2x4 Framing Question...

[Derek Arita]

Across a particular span, does a 2x6 sistered to a 2x4 equal the strength of a 2x10? This is for my shop shelving project again. As I was thinking about gluing and screwing the 2x4 to to the 2x6, it occurred to me that it would be just like a 2x10, but I wasn't sure.

[Brett Nelson]

No it doesn't. The glue changes the modulus of elasticity, as do the screws.

But, I'm assuming that you are talking about laying the glue-up flat so that it becomes a 9" deep shelf (as opposed to tipping it on edge and expecting it to be a 9" deep beam).

Using it as a shelf that way is certainly kosher, but the alternative use as a beam is not kosher. If you'd like, I will run the numbers for you. Tell me you span and the max amount of weight you'd be placing on that shelf, and I'll tell you what your deflection will be under max load.

BTW, I'll also need to know if the weight will be evenly spread across the shelf or if it will be concentrated point loads (like heavy metal machine parts)

[Derek Arita]

Brett, actually, I'm adding a 2x6 to an existing 2x4 front "joist" of a shelf that spans the width of a 1 car garage that is 12' in width. So, my thought was to sister the 2x6 to the existing 2x4 with glue and screws. I was hoping to prevent some sag.
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[Jamie Buxton]

If you're fastening the 2x4 to the 2x6 so that the result is about 1 1/2" by 9 1/2", just like a 2x10, yes, the composite beam is as stiff as the 2x10. However, if you're fastening the 2x4 to the side of the 2x6, so the result is only 5 1/2" tall, no, the composite in nothing like the 2x10. There's some math involved, but the stiffness of that arrangement isn't much better than the stiffness of the 2x6 alone.

You might think about it this way. A 2x10 is stiffest against gravity when the 10 dimension is up-down. If you turn it so that the 10 dimension is horizontal, the 2x10 is nowhere near as stiff. That is, while the cross-sectional area is the same, one configuration is much much stiffer.

[Graham Wintersgill]

Hang a tennis ball from the ceiling with a bit of string so that it touches the screen when the car is in the right position then use that as a marker when parking the car. Put a hook on one of the side walls so you ccan put it out of the way when you are working.

Graham

[Brett Nelson]

Brett, actually, I'm adding a 2x6 to an existing 2x4 front "joist" of a shelf that spans the width of a 1 car garage that is 12' in width. So, my thought was to sister the 2x6 to the existing 2x4 with glue and screws. I was hoping to prevent some sag.


OK... A couple things I would suggest. If you sister that 2x6 under the 2x4, then you are giving up valuable head room, and without proper end support you won't get as much benefit from it, IOW it wouldn't be even close to the strength of a properly supported 2x10.

OTOH, if you take 2 more 2x4s and sister them to the back of the existing 2x4, then you'll get most of the benefit. By my calculations, you can load up that shelf with 2,000lbs of "stuff", and you won't get any more than 3/8" deflection at the center of the long span. You'll still want to glue and lag the 2x4s together though.

[Brett Nelson]

If you're fastening the 2x4 to the 2x6 so that the result is about 1 1/2" by 9 1/2", just like a 2x10, yes, the composite beam is as stiff as the 2x10. However, if you're fastening the 2x4 to the side of the 2x6, so the result is only 5 1/2" tall, no, the composite in nothing like the 2x10. There's some math involved, but the stiffness of that arrangement isn't much better than the stiffness of the 2x6 alone.

You might think about it this way. A 2x10 is stiffest against gravity when the 10 dimension is up-down. If you turn it so that the 10 dimension is horizontal, the 2x10 is nowhere near as stiff. That is, while the cross-sectional area is the same, one configuration is much much stiffer.

Ahhh... I'd assumed he was talking about stacking them vertically. But I think you're right in that he is suggesting sistering them on their vertical axis but in a side-to-side fashion.

In that case Jamie is correct. In my above example of 2,000 uniform load on that shelf, you could expect a little more than 5/8" deflection if built as suggested. Whereas, a 2x10 would result in less than 1/8" deflection. Big difference.

[Derek Arita]

So what if I used 2 2x6's sistered side by side with the existing 2x4? Would that be stronger than 3 2x4's sistered side by side? Because the shelving is already in place, it would be simpler for me to add to the front of the existing 2x4 than to sister to the back of it. All would be supported by cripples on the sides, to the slab. Thanks for hanging in with me on this one.

[Alan Schwabacher]

What goes on the shelf? Note that in the example given, if you load a ton onto the shelf, it will sag by 5/8" at the center. My guess is that even though it's less stiff than a 2x10, it's plenty good enough.

[Mort Stevens]

Take a look at a steel section - a small piece of steel can do the job of a large piece of wood.

[Henry Ambrose]

Unless you're storing gold ingots I don't think you need to worry about what you've got now. Sistering on another 2x4 would be easy and make it some stronger. What's the span 6 or 7 feet?

