Number of stories 1 or 2?

[m1208]

I have a one-story building built on a raised wood floor system with crawl space. It seems the attic space was converted to the second story in the past and now is a living area. It has a-frame style construction as shown in the attached sketch. For lateral analysis I have combined the roof and the second story floor loads together and applied at the second story floor line; and the first story floor loads act along the first floor line. Is the assumption conservative and is there any other way to deal with the number of stories?
Also, it appears the seismic loads obtained from ASCE 7 for lateral analysis, are lower than the UBC 1997. Is this true?
Thanks

 

[msquared48]

For seismic forces normal to the sketch, I would agree with your assumption as I feel that the roof diaphrabm will resolve any overturning to the top plate level.

However, for a gable end condition and the forces parallel with those walls, I would take the seismic portion of the roof through the gable end walls to the top plate below, with the associated overturning to be resisted by the gable end upper shear walls. If it is a hip roof, then the former comment would apply and I would agree with your original assumption.

As for the total seismic forces, I am finding them equal to or less than '97 for the work I do, not more.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[m1208]

Thanks for your reply. Based on ASCE 7, I get the seismic coefficient of 0.045 for the seismic base shear and with UBC 1997 the seismic coefficient is 0.10. I do not know why the coefficient is so different.