Design and analysis of walls coupled by floor diaphragms

Student: Katrin Beyer
Supervisors: Prof. M.J.N. Priestley, Prof. G.M. Calvi, Dr R. Pinho

Abstract

An eight storey RC coupled wall structure with two walls of different length which are coupled by floor diaphragms with in-plane stiffness only is designed for a drift limit of 2% using the direct displacement based design approach. The design is verified by non-linear time history analysis with five artificial accelerograms which in average match the design spectrum for 5% damping with a spectral displacement of 0.6m at a corner period of 4s. For the numerical analyses the element program "RUAUMOKO" was used. The analysis results agree very well with the design displacement and drift profiles. The  structure is designed and analysed assuming a rigid foundation but the effect of foundation flexibility on the behaviour of the coupled wall system is also briefly investigated.

Since yield displacement is inversely proportional to the wall length and since the longer wall is also the stronger one the longer wall will start yielding before the short wall. After onset of yielding in the long wall the displacement profile of corresponding de-coupled walls will differ. The floor diaphragms connecting the walls enforce a compatibility condition onto the displacement profiles of the walls and lead to an increase in base shear demand on the short wall. This was first recognized by Rutenberg and his coworkers who described the mechanism behind the compatibility forces in several papers ([25] to [28]). It is shown that the coupling effect is largely dependent on the location of the resultant lateral force with respect to the lower floor diaphragms which transfer the greatest coupling forces. In addition it was found that coupling can also cause a reduction in dynamic amplification of shear forces and bending moments in the short wall.

In this study the effect of different modelling assumptions (hysteretic behaviour of walls, flexibility of floor diaphragms, shear stiffness of walls, modelling of plasticity in structural walls) on the analysis results (displacement, drift, shear and bending moment demand) is investigated. It is found that common modelling with rigid floors, unrealistic high shear stiffness of walls and lumped plasticity might overestimate the base shear demand on the short wall. The greatest effect is attributed to the assumptions of lumped plasticity which leads to a "locking-in"of compatibility forces transmitted by the floor diaphragms. This can be accounted for by using a fibre element model when determining the base shear forces on the walls by means of a force-controlled pushover analysis as suggested by Rutenberg [28]. In this case the dynamic amplification factor should be determined according to the "Intensity based" method which links the amplification factor to effective period and the displacement ductility rather than the "traditional method" which relates the amplification factor solely to the number of storeys and which tends to underestimate the system shear demand.

You may download a digital version of this MSc dissertation here.