But if you really need more strength add a section of angle steel to the bottom edge of the 2x4. You'll loose no clearance to speak of. Screw every six inches on both edges, alternating. If you want to get real fancy throw some epoxy in there.

[Brett Nelson]

well I agree and disagree with some of the posts here.. First, it is surprising how much people underestimate the weight of storage items. Guaranteed that the stuff on there in the picture is over 1,000lbs. Start storing books and tools up there and it gets heavy in a hurry. But I don't think you would go over 2,000lbs which is why I used that weight for my calcs. I also assumed an 8 foot long span. As I look at it now though, it looks like it might only be about 6'. If it is only 6', then you would only be getting 3/8" deflection with 2,000lbs on that shelf if you just stuck on another 2x4.

Interestingly enough, given the design of your shelf, you should be getting greater deflection in the middle joist than you are on the front joist. If I were you, I would just sister another 2x4 onto each of them and call it good. I think it is adequate for what you are using it for. From there, cripples on the ends for bearing would be the best thing you could do here. If you are like most people you are currently counting on 2 nails in the end of each joist to carry that load. Luckily, that post takes a huge portion of it.

[Steve Schoene]

Look at the picture again. The longest unsupported span is about 4 feet, The support comes from the ceiling, as well as from the post to the floor. You have a 2x4, presumably attached to the ceiling at 16" or 24" intervals, with two sets of vertical supports dropping from the top. While somewhat limited by the capacity of the nails holding the top to the ceiling joists, those vertical supports add significantly to the holding capacity of the shelf. This is by no means a simple system that you can calculate sag from "sagulator".

[Brett Nelson]

Look at the picture again. The longest unsupported span is about 4 feet, The support comes from the ceiling, as well as from the post to the floor. You have a 2x4, presumably attached to the ceiling at 16" or 24" intervals, with two sets of vertical supports dropping from the top. While somewhat limited by the capacity of the nails holding the top to the ceiling joists, those vertical supports add significantly to the holding capacity of the shelf. This is by no means a simple system that you can calculate sag from "sagulator".

Ah, you're right. At first glance my brain said that the vertical supports were just a railing.

It would be relatively easy to calculate the contribution of these additional members, but at this point it is completely unnecessary. The point still stands. The weakest part of the entire structure is the middle joist. I would beef that one up a bit with another 2x4, but the front one is probably ok assuming that those vertical members are indeed helping to prevent deflection.

[Nick Laeder]

Look at the picture again. The longest unsupported span is about 4 feet, The support comes from the ceiling, as well as from the post to the floor. You have a 2x4, presumably attached to the ceiling at 16" or 24" intervals, with two sets of vertical supports dropping from the top. While somewhat limited by the capacity of the nails holding the top to the ceiling joists, those vertical supports add significantly to the holding capacity of the shelf. This is by no means a simple system that you can calculate sag from "sagulator".


Wait a minute. You mean there could be more to engineering than just putting data into a web based application, pushing the magic button and then regurgitating the results?

Poppycock.

[Brett Nelson]

Wait a minute. You mean there could be more to engineering than just putting data into a web based application, pushing the magic button and then regurgitating the results?

Poppycock.

Yeah, there is a bit more to it than that. You have to sit through 4 years worth of lectures, then put data into an expensive desktop application, hit the magic button, and regurgitate the results. :D

[Nick Laeder]

Yeah, there is a bit more to it than that. You have to sit through 4 years worth of lectures, then put data into an expensive desktop application, hit the magic button, and regurgitate the results. :D

Yeah, and take two weeks doing it.

Nick
BSCE

[Lee Koepke]

Yeah, and take two weeks doing it.

Nick
BSCE
Then plug in the lowest bidder

Lee
Certified Professional Estimator
:cool:

[Derek Arita]

Actually, maybe the picture I posted is deceptive, because the span that the 2x6 sistered to the 2x4 has to cross is just short of 12'. The idea is to get rid of the post completely. Now what do you think?

[Tom Ewell]

visualize your shelf as being a floor system.

are the 2x4 "joists" spanning the across the garage... wall to wall?
(left to right in the picture ) and is that the 12' span you speak of.

or are the "joists" being supported on one end by the "beam" that you're trying to beef-up (front to back in the picture)

hard to tell by the picture but if it is front to back and your beam is carrying the half load then how deep is the span front to rear?

[Brett Nelson]

Yeah, and take two weeks doing it.

Nick
BSCE

Sure, if we work faster than that, it would de-value our services.:eek:

Seriously though, I didn't graduate in structural (although I'm trying to now), so if I mislead, then call me on it.

[Brett Nelson]

Actually, maybe the picture I posted is deceptive, because the span that the 2x6 sistered to the 2x4 has to cross is just short of 12'. The idea is to get rid of the post completely. Now what do you think?

That changes everything!

Most importantly, I don't know how you have it fastened to the wall on the left and right. If you simply end nailed with 2 16D nails, then that is not adequate for this structure. Hangers or cripples are needed for sure.

