Does lateral restraint attached to only web prevent Lateral Torsional Buckling 1

[abhussain]

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.

 

[BAretired]

If the connection prevents the

BA

 

[abhussain]

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

 

[BAretired]

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

 

[abhussain]

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.

 

[abhussain]

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

A.

 

[BAretired]

Could you provide a sketch of the connection?

BA

 

[glass99]

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.

 

[abhussain]

Hi BA,

I have attached a sketch.

A.

 

[abhussain]

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.

 

[glass99]

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.

 

[abhussain]

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.

 

[BAretired]

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

 

[KootK]

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.

 

[TLHS]

http://www.scribd.com/doc/63965314/Fundamentals-of-Beam-Bracing-1

 

[hokie66]

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

 

[KootK]

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.

 

[abhussain]

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

A

 

[KootK]

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.

 

[KootK]

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.