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    Conduit Run Deflection

    After submitting point loads, shop drawings etc we began our installation and ran into a problem with the Architect. They are claiming the building design allows for three inches of deflection in the floor slabs and beams. In turn they now want us to install deflection fittings where any horizontal runs turn vertical so as not to put downward pressure on termination points. We are using EMT conduit on trapeze hangers. Is there a standard amount of deflection EMT can withstand when rigidly braced? Any other solutions? This has noting to do with expansion joints etc. Seems to me the drop ceiling is going to be awfully wavy when the floor above gets loaded......

    #2
    Is any flexible metallic conduit allowed? If so, then some judicious use of it in limited portions of the runs might help.

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      #3
      We are using EMT conduit on trapeze hangers.
      It seems to me that making some sort of suspension for the hanger rods is a better option. Conduit and Unistrut stays put. Hanger rods extend and contract with the movement of the slab they are attached to.

      Maybe something like this though the travel isn't enough:
      https://www.zoro.com/mason-vibration...45/i/G0327205/

      -Hal
      Last edited by hbiss; 11-14-19, 08:07 PM.

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        #4
        Originally posted by mak134 View Post
        They are claiming the building design allows for three inches of deflection in the floor slabs and beams. In turn they now want us to install deflection fittings where any horizontal runs turn vertical so as not to put downward pressure on termination points......
        Depending on your constraints, another possible option is to turn the linear vertical movement into a rotational movement so that the horizontal runs can stay at the same distance from the ceiling. This could be done if you can offset the vertical drops at least a foot or two sideways from the horizontal run. You could then come off the horizontal run with a tee or 90 bend for a few feet, and then go into back-to-back LB's or pulling elbows turned down for the vertical drop. The back-to-back fittings could then rotate a little to accommodate any vertical movement. Combination EMT/rigid LB's or elbows such as the following could be used and connected together with a threaded nipple:

        https://www.topaz-usa.com/conduit-bo...ation-lb-type/

        You would also preferably have a threaded fitting on the horizontal run to accomodate a slight rotation.
        The principle is similar to that of "swing joints" used in plumbing, especially with steam, to accommodate the expansion.
        The devil is in the details so it's hard to make any more specific recommendations.

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          #5
          three inches of deflection in the floor slabs and beams

          what type span are you talking about? Even at L/120, 3"= a 30 foot span, 30 ft of EMT length changes from about 360" to about 360-1/16". where is the problem?

          Are the archys talking about floating end hangers? Hard to imagine 3" vertical 'deflection' at a wall unless suspended?

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            #6
            I was thinking that the OP was concerned about the ceiling to floor distance changing, which would be the highest near the center of the span. But the OP should clarify more precisely what the problem is.

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              #7
              3" of deflection seems excessive to me but I'm not the designer. With that said, why couldn't you just use an expansion coupling where the runs transition from horizontal to vertical? I would also assume that this maximum deflection is only at the center of the open slab. Anywhere around the perimeter you would never be able to have that much to deflection so couldn't you just make most of the conduit runs go vertical at the perimeter. If you had to make a vertical turn anywhere near the center, then use an expansion coupling.
              The world is round, you will get there no matter what path you take.

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                #8
                Originally posted by rlundsrud View Post
                3" of deflection seems excessive to me but I'm not the designer. With that said, why couldn't you just use an expansion coupling where the runs transition from horizontal to vertical? I would also assume that this maximum deflection is only at the center of the open slab. Anywhere around the perimeter you would never be able to have that much to deflection so couldn't you just make most of the conduit runs go vertical at the perimeter. If you had to make a vertical turn anywhere near the center, then use an expansion coupling.
                I agree but that is a very expensive solution.

                Comment


                  #9
                  Originally posted by rlundsrud View Post
                  3" of deflection seems excessive to me but I'm not the designer. With that said, why couldn't you just use an expansion coupling where the runs transition from horizontal to vertical? I would also assume that this maximum deflection is only at the center of the open slab. Anywhere around the perimeter you would never be able to have that much to deflection so couldn't you just make most of the conduit runs go vertical at the perimeter. If you had to make a vertical turn anywhere near the center, then use an expansion coupling.
                  That would work but is an expensive proposition

                  Comment


                    #10
                    Originally posted by junkhound View Post
                    three inches of deflection in the floor slabs and beams

                    what type span are you talking about? Even at L/120, 3"= a 30 foot span, 30 ft of EMT length changes from about 360" to about 360-1/16". where is the problem?

                    Are the archys talking about floating end hangers? Hard to imagine 3" vertical 'deflection' at a wall unless suspended?
                    They are using a slotted top track on the walls. How are you calculating the deflection above? We are going off of a set of structural plans that show what the maximum deflection will be in a given area.

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