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I have a support post in my attic that is holding up one side of a minor ridge beam. The post is sitting on two 2x6s on their backs, and which span 20 inches. Yes, these 2x6s are not on edge -- they are on their backs (height = 1.5 inch, width = 11 inch). I want to replace the 2x6s with a cantilevered beam and am wondering if I am thinking about this correctly.

Okay, so there is no visible deflection in the 2x6s (maybe I should say 6x2s) but since I have no idea what kind of load the post is holding up, I found a calculator that determines deflection and put in a heavy load to establish a baseline.

(Beam Supported at Both Ends, Load at Center). I entered 4500lbs, 20inch beam, a modulus of elasticity of 1,400,000 (typical for wood joists?), and 37 for the moment of inertia. I calculated the 37 with w*(h^3) or 11*(1.5^3). My end result is a maximum deflection of 0.0145 or less than 1/64". Since I don't detect that, my assumption is that the load is less than 4500lbs.

Stop right here. Are my assumptions correct so far? Are any of my values suspect? What about calculations?

Okay, assuming that is correct, I move to the cantilevered option. It needs to support 4500lbs with at worst a deflection of 0.0145. I use this calculator

(Cantilever Beam - Single Load). I use 4500lbs, a length of 36 inches (it won't be that far, but I want to be safe), 0 for b (the post will be on the edge), 1544 for I and the same 1,400,000 for E.

The 1544 for I is assuming four 2x10s sandwiched together (6*(9.5^3)). I may actually get a glulam that is beefier than that, but again, I want to be a little conservative.

With those, I come up with a deflection of 0.0097 or 1/128".

That tells me that even with those extreme measurements, I'd still have a beam that is TWICE as strong as the existing support, even though it is cantilevered. If I went with a 8x12 glulam, then my deflection would be 1/256", twice again as strong.

So.. am I wrong?

The entire reason for this is because I am expanding the room that happens to be under that post. The room is going to expand out by about 2 feet, which leaves the post with no support on one side unless I do something about it (which I have to).

Here's roughly what the situation looks like now:

Current Support

The doubled up 2x4s are walls. The joist-like beams are joists. That's the two 2x6s holding up the post, which is three 2x6s sandwiched together. The wall going off to the left will be completely removed and the other three walls will be moved to the right by about 2 feet. That means that all of the walls will be to the right of the post. The joists are all going away, too.

My proposed plan looks something like so:

Cantilevered Support

The supporting beam is either four sandwiched 2x10s (maybe 2x12s) or one big glulam. The two side walls are shortened by two feet and the perpendicular wall is moved those two feet. The back bolstering will be done by two 2x10s (or 12s) that are attached to the walls with hurricane straps. The support beam would use a joist hanger on one end and straps on the other.

Confession time:

I actually did bring a structural engineer out earlier for an informal consultation. He gave me some solid ideas on how to support that post with an expanded room -- basically install a 8x12 glulam from the other side of the room (not shown) to the new location of the moved wall. That's definitely an option. HOWEVER, since then, I also realized that I want to raise the ceiling by a few inches and if I do so, the option of resting the long beam on the opposing wall is gone (exterior wall with a low pitch roof). So I'm trying to figure out a way to support the post entirely from one side -- hence a cantilever.

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I'd really like to see a photo of the existing situation as well as a sketch of the proposed solution. Also, have you considered how you'll be attaching the cantilever? –  Chris Cudmore Dec 17 '13 at 15:07
    
I agree with Chris, a sketch would be useful. What problem are you trying to solve? –  Henry Jackson Dec 18 '13 at 1:33

2 Answers 2

up vote 4 down vote accepted

OK first let me say that if you're going to be making major structural changes to a building (and this counts as a pretty serious structural change in my mind) it would be well worth your while to get an engineer or other qualified building professional to help you with your design. If you just "wing it" you may be endangering the house and its occupants. There may also be legal or insurance-related ramifications to doing this outside the proper channels. However, let me offer these pieces of advice:

