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reinforcing existing bar joists
4

reinforcing existing bar joists

reinforcing existing bar joists

(OP)
I am placing three roof top units on a tilt up building with non-bearing masonry interior walls. To support my roof tops I am planning to reinforce the bar joists if necessary. I have heard of it done but never tried it myself. Any suggestions in the design is appreciated.
kandg

RE: reinforcing existing bar joists

A few thoughts to consider:

1. Find an ID tag on the existing joist to verify size and type.  See if the original manufacturer will do the design for you once you provide him with the loads.  Joist are proprietory designs and design of repairs/changes are best left to them.  If you insist on doing the design yourself, you will have get the dimensions and material of each joist member and compute the capacity before and after changes. Joist are not always made of "typical" cross section members or of A36 or A50 steel.

2.  Add new additional joist along side existing joist to carry the roof top units.  Depending on loading and spans, you can use joist, beams or lt. ga. purlins.

3.  Check expected deflection caused by the added weight of the units, it can affect the roof drainage and you don't want to cause ponding on the roof.

Just a few things to think about.

RE: reinforcing existing bar joists

2
You can get the moment and shear capacity of the existing joists from the uniform load capacity in the joist tables.  M=wL^2/8, V=wL/2.  Then you draw the existing moment and shear capacity diagram.  The joist shear capacity does not fall below 0.25*the max so the SFD will be slightly different than 2 triangles.  Over this draw the new BMD and SFD according to the new loads (including all existing loads).  You can identify readily the areas that need reinforcing, those where the load diagram exceeds the capacity diagram.

To reinforce for moment you can add rods to the top and bottom angle chords.  Remember that excess moment can be resolved into a couple force to give you a design force.  These rods will be nested inside the "L".  I almost always use 5/8" dia. rods.  Even though you might need smaller rods for the tension, the compression side works better with bigger rods so they don't have bckling issues between welds.  Use one each side top and bottom for the area that needs moment reinforcement and flare bevel weld to the joist angle.

To reinforce for shear you can add small double angles each side of the web member for the range that needs it.  Typically I find that small angles, such as L1.25x1.25x0.25, work fine.  You may also need to add bearing reinforcement.  That will be a little 0.25" plate stiffener at the joist seat each side.  If you don't have bearing capacity information use the max shear force as the max bearing.

Usually if you need reinforcing the moment reinforcing is for so many feet of the middle and the shear reinforcing is for so many feet from the end.  You may not necessarily have to have reinforcing at each end depending on how your strength works out and just where the RTU is placed on the joist.

All of this information I read in Vulcraft's book "Designing with Vulcraft Steel Joists, Joist Girders, and Steel Deck".  It an exceelent sourse for more than just reinforcing and is only $15 for the 2nd edition.  I've used it many times for retrofits for grocery stores, adding RTU's, and it's a pretty easy procedure.  It will have this procedure outlined and have examples as well as a great deal of other information about designing with joists.  It's well worth ordering IMHO.

http://www.vulcraft.com/

RE: reinforcing existing bar joists

Be careful of a web member that changes from originally a tension member to a compression member. This may happen toward the middle of the joist where the point of zero shear has shifted from the original location because of the new load.

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