Bowstring Truss - Reinforcing Bottom Chord 1

[JRWY]

Hi all;
I've been tasked with ensuring that some large wood bowstring roof trusses built 40-50 years ago are reinforced and bought up to current Canadian design standards.

In particular, the strength of the glulam bottom chord is considerably lower than what's currently required. We've designed some high strength steel tension rods connected at the ends and supported mid span by wing plates similar to what's been done in this thread (

https://www.eng-tips.com/viewthread.cfm?qid=293002)

here.
Currently the plan is to apply tension to the rod to remove sag, and then tighten nuts on either side of the wing plates. Jacking the bottom chord up before installing the rod is unfortunately not an option.

My question is this: How should I determine the amount of tension force that should initially be applied to the steel rods? We're currently thinking that they should be pre-tensioned to 80% of the dead load force in the bottom chord, but we're still a little concerned about accidentally inducing compression in the wood. If we were to tension them to only, say, 10% of the axial dead load just to remove sag, is there any reason why they wouldn't still take snow and live loads in place of the bottom chord?

Any help or guidance would be appreciated - please let me know if anything needs clarification.

Thank you!
Engineer in Training

 

[oldestguy]

I'd use a precision dial gauge with a sufficient distance between grips attached to the rod and measure the strain in this distance. You get stress by assuming the coif of strain to stress for your steel.

 

[phamENG]

It's all a matter of stiffness. If your new tension members are stiffer than the original bottom chord, then yes - they'll take the live load in lieu of the bottom chord (assuming your connections are configured in such a way that this is possible). A sketch of what you're planning may help.

A couple things I would be concerned about:
1) Eccentricity in the bottom chord. If the bottom chord cannot be shored and has deflected (under self weight, weight of a ceiling, etc.), then applying the rods with tension may induce bending in the bottom chord akin to secondary moments in a column and accelerate failure. Of course this could be mitigated by ensuring the bottom chord never goes into compression, but this could take a pretty detailed analysis to make yourself comfortable.
2) Bolt Stress Relaxation. Bolted assemblies, including threaded tension rods, will relax after they've been tightened to the desired pretension. In other words, you tighten it down to where you want it, and when you come back 2 months later you no loner have your tension. This could render a repair such as this ineffective or completely useless. To be clear, I'm not talking about the nut backing off or anything of that sort - the sresses in the rod itself relax as the rod creeps under tension.

Best of luck.

 

[MotorCity]

What is the reason for the reinforcing - is it to merely bring it into compliance w/ current codes or is additional load being added?

I doubt you will be able to remove the sag without jacking or shoring the truss. You run the risk of the bottom chord suddenly "snapping" upward which could be catastrophic.

Essentially what you are trying to do is external post tensioning. I don't think that introducing compression into the bottom chord is necessarily a bad thing IF that was the original design intent and it was constructed that way (like a prestressed or post tensioned concrete beam). But in your case, its being done under service loads which means that some of that load needs to be relieved (i.e shoring)

 

[Sawbux]

I've had success in cable reinforcing bowstring trusses, prestressing them to reduce the dead load stresses. 80% as you mention seems like a reasonable number.
In the building I did, the ends of the trusses were accessible by cutting out a bit of siding. I put a bearing plate across the ends of the truss, fed through some swaged rod ends attached to the cables. The cables were then tensioned by extending the rods through another plate, putting a portapower hydraulic cylinder between that plate and the plate at the truss end. Then you pump up the cylinder to the desired load and dog down the nuts.