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Old 01-18-2010, 12:30 PM   #1
azone
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Default reinforcing joists for deck above garage

I'm working on part2 of my garage remodel.
I'm insulating and dry-walling all the walls and ceiling.
Before I do the ceiling I'd like to see if there's a way to reinforce the joists since there is a deck above the garage. The deck will remodeled next. It's fairly bouncy as it is now.
The garage ceiling is 2x10 @ 16" spacing with inserts between each row as shown in the pic. Other than these inserts there is no support perpendicular to the ceiling joists. Is there anything I can do to improve the structure to make the ceiling more stable? some sort of joist brackets or something?
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Old 01-18-2010, 12:37 PM   #2
mtwaterguy
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Default Re: reinforcing joists for deck above garage

How long are those 2x10's? From the pic, it looks like they're pretty long. You may need to run at least one support beam under them from side to side.
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Old 01-18-2010, 01:24 PM   #3
CraigFL
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Default Re: reinforcing joists for deck above garage

If you can stand it, screw 3/4" plywood into the bottom of the joists instead of drywall and paint it white. Probably 2" deck screws every 4" would be fine. Boxing it in like this will give it a suprising increase in stiffness.
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Old 01-18-2010, 02:46 PM   #4
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Default Re: reinforcing joists for deck above garage

A beam would do it, but it looks like the bridging is incorrectly installed. Yours look like they are alternating one material thickness from one to the next. I believe the correct way is to have them all aligned. This makes the floor more of a box structure.

Plywood would help too. A Structural engineer would have better ideas
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Old 01-18-2010, 03:43 PM   #5
azone
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Default Re: reinforcing joists for deck above garage

the span is 22'.

The bridging (I guess that's what you call it) is also 2x10 pieces, they are staggered back and forth because they are nailed through the joists into the ends of the bridging pieces as opposed to screwed from the surface at angles.

thanks for the insights so far.

Last edited by azone; 01-18-2010 at 03:46 PM.
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Old 01-18-2010, 03:53 PM   #6
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Default Re: reinforcing joists for deck above garage

There are code limits related to the span length of your lumber size, in this case 2X10's. That will give you an idea of the MINIMUM requirements but if you plan to have parties up there or other heavy loads you really should exceed the minimum code requirements. If it is bouncy that is an indicator that you are beyond what is safe for the span using 2X10's. It looks from the pic that your floor boards are laid on a 45 degree angle which is good and helps tighten up the structure and prevents racking. My guess is that you are going to end up with a couple of load bearing beams perpendicular to the joists and some posts to transfer the weight to some footings you will need in your floor. By the way and I am NOT a structural engineer. I agree with MadMechMaster - have an SE look at it. Good luck.
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Old 01-18-2010, 03:55 PM   #7
Daniel Dudley
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Default Re: reinforcing joists for deck above garage

You could run a stud up from the floor joists to the roof rafters at the peak, and space them every 8 feet.

What is the span for those 2x10s anyway ? Sixteen feet would be about the max span for a 2x10.

Of course you could always pull the blocking and double the joists. That would do it for sure. Keep in mind though that modern structural engineering is designed for about zero deflection at rated capacity. Just because something is springy doesn't mean it is absolutely going to fail.
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Old 01-18-2010, 04:05 PM   #8
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Default Re: reinforcing joists for deck above garage

That' s not bridging, its blocking and it is correctly installed. While adding some bridging will stiffen the floor, 22 feet is quite a span for 2x10 material. Both blocking and bridging function to transfer point loads to adjacent joists and constrain the joist from deflectiing from side to side along its' bottom edge as load is applied. I am going to assume that they are probably pitched if they function as roof rafters unless you mean that the "deck" is just the floor of the attic or 2nd floor of the structure. Just guessing that if it is the 2nd floor or attic floor, it was designed for light loads.

A beam placed mid span would certainly help, however, the dimension of the structure 90 degrees to the joists may be quite long and a beam to carry the load with no columns may end up being quite a chunk of wood. Go to a lumber dealer that sells LVL or Gluelams or other engineered lumber. They have software that will specify the size of the beam needed to go from one end to the other without intermediate supports. If the beam is quite large, you could "flush frame" the beam install by cutting back the existing joists to provide a channel to install the beam into and then hang the existing joists from the beam with approved metal connectors. This will add 9 1/4 inches of clearance measured from the floor to the bottom of the beam as opposed to just running the beam under the joists.

Putting plywood on the underside of the joists will add minimal stiffness to the frame and you will never know what the assembly is truly rated for. If you go the engineered route you will have valuable information as to how much load the structure can really handle.
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Old 01-18-2010, 05:48 PM   #9
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Default Re: reinforcing joists for deck above garage

First thing, does the floor need to be to code for any reason (i.e living space, new walls, etc)?

If so a whole lot of work has to be done. In general 2x10's @ 16" o.c. should span about 15' to 16' as controlled by deflection (bounce) @ 40 psf live load or about 16' as controlled by stress in the 2x10's (@40psf live load). Cutting down the span of the 2x 10's will have a huge effect on the deflection/bounce. Not to get too technical but if you were to jump up and down (point load at center of span) at the center of the 22' span versus the center of a 16' span you would have 2.6 times the deflection/bounce in the 22' as compared to the 16' span. Point being, depending on what you want to accomplish, cutting the span down to say 16' looks to be the most efficient way to solve your problem.

Even "sistering" new 2x10's (perhaps new 2x12's/14's instead ?) in next to the existing 2x10's will not get you where you want to be in my opinion but you could always do a few and then walk around up there and see how comfortable it is. The sistered 2x10's may not need to run the whole length either but only an engineer could tell you that.

