ASCE 7-05 Table 15.4-1 "Height Increase"

[eisparky]

What allows you to say that you have a system with "permitted height increase"?

I am analysing a structure used as a cooler along a conveyor system and the owner would like to install heavy item supported above it. ASCE table 15.4-1 does not permit the use of an OMF in seismic design category D unless it is a "permitted height increase" which would mean I can use an OMF up to 100ft as long as I use R=2.5, omega=2, & Cd=2.5. However, what permits me to say that I have an allowable height increase? I can't seem to find anything on this.

Thank you for helping,

 

[jittles]

A quick search popped up this thread:

Link

 

[eisparky]

Thank you jittles. I did read that thread before posting mine and I would agree with azcats on saying that I as an engineer permit the height to be increased as long as we design the structure for the lower R values. However, our plan checkers in our area are getting to a point where they want everything proven mathematically or academically and will not accept an engineer's judgement on simple or obvious things.

So my question still stands. Also section 12.2.5.6 & 12.2.5.7 for building structures does say I can use ordinary systems as long as the tributary load to the frame from the roof does not exceed 20psf and in light frame construction with floor or roof loads not exceeding 35psf.

 

[DCBII]

For non-building industrial structures I almost always use the lowest R value possible, preferring to take the penalty on my seismic loads rather than conform to AISC 341.

Contrary to popular belief, the cost is often in the complexity of the details rather than the magnitude of the seismic load. The idea that a smaller earthquake forces (higher R values) lead to smaller members doesn't usually hold true because members get held to tight compactness criteria and slenderness ratios that usually force you into a huge size.

That said, why not use an R of 1 per table 15.4-1 and avoid AISC 341 altogether? R=1 removes height limits altogether and only requires you to detail to AISC 360. R=2.5 still requires you to detail to AISC 341. Why make it harder than it needs to be?

To answer your original question, I believe the permitted height increase is addressed in the footnotes of Table 15.4-1 isn't it?

 

[eisparky]

DCBII I just wanted to get away from using a higher seismic load, since I am analyzing an existing structure. I guess I'll just have to let the owner know that his structure was not designed to carry additional loads. Specially when those loads are 50 - 60% of the effective weight.

 

[DCBII]

You'll have an even harder time getting an existing structure to conform to AISC 341 if it wasn't built to 341 to begin with.