Jian Zhang and John Zhao
Nonlinear seismic responses of 2-dimensional (2-D) soft-soil basins, with 3 width/depth ratios and excited by 186 accelerograms (including 78 scaled accelerograms) recorded on rock sites, have been evaluated by using 1-D and 2-D models. The curves of mean 5% damped response spectral acceleration amplification ratio for various spectral periods are presented and are used to evaluate the effects of basins and nonlinear soil response. The mean response spectral amplification ratios for peak ground acceleration (PGA) and other spectral periods are found to vary considerably across a basin and for basins with different width/depth ratios. For spectral periods up to 1.0s, the amplification ratios decrease with an increase of input response spectral acceleration, but are nearly constant for longer spectral periods. For each basin at short spectral period, the crossover points vary considerably at various locations. The input response spectral accelerations at the crossover points for spectral periods larger than 0.2s are larger than those from PGA plots. The 1-D models underestimate the amplification ratios for small input response spectral accelerations and provide an approximation for large input response spectral accelerations at the centres of basins with width/depth ratio 6 or larger. The variation of amplification ratios in a basin under weak excitation does not bear any resemblance to that under moderate or strong excitation.
Paper P12: [Read]
Mohamed El Gawady
This paper presents an analytical model for in-plane shear behavior of unreinforced masonry (URM) walls retrofitted using fiber reinforced polymers (URM-FRP). The proposed model idealizes masonry, epoxy, and FRP in a URM-FRP as different homogenous layers. Then, using principles from the theory of elasticity, the governing differential equation of the system is formulated and solved. A simple computer program was developed to combine the solution of the differential equations with material nonlinearity. The material nonlinearity was represented by step-by-step layer stiffness degradation; after each step the equations are resolved linearly. The proposed basic analytical model allows the fundamental investigation of in-plane shear behavior of URM-FRP. Finally, effects of epoxy and masonry allowable shear stresses and FRP axial rigidity on the shear strength of URM-FRP are examined. In addition, comparisons with three existing models are carried out.
Paper P13: [Read]
Alessandro Palermo, Stefano Pampanin, Andy Buchanan and Michael Newcombe
The recent development of laminated veneer lumber (LVL) as an alternative to solid timber or glue-laminated timber has greatly improved the viability of structural timber for the seismic design of certain types of buildings. The low mass, flexibility of design and rapidity of construction all create the potential for increased use of LVL timber in low-rise multi-storey buildings. Based on recent developments in the seismic design of precast concrete for multi-storey buildings, proposals are made for innovative types of jointed ductile connections in LVL timber buildings, based on post-tensioning techniques to assemble structural members for both frame and wall systems. This contribution gives an overview of an going comprehensive research project involving both numerical and experimental investigations. The extremely satisfactory preliminary results of quasi-static cyclic tests of exterior beam-column joint subassemblies are presented as a confirmation of the expected high seismic performance of the proposed solutions for LVL seismic resisting systems.
Paper P14: [Read]
Robert Davey and Ted Blaikie
If a concrete member is very lightly reinforced the ultimate moment capacity may be less than the bending moment required to crack the member. In these circumstances only one crack (or possibly a construction joint) may open at the highly stressed part of the member, and strains in the reinforcing will be concentrated at that location. If the steel yielding is concentrated at one location rather than distributed over a plastic hinge length, as in a normally reinforced member, the strains are much higher and, with low-cycle fatigue effects, could lead to fracture of the reinforcing steel. In this paper a method is proposed for estimating the maximum displacements that members of this type can sustain without steel fracture, and the process is illustrated by applying it to a very lightly reinforced concrete spillway pier. The maximum rotation at the cracked section is controlled by the maximum yield strain that the reinforcement can sustain without fracture, and how far the yield strains can penetrate into the concrete above and below the crack (i.e. yield penetration). The maximum yield strain that the bars can sustain depends on the low-cycle fatigue capacity of the bars. The yield penetration above and below the crack depends on the bond-slip relationship of the reinforcing bars. The method of assessing the displacement capacity is based on the investigations by others into the low cycle, large strain fatigue performance of reinforcing steel, and into the bond slip and yield penetration of reinforcement at crack locations. It is believed that the method provides a useful tool for assessing the ductility of very lightly reinforced concrete sections.
Paper P15: [Read]