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I am working on a city permit for a shade sail structure. The structure is two rectangular shade sails, 16x20, and 12x20, on six wooden 6x6x10 beams. Beams are installed in concrete slabs in the ground.

I am working with a (obviously inexperienced) civil engineer to make this permit. We have a disagreement, and I don't know how to resolve it, because I am not a civil engineer but whatever I am being told makes no sense.

The broad process seems straightforward: You calculate the force that the wind exerts on each sail, and then calculate whether the volume of concrete is sufficient for that force (I guess we assume that the beams and the tension hardware can withstand all of that - fine, I am now aware that there are two methods, MWFRS and C&C, and we are skipping over C&C for now, ok)

So, how to calculate the force on the sails makes sense to me: calculate the wind pressure (take the highest wind speed you want to protect against, square that, multiply by some empirically established constants), multiply wind pressure by area of sails to get force, divide by 4 to get force that each beam has to withstand.

Please see the attached calculations.
The current plan

The current plan claims to calculate the volume of concrete needed to resist that force in a way that makes no sense. The current plan says that the density of concrete is 150lb/ft^3 (ok so far, that seems standard), then multiplies that by volume, and gets the force. That can't possibly be right - that's the gravitational force that is exerted on the concrete block, not the amount of force that the concrete block can withstand.

So, how do you calculate the volume of concrete needed?

brhans
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Yuri Niyazov
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1 Answers1

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In agreement with comments to the question, I believe the real question is how deep must the posts be. The concrete is relevant in terms of increasing the effective diameter of the post when resisted by the soil, and what is more important is whether the ground is sand, organic, compacted, undisturbed etc...

A post with bulky base in undisturbed soil will be much more difficult to move laterally than a thin post set in sand.

A rule of thumb for fences is 2:1 above/under ground, so 1/3 of the post is under ground, and the post is set in concrete or gravel filling a hole with diameter 2x that of the post.

Alternatively, the posts in your structure are set and integrated into a concrete pad, and the dimension of the pad, based on moment arm and integrated weight, determine its wind rating. The concrete PSI rating would come into play to determine the strength of the pad where it would be "tipping" without cracking the concrete.

Otherwise some sort of strutting will provide the rating required, albeit such strutted structure would be quite unsightly.

P2000
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