Replies

1. 
   Tim_H | Jul 03, 2000 08:11am | #1

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   Gordy

   First try something besides all caps.

   I have used a simpson clip that is like a "L" bracket. you nail one side tight and leave the other side loose to allow truss uplift. just rember , don't nail the rock near the wall.

   I seriously doubt that corner bead is going to stop movement. But hey i never tried it.

   I know that truss uplift is somewhat caused by the lower chord being buried in insulation and the rest of the truss being exposed to a very diffrent tempature, but what would happen to a truss that did not have any insulation surronding it? would there still be movement?

2. 
   Guest_ | Jul 04, 2000 03:54am | #2

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   Hmm truss uplift...

   I have heard of this phenomenom for as long as I have been building. I know of many techniques to prevent it. I have never seen a clear cut case of truss uplift and I can tell you that in my neck of the woods it is standard procedure to toenail the trusses to the interior walls to hold them on layoff. I know of no drywaller who leaves screws in the ceilings back from the partition. We also cover the bottom chord with insulation.

   I don't know why we never have this problem but I can speculate.

   Maybe someone can explain to me how wood shrinkage in a truss would cause uplift. If the insulation is a factor, then how does expansion/contraction explain truss uplift.

   Is it possible that truss uplift would better be described as "beam downpull" due to shrinkage in wood beams in basements and first floors. How about settling of the interior of the house vs. the truss bearing exterior walls.

   I just have a hard time understanding this one, because I have yet to see a logical explanation of why truss uplift occurs.

   Anyone care to educate me?

   Tom

1. 
   Guest_ | Jul 04, 2000 01:33pm | #3

   *> Maybe someone can explain to me how wood shrinkage in a truss would cause uplift.The bottom chord is warm and dry. The top chord is cold, gets condensation on it, and is damp. The bottom chord shrinks, the top chord expands, and the truss raises up. This has been covered in more detail in the past, if you care to search the archives.> Is it possible that truss uplift would better be described as "beam downpull" due to shrinkage in wood beams in basements and first floors.Yup - I'm sure it's the case at least part of the time. But truss uplift is a more popular thing to blame drywall cracks on. People like to have someone to blame things on. If your floor joists and/or beams shrink, who do you blame ? If you have truss uplift, you can blame the truss manufacturer.

1. 
   Guest_ | Jul 04, 2000 03:13pm | #4

   *Ron, I value your expertise as a truss designer, so a couple of follow up questions.I am not familiar with values associated with would expansion and contraction due to temperature, but I would think it to be negligible since would is such a good insulator. Shrinkage and swelling due to moisture levels is an known phenomenom to me and I understand that it can be 5% of the boards width. This effect would seem to be negligible in a truss design because the members are generall 3.5 " wide. As to shrinkage/swelling with the grain, I have seen some evidence of that in boards with wavy grain or cut on a bias, but generally I would think it does not come in to play.i The bottom chord is warm and dry. The top chord is cold, gets condensation on it, and is damp. The bottom chord shrinks, the top chord expands, and the truss raises upAlthough this statement makes sense at first, on a standard fink type truss, I wonder what effect the purlins have on this scenario. Basically, if the strength comes from the creation of independent triangles it would seem to me that the length of the members in any given triangle would also come into play, and in the above example it would seem that the pulins would expand and cancel the alleged uplift effect. If this phenomenom occurs the way you describe, it would seem that in the summer, there should be an opposite effect, pulling the trusses off of one or both bearing walls.I think there are other causes that may be responsible for this phenomenon, at least on some occasions. If the bottom chord of the truss is crowned, or the partition wall in the center is low, forcing it down and nailing it will probably result in temporary attachment.Assuming truss lift does occur, why not use hurricane straps and really attach those buggers to the walls. I'm not convinced that the forces of truss uplift are great enough to lift the walls off the floor.Yeah, I agree with you on the blame thing, but it makes me wonder why I have not seen any good research on this subject. Seems like one day somebody said that drywall cracking is caused by truss uplift and everyone just jumped on board.I got a question for you on piggy back trusses I'll post under a new topic. I'd like to hear your thoughts.Tom

1. 
   Guest_ | Jul 04, 2000 06:58pm | #5

   *I have expierinced truss uplift - but only on the longest spans ( those over 36' ). And yes it will pull the top plate and/or wall up with it.It is a seasonal thing, the truss comes back down in the spring.I haven't had any problems with it on the smaller spans and quite often still catch myself nailing the truss down solid on the partition walls ( old habits die hard )

1. 
   Guest_ | Jul 05, 2000 10:58pm | #7

   *> Although this statement makes sense at first, on a standard fink type truss, I wonder what effect the purlins have on this scenario.I'm not sure what you mean by "purlins". Are you referring to the webs ? (the interior members that connect the top and bottom chords together) I would assume the moisture content of the webs doesn't change much. > Assuming truss lift does occur, why not use hurricane straps and really attach those buggers to the walls. I'm not convinced that the forces of truss uplift are great enough to lift the walls off the floor. I can tell you from experience that you're wrong here. I've seen huge cracks in walls where they were lifted up by trusses. (The worst one was a $450,000 house that was a mess anyway) Think about it - a 30' truss designed for a 50# total load is designed to carry a total of #3,000. So picking up a 2X4 wall isn't a big deal. > Yeah, I agree with you on the blame thing, but it makes me wonder why I have not seen any good research on this subject.The Wood Truss Council of America (WTCA) has done some research. But they didn't come to any great conclusions that I know of. Most of their publications are just designed to shield themselves from any blame.

3. 
   gordy_simpson | Jul 05, 2000 10:58pm | #6

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   I WOULD LIKE SOME FEEDBACK ABOUT FASTENING INTERIOR PARTITIONS TO TRUSSES. UP HERE IN CANADA THE TRUSS COMPANIES DON'T RECOMMEND NAILING DIRECTLY FROM TRUSS TO WALL PLATE BACAUSE OF TRUSS UPLIFT. I HAVE HEARD OF SOME BUILDERS FASTENING A METAL OUTSIDE CORNER BEAD AT THE INTERSECTION OF THE WALL PLATE AND TRUSS. THIS IS SUPPOSED TO ELIMINATE CRACKS IN DRYWALL. PLEASE RESPOND WITH YOUR THOUGHTS AND IDEAS. THANK YOU