What can be done to help eliminate hanger gaps when attaching end jacks, or common trusses, at 90 degrees to the girder truss?
This month’s installment is a good question with a really easy solution. I was on another jobsite recently when I noticed a series of small (about 8’ long) end jacks attached to a two-ply hip girder, with rather serious gaps between the end of the jacks and the face of the girder in the hanger seats. I asked the framer about the condition and he proclaimed that the end jacks were not all the same length. I explained how difficult that would be to do in a modern truss manufacturing plant and asked if he had a string line. He wanted to know how a string line was going to reduce the gaps in the hanger connections, or make all the jacks ‘the same length’.
Pulling a string line spaced on the back face of the girder truss showed the girder to be bowed away from the end jacks by about 5/8” at the center of the girder. The framer said that must have been the way it came from the shop because they didn’t do anything to cause it to bow. I asked if they did anything to prevent it from bowing during installation and he looked at me perplexed. He had never done anything like that before.
The easiest possible way to eliminate the potential for hanger gaps when attaching trusses at 90 degrees to the girder is to place a short length of metal strap into the hanger seat prior to setting the carried truss. This piece is normally only 16–18” long and wraps up the back side of the carrying members. Placing these straps at 6–8’ on-center will generally eliminate the main cause of hanger gaps at girder trusses.
I suggested to him that while setting the trusses, perhaps he could do something as simple as placing a tie-strap on the few jack trusses that they used to align and plumb the girder (normally placed in line with where the common truss Top Chord Temporary Lateral Restraints would be placed or about 6–8’ on-center) would eliminate the tendency of the girder to want to bow during the installation of all the jack truss hangers. In his case, the jack trusses were supported by 2x4 face-mount hangers, each requiring four nails into the girder. When we calculated the probable number of hammer blows the girder had been subjected to just while the hangers were being installed (4 nails x 4 hammer blows x 13 installed hangers), the otherwise unrestrained girder was hit over 200 times, not including the nails installed to fasten the plies together. Yes, possibly the framer did in fact do something to cause the girder to bow.
Perhaps a specification for a few of these straps on the Truss Placement Diagram would prevent this kind of hanger mis-installation in the future. Otherwise, a specification requiring lateral restraints and diagonal bracing of the girder bottom chord is in order to help keep it in its assumed location and perfectly straight to receive the same length end jacks or common trusses framing into the opposite face.
Thanks for reading.