Unbraced Length & the 2% rule (Not again! ......) 1

[GalileoG]

Hi all,

I have a beam that spans approximately 10m. I have wood trusses @ 600mm that sits on a wood plate that's bolted to the top flange of the beam. The hurricane ties that I am using would not take anything close to 2% of the compression force in the flange of this beam. Obviously, I can not have an unbraced length of 600mm.

Let's say the brace force is 10kN and each hurricane tie can take a lateral force of 2kN. Can I then rationalize and make the judgement call that my unbrace length is equal to (10kN / 2kN) * 600mm = 3000mm?

It's a judgement call, but I don't see why this wouldn't work?

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[msquared48]

I wouldn't. If possible, I would modify the situation to make it work without having to rationalize.

Are these top or bottom bearing trusses?

If they are top bearing, you could add blocking that is also boolted to the top flange, and nail the diaphragm off to the blocking.

If the trusses are bottom bearing, I would consider trying to install some diagonal wood stuts from the top flange to the underside of the diaphragm. If this is the case, how deep are the trusses?

As another alternative, use a different, higher capacity connector of the truss to the wood sill.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[GalileoG]

msquared, it's a top bearing truss. I was thinking of that solution, but it would be hard to nail the diaphragm to the blocking because it is a slopped roof.

I doubt there are hurricane tie anchors that can take the brace force that I am coming up with. It can still work if I spec a whole bunch of them per truss (4)

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[DaveAtkins]

I would consider the beam braced @ 600 mm. This is assumed all the time, without worrying about whether each hurricane tie can resist 2% of the buckling force. There will be friction between the bottom of each truss and the wood plate. The higher the beam load, the higher the buckling force--but also more friction.

Anyway, 2% is a rule of thumb.

DaveAtkins

 

[BAretired]

Clansman,

Read CAN/CSA S-16-01, Article 9.2.7. You need to resist a lateral force of 0.05 times the force in the flange, uniformly distributed along the compression flange. You do not have to take 2% at each connection. In fact, what you have to take at each connection is:

0.05*F_f/10*0.6 = 0.003F_f

where F_f is the maximum flange force.

BA

 

[GalileoG]

BAretired,

Thanks for that! I was looking at Article 9.2.5 which states, "Bracing systems shall be proportioned to have a strength perpendicular to the longitudinal axis of the braced member in the plane of buckling equal to 0.02 times the factored compressive force at each brace point in the member being braced, unless a detailed analysis is carried out....."

The requirements in Article 9.2.5 and 9.2.7 have vastly different requirements, I'm not sure why.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

Clansman,

I believe that Article 9.2.5 is written for the situation where a member is braced only at required brace points. If braces are placed at much closer intervals, it does not seem reasonable that Article 9.2.5 must be satisfied at each and every brace connection.

Suppose you changed the joist spacing to 300 mm. That would mean twice as many connections, each resisting the same force, namely 0.02*C_f which in my opinion, would be absurd.

Article 9.2.7 relates to bracing effected by a slab or deck, so perhaps it is a bit of a stretch to apply it to your situation. However, I believe it is normal practice to regard a beam supporting joists fastened to a nailer on the top flange at two foot centers to be laterally braced.

BA

 

[a2mfk]

Without looking anything up, I agree with BA. That 2% rule would apply if there were say one or two braced points. I have used bottom chords of trusses to brace beams just as you are describing without much thought to that buckling force being very large for trusses at 2ft on center (standard in the US)...

 

[rscassar]

I had a detail very similar to this recently. My example was a 410mm deep steel beam (16") with a maximum flange force of 250kN. Therefore to provide bracing the roof trusses need to be fixed to the steel beam with 5kN lateral resistance to be considered effective in providing lateral restraint.

Each truss is fixed to the nailing plate with 2 skew nails for stability (as well as the hurricane strap for tie-down). So why wouldn't four trusses fixed with 8 skew nails which can laterally transfer 8kN into the roof diaphragm provide lateral restraint to the top flange of the steel beam?

 

[a2mfk]

Two things popped into my head:
-Uplift situation, does your bottom flange need bracing?
-Whenever you have to ADD bracing that is not already there, consider changing the beam size. Per ton, steel is pretty cheap versus the labor of bracing. Worth a look...