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Design of fence with posts bolted to face of retaining wall

Design of fence with posts bolted to face of retaining wall

(OP)
Hi All,

I need to calculate whether or not the bolts used to attach a fence to the face of a retaining wall are adequate to accommodate applied loads (primarily dead load of fence, wind load, and vandal load). Due to existing services in the ground meaning that the fence posts could not be embedded in a certain area, the contractor instead bolted the fence to the face of an existing retaining wall structure (~1.5m high).

The fence will be attached to either UC or SHS posts which are then bolted to the retaining wall (see attached photo of corner section for example).

Can you please suggest a method of calculating the adequacy of this design? How would you suggest calculating pull out force on each bolt?

If additional information is required, let me know and I can provide.

Thank you in advance!

RE: Design of fence with posts bolted to face of retaining wall

Wow... that's tough..

I'm not sure on the strength of the connections, but I always start working on the parts of the problem I do know, and then hope the parts I don't know will come to me.

If you have a finite element program that you can use to determine your reactions at the wall, then I would use that.

If not, then I would do the following:

I would figure out the load first, and apply it to the post. After you have that figured out, then start worrying how the post is going to attach to the wall. Are you going to have 2 hinges or 3? Also, are the corner CMU cells grout filled or empty?

The hanging gate is going to create a moment and shear about the post. You can use AISC's Eccentrically loaded bolt group methodology to figure out the load going into the post (Page 7-6 in 14th edition). They have two methods, the Elastic Method and the Instantaneous Center of Rotation method. The elastic method is easier and more conservative. The instantaneous center method requires an iterative solution.

For the reactions at the wall, i'm not sure of how to easily do a hand calc; but maybe some other people on here will have an idea.



RE: Design of fence with posts bolted to face of retaining wall

I'm not sure if I follow your question.....are you wanting to know how to figure the load on the bolts or their capacity in the masonry? If it's the latter, see ACI 530. There are embedded bolt capacity equations in it.

RE: Design of fence with posts bolted to face of retaining wall

(OP)
Thanks for your responses guys.

JoelTXCive,

The CMU cells are filled in the existing retaining wall. Unfortunately I don't have access to a finite element program so was hoping to be able to calculate by hand, if possible.

The contractors have installed two sets of two bolts (four bolts total) per post, effectively acting as two hinges I suppose.

Fortunately, the section that contains the gate was not situated over services so the posts were embedded in the ground with footings etc as per the original design so the capacity there should be adequate.

WARose,

I'm trying to figure the reaction load on the bolts resulting from the applied loads (ie: wind, weight, vandal, etc.). The capacity of the bolts is fairly readily available in product brochures etc so shouldn't be an issue to find.

I was hoping I would be able just to create a simple free-body diagram and calculate that way? Or perhaps there is a suitable equation to use in this situation?

Thank you for your inputs!

RE: Design of fence with posts bolted to face of retaining wall

Quote:

I'm trying to figure the reaction load on the bolts resulting from the applied loads (ie: wind, weight, vandal, etc.). The capacity of the bolts is fairly readily available in product brochures etc so shouldn't be an issue to find.

I was hoping I would be able just to create a simple free-body diagram and calculate that way? Or perhaps there is a suitable equation to use in this situation?

Looks pretty straight forward to me. (If all you are after is reactions.) Cantilever any lateral loads to the edge of the bolt group. Then figure your reactions. You've got 4 bolts per post, but I would only consider the ones that would go into shear from a lateral load as effective to resist it. (In effect, just 2 bolts. The bolts on the other side probably wouldn't bend the connection plate until those other 2 had engaged in shear.)

Any vertical load just divide up between all 4 bolts. (With a bit of eccentricity accounted for by the method I describe above.)

Where it would get tricky to me is if there was some consideration of active pressure/displacement of the wall. But as I read your post (and look at the pic) I would think that would not be a consideration as the wall looks like it has been there for a while (before the fence was placed). So the wall has done everything it's going to do already.

RE: Design of fence with posts bolted to face of retaining wall

Dead load of fence is probably inconsequential, but you can include it as a bolt shear load if you want. Vandalism, well that might occur no matter how strong you make the connections.

So to me, that leaves wind. I would just apply the wind pressure, find the moment about the bottom bolts, and resist with the bolt couple. Sum of forces must be zero, and sum of moments about bottom bolts is also zero. Typical cantilever situation.

RE: Design of fence with posts bolted to face of retaining wall

Agreed with what hokie66 said. Do the calculation for an orthogonal wind load, as well as a 45 degree wind load, in both directions, and you're in business.

RE: Design of fence with posts bolted to face of retaining wall

(OP)

Quote (WARose)

Looks pretty straight forward to me. (If all you are after is reactions.) Cantilever any lateral loads to the edge of the bolt group. Then figure your reactions. You've got 4 bolts per post, but I would only consider the ones that would go into shear from a lateral load as effective to resist it. (In effect, just 2 bolts. The bolts on the other side probably wouldn't bend the connection plate until those other 2 had engaged in shear.)

Is this assuming a corner connection? Would the approach for a section bolted directly to the face of the wall be different?

Quote (hokie66)

Dead load of fence is probably inconsequential, but you can include it as a bolt shear load if you want. Vandalism, well that might occur no matter how strong you make the connections.

I agree, the dead load and vandal load (effectively the weight of someone climbing the fence) are fairly inconsequential as you said as the shear strength of the bolts is far greater than the applied loads, i just mentioned them for the purpose completeness. The critical thing will be the pull-out of bolts from the wall.

Quote (hokie66)

So to me, that leaves wind. I would just apply the wind pressure, find the moment about the bottom bolts, and resist with the bolt couple. Sum of forces must be zero, and sum of moments about bottom bolts is also zero. Typical cantilever situation.

Quote (atrizzy)

Agreed with what hokie66 said. Do the calculation for an orthogonal wind load, as well as a 45 degree wind load, in both directions, and you're in business.

Okay excellent, this is the approach I have taken. Would you treat the bolt couples as hinge connections in this case? I assume so as you have said to take sum of moments around bolts as zero. Just wanting to clarify this is appropriate?

RE: Design of fence with posts bolted to face of retaining wall

Yes, they are hinged supports.

RE: Design of fence with posts bolted to face of retaining wall

Quote:

Is this assuming a corner connection?

Yep.

Quote:

Would the approach for a section bolted directly to the face of the wall be different?

Yes. I'd use all 4 in that case.

RE: Design of fence with posts bolted to face of retaining wall

Is that a walkway behind the fence? Would handrail loads (50#/lf / 200# point load, or equivalent if not in the US) need to be applied?

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