Fence installed through center of SRW Wall

[kobedog]

I have a contractor that due to lot line problems wants to install fence posts directly into the center part of a Vertica pro (anchor wall) segmental retaining wall. When I calculate the overturning forces from the wind load (assume 6' tall posts' 8 feet apart, then use UBC to get coefficients)my driving moment exceeds the resisting moment. The driving moment from the wind load I calculate is 2 k lb-ft/ft and the resisting moment is only the first 4 layers of block solid grouted 4 x140 - 560 lb-ft/ft. The only way to really get it to work is to add a large concrete strip behind the wall to get extra overturning moment resistance. I have seen some contractor install the fences into block walls without adding the extra concrete and they seem to be holding fine. Am I over-calculating the driving moment form the wind load - or are some other contractors just cutting corners. Thanks in advance for the help.

 

[SlideRuleEra]

The contractors are probably cutting corners. Most structures never "see" the loads used during design. If they do, components of a well built (but pooly designed) structure often interact with each other in ways not foreseen to resist loads.
However as the engineer, you are responsible - trust your numbers.

 

[MSEMan]

You contractor's idea seems pretty ill advised to me. It might work for a 3 rail fence, but not for a solid panel fence.

In addition to the moment, there is also the shear force to consider and this will be applied locally at the bottom of the fourth row of blocks. Since the wall is the stiff element, I wouldn't count on the soil reinforcement being much help in resisting shear since the amount of strain needed to mobilise the geogrid strength is likely to be consistent with facing failure.

The solution you've come up with (a concrete strip footing) is the standard tried and tested solution. Stick with it.

 

[ZipperZeman]

Is this a geogrid reinforced SRW? If so, if your posts penetrate through geogrid layers, then your geogrid will resist overturning and shear in pullout.

this is why you see some contractors just putting the fence post into grouted blocks with nothing else. You can use the geogrid pullout resistance.