Restrained/unrestrained beam 2

[Kurtisreeve]

I have the folowing situation, and I am not sure if the beam I am designing is prone to LTB;

If I have a pin jointed truss, with a back to back channel section upper chord. To the web of this top chord I have a fin plate to which a secondary beam is connected (pinned).
On this secondary beam's flange lies 5 equidistant purlins (also pinned).

Lateral stability is provided with two horizontal wind girders at each side of the structure.

I am concerned about the secondary beam connected to the top chord (pinned)... is it restrained against LTB by the purlins? should I check LTB, for every span between purlins?

Thankyou, help much appreciated

 

[Ron]

Yes, particularly if you are in a high wind area and/or you have axial compression from diaphragm.

 

[BAretired]

There is insufficient information to answer the question. It seems like an unusual framing arrangement. The secondary beam is restrained against lateral torsional buckling only if the purlins meet all of the bracing criteria set out in the code. They must meet strength requirements and deformation requirements. If they do not, they do not qualify as bracing against LTB.

BA

 

[Kurtisreeve]

Thankyou for your help... I understand that it is not as clearly explained as I wanted... But I get your answer. I will be chacking for ltb afterall, since the purlins resting on the secondary beam are not really restraining the beam below it, and even if they were I can have ltb between these points.

Another thing I wanted to ask, is it safe to choose a purlin from load tables and specifications?
I have a table stating that for 2Kn/m2 of imposed load and 10m span an IPE 140 would suffice... Not sure if I should still design as a beam (i.e make the checks we do for simply supported beams with UDL roof sheet)

Thanks for your kind help... (not very familiar with long span steel structures)

 

[BAretired]

If purlins provide adequate bracing to the top flange of the secondary beam, you would check LTB with an unbraced length of Sp where Sp = purlin spacing. It seems unlikely that this would be critical.

If purlins do not provide adequate bracing to the top flange of the beam, you would have to check LTB with an unbraced length of 6*Sp, a completely different proposition. The sensible approach is to ensure that the purlins do provide adequate bracing to the beam.

Not sure what you are asking in your second question. Perhaps a framing plan would be helpful.

BA

 

[hokie66]

I don't know what an IPE 140 is, but roof purlins are usually selected by the use of manufacturer's load tables...assuming you mean cold formed steel purlins.

 

[kingnero]

IPE 140 is a beam with cross-section resembling a capital I and a height of 140mm

http://www.metaalchristiaens.be/wordpress-christiaens/pdf/116-117.pdf

 

[hokie66]

Whether or not a purlin braces its supporting beam depends on how the connection is made, and whether of not the supporting beam is stiffened. I sections are infrequently used as purlins where I am, as connecting only the flanges doesn't provide a lot of stiffness. If cee or zed section purlins are connected to welded cleats, the stiffness of the cleats enhance the bracing of the opposite beam flange, dependent on the geometry. As an option, you could also use diagonal "fly braces", similar to knee braces.

 

[Kurtisreeve]

Thankyou for your help. much appreciated.

Last question, if you don't mid ,

As a top and bottom chord, I am using double channel sections back to back, with a gusset 12mm in between, and battened along the 5 mm span between gussets.

When doing the checks, the thickness of flange (tf) and web (tw), are the summation of both sections and gusset.... or just the thickness of one C- section?
and any guideline on the amount of battens required between gussets provided at each node of truss?

Thank you, for all feedback

 

[Ron]

My "yes" was intended for your question on whether you should check LTB. Just noticed you had a question before that which would be a "no" in my opinion......

Doesn't matter...you've gotten good answers otherwise, just wanted to clarify!

 

[hokie66]

Attempting to answer your last question, which I don't completely understand...the battens are for the purpose of making the entire section control, preventing local buckling of the individual members before the entire chord can buckle. I assume that the spacing between gussets is 5 metres, not 5 mm.

 

[Kurtisreeve]

aa yes so sorry... I meant 5m span between gussets (Nodes).

Am I right to take thickness of web (tw), of the welded built section ]|[ as ;

tw of Channel (9) + Gusset thickness (12) + tw of channel (9) = 30mm


Thanks hokie66

 

[hokie66]

Not sure what you are asking. You have two channels, thus two webs. So the gross area of your section is the sum of the channel areas, but each channel has its own web. Not sure what check the combined 30mm would be used for.

 

[Kurtisreeve]

Since it is a top chord of a truss in compression, I just have to check for buckling Nbrd... So you are right, not particularly important in this case.

However, I was in doubt about tw, for section web classification. Since I have always made use of rolled sections.

 

[hokie66]

The section is a single channel.