Existing Wood Trusses are Failing... 3

[StructuralJoe]

I have any interesting project...

It is a 1-story plaza in Hernando County, FL

It has wood roof trusses spanning ~50' between load-bearing CMU walls

It was built in 1985 and from discussions with the owner the roof originally was constructed of plywood sheathing with Clay Roof Tiles...

Upon inspection the roof has 1 member on each truss that has buckled, typical on every truss

The owner had originally contracted a "handyman" to provide "repairs" although no engineer specified the repairs.

Members where added randomly as shown in attached photos.

I have since been contracted and modeled the roof truss with all applied loads to current code...

It appears that the trusses have no horizontal bracing

My questions:

The only member that fails (on-site) does not fail upon analysis... but other members do, why?

Bad wood grade on every truss?
Load transfer?
Lack of proper permanent bracing?
Current wind loads to excessive?

I have modeled the wood grades to be the minimum that will work for the minimum loads applied and not fail any members... but I don't feel comfortable assuming that the trusses where originally designed to not have any lateral bracing (rat-runs)

Any thoughts on this process...

Any suggestions or recommendations?

 

[Ron]

Joe...all trusses in your area require a grade stamp on the lumber. It should be visible. It is a grade stamp from the Southern Pine Inspection Bureau. The top and bottom chords are typically required to be at least No. 2 KD (kiln dried), while the diagonals may be No. 3 grade.

Hopefully you have modeled it without all the crap the handyman added. That should tell you what it "should have been". As hokie66 noted, removing the tile would make it more susceptible to uplift.

And then....you just might have a bad truss design!

 

[kslee1000]

StructuralJoe:

It maight not deserve the highest attention, as you can see the defects onsite, and make necessary modifications by judgement. But a valid analysis can check and backup your actions. From information you provided, the truss itself is inherently unstable, therefore member stress couldn't be predicted accurately. Someone before me has pointed out, that the reason for the truss to having survived for so long colud be attributed to the "hidden" redundancy built in the wood structures, however, it's not something you can rely upon. Also, someone has pointed out that the change in roofing material may have considerable impact on how the truss behaves. Bad wood theory? Difficult to image it happens on the pattern of one member on each truss.

How the truss was supported on the left end? Especially the upper panel joint directly above. Was it attached to something not shown, and/or could it be strengthened to take out the likely excessive lateral displacement?

Finally, with a limit budget, you may go ahead modify the roof system based on your best judgement. Otherwise, the truss merits a thorough structural evaluation, changes and $$$ are anticipated.

 

[snaggle]

You may consider having the local truss plant assist you with designing and repairing the trusses. They will be able to model the existing truss for you. Have them build a repair frame (partial truss) that will transfer the loads across the damaged sections. This is also the preferred approach to truss repairs (using metal plates), especially when there is more than (2) damaged trusses in series.

The repair frame should be continuous across panels. Fasten the frame to one face of the truss with 10d common wire nails. Leave all of the existing truss members in place (of course you can remove the broken ones)

Nail qty is determined by the forces in the top and btm chords in the end panels of the frame.

The drawback is that sometimes the required frames are too large to install and there is no choice but to use plywood.

 

[kslee1000]

Unless it serves a special purpose, the direction of the diagonal in the left end panel shall be reversed to provide better stability. Also, at the right end panel, addition of a vertical my help to stiffen the truss. The analysis is straight forward, give it a try.

 

[kslee1000]

Correction: To the left end panel, try "y", or "X".

 

[BAretired]

As noted by mudflaps, your photograph reveals that the second vertical from the left, shown in green on your elevation appears to be the member which buckled. That makes sense as it is a compression member under gravity load whereas the member you flagged is a tension member.

If your elevation is drawn to scale, and the span is 50', the height of that member is about 7.1 feet. The maximum slenderness ratio you can use for a compression member is 50 in CSA 086-01 (the Canadian timber code). For a width of 1.5", the maximum length of an unbraced compression member is 75" or 6'-3". The member should have been braced to meet the minimum standard.

Based on your geometry which I scaled, I believe the compression in Vertical #2 is about 1.7P where P is the panel point load, in this case w*2*50/8 = 12.5w where w is the uniform load per square foot.

If your roof load is 20 Dead + 25 Live = 45psf, P = 562# and the load in Vertical #2 = 956#, far too much load for such a long member.

In all probability, the original design required bracing but it simply wasn't installed. It happens all too frequently.


BA

 

[hokie66]

StructuralJoe,

Mudflaps and BA seem to be correct that you have misidentified the member which failed, but it is hard to tell from the photo. It would be good if you can clarify for us, perhaps with another photo.

 

[BAretired]

hokie66,

I agree. When I first looked at the photo we have, I thought the "X" in the middle was some kind of cross bracing. After I stared at it for a while I realized that it was a web member from each truss which had buckled over until they met in the middle. That's one way of reducing your slenderness ratio!

I think the "handyman" added a couple of verticals to each truss but didn't remove the buckled members. Just as well! They may be carrying as much load as the repair.

It would be interesting to see a photo from the side, but it might end up looking like a forest of two by fours.


BA

 

[BAretired]

I marked up the photo showing my interpretation of the buckled member and the repair for the left-hand truss.


BA

 

[kslee1000]

StrJoe:

Can you provide location of the supports, and details on connections? Was there a change of support conditions after erection? I don't think the original designer/supplier/erector are all ignorant.

After re-exam the photo, I guess the top and bottom chords are still in position, means there is no obvious problem with lateral stability, but how and why so much compression gets into this member, if the chord is free to deflect in vertical direction?

 

[BAretired]

kslee,

For the geometry of the two trusses in the photo to accommodate buckling of Vertical #2, the top and bottom chords would have to move about 2.7" closer together. Whether the top went down or the bottom went up or a combination of the two is not known.


BA

 

[BAretired]

The upper halves of the two members which failed clearly are bowed toward each other. The bow may have been present from the beginning or could have occurred as a result of drying shrinkage. This would have added eccentricity to these members when loaded and likely contributed to their failure.


BA

 

[kslee1000]

Yes, it is very difficut to detect the 1.35" deflection as compared to the huge displacement of the buckled members, provides there is not much evidence on anticipated deformations (tear, pull, warp) around the bottom chord. Thus, I think it is imperative to set the investigation of load path on higher priority than laterl stability. Though ultimately it might be found that lacking of lateral braces has significant impact.

 

[msquared48]

Could you tell from your field exam if there could have been either excessive waning or an oversize knot at or near the point of failure in the two chord members?

Mike McCann
MMC Engineering

 

[BAretired]

Mike,

I assume you mean the two web members? Right?


BA

 

[msquared48]

Here is some info on that bad wood. It was first found here in 1990, but could have been in the mix prior to that date. I will have to post several times here to get all the info in.

Mike McCann
MMC Engineering

 

[msquared48]

Next:

Mike McCann
MMC Engineering

 

[msquared48]

And again:

Mike McCann
MMC Engineering

 

[msquared48]

Finally:

Mike McCann
MMC Engineering

 

[BAretired]

Mike McCann,

I am not in a position to defend the Canadian lumber market because I know very little about it. Perhaps there was some serious skulduggery going on in that industry. I don't know.

What bothers me is your assumption that the problem addressed in this thread is in some way connected with bad lumber from Canada. I really thought you had more on the ball than that!

If you had taken the trouble to read my earlier posts, you would know that the design of Vertical #2 could not possibly have been justified, even if it had been grown in the good old U.S.A.

Mike, I really expected more from someone of your standing in the engineering community.

Bruce Alexander, P. Eng. (retired)


BA