Raising Ceiling Joist and removing I beam in garage
I have a garage that is ~20.3′ wide. Current roof design has stick built rafters with ridge board at 18″ oc and ceiling joists at 24″ oc. Due to the 20.3′ span, there are 2 2x6x12’s for each ceiling joist that meet in the center of the garage, are overlapped and bolted together, and supported by a steel ibeam. There is a post right in the middle of the open area of the garage to support the middle point of the ibeam. The roof is a 6/12 pitch.
Most of the tables I looked at showed that at a 20.3 foot span, there is no way to use standard lumber for the 20′ span without any center support. I came across several web sites that talk about being able to move the ceiling joist 1/3 the total distance from ridge board to wall. So my ridge board to wall length is ~135 inches which means I could measure from the wall, up the rafter ~45 inches and place my rafter there. This would reduce the span down to about 13.5 feet.
I then looked at one of the tables I found and see that I could actually use a 2×4 for the span under a 20# psf dead load, if I am looking at it correctly when 19.2″ oc. I would use a 2×6 just to be on the safe side.
Here is a link to the tables I am looking at.
http://publicecodes.cyberregs.com/st/ny/st/b400v07/st_ny_st_b400v07_8_sec002_par020.htm
I am just trying to get a rough idea before I approach an architect to write up plans and verify.
Finally, there is storage in the ceiling are now that I am fine with getting rid of.
I will try and post some pics tomorrow.
Replies
If you're hiring an architect, figuring this out should be his problem. Explain that you don't want a center post, and ask how he'd make that happen.
Brace the ceiling with a temporary wall, pull the steel beam out one end wall, cut another hole in the gable end and slide the steel beam in on top of the joists. Run strapping from each joist over the steel beam and back to the other side of the joist. Remove temporay wall.
You may just need a bigger beam. I spanned 25' in my garage in Md with a room overhead and no posts but it was a pretty thick beam. It was bearing on masonry on both ends and 10" x 34-36# as I recall
Rafter Ties
You're attempting to install rafter ties and remove the ceiling joists. From the way you described it, there are no vertical structural members between the rafters and the ceiling joists. This is normally a perfectly acceptable alternative within the 1/3 height restriction you noted.
You don't mention what part of the country you live in (snow load?) and you don't mention the size of the rafters. If they are only 2x4's, they may not be adequate to take the stress. If the rafters are at 18" oc, then your rafter ties will also be at 18" o.c. An engineer might ask for a rafter tie on each side of the rafters and will designate the type of connections (nails, bolts, clips, etc...). He may also want to revise or add to the rafter/exterior walls connection and at the ridgeboard - perhaps the addition of a collar.. Google "rafter collar tie" and you'll get a many good descriptions of the terms.
Another option...similar to a suggestion earlier...open a hole in the gable end and slide in a ridge beam just under the ridge board. Once the beam's installed and supporting the rafters, remove the ceiling joists, steel beam, and column. This solution requires adequate bearing at each end of the beam (ie. if there's a wide door on the gable end - is the header adequate?), but might be less work.
BTW, I'm an architect - I would bypass the architect and head right to a atructural engineer familiar with light frame construction...
You just might be able to do that with 2x4s but look further in the code and find the nailing rquirements for the rafter tie to rafter connection. I think you'll have hard time believing a 2x4 with 4 or 5 (or is it 6?) nails through it into the rafter will be anything but swiss cheese that is trying to carry a shear load. Use at least a 2x6. 2x8 around here is a much high quality than 2x6 so something to consider for your future uses.
The top of the rafter tie should be in the lwoer 1/3.
I don't see your problem.
The tables in your link are for rafter spans. Here are the ones for ceiling joists.
http://publicecodes.cyberregs.com/st/ny/st/b400v07/st_ny_st_b400v07_8_par021.htm
You can easily span 20.3 with many species of 2x8. Unless you are going to put in a ceiling you could make due with collar ties.