Lateral Pressure from privacy fence footing (Fence footing is Short Pile or Column)

[jagti]

Can anyone provide me reference or give guidance on how to account for additional Horizontal Loading due to Wind loading from Wood Fence say 6.0ft high fence (fence footing Founded on Short pile/column)behind my SRW Retaining wall.
I will have to add this force in my Base Sliding & Overturning calculation. We have extended base of footing close to the base of the wall so Overturning of footing wouldn't be of any issue i think. The fence footing offset is 2.0ft behind wall.

I would also like to know ways as to how this would be built assuming retaining wall does not have geogrids its designed as gravity wall i can get permission to incorporate sacrificial geogrid layers for increased wall stability.
On one side Pile would get resistance from retaining wall and on other side it would develop resistance from retained soil side.
Would they auger the piles after retaining wall is built or they would place hollow pipe formwork and afterwards fill that with concrete.
Any help would be much appreciated.

Thank you in advance.

 

[oldestguy]

Your description is a little hard to follow. Add a sketch of a cross sectional view. Any soils data also would help. Dimensions also.

 

[fattdad]

there are two questions: What's the wind load for design; and, how to determine the soil's resistance. One way to determine the ultimate capacity of an embedded pile is by multiplying the Rankine passive coefficient by an arching factor. For simplicity's sake, just use phi/10. So, if the pile is embedded into some sand with a friction angle of 30 degrees, you'd end up with 3. If the Rankine passive earth pressure coefficient is 3 (which it'd be for a friction angle of 30 degrees), you'd have a maximum resistance of nine times the gamma*H value with depth.

Now for a laterally loaded pile there are changing forces (and force directions) with depth, owing to the nature of the load applied at the surface (i.e., in consideration of moments). That's a unique solution.

I have no idea how to calculate the actual wind load though. . . I'd need a structural engineer for that!

f-d

ípapß gordo ainÆt no madre flaca!

 

[jagti]

Hi Oldestguy,

The pier diameter is 18" & it extended down to match base of the wall. The wall design is Gravity wall.I am thinking to add few layers of geogrids to improve my base sliding factors of safety.

Fattdad,

The horizontal load on foundation due to wind is 1800lbs.(9.0kN). My question is how to account for any additional load on my retaining wall.

 

[oldestguy]

I think some info on the block wall is needed. What kind of shape is it in? Wall backfill is what?

The 1,800 lbs is on the post?

I dis some fixing on the scan. I could not turn it right side up as it came. I assume wind blows both ways.

 

[msquared48]

Either throw on a uniform surcharge load that creates the same moment at the base of the retaining wall as the point load from the reaction at the fence base, or add a point load per foot of wall from the wind to your program at the top of your concrete retaining wall and run it.

Mike McCann, PE, SE (WA)

 

[Doctormo]

See attached for very simplified post foundation load diagram which gives you a thrust on the wall (from memory). You should be able to see that the thrust is roughly 3X the wind load which you have to figure out how to distribute, over 8 ft. or narrower depending on block's ability to transfer load laterally from the post loading.

You can also make this real complicated and run a L-pile analysis and get a more correct pressure but this is the simplest way. You could also take the mass of the foundation in account for a little overturning resistance contribution when they get larger. The method is actually in the AASHRO sign and post foundation guidelines somewhere...

 

[jagti]

Oldestguy,

The block is precast modular block with variable depths. Back fill is gravel.1800 lb is applied on the post. Wind will blow both the ways.

Doctormo
i found reaction thrust is around 2 to 3 times based on the post length. In doing further research i found Prasad & Chari have given earth pressure distribution for short rigid piles. Has anyone used this it shows a triangular distribution & the lower portion thrust is 1.7 times the upper portion thrust also the point of rotation lies around 0.7L.

 

[KootK]

Similar to Mike's suggestion, I think that a simple approach would be to pretend that your privacy fence is mounted on top of the retaining wall. I'm also not convinced that it's necessary to take your foundation all the way downt to match the retaining wall.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.