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Creep in 1920 wood truss

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

RE: Creep in 1920 wood truss

Can you clarify something?  You mention that the owner noted 11 7/16" of movement since 2003.  Is this accurate?

RE: Creep in 1920 wood truss

(OP)
Typo, 11/16" movement total, 1/8" in two years

RE: Creep in 1920 wood truss

If I read it correctly, that's L/50, way too much.  With plaster, the total load deflection should be no more than L/240, and the live load (snow) deflection L/360.  That's 2.5" max under TOTAL load.  The dead load deflection should be no more than 1".   

"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

Well, I guess things change... I was typing as the update cam in...

Just check the connection you mentioned real close.

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

RE: Creep in 1920 wood truss

(OP)
The re-roof was around 2003 and the owner is trying to find out what (if any) of the existing was removed prior to the new membrane install.  From the attic/underside the original planks are visible.

RE: Creep in 1920 wood truss

Sounds like it's deflecting too much under dead load.  I would strongly consider strengthening all of the existing trusses and devising a repair detail for the split top chord.

BA

RE: Creep in 1920 wood truss

This type truss deflects primarily due to loosening of the joints.  As the chord members shrink across the grain, the rod tension members loosen, and of course nobody retightens them.  This causes deflection and slippage of the fitted compression webs.  The strength of the timbers is not likely to have changed appreciably unless the water damage was substantial...it is all in the joints.  So creep is probably not the best description of what has happened.  To confirm whether or not the timber member strength is an issue, you should probably have them assessed by a grading expert.

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

I agree with hokie, this level of movement is most likely due to the connections loosening up.

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

csd72,
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

(OP)
OK so connections, I did see some gaps (not large, maybe 1/4") between the the bottom of the top chord and the angled 4x webs which I assume to be s result of the rotation due to deflection.  I can clearly see the rods and threaded portion with nut above the top chord (at least in some areas), bottom is mostly inaccesible.  Can that be tightened on a truss this large?

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

Not that it has much of an effect, but moisture in wood will exacerbate creep. Just did some research for a job and found a couple articles on it. Wood is one odd building material when you really get into it.

RE: Creep in 1920 wood truss

Yes, the rods should be able to be tightened, provided there is sufficient thread.  Tedious but necessary if the trusses are to be saved.  A 1/4" gap between the fitted compression webs and the chord makes for large deflections when geometrically applied over the length.

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

My experience with old wood trusses is that most are overstressed in the bottom chord.  Older designs used higher allowable stresses for tension.  Research in the 80's (?),( on full size members) has greatly reduced the allowable tension values for wood.

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

I would be very suspicious of "nailing" the truss joints back together.

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

I had a church project a few years ago with a similar truss - sagging at midspan and cracked plaster ceilings.  There was concern over the ultimate capacity of the trusses and really no easy way to evaluate the safety.

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

(OP)
Thanks all for yout input, will keep you posted.  

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

(OP)
I reinspected the trusses that are accessible (about half of total)  Found three connections in bottom chord where the plies were bolted together (staggered 18" o.c.) and bolt was too close to the end of the ply and thus failed.  See pic with the gap in the ply. There's likely more at the inner plies.  Several bolts appear to be bending (there's prying to one side of the washer/bolt). The bottom and top chords are both sitting in pockets in the brick wall each end, but there's a gap between brick and the top chords so all the support is now at the bottom?  Bottom bearing is hard to see, but there's steel bolted plates and connection has varied gaps (can see bolts).
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

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).

 

RE: Creep in 1920 wood truss

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.  

RE: Creep in 1920 wood truss

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

BA

RE: Creep in 1920 wood truss

(OP)
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.

 

RE: Creep in 1920 wood truss

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?

RE: Creep in 1920 wood truss

(OP)
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)

RE: Creep in 1920 wood truss

(OP)
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.

RE: Creep in 1920 wood truss

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

RE: Creep in 1920 wood truss

(OP)
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.

RE: Creep in 1920 wood truss

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

RE: Creep in 1920 wood truss

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

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

RE: Creep in 1920 wood truss

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.
 

RE: Creep in 1920 wood truss

(OP)
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.

RE: Creep in 1920 wood truss

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.

RE: Creep in 1920 wood truss

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

RE: Creep in 1920 wood truss

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.

 

RE: Creep in 1920 wood truss

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.

RE: Creep in 1920 wood truss

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.

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