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[SEAOC] Re: [SEAOC] Basic Roof Rafter Analysis Question

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Mark ---

I don't agree with your presumption of seismic vs. wind risks.  My
understanding is that the code authors have attempted to equalize the
natural hazards risks.  The code-prescribed wind and seismic forces are
both set at levels which have a 10% probability of exceedance in 50
years.

I do agree that the engineer's risks are not commensurate with his
compensation.  Once an owner is financially injured, he(she) will seek compensation, and will certainly recall that "the engineer said it was
OK".  I still contend that it is a violation of the UBC to alter a
structure to a condition that is less safe, such as adding significant roof mass without other strengthening.   A smart attorney would argue the
same.

My experience with adding heavy tile or cement shake roofing is that
seismic will often now govern over wind, even if wind governed before. 
Best to check carefully.

Russ Nester, SE, GE
rnester(--nospam--at)juno.com

__________________________________________________________

On Mon, 12 Aug 1996 20:23:41 -0400 Shake4bake(--nospam--at)aol.com writes:
>Dennis writes:
>
>"2. I have not considered the added weight as having an effect on the
>lateral stability of the structure for two reasons. First a structure 
>this
>size - conventionally framed is governed by wind rather than seismic
>loading (from years of experience with lightweight wood structures).
>Therefore the difference in weight would not be sufficient to cause 
>seismic
>to govern over wind."
>
>No argument here, however... from a practical standpoint, which force 
>is the
>building likely to see, code prescribed wind or code prescribed 
>seismic
>force?
>
>Assuming we agree that the majority of single story wood framed 
>residential
>buildings will be more likely to experience the code  prescribed 
>seismic
>force, how will it perform?
>
>Consider John Q. Public who has a 15% deductible on his $100,000 
>earthquake
>rider. John's home sustains $15,000 worth of  damage in the form of 
>cracked
>stucco as a result of the most recent earthquake (post tile). Will it 
>occur
>to John that he had an engineer say it was o.k. to put on the tile?
>
>Finally, how great a disparity is there in the fee being charged for 
>your
>blessing vs. the assumed liability.
>
>Just thought I'd throw this out there.
>
>Mark
>
>
>...
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