Joist bottom chord out of plane bowing

[knelli]

Hi all,

I received an email today from a jobsite asking what the tolerances are for a joist bottom chord to be bowed 1" to 2" out of plane compared to the top chord. The bridging has already been installed.

Has anyone else ever run into this? I would assume a joist is not designed to perform while bowed as described? Would you recommend diagonal cross bracing at the center?


FYI, we are design-build and this project was actually designed by another firm. Of course I want to tell the PM to ask the engineer of record, but they like to have an opinion from us, the in-house engineers, before going to the next level.
Thanks! -K

 

[civilperson]

Depends on the depth of the joist. Less than 5% lean is tolerable.

 

[frv]

If you contact a local joist manufacturer or regional supplier and ask to speak with someone in engineering, they'll generally be very nice and point you in the right direction.

I only suggest this because regardless of the opinion of other engineers (for example, I'd thikg the bowing could be problematic if the joists are exposed to uplift pressures and the bottom chord goes into compression), these people do joists day in and day out and may have some practical advice.

 

[SteelPE]

I second the thoughts of frv.

Every time I find something wrong with a joist onsite I refer the problem back to the manufacturer. If there is a problem they will typically suggest some sort of repair. If some sort of repair is required is should be approved by the EOR.

 

[dgkhan]

Cross check with this sheet also by Alex T. If I am involved in some problem as engineer, I will also like to dig in ... sorry the file is attached with #8x3 pullout .. thread

 

[knelli]

Hi all,

Thank you for your responses. As a follow up, we contacted SJI and their response was

"There are no set standards except that the erector is to utilize the bridging to assure that the joists are reasonably true and plumb."

So, who is to determine what is reasonably true and plumb? I guess the EOR, too bad there are no standards to refer to!! Especially if something were to go wrong. We will have the PM contact the joist mfg.

I have also attached a photo of the condition.

 

[JedClampett]

I can't tell from the picture, but how do you know that the whole joist isn't bowed, not just the bottom chord?

 

[hokie66]

Is it an optical illusion, or is the joist to the right of the bowed one damaged? Looks like part of the bottom chord is missing or bent.

 

[haynewp]

Is it a roof or floor joist? I expect the bottom chord to kick out to the right further under increasing load, probably stressing the top chord and web members more than what they were designed for. There is a lot of bridging that would send some load over to adjacent joists.

Why don't you send the issue to the joist supplier for this job? They designed it and know how close it would be to capacity under the design loads. As a gut feeling, I would expect adding a couple of additional rows of diag. bridging would bring it back to design capacity. But just check with the joist supplier.

 

[AJGIII]

What is the reference for 5% as tolerable?

For the wood truss industry, the Truss Plate Institute specifies 2% as maximum vertical lean and L/200 for bow (as in the photo) where L = length of truss in inches.

Er, wouldn't L/200 = 0.5%???

The old source doc was HIB-91, but I can't recall new name/number of the doc that replaced it.

J.

A John Gironda III, P.E.
"As long as you are in the seat...make a difference"---Capt James T. Kirk

 

[MikeE55]

I had a similar problem. Contacted Vulcraft, the supplier, and was told nothing other than the joist must be straight and plumb. They did send a representative out to look at it, who suggested the bridging and deck be removed and joists be straightened and plumbed. Although they would not give any tolerances, they did point me to the Canadian Standards Association, who give the tolerance for sweep on a joist as L/500. But this is a manufacturing tolerance, not a construction one. SJI needs to get a backbone and establish some tolerances.