Top Flange Bracing
Top Flange Bracing
(OP)
Do wood roof or floor trusses at repetitive spacing, nailed to a continuous, single or double 2x bearing plate, bolted at a repetitive, staggered spacing to the top flange of the supporting wide flange steel beam, qualify as sufficient compression flange bracing of the steel beam?
I've been under the impression that only steel or concrete can brace steel. Is this a misconception?
Thank you!
I've been under the impression that only steel or concrete can brace steel. Is this a misconception?
Thank you!






RE: Top Flange Bracing
Be sure that the bracing is on the compression flange however. For fixed, continuous or cantilevered beams the top flange may not be the compression flange for the entire length.
RE: Top Flange Bracing
I would consider this as full restraint, as long as there is an adequate horizontal diaphragm present fixed to the trusses.
This may take the form of ceiling cladding, if this is to be applied to your structure.
RE: Top Flange Bracing
If it is a roof, and the wood is on top of the steel, wind uplift can reverse the stresses and cause the bottom flange to go into compression. This will really come into play if you are doing a project where you have to design the purlins or girts(leeward side), using channels. Just remember you are allowed an increase in the allowable stresses for wind loading. I've got a few that I've overdesigned because I was focused on not exceeded the allowable stresses and neglected (forgot) about my allowable increase.
SlideRuleEra,
It's been my understanding that it's 2-5% of the force in the compression flange, not of the applied force as that could be significantly different.
For instance:
W12x26 member simply supported, 20' between supports, with a uniform load of 1k/ft.
Using 2% of the applied force would mean the bracing members capacity would have to be 20lbs/ft between bracing members. (?)(If I understood correctly)
Using 2% of the force in the compression flange:
M=51.3k*ft
D=d-Tf=12.22-0.38=11.84"
F=M/D=51.3k*ft*(12"/ft)/11.84"=52k (approximate)
2%(52k)=1k
This is the force which the bracing member connecting at mid-span (where M=Mmax)of the primary member, would have to withstand.
Which is correct?
RE: Top Flange Bracing
RE: Top Flange Bracing
I'm not understanding your example. It appears as though, since you have subtracted the "tf" from the "d", you are then determining 2% of what the beam section minus the top flange can handle.
Am I misunderstanding?
RE: Top Flange Bracing
You could also do it by calculating your extreme fiber stress and muliplying by the area of your flange:
Area of Flange = 6.49"*0.38"=2.47 in^2
sigma=M/S=51.3k*ft(12 in/ft)/33.4 in^3=18.43 ksi
F=sigma*A=18.43ksi*2.47in^2=45.5kips<52k
So it was quicker for me to run the numbers as I did previously, resulting in a conservative number.
Hope this helps,
Chip
RE: Top Flange Bracing
Wood floor framing is used to stabilize the flange.
In ChipB's example, the connection or bolt would need to transmit 1000#. Thats a tall order for cross grain bearing and shear for a wood plate.
Even Smeared over 5 ft thats 200 plf. Almost 10 X the less conservative load. But that shows the importance of the joist connection to the plate. At 55# per toe nail thats 4 per foot.
RE: Top Flange Bracing
For 1 midpoint brace in a compression member.
PBrace > 0.8% Pcr
For continuous bracing.
PBrace > 1.6% Pcr
RE: Top Flange Bracing
RE: Top Flange Bracing