Post Tension Timber Beam - Rod Strength 1

RFreund · Apr 22, 2022

I came across an existing warehouse (old building maybe 70's) with post tensioned timber girders. The entire structure seems to be designed for some fairly high loads. However, when I check the post-tensioned girders, the rods seem to fail badly. The girder's consist of 6x12 beam (top chord) and a 3/4" diameter steel rod with turn-buckle. Depth from top of beam to centerline of rod is 30". Seems very odd that these would have been under designed given the "beef" of everything else.
A few questions come to mind:

[ol 1]
[li]What grade steel was used?[/li]
[li]Would they have used ultimate strength or yield strength?[/li]
[li]Were these tensioned or just acting as a bottom chord?[/li]
[li]Any ideas on why these rods would be so far overstressed?[/li]
[/ol]

phamENG · Apr 22, 2022

Do they look like they were part of the original design or a retrofit? Someone may have added them after the fact due to large deflections without a thought to stresses at even higher loads.

KootK · Apr 25, 2022

My guesses:

-36 ksi
-Yield
-Just bottom chord save some pre-stressing of the turnbuckles to get things taught. This very much depends on whether or not the rods are original construction vs retrofit.

24" effective depth on a 12" beam seems like a pretty shallow truss for this kind of setup.

Span?

One deviator at the middle or two at the 1/3 points?

phamENG's theory is appealing. Maybe the wood was creeping in a way that was deemed unacceptable.

RFreund · Apr 25, 2022

phamENG said:
Do they look like they were part of the original design or a retrofit?

I'm thinking original b/c the rods go up through the center of the beam not alongside & bolted. Also the steel and deviator look fairly old, but hard to really say.

Kootk said:
Span?

25'
I'm doubling checking measurements though, again, just very odd. Your assumptions are what I used in the analysis. I played around with post-tension which helps things overall, but still the rod doesn't seem to work.

Kootk said:
One deviator at the middle or two at the 1/3 points?

1/3 points

Kootk said:
24" effective depth on a 12" beam seems like a pretty shallow truss for this kind of setup.

Agreed. again, I'm double-checking dimensions.

KootK · Apr 25, 2022

With regard to the truss depth, if it's really that shallow, the beam may be competing with the truss as a full span member in its own right. It's a long shot but maybe that might draw some demand away from the rods. You're likely already picking this up if you're modelling it though.

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Post Tension Timber Beam - Rod Strength 1

RFreund

Structural

phamENG

Structural

KootK

Structural

RFreund

Structural

KootK

Structural

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