Replies

1. 
   DanH | Mar 29, 2011 12:00am | #1

   In theory, anything (substantial) added to the bottom chord of a joist will help.  If these are trusses, though, then it's hard to believe you could add enough to do much good.

   What is the joist spacing, and what's the composition of the floor on top?  It could be that the bounce is due to the floor flexing between joists?

2. 
   BossHog | Mar 29, 2011 05:03pm | #2

   Floor trusses that are 16"

   Floor trusses that are 16" deep and span 19' should not have vibration in them.  

   That makes me wonder if something is wrong.  Like if there's an off center bearing point and the trusses were installed backwards, or a center beam is undersized.

   Were 2x6 strongbacks installed in the trusses?    Have you asked the truss manufacturer to come out and look at them?

1. 
   bski | Mar 29, 2011 07:18pm | #3

   Someone from the truss company did come and look at them.  There were strongbacks installed, but not as frequently as they should have been.  They are mostly bearing on a 2x6 center wall, spaced 19.2 oc.   The wall is in the proper place.  They were manufactured to L370 I think, so is there any way I can beef them up after the fact?  Like I said, adding anything to the sides is not an option at this point.

1. 
   BossHog | Mar 30, 2011 05:47pm | #4

   Any chance of getting a picture or two?  Or maybe a PDF copy of the truss design?

   Something is missing in this puzzle.  As I said earlier, trusses with the span/depth ratio you gave should be very stiff.   Much more than the L/370 figure that you mentioned.  

   If the trusses span 19' and have a center bearing wall, does that mean the house is 38' wide?

   Is there a rectangular duct chase in the trusses?  If so, where is it located, and how wide is it?

   I know I'm asking a lot of questions.  But I want to know what exactly  the problem is before I take a crack at solving it.

2. 
   bd | Mar 30, 2011 07:54pm | #5

   Just in case you haven't been around enough to know Boss's background. Boss knows what he's talking about on trusses. Worth it to spend the time answering his questions, taking some pictures. You'll get a knowledgeable response.

   Ed

1. 
   BossHog | Mar 31, 2011 08:49pm | #6

   Thanks

   I appreciate the kind words, Ed. 

   I hope the OP brings some more info back so we can figure this out.

1. 
   bski | Apr 01, 2011 06:40am | #7

   will get some shots up this weekend.

2. 
   bski | Apr 03, 2011 08:27am | #12

   PICS UP

   Pictures from right to left where the span is 19', the problem area.

   
3. 
   bd | Apr 02, 2011 03:49pm | #11

   You're welcome!!

   Ron,

   You've contributed an awful to this forum thru the years. No way for relative newcomers to know that. Given the spread of opinions that sometimes popup here, it's much better, IMO, to let people know when they're talking to a real expert.

   Hope you're doing well.

   Ed

3. 
   junkhound | Apr 01, 2011 08:33am | #8

   Plus, having been one of the few to actually have been in the 'spec house from he11,  IIRC Rons floor has 16" floor trusses.  Proabably could park a loaded dump truck in the living room.

   My personal method of beefing up any beam is to bolt a steel strap across the bottom face, with appropriate spacings for the lateral shear.

1. 
   BossHog | Apr 01, 2011 06:29pm | #9

   Art, you're actually the only one from BT that was ever in that house.  So you oughta feel privileged. 

   (-:

   The trusses in that house were actually 12", 14", and 24" deep depending on where you were at in the house.  The 24" deep ones clear spanned 30' - That's why they were so deep.

2. 
   bd | Apr 02, 2011 03:41pm | #10

   JH,

   I've thought of adding that a few times through the years to my living room joists just to stiffen them up a bit.  Since it's not really an issue in that I don't have a bouncy floor, I've never got around to it. Just another "gonna do" on the list of things that I want to do sometime if the future, if I live long enough. How do you tension the bands? Do you jack the joists a little first??

   Ed

4. 
   DanH | Apr 03, 2011 09:48am | #13

   Uh, I don't think skin ointment would make a very good glue.  (Did you maybe mean resorcinol?)

5. 
   BossHog | Apr 03, 2011 12:09pm | #14

   I don't think that adding something to the BC would help.  The trusses are plenty deep enough already.

   So I'm curious why you are suggesting that.

1. 
   junkhound | Apr 03, 2011 04:24pm | #15

   curious why you are suggesting that

   Deflection proportional to moment of inertia.

   Simplistically, moment of inertia proportional to cross section area of  this truss chord section, so at the least the deflection will be reduced, plus the least labor intensive method (to me at least).  

   With the floor on top added into the equation, the centrod of the trusses with the added BC may actually stay close to the center, providing for some additional improvement in the moment of inertia, as much as cutting deflection in half (this if the floor is glued, if not, too lazy to due the calcs on how much the centroid moves with just the BC doubled, etc, but still would guess >50% deflection reduction)

1. 
   DanH | Apr 03, 2011 05:03pm | #16

   Well, obviously there's vibration because something is bending.  Since this is a truss, in theory at least the "bending" would be pure compression or stretching of a member.  The top chord is, we will assume as a first approximation, "quite stiff" due to the flooring.  So the bulk of the elasticity must be in either the bottom chord or the the angled "truss thingies" (I'm sure there's a name for them but it's escaping me right now).  Rather, let's make that:  The bulk of the elasticity must be in the combination of the two.  And, when considering the strength of the angled members we need to take into account the truss plates and the joints, since there's no doubt considerable motion there.

   What's not clear to me is that stiffening the bottom chord would accomplish that much -- it seems to me that more elasticity is apt to be present in the angled members and the truss joints.

1. 
   junkhound | Apr 03, 2011 05:17pm | #17

   I wonder if we are all looking at the trusses and forgetting about the plywood above.

   Could read the APA on the panels in the pics, but not the span rating.  Could be just 5/8" ply above the trusses spaced on those little diamond thingys on the tape and the 'bounce' is plywood deflections vs. the trusses???? 

   1/2 ply on 16" has detectable bounce, have to admit have never framed anything on 19.2" little blakc thingys in my life.......

1. 
   DanH | Apr 03, 2011 07:50pm | #19

   I wonder if we are all looking at the trusses and forgetting about the plywood above.

   Look at the very first response to this thread.

2. 
   BossHog | Apr 03, 2011 07:04pm | #18

   I can see where that might help if the length over depth ratio was bad.  But since it isn't the problem on this truss, I don't really think it would make a difference.