Return to index: [Subject] [Thread] [Date] [Author]

Re: cable x-bracing

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Mark,
 
Tension-only bracing under hillside houses did very poorly in the 1994 Northridge earthquake.  Typically, if the tension-only bracing yielded, it allowed the main-level diaphragm of the house to rotate until something worse happened.  A few hillside houses collapsed because yielding of tension-only bracing allowed the diaphragm to rotate away from and damage its shear connections to the uphill foundation, and then everything came apart.  A couple was killed by one of the collapsed houses.  Los Angeles City does not allow tension-only bracing of hillside houses any more.
 
Bracing for a deck probably poses considerably less risk than bracing for a house.  However, since there is really no upper limit to possible earthquake forces, you might consider one of two possibilities: design the braces to have tension-compression capacity (the L.A. City approach) or give the cable braces a yield strength about 5 times the code-determined required strength for seismic loading with connections to develop that strength, AND check the deflection/rotation that results from deflection of the cables, determine the resultant forces in the other bracing components, and design them appropriately. 
 
I don't know what the effective E of a cable is, but my impression, based on the way cables are made, is that a cable may need to have greater cross-sectional area than a solid rod to have an equivalent limitation-of-deflection effect.
 
Do cables take more maintenance than structural steel to protect them against corrosion?  Again, because of the way they are made, I suspect so.  Maintenance is too easily deferred and forgotten.  Corrosion of connections in houses built in the 60's contributed to hillside damage -- that's only 30 years.
 
That was my long answer -- my short answer is, "I don't like the idea -- be careful". 
 
Nels Roselund
Structural Engineer