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Wood Truss Top Chord Buckling

Wood Truss Top Chord Buckling

Wood Truss Top Chord Buckling

(OP)
Hello -

I was recently sent out to look at a agricultural facility where the owner noticed significant deflection in the ceiling of their buildings. The site is a group of nearly identical livestock confinement buildings of varying age (oldest building being from 1991, newest from 2001). This is all post frame construction with the roof being constructed with metal plate connected wood trusses with 2x4 purlins on top of the trusses and the roof is sheathed with corrugated steel panels.

The metal plate connected trusses span 70' and have 2x8 top and bottom chords. In the newer buildings, the deflection in the ceiling is not as noticable and the deflection in the ceilings certainly becomes more noticable as you look at the older buildings.

When in the attic space, I could see the top chords in the buildings were buckling out of plane. In the newer buildings, where the buckling was not as advanced, the top fibers of the top chord were still restrained from lateral movement by the purlin connections but the bottom fibers of the top chord were clearly buckling laterally.

In the older buildings, the entire top chord was buckled and displaced laterally (broke free of the purlin connection), sometimes by as much as 18".

The ownwer attempted a repair using their local contractor of choice. I'm not convinvced the repair is at all adequate. The repair consisted of running several rows of continuous bridging (2x4's) along the bottom of the top chord. Then, they placed blocking on top of the bridging anywhere top chord buckling had occured to prevent the top chord from displacing laterally any further.

Two things I'd appreciate some opinions on. First being the cause of this type of failure. I suspect the trusses were undersized at original construction and the bucking is occuring due to creep. Anyone agree/disagree? Or have other insights?

Second thing: repair. Is their anything other than total roof system replacement that you would reccomend for these buildings?

Thanks you for your help and insight!

PS. I'll try to get a few pictures up shortly...

RE: Wood Truss Top Chord Buckling

I would probably reconstruct the roofs using positive bracing... it is possible that the deformation may be permanent due to the time period. They can be braced to maintain their strength. With the P-Delta effect and added torsion the metal plates may have been dislodged; these should be checked. The TPI has guidelines.

If the roof is failing under dead load only, then action should be undertaken quickly. Once deformation has occurred the forces increase until failure.

Barricades should be put up for no human access... (and maybe cows <G>).

Dik

RE: Wood Truss Top Chord Buckling

I doubt the trusses were originally undersized. I would say that most likely the purlins to truss connection, as it appears to be only nails from above, is the more possible problem. With wind uplift pulling the nails loose over time.
As for a total roof replacement, it would depend on how much cost it will be to correct the placement of the trusses, provide a better purlin to truss connection, and inspect/repair the trusses vs doing the total replacement.
Inspecting the new buildings and upgrading the purlin to truss connections as required should also be done.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood Truss Top Chord Buckling

(OP)
When I first saw pictures of the trusses, my very first reaction was the same as yours, woodman88. I thought for certain the issue here was the fastener selection for the purlin-to-top chord connection. The fastener used was a smooth, common nail. I was initially of the opinion that uplift forces and the humid environment combined to remove the top chord bracing provided by the purlin.

After being on site and inspecting, I still think the fastener selection was inappropriate, but not at the core of the problem here. I say this becasue in the newer buildings, the buckling is very evident at the bottom fibers of the top chord yet the top fibers of the top chord are still connected and restrained by the purlins.

With top chords that are this deep (2x8's), would a truss manufacturer typically specicy some means to also stabilize the bottom fibers of the top chord?

I'll post some pictures from the newer buildings as well...

Also a interesting side note from the site visit. This site has 9 buildings total. All more or less identical. 3 buildings have no issues with the roof trusses and are about mid-range as far as age (some buildings on site are newer, some are older). The 3 buildings with no issues have MSR graded lumber for the top and bottom chords. The 6 buildings with issues have Select Structural lumber used for the top and bottom chords.

RE: Wood Truss Top Chord Buckling

I have not ran into this situation before but here are some more things to consider.

"... the buckling is very evident at the bottom fibers of the top chord yet the top fibers of the top chord are still connected and restrained by the purlins."
This condition I would say could only result from an upward movement of the top chord (placing the bottom edge in compression) or extreme warping of the lumber. What species and grade is the lumber?

One thing is about MSR vs Visual Graded lumber. The MSR lumber having about the same strength as Visual Graded, in my experience, has always been of better appearance and also seems to weather better than the Visual Graded lumber.

The truss designs probably specified the top and bottom chord oc bracing requirements. With 2x8 chords the bracing could be attached to just one edge of the chords.

I assume that the owner does not have copies of the plans or truss design for you to review.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood Truss Top Chord Buckling

Disagree with woodman88 about the buckling being caused by uplift. I think it is just buckling caused by the chord compressive force, with no effective bracing. The chords which are now bent only at the bottom will eventually pull free of the nails and the entire chord will spring over, as has occurred with the others. I think this can be repaired, but I would require all the members to be straigtened rather than just bracing them in place.

