Too spongy and springy?

14 inch I-joists, 21 feet long, 16 inch on center, no supports under the floor, 1 double wall down the center of them. Too springy? Should I go 12 inch O.C? I'm getting the I-joists for $51 a piece, so if I really need to add a few more it wont kill me. But adding 4 more is also cutting me out of a Realtrap. Its not all about money, but if 16 o.c. is fine, I'd like to go with it. If you guys think 12 is the way to go, I'll prob do it. Thanks Paul

Oh yeah... the last couple of joists where the stair hole will be will be shortened to 14 or 15 ft to make room for the stairs, and I was planning on using regular 2x12's for that on 16 inch centers. Is that ok too or should I go 12 on those?

You haven't really provided enough information for anyone to help. What is the floor going to support? Another floor above? Two? Three? We need to know a little more about the bigger picture before we can provide any details. len

Hey DemoKing - I'm making some assumptions, like typical household loading. I'm pretty familiar with I-joists. 14 feet spanning 21 feet is a bit of a stretch. I realize that it may calc out for strength and deflection, but your span-to-depth ratio is 18... a little risky... especially if it's an active floor. I would encourage you to go 16" deep. Installing them at 12" o.c. will not help that much (stiffness/bounce-wise). I know that's counter-intuitive but it's true. If you have a couple walls that run perpendicular to the joists, that's probably no big deal. If you have a wall that is parallel to the joists I would add one extra joist under the wall. Creatch

Its supporting the studio upstairs and the double wall will cross across the middle of the beams, all of them, almost dead center. 1 double wall, two rooms. There are already two 14inch i joists supporting the garage door (hanging) so I'm kind of stuck using 14 inch joists. You're scaring me.

Hi DemoKing - Sorry... didn't mean to scare you. A couple things.. 1) When you say supporting the studio upstairs, I assume you mean that the joists are supporting the studio immediately above them... and that there is not another story above that. If that's the case then sounds like you're okay, structurally. 2) If you mean that there's a wall above the joists and the wall in turn holds up another floor above that, then you may have a problem.. or at least something that needs to be checked out by a professional. Hopefully you're in the item 1) category. The 14-inch joists are probably fine structurally. I'm just saying that if you have a choice, use 16-inch joists as a precaution. If you don't have a choice then you have to use 14-inch joists.. you may have a little bounce in the floor, and you may not... but going to a closer on-center spacing will not gain you much so don't waste your money doing that. You'd be better off spending your money on a heavier deck like 1-1/8 plywood. Regards, Creature82

Creature82 wrote:

You'd be better off spending your money on a heavier deck like 1-1/8 plywood.

...although I think isolation-wise you'd be better with two 5/8" layers with staggered seams, as opposed to one layer of 1 1/8". I think the resonance frequency of a single thicker, heavier board would not bode well for you. That being said, I'm not seeing enough detail for anyone to be able to provide conclusive advice on loading. I'm going to play my official "Please please please, in the name of gravity -- consult with a local structural engineer" card here. I understand and appreciate that you've got a lot of background in this, Creature82, so I'm not questioning the advice you're giving... But I am questioning (and I believe rightfully so) the amount of input upon which these conclusions are being drawn. So often I've seen cases where the questions we're asked deserve one answer, but then when we dig deeper and learn more details, often in "like pulling teeth" fashion, the answer often changes, sometimes quite dramatically. When lives are at stake (and they are), we need to be extremely careful not to shoot until we're absolutely positively certain what the target is. --Keith :mrgreen:

Fair enough, Keith. I guess I was just anxious to help. I'll try to get more details next time. Creatch

No problem at all, Creatch!! 8) Don't be shy. I've learned so much by helping others -- because often an idea I have isn't quite spot on and someone else joins the conversation with another point of view. :) The words "mere mortal" that appear above my avatar? That's actually something that John humorously stuck in there in response to a statement I made about my own humility. :twisted: --Keith :mrgreen: