Cross Bracing Deflection 5

[TxGraeme]

My office was having a discussion about the design of rod bracing as a lateral resising system. The modeling software we are using (RAM Advanse) checks for bending + axial load stresses, which indicate the member is overstressed. The deflection in the model is over 10 inches, which I'm assuming is the cause of the high bending stresses. However, I have heard that braces are typically pretensioned to eliminate sag (and tensioned even further when lateral load is applied) and the brace straightens out so in reality there is no bending.

It is a light seismic zone and the rods are assumed to be tension only. The building is single story. Does anyone have any experience with this or have a reference we can check? I posed the question to AISC steel solution center but that didn't get me anywhere.

Thank you,
Graeme Sharpe

 

[Lion06]

Where do you draw the line between a thin rod and a cable? A cable certainly can't support any bending stresses.
I think this long, thin rod more closely approximates a cable than a beam.

 

[Tomfh]

I think this long, thin rod more closely approximates a cable than a beam.

I agree. I was just commenting that there will be *some* bending stress, albeit a fairly negligible amount. You can't have zero bending stresss unless the rod is tensioned perfectly straight, and that's impossible.

 

[haynewp]

A rod is not a zero rigidity bending member. The ends are also being forced against the natural slope of the hanging rod by the tensioning attachment at each end. I wouldn't go so far to say negligible bending, though I agree somewhere between catenary and bending. I don't know what effect the pretensioning of the rod (stiffening against lateral movement) has on its catenary ability.

 

[kslee1000]

For 3/8" dia. steel rod with 20' horizontal span length, it yields 10" deflection in the mid-span.

The 10" sagging is not accept to me, I guess your client wouldn't either. Suggest to provide turn buckle or connection plate in the middle to control the deflection due to selfweight (assuming tension capacity of the rod is adequate).

 

[kslee1000]

Correction: 13.5' span, not 20'.