The Influence of Diaphragms on Strength of Beams

In the seismic design of ductile multi-storey moment resisting frames the majority of potential plastic regions are located in the beams. Capacity design requires the maximum likely flexural strength of these zones to be determined. The remainder of the structure is then proportioned to resist the maximum actions that these regions can apply without exceeding their nominal strengths. This process is intended to ensure that inelastic deformation is confined to the potential plastic regions, which have been detailed to sustain the required deformation. Recent research at the Universities of Auckland and Canterbury on the performance of perimeter reinforced concrete frames has shown that interaction of typical floors with the beams can lead to a substantial increase in the flexural over-strength of potential plastic regions. This increase is considerably greater than that predicted by the current Structural Concrete Standard (NZS 1995). An under-estimate of beam over-strengths may have serious consequences in that non-ductile failure mechanisms, such as a column sway mechanism, may form leading to premature collapse of the building in a major earthquake. This paper describes basic theory related to determination of over- strength actions together with some test results obtained from a recent large scale test. 1 BACKGROUND In the last three and a half decades major advances have been made in our understanding of the behaviour of concrete structures in major earthquakes. Research in New Zealand has contributed very significantly to this knowledge. The behaviour of beams, columns, beam-column joint zones, walls and other individual components of concrete structures have been extensively studied and tested. However, the behaviour of floors, which act as diaphragms, has received relatively little attention. There would appear to be two main reasons for this, namely; • Diaphragms are expensive to build in realistic sizes and difficult to test and there are relatively few laboratories that have the resources to undertake such testing; • The behaviour of diaphragms is complex and it is difficult to obtain a quick return on research in terms of published papers. However, the good performance of diaphragms is essential if a building is to have a satisfactory performance in a major earthquake as it is this component which holds the structure together, distributes forces to the different load resisting elements and gives the structure toughness by allowing structural actions to be redistributed when elements lose strength. Diaphragms also restrain columns and walls from buckling. In the last few years four tests have been made to investigate the interaction of diaphragms with ductile concrete moment resisting frames. Three of these were made at Canterbury University and one at Auckland University. At Auckland the main concern was with the potential increase in strength of the beams in the perimeter frame due to interaction with floor slabs. Some of the results and conclusions from this test were reported in the previous conference (Fenwick et al. 2005). At Canterbury, due to the damage that was sustained by the hollowcore units at small displacements and the premature collapse of the floor at displacements less than the design value, concern was focused on developing detailing which would give acceptable behaviour (Matthews, Lindsay, MacPherson).