Creep in 1920 wood truss

[tdse]

Without any forensic structural engineering experience I've been asked to look into an HVAC remodel on a roof with problems. The building is a 90 year old theater with wood trusses spanning 50' between brick piers, 13'6" on center, about 4' tall, Howe style (I think, and top chord bearing from what I can see). T/B chords are 4-2x12 through bolted, angled webs are 4x4 and 4x6, vertical webs are steel rods through T/B chords with steel plates/nuts. The owner tied a string to monitor movement several years ago, and there is siginificant deflection at midspan (4 3/8" = L/137) under dead load (no snow when observed, design sl is 30psf). He's noted 11 7/16" movement since 2003, 1/8" in the last two years. It last went under a remodel in 1997 when the current HVAC was installed. There's no previous information on the project and the "structural plans" from the remodel are by an architectural firm. My question is, how much creep is too much. The plaster is cracking along each truss at or near midspan. There is also cracking at or around each brick bearing pier. I observed three trusses and all look Ok with one exception where there is splitting along a top chord through bolt connection.

There are several "collateral" loads which could not have been originally designed (mechancical ducts, lights, stage lighting, sprinklers). In addition I can only assume the "attic" is accessed for the recessed stage lighting possibly more often than any original intent as well. Lastly, before the current owner bought the building there was water damage, and the tar roof was replaced with membrane.

I plan on taking measurements and trying to find the capacity of the existing trusses. Does anyone know a good refence for calculating a decrease in capacity of the wood due to the creep? thx

 

[JAE]

tdse - We installed the beams, dropped the hanger rods through the roof and connected (bolted) them to the cross tubes under/near the top chord panel points.

These rods were then tightened but we didn't attempt to torque them such that the wood trusses would be brought back up to a straight condition.

We felt that doing so might cause additional damage to the underside ceiling plaster and also could possibly cause stresses -fighting against perhaps a newly deformed condition of the trusses.

The goal for us was public safety. We told the church that this method would provide a safe, understood load path to the bearing walls but would not necessarily eliminate the on-going creep in the trusses.

In fact, as the trusses continue to creep, they will slowly deliver load into the beams, causing them to deflect. So the downward deflection would be reduced and minimized but not eliminated.

We also recommended that the owner (the church) hire someone with specialized knowledge of plaster and sound out the ceiling areas with distress to ensure that there were no loose sections of plaster that could let go and fall on people below.

An on-going monitoring program was also recommended (i.e. sound out the ceiling at periodic intervals).

 

[hokie66]

The truss can't be top chord bearing unless there is a tension rod within the brick wall. The end panel top chord of a Howe truss is a zero force member.

 

[BAretired]

The end diagonal does not appear in the photo. This looks like a bottom chord bearing truss.

BA

 

[tdse]

OK, I had a hard time justifying the top chord bearing. Assumed it wasn't when I modeled it, but didn't know why they'd pocket it in the brick (except for stability).

Going to recommend reinforcing the bottom chord, repairing connections, investigating the plaster for loose pieces. Then we can address the rest of the support needed for the HVAC.

 

[hokie66]

Yes, the top chord is embedded for stability.

You need to tighten up the joints. Hopefully, you can retighten the tension rods. Grouting the compression members may be possible. Have you analyzed the member capacity of the trusses assuming that all joints are tight?

 

[tdse]

How much torque would you need to retighten the tension rods?

I did analyze the member but without taking a specimen for eval on the wood I don't know for sure if it would work. Needed DF No. 1 or better. Also, the dead/collateral load is more what was there originally with HVAC, stage lights, sprinklers, plywood catwalk (to stage lights)

 

[tdse]

The owner's asking about the "required torque" but I can't find where that would be applicable (hence the question). I am suggesting 1/4 to 1/2 turn of the nut, starting in middle where there's the least amount of axial load, proceeding symmetrically to outer bearing locations, and making a few passes if needed.

 

[frogit22]

You need to be careful here. Many of these systems are not properly designed. I witnessed one in CT where the bottom chord suddenly failed.

http://woodenroofinc.com/articles/Vintage.pdf

 

[msquared48]

tdse:

You said that "Here's a pic of the gap in the top chord bearing at brick. The outer ply has slipped to be offset-vertically from the others."

I wonder if that is the case or if you are merely seeing a mix of Rough Cut and Dimension lumber in the top chord, perhaps even the bottom chord?

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

 

[tdse]

mssquared48 Why would they mix rough cut and dimensional? Was that common back in the day? This is a pic of the top chord, albeit the adjacent truss but near same end. It shows a gap between the outer ply at bottom... This is also the truss where the water damage occurred.

 

[msquared48]

Like... It may have been what was available? In that these trusses may have been, oh heck, most likely were, field constructed, that is very possible.

Also, it appears from the photo above that the roof purlins were notched in the bottom to bear on the top chord. It would not be normal to do that unless there was a dimensional mix of lumber for these members, let alone the truss members.

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

 

[msquared48]

They may have even milled their own lumber on-site for that matter...

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

 

[JAE]

I would be careful about torquing any of the bolts on these trusses. The wood is deformed differently from where it started years ago. Re-tightening the bolts in an attempt to close gaps, etc. in my mind is a dangerous idea - you might be essentially forcing a deformed shape back towards its original geometry and developing stresses in the wood elements that you would know nothing about.

Key in on the safety of the trusses and don't worry about their shape.

 

[tdse]

Thanks all for your valuable input! Learnimg much through this experience. I'm sticking by reinforcing all the trusses, will have to see which way the owner will want to proceed.

 

[hokie66]

I hear what JAE is saying, but if the joints are not tight, you don't have trusses, but rather the top and bottom chords spanning the 50' in bending. So maybe a maximum of 6-2x12's effective as a beam rather than a truss.

 

[msquared48]

Hokie, I hear what you are saying, but both you and I know that (6)2X12's will not span 50 feet with a tributary width of 13'-6" the stipulated loads, so there has to still be a lot of truss action.

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

 

[JAE]

I was basing my comment on using secondary supports (i.e. beams above the trusses per my post above). I agree that if you don't supply a "new" support path then you have to strengthen the truss. With a truss there are so many parts to it (members, splices, joint connections, vertical rods, etc.) that it is sometimes difficult, if not impossible, to know whether you have a safe condition or not.

 

[hokie66]

Yes, I would agree yours is the best approach. With access difficult and some connections not even visible, it would be a lottery. But doing that sort of work would bring up the question of whether the entire building should be assessed and upgraded if necessary, which might not be feasible at all.

 

[Sawbux]

To answer your question on cable reinforcing trusses:

I have done the cables, one on each side, the full length of the trusses. Best connection detail I have found is to fit a bearing plate behind the ends of the bottom chord and passing threaded rod ends though that plate for tightening. There is usually too much load to transfer to truss chord by way of bolting to the chord.

It looks from your photos that this is a "carpenter-built" truss and likely not "engineered". The plies of the bottom chord are slipping as the bolting holding the plies together are inadequate. It appears to be a dangerous situation.

The solution of putting beams above the roof and suspending the old trusses from it may be a practical approach. I was involved in doing this method a couple of times. Once to hold up the roof of a plywood plant that the glulam roof beams had deteriorated due to high heat and humidity.

Main things to consider is availability of end support of the beams on the brick walls and lateral bracing for the long span steel beams.