TJL Floor Joists and Long Term Creep 2

[msquared48]

Got a project constructed under the 1976 UBC that has three stories of wood and steel framing over parking on a sloping lot.

The ceiling GWB is cracking in at least the second story end unit about 4 feet in from the end wall and I see no settlement issues with the foundation - on piling and no cracking. Don't know about the other units... yet.

I am leaning toward long term creep as the floor joists have a 1.5" thick topping spec'd at 9 psf. I am suspicious about the actual density of the topping too which would make matters worse... When coupled with the other framing, the dead load is 20 psf and the dead to total load ratio is 33 percent, exceeding the 22 percent guideline for table use, triggering a special computer analysis according to the November 2000 catalog, the oldest I have available. If you ignore the 22% issue, the joists appear fine for the loading and span seen.

Has anyone experienced similar long term creep issues in similar circumstances?

Mike McCann, PE, SE (WA)

 

[JAE]

Mike - not much numerical/technical information HERE(Weyerhauser link) but some general discussions that appear at first glance to match your situation a bit.

The standard TJI catalogs state that long term creep is not considered in their tables...either suggests that creep isn't an issue or they are simply covering themselves and don't know much about creep in their product.

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[msquared48]

I did find a one page Technical Bulletin (11/2016) TB-101 from Trus Joist, but it is dealing more with roofs and long term creep due to ponding issue.

This is one of those problems where the cause does not immediately hit you in the face, but creeps up on you instead,(no pun intended).

Mike McCann, PE, SE (WA)

 

[Triangled]

1988 is the 1st year I find any guidance for deflection calculation in their code reports. Section V "conditions of use". item 4

 

[XR250]

I find long-term creep in TJI's regularly under Kitchens. Usually, it is in houses with crawlspaces so I don't know if the elevated humidity levels exacerbates it.

 

[msquared48]

If long term creep is the problem here, then the only solution I see would be to remove the concrete topping, not acceptable though considering the owners of the condos.

Otherwise, unless there is another way, they would just have to live with the situation, periodically making Crack repairs.

As far as I know, this is just a serviceability situation structurally, and not a life safety issue.

Any other ideas?

Mike McCann, PE, SE (WA)

 

[msquared48]

I guess I am wondering how much creep deformation a truss can take before it fails, assuming that creep can initiate failure. Is there a limit? Or is this only serviceability related?

Mike McCann, PE, SE (WA)

 

[msquared48]

JAE:

I guess what I found is what you found too.

Not much info here. I have contacted a local technical rep from RedBuilt. We'll see what he says.

Thanks all.

Mike McCann, PE, SE (WA)

 

[msquared48]

Triangled:

I looked tat the reference you cited and there is still no specific mention, even then in 1988, of long term creep issues, only referencing the applicable building code - would have been the 1985 UBC for that one...

So addressing the issue had to have occurred between 1988 and 2000, as it was addressed in 2000.

Mike McCann, PE, SE (WA)

 

[msquared48]

In thinking about this, does creep accentuate bending stresses over time eventually leading to a tension failure,

OR is the creep mechanism merely related to a relaxing of "E" over time, allowing further deflection not associated with any stress increase?

Mike McCann, PE, SE (WA)

 

[Shotzie]

Creep is an increase in strain under a constant load. I believe you could get a rupture failure over time as a result of creep. Whether or not that would actually happen with wood at room temperature is another question.