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URM in seismic zones after 1933

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Up until the 1985 UBC, Chapter 23 had the requirement that "... all concrete 
and masonry that resists seismic forces is to be reinforced to conform to 
the requirements for reinforced concrete and masonry."

1997 UBC Section 2106.1.12.4 has the minimum requirements for masonry 
construction in Seismic Zone 3 and 4.

1997 UBC Section 2106.1.12.3 has the minimum requirements for masonry 
construction ins Seismic Zone 2, however, I would caution against using these 
minimum requirements.  These requirements were based on tests done for HUD to 
determine the effect of Seismic Zone 2 earthquakes on small, lightly loaded 
single family, single story houses in the Phoenix, Arizona area having a 
maximum roof area of 2500 sf.  Somehow, these limitations got lost, and the 
requirements were extrapolated to *any* masonry structure of unlimited size.  
I would recommend that the same reinforcing requirements stated in Section 
2106.1.12.4 be used for masonry construction in Seismic Zone 2.

It should be remembered that masonry is a brittle material, and that to 
resist earthquakes, the material must be able to absorb energy.  Since a 
brittle material is ineffective at absorbing energy, it must be made to act 
as a ductile material, which is what reinforcing does (as well as hold the 
broken masonry together).

Masonry (and concrete) subject to seismic forces is once again evidence that 
everything mathematically calculated is not always correct.  Much 
unreinforced masonry will calculate out to comply with code stress 
requirements, but no one has told Mother Nature that.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Mark Gorman wrote:

. > It is usually said that the use of URM was not permitted after the 1933
. > Long Beach earthquake.  Other than references to the Field Act for
. > schools,  I have not been able to find the particular code (UBC) section
. > that outlaws URM for commercial buildings.  Can some one provide the
. > reference?
. >