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4) Structural characteristics
R is a factor to account for ductility of the structure and varies between 4 and 8.5 for high ductility
buildings and as low as 1 for low ductility buildings.
5; Weight of Building
W is the seismic weight of the structure. Engineers use force rather than mass units and so when
accelerations are used care needs to be taken with the units to ensure the results are not incorrect by a
factor of g (9.81 m/sec 2 ). Codes use different percentages of live load to be included for the building
weight, which leads to some confusion. Structural engineers would prefer clear guidelines as to the
appropriate factor.
Base Shear
In Hong Kong for earthquake design when the above variability is considered the value of V could be
anything between .0014W and .225 W (a factor of 160). Fortunately very few buildings would be
designed for the low value since they would tend to be over 100 metres in height where the wind loads
are likely to be the dominant lateral load. In the Australian / New Zealand draft seismic loading code it
is recommended that all buildings should be designed for a minimum horizontal load of .01W
Building period & structural response spectra
It is important that structural engineers understand the importance of the building period and the
spectrum response when carrying out the seismic design of building structures.
One concern is the accuracy of calculating natural frequencies. A plot of fundamental frequency versus
height for 163 rectangular plan buildings is shown in Fig 3 (Ellis, B.R. 1980). If the actual measured
frequency is checked against the theoretically calculated frequency the spread of frequency is even less
accurate (Ellis B. R. 1995).
E.
Fig 3. Plot of fundamental frequency V, height for 163
rectangular plan buildings. ISO 4866 'Evaluation and
measurement of vibration in buildings' Annex A -
'Predicting natural frequencies and damping of
buildings.'(Ellis 1995)
7=
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I
T2.5-
O.Z5 0.5 r 0
Frequency
2.0
Most structures are analysed assumed the frame to be skeletal made up of beams, columns and shear
walls. In measurements of real buildings so called non-structural elements can contribute significantly
to the stiffness of the structure resulting in a wide scatter of results.
A comparison of acceleration spectrum responses for different codes normalised to O.OSg for rock and
soil is shown in Figs, 4 and 6. From fig. 3 the rule of thumb calculation for a building height of say 50