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structural engineers. Although deflection calculations in reinforced concrete design only predict actual
deflections within 25 to 30% this is usually not of concern to structural engineers since they only have
the need to use simplified code methods of checking using BSS110 Part 1. In BS8110 Part 2 the more
detailed methods of calculating deflection are not frequently referred to and are generally only used in
automated structural computer programs.
It is likely that the uncertainty, variability and perceived requirement to have a better than normal
understanding of geotechnical engineering will be one of the hurdles preventing structural engineers to
easily make the changes required to incorporate seismic loads into their structural design.
Unfortunately there are a number of terms and different ways of presenting seismic risk that can be
confusing for the non-specialist. It is important therefore when discussing the subject that all parties
have a common understanding of the definitions adopted.
The best way to train structural designers may be to play down the load variability and all of the
geotechnical complexities and concentrate on the importance of detailing for ductility, providing well
defined load paths and economical yet robust design. As Bubb (1998) said in a keynote address it
should be realised that in building structures that "if there is a weakness, the earthquake will seek it out
and even cause collapse and therefore loss of life. The earthquake will also find the weakness that the
ignorance builds in." As found in Kobe, Taiwan, and Northridge, failures commonly occur at changes
in strength or stiffness, because non structural walls affected the structural response or because the
joints have not performed as well as expected. Bubb (1998) follows on by saying "It is the earthquake
and the principles of mechanics that determines which are structural and which are non structural
elements. Common sense and 'normal' practice would wrongly assume that the architect, the owner or
the builder could determine which are non-structural elements".
Seismic loads
The main difference in seismic horizontal loads to wind loads is that for wind the load at right angles
to the longer side is usually the critical load case. However for long narrow buildings seismic loads are
dominant in the direction normal to the short side i.e. at right angles to the maximum wind load
direction because the load is dependent on mass, which is the same in any direction. This means that
the required relative seismic capacity of medium height and tall buildings in the long direction will
need to be higher than that required for wind. However this may not control the design since concrete
core buildings frequently have much greater capacity than required in the long direction because the
reinforcement provided for the short direction controls the design.
Static Equivalent Seismic Design Equation
For concept design the lateral loads should be calculated from the static equivalent design equation
Eqn. 1. but can be visualised most easily using a spectrum response curve see Fig. 4 and 5.
7TC
^~W (1)
where V - Base shear, 2 = Zone Factor, I = Importance factor (Building use),
C = Dynamic factor, R = Structural Characteristic, W = Weight of the building.
Although the equation is simple it is somewhat difficult to understand in detail because the variability
of the factors is greater than usually encountered by structural engineers.