Precast Concrete Blocks versus Reinforced Concrete Slab

[vvs9770]

I have designed an electrical equipment concrete pad consisting of reinforce concrete- see attachment (slab is on top of page). The client has asked for a design alternative consisting of a series of precast concrete blocks bearing over compacted stone-see attachment. They said that they have seen this done in another project, the details of which I do not have. My concerns are two folds: The first one is over turning due to wind. What sort of uplift can I expect obtain from a 16"x8"x4" concrete block? The second concern is related to the seismic coded compliance. Does this alternative meet the IBC seismic code requirements? I have looked on the IBC2009 book for anything that might hint noncompliance but have not found any. This precast block foundation system seems logically "flimsy". In an event of an earthquake wouldn't the unattached precast blocks slide and become unstable? How can I convince the client not to go this route? Or maybe this alternative design is fine and I am overly cautious. What are your thoughts?

best,

vvs

 

[bridgebuster]

Maybe this thread will e of some use.

http://www.eng-tips.com/viewthread.cfm?qid=352793

 

[hokie66]

I think you have to resort to common sense, which unfortunately is in short supply these days. You will have to explain to the client the distinction between gravity loading and lateral loading, and have him understand how individual small blocks will not provide sufficient mass to prevent overturning. The footing for electrical equipment almost always represents a small percentage of the total cost, so that is not the place to scrimp.

 

[cvg]

first of all, your reinforced slab seems a bit of overkill but otherwise hokie nailed it

 

[vvs9770]

Brigebuster,
thank you for the link. It has been very helpful and informative and addresses many of the concerns that I may have.


cvg,
I agree the slab may be seem overkill. It was not designed by me but rather it was an off-the-shelf design submitted by a colleague who's no longer with the firm. I was asked to pick up the project after his leave. This could be the reason our client came back with requests for the alternative design.

 

[bridgebuster]

you're welcome.