Creep in 1920 wood truss
Creep in 1920 wood truss
(OP)
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
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






RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
"there is splitting along a top chord through bolt connection." BIG red flag here!!!!!!!!!!! That, coupled with the roof leaking and wetting/drying the wood under load could be the main problem. There may be slippage in the connection. IT NEEDS TO BE CHECKED ASAP!
When was the last time that the old roofing was removed from the structure prior to re-roofing?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Creep in 1920 wood truss
Just check the connection you mentioned real close.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
BA
RE: Creep in 1920 wood truss
Where there has been splitting of a chord, that can probably be rectified by adding bolts or screws to pull the members back together.
The L/d of the trusses is 12.5, which is quite comfortable. Is the roof essentially flat or sloped? If flat, ponding could be an issue with the deflection which has occurred over the years.
RE: Creep in 1920 wood truss
Creep does not usually weaken the timber, although the opposite could be true i.e. weakened timber would tend to creep/deflect more.
Nailing plates may solve the problem of the splitting, I would be careful about adding any more bolts unless this is rectified.
RE: Creep in 1920 wood truss
Yes, nailing plates may be the answer for the splitting. I was thinking he meant the 4 chord members were separating at the hanger, but you are probably right, there is splitting on the wide face.
Another issue is to check the joints in the chords. The 2x12's wouldn't be in 50' lengths. If only one is discontinuous at a given location, this may not be a concern.
RE: Creep in 1920 wood truss
Also on the L/d comment, the 4' height was not measured but only estimated ( and poorly, apologies). Am heading out to take measurements, inspect all the connections I can get to.
The chords 4-2x12 are not continuous, however the breaks are staggered. Added pic of the split at connection...
Each truss steps with the pitch of roof, probably about 0.5/12 pitch or larger, we already discussed ponding.
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
The joint shown needs to be reinforced, and I would suggest if other joints are fitted in that manner, all the joints near the ends of the trusses should be side plated. The cut in the chord is so shallow that longitudinal shear has split the chord, at least the outside member which is shown. The web is cracked also.
These trusses have no capacity to resist wind uplift. If you must follow current codes, does the dead load of the system provide sufficient ballast against uplift?
RE: Creep in 1920 wood truss
You also indicated that the bottom chord is made up 4- 2x12 laminated, the net section of the bottom chord is only 3-2x12 in that case. The tension in the member is then governed by how much load can be transferred by the fastenings between the laminations. The deflection that is measured is probably due more to slip in the inter-lam connections than creep in the wood.
The deflections in these trusses can be removed by tightening the vertical rods- if the nuts will turn.
I have reinforced many old trusses using cables, which I tighten to reduce a portion of the dead load stresses. I limit the preload to about 1/2 to 3/4 of the calculated dead load to avoid putting the bottom chord in compression, which can make it buckle. I size the cables to take the full D plus L load for safety. The wood chord will still carry the live load because the EA of the wood is much greater than the cables will be.
The split you have is caused either by horizontal shear due to the thrust of the diagonal member ( I suspect it is at one of the outer panel points?), or cross grain tension caused by rotation of the joint. It is best repaired by bolted clamp plates and drawing the split closed.
I recommend you check for slip in the lower chord. Look for gaps between the ends of the discontinuous 2x12's, any fresh looking wood - indicating movement, deterioration or slip in the fasteners, etc. It sounds to me like they might be a good candidate for the cable fix.
S. Wagner, SE
RE: Creep in 1920 wood truss
The wood is already noted as splitting, and the joints are noted to be separated by as much as 1/4 inch.
Instead, consider adding 3/8 (or 1/2) plates on both sides of each truss joint. Then drill through the joint (through the 1st plate, the wood, the then second plate) and bolt the joint back together by requiring at least 2x bolts in each compression member of wood going into the truss joint, and at least 3x in each tension member.
Painted flat black, the new plates will be visible but won't be too ugly - rather striking in their own way - and the repair will allow you to pull members back into position. For example, put a jack under the sagging pieces, and tension up the joint plates when the sag is removed.
Tension cables can be run between nearby plates if a tie point is provided.
RE: Creep in 1920 wood truss
We ended up adding W24 steel beams OVER the roof, directly above the trusses and dropping rods down from the beam to cross tube brackets - essentially picking up the truss at its panel points and taking the overall load into the above-roof steel beams which spanned across the main room and landed on the existing load-bearing brick walls.
An awkward, sledge-hammer fix but it gave us the assurance of safety and avoided massive interior work which would have cost a lot.
Here is the cross bracket detail/photo:
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
JAE the owner and I discussed a similar fix at the original inspection, using a steel truss conficuration over each existing. If it comes to that it's good to know it's feasible.
RE: Creep in 1920 wood truss
Sawbux, do you use the cables over the entire length of truss? The inacceblilty of the bottom chord of the trusses may make this impossible without demoing plaster (we have stacked/stepped ceiling joists on top of bottom chord, not to mention mechanical)
What torque would be required to tighten the nut on the vertical rods (provided there is sufficient thread)?
If ponding is not a concern, could I reinforce the truss in it's current deflected condition without returning it to it's original profile (assuming no additional deflection will exceed the connections once reinforced)
JAE, when you did the reinforcing from above did you attempt to "pull-up" the truss to overcome the deflections?
Will post additional pics
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
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).
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
BA
RE: Creep in 1920 wood truss
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.
RE: Creep in 1920 wood truss
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?
RE: Creep in 1920 wood truss
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)
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
http://woodenroofinc.com/articles/Vintage.pdf
RE: Creep in 1920 wood truss
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
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
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
RE: Creep in 1920 wood truss
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Creep in 1920 wood truss
Key in on the safety of the trusses and don't worry about their shape.
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
RE: Creep in 1920 wood truss
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.