strengthening 112ft span timber truss

[calculor1]

Currently assessing the capacity of a 80 year old 112ft single span timber truss. The analysis indicates excessive deflection based on today's code. We are recommending exterior post-tensioning. Any other solutions?

 

[registeredpe]

Greetings:

Are intermediate supports an option? Although you would have to reinforce/modify accordingly due to stress reversals in truss members.

Best regards, registeredpe in AZ

 

[calculor1]

No intermediate supports permitted.

 

[JAE]

I had two projects with old timber trusses.

bowstring truss
This was about a 150 ft. span I think - flat bottom chord and curved "arched" top chord. Split ring connections with bolts and the bottom chords had in many places split at the nodal connections. Top chord was gluelam I think.

We used twin steel tubes, one each side of the bottom chord, and attached them to the bottom chord with new side gusset plates through-bolted away from the panel points but then used additional plates at the existing panel point gussets to attached there as well. The truss was shored and jacked up a bit to bring it back to the original position. The steel tubes were spliced with full pen welds and essentially substituted for the bottom chord. The trick was getting the diagonal forces to flow into the new tubes.

Old Church Trusses
100 ft. long built-up wood trusses with verticals comprised of steel pipes bolted to at the ends and all wood compression diagonals. Couldn't fit any strengthening on the sides of the trusses or bottom due to existing framing and finishes. Ended up adding two new wide flanges over the trusses and dropping twin hanger rods on either side of the truss to brackets at the tops of the steel pipe verticals...essentially grabbing the truss with "sky-hooks" at each panel point.

 

[apsix]

Why is deflection suddenly a problem after 80 yrs, or are you wanting to increase loading?

 

[dik]

add 2x4's on each side? <G>

Any chance the truss is of historic significance. Have you run a detailed analysis? coupons for strength tests?

About 25 years ago, I encountered an old truss that an engineer was having difficulty with. A very shallow sloped member near the support was overstressed by a factor greater than 3 and the large force was having an effect on adjacent members. I suggested he re-run the frame and eliminate the offending member... everything else then worked.

You have to carefully look at connections, too. These are often where failures are initiated. Some bolts / sideplates / castings can be overstressed. These can be improved and I've often used 'glulam rivets' for added load capacity. Also can use FRP reinforcing. If historic, then you should be looking at concealed reinforcing and it's costly.

Dik

 

[Sawbux]

I've repaired many trusses with cables. Typically bowstring types.

The post-tensioning will have little effect on the live load deflection. On a typical truss, the AE (as in PL/AE) of the wood member will greatly exceed the AE of the added cable steel. The live load will continue to be resisted mainly by the wood.

I typically use the cable to reduce the dead load stress- but not completely, since you don't want the bottom chord to go into compression with the long unbraced length. The dead load stress relieved can be used to decrease the overstress in the live load condition.

 

[MichiganPE]

I've also used post tensioning of the bottom chords to reinforce large span wood bowstring trusses. I also concure that you have to closely monitor the installation so that the bottom chords do not go into compression. I used a thick steel plate attached to the outside face of the truss heels to anchor the cables at the ends.