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Re: Multiple loads on a pad foundation question

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Dennis:
 
Find the resutant location of both loads for all conditions of live load and dead load and design the footing or pad for the soil bearing that creates the worst punching shears and moments on the footing.  Another approach if you have the space is to determine the footing size on the maximum total load for both columns and locate the center of the footing at the resultant location of the maximum column loads and then check soil bearing for DL one column and DL + LL the other column.
i hope this helps.
 
Ray Shreenan
   
----- Original Message -----
Sent: Thursday, January 18, 2007 2:31 PM
Subject: Multiple loads on a pad foundation question

I am designing a pad foundation for a prefabricated steel building. The steel building is used in the middle of a wood building that is a private school for a local church. I have the deflection calculations for the steel building (story drift) and am using proprietary Hardy Frames to control the drift around the core of the wood building. Using the building separation analysis of UBC 1633.2.11 (similar in CBC) I come up with an allowable building separation of 2-inches.

 

This creates one problem for me. The wood structure is designed as a typical slab on grade. All four walls surrounding the center of the building are bearing (except for the steel building in the middle). I will integrate the turn-down edge of the slab (or interior grade beam for bearing walls). There are a number of pads that will have a reaction from heavy timber loads that need to rest on the same pad footing as the columns for the steel building rests. If the foundations are placed in a monolithic pour I have to design the pad food for both a concentric load from the steel building and another eccentric load from the wood building. So the question is how to address the analysis? I?m using one of the typical foundation design programs that allow only one load whether it is eccentric or concentric.

 

So my question is if it is feasible to locate the eccentricity of the resultant of both loads by proportional interpolation? There is no uplift on the structure since the steel building is fully enclosed in the wood structure and a flexible membrane or flashing is used to seal the gap between the two structures. This allows movement without worry about pounding between the two structures. The design is based on worst case allowable deflections rather than actual deflections which is calculated by the steel building manufacturer and controlled by the stiffness of the Hardy Frames. The question is really limited to identifying the bearing point or its eccentricity on the pad footings due to two column loads applied within about 12-inches of each other center to center.

 

Any suggestions?

 

TIA,

Dennis

 

P.S. I am not the Engineer in Responsible Charge on this project. The project was designed by another engineer who I am consulting for. He will wet seal the design and I will seal (along with him) the separation calculations as well as the pad foundation calc?s.