Top Flange Bracing 3

[EIT2]

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!

 

[SlideRuleEra]

I'm not aware of any restrictions on the material that can be used to brace a steel member. The situation that you describe sounds reasonable for a steel beam with simple supports. A typical "rule of thumb" is for the bracing to be able to resist 2% of the applied forces.
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.

 

[valleyboy]

EIT2

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.

 

[ChipB]

EIT2,
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?

 

[AlohaBob]

Consider the interval that the mechanical connection is made into the beam flange. Although the floor joist spacing is 16 or 24 inches, the bolt from the plate into the flange might only be occurring at 48 inches on center. Conservatively, you might use 48 inches then as your unsupported length.

 

[EIT2]

ChipB,

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?

 

[ChipB]

I am conservatively calculating the axial force present in the flange by treating it as a truss. The distance I calculated is the distance from the middle of the top flange to the middle of the bottom flange.

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

 

[AlohaBob]

I think you are right ChipB. The restraining force is the force basically which would restrain a column from buckling in compression. The column is the flange of the W12, the depth the centroid of the developed force.

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.

 

[AlohaBob]

March 2004 Steel Construction page 12 has a reference to a reference. The 4th Edition of the Guide to Stability Design Criteria for Metal structures......(Don't know what that is) p. 55 and AISC Spec Chapter C3 at

http://www.aisc.org.

confirms the 2% rule and defines where it came from.

For 1 midpoint brace in a compression member.
PBrace > 0.8% Pcr

For continuous bracing.
PBrace > 1.6% Pcr

 

[LPPE]

I agree with ChipB. Top flange is braced unless the uplift forces reverse the bending. Then you should design your beam as unbraced the entire length.

 

[cap4000]

Review the Joseph A. Yura Engineering Journal Article-1st 1/4 in 2001. On pg.26 you will find some worked-out examples he did. AISC started in 1923 with its first code and this bracing has just now caught a lot of attention starting in the 9th edition. AASHTO also has a formula for its Mr value. See Equation 10-102c (1990) for this amazing buckling equation.I believe in addition to your wood framing you may just want to add a couple of small brace WF beams if the span, moments and loads are quite large. The 2%rule in general is ok, but it can get much larger. Good Luck.