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Re: Lateral Distribution for Sloped Foundation

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"Findings and Recommendations of the Hillside Buildings Subcommittee" Final 
Report, September 9, 1996, by the City of Los Angeles Department of Building 
and Safety and the Structural Engineers Association of Southern California, 
based on investigation of damage to hillside buildings by the Northridge 
Earthquake of January 1994 is the most thorough discussion of the problem 
that I know of.  Check with the SEAOSC office to see if its still available.  

Los Angeles City Building Code Chapter 94 provides retrofit methods for 
existing hillside buildings.  It apples to buildings on slopes steeper than 
3:1.  I don't know of a similar Code for new buildings, but the problems of 
wood-framed buildings on sloped or stepped foundations are important, and 
should be more widely understood than they are.  

A wood-framed shear wall on a stepped or sloping foundation is most highly 
stressed at the short end, and tends to un-zip down the slope when loaded by 
strong lateral forces.  The damage can occur in the nailed sheathing 
connectors or in the anchor bolts, beginning at the short end of the wall.  

If the up-hill foundation (oriented perpendicular to the down-slope loads) is 
connected to the diaphragm, but the diaphragm is braced against down-slope 
loads by wood-framed shear walls below it, deflection of the shear walls 
results in deflection of the diaphragm away from the uphill foundation, 
damaging the connectors that restrain the diaphragm for forces in the 
parallel-to-slope direction.  This kind of building is a disaster waiting for 
a quake.  In the 1994 Northridge quake, homes were destroyed or seriously 
damaged that would otherwise have done fine on the flat.

Nels Roselund
Structural Engineer