Fabric Covered Structure

[SteveMort]

I have a client who wants me to design a 60' x 75' open sided canopy structure that will be covered with fabric. The fabric is to be removed in the fall and therefore he does not want us to include snow loads in the design. is this allowed? Does anyone have any experience with that kind of request?

 

[flash3780]

I dont see what the issue is, there will be a visible warning tag when it is loaded with snow - its called sagging! Lay people are much more paranoid about these things than we are and I guarantee no-one will go under it if it is that overloaded.

Also, a fabric structure is much less likely to accumulate snow because it moves in the wind.

So... your suggestion is to assume that the users recognize the inherent danger? Really?

This thing's going to be the size of a large army tent. If it collapses on someone it can kill or injure them. Would you really be comfortable approving something like that? It seems unethical to me.

There are plenty of things that can be written off as misuse - improper assembly for example. However it is our obligation as engineers to - first and foremost - assure that we do everything in our power to ensure that our designs do no harm to the users.

This thread reminds me of a quote that I read:

Structural engineering is the art of molding materials we don't wholly understand, into shapes we can't fully analyze, so as to withstand forces we can't really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance.

The fact is, you don't know what's going to happen with this thing down the road. A prudent designer will do their best to design for plausible worst-case scenarios. In my mind, a snow load combined with a wind load would be plausible for this structure unless it's located in south Florida or something. Why not design for it? Regardless of whether it's required by code (which it may be), why would you take the risk of under-designing your structure? If cost is an issue, focus on optimizing your design rather than on neglecting plausible loads.

 

[csd72]

flash3780,

If we had to allow for every possibility in the design of structures we would never be able to build one of them.

The point that I was making was in specific reference to snow loads, which occur over a longer period of time than storm wind and would cause considerable deflection prior to any failure.

This is obviously a temporary type structure and therefore it would be reasonable to place limitations on its use in line with that briefed by the client.

If it was of a more permanent nature then I would 100% agree with you.

 

[flash3780]

This is obviously a temporary type structure and therefore it would be reasonable to place limitations on its use in line with that briefed by the client.

It sounded to me like it's a permanent or semi-permanent structure with a removable roof. The thing takes up a 4500 sq ft - the average house in the US is under 2500 sq ft. It seems like a large public gazebo or large commercial awning to me. No further details were given.

The added material costs for beefing up the supporting members isn't likely that great in the scheme of things (it probably costs more to hire the structural engineer). Unless the structure is to be built in a warm weather area, it would be prudent to consider the possibility that someone forgets to remove the roof. Or even the possibility that a freak spring snow storm blows in after the roof has been erected. It seems like an entirely plausible scenario.

 

[csd72]

flash3780,

there is no need to requote me if your post is following immediately after mine.

It appears that you may not have designed many of these fabric structures, but the required restraint forces can be horrendous.

I agree that a spring storm is not inconcievable, but it is also not inconcievable that the client would shake the snow off prior to allowing it to be used.

By your theory we should also be banning motor cars as it is not inconcievable that someone could drive one drunk or over a cliff.

I really do not see how this can be a problem if you have informed your client of the implications.

In my experience, the vast majority of these types of structures do not have any engineering input at all.

Anyway, I do not see any point in further debate on this subject, I understand your point of view and agree that it is a valid point of view, I just happen to disagree with it. Yours is the more cautious approach and mine is a bit more pragmatic.