Need a book? Engineering books recommendations...
[SEAOC] Re: [SEAOC] [SEAOC] Gyp. board shear walls in LA[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: tdh(--nospam--at)capella.dwp.ci.la.ca.us
- Subject: [SEAOC] Re: [SEAOC] [SEAOC] Gyp. board shear walls in LA
- From: IteUrsi(--nospam--at)gnn.com
- Date: Wed, 04 Sep 1996 11:19:15
- Cc: seaoc(--nospam--at)seaoc.org
>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. ...
- Prev by Subject: [SEAOC] Re: [SEAOC] [SEAOC] [SEAOC] [SEAOC] terminology -Reply -Reply -Reply
- Next by Subject: [SEAOC] Re: [SEAOC] ] Plywood: sinkers beat common nails?
- Previous by thread: [SEAOC] Re: [SEAOC] Type M vs Type S mortar
- Next by thread: [SEAOC] 9" Anchor Bolt Embedment