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I asked this same question of a close friend, but thought I might get a wider range of references by posting this notice on the SEAINT List.

I have a need to write my own spreadsheet tool or TEDDS template that will allow me to design a spread footing with multiple eccentric loads (say two or three columns that share the same pad that might be common in custom home remodels).  One of the problems stems from doweling two slab on grades together where a room addition is added and removing a wall to allow a pass-through. In some cases the framing is such that the reaction at each header column exceeds the soil bearing capacity of a typical turned down 12-inch wide continuous slab on grade foundation when considering the  maximum distribution on the foundation should not exceed a 4:1 ratio. So if the combined reaction is 6-kips and the soil bearing pressure is only 1000 psf then the maximum bearing pressure you can distribute over the length of the slab foundation is 4:1 or 4-kips. However, if you provide a similar turn-down edge to the adjacent new slab on grade (rather the more common 8-inch thick slab edge that is used to dowel the two together) then you can double the bearing pressure by considering the width of the combined slab foundation to be 2-feet in width and the 4:1 max ratio will allow for an 8-kip reaction. The problem is that the column now has an eccentricity that must be considered.

The second condition is if the new and the existing are independently supported on two columns where the framing is parallel to one another. This provides two columns on two slab edges that are doweled together or on one rectangular (or square pad) with each column eccentric and not necessarily balanced in loading.

To be frank, after 21 years in private practice, I’ve relied on software to do the work and while my intuition is good, I would like to relearn my skills and go back to the basics so that I can write a spreadsheet for up to four of six eccentric loads on one pad foundation.

 

I’m looking for a back to basic soils or foundation design manual to help me work out the algorithms that I need to write the spreadsheet. I think I understand how to find the center of mass based on the load distributions but would like to compare it against a text. My foundation design manuals that I used in school seem more than complicated and I am looking for something fairly straight forward.

 

If I would prefer, I would also like the text to cover other types of foundations; continuous wall foundations; friction piles, spread footings that are used installed below an existing foundation (remodeling situation); distribution of shear from eccentric loading conditions where new foundations are doweled or added to the existing etc.

 

Please post your favorite references for foundation design. I think the Retaining wall reference published by Huge Brooks is the best I’ve seen and I use this as a primer to refresh my memory on retaining wall design, but for the other conditions I am seeking a text rather than a reference for a computer program that will do the work for me.

 

One of the problems with relying on computer software that I take for granted because I write most of my own tools is writing one that you have been used to designing by shortcut (for example a grade beam that takes the moment from each end at the base of a moment frame). I’ve designed these using a half page or less shortcut, but I want to restore my understanding of the basics as I have always done with code programs such as MultiLat™ that assisted me to understand what was in the mind of the code writer while I was recreating the spreadsheet.

 

At nearly sixty years old, I am not too proud to want to return to the basics and admit that I forgot some of my elemental skills. Help me restore my memory by suggesting the reference that you use when you need to do an analysis manually that you have not done for a long time.

 

Thanks

Dennis

 

Dennis S. Wish, PE

California Professional Engineering – C 41250 Exp 03/31/09

Structural Engineering Consultant

La Quinta California


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