Ok, who can explain this so I can understand it? L is the span in inches, 360 is an arbitrary number (arbitrary because you can also use 480 etc). So if the span is 10 feet, then L=120 and L/360 = .333 and if span is 30 feet then L=360 and L/360 = 1. So? When you are limited by L/360, does than mean that the mathematical answer can’t be more than 1 ?
Do it right, or do it twice.
Replies
L/360 is the measurement of the maximum deflection at the middle of a span. Using your example, on a 10' span, .33" of deflection would be acceptable, and on a 30' span, 1" of deflection would be acceptable. On a 60' span, 2" of deflection would be acceptable (60x12=720. 720/360 = 2"). Basically, by increasing the number you are dividing by (from 120 to 360, say), you decrease the allowable deflection for the system, and thereby create a stiffer floor. That's why you'll usually see several charts with different L/xx ratings- some people are wiling to accept a bouncier floor.
Bob
Did Bob's explanation do the trick? He's correct in what he explained.
If you're still in the dark, it might help if you explain why you're asking. That way you could get a more specific response.
Death to all fanatics!
Ok, so the mathematical answer of L/360 is not a magic number, but it is the maximum deflection in inches. That's easy enough. Now, how do you calculate the anticipated defdlection to see if it is less than L/360?
I'm asking because I don't know. People talk about it here, and the span tables mention it, but I never knew what it really meant. "You realize that your joist design can't exceed L/360, right?" "Oh, yeah, of course. I always do that. Very important." Sounds like Billy Crystal.
Do it right, or do it twice.
OK, now you're getting deeper into the engineering side of things....lol. If you compare your joist system to any of the tables, you'll see where you fall. For example, if you've got 2x10 DF at 16" o.c. with 3/4" decking, and the L/360 max span for that system is 14'-6", you're OK as long as you are less than 14'-6".
If you're doing something unusual, or you need to know the actual deflection amount for some reason (tile flex, etc), then you need to use beam formulas to calculate the actual deflection. If memory serves me correctly (I haven't sized beams this way in about 10 years), the maximum deflection in a single-span beam is :
Defl.= 5wL^4/384EL
Where w= the uniform load on the beam, L= the length between supports, and E= the Modulus of Elasticity for the wood species and lumber grade.
Know more than you ever cared to about calculating deflection now? LOL
Bob
Ok, so then you almost never do any calcs, just read the span tables. Works for me.
Defl.= 5wL^4/384EL Lost me a little with the receipe. 5wL raised to the 4/3 power? No...5wL to the 4th power all divided by 384 times E times L again?
Do it right, or do it twice.
Edited 5/24/2003 12:19:27 AM ET by ELCID72
That should be 5wL^4 / 384EI
Only L is raised to the 4th power. The 384, E and I are in the divisor.
I = Moment of Inertia of the cross section. This is the stiffness of a structural shape, which is why it goes on the bottom of the equation....that's not a mistake, it's rustic
Good catch Jeff- I thought something looked funny. That's why I shouldn't be doing engineering at 11 pm...lol.
Bob
I wouldn't mess with trying to calculate the deflection on anything - Just stick with the span charts and use your head.
There are cases in the span charts where bending is the limiting factor, and others where deflection controls the max span. In general, wider spacings (like 24" O.C.) will be controlled by bending, and narrower spacing (like 16" O.C.) will be controlled by maximum allowable deflection.
BTW - Ever read the thread on Floor Vibration? Lots of info there on designing floors, deflection, and vibration.I am the bad thing that happens to good people.
I really don't intend on trying to calculate anything, especially now that I (better) understand the def of L/360. My calculating days are long since passed. I'm not sure if I read Floor Vibration. Since nearly 100% of our houses are slab on grade, the calcs would only apply to the second floor. And since practically all of my work is remodeling, I don't get involved in that phase. Again, I was just trying to learn something. Thanmks to both of you for the enlightenment.
Do it right, or do it twice.