Really depends on what your intent is or the "big picture"
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Old 01-18-2010, 05:52 PM   #10
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Default Re: reinforcing joists for deck above garage

Lumber design tables for rafters:
http://www.awc.org/pdf/WSDD/C5.pdf

I'm not an expert on this; I just found these tables while convincing myself that the loft I built wouldn't fall down when I set the air compressor on it.

According to the table on p. 256 (it looks like you have 16" spacing), the longest span your 2x10's should be making is 12'7". And that's only if you intend to have a 50 lb per sq ft load; the minimum load offered in the tables.

I used the table's column with the weakest mechanical properties since we can't be sure of your wood's strength. When in doubt, you have to assume you've got the weakest material.

I agree with what the others have said. You could divide the span in two with a beam that runs perpendicular to the joists and goes across the whole garage. It doesn't have to be centered, so you can position the columns in between door openings so as not to obstruct any access points into the garage. Just as long as it lands within the middle 1/3rd of the span.

That document I linked to has tables and formulae for designing every aspect of your situation there if you explore the bookmarks.

Like tcianci said, you would need a very tall beam to span the whole garage without any columns along its length. If you look at the picture of the beams that support the roof in my shop, you'll get an idea of what it takes to make such a span. The column is a 6x6 but it's also bolted to a concrete filled wall, which helps keep it from buckling, greatly increasing its capacity. The large beam is 6x26 spanning 50 feet, and the smaller beam is 4x16 spanning 25 feet. Ignore the 2x4's on 24" centers; the roof feels springy when you walk on it except when you're standing directly over one of those beams of course. The 4x16 is something along the lines of what you would need.

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Old 01-18-2010, 05:57 PM   #11
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Default Re: reinforcing joists for deck above garage

WNYflyer's avatar reminded me of something. You could save a lot of headroom over using a lumber beam by using a steel I-beam instead. I bet WNYflyer can tell you what size beam you would need. The height of a steel I-beam would be a fraction of that of an equivalent timber.
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Old 01-18-2010, 06:20 PM   #12
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Default Re: reinforcing joists for deck above garage

While steel could certainly be used, it has a few drawbacks. 1st believe it or not, a steel beam wouldn't be too much smaller than an engineered wood one. 2nd The steel would have to be specified by a structural engineer and bear his stamp, around here that runs about 400 bucks. 3rd, if you wanted to "flush frame" with a steel beam, you would have to "pad out" the sides of the beam so that the correct hangers could be used. This involves having the beam drilled and then in filling the sides with 2x material to take the hangers. 4th, if you decided not to "flush frame" you would not really gain any head height over a flush framed wood beam. 5th, Steel is much "springier" than wood and although you would have an actual, calculated PSF rating for the floor, the bounce may still be un acceptable for your application. A padded out steel beam exhibits much better bounce damping characteristics. 6th, steel beams don't float. Remember that once you get the beam there, you need to get it into place. This is where the wood product shines because you can often get the structural specification to be for multiple engineered beams laminated together in place instead of having to lift a huge chunk at once. For example, say your specified beam was a 6 x 14 glue lam. The could probably specify say 3, 2x16 LVL's or some other combination of pieces that are very easy to handle on site by just a few guys with a few beers.
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Old 01-18-2010, 08:05 PM   #13
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Default Re: reinforcing joists for deck above garage

depending on your desired loading/usage.
I think I would go with putting 2 or 3 engineered (the I type) beams running down the middle with posts at each end.
that would give you 2x10s effectivly spanning 11' which should be very stable

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Old 01-18-2010, 08:28 PM   #14
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Default Re: reinforcing joists for deck above garage

I would add a couple 6 x 10 parrallam beams at 1/3 and 2/3 the width
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Old 01-18-2010, 10:54 PM   #15
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Default Re: reinforcing joists for deck above garage

Check out Lite steel beams in Virgina. New product for the structural stuff but used many years down under. Put in the welder for this stuff and was very impressed by it. It is a lot lighter than I beams. Two guy's can handle it with out a crane and it can carry a hellva load. They make different sizes to 40 foot.

http://www.litesteelbeam.com/
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Old 01-19-2010, 04:45 AM   #16
Daniel Dudley
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Default Re: reinforcing joists for deck above garage

Quote:
Originally Posted by azone View Post
the span is 22'.

The bridging (I guess that's what you call it) is also 2x10 pieces, they are staggered back and forth because they are nailed through the joists into the ends of the bridging pieces as opposed to screwed from the surface at angles.

thanks for the insights so far.
Wow, didn't see that. That is a long span, and I assume a beam would wind up right over the garage door. Maybe consider making the roof rafters into part of a storage truss type of setup, or doubling the 2x10s with 2x12s. That gets you into the range of strength, but a beam would be your freind.

Lots if ways to skin a cat, depends on what you need in the end. What is the use of the upper space, and do you have a picture of it ?

BTW, I am not an engineer, and I do not pretend to be one. Overbuilt is a concept that I like to build to, if there are no engineers around.
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Old 01-19-2010, 08:48 AM   #17
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Default Re: reinforcing joists for deck above garage

According to my rough calculations...

Stiffness of 2X10 is 107

Stiffness with some subfloor included with the joist is 207 (twice as stiff, 1/2 the deflection)-- Realistically, the way it is now.

Stiffness with another 2X10 joist sistered is 314 (1.5 times as stiff, 3/4 the deflection)

Stiffness with present system and 3/4" plywood on bottom is 380 (almost twice as stiff, 1/2 the deflection)



So, it's a matter of how springy it is. If half the springyness(deflection) is OK, then the 3/4" plywood is the best of the above.

If you can stand the loss of headroom, the options get better.
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