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?

 

[youngstructural]

Are you modelling the trusses as purely pinned? They aren't... Timber truss plates do transfer minor moment loads, which can have a significant effect on the load patterns...

What size are the plates? I have a 1980s text on the capacity of the plates. Perhaps modelling with a modified spring stiffness or nodal moment loads will give you more realistic load patterns. As you've already seen, our models don't always match reality. The designs we produce often work more because of the general rule that if a total system has sufficient capacity it will redistribute as needed in service to achieve stability.

Very interesting situation... Keen to see how it all works out!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[SlideRuleEra]

Have any localized distributed or point loads been added to the trusses - either on the roof or hanging from the ceiling underneath?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ron]

StructuralJoe....those trusses were probably locally produced (Lakeland, Tampa, Brooksville, Dade City, etc.) so you can probably find the original truss drawings, even if the building department doesn't have them. Many of the larger lumber companies have small truss manufacturing operations tied to them.

The trusses were probably designed for bottom chord bracing and it was likely shown on the truss plans, but the contractor might have left them out.

If I'm understanding your photo, the failed member failed in compression. Further, it looks like either a gambrel roof or a mansard. Is that correct? If so, both pick up large lateral wind loads.

I agree with YS....our models don't always equate to reality, and the most common error is that of connections not acting like they're modeled. Do as YS suggested and see how your distribution changes. Don't necessarily look for failure in that member, but look at its axial load relative to the other members. If it is higher than the others of the same configuration, then you have an anomaly that might be pointed toward a single side loading (wind from one direction, for instance), ponding on one side, etc.)

 

[kslee1000]

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

Can you post a sketch of the truss, identify the loading mechanism, the failed and the survived members?

 

[msquared48]

If my memory serves me correctly, I believe that the Northwest received some bad lumber from Canada around that time. I don't recall the particulars, but there was a big flap about it. You might look into the grades provided.

Mike McCann
MMC Engineering

 

[mark1234]

Almost every roof truss failure I hear about during heavy wind loading happens because the web bracing was inadequate or not installed.





MDJ

http://www.windspeedbyzip.com

http://www.groundsnowbyzip.com

 

[vandede427]

Never ask "why" with wood. I've seen so many things that should have failed but didn't.

I've seen a rectangular shaped house have one of its perimeter basement walls completly collapse leaving the house to "span" the long distance from one side to the other.

Wood structures have so much crazy redundancy that they don't always follow traditional stability logic.

 

[StructuralJoe]

Thanks everyone for all of the input,

Attached is a jpg of the truss along with failed members from the program I am analyzing it in...

All I can determine is that the "other" members are overstressed but due to some of the modifications present (from the "handyman"... that just cracks me up every time I type it... I can hear it now "yeehaw I sur can fix yo probem... I'll nail me some 2x8 here and there and you's be good to go...) it isn't as noticeable on-site.

As to the cause I would think that even if this structure had experienced full DL and maybe full LL or WL I doubt it has ever seen current design load combinations DL + LL + WL (NDS - LFRD)...

I intend to add the lateral bracing and reinforce some of the members that fail in my model and go from there...

The buckled members appear to fail due to P-Crit axial loading...

If anyone has any design examples for wood truss plywood gussets that would be helpful also...

Thanks again,

 

[StructuralJoe]

Does anyone know of any procedure to determine the grade or strength of the wood in the truss?

I have designed the modeled the truss with current loads applied and based the lumber grade on the effectiveness of the truss... but I have no way of knowing if the truss was properly designed in the first place and if it was truely designed for a clay tile roof... the owner says it was originally a clay tile roof but that wouldn't be the first time I've been lied too...

 

[vandede427]

that looks like RamAdvanse

 

[StructuralJoe]

Thanks,

I have modeled everything as pin-pin everything as fixed-fixed and also fixed at continuous members (top & bottom chords) with pins at web members and TC or BC joints...

All with no luck... there where some additional memebers added (by the "handyman") and I am now wondering if that is what is throwing off my model?

 

[snaggle]

To size the plywood you need to know the member forces. 1)Calculate the number of nails needed in each member based on member forces. Multiply the nail values by 0.6 at the heels (Hankinson's formula).
2)Space the nails at 2" o.c. for 2x4 members (2) rows for larger members. This will size your plywood.
3)The nail spacing generally governs the plywood size, however it is wise to check the shear length of the plywood at the heels along the plane of the top chord.
4)Fasten the 3/4" CDX plywood to both faces of the truss w/ clinched nails.

Example:
clinched nail value = 200#/nail (fictitious value)
F_topchord = 4000 comp. -> 2400/(.6)200 = 20 nails
F_btmChord = 3700 tension -> 2220/(.6)200 = 19 nails

Plywood will need to be 19*2= 38" wide spanning from the btm of the btm chord to the top of the top chord

FYI- truss designs generally run D+L and .6D+W to obtain the best stress reversals and uplift calculations.

 

[mudflaps]

Joe, your colored stress diagram doesn't indicate the same member that the picture shows as failed. The failed member in the picture is the green vertical to the left of the one your note points to. The failed vertical in the picture has a pair of knots in it, small to be sure, but close together, in a compression member. I'm guessing a P-delta failure due to localized warping in the area of the knots. Also check for slippage of the press plates on the top chord of the vertical although I think the "handy man" would have noticed that and fixed it with plywood gussets. I suggest you get on a ladder and closely inspect the top connection of the green and blue web members. Something is moving up there.

Old CA SE

 

[kslee1000]

I have some difficulty to picture your analysis result.
-Do you have a column or wall directly below the vertical ridge member? The stress ratios are high for members around the bottom chord panel joint.
-Why is no, or negligible, stress on members near the left end?
-The stress ratios on members around the 2nd bottom chord panel joint from the left don't make sense either.
-Is the right end supported on roller for this particular case?

Any one has better clues? Or I am plan wrong?

 

[kslee1000]

The most intrigue point is question 2 above. Say there is 1 kip vertical load on the ridge, shouldn't the left vertical member take the entire reaction from the support below? However, your stress diagram shown the stress is close to zero, why?

 

[youngstructural]

I think kslee is on to something here... Several of your members appear to have "unlike" loads. Are you sure there isn't anything wrong with your model?

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[snaggle]

Correction: 1/2" CDX is typical not 3/4" as mentioned above.

 

[youngstructural]

lol... Make that "unlikely" loads in my previous post...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

Assuming Joe has identified the correct member as the one which failed, that diagonal is a compression member in uplift loading. If the trusses were designed for a tile roof as he has been told, removal of the tiles would have made that member more highly stressed in uplift than in the original design.

I think there is no doubt it should have been braced. I don't think that pinned or fixed connections make much difference, as the member would have buckled laterally, not in the plane of the truss.

Defective timber, as Mike suggested above, could be involved. A lot of pine bark beetle infested logs were slipped into the market a few years back. Deterioration due to heat and humidity is another possibility, especially if the roof space is not vented. The white spots visible on some of the members suggests fungal attack.