How to Reinforce 2x6 Ceiling Joists to Handle Heavy Loadscomments (17) February 9th, 2011 in Blogs
That's Leo's delimma, as he explains in this Breaktime post. The recently completed 16- by 24-foot building has 2x4 walls and a roof with a 2x10 ridge and 2x6 rafters.
It was only after the building was up did Leom realize he hadn't figured in the added weight of the lumber on the ceiling joists.
"The 16-ft. 2x6s are there to keep the 2x6 rafters from spreading," he writes. "This is a pretty basic design, designed by an engineer and approved by the town. I only wish I mentioned it earlier that the attic might be used for storage. If I had mentioned it earlier what would I have done differently? I don't want a beam or posts running down the middle for obvious reasons."
If he doubled up every other joist, would it be enough to handle the weight of "a lot of hardwood." His engineer does, but what about everyone else?
A little sag, but it won't collapse
The 2x6 joists will sag, no doubt, says DanH. "They will probably not collapse, and the load will probably not compromise the structure. Many a garage has been loaded down for decades the way you suggest, and out-and-out failures are rare, in spite of the rather inferior garage construction techniques used 60-80 years ago."
But, DanH adds, that assumes Leom uses common sense and doesn't overload the overhead storage area with wood.
The sag could be reduced or eliminated by installing king posts, or Leom could sister 2x8s to the 2x6 joists to "roughly triple" the load-bearing capacity of the ceiling.
"Since this isn't a floor, then the allowable deflection tables don't really apply," adds Jigs-n-fixtures. "They are mostly there to keep people walking on the floor comfortable, or prevent inflexible flooring like ceramic tile, from being damaged. The structure would be sound with considerably more deflection."
Even a stack of wood a foot deep would mean a load of 50 lb. per sq. ft. (psf).
Engineer 10 is skeptical.
"Tell your engineer that another engineer says that doubling 2x6s doesn't work for heavy loads," he writes. "If he/she still insists that doubling is OK then ask for a signed and sealed letter to this effect."
Engineer 10 adds that a No. 2 2x6 over a 16-ft. span can carry 10 psf while a 2x8 will carry 20 psf. Sistering the two together means a load-carrying capacity of 30 psf.
"This is based on bending strength only not deflection, therefore a load larger than that will cause collapse," he writes. "The load can be considered permanent dead load since it will be there for months at a time which makes everything worse because of material fatigue.
"In addition to that the deflection will be so noticeable from below that it will look frightening to a casual observer."
I-joists would have been a good bet
Calvin suggests that I-joists would have been an alternative to consider at the start of the project. In his own garage, he used I-joists with 2x4 flanges for a 30-ft. clear span.
"No sistering or modification of I-joists to the existing situation," he says.
There's a problem with I-joists, Sandcastles points out. Leom is already concerned there isn't enough room between his double top plate and the roof to wedge in the additional 2x8s, and the minimum I-joist Sandcastles knows of is 9 1/4 in.
"You probably have to even more significantly trim the 9 1/4 I joist, cutting it more than manufacturers will allow," Sandcastles says.
Also, says Leom, the load rating for an I-joist is 100 psf live load, 20 psf for a dead load, and that doesn't help his situation much because lumber stored overhead for long periods of time is dead load.
Piffin had this variation on the I-joist option:
"What I would do is to add a 2x4 to the top and bottom of each joist, using glue and structural screws to convert each joist into an I-joist in place," Piffin writes.
"You could even possibly introduce a good crown to each by holding the bottom chord in place with a slightly long 2x4 post under the center as you install it. Do bottom chords first, then the tops. Leave posts in place until all fasters are in and the glue kicks well."
Or, use LVLs in a limited area
JAlden wonders whether Leom actually needs storage over the full 24-ft. length of the shop. Would, say, 8 ft of storage be enough for the amount of wood he has?
"If so, go buy 4 pieces of 9 1/4 inch [laminated venner lumber]," JAlden writes. "Last time I bought them they were about 3 bucks a foot so we are talking 200 dollars here.
"Cut the ends at the angle of your roof and see how long you can make them and still get some bearing on your top plates. You may get full bearing if you have some roof overhang to slide one side into. Get as much bearing as you can. Even half of the plate is sufficient.
"Sister these to every other 2x6 and you'll have 8 feet of beefed up area."
Come back next week to find out what Leo did to solve this deflection dilemma
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