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RE: EQ resistant Design

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This whole topic has been made much more complex than it really is.

The basic provision is that you must consider the effect of the earthquake
acting in the most critical direction for each member.

For many structures this requirement is satisfied by analyzing the
structure in the principle directions.  This assumes that the lateral
resisting elements are parallel to the principle directions.  When dealing
with non perpendicular systems or when the same element participates in
non-parallel walls, braced frames, or moment frames then you need to
consider the effects of most critical direction.

In order to simplify this process the  code provides several methods by
which you can approximate the expected forces using the results of analyses
in two perpendicular directions.

The extreme situation is when you have 4 columns in a square with braced
frames on each of the sides.  In this situation when the direction of
seismic forces is at 45 degrees with respect to any of the sides two of the
columns will see a reaction 1.414 times that that you would obtain when the
forces are parallel to one of the sides.  The critical direction for the
braces would probably be one of the principle directions.  Each of the
approximations is based on this relationship.

If the force in each direction is the same (fx=fy) the SRSS method gives
you (fx^2+fy^2)^0.5=1.414*fx

The 100% and 30% gives you fx*1.0+fy*0.3= 1.3*fx.  The difference between
1.414 and 1.3 results from the fact that some SEAOC Seismology Committee
members believed that 1.3  was conservative for most situations and that
there was little reason for concern in those situations where it wasn't.

Some others will use 0.707*fx+0.707*fy= 1.414**fx.

This is as simple as it gets.  Forget about fancy dynamic effects.  Their
is no difference between the concept of orthogonal effects and most
critical direction.

Mark Gilligan

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