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[SEAOC] Re: [SEAOC] [SEAOC] Gyp. board shear walls in LA

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>Over and over, three story (semi-subterranean garage) multi-family 
>construction appeared to be  extremely vulnerable. The gyp board 
>main floor shear walls and the second floor shear walls in most 
>cases appeared to be relatively intact- often the entire upper 
>structure moved as a 'block' as the ground floor shear walls 
>failed.

The same damage pattern was observed in Loma Prieta quake in the 
Marina district of San Fancisco.  However, the minimal damage 
within the "block" doesn't mean gyp board should be given credit 
for superior performance.  The soft story at the garage level 
provided unintentional base isolation for the upper stories, and 
together with the smaller room sizes (and therefore large lineal 
feet of gyp board partitions within the block), the shear forces 
seen by gyp board partitions were below 30 plf in most cases.  For 
such damaged buildings that were rehabilitated, analyses often 
showed that the "block" would benefit from additional bracing when 
the ground floor is strengthened (and stiffened), and the block 
sees greater forces with the elimination of the soft story.

>Inspection is always a critical element in construction. Without a
>verification that actual construction meets the design 
>requirements stated in the construction documents, performance of 
>the constructed structure cannot be relied upon. I think this is a 
>strong argument for structural observation requirements. It is 
>unfortunate that the economical option of using typical wall 
>sheathing material as the primary lateral force resisting element 
>has been effectively eliminated as a result of poor follow-up in 
>the field.

Amen.  There have been many projects in which gyp board were 
installed with "floating" edges at partition boundaries to reduce 
the problem of joint failures caused by shrinkage of green lumber.  
This common practice reduces the shear capacity to next to nothing. 
Without attentive field inspections, such installations could pass 
undetected.  The alternative to providing attention and time is to 
not count on the gyp board, which will also allow the designer to 
sleep better.

>>The truth is that a gyp board is worth much more than 30 plf. 

Only if it is properly installed AND inspected.  The environment 
that many building officials work in include city managers who want 
the building department to be a "revenue center" (never mind that 
in California, this is against the law), and at the same time city 
councila that wants to keep the fees low in order to be 
business-friendly.  Caught in between, inspectors often just don't 
have the resources to do a thorough job.  I had an interesting 
conversation recently with one of my inspection supervisors.  He 
showed me two sets of plans for tract houses, from the same 
developer.  One was for a permit issued twelve years ago, and the 
other is for a currently active permit.  The newer plans contain 
significantly more seismic details, and he said to do an adequate 
job now on underfloor and frame inspections requires at least twice 
as much time due to the additional details.  Our permit fees 
essentially have been unchanged over this period when adjusted for 
inflation.

>>Actually, what I heard is that after Northridge earthquake, they 
>>were going to prevent use of gyp board shear walls. However, the 
>>gyp board industry lobby negotiated a 30 plf value. Something is 
>>better than nothing.

A few years after shear values for gyp board first appeared in the 
1961 UBC, clients started hearing about it and asked me to take 
advantage of this provision.  Based on my observations of the 
practices used in the sheetrock trade, I had grave misgivings over 
doing so, and generally wouldn't unless the client agreed to pay 
for me to inspect the work.  Even then, I had hassles with 
contractors when deficiencies were pointed out.  The decline in 
workmanship and materials quality over the years, and the use of 
green lumber, have helped maintain my skepticism of the reliability 
and trustworthiness of gyp board shearwalls.  Los Angeles's 
concerns are not unusual among building officials who have 
technical backgrounds.

Franklin Lew, S.E.

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