Column Foundation Design - kern issue 2

[LearningAlways]

Hello and thank you for any help.

I'm designing a monument sign foundation. The sign itself is about 20' long and 6' tall. It has two 16" square masonry columns on each end and the middle span is a single 8" wythe bearing on top of a 16" wythe, the 16" wythe bears on the foundation and is about 2'-0" tall.

My approach was as follows: [ul]
[li]the middle span is supported laterally by the columns (essentially, the wall "bears" on the columns for lateral support)[/li]
[li]there are no connections between the foundation and the wall between the columns[/li]
[li]columns are cantilever[/li]
[/ul]

I'm in Case C due to the sign aspect ratio. Case C has about 3 times as much wind as Case A or B.

There is a high amount of O.M. on the columns. I exceed the kern distance in wind service loads. My calculated eccentricity (M/P) is beyond my footer in ultimate load wind case 0.9D + W. I feel like I am doing something wrong by using kern and qmax at ultimate loads, it just seems unreasonable.

Attaching the wall section to the foundation might be a possibility but the load path isn't clear to me at the transition between the 8" wide and 16" wide sections.

How do I design a foundation with large O.M. without using finite element?

 

[Settingsun]

Are you trying to say it's unstable in overturning? If so, make the footing larger or tie it down.

 

[LearningAlways]

Steveh49 - Tying the footer down is not an option given the scale of the project. Yes, I can make the footer larger but that is the easy (and costly) way out. And to answer your question, the footer is stable in overturning.

My question is regarding flexural and shear strength of the footer. Using qmax with the larger eccentricities (and subsequent small 'a' value) is putting a lot of force on the footer, but qmax assumes a linear analysis.

I think I know what to do. I will tie the wall 16" wythe down and assume that the 8" wythe wind loads are transferred to the columns, which is a conservative assumption. The 16" wythe will receive the load of its effective width, another conservative assumption. If the grout fails between the 8" and 16" sections than the 8" lateral sections would transfer over to the columns. This will cover me on the column and the wall portion of foundations and reduce the load on the column foundations.

 

[JoshPlumSE]

So, the footing is sized for overturning for the service level or allowable level cases. But, it becomes "unstable" for the factored load cases. The issue isn't so much related to calculating soil pressures, but one of calculating footing shear and moment demands. Correct?

I dealt with this issue a number of times over the years. When I was was at RISA (working on RISAFoot) I wrote up a topic on how to handle it. See the link to RISAFoundation's current help file.

https://risa.com/risahelp/risafound...Stability and Overturning.htm?TocPath=_____11

Scroll to the bottom to get to a sub-topic titled, "Calculation of Moment and Shear Demand for Unstable Footings".

 

[Settingsun]

"Stability is essential, but otherwise unimportant."

 

[Settingsun]

I should mention that the Aus bridge code uses unfactored loads for foundations in a specific case. The approach then is to calculate bending moment, shear force etc from the unfactored loads and reactions then multiply by 1.5. In software, you can use unfactored loads then check the structural utilisation is less than 67%. I prefer that to the Risa method, but the Risa approach sounds like it is forced because of US codes being half allowable design and half limit state.

 

[southard2]

I've designed a lot of similar structures and I don't remember running into this problem a whole lot. Usually the soil capacity will have issues as you approach an OT problem. Then again I've depended on software a lot and may not have noticed this when it came up.

I'll be honest concrete isn't that expensive and the labor won't change a whole lot. I'd recommended making the footing a bit wider. When in doubt make it stout. I've almost always regretted when I've bent over backwards to make something work. You'll be getting a call in a few weeks about how they want to make the sign larger or smaller, etc... Just make the footer larger and don't waste anymore time on this.

And just a bit off topic. But case C in ASCE 7 has annoyed me to no end. Just seems like the loads jump up way way too much in relation to cases A and B. I haven't opened up my new ASCE 7-16 yet to find out what new horrors await us.

John Southard, M.S., P.E.

https://www.pdhlibrary.com/