|Session 7.1 - Structural Design Methods|
Performance based design of structural systems requires realistic predictions of displacement response to earthquake excitation. Considerable research effort has been undertaken internationally with approximate methods published to enable practitioners to make some prediction of peak displacement demand. This paper describes some of the research that has been undertaken to examine the nature of deformation and damping in reinforced concrete frame systems based on a large data base of experimental testing with a particular focus on the substitute structure method. The findings have been incorporated into detailed time history analysis models that have, in turn, been used to calibrate a linear idealisation of response. Comparisons made with other published predictive methods have been encouraging for the limited structural range considered. The results suggest that a substitute structure methodology can be calibrated to provide an improved prediction of maximum displacement response when compared with other published methods.
Keywords: displacement, prediction, concrete, damping, substitute, structure
H.J. Judi, B.J. Davidson and R.C. Fenwick
Two methods of seismic design, namely Direct Displacement Based Design and Displacement Focused Force Based Design, are reviewed together with the Capacity Spectrum Method for the analysis for existing structures. Modifications to these three approaches are proposed to enable them to more accurately predict the influence of different hysteretic behaviour on response. To assess the relative accuracy of these methods, a range of single degree of freedom structures were proportioned by each of the methods for different hysteretic behaviours. The responses of these proportioned structures to four different earthquake records were then determined. A comparison of the maximum displacement recorded in each time history analysis, shows that there is little to pick between the three design approaches in terms of accuracy.
Keywords: displacement focussed force based design, direct displacement based design, capacity spectrum method
A parametric study was conducted on reinforced concrete walls with irregular openings to investigate the influence of critical parameters on the behavior of the walls subjected to earthquake loading. The parameters included the flanges, the axial load, and the sizes and positions of the openings. The parameters were added to the specimens tested by the previous researchers. The study was conducted using a reliable non-linear finite element program. The load paths indicated by the principal compressive stress flows, obtained from the finite element analysis, were studied to understand the influence of these parameters on the force transfer mechanisms of the walls. Analytical results show that the flange and axial loads increased the load carrying capacity of the walls, but caused a decrease of the structural ductility. The sizes and positions of openings showed direct relationships with the levels of strength, stiffness and ductility the walls, with walls containing larger and more frequent openings exhibiting poorer performance. However, the walls could perform satisfactorily if the opening areas were limited within certain levels and the walls were well detailed to ensure the force-transfer mechanisms in the walls could function suitably.
Keywords: wall with irregular openings, parametric study, load path, force-transferred mechanism
Ten models of structural walls were analysed using a reliable non-linear finite element program. The model walls involved six walls with low aspect ratios; including one solid wall, three with irregular openings and two with regular openings, and were identical with the specimens tested by Yanez (University of Canterbury). The other four walls were slender; one solid wall and three with staggered openings, were tested by Ali (University of Michigan). The models were analysed under reversed cyclic loading to simulate the behavior of the walls subjected to an earthquake. Based on the principal compressive stress flows obtained from the finite element analysis, the load paths in the walls with irregular openings were proposed to follow the force transfer mechanisms in the walls. The load paths showed a good correlation with the strut-and-tie models proposed to design these walls. The typical principal stress distributions and internal forces of the sections in some critical zones, at each ductility level, were studied to verify the load paths. Similar force transfer mechanisms were found in the walls with regular openings. A cyclic strut-and-tie model for walls with openings was proposed based on the analytical results that were also analysed.
Keywords: wall with openings, load path, principal compressive stress flow, strut-and-tie model, stress distribution