Replies

1. 
   Guest_ | Feb 17, 1999 08:32pm | #9

   *
   I was on a recent bathroom remodel job over the summer and my boss had me sister the floor joists with regular galvanized screws. I told him screws were no good in the situation because they have no shear strength. I showed him by driving a 3 inch screw and a 3 inch nail through a 2 by 4 and banging the screw once and it would snap....the nail would hardly move, and would only bend..not snap like the screw. He just ignored me though. Was I correct?

1. 
   Guest_ | Feb 17, 1999 05:44am | #1

   *Screw strength aside, if you were sistering the joists it wasn't very important that the joists be bound to each other, right? Just that the new joists not twist out of plumb, which screws could prevent better than nails. Since the old and new joist would move in unison, it seems to me that shear strength would not matter....

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   Guest_ | Feb 17, 1999 06:14am | #2

   *good point Andrew..I forgot to mention that the new joists were not extended to the plates for whatever reason. I dont remember why, but I dont beleive we had access to them. So the load would not be taken to them but to the screws....which would not have the shear strength needed.

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   Keith_Utter | Feb 17, 1999 06:53am | #3

   *Although screws don't have as much shear strength as a nail, it sounds like the application called for a method that would perhaps limit damage to a ceiling below. If that's the case you could consider that 2 or 3 screws may equal 1 nail or use construction adhesive in conjuction with the screws. The only reason the sistered joists don't rest on the plates should be that it was imposible to get the full length in the space. I know there's some assumtion here on my part but sometimes you have to do the best you can and if what you end up with is better than what you started with then that's a plus. If your point is that to much faith is put in drywall screws, your right and when there's a failure you'll have to remember it said "drywall" or "deck" screw on the box. Hope this helps.

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   Guest_ | Feb 17, 1999 08:04am | #4

   *deFlash: Yes, drywell screws do not have nearly the shear strength of nails, as you demonstrated. And, as you commented, there's also a difference in brittleness. Can't imagine why you couldn't use a nail gun to place several nails without bumping things around very much, unless you had no pnuematic nail guns around. I'd look for another boss who wasn't 1) so closed to an obviously better idea and 2) either more knowledgable and able to teach you something or had less ego on the line and was able to learn from you. -David

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   Guest_ | Feb 17, 1999 04:16pm | #5

   *David....thats what I thought...use a nail gun.....we didnt want to damage the plaster below so hand nailing was out of the question....but a gun would have worked great I thought. And your also right about the ego thing..that was more an issue I thought. Thanks.

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   Guest_ | Feb 17, 1999 04:24pm | #6

   *Just a side note to this discussion. The reduced shank diameter of screws is why they shouldn't be used to attach hangers.

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   Guest_ | Feb 17, 1999 07:28pm | #7

   *Flash,I don't have a trouble understanding why a "sister" doesn't reach the plates. Most likely, your boss detected excessive deflection of the floor at mid span, the point in any beam....wood, steel, whatever.... where bending moment is the greatest. It's the increased section modulus AT THAT POINT which resists deflection....the sister provides that increased section modulus. Look closely at the structural steel on the next bridge span that's convenient for you to inspect. I think you'll notice that at midspan, there have been some additional plating added to the flanges. Quite frankly, IMHO, such use of materials is environmentally friendly...but of course, it does require more time and labor.Brian

5. 
   Guest_ | Feb 17, 1999 07:59pm | #8

   *I wondered what the purpose of scabbing on the extra joist was, and question how much stiffness it would contribute -- isn't stiffness mostly a function of joist depth? I can see that it would strengthen the joist, but how much stiffness would it add?

6. 
   Guest_ | Feb 17, 1999 08:32pm | #10

   *Plus their brittleness! FH had some story about a guy who was sitting on a ceiling joist that gave way -- someone fastened the hangers with drywall screws. He was hurt plenty.