From the picture I'd assume 12'x4' platform. Attic storage would normally be limited to 20lb live load, but I think 30lb would be safer here, cause you'll be stacking that shelf high with all sorts of stuff.

To make things simple for you, building codes would spec 2x8 @ 24"oc, if you wanted to make the full 12' span. Even then, under full load, you might see 1/2" sag in the middle. You can't reasonably put in enough 2x4s to accomplish the task. Sorry.

[Thom Porterfield]

Assuming (from looking at your photo):
shelf span = 12' or so
shelf depth (wall to open edge) = 24" with no center joist.
The back of the shelf is fastened to the wall.
(This ignores connection to the ceiling which doesn't look to me to be anything more than a positioning device for the upright members.)

Then:
(3) 2x4 glued and screwed together = 4.5" x 3.5" will support a total load (that's stuff evenly placed everywhere on the shelf!) of about 1000 lbs. (The actual structural calculation allows 1056 lbs, but part of that has to be the weight of the shelf itself!)
This assembly will deflect a little over 3/4" in the middle.

Increase the assembly to (3) 2x6 glued and screwed together (4.5" x 5.5") and the capacity of the shelf rises to 2700 lbs.

Sistering a 2x6 either side of the existing 2x4 will have carrying capacity somewhere between these two figures.

Brett's comment that the middle joist is under greater stress than the front one is valid. It isn't clear, however, how deep the overall shelf is. Assume somewhere less than 4 feet (based on the white cabinet under that shelf on the right--and I hope you aren't using it for support!), then the above calculations apply to that center joist.

I would triple up the center joist (with 2x4s) and add one to the front. I would use ordinary yellow glue (even white glue will do the work just fine), and secure (permanently clamp) the members together with 3" wood screws placed every 12 inches staggered top and bottom (and in the triple member, from both faces).

It is very important in these scenarios that the ends of the joists have adequate bearing/connection. At the ledger shown on the left wall, I would use a metal hanger designed for the beam assembly. Simpson Strongtie makes such hangers and they're available at most decent lumber yards (maybe even at the borg). However, it isn't clear how that ledger is connected to the wall. Assuming it has at least (2) 16d nails in each of (4) studs (driven through the drywall?), it can carry about 800 lbs, total load. That limits the load on the center joist to less than 1600 lbs, because part of that ledger's load carrying capacity has to be the end reaction of the front joist.

Still, let's say you beef it up as suggested here. You wind up with a carrying capacity of that shelf system of something like a ton. How much camping gear do you have? :D

[Nick Laeder]

Seriously though, I didn't graduate in structural (although I'm trying to now), so if I mislead, then call me on it.


Not me man. I'm just being a smarta$$. I've got the paper, but haven't followed up on the initials (PE) because I work in Construction Management. I value my education because it taught me to think like an engineer, but when I need something designed, I call a real engineer.

[Brett Nelson]

Not me man. I'm just being a smarta$$. I've got the paper, but haven't followed up on the initials (PE) because I work in Construction Management. I value my education because it taught me to think like an engineer, but when I need something designed, I call a real engineer.

I'm only following up on the initials cause I have some interesting designs in my buildings and structural engineers are hard to work with and costing me a lot of money. I need to be able to engineer my own plans.

[Derek Arita]

visualize your shelf as being a floor system.

are the 2x4 "joists" spanning the across the garage... wall to wall?
(left to right in the picture ) and is that the 12' span you speak of.

or are the "joists" being supported on one end by the "beam" that you're trying to beef-up (front to back in the picture)

hard to tell by the picture but if it is front to back and your beam is carrying the half load then how deep is the span front to rear?

I guess I'm confusing everyone by misusing terminology. The beam I want to beef up is going wall to wall, right to left in the photo, which is 12'. I will add a "cripple" stud(if that's right), to both sides, under the beefed up beam to take the load of the beam. I will also leave the two center supports that go up to the roof rafters via the 2x4 on the ceiling. Sorry for the confusion...I wish I were a framer and could use the proper terminology.

[Tom Ewell]

What most seem to have a problem with is how the current structure is put up. If your "beam" is the only outboard support for a shallow shelf, say two feet or less then yes use the recommendations posted for the load intended.

If your shelf is something like four feet or better then your best bet is to treat the whole system as a storage ceiling or floor system and figure dead and live loads accordingly. The beam is just another joist.

You'll need to properly support both ends of your floor system, at the walls. Bolt or cripple the 2x4's at the walls. Add more Joists, sisters or whatever you need to at the 12' span to get up to storage specs.

Here's a calculator to fiddle with.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Spruce-Pine-Fir+%28South%29&size=2x8&grade=No.+2&member=Ceiling+Joists&deflectionlimit=L%2F360&spacing=24&wet=No&incised=No&liveload=10&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span

Brett and the others are suggesting similar.