On the truss drawings and your website it says your trusses are loaded “30-10-10”. What does that mean?
Short answer: It means 30 pounds per square foot of top chord live load, 10 pounds per square foot of top chord dead load, and 10 pounds per square foot of bottom chord dead load
Longer answer: Loading can be a little confusing if you don’t work with it every day. We use numbers like “30-10-10” as a kind of shorthand.
In general, loads on our trusses are either “live”, or “dead”.
Live loads are dynamic. That is, they come and go, fluctuate in size, show up unexpectedly, and may be unbalanced. I bet we could come up with a joke about someone’s in-laws to follow that sentence, if we tried hard enough.
Dead loads are static. In other words, they don’t move or change. They’re just always there, kind of like the 10 lbs you resolved to get rid of the first of this year. Dead loads include things like sheathing, shingles, drywall, flooring, etc.
Loading numbers are always noted as: TC Live – TC Dead – BC Live- BC Dead
“Wait!” I hear you saying, “There’s only three numbers up there in my question, and you just said it’s always noted as FOUR different numbers? Can’t you count?”
Here’s the deal: Trusses very seldom have a live load on the bottom chord. So we don’t bother to note anything for it UNLESS the truss DOES have a bottom chord live load.
What does this mean: Deflection L/360?
Deflection is simply how much a truss will bend or sag under load. The L/360 tells us how much the truss is allowed to deflect under load in our particular design. While it’s easier to ignore numbers that look like formulas, this is an important number to understand. And it can be changed to any number, as we’ll see.
Example: On floor trusses we design to L/360 Live Load Deflection.
Let’s look at what L/360 means. L/360 means that the deflection (i.e. bending or sagging) of the truss from the live load is limited to 1/360th of the overall length of the truss. To calc that out, a truss that is 240″ long (or 20′ 0″), under total live load, may deflect only 1/360th of 20′ 0″. In this case, that is is equal to 0.67″ (240″ / 360 = 0.66667″).
If the same truss were designed to L/480, it would be allowed to deflect only 1/480th of 20′ 0″. Under that design criteria, it could only deflect a half inch. (240″ / 480 = 0.5″) So we can see that designing to a higher deflection ratio makes a real world difference in how stiff our truss will be.
Here’s the thing to remember: Higher deflection ratio = stiffer floor.
As with anything, you should weigh the costs and benefits (please notice that clever pun) when considering increasing your deflection ratio. In our example, changing the deflection ratio from L/360 to L/480 changed the amount our 20′ 0″ truss can deflect by only 0.17″ That is a difference of just over 1/8″ of an inch. If that change increases the cost of the trusses by 10%, well, you have to make the call whether it’s worth it.
Just how much weight is our hypothetical floor truss carrying? We’ll assume spacing of 24″ O.C. for easy figuring. The truss carries half the distance to the next truss on either side, which equals 24″. 2′ of spacing X 20′ of span = a total carried area of 40 square feet. We can multiply that times the Live Load of 40 psf. 40 X 40 = 1600. Therefore, according to our design standard of L/360, our 20′ truss can carry 1600lbs of live load without that load causing it to deflect, or bend, more than 0.67″. You can do these same calculations to any truss span, and design to any deflection ratio you would like.
We have a 30 lb Live Load, and it also says the truss is engineered for a 30 lb Snow Load. So, it’s engineered for 60 lbs Live Load?
No. It isn’t. Here’s how it works. Yes, snow load is a “live” load. However, within building codes, there are deductions from the snow load that can be taken depending upon factors such as how exposed the roof is, what pitch the roof is (for example, snow doesn’t sit very long on a 12/12 pitch roof), etc. Fortunately the engineering software takes all that into account so that we don’t have to figure it out ourselves, because that gets really complicated.
While snow loads can be reduced depending on a variety of factors, the “Live Load” of the roof truss cannot be reduced. If it says 30 lbs, it will always be 30 lbs, with no reduction.
The engineering software does not “double up” the live load and the snow load. It engineers to the worst-case scenario, and uses that “worst case” load in designing the components of the truss.
Anyone still reading is smart enough to figure out then, that we don’t get to reduce our 30 lb snow load, ever. Because we also have the 30 lb live load. You might wonder why even build it into the code if it’s not something we can take advantage of. One reason it’s built into the code this way is that a lot of areas have a higher snow load than we do in central Nebraska. Depending on your area and elevation you may have a 40, 50, 70, or even 100 lb snow load. In that case, we’d still only design our truss with a 30lb live roof load, but make the snow load whatever amount your code requires. In THAT scenario we would be able to utilize any reductions in snow load due to roof factors, because the roof snow load would then become the “worst case scenario” for the engineering software.
If you’ve read all this, we congratulate you. You now know as much as much about truss loading as many truss designers. In other words, enough to be dangerous. We don’t know everything about truss engineering, but don’t let that stop you, just get in touch if you have more questions. We’ll either know the answer or know someone who does.