Two span floor joist

[gcs3pe]

I've got a 24' wide house with a bearing wall dead center down the middle. StruCalc and Code say 2x10 floor joists (#2 DFL) at 16" are way more than fine for 30/20 loading at 12' span. Why then, does adding a second, equivalent span in StruCalc fail? I'd like to run continuous 24' joists over the bearing wall. I'm staring at the VMD diagrams right now, and see nothing that changes dramatically enough to warrant a "good by 69%" to flip to a "bad by 11.3%" Is it a glitch in the software, or am I missing something?

 

[MiketheEngineer]

Yeah - I think you are.

First you will play hell finding 24' floor joists.

Second - with a center bearing about 5/8 of the total 24' load will go into the center bearing versus 1/2 of the load of two 12' joists.
Also, your moments and deflections go a bit crazy but easily definable.

Do the math and you will see what happens.

Are you really structural?? Not to be impertinent - but this is found in all structural books, beam calcs and steel books, etc.

Buy the 24' footers and cut them in half. Just kidding. But I have actually shown that detail on multi-span roof trusses to relieve the stress.

You could use TJI joists that are properly selected to reach the 24' or use 2- 14' sections and overlap at the center beam. VERY typical house construction.

 

[msquared48]

Also, lateral support of the bottom flange of the joists coule become an issue for a distance to either side of the bearing wall

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[gcs3pe]

It seems like full lateral bracing across the bottom (in the form of sheetrock) reduces the bending stress to nearly that found in the spliced condition (two single spans). That's what I was missing.

 

[hokie66]

I'm not seeing that gcs3pe's question has been answered. The maximum moment for a continuous joist is the same for a simple span, and the deflection is less. Provided it is braced by the ceiling, what is the issue, strengthwise at least? I know nothing about StruCalc, but the output seems screwy to me.

 

[bylar]

I think your program is in error!

 

[MiketheEngineer]

Double checking indicates it may be in the software UNLESS it is "worried" about deflection. The simple spans show a bit more than twice the deflection as the continuous spans. The center reaction is also more than doubled. Also it may be checking different loading "cases" that could be suspect.

Maybe these are some how coming into play.

 

[AELLC]

It must be the software taking the bottom of the joists as laterally unsupported.

I have my own Excel and got similar results. But if I input the joists as supported at the bottom, it returns good by 68%. (there is no repetitive factor since there is no wood sheathing on the compression edge of the joists in the negative moment region.

But as Mike the Engineer says, it's a moot point since you won't find 24' long 2 x 10.

 

[msquared48]

Mike:

The center reaction continuous span should only be 25% more than the two simple spans, not doubled.

To have a problem with this simple case is ridiculous. Throw out the program and do it by hand.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

I agree - it is 25% and more easily done by hand then spending time trying to model it in some software program that may/may not be right.

You can look up the moment and deflection equations in any good structural or steel handbook.

 

[gcs3pe]

Are we having fun yet, or what?

I ignored the program's results within 10 minutes of the post. The software was, in fact, taking the bottom of the joists as laterally unsupported (Cl=.53 => Fb'=605). Modeling it as laterally supported follows AE's calcs at good by 68%.

My questions tend to come from quirky field issues. In this case, the builder had leftover 24'x2"x10" rafters from another job and wanted to use them. Otherwise, ummm, yes, lapping them on the bearing wall would have been a fine way to proceed.

And I can have up to 28'x2"x10" No. 2 DFL delivered here (SW CT) in 2 day's time, all year long.

 

[MiketheEngineer]

Really!!!

BTW - Anywhere near Danbury

 

[boo1]

Since you want single 24' floor joist (vs 2-14' beams), consider wood I beams. They are easy to handle and are available.

 

[woodman88]

AELLC you state "(there is no repetitive factor since there is no wood sheathing on the compression edge of the joists in the negative moment region."

How/why do you get this? As I read the NDS the repetitive factor is still good to use in this situation.

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.

 

[msquared48]

Woodman:

I don't know where he gets the interpretation either, but it makes sense considering that the repetitive factor has to do with the ability of plywood sheathing to spread out a load over more than one joist, effectively reducing the load to any one joist. Could also technically apply to the bottom flange too, although plywood is never applied there, only sheetrock.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

msquared:
The floor loading is being applied to the top of the floor joist, where the load distributing element (the floor sheathing) is applied, not the bottom of the joist.

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.

 

[msquared48]

Yea - mixing apples and oranges.

The tension flange stability has nothing to do with the application of the repetitive factor. Two different issues.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

Neither the tension flange nor the compression flange stability has anything to do with the application of the repetitive factor.
The stability of bending members is determined (per the NDS 2001, section 3.3.3, I doubt if this has been changed in the more current versions) by the Beam Stability Factor and per Table 4.3.1 the Beam Stability Factor and the Repetitive Member Factor can be used together.

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.