An experimental investigation into the degradation of load-paths in damaged diaphragms was conducted to provide
answers to the New Zealand structural engineering community following concerns that strut-and-tie load-paths could
not cross wide cracks that develop around the floor perimeter during earthquake loading demands. A full-scale superassembly concrete moment frame specimen with a hollow-core flooring system installed was subjected to realistic drift
deformations to induce damage in the floor, followed by in-plane shear deformation demands to assess the ability of
the diaphragm to transfer load between frames at different floor damage levels. It was found that compression struts
could form across much wider cracks in floors than previously anticipated. This was due to contact compressive
stresses forming via loose aggregate that lodged within rugged sinusoidal wide floor cracks. Additionally it was found
that diaphragm compression struts can only transfer to the primary lateral load resisting frame through beam plastic
hinges acting in minor axis shear following gaps opening between the floor and columns at moderate drift demands.
Smooth floor-to-column interfaces did not provide the same residual rubble aggregate binding compressive load path
observed in cracks within the floor. The primary driver of diaphragm shear stiffness degradation was found to be
torsional softening of the perimeter beams of the floor. This was caused by simultaneous bi-directional demands applied
to longitudinal beam bars and a phenomenon known as the bowstring effect applying large torsional demands through
the beam-floor continuity reinforcement. The diaphragm strength and rate of shear stiffness degradation was found
to be highly reliant on earthquake directionality. A set of generalised equations was developed to describe the rate of
diaphragm shear stiffness degradation with respect to magnitude and directionality of drift demands.
Part II of II in this journal series provides analysis of the cause and rate of diaphragm stiffness degradation based on
instrument data and visual observations as described in Part I (Parr et al. 2022). Analysis of the formation of residual
diaphragm load-paths and the relationship between torsional softening of beams with simultaneous bi-directional
demands is also investigated
