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Re: City of Los Angeles hillside ordinance

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The function of the Primary Anchor is to provide a rigid link in the load
path from the diaphragm to the foundation through the foundation stem wall
under it.  In the past, the links between the diaphragm and the foundation
stem wall were often plywood shear walls.  The plywood shear walls tended to
be flexible and/or non-rectangular.  Flexibility allowed the diaphragm to
deflect away from the uphill foundation; non-rectangularity resulted in a
tendency toward progressive damage, sort of like unzipping.  The foundation
stem wall, then, is the rigid lateral load resisting element that works with
the rigid Primary Anchor load to provide a rigid load path to the foundation
for down-slope lateral loads.

Secondary anchors are intended to interconnect the diaphragm and the uphill
foundation at the anchor bolts that transmit the cross-slope lateral loads
to the uphill foundation.  The function of Secondary Anchors is to protect
the cross-slope-shear-resisting system by preventing separation of the
diaphragm from the uphill foundation due to down-slope forces.  There needs
to be no load path to the foundation beyond the Secondary Anchor
connections; the secondary anchors are not a part of the primary lateral
load resisting system that provides the load path to the foundation for
down-slope lateral forces.

Both Secondary and Primary Anchors are required by the Code.  More research
is needed to determine the real required capacity for Secondary Anchors and
to determine whether or not there might be a way to minimize or eliminate
Secondary Anchors by increasing the rigidity of the diaphragm between
Primary Anchors, or by minimizing the Primary Anchor spacing.  I had hoped
that the CUREe program would look into that, but it'll have to be done by
someone else.

Nels Roselund
Structural Engineer
South San Gabriel, CA

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