The Builder's Engineer Goes Green
I’ve read so many articles about green construction lately I’m beginning to believe that our industry really does give a rip about it. For a long time I passed the whole notion off as someone’s political agenda no real person actually cared much about.
Don’t get me wrong, I think green is great. But in America if it doesn’t pay, it ain’t gonna fly. Finally, green pays. Energy and materials simply cost too much for builders to be inefficient any longer.
I have a very self-serving, very green idea to share. It has to do with lumber waste on the job site.
I bet 90% of all new homes waste at least $1,000 worth of over-designed framing. That’s $1,000 worth of profit.
Here’s what I’m talking about:
- Using 6x beams where two 2x’s would work
- Using two 2x10’s where two 2x4’s would work
- Using three rows of posts and piers where two would work
- Using new PSL where boneyard 6x or 4x would work
- Using big headers over non-load bearing doors and windows
- Spanning three or more windows in a row with a big herking glu-lam, even though the 2x mullions between the windows will act as bearing posts whether you intend them to or not
- Using a 12”-deep floor system where 10” would be plenty strong and stiff
- Using rafters, joists and studs at 16 inches when 19.2 or 24 inches would work
- Throwing out perfectly good LVL, LSL, glue lam, PSL or sawed odds and ends because you’re not sure if they’ll calc for a given span
Here’s the solution: Calc your own beams, headers, joists, post and studs. With today’s software it’s a snap. Really.
Certainly, it doesn’t make sense to calc every single stick on the job. But an hour or so spent with your plan set could easily save you over a grand. To me, that’s a no-brainer. The really smart contractor will even go so far as to bring his laptop to the job site for on-the-spot changes and quick calcs.
Now here’s the self-serving part. I happen to own a software company that sells just such software. It’s called ConstructionCalc: www.constructioncalc.com. I started the company in 1999 out of frustration with brands that were just too difficult to learn and too cumbersome to use. I can remember both hands knuckle-deep in my crew cut, thinking “Gaaa — I just want to size a simple beam, for Pete’s sake! This should be a 15 second quickie, not a 15 minute frustration.”
But what about span tables? The main problems with span tables are:
- They only work with “uniform distributed loads.” In other words, there can be no point loads (aka concentrated loads), nor can the loads vary over the length of the member. For example, say you’re designing a garage door header. You could use a span table as long as there wasn’t a girder truss or other intersecting beam bringing a point load, and as long as the roof and ceiling members being supported did not vary in length or support an HVAC unit or some other oddball load.
- Span tables can be confusing in terms of live and dead load applied. The answer you get from a span table has everything to do with your choosing the right table from the 20 or so in whatever publication you’re looking at. Pick the wrong table — the wrong live or dead load — and your design is wrong.
- Span tables generally only give you sizes for one type of material. So if you want to use hem fir #3, or an I-joist, for example, and you’ve got a Douglas Fir #2 table, you’re sunk.
- Deflection (a measure of bounciness and sag) is usually restricted to code maximums. Say you want a really stiff floor system. Don’t use a span table, especially if you’re dealing with spans over about 12 feet. Span tables generally ensure that drywall and other coverings don’t crack, but won’t ensure a really stiff system. Conversely, there may be times when you don’t care about deflection, say for a shed roof that won’t have a drywall ceiling. In that case you might want to relax deflection criteria, which could result in, say, a 2x8 rafter system as opposed to a 2x10 system. You’d save big bucks and still have a plenty strong roof that’s code-compliant for strength.
- You can’t use span tables for cantilever (overhanging) members. Span tables assume the member is supported at each end with no overhang.
In short, there are too many variables that go into the design of a structural member for a simple span table to really get the job done.
Well-designed software, on the other hand, will walk you though the process, step-by-step, ensuring that you don’t forget something, and that the numbers you input are correct. Most brands, however, don’t work that way. They make you jump and flop around from screen to screen so you’re never sure if you’ve done all you’re supposed to, or have even done the right thing to begin with. “Garbage in equals garbage out,” comes to mind.
Well-designed software will have the help you need for every input, right there on the same screen so you never have to open a manual or press F1.
Well-designed software will show you all possible alternatives at the same time: sawed lumber of any species or grade, glu-lam, LSL, LVL, PSL, I-joist, steel tubes and steel wide flanges.
Well-designed software will allow you to input a cantilever, then, and most importantly, check unbalanced live loads automatically. For example, say you’re designing a deck joist that has a 3-1/2 foot overhang (cantilever). We all know how decks work. When there is a party, everyone congregates at the railing to gawk at whatever spectacle is unfolding below. That’s a lot of unbalanced live load at the end of the cantilever. Some brands don’t automatically check that. You have to make several runs, placing live load in different places each time. What a hassle — if you even remember to do it.
Well-designed software will allow you to input the member’s pitch if not flat.
Of course, no software is worth a hoot unless the user understands it. And the best way to learn something is to work through solved examples. Whether you use ConstructionCalc or some other brand, you can go to the ConstructionCalc Web site and download any of 40 solved, illustrated examples for free.
So, to summarize, if you want to save serious money and build green, it’s time to crank up that computer and optimize your framing plans. The best part is, once you learn the software, you own it for life.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to email@example.com. Tim reads every one.
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The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.
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