Lateral bracing of flanges 1

[ranawaseem]

Do you guyz think that this stiff plate (lets say 10mm) is adequate to provide resistance to lateral torsional buckling of flanges? This is the part of a long truss.

 

[paddingtongreen]

If you are you really asking if that detail is as effective as the ones where the bottom flange of the purlin is not coped, I would say yes. But there is not enough information to say on an absolute level.



Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[BAretired]

The bottom flange of the top chord is not adequately braced against lateral buckling. The stiffener plates are ineffective because they do not connect to a stiff web member beyond the flange. Thus, they do not significantly restrain rotation of the top chord.

Why not use a structural tee for the top and bottom chords and bend to suit the shape of the roof, allowing purlins to bear on the flange of the tee and web members to connect to the web of the tee?

BA

 

[hokie66]

If the purlins were connected with two bolts in the web rather than the one shown, I would consider the chord braced, provided the geometry is as shown.

 

[ranawaseem]

BAretired, we cannot use here a Tee, but as hokie66 said, I have added another bolt and the connection looks like too stiff now. Personally I feel like its more than enough laterally restrained, but thanks a lot for your views.

 

[hokie66]

Why don't the purlins go over the top chord as shown in the elevation of the truss? And why don't you use zed purlins so you can lap them? Much more efficient use of the material.

My comment about the purlins bracing the chord was based on the cleats being welded to the beam. With a bolted connection, which I now see is the case, I would be less confident in the stability.

 

[ranawaseem]

hokie66. Yes this is the top chord shown in image 1 and purlins go over this chord.

 

[JLNJ]

I am always a little suspicious of these truss chords where the entire WF section is fully stressed in compression and all you have to brace it is a light guage member sitting very eccentric to the compression member. I have the same feeling about bracing column sections with girts.

I have been to Joseph Yura's seminars and read the literature, but I can't claim to fully understand the adequacy of certain bracing schemes.

 

[a2mfk]

To add onto what several others have said, how often does the top chord need bracing to work in your design? And then, what is the buckling restraint force?

I would take a look at bracing it at 1/4 points (or maybe even only mid-span) and then using a WF member framed perpendicular to your trusses for this.

If you stay with your original bracing scheme, I think a more common detail may be use Cs or angles welded to the bottom flange and then diagonal kicker up to the roof member. Your detail seems to create restraint that would develop moment in the roof purlins and cause torsion in your WF, which is probably not the way you are designing it.