Originally Posted by TMcG
Next up is boxing out the beam....
To preserve maximum sound isolation, I had to find a way to prevent sound from 'escaping' through / around the steel beam. Decoupling was key, but how?? The solution had to be low profile because I am constantly fighting for every inch of ceiling height and the lower the isolation extends below the beam, the lower the projector must be mounted to project an image past this obstruction. And ideally I'd like to keep the total width of this assembly on either side of the beam as thin as possible.
I considered a number of options to achieve these design goals and decided engineered TJI joists were my best option. Since weight is going to hang from these joists, I opted for the plywood inserts shown above, only giving up a slight amount of soundproofing mass in exchange for solid wood to mount the TJI Joist hangers. This also achieves my decoupling goals because the TJIs will be attached to the fully decoupled theater walls. Mounting these hangers as close as possible to either side of the steel I-Beam is the rationale for the additional blocking you see I installed inside the beam pocket above. If the TJI is right along side the steel beam, the hanger bracket nails would be inside the steel beam pocket. Here's the process I took:
Unfortunately my steel beam is not dead level. There is a 'low' side. The beam is also slightly twisted / canted to one side. I installed a small piece of 3/4" plywood dead level under the low point of the steel beam, leaving just enough gap for the bottom of a mounted joist to protrude beyond the beam by less than 1/8".
This is the first time I had an opportunity to use my Bosch auto-leveling laser level.
I marked off the equivalent distance on the opposite wall and attached another piece of plywood so my two plywood ledgers were dead even with one another.
Next, I used a brand new laser measuring device to determine the length I needed to cut the TJI joists. I drew a line vertically with my bullet level on the opposite wall and shot a measurement at the top, middle and bottom of the line. I was on the short ladder and this was all done above my head. When I pulled down the laser level I had to laugh because all three measurements were identical down to the 32nd of an inch. I know I spent time squaring the room, but dang...
I brought the two 18' TJIs into the room and set them up on my sawhorses.
Since I had plenty of scrap to spare, I first squared one end before cutting to final length
I cut the TJI to length, leaving 1/16" gap on either side
The TJIs weren't especially heavy, just a bit awkward. Working by myself, I did what I could to lift it into position
Sitting on the two plywood cleats. It was a perfect fit.
I then repeated the process for the other side of the beam. This side was out 1/16" at the top. I briefly considered inviting
over to help rid me of this shoddy workmanship by demo'ing the entire theater again...but I reluctantly decided to push forward....
I made my cut and lined up the measuring device with one end...
Then put the other end flat against the wall to get my measurement
This time I cut the TJI to exact length with no gap
...and installed. Again, fit was bang on.
I initially lined up both TJIs in parallel with the steel beam...
But upon further thought, I decided to use this as my opportunity to square this structure with the front and rear walls. Not only was the steel beam slightly sloped and tilted, but it was also not parallel with the concrete foundation and therefore my theater walls.
Here's what the same TJIs looked like when squared with the room but as close as possible to the steel beam:
Using my combination square, I penciled in the position of the TJIs on the plywood so I knew where to install the joist hangers
I installed the joist hangers, nailing on one side only. I need to install orange conduit on either side of the beam, insulate and then temporarily bend back the bracket slightly to slip the TJI into place
All four joist hangers installed and ready to go
Next up is installing orange conduit, insulating the beam pocket and covering with two layers of sheet material / Green Glue...