Seismic Performance Of Reinforced Concrete Coupled Walls

Following the 2010/2011 Canterbury Earthquakes, an investigation by the Canterbury Earthquakes Royal Commission (CERC) considered the performance of a range of buildings in Christchurch. Several of the buildings investigated by the CERC included reinforced concrete coupled walls, which are comprised of two wall piers linked (or coupled) by a series of coupling beams at each floor level. Notably the coupled wall buildings investigated by the CERC were observed to have performed undesirably when compared to their design intent. It was found by the CERC that these coupled walls tended to display higher strengths and lower ductility capacity than was intended in design. The postulated reason for this behaviour was that interaction between structural components strengthened the coupling beams by restraining the tendency of the coupling beams to axially elongate. To better account for this interaction in design practice, it was recommended by the CERC that the behaviour of coupled walls be investigated further. In this study, structural interaction between coupling beams and floors was first considered using finite element software VecTor2. It was found that the floors tended to restrain the elongation of coupling beams and to cause large coupling beam strength increases. The extent of floor that was activated to restrain coupling beam elongation being found to be dependent upon the arrangement of the floor. Existing provisions of NZS 3101:2006 for upper bounds on floor effective widths were found to be valid for assessment of the maximum coupling beam strength amplification caused by floor interaction. Analysis of a series of seismically loaded coupled walls interacting with floors was undertaken using VecTor2 software. In agreement with the findings of the CERC, axial restraint of coupling beams was found to have a large impact on coupled wall performance. Coupling beam strengths were measured up to 300% of their design strength, which tended to change the strength hierarchy of the coupled wall. In particular it was found that many existing coupled walls would have behaved similarly to a single cantilever wall with penetrations because the coupling beams were too strong to yield. These coupled walls tended to display lower energy dissipation and higher wall pier damage than assumed in design. The coupled wall provisions proposed (at the time of writing) in the 2014/2015 NZS 3101:2006 Amendment were found to over-estimate the impact of the floor systems on restraining coupling beam elongation. However these provisions did not include the effect of the wall piers restraining coupling beam elongation, so overall coupled wall overstrength capacities tended to be under-predicted. As an approximate method of accounting for axial restraint in design of coupled walls, it was recommended that redistribution of design demands be used to reduce the coupling beam design capacity and to achieve a more desirable coupled wall behaviour.