Deflection limits - Steel frame clad with tension fabric
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Question

Deflection limits - Steel frame clad with tension fabric

thendzor (Structural)

(OP)

22 May 20 12:33

Hi,

I have approached a problem regarding the deflection of the structure clad with tension fabric.
We have a team of new engineers who are quite conservative, which results in very heavy structures.
After my investigation, I have realised that it is mainly driven by SLS - deflection. Structural members are at 50-60% utilisation ratio.

Our structures have pinned bases and are made of a lattice steel frame. On many occasions do not have gable ends or have only one gable end.

According to the BS, there is not design limitation for deflection, and it will vary according to the project, client requirements and cladding type.

As we use tension fabric which is very flexible, I believe the deflection limits are no applicable, and our new engineers use L/150 - L/200 - I believe it is a bit excessive.

Can you please advise what deflection limits would you recommend to consider? Or to not consider the deflection limits at all? (if the client does not have any requirements and deflection of the structure will not collide with other buildings etc).

Thanks!

canwesteng (Structural) · Answer 1 · 2020-05-22 14:11:54

Well at some point it's just unsightly. I'm guessing this is a fabric building, like some kind of large tent? As long as you do a second order analysis and satisfy stability requirements, and are convinced that the occupants won't panic, I'd let anything go.

thendzor (Structural) · Answer 2 · 2020-05-22 14:23:04

Yes, it is a Tent type structure.

Most of the structures are used for storage and maintenance, so there is not a lot of occupants in there.

The question is how to convenience our structural engineers who keep saying that they need to use deflection limits?

EdStainless (Materials) · Answer 3 · 2020-05-22 14:52:16

At some point the deflection will cause excessive stress in the frame, particularly the joints. Often it is easier to make something strong and stiff, as opposed to strong and compliant.
There is no reason for your structures to be this rigid, but there may be some real difficulty making them strong enough (wind, snow, whatever) with greater deflection.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

jayrod12 (Structural) · Answer 4 · 2020-05-22 15:16:09

I could probably be persuaded down to L/90 depending on the spans. You don't want something deflecting 75mm however, so you need to keep that in mind.

thendzor (Structural) · Answer 5 · 2020-05-22 15:20:34

I have explained it incorrectly...
The main problem is a lateral movement of the entire structure due to the wind load and pinned bases. As it is a lattice steel frame, deflection itself is not that big due to the rigidity of the structure.

jayrod12 (Structural) · Answer 6 · 2020-05-22 15:24:55

That's what I had assumed, they were talking about lateral. Again, for a building with cladding that won't crack/break etc from excessive lateral movement I could be persuaded to go down to L/90, within reason. Again, I wouldn't want to see my structure sway a couple of inches each way in a wind.

strucbells (Structural) · Answer 7 · 2020-05-22 15:35:12

Edit- I get it now. If you want to dial in the design you could probably assume some partial fixity at the base instead of fully pinned. You'll have to determine what percentage of the moment will be resisted at the column base and design the anchorage and foundation for that, but this can help reign in your wind drift.

The analysis is using the 0.42 * C&C load per IBC 2018 1604.3 for your serviceability check correct? If not, you can probably reduce the wind load a bit which will bring your member sizes down (and increase your UT for strength load cases).

thendzor (Structural) · Answer 8 · 2020-05-22 15:56:23

As we are in the UK, the structure was designed according to Eurocodes and BS.

strucbells (Structural) · Answer 9 · 2020-05-22 16:25:01

I'm in the US so not familiar with Eurocodes/BS approach to mean return intervals and how that plays in to the load factors used for serviceability checks or what the typical drift limits are and how they are presented.

Did a quick google and it looks like you can assume up to 20% of column stiffness for partial base fixity for the serviceability limit states. This will reduce your drift but you will have to make sure the anchorage and foundation details actually calc out for the moment reaction induced by this assumption.

KootK (Structural) · Answer 10 · 2020-05-22 16:51:34

The Metal Building Manufacturer's Association (MBMA) recommends a drift limit of h/60. If that's good enough for an all metal building, I'd have to think that it would suffice for a fabric clad metal building. Such flexibility obviously limits future usage with respect to potential mezzanines, exterior finishes etc but, if you're confident that those are not pertinent concerns for your projects, so be it.

KootK (Structural) · Answer 11 · 2020-05-22 16:53:51

Quote (strucbells)
Did a quick google and it looks like you can assume up to 20% of column stiffness for partial base fixity for the serviceability limit states.

I wasn't aware of this and would like to check it out. Any chance you can share/reproduce what you found?

strucbells (Structural) · Answer 12 · 2020-05-22 17:50:04

KootK-

I couldn't find the code clause referenced (BS5950 5.1.3.3)in the below link(s) as I have 0 experience with Eurocode/BS and no access to these standards, but I found that reference on pg. 2 (Section 1.2.2) here: https://cads.co.uk/wp-content/uploads/2017/06/A3D-... and also here: https://www.masterseries.com/blog/2019/utilising-t...

On re-reading, it appears to be saying that you don't even need to design the column base for this limited moment restraint. But I would probably check that anyway barring looking more into this. I'd think at the very least some engineering judgement goes into determining if the foundation/anchorage is capable of resisting that amount of moment.

KootK (Structural) · Answer 13 · 2020-05-22 17:53:17

Thanks so much strucbells. In searching for what you found, I found this: Link. Old as dirt and surely superseded by newer work but, none the less, comparable conclusions.

retired13 (Civil/Environmental) · Answer 14 · 2020-05-22 18:23:40

Note that the wind on a flexible surface is quite different than that on solid/rigid surface, localized stress due to highly distort shape could be critical. What is the use of this tent, one time recreational, or long term/permanent setup? Can you add braces to minimize the demand on the main members, while satisfy more strict deflection limit? Without knowing how the wind-fabric interact, I rather be more conservative.

thendzor (Structural) · Answer 15 · 2020-05-23 06:29:09

Permanent setup. There is a lot of braces but wind loads are significant.
We do consider deflection of the fabric and increased stresses caused by that. Also, pretension forces of tensioned fabric are being considered.

retired13 (Civil/Environmental) · Answer 16 · 2020-05-23 19:39:17

For tent walls, I think you can use IBC deflection limit for exterior walls with flexible finishes, L/120. Since the fabric is much flexible than typical building wall finishes, it is arguable that the limit can be relaxed further.

BAretired (Structural) · Answer 17 · 2020-05-23 21:14:30

Are we talking about buildings like the one shown below? If so, what is L? The height of eave? What rationale can we offer for setting a particular limit on deflection due to wind? If there are no materials to be damaged, such as drywall or brickwork, which would crack if deflections got too high, why bother setting a limit?

If there is any danger of flutter developing in the roof as a result of wind, that would be something to worry about, but I'm not sure that could be readily analyzed. If the ends of the building are open, or perhaps closed at one end and open at the other, internal wind pressure could introduce a potential problem. The building designer may want to consider limiting deflection in certain unusual cases, but lateral deflection at the eave of a building similar to the one shown below, to me, seems pointless.

BA

thendzor (Structural) · Answer 18 · 2020-05-26 07:13:50

Yes it is this type of building, but there is no steel cladding on walls, pvc fabric all around.
Eave Height 4.5m, apex 10m, 20m span x 20m long

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