Bottom flange bracing of prefabricated wood "I" joist 1

[ajk1]

For a roof with no ceiling on the long span prefabricated wood "I" joists soffit, what spacing would be required for bracing to the bottom flange? The span is about 36 feet. I would think that some horizontal bridging would be required to the bottom tension flange to stop the joists from twisting, but I have not found any such recommendation as yet. Is none required? Can anyone help?

 

[CANPRO]

your best bet is probably to talk to a local joist supplier....the requirements may vary between manufacturers. Strapping at some regular spacing would be my first suggestion since they will likely need it for drywall attachment at some point anyway.

See link below for some more info.

http://www.woodbywy.com/trus-joist/tji-joists/

 

[precast78]

I think you only need to brace the bottom when you have a support in the middle where the bottom flange will be in compression in mid span.

 

[structSU10]

I would imagine you need a bottom chord brace in a number of locations unless it can be shown that the bottom chord, with no bracing, is strong and stiff enough to act as a bracing member for the web members.

Attached is a paper, which was for steel trusses, but the same fundamental issue exists no matter the construction type.

I would imagine something will be needed, as that seems like a long span, but following the concept in the paper something non-prescriptive can be determined.

 

[manstrom]

Interesting paper.

I have never considered needing and tension chord bracing on beams or joists in general.

Wood floors typically have a ceiling, so restraint would be provided by drywall (not much). In some cases, unfinished basements don't have anything at all. If you need something, I don't think it is much at all.

For the 36' joist in question, you may want to consider either shortening the spans or providing something else to stiffen up the system and reduce vibration.

 

[structSU10]

I guess I was more thinking of a truss than a joist to be honest - don't know why. I would think for a beam the continuous nature of the web changes things quiet a bit. I would think standard blocking or bridge would be wanted, but I don't know of a way to calc that.

 

[jayrod12]

Manstrom, you may be underestimating the strength of drywall ceiling.

And on many of the unfinished basements I have seen there is 2x2 cross bracing at midspan or something similar.

 

[ajk1]

THIS IS A SPECIAL TEMPORARY STRUCTURE THAT WILL NEVER HAVE A DRYWALLL CEILING. IT IS ERECTED ANNUALLY AND LEFT IN PLACE A WEEK AND THEN TAKEN DOWN AND RE-ERECTED THE FOLLOWING YEAR. THE ACTUAL SPAN WHICH WE MEASURED THIS MORNING IS A BIT SHORTER THAN I SAID EARLIER. IT IS 30 FEET SPAN. THERE ARE CURRENTLY 2 LINES OF BOTTOM FLANGE BRACING, BUT THEY PUT THEM AT 8 FEET AND 5 FEET FROM THE LEFT AND RIGHT ENDS, LEAVING ABOUT 17 FEET IN THE SPAN BETWEEN THE BOTTOM FLANGE BRACES. should I TELL THEM TO ADD A LINE OF BRACES DOWN THE MIDDLE, OR IS IT OK AS IS?

 

[CANPRO]

1st off, all CAPS makes your post difficult to read.

2nd - if this is to be taken down, stored and erected again every year I would not recommend I-joist as they are at their weakest and most susceptible to damage during erection (and I can only assume during dismantling as well). How is the joist fastened to their supporting members? I don't know if you have ever nailed the top/bottom chords of these joist, but they split quite easily and I would not want the same joist nailed over and over again.

Is this an open canopy? You haven't given enough information. And it also depends on the manufacturer and size of joist. I still think your best bet here is to contact the manufacturer of the joist and get their opinion.

 

[woodman88]

Roof joists/trusses bottom chords are braced due to reverse stresses caused by wind up lift creating compression in the bottom chord. I would do the required calculations to determain the bracing requirements.

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.

 

[KootK]

You raise an interesting point StructSU10. I've been wondering about that paper for years. I don't understand why a steel truss requires tension chord bracing but a steel beam does not.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CANPRO]

KootK, I think the major point they're making is that you need to provide lateral restraint to your tension chord so that the tension chord can properly brace the compression webs. For a wide flange beam, tension is introduced into the flange via transverse shear so there is no compression web to be braced. Having read many of your posts over the last few months (kudos to you for participating so much)I know you like to get into the gritty technical details, so I suspect you may be thinking deeper into this than I am...but that is my simple take on the matter.

 

[KootK]

Yeah, I do have a tendency to wax philosophical. You probably won't be surprised to hear that I'd like to discuss this further. I'll leave that for a dedicated post on the matter, however, as I don't want hijack ajk's thread. Thanks for sharing your thoughts on Canpro.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ajk1]

These joists have been used for this purpose about 15 years so I don't think they are all that delicate. I think Woodman88 has the practical answer for this structure...I ought to have realized that myself. I must be getting too old or too busy. I will pursue that. Thanks everyone for the time and effort. It is much appreciated.

 

[KootK]

Yeah, I do have a tendency to wax philosophical Canpro. I eat a lot of refined sugar. You probably won't be surprised to hear that I'd like to discuss this further? I'll leave that for a dedicated thread on the subject, however, so as not to hijack AJK's thread. Thanks for sharing your opinion with me.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[bigmig]

woodman88 is correct. You brace the bottom chord for internal pressure uplift.