Replies

1. 
   slykarma | Mar 30, 2007 07:38am | #1

   It may be that joist bridging is not required, depending on some physical factors. What is the span of the joists? What wood species? What size of joist stock?

   Lignum est bonum.
1. 
   Noidea | Mar 30, 2007 01:59pm | #4

   The span is 14' and the joists are only 2x8's at 16"o.c.  The floor seems a little under engineered to me.  I noticed a sag in the ceiling in this bedroom so I tore out the ceiling and found that 1 of the joists had been notched down to 2x2 dimensions to allow for 3" drain.  The plumbing was all redone by a licensed plumber after I tore out the old plumbing, jacked the sagging ceiling up,  headered around the area where the 3" toilet drain turns, and sistered some new joists to the notched joists.  I also took the 300lb cast iron tub out of the bathroom that is above this ceiling and replaced it with a fiberglass tub to help ease the load in this area.  But, since the floor seems a bit flimsy, I was wondering if bridging would strengthen the floor?

    

    

1. 
   Danno | Mar 30, 2007 03:20pm | #5

   Someone asked a similar question yesterday about a bouncy floor--maybe you can find that discussion or someone else here can lead you to it. Anyway, there was a suggestion in that one to screw strips of steel to the bottoms of the joists. That would work well where the joists have been notched. Someone else mentiond that you can add 2x4's flat to reinforce the joists, if you can spare the room (it lowers the ceiling by 1-1/2").

2. 
   BossHog | Mar 30, 2007 03:37pm | #6

   Ouch - That's really overspanned for 2X8s. Bridging or blocking ain't gonna help that.BTW - Fill in your profile, would ya ??? We gotta know where to send the black helicopters...(-:
   Never Put Off Until Tomorrow What You Can Avoid Altogether

3. 
   slykarma | Mar 30, 2007 05:10pm | #7

   Yeah, that is overspanned. Even with bridging AND strapping, max span for 2x8 @16" o/c is barely 12 ft. And that would be #2 & better fir, deduct 5-8% for SPF.Lignum est bonum.

1. 
   junkhound | Mar 30, 2007 06:15pm | #8

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=87830.1&maxT=6#a7

   is a link to the discussion Danno mentioned.

   Steel strap pix on a 6x12 beam below more than doubled the strength of the beam, bolting is critical to develop full strength.

   In the case of 2x8s, a 1/8" thick or thicker 1-1/2" wide steel strip properly bolted will exceed the strength of a 2x10, which is needed for 14 ft like Boss said.  You can span notches on the bottom with the strap and dont need to replace notched joists either.

   Again - THE BOLTING IS CRITICAL, YU NEED TO GET A STRUCTURES BOOK AND LEARN HOW TO CALCULATE LATERAL SHEAR AT THE ENDS OF THE JOISTS FOR THIS TO WORK. Too much calc. detail to try to put in an internet post. Look up "built up beams' on the internet, there are a couple of places that do describe the calculations. Or get "elements of strength of materials', by Timeshenki and Young from the library.(or similar text) 

   View Image

4. 
   Piffin | Mar 30, 2007 11:05pm | #14

   14' span should have 2x10 at least so you need to sister more to get rid of the flimsy 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

2. 
   Piffin | Mar 30, 2007 01:31pm | #2

   When load is applied to the floor above, as joists are pushed down, they naturally want to deflect one way or the other. They are stronger and sag less when they are kept vertically aligned with blocking or bridging.

   Generally speaking, any span over 8' is divided in half with blocking or bridging and less than that it is not needed. Also, if joists cross span walls below and are attached there, then that attachment does the same thing in maintaining alignment.

   So generally speaking a bathroom is small enough that blocking may not be needed.

   But5 if the jjoists were hacked up, or things feel bouncy, adding some can help - sure won't hurt.

   Another way of dealing with this is to add strapping ( 1x4s) to the bottom of the joists, perpendicular to them at 16" OC and lowering your ceiling by that much. This also gives you a chance to shim and straighten things if need be.

    

    

   Welcome to the
   Taunton University of Knowledge FHB Campus at Breaktime.
    where ...
   Excellence is its own reward!

1. 
   segundo | Mar 30, 2007 01:56pm | #3

   agree with what you say, but feel blocks add more, and are superior to "bridging".

   you didn't specify in your post, and i just thought it should be said, and wonder if you agree?

    

1. 
   Piffin | Mar 30, 2007 11:03pm | #13

   Probably true and I normally use solid blocking. Two other reasons - we get to scrap up the cut ends and poorer pieces that way, and it is less likely to end up with squeeks, IMO.The reason I kept options open is that with a bunch of pipes running around this space, bridging might be easier to get in 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

3. 
   DanH | Mar 30, 2007 06:38pm | #9

   Bridging never hurts (except for getting in the way of mechanicals). In addition to keeping the joists from "rolling", it transfers load from one joist to adjacent joists, spreading the effect of point loads. This is especially helpful in removing "bounce", or when there may be a large static load such as a tub.

   Bridging won't signficantly increase the depth of sand you can pile evenly on the floor without it collapsing.

   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
1. 
   karp | Mar 30, 2007 08:27pm | #10

   Ditto

   Dan's pretty much covered the structural aspect of bridging, and an earlier post has determined that even with bridging, the joists are overspanned.

   I think our work here is done.

1. 
   Noidea | Mar 30, 2007 08:37pm | #11

   Thanks to all who gave me advice and opinions.  I'll have to decide the next course of action.  I really want to make it safe since we will be sleeping directly beneath this ceiling.  Thanks again.

1. 
   DanH | Mar 30, 2007 10:17pm | #12

   It should be noted that the likelihood of catastrophic failure is vanishingly small. What could happen, though, is a steadily increasing sag in the floor (eliminating the need for a separate tub ;) ), prossibly accompanied by some failure of the ceiling plaster below.
   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

2. 
   Piffin | Mar 30, 2007 11:10pm | #15

   I would not worry about sleeping under it or dancing on top of it, the failure will merely show is sagging, cracks of finish surfaces, and bounce. Steel tends to fail catastrophicly when it reaches the point of no return, but wood fibres act differently and gradually change their orientation so failure is almost always a gradual process. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

4. 
   Framer | Mar 31, 2007 04:01am | #16

   See if you can sister them with 1-3/4" x 7-1/4" LVL's.

   Joe Carola