×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Fabric Covered Structure

Fabric Covered Structure

Fabric Covered Structure

(OP)
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?

RE: Fabric Covered Structure

Never been in that situation - I think you have a valid concern that someone, somewhere, sometime, will forget that you qualified your design to not include snow and leave the dang think up there over the winter.

However, it is valid to qualify a design and require the removal - full disclosure to the owner, documented, etc.

 

RE: Fabric Covered Structure

I agree with JAE that the design be qualified an full disclosure regarding the necessity for removal of the cover during the times agreed.

Good luck  

RE: Fabric Covered Structure

Yeah, they're going to roll up that tent and reinstall it just about once.

Be sure the tent itself includes visible and prominent warning tags.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Fabric Covered Structure

I've noticed on some similar posts in the past that people recommended designing the structure for the breaking strength of the fabric plus some margin- this might be a place to try that approach.

We lived up in Colorado for a while, and you could get fairly early heavy snows there- it would be easy to get taken by surprise.

RE: Fabric Covered Structure

JAE... in some jurisdictions, it's possible to put a caveat on the property that identifies the 'temporary' structure.

Dik

RE: Fabric Covered Structure

Not a good idea.  For one thing, they will forget to remove the fabric in the winter.  For another, spring snow can be very heavy, and can arrive when least expected.

BA

RE: Fabric Covered Structure

Hi, Steve,

The structure must be designed for minimum live load anyway (20 psf, I believe), so I don't think there is much to gain here.

And I don't thing a code official will allow it.  For example, a few years ago I went to a code official with a proposed floor plan for an existing second floor room, to be used for storage.  I showed him how the owner would only store boxes in a certain way, so as not to overload the floor.  He wouldn't approve this.

DaveAtkins

RE: Fabric Covered Structure

I do it ---- but also include all the warnings.

Usually check it for at least a 12 psf load - in the hopes that wind, slope etc. will allow this....

And people do take them down!! esp restaurants, nurseries, etc.

RE: Fabric Covered Structure

Actually per IBC, you can design it for a 5 psf live load for fabric roof. There's no way around the snow load though unless you classify the structure as temporary.

I'm not a fan of using the temporary structure designation as I've seen a number of them being used year round. You can put all sorts of disclaimer and warnings but you don't really have control if they take it down or not. When that thing collapses, that's the only time you'll ever hear from them again. The fee on these types of structures is typically a small dollar amount that I normally walks away than be painted in a corner. You're taking increased liability for peanuts if you're going the temporary structure route.   

RE: Fabric Covered Structure

What if you designed it with a known weak link (grommets & stitching) that wouldn't allow much snow to build up.  It would tear apart in the middle or something and dump the snow.  Just don't stand under it when it's snowing.

RE: Fabric Covered Structure

Perhaps it would be better to avoid the problem altogether by simply not getting involved in the first place.  Consider the merits of accepting or rejecting the assignment and choose wisely.

BA

RE: Fabric Covered Structure

If you aren't a "fabric structure" expert, recognize that you "don't know what you don't know" and steer clear of large fabric structures.  I say that with no disrespect intended.

Personally, if this was a "two day event" type structure that was legitimately taken down, I'd probably design it for now snow.  However, if it was supposed to be a "only in the spring/summer/fall", I wouldn't trust maintenance guys timing the weather.  I'd inform the client I would only design a permanent structure for all loads including snow.  They may need to find another engineer to do this structure in that case.

RE: Fabric Covered Structure

You might consider talking the client into using a steel or wood framed roof with steel roofing.  You could lighten the roof snow load based on the ability of the roof to shed the load.   

RE: Fabric Covered Structure

I assume that you are signing off on this thing? I wouldn't feel comfortable signing off on something like that - people could be hurt or killed in the event of the structure collapsing from the snow load because someone forgot to roll up the roof. Who's liable?

I suppose that you could design the fabric to tear before the structure collapsed... but that might not work for ice buildup.

RE: Fabric Covered Structure

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.

RE: Fabric Covered Structure

Let's not forget the Dallas Cowboys Training Facility collapse.

BA

RE: Fabric Covered Structure

BA, that is a very different scenario to this one.

It was a framed structure with an air supported roof and it was brought down by a sudden storm. What is being described in this thread is basically an oversized tent an any impending overload from snow will be clearly visible from the start.

RE: Fabric Covered Structure

The Dallas Cowboys facility was not air supported.  But I agree that this is a different scenario.

DaveAtkins

RE: Fabric Covered Structure

Different in what respect?  They are both framed structures covered with fabric.   

BA

RE: Fabric Covered Structure

Quote (csd72):

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:

Quote (James E. Amrhein):

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.

RE: Fabric Covered Structure

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.

RE: Fabric Covered Structure

Quote (csd72):

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.

RE: Fabric Covered Structure

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.

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources