Freestanding site Wall Design

[Revv]

Hey Guys,

So I'm working on a freestanding site wall that is 8' tall above grade. I'm coming up that I need a 6' wide x 14" deep footing which just seems really large but I can't figure out what I'm doing wrong. It qualifies for Case C so I am averaging the load from 0-s and the load from S-2s as my windload. Does that seem reasonable or what techniques do you use? Does the 6' footing seem too large?

 

[palk7 EIT]

Hi Revv,

What is the surface area of the free standing wall and ur design wind pressure (psf), the overturning, sliding stability depends on it...also the soil bearing pressure... If u already did it, u can post the calculation here for reference as others might able to go through it and guide you

 

[phamENG]

I am averaging the load from 0-s and the load from S-2s as my windload.

If you're using that as your wind load for the entire length of the wall, that may be too much. How long is the wall?

How deep is the footing buried? As long as the back fill is suitable and properly compacted, you may be able to use some active pressure from the soil to resist overturning.

 

[le99]

What is the depth of the wall below the grade? Did you utilize the lateral soil pressure in stability analysis? You can rely on h - 1' as the effective depth in developing OT resistance. (h is the embed length, 1' accounts for bad soil close to the ground)

 

[Roukkia]

Seems like the footing size could be reasonable, considering how high the Case C load values can get. It does seems like averaging the values of 0-s and s-2s could lead to an undersized footing at the end of the wall, where the highest load occurs.

One thing that I have done on a recent project (has not gone into construction yet so not sure about the constructibility or if this would be difficult for the contractor to do) is provide footing sizes based on a dimension from the end of the wall. The reason for this is to prevent them from pouring a 6’ footing for the entire wall when a 3’-4’ footing for the majority of the wall would suffice.

For example, within 8’ of end of a wall, use x size footing, from 8’-40’ use y size footing, then the rest of the wall has z size footing.

Although, I am curious if others have done this and whether this is a practical solution or not.

 

[BridgeSmith]

Maybe the term "site wall" means more to others than it does to me, but for me to have an idea of whether the footing width is reasonable, I'd need to know how much the wall weighs. For a wood wall or a thin (4") concrete wall, the 6' width could be about right, maybe even small. After all, that's only a 3' 'heel' for an 8' height. For an 8" or 12" thick concrete wall, it could be conservative.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Aesur]

Case C creates extremely high wind forces on the ends of walls. I would not use those zones for the entire wall.

Is there a return wall at the ends of your wall? Many times site walls aren't just straight segments and will have returns at the ends. The returns can reduce the forces and many times a return wall foundation will provide more than enough to resist overturning where the higher loads are. Also take a look at Case C reduction where s/h > 0.8.

What safety factor are you using for the overturning and sliding? Many times I see engineers using "retaining" modules in software to design site walls and not realizing that typically the 1.5 factor of safety applies to retaining walls, not freestanding site walls.

I would expect your wall footing to be in the neighborhood of 3' to 4' wide and 12" thick.

 

[azcats]

I'm not a fan of Case C for screen walls. It's not as though there was a spate of failures in properly engineered and constructed screen walls before Case C existed. Clark County, NV even exempts screen walls up to 10' from the Case C.

What's your wind speed and/or seismic load & allowable bearing pressure?