Eccentrically Loaded Monolithic Foundations 1

[egc]

I am looking for information on the design of eccentrically loaded monolithic foundations (footer and slab poured at the same time). In Florida, such foundations are utilized commonly for residential as well as light commercial and pre-engineered buildings.

In particular, is the "keep resultant in the center third" valid?

 

[UcfSE]

I say yes. You should always keep the resultant in the middle third, IMHO. I'm in FL also and that's how I design mono ftg's. That's why I avoid them as much as possible; keeping the resultant in the middle third results in large footings where a small stem wall footing will use a great deal less material.

 

[MikeE55]

This type of footing used for a metal building foundation will normally have the size governed by uplift. Should the uplift load also be in the mddle third of the resisting mass? Just curious about how others are handling this.

 

[UcfSE]

I'm not really sure what you should do with uplift like that. It's not as simple as just using the weight of the footing with 1.5 FS against the uplift since in this case the line of the force does not coincide with the c.g. of the footing. So it seems you should have to check sum of moments = 0 in order to evaluate how well the uplift is resisted. The thing is it isn't clear where the center of rotation should be taken (maybe the footing edge?) so that when you check sum M = 0 you are using the correct moment arm. That's my logic but I'm not sure how "clear as mud" it is. Does anyone have any thoughts? Since having the uplift in the middle third has to do with avoiding having "soil in tension" I don't think it applies to uplift, but I don't know obviously lol. It's my opinion.

 

[egc]

Thanks for the comments. I don't believe that a footing monolithic with a slab responds to loading the same as a strip/pad footer would. The big question is, how do we qualify the difference? Some input from the geotechnicals would be helpful.

Since 1984, I have routinely used utility-grade earth anchors to resist uplift in pre-engineered buildings (the anchors and installation are more economical than concrete). Issues you have to be concerned about is (of course) corrision and water table elevation relative to the anchor contact zone. I also think summing moments to equal zero is not neccessary as long as the resisting moment is greater than the uplift moment. And, if the column pad is indeed monolithic with the slab, what affect does this have?

Ucfse, where in Florida do you do these stemwall foundations (Orlando, CF)? This type of foundation is not competitive with monolithic construction on the East coast. And the 1.5 FS has been replaced with the 0.6DL + WL load combination (or 1.67 FS) in both ASCE 7 and FBC.

Come on Structural/Geotechnical Dudes/Dudettes, post some comments!

 

[UcfSE]

I'm in the Orlando area (University of Central Florida . Most of my work is on commercial structures where the size of a monolithic footing would be much larger than that for a stem wall. I know it's cheaper to use a mono when it's say 16" wide or maybe 24". I haven't used stem walls for metal buildings ever, but I don't do many of those either. I know that monolithics are used frequently for track homes around here. When I worked in the field in masonry that's mostly what I saw. For large expensive custom homes they still used stem walls.

What kind of earth anchors do you use? Is there a web site for them? That sounds like a really good idea, I haven't heard that one yet.

 

[Lutfi]

Well, I am going to be a nice guy to the forum and share the following:

1. This link to USACOE manual "Concrete Floor Slabs on Grade Subjected to Heavy Loads", chapter 3 has the information that you need.

http://www.usace.army.mil/inet/usace-docs/armytm/tm5-809-12/chap3.pdf

2. I use and old DOS program that is very wonderful to design the monolithic footing loaded with walls. It has great way to check the soil bearing pressure, steel required for the footing and also spits out bar requirement that projects from the footing into the slab to prevent the weak plane between footing and slab from cracking.

Regards




Lutfi

http://www.cdeco.com

 

[egc]

Hey Lutfi! Give JWallace a hard time for me and please let him leave eraly for lunch next week.

 

[Lutfi]

I never hold Mr Wallace espcially from lunch.

Lutfi

http://www.cdeco.com