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Old Bowstring Wood Truss Bottom Chord Split. Worried.
6

Old Bowstring Wood Truss Bottom Chord Split. Worried.

Old Bowstring Wood Truss Bottom Chord Split. Worried.

(OP)

I've got this 50 to 60 year old warehouse with these existing bowstring trusses. California, light wind, no snow.



The owner is putting up a structure interior to this existing one, I am doing calculations for the interior structure only.

However, the architect just sent me this picture.



The architect is wanting to pay me to come up with reinforcing. I did some quick calculations, and figuring 20 PSF DL + 12 PSF LL, the ASD force on that bottom chord is approx #70,000, putting about 1800 PSI on the bottom chord.
I can reinforce that easy enough, but I'm worried the bottom chord itself isn't sufficient (I mean duh, it broke).

If it broke it'd be a catastrophic failure, and I'm guessing all of the other truss bottom chords are not too far away from looking like this.

I don't want to take responsibility for the existing structure. I also don't want the building to collapse from people trying to fix this. I don't want any body getting themselves killed either.

What are my options?

A. Locally reinforce around split and write disclaimer that I'm not responsibility for remainder of truss or any other exisitng trusses or existing building? Will that disclaimer just make for a good laugh in a court of law?
B. Reinforce every truss bottom chord across full length. Write disclaimer that I'm not responsible for trusses or (e) building.
C. Tell architect the entire building is not fit for occupation until after full scale structural analysis on existing buliding is performed and reinforced as needed?

What's my responsibility? Do I notify the city of this and ask that it be red-tagged? Am I over-reacting?

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

Start with shoring.

I am of the opinion that you should be responsible for shoring, reinforcing that instance, observing other trusses, and notifying the owner of any structural findings, such as undersized truss chords. However, you can't say that the owner has to fix an undersized truss chord in a month or it will collapse, and reinforcing the whole structure isn't urgent. Take care of the urgent things, and then write a little report for the owner.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

Have you determined the cause of the split. It looks like more than they regular checking, so there is some reason to be concerned.

If I were you I would do do something between A and B. Obviously, your primary job is to come up with a repair scheme for this specific issue. As part of that, I would evaluate the cause and determine if further measures are required.

If you do calculations for the capacity of the chord vs demand, how much over or under is the chord? You probably have to make some assumptions about the wood, but you should be able to get reasonably close.

It's not a great picture, so it's hard to tell. But, my gut feeling is there is a connection issue locally at that connection that caused the split. If so, then all the connections (or the most highly stressed ones) may be susceptible to a similar failure.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

By the way, option C means that they're just going to get a different engineer to look at it and they might refuse to use you again.

Option D: Reporting it to the city and suggest it be red tagged. I doubt that this structure is in imminent danger of collapse or poses an immediate threat to the public. So, you have no obligation to report it. If you have done an analysis that uses a reasonable allowable stress and shows that the chord is dramatically overstressed and this is merely one of many signs that the structure is in trouble, then yes you would probably be obligated to report it.... but, only if the client / owner was not willing to take some action to repair it.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

The stress levels used that long ago were much higher than is allowed now days, probably 1500 psi with duration of load factor of 1.25 on top of that. So you'll likely find the bottom chords highly stressed.
Looks to me that the chord may have slightly more than the allowable slope of grain, which was 1:12 I think, for tension members. All this lumber was visually graded, so many times this slipped through.

It is difficult to splice with enough bolts to develop the tension in the member. If you do splice it in one place, it may split again in a another location. Since this is basically a tied arch, the tension is constant the whole length of the chord. I've had success using rods, steel bars or cables the full length of the chord and welded to the heel plates at the ends.

I see small chances of this being an immediate danger, as the roof decking acts to bridge the broken truss and distribute the load to the adjacent ones. In CA, it shouldn't be a problem unless you get snow, or the Owner puts on another layer of roofing, or you get a significant seismic event.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

Presuming that there is no incipient failure...

Make the offer to the Owner to evaluate the other trusses, stating what your visual assessment has uncovered. Tell him explicitly that you have not performed any engineering to delve into the remainder of the building and the problems it may contain.

Once the Owner is fully apprised of the situation he can make his own judgment as to whether or not he wants to invest in his structure.

