Uplift factor of 1.5?? 4

[EngineerofSteel]

I received a review on a simple foundation project. I calculated a perimeter footing and column footings for a pre-fab steel building.

The reviewer (who is a good engineer) is requiring that I increase the metal company's uplift by 1.5. I cannot find any mention of this in the UBC/CBC.

He said it is because:
"note that the 1.5 factor is based on the height to width ratio of the building being greater than .5"

I think this is based on sect. 1621.1:

"The base overturning moment for the entire structure, or for any one of its individual primary lateral-resisting elements, shall not exceed 2/3 of the dead-load-resisting moment."

[That part obviously does not refer to uplift.]

(1621.1 cont.) "For an entire structure with a height to width ratio of .5 or less in the wind direction and a maximum height of 60 feet, the combination of the effects of uplift and overturning may be reduced by one third."

This second part refers to a potential reduction. I postulate he has mentally fabricated a new requirement by jumbling the two parts. I found another thread on eng-tips in which another person believed the same as my reviewer, but no support was found by him or any other engineer. see it here:

http://www.eng-tips.com/viewthread.cfm?qid=75910

CAN ANYONE JUSTIFY THE REVIEWER?

Thanks, DD

 

[EngineerofSteel]

UcfSE,

You are TOO funny. My brick "lab" was actually only an analogy. Of course, burying a brick only a few inches will not give a true proportionate relationship due to the nature of soil. The top one-foot tends to have much more disturbance and less compaction, and greater variability. this is the reason the 1st foot is not calculated as having any resistance in this area.

Here in the Central Valley, most soils are well compacted. The soil (except for near rivers and creeks) is type 3, but local agencies only allow engineers to claim type 4 without geotechnical reports. Since the geo studies and lab fees are expensive, testing is rarely done and extra concrete is used. The difference is 1500 psf bearing vs. 2000 psf. So, (w/o depth and width increase) this is a 33% increase in bearing already.

I already finished the project. I designed with a 1.5 FS, as requested.

Thanks, DD

 

[haynewp]

I wouldn't rely on friction for uplift either.

 

[UcfSE]

I can just see the looks we'd get standing in the front yard with a beer burying bricks with a string tied to them, much less having a fish scale on the other end of the string.

 

[haynewp]

My county reviewer wouldn't buy into the buried brick analogy either, so I would have to invite him out to watch too. Maybe if I got him real drunk he wouldn't remember exactly what happened and feel obligated to give the permit.

 

[EngineerofSteel]

Thanks, guys. I really appreciate the laughs. A smile is always good. Did I mention my brick is 3'x3'x2'? Maybe I forgot to say....

I ommitted the allowance for resistance due to the friction of earth pressed against concrete. The reinforced concrete in the integrated perimeter footing was more than able to transfer additional resisting load.

Thanks. -DD