A contribution to seismic shear design of R/C walls in dual structures

Abstract

The paper focuses on the seismic response of walls in dual (frame + wall) structures, with particular emphasis on shear behaviour. Although dual structures are widely used in earthquake-resistant medium-rise and high-rise buildings, the provisions of modern seismic codes regarding design of walls for shear are not fully satisfactory, particularly in the (common) case that walls of substantially different length form part of the same structure. Relevant provisions of the leading seismic codes are first summarised and their limitations discussed. Then an extensive parametric study is presented, involving two multistorey dual systems, one with identical walls, and one with walls with unequal length, designed to the provisions of Eurocode 8 for two different ductility classes (H and M). The walls of the same structures are also designed to other methods such as those used in New Zealand and Greece. The resulting different designs are then assessed by subjecting the structures to a suite of strong ground motions, carrying out inelastic time history analysis, and comparing the results against design action effects. It is found that although modern code procedures generally lead to satisfactory performance (differences among them do exist), the design of walls seems to be less appropriate in the case of unequal length walls. For this case a modified procedure is proposed, consisting of an additional factor to account for the relative contribution of walls of the same length to the total base and an improved envelope of wall shears along the height; this improved method seems to work better than the other procedures evaluated herein, but further calibration is clearly required.

Seismic shear force magnification in RC cantilever structural walls, designed according to Eurocode 8

“…For walls which enter far into the inelastic range (ductility class high-DCH structures), larger shear magnifications are expected, and the factor ε should be calculated using the expression proposed by Keintzel (1990), which explicitly takes into account the effect of higher modes in the inelastic range and flexural overstrength as explained in Fardis (2009). However, recent research work performed by Rutenberg and Nsieri (2006), Kappos and Antoniadis (2007) and Priestley et al (2007) has shown that the Eurocode procedure needs some modifications in order to provide an improved estimate of shear magnification factors. In particular, the work of Rutenberg and Nsieri has demonstrated that the ε factor is too low for DCM walls, and is also frequently conservative for DCH walls.…”

Evaluation and suggestions for improvement of seismic design procedures for R/C walls in dual systems

“…Figure 1 shows the configuration in plan, as well as in elevation, of the 15‐storey building with the dual system studied here. In a previous study by the authors 7 two similar configurations of medium‐rise (9‐storey) dual systems were studied. In the first configuration only two central (6 m) walls were present, whereas the second one includes six wall elements of different lengths (top of Figure 1).…”

“… Configurations in plan and in elevation and geometric data for the 15‐storey building (same plan configuration applies to the 9‐storey building, see 7). …”

“…In the following, each of the 15‐storey structures is denoted by its ductility class and the a g considered for design, e.g. DCH0.16 g. Details for the 9‐storey dual systems are omitted here for economy of space, but are given in full in 7; it is noted that all four 9‐storey structures were designed for a g = 0.16 g .…”

“…Meanwhile, the proposal of Keintzel 4 was adopted by Eurocode 8 for high‐ductility structures (DC H). Other researchers have addressed issues not properly covered by codes, such as the co‐existence of walls of substantially different lengths 5–7 or the (direct) displacement‐based design of dual systems 8, 9.…”