The distribution of lateral forces is proportional to the gravitational load of the structure (Dead Load + 40% Live
Load). Gravity loads dominate the structure’s design, and as such, the beam sizes did not alter. Any dimensional
reductions of columns gained through the use of high strength concrete did not translate into a major reduction in
earthquake loading (reductions of 1.8% and 0.8% in gravity loads for OMRF and IMRF respectively).
Dynamic Analysis Results
The El Centro earthquake (PGA=0.33g) was selected to represent a major earthquake. The time history analyses
were performed using the non-linear dynamic structural analysis program RUAUMOKO [Carr, 1999]. The
members were modeled using the standard beam and column elements provided in the program.
The following response parameters are presented to illustrate the behavioural nature of the structures designed
for different ductility levels. The roof displacements of the gross section stiffness frames do not differ
substantially and are in all four cases less than 0.85% of the building height (0.0085h). It has been suggested by
De Stefano et al. [1995] that the onset of severe structural damage occurs approximately at an overall (roof)
displacement of 0.01h, hence the damage levels in the frames considered here are expected to be moderate. The
frames analysed with the effective stiffness showed much greater variability in displacements, being as high as
1.38%, thus indicating severe damage levels.
In both the OMRF and IMRF, the response with the higher steel reinforcement yield value showed an increase in
column hinging. The relative increase of moment capacities in the sections is not uniform. The beams exhibit a
substantial increase in capacity allowing the columns to yield first. Figures 6 and 7 show the locations of hinges
that have occurred through the earthquake input. The utilisation of HSC lead to a smaller section and a reduction
in longitudinal reinforcement, which has lead to a lower yield moment. It can be clearly seen that there is a shift
to column hinging with higher strength concrete columns. This is not the favoured seismic response, although,
the curvature ductility capacities of all sections were not exceeded. The only exception being frame 3 with EIeff,
where R10 lateral reinforcement would not provide sufficient capacity, although Y12 lateral reinforcement at the
same spacing is sufficient to prevent flexural failure due to the increase in confinement pressure of the core.
Generally, the response and extent of hinging of frames using the effective stiffness was very similar to those
with gross stiffness. The trend indicates that due to reduced stiffness, smaller forces are attracted by the sections
and consequently have reduced curvature ductility demands. The exception is frame 3 where there is a shift from
high beam demands to high column demands. The response with fsy=500MPa was extensive external column
hinging (GL-L5), internal column hinging (L4-L5) and beam hinging (L1-L3). With fsy=650MPa, there was
additional internal column hinging all the way up the structure with reduced beam hinging. The moment