Span Length-to-Cost Threshold
andyfrog
| Posted in General Questions on
What are some common length thresholds at which open spans become exponentially more expensive?
I know for example, a 20′ span is much easier than a 40′ span, but for just a general mental framework when drawing floorplans, I was wondering what the length to cost thresholds looked like.
I’m guessing it gets dramatically more expensive past 24′ or so, but how does 28′ compare to 32′, for example?
This is definitely not a specific question; just trying to get a feel for things to avoid wildly unrealistic floorplans. I know any specific application calls for an engineer.
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I'm very interested in this question as well. I'd add also "constructibility" issues, such as whether there is some length that becomes problematic to transport to the site through city streets.
Ah yes, other logistical constraints are good considerations too.
If you want an idea of costs, take a look at a TJI span table and from there you can easily calculate the material cost.
Anything over 22-24'', you'll probably have to switch to floor trusses or add a beam and a couple of columns. Floor trusses are usually a bit more expensive (depends where you live). Longer trusses are heavy and may require a crane.
Empirically it seems that professionally designed plans rarely go beyond about 20'. In DC many houses are 34' wide, 50' is a standard lot size with 8' setbacks on each side. It would be very unusually to do that as a single span, most common would be a beam or bearing wall splitting it in half.
Longer spans are usually accomplished by using steel beams to break it into several shorter spans.
If you want a quick rule of thumb, I'd look at the span tables in the code book. When you get past what you can safely span with a 2x12, expect a cost jump at that footage, which will be around 15 to 17 feet or so depending on lumber grade. Your next big jump is probably going to be up around 20-25 feet or so when you have to switch from "standard" engineered lumber products (TJIs) to "fancy" engineering products (floor trusses, etc.), or steel.
My guess is the difference between 28 feet and 32 feet won't be too bad, but keep in mind that those price "jumps" are going to whenever you have to go from one type of structural product to the next up product. For long clear spans, you really want to involve an engineer too -- there are ways to split things up so that you can minimize how many fancy structural elements you need. It may well be cheaper to run one big steel beam and hang a bunch of 2x12s off of it than it would be to use a bunch of long floor trusses, for example. The best mix of structural elements to arrive at the least cost floor design for a project is an engineering decision.
Bill
Floor trusses can clear span quite a distance, if I remember correctly the longest span I have built is 32'. The general rule of thumb is the span less 8 for the floor truss height. A 32' span would require a 24" deep floor truss.