Factor of Safety for overturning of footing with load in 2 directions

[structural-eng]

I have a footing that has overturning in 2 directions and I'm wondering about meeting the factor of safety requirements for overturning. I have the footing.xls spreadsheet from Alex Tomanovich which I've used in the past for non-concentric foundations and found to be very accurate. It appears when overturning moments are applied in 2 directions it checks the FS in both directions using the full resistance load but wouldn't that be "double dipping" to use the same load to resist forces in both directions? I've always found the Tomanovich spreadsheets to be very accurate so maybe it's acceptable. The spreadsheet indicates that it's based on "Analytical Approach to Biaxial Eccentricity" by Eli Czerniak which was published in an ASCE journal in the 60's. I've tried finding a copy of that article but haven't had any luck.

In the past when I've had footings with biaxial bending I've use a proportion of the resistance load to resist the overturning in each direction. I was comfortable with this approach because it's conservative but for this particular case there could be large cost implications and design challenges so I'm looking for confirmation that it's ok to use the full resistance load to resist the overturning in both locations is acceptable.

Thanks.

 

[JAE]

Assuming the orthogonal loads occur simultaneously in the same load combination, then I would agree that you can't use dead load for one direction to calculate a SF and the same dead load in the other as well.

One way to look at this is to assume that the two loads have a singular resultant which should be used with the dead load (once) and with the "odd" shape of the footing in the resultant direction.

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[fattdad]

take all the, "X" loads and perform in-plane analyses. Take all the, "Y" loads and do the same.

I'd think that'd inform the stability of the x-y loads?

f-d

ípapß gordo ainÆt no madre flaca!

 

[rapt]

I am with JAE, do a "biaxial" analysis of the footing at the angle of the resultant!

 

[structural-eng]

If you use the resultant overturning moment would you calculate the resistance moment to the corner/tip of the footing or to some arbitrary line back from the corner?

I've looked at a couple hypothetical conditions using the resultant overturning moment and if you calculate the resistance moment to the corner than the SF isn't any worse than looking at it in the 2 orthogonal directions which may be way some people just do that.

 

[VoyageofDiscovery]

I use the example in Tomlinson, thus checking footing bearing pressures under bidirectional moments.

 

[dik]

How would you design the column? Design the footing the same way.

Dik

 

[structural-eng]

There is no column. What I'm looking at is a precast t-panel that they want to put a wood building on top of. These t-panels are commonly used for salt storage in our area and most of the time they are inside a fabric covered building but in this case they want to building wood walls and a monosloping roof on top. The building is 3-sided with one end open for loading and unloading salt. The case that creates bending in both directions to the t-panels is when the wind blows in the open end of the building and fills it up like a balloon. The results are a wind load parallel to the panel from the shear wall, a wind load perpendicular the to wall and an uplift load from the roof. I can calculate the bearing pressure on the bottom of the t-panel for the bi-axial condition as less than 900 psf. I just want make sure the SF for overturning is ok as the footing.xls spreadsheet that I have uses the full dead load to resist the overturning in both directions.

 

[dik]

Can you post a small plan showing the T panel and how it is connected to the rest of the structure? Only type of structure I've encountered for this is the T panel stem is part of a transverse wall system and the T 'top' provides passageway for loading/unloading.

Dik

 

[structural-eng]

Sorry for the delay. We've been busy with another project. I believe I have the pdf attached correctly.