Paranoid About Uplift on light weight Flat Roof

[Contraflexure74]

Hi,

I have a flat roof on a steel frame building. The structural flat roof is light weight and will be constructed as per the attached sketch. In terms of wind load I have nothing but suction forces on the roof.

Originally I had no strapping of the bottom flange a midspan. My colleague thinks strapping of the bottom flange (compression flange in the uplift scenario) is not required as the joists are effectively stopping the UB from going side ways in this loading condition.

I have a suspended ceiling scenario on the soffit of the steel frame/timber joists.

I'm just wondering if the attached detail gives me full lateral restraint in the uplift scenario (thus giving me the full buckling capacity of the UB) or should I provide some sort of strapping of the bottom flange as shown in red on the attached sketch......or is this overkill.

Any thoughts welcome.

John.

 

[jayrod12]

Your detail would provide full bottom chord bracing in my eyes.

 

[DaveAtkins]

Straps are overkill. Since the joists are almost as deep as the beam, they brace both flanges.

DaveAtkins

 

[Contraflexure74]

Thanks guys.

John.

 

[Contraflexure74]

Hi Dave,

On one of my roof rafters the beam is loaded 6m on one side and only 3.9m on the other side. The dead load is only 0.9kN/m2. I assume I don't have to worry about the beam twisting due to this uneven loading as if it tries to twist the floor joists will hold it in position? Do you agree?

The rafter is a 254x146x43UB

Your thoughts are welcome.

John.

 

[msquared48]

Seems logical to me.

Mike McCann, PE, SE (WA)

 

[XR250]

Make sure you specify that your first sheet of roof decking be centered over the beam to tie the assembly together.

 

[Contraflexure74]

Thanks guys.

John.

 

[jwilki]

Bottom flange is restrained, it cannot twist without pulling out the timber joists. It sometimes may be more economical and less of a hassle to increase the size of the UB to avoid the fly braces. Just ensure the infill framer is full height (I imagine if the infill is not touching the bottom flange, the beam would still have a partial restraint).

 

[BAretired]

With the detail shown, a hump will form in the roof over each steel beam because the deck is in contact with the top flange of the beam. To avoid this problem, a gap should be provided between flange and deck large enough to accommodate joist shrinkage.

BA

 

[DaveAtkins]

I would not worry about torsion in the steel beam due to the two spans being different. As you said, the joists themselves prevent excessive twist of the beam.

DaveAtkins

 

[Contraflexure74]

Thanks Guys.

John.

 

[BAretired]

I agree that the beam is adequately braced for lateral torsional buckling, but I will repeat my warning: if you are using dimensional lumber as opposed to kiln dried, joist shrinkage will result in a visible ridge over each steel beam unless you provide a gap between the steel beam and the deck.

BA

 

[Contraflexure74]

Hi BAretired,

Good thinking about providing gap for joist shrinkage.

John.