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Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Hi guys,

We make our roof structure for our circular tanks out of variously sized purlins. We have one end of 2 main purlins attached to a centre bracket and the other ends bolted to the tank wall. Now we have secondary smaller purlins regularly spaced attached to the main and the tank wall. Then we have bridging purlins regularly spaced and attached to the second. and the tank wall.

Now my question is the bridging purlins are bolted to the secondary (inside web) via plates bolted to the web (for both). Given that the spacing is within limits, would that prevent lateral torsional buckling, even though the top (and bottom) flange is unrestrained?

Thanx.

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

If the connection prevents the

BA

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Minor correction to my post above, the bridging are connected between the secondary purlins, not to the tank wall.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

I accidentally hit the "Submit" button before I was finished.

If the connection prevents the purlin from twisting, then yes it braces the purlin against lateral torsional buckling. That would require the connection to have two bolts.

BA

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Hi BA,
Thanx for the reply. All the literature I've been reading suggests that the compression flange needs to be restrained. Hence, my doubt. Yes 2 bolts are used to the bolt the L Plate to the webs of the secondary and main.

One more question- Would the spacing equations for the lateral restraints given in AISC still be used for this setup?

Thanx.

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
2 bolts /web (i.e 4 per plate)

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Could you provide a sketch of the connection?

BA

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

I'm not sure how this works for the formula you are using, but you would have the "lateral" but not necessarily "torsional" component restrained. If you have two bolts each side, they would have to be a moment connection and the restraint would have to have high bending stiffness for it to restrain the section properly.

I face unusual buckling restraint conditions quite often, and I normally do a shell FEA model. As a first pass you can do an eigenvalue buckling analysis. Hopefully this isn't too onerous for you.

I'm not sure how close your beam is to buckling, but a restraint of the web only will provide substantial restraint, certainly much more than if it were completely unrestrained.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Hi BA,

I have attached a sketch.

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Sorry now I have

Basically the attachment setup between purlins and b/w purlins and the tank wall is the same. I understand what ur saying glass99, but the objective is to prevent lateral torsional buckling by preventing twist and lateral movement. I can see that the compression flange may twist to a certain degree but I don't see how that would affect the setup much, coz I would imagine there would primarily be an axial tension force on the lateral restraint and some moment, but not much, or is it the other way round.Either way I don't see how other than may be more pronounced deflections, that the roof would collapse.

Also, if you could provide me a program file (whatever the software may be) sample of your calculations that would be great.

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Abhussain: you only have one bolt into the bridging/restraint, so it can't carry moment. If you give it a second bolt, you need to make sure that the holes are only minimally oversized relative to the bolt diameter so that its a proper moment connection. If you could make the connecting bracket the full height of the web, it would ensure it was a stiff restraint of the flanges.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Hi Glass

It has 2 bolts I guess it's covered by the flange. Thanx for the tip. Ye the hole dia. is only 2mm over the bolt dia. I wouldn't be able to make the bracket the full height atleast not for the secondary purlin coz its larger than the bridging. Likewise secondary to main.

Regards,

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

The connection shown will provide considerable resistance to rotation, but it is limited by the flexibility of the bent plate and the flexibility of the purlin web. To qualify as a brace, there are two checks required by code:

1) The bent plate must be strong enough to resist a lateral force equal to 2% of the compression in the purlin flange acting normal to the web at the flange location.

2) The lateral deflection of the flange, including deformation of the bent plate and purlin web, cannot exceed the value given in the code (I am more familiar with the Canadian code than the American code but they are similar).

Because the bracing members are not attached to the tank wall, they cannot be considered as lateral braces for the compression flange.

BA

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

So long as the far end of the bridging purlin has some manner of vertical support, and I'm sure that it does, the bridging purlin should be able to provide LTB restraint to the braced member. Strength and stiffness checks ought to be made of course. Just eyeballing it, I'd like two see the two bolts in the bridging member centred vertically and on a common horizontal line. Is there no diaphragm element here that can be used for bracing?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

KootK,
Why would you orient the bolts horizontally? They provide resistance to twisting by their vertical separation.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Other bolts Hokie. Certainly, on the braced member, I'd spread the bots apart vertically. As it's drawn, I feel like the bolts in the bridging member create a bit of a vertical hinge between the bridging member and the clip. I've been envisioning light gauge here too. OP: is that true? If it's hot rolled, that might change my feeling on it some.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
In reply they are C25024 purlins cold formed. But I still cant imagine using horz. bolts. WEudnt that reduce the twist resistance.

A

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Nope. It might improve bending resistance as you could increase the distance between the bolts. If you do this, I recommend posting a revised detail here for review.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Also, back to your original question. AISC spacings would not apply. You'll want to adhere to the cold formed metal specification in force in your jurisdiction, probably S136 .

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

I'd still like to know what kind of roof diaphragm we're dealing with here.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
Hi

Could you elaborate a bit more what you mean by diaphragm.

I am a mech. eng. so am not familiar with all the structural terms.

A.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Quote (abhussain)

In reply they are C25024 purlins cold formed.

abhussain,

1. Are these C25024 secondary purlins part of a proprietary roofing system like LYSAGHT purlins incorporating HOOK-LOK II bridging, or STRATCO purlins with RAPIDFIX bridging etc?

2. Are the C25024 secondary purlins framed into the primary/main purlins in approximately the same plane (web to web) or seated on the top flange of the main purlins?

3. Where is the roof cladding (aka diaphragm) connected to - direct screw attachment to top flange of C25024 secondary purlins?

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Don't call roof cladding, as opposed to roof deck, a diaphragm. It is not considered to be a diaphragm, at least in the global sense. It does brace the top of the purlins.

Purlin systems like the Lysaght and Stratco ones which Ingenuity referenced are proprietary systems, including the bridging, and the capacities are well recognized and based more on testing than on calculation. These purlin systems are basically commodities, and the manufacturer's details need to be followed.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

Quote (hokie66)

Don't call roof cladding, as opposed to roof deck, a diaphragm. It is not considered to be a diaphragm, at least in the global sense. It does brace the top of the purlins.

I meant to place inverted commas/quotes around "diaphragm". I was trying to give the OP a loose terminology cross-link that KookK referred.

But totally, agree - roof cladding is NOT a diaphragm, in that it provides lateral restraint to the top flange of the purlin to which it is fastened.

RE: Does lateral restraint attached to only web prevent Lateral Torsional Buckling

(OP)
1. Are these C25024 secondary purlins part of a proprietary roofing system like LYSAGHT purlins incorporating HOOK-LOK II bridging, or STRATCO purlins with RAPIDFIX bridging etc?

Similar, but not the same. Our bridging is similar to that shown in their manual, except we donot use tie-rods

2. Are the C25024 secondary purlins framed into the primary/main purlins in approximately the same plane (web to web) or seated on the top flange of the main purlins?

All Purlins are attached web to web (90 deg.)> The 2 main are attached to a centre brkt. and far end of secondary and main are attached to the tank wall.

3. Where is the roof cladding (aka diaphragm) connected to - direct screw attachment to top flange of C25024 secondary purlins?

Yes roof sheeting (polyvinyl) is directed screwed onto the top flanges. I cant imagine it of being reliable enough to be considered as a lateral support.

Anyway thanx guys for your input, much appreciated. We had the designs checked by a structural engineer (certified) and he Oked it.

Regards,

A.

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