RE: Wood Truss Top Chord Buckling

One thing that has not been discussed here is what is happening to the diaphragm. The diaphragm probably has a stronger connection to the purlins than the purlins do to the trusses. Also, as the purlins are flat 2x4(?)'s they will more readily deflect perpendicular to the diaphragm. If the diaphragm does buckle downwards it can only be in the area between the trusses as the purlins are blocked by the top chord at the truss from going downward. It would be easier for the diaphragm to buckle upwards pulling the purlins up with it and than the top chord or away from the top chord depending on the strength of the purlin to chord connection. A close look at the diaphragm and any buckling in it, may help explain the separation of the purlins from the chord and/or the truss top chord buckling.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood Truss Top Chord Buckling

What is the loading history? High snow load area?

RE: Wood Truss Top Chord Buckling

(OP)
To my knowledge and the site managers knowledge, there have been no unusual loading events. Facility is not located in a high snow load area (Southern Indiana). I was alsso expecting to see alot of equipment mounted to the bottom chords of these trusses but upon examination during the site visit, there is no equipment mounted to the bottom chords. The only bottom chord loading is a light gage corrugated steel liner and about 8" of blown-in insulation.

The owner does not have any drawings for any of the buildings. My experience has been that if it's an agricul;tural building of any kind, they won't have plans :(

I'll have to get the additional photos added tomorrow. I apparently had the camera resolution set on very high and the file size is too big to upload. I'll reduce the file sizes at work tomorrow and post.

Thanks for you help thus far guys!


RE: Wood Truss Top Chord Buckling

(OP)
This attached photo shows the attempted repair to the trusses - I think you can see all the components of the repair in this picture. Each truss top chord was attached to each purlin with light gage angles and fastened with screws. Also, continuous rows of bridging (2x4's) were installed on the bottom of the top chord and blocking was placed on top of the bridging to prevent the top chords that were bowed/warped from moving out of plane any further.

RE: Wood Truss Top Chord Buckling

(OP)
So I would appreciate your thoughts on that repair. The repair has stabilized and prevented the top chord from buckling out of plane any further, but the buklcing/warping was not straightened out or removed prior to the repair. Do you think it is a safe roof system with this repair?

RE: Wood Truss Top Chord Buckling

For information on the allowable bow in wood trusses you can download the technical info on temporary bracing from http://www.sbcindustry.com/bracing.php.
The more you allow the truss chords to bow, the greater force is needed to keep the chord in place. In the top chord this force is being resisted by the roof diaphragm. Whether you have a safe condition now depends the ability of the diaphragm to resist this force in addition to the other code require loads. I would say that the trusses should be bought back to the Wood Truss Industry Standards (if they do not meet these standards) rather than to rely on the connection of the truss chord to purlin to maintain itself over time.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood Truss Top Chord Buckling

I wouldn't judge the repair to be appropriate without straightening the chords. Otherwise, the idea is good.

RE: Wood Truss Top Chord Buckling

(OP)
Here is another picture that has me re-evaluating the casue of the buckling...In this picture, you can see evidence of a continuous drip line in the blown-in insulation directly below each purlin. This building can be extremely humid given its usage (there are mountains of manure in the space below the roof trusses).

Anyone think that maybe this buckling could be warping from extreme humidity rather than buckling from compressive stress?

RE: Wood Truss Top Chord Buckling

At this website is some information on bending wood with heat and moisture, http://en.wikipedia.org/wiki/Heat_bending_of_wood. This is what I think is happening in your picture, with the action of compression force of the chord.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood Truss Top Chord Buckling

(OP)
A ha! Now we are really on to something, Woodman88! I did a little research on heat bending of wood and a common technique used in the process is chemical treatment of the wood to soften the wood before bending. One of the most common chemicals used...Ammmonia!! And there is a great deal of ammonia in the manure!!!

Now the question is...Does this long term exposure to ammonia and softening change the engineering properties of the wood....

RE: Wood Truss Top Chord Buckling

The following is from the "Wood Handbook 2010" A free download from http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

"Exposure to Chemicals
The effect of chemical solutions on mechanical properties
depends on the specific type of chemical. Nonswelling
liquids, such as petroleum oils and creosote, have no appreciable
effect on properties. Properties are lowered in the
presence of water, alcohol, or other wood-swelling organic
liquids even though these liquids do not chemically degrade
the wood substance. The loss in properties depends largely
on the amount of swelling, and this loss is regained upon
removal of the swelling liquid. Anhydrous ammonia markedly
reduces the strength and stiffness of wood, but these
properties are regained to a great extent when the ammonia
is removed. Heartwood generally is less affected than sapwood
because it is more impermeable. Accordingly, wood
treatments that retard liquid penetration usually enhance
natural resistance to chemicals."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood Truss Top Chord Buckling

How many trusses? (did I miss the answer in my quick read?)
I have repaired similar truss problems, and left many in a deformed shape because there was no other option. The axial force in the top chord needs to stay in the truss to maintain equilibrium in the plane of he truss, so the goal is to stabilize the truss. This requires significantly more bracing force than for a straight chord (just use trig to figure out how much). If there is good access, your best solution may be to insert new trusses into the attic space between the existing. This may require the use of 2-piece trusses or other methods to allow salvage of the roof.

No matter what method you use, it will likely cost nearly as much to fix as replace.

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