Two span floor joist
Two span floor joist
(OP)
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?





RE: Two span floor joist
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.
RE: Two span floor joist
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Two span floor joist
RE: Two span floor joist
RE: Two span floor joist
RE: Two span floor joist
Maybe these are some how coming into play.
RE: Two span floor joist
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.
RE: Two span floor joist
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
RE: Two span floor joist
You can look up the moment and deflection equations in any good structural or steel handbook.
RE: Two span floor joist
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.
RE: Two span floor joist
BTW - Anywhere near Danbury
RE: Two span floor joist
RE: Two span floor joist
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.
RE: Two span floor joist
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
RE: Two span floor joist
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.
RE: Two span floor joist
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
RE: Two span floor joist
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.