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Ductility demands of filtered earthquakes on reinforced concrete frames

University of British Columbia

The Mexico City 1985 and the Loma Prieta 1989 earthquakes demonstrated the destructive nature of seismic waves filtered through soft soils. Key characteristics of filtered ground motions are generally amplified acceleration, increased predominant period, and lengthened duration of strong shaking. Filtering of seismic waves through the soft soils in the Fraser River delta in British Columbia was investigated using the two-dimensional, dynamic analysis program SHAKE. It was found that the peak acceleration and the predominant period of the bedrock vibration was increased by about fifty percent. Surface ground motion amplification similar to the strong motion that occurred in Mexico City in 1985 only occurred when the bedrock motion was of low intensity and vibrated predominantly at the inherent natural period of the soil layers, two conditions which may be brought on by large earthquakes at a large epicentral distance. Filtered earthquakes can result in increasing displacements with decreasing yield strength of a structure, in contrast with the usually accepted equal-displacement criterion. In other words, in a filtered earthquake, the inelastic displacement response curve, which is a line connecting the maximum response of a given structure with different yield strengths in a lateral load-deflection plot, is, for most structures, outward-sloping rather than vertical, implying that higher strengths are required to maintain a certain ductility. These results were indicated by the response of single-degree-of-freedom, elasto-plastic systems in time-step computer analyses. They can also be predicted reasonably accurately by the R-μ equation, a relationship derived from energy principles applied to simple elasto-plastic vibrating systems and developed in this thesis. The strength and ductility demand analyses using the single-degree-of-freedom systems and the R-μ equation further revealed that the response of a structure in any earthquake, filtered or unfiltered, is influenced mainly by the relationship of the fundamental structural period to the predominant period of the ground motion, or by the slope of the velocity response spectrum. These findings point to possible adjustments to the seismic design philosophy of the building code for design of structures for filtered earthquakes. Inelastic, time-step analyses of reinforced concrete frames and shear walls, designed in accordance with the Canadian codes, showed that filtered earthquakes caused more cycles into the inelastic range than unfiltered earthquakes, probably as a direct result of the longer duration of strong shaking of the filtered ground motion. Also, it was calculated that code-designed frames would respond to acceptable ductility levels in the filtered version of the design earthquake for the Fraser River delta. Based on results of the computer analyses of prototype six-storey frames, cyclic loads were applied to five large-scale, reinforced concrete exterior beam-column-slab subassemblies designed and detailed in accordance with the codes. The tests indicated that structures would behave well up to the deflection limits that they were designed for, with only moderate damage and with no influence from differences in the loading history, but larger displacements could cause severe structural damage and could lead to a significant loss of load-carrying capacity. Most importantly, it was shown that maintaining the integrity of the joint core is required to develop the required strength and stiffness of a reinforced concrete frame in a code-level seismic event and that joints designed to satisfy the National Building Code of Canada suffered considerable damage. In order to behave well in a filtered surface ground motion, many structures need to be designed for higher strengths or for larger ductility capacities when compared to structures on rock subjected to the same earthquake.

Thesis/Dissertation

application/pdf

2008-07-29

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http://hdl.handle.net/2429/1190

Civil Engineering

University of British Columbia

UBCV

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