  • Do not confuse "service loads" (the day-to-day loads on a structure) with "design loads" (the loads the structure is built to withstand — typically much higher). Whatever condition you observe on a given day is probably not the worst-case scenario.
  • Deflection (stiffness) is not the only consideration when designing a structure, you must also consider its strength - i.e. how much weight it can take before it breaks. Strength and stiffness are both important.
  • I don't know where you came up with the 4500 lbs quantity, it sounds somewhat arbitrary since you haven't provided any background info. The amount of weight you design the structure for is based on your local building code, which includes factors like snow loads, wind loads, etc. which may be difficult for you to estimate. The weight of the roof itself is only a portion of the load you must take into account (usually less than half). These loads are typically specified as pounds per square foot (probably in the range of 30psf to 60psf) in addition to the weight of the roof.
  • Your existing 2x6 situation sounds weird, I wonder if someone added an addition and the columns didn't quite line up. I would not assume that it's OK as it is now unless you see some reliable documentation. You should design your new solution for the geometry of the house and the requirements of your building code, not just matching the existing solution.
  • Stresses on a cantileved beam are more complicated than a simple span between two supports. You haven't described your proposed cantilever at all but the backspan is an important part of the design. And again, you must consider the strength of the beam, not just its stiffness.
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Yeah, I understand that the correct way to do this would be to get a signed engineering plan, but I fear that any remediation would require a huge amount of work, since I suspect my house isn't that well made (e.g., cascading changes would be needed). I'd prefer to upgrade in place. The 2x6 situation does sound weird, but is what has been in place for 30-odd years. My thinking is that if that clearly works, then all I need to do is replace that support with something that's as good or better. –  Kurt Granroth Dec 17 '13 at 13:46
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+1 for the difference between strength and deflection. My guess is that you actually don't care much about the deflection here--within reason. You should be designing for a factor of safety with respect to loading, and then simply checking that the deflection at load is acceptable. +10 for get professional help. –  mac Dec 17 '13 at 14:32
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While you are correct re: strength vs. deflection, for most household materials the deflection limit typically 1/240 or 1/360 IIRC, are usually the limiting factor. That is, a safe design will typically violate the deflection limit, but a deflection based design will usually be safe. (Although, all bets are off on this short span). –  Chris Cudmore Dec 17 '13 at 15:09
    
@KurtGranroth: if you have concerns about the rest of the house, that's an even greater reason to ask for professional advice. Ignorance is not bliss in this case. Just getting someone to come take a look doesn't mean you need to tear the whole building down. –  Henry Jackson Dec 18 '13 at 1:32
    
When we're talking about strength, is that referring to 'I' (b*h*h*h) in the formulas or is there a different way of calculating it? I'm reading that 'I' is a measure of how rigid the beam is and two beams of similar materials can be directly compared that way. If so, then my proposed beam is some 500 times more rigid (from memory when I did the calculation) than the existing solution. I suspect that's now what we are talking about, though, since a weakness of a cantilever seems directly related to how much it is extended, too... hence my focus on the deflection formulas. –  Kurt Granroth Dec 18 '13 at 6:29

As Chris says, a picture would be very useful.

At present I'm completely missing the point of replacing a spanned beam support with a cantilever. A picture might make the constraints that affect this idea more obvious. If there is support for the present beam, use it. Or do you have un-mentioned plans to remove the support from one side?

In the absence of a better understanding of the constraints, if the present system is sound, however kludgy it may be, I'd try something like placing vertical 2x stock beside the post, on top of the existing 2x6 laid flat, attached (nails and framing adhesive, perhaps a few steel L brackets as well) to the post and flat 2x6, converting the 2x6 from a plank support to a pair of L supports, without having to pull anything out (life gets too interesting too often when you start pulling supports in old houses. If they are not rotten, I avoid that if at all possible.)

If the post is all the way out to the edge of the 2x6, similar approach, but the new 2X going beside rather than on top of the 2x6 - which is actually easier to fasten in place.

You get all the benefit of "worked for 30 years" along with some extra security from serious reenforcement.

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I just updated the original post with some pictures and a more detailed description. TL;DR - I'm expanding the room and so the support structure under the post is going to disappear no matter what. –  Kurt Granroth Dec 18 '13 at 6:23

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