His non-robust building shouldn't be your liability, it should be HIS liability.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

2
Cal91:
I wonder if that split didn’t start at that knot, where it probably comes out the other side of the member, up near the upper corner of the lower chord member. The left and right parts of the member have already moved/separated by about 1.5” or 2” w.r.t. each other longitudinally. The trusses are spaced at 20’ o/c, so I’m not sure how much deck system bridging you’ll get. I suspect the deck system is 2x’s spanning btwn. the trusses, simple spans, can’t make out the actual decking. Otherwise, I agree with Sawbux, that will be better lumber than we have today, and will have been designed at a higher allowable stress than we can use today. There are a lot of those type trusses around, and the problem is usually the joinery and movement in the connections. I would use harped steel bars to reinforce the bottom chord. Wire rope has the disadvantage of stretching too much during the stressing operation, both mechanical and elastic elongation. You will have to fab. some new end shoes to work with (fit around) the existing end hardware and anchor the ends of your new tension rods. Harping, setting the ends higher and the middle below the bottom of the bottom chord also helps a little. The harping low points may occur at the first or second vert. chords away from the truss center. You will also want to jack the truss up and try to realign and pull together the split before you start tightening your tensioning rods. You probably can’t completely pull the bot. chord back together just be tightening your tensioning rods. You sorta have to unload the bot. chord (entire truss) by jacking it up, and pulling the bot. chord back together. You might look at epoxy in the split, then vert. through bolts up through the split, on the centerline of the member, not to large, to maximize the net area. Clamp the split, then tighten the vert. bolts.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

In addition to the comments above regarding the strength of older lumber, the 20 psf dead load used in your initial calcs seems a bit high. It would be common for an older warehouse like this to be designed using a dead load of around 12 to 15 psf including the weight of the bowtruss. Once you figure out an allowable stress to use, back calculate the allowable dead and make an assessment on whether it has been exceeded.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

Quote (dheng)

You might look at epoxy in the split, then vert. through bolts up through the split, on the centerline of the member, not to large, to maximize the net area.

That might be a really good idea. I haven't done that sort of thing before though. Since you seem to have a lot of experience with this sort of wood issue... Why not use screws instead of bolts? Just curious.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

I'm guessing that you'd drill through for bolts, but screws can potentially split the wood yet again.

Given that as an option, one would think that running a steel strip along the bottom and bolted through the beam would be a plausible thing to do.

TTFN (ta ta for now)
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RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

I would make sure you analyze the truss for an unbalanced loading condition, a load condition that was not uses for bowstring trusses of the 60's era and before. Seems like this is not a shear failure, but a bending failure in the bottom chord. I suspect that the knot and grain orientation had much to do with the failure too.

With the failure of the lower chord, and other than the rest of the roof structure, what is holding this truss up presently?

Do you have a picture of the rest of the split you can post?

In looking at the photo, it seems like this is the last truss before the sloped rafters to the endwall.

Mike McCann, PE, SE (WA, HI)


RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

(OP)
Thanks everyone for your responses, very helpful. Busy at the moment but will responding later today be posting other pictures, none are a whole lot better unfortunately.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

3
Have you read this white paper?

Link

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

In addition to dhengr’s recommendations, I would wrap the entire split area with FRP.

The presence of additional bolts will reduce the effective area, and the addition of the FRP should make up for that.

Mike McCann, PE, SE (WA, HI)


RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

(OP)
Turns out there's (2) 1" Ø turnbuckle steel rods at each bottom chord as well. I'm not sure how I'd know how much force is going into the rods vs staying in the wood, but it's definitely a relief that they are there.

dhenger, great suggestion, and PropertyGuy67, thanks that is super helpful.

I'm talking with the architect and contractor later today. I'm going to have them shore it up and look into adding more post-tensioning steel bars as well as locally repairing the split, following dhenger's suggestions.

I'll talk to them about having a discussion with the owner about evaluating the trusses and costs for repair, etc.














RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

(OP)
That last picture clearly shows the split propogated from the connection bolt, and then through the knot.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

With the presence of two turnbuckles, one on either side of the bottom chord, has this truss been retrofitted in the past?

Where are the splices in the bottom chord?

OP: How long have the cracks seen been present? The presence of the cracking seen in the last two photos is different than originally seen... Is the cracking in the second photo up from this post on the opposite side of the crack seen in the first photo you posted?

Bottom chord is shot.

Mike McCann, PE, SE (WA, HI)


RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

Assume the rods have taken the full tension away from the wood bottom chord, are they adequate? Safety factor?

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

The two steel rods are probably an earlier retrofit and should now be considered as your main chord. You can may be able to band the existing split while using a good adhesive when installing the bands. Was involved in a similar situation in the early 90's when a building with similar but as not as well designed collasped. Those trusses had been put up in the 1950's in Richmond, Ca. Trusses all came down on top of the many cars stored in the building. There were eye-witnesses at the time because the cars were available for parts. An adjacent similar building, but not collasped, was retrofitted similar to the one you have.
It looks like there are channels near the ends of the trusses bolted to the original section of bottom chord with an anchor plate. That should be checked for adequacy. The one in California took the new rods to the top of the tilt-up walls with some sort of connection at that point. Check your local building department for possible drawings or even a permit.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

You might look for the last time the property changed hands and see if it was noted during any inspection. Also try and find if any retrofit is documented, see how far back the crack goes.

RE: Old Bowstring Wood Truss Bottom Chord Split. Worried.

Judging by the boot print on the rod anchor, I'd date that retrofit to 1974 or later.

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