF system. Based on this, an energy balance equation is established, and a method used to estimate normalized hysteretic energy (NHE) is proposed. Analysis of the ratio between NHE (γ ≠ 0) and NHE (γ = 0) is suggested in order to analyze the influence of bidirectional ground motions on hysteretic energy demand, and then, “α1 = NHE (γ ≠ 0)/NHE (γ = 0)” is defined, and bidirectional ground motion records for different soil sites are selected for establishing superimposed excitations. In addition, the period range of 0–5 s for the energy spectrum is divided into 6 ranges. In each period range, the means of α1 are defined as α. The curves of α of constant ductility factors for different soil sites are established, in which α is the vertical coordinate and γ is the horizontal coordinate. Through nonlinear response history analysis, the influence of soil types at different sites, the ductility factor, the ratio of modal participation factors, and the period on the values of α are analyzed. According to the analytical results, correction coefficient αs (the simplified value of α) is obtained so that the hysteretic energy demand under bidirectional ground motions can be determined.

Recycling of damaged RC frames: Replacing crumbled concrete and installing steel haunches below/above the beam at connections

Experimental and numerical studies are presented evaluating the efficacy of a recycling technique applied to a 1:3 reduced scale damaged RC frame. The crumbled concrete at the beam-column connections was replaced with new high-strength concrete. Epoxy mortar was applied at the interface to secure bonding between the old and new concrete. Additionally, the connections were provisioned with steel haunches, applied below and above the beams. The retrofitted frame was tested under quasi-static cyclic loads. The lateral resistance-displacement hysteretic response of the tested frame was obtained to quantify hysteretic damping, derive the lateral resistance-displacement capacity curve, and develop performance levels. The technique improved the response of the frame; exhibiting an increase in the lateral stiffness, resistance and post-yield stiffness of the frame in comparison to the undamaged original frame. This good behaviour is attributed to the steel haunches installed at connections. A representative numerical model was calibrated in the finite element program SeismoStruct. A set of spectrum compatible ground motions were input to the numerical model for response history analysis. The story drift demands were computed for both the design basis and maximum considered earthquakes. Moreover, the technique was extended to a five-story frame, which was evaluated through nonlinear static pushover and response history analyses. Overstrength factor WR = 4.0 is proposed to facilitate analysis and preliminary design of steel haunches and anchors for retrofitting the low-/mid-rise RC frames.

Developing a simplified method for analysis and design of isolated structures with the novel quintuple friction pendulum system under bidirectional near-field excitations

Simplified analysis methods for seismically isolated structures proposed in recent structural codes and specifications are frequently used to reduce the computational effort and to simplify the design procedure, either directly for special cases or for checking the results of nonlinear response history analysis. Of the approximate methods, the equivalent lateral force procedure using the effective stiffness and effective damping is one of the best known. In this study, the simplified method is developed by combining the equivalent lateral force procedure with the capacity spectrum method and evaluated in terms of maximum isolator displacements and base shears for isolated structures with recently invented quintuple friction pendulum isolators , with different geometrical and frictional properties, under two different response spectra with corresponding two different sets of bidirectional near-field ground motions for stiff and soft soils site classes. In order to assess the accuracy of the simplified method, the delivered results of the ELF procedure are compared to those of nonlinear response history analysis, by modelling the quintuple friction pendulum isolator 3D element in OpenSees. Eventually, comments on the accuracy of the simplified method are given to make its applications more appropriate in practical design of base isolation systems.

COLLAPSE SEISMIC RISK ASSESSMENT FOR IRREGULAR MID-RISE BUILDINGS

Mid-rise buildings, which may be used as office or apartment buildings, are very common structures in urban areas. Because these buildings are usually heavily populated, the casualty caused by the collapse of these structures in an earthquake could not be overestimated. Therefore, developing a suitable assessment method to identify these buildings with high collapse risk is an important issue. This paper presents a probabilistic assessment method, which involves nonlinear response-history analysis together with incremental dynamic analysis (IDA), to assess the collapse risk of a mid-rise building, so high-risk buildings and their damage patterns can be identified. This methodology is developed based on the procedure of collapse fragility analysis proposed by FEMA P-58, while the local and damage global criteria that define collapse failure are adopted from ASCE 41-13 and PEER-TBI, respectively. Finally, for demonstration, the proposed procedure is applied to assess the collapse risk of a mid-rise RC building that collapsed in a major earthquake occurred in Taiwan, 2016.

Floor Acceleration Demands in a Twelve-Storey RC Shear Wall Building

The seismic response of acceleration-sensitive non-structural components in buildings has attracted the attention of a significant number of researchers over the past decade. This paper provides the results which improve the state-of-knowledge of the influences that higher vibration modes of structures and nonlinearity of non-structural components have on floor acceleration demands. In order to study these influences, a response-history analysis of a code-designed twelve-storey reinforced concrete building consisting of uncoupled ductile cantilever shear walls was conducted. The obtained absolute floor accelerations were used as a seismic input for linear elastic and nonlinear non-structural components represented by simple single-degree-of-freedom systems, and the main observations and findings related to the studied influences along the building height are presented and discussed. Additionally, the accuracy of the method for the direct determination of peak floor accelerations and floor response (acceleration) spectra recently co-developed by the first author was once again investigated and validated. A brief summary of the method is provided in the paper, along with the main steps in its application. Being relatively simple and sufficiently accurate, the method (in its simplified form) has been recently incorporated into the draft of the new generation of Eurocode 8.

A subset of CyberShake ground-motion time series for response-history analysis

This manuscript describes a subset of CyberShake numerically simulated ground motions that were selected and vetted for use in engineering response-history analyses. Ground motions were selected that have seismological properties and response spectra representative of conditions in the Los Angeles area, based on disaggregation of seismic hazard. Ground motions were selected from millions of available time series and were reviewed to confirm their suitability for response-history analysis. The processes used to select the time series, the characteristics of the resulting data, and the provided documentation are described in this article. The resulting data and documentation are available electronically.