Replies

1. 
   JeffBuck | Jun 13, 2002 02:29am | #1

   bigger trusses, midspan beam, lally columns, bearing wall. Jeff

                    Genius has it's limits.....but stupidity knows no bounds

    

2. 
   UncleDunc | Jun 13, 2002 05:15am | #2

   Or you can reduce the spacing. Most of the span tables I've seen have columns for 12", 16", and 24" spacing, but you (or your engineer) can calculate the spacing corresponding to a 30' span for whatever deflection limit you specify.

   I think I calculated once that 2x8's would span 40' if they were installed 1.5" on center. But I probably overlooked something. :)

3. 
   WebTrooper | Jun 13, 2002 09:25am | #3

   You could probably get truss type joists engineered for the span.

4. 
   BossHog | Jun 13, 2002 02:26pm | #4

   Wood webbed floor trusses can span that far with no major problems. But anything with a clear span over 28' has SOME potential for vibration problems. Make sure you (Or your builder) install strongbacks at 10' O.C. to help control this.

   The reason you've seen problems with bouncy floors and cracked sheet rock is probably due to people doing what Uncle Dunc suggested - reducing height by reducing spacing. This gives you a longer, more slender truss that's more likely to bounce.

   Think of it this way: You CAN get more strength by reducing spacing. You WILL get more stiffness by increasing height. Strength and stiffness are only loosly related.

   I'd suggest talking to a truss manufacturer about some wood webbed floor trusses 24" deep. They'll probably pull out some span charts that say if you set them 16" O.C. you can get by with 18" deep, or 12" O.C. and make them 16" deep. Tell them to roll those span charts up and stick 'em where the sun don't shine, and quote you some 24" deep trusses. Set them 16" O.C if you want. But don't accept anything shallower than 24" deep.

   The urge to scream tells me I must be at work.

1. 
   UncleDunc | Jun 13, 2002 11:36pm | #5

   I figured I had missed something, but I'm still not sure what it was. I know stiffness varies directly with width. Double a joist, it halves the deflection. I assumed that putting in twice as many joists at half the spacing would have the same effect as doubling each joist at the same spacing. Not so? Or is it that 'bounciness' not very closely related to static deflection? Would a joist that felt bouncy at 16" OC still feel bouncy at 8" on center, even though the actual deflection was cut in half.

   Increasing stiffness with deeper trusses is probably cheaper than reduced spacing, even considering the extra sheathing, siding, stair framing, etc, but there could also be design reasons not to want a very deep floor assembly, such as absolute height limits or solar encroachment regulations.

   Edited 6/13/2002 6:24:54 PM ET by Uncle Dunc

1. 
   BossHog | Jun 14, 2002 01:48am | #6

   Doubling a joist halves the THEORETICAL deflection. But it does almost nothing to change the vibration characteristics of a floor. That's more a function of the depth of the floor system.

   Let me give it some more thought, and I'll try to explain it better tomorrow............

1. 
   UncleDunc | Jun 14, 2002 02:05am | #7

   >> But it does almost nothing to change the vibration characteristics of a floor.

   I can certainly believe that, and I had not considered it while I was playing with the numbers. That would explain the uneasy feeling I had about my 40' span with 2x8's nailed cheek to cheek. Uneasy as in "That can't be right." :)

   I suppose it would be too much to hope that there are some simple equations for describing bounce, like the square and cube rules for strength and stiffness.

1. 
   BossHog | Jun 14, 2002 04:48pm | #8

   Take a look at this thread, where I tried to explain how I think regarding floor design:

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21010.1

   It's a good thing we have gravity, or else when birds died they'd just stay right up there. Hunters would be all confused.

2. 
   Hector45 | Jun 14, 2002 10:52pm | #9

   >..I know stiffness varies directly with width. Double a joist, it halves the deflection. I assumed that putting in twice as many joists at half the spacing would have the same effect as doubling each joist at the same spacing. Not so?

   Doubling the width of a beam will definitely double its stiffness and half the deflection.  Nailing and/or glueing two beams side by side will give you the same effect.  However, halving the OC spacing will not neccessarily accomplish the same thing. 

   Why not?  Because point loads will not be evenly shared.

   If you laid a stack of 12' sheets of drywall across the floor, doubling the number of joists by halving the OC spacing WOULD cut the deflection in half.

   But "bounciness" arises from the weight of a person, and that is essentially a point load.  Stand on top of two sistered beams and the beams will share the load nearly equally.  But stand on a single beam and you rely on the stiffness of the subfloor to transfer load to the neighboring joists.  With any normal plywood or OSB subfloor, you'll get the vast majority of the weight going into the joist below your feet and only a fraction of it being transferred to neighboring joists.  If 75% of the load goes into the beam below your feet, (quite possible) then you've only reduced bounciness by 25% despite doubling the number of joists.

   If you had a rigid subfloor, the load would be shared equally and you'd get the 50% reduction you expected.

1. 
   Hector45 | Jun 14, 2002 11:13pm | #10

   BTW  This is not to contradict BH's information, which I believe is accurate.  I'm only pointing out why doubling the number of joists will not reduce a floor's bounciness by half.

1. 
   south48 | Jun 15, 2002 04:07pm | #11

   Keek    One  of the ways to take bounce out of a floor is after you have formulated the proper size and spacing   for your joists is to deck with 1 by 12 boards on a diagonal then sheet with tonuge and groove plywood this will help to evenly carry the load                                               

                    jackpine                                                                      

2. 
   Ablesupe | Jun 17, 2002 04:11am | #12

   First time I've ever responded to anyone on any forum. My kids call me a low tech guy in a high tech world. I never did see any practical reason for installing sheathing diagonally. The span is almost 50% greater and theres always more waste when cutting. If the joists,trusses,TJI's,etc. are sized properly for span it seems an unnecessary additional expense. TJI literature is explicit in it's explanation that a properly sized joist to carry the intended load may result in unsatisfactory "bounce", hence the need to reduce the o.c. spacing,or use a wider or taller joist. The principles are the same for the other structural members