Live Load, Dead Load

VannL · November 01, 2016, 09:13:50 PM

As I am getting ready to get the foundation finished...finally...long story I'll save for another time and place, I am looking at the floor joists.

I am building a 20' X 40', based on the 1 1/2 story plans. My intention is to use 2X12 X 20' across the 20' span, 12" on center to achieve less deflection and provide a more solid situation. I have a full perimeter block wall for a crawl space and no center support.

As I am attempting to double check the viability for using 2X12 like this, I see in the IRC that they are asking the live load, dead load question. Guess what? I have not a clue as to what they are talking about. Can someone clue me in, and what would be a normal house live load and dead load?

My thanks as always.

MountainDon · November 01, 2016, 10:29:58 PM

Dead load, remains constant over time = the weight of the materials used to build the structure; joists, studs, sheathing, flooring, HVAC equipment, etc.

Live load, can change over time = the weight of the furniture, contents, water beds, people, etc. Big party = big live load, sometimes bad news for underdesigned porch, balcony or deck

November 02, 2016, 06:01:48 AM

For dead load... the self weight of the permanent materials, you can add up the weights of all the materials, or, the default is generally to use 10 pounds per square foot.

For live load the building code specifies the weights to be used. On the main floor and "public" rooms use 40 psf. Sleeping rooms can use 30 psf. Most engineers go ahead and use 40 psf throughout to avoid problems.

Add the LL + DL together to get the total load, generally 50 psf.

Wading into the deep end, bending strength and horizontal shear are checked for the total load. Deflection is correctly checked on just the live load.

Look into engineered I joists or open web trusses for that span as well, they will be flatter, stiffer and stronger than solid sawn joists. The trades will thank you for not going to 12"... I just helped sparky and the plumber getting drills into 12" bays and boring a bunch of holes, less than fun.

If you look at pictures of deck collapses I've never seen broken joists as being the cause of the collapse. Despite what the news reports say, those decks are never overloaded by the numbers above although they certainly overloaded the built assembly. The connections are almost always the culprit. We tend to check the members and forget to check the connections.

VannL · November 02, 2016, 11:48:45 AM

Thanks to the Dons for that quick and dirty on the DL/LL. That helps. I hear what you are saying about 12 vs 16 on center and will keep it in mind.

According to this post http://countryplans.com/smf/index.php?topic=14479.msg189384#msg189384 from ChugiakTinkerer, which I found after posting my question last night, my calculations are coming out to:
20X40, equalling a 19X39 for 741sf.
Live load (@ 40) = 29640 lbs
Dead load = 7410 lbs
Total load 37050 lbs
Divide that by the number of supporting joist (39 at 12" on center) = 950 lbs per joist.
Divide that by the supported length of the joists = 50 pounds per linear foot.

And that is the number I take to my lumber yard when shopping for LVL or other engineered lumber?

ChugiakTinkerer · November 02, 2016, 02:06:16 PM

VannL,

Wow, somebody reads my posts! I feel like John Candy in Splash.

There are a lot of ways to build the floor you want. Looking at the IRC span tables for floor joists, 2x12 in # DougFir can span up to 19'-1" when placed at 16" spacing. That table is for the allowable deflection of L/360. Allowable by code but not the stiffest floor by far. Reducing the joist spacing to 12" will add a lot of stiffness, but like Don_P mentions, it makes installing electrical, plumbing, and HVAC much more challenging. The 2x12 at 20' in #1 or select grade will be much more expensive than an engineered wood joist. Add in the fact that you can go deeper with the I-joist and more options open up for you. I suspect you would be happier with a stiffer floor, but cost and constructability are always a concern.

Were I looking to build a 20x40 clear span floor, I'd see what the cost and availability is of the 2x12 at 20', and calculate the cost of building that floor. I would also calculate the deflection in a beam calculator with engineering properties of the wood and grade that your lumber yard has. Assuming DougFir/Larch #1 or better at 12" spacing, I calculate a downward deflection of about 0.48 inches, which equates to L/480. This is a relatively stiff floor provided you don't intend to put in slate or marble tile. I would take this deflection value to your lumber yard I-joist specialist and ask them to design a floor that meets or exceeds the L/480 standard. Then compare costs, availability, etc.. Hopefully they can draw up an I-joist floor at 16" spacing that meets your needs and will be considerably less expensive than the sawn wood joists.

November 02, 2016, 02:30:05 PM

If L/480 is .48 inches, I would assume, probably incorrectly, that L/360 would be .36 inches, but your reply reads to me that my assumption is not correct.

I am scheduled to work on availability of engineered products next week. In the morning, my wife and I are headed to the mountains for a few days of R n R.

VannL · November 02, 2016, 02:31:01 PM

And yes, ChugiakTinkerer, your post was a great deal of information and very helpful. Thanks for sharing!

ChugiakTinkerer · November 02, 2016, 03:52:02 PM

Quote from: VannL on November 02, 2016, 02:30:05 PM
If L/480 is .48 inches, I would assume, probably incorrectly, that L/360 would be .36 inches, but your reply reads to me that my assumption is not correct.

I am scheduled to work on availability of engineered products next week. In the morning, my wife and I are headed to the mountains for a few days of R n R.

Have yourselves a wonderful time. The deflection calc is the span (L) divided by the reference number. So...

L/360 = 19'/360 = 228"/360 = 0.633"

L/480 = 19'/480 = 228"/480 = 0.475"

L/720 = 19'/720 = 228"/720 = 0.317"

The less deflection, the less bounciness.

VannL · November 02, 2016, 07:28:05 PM

Ok, so the numbers are inverse per the math. You are a wealth of info. Thanks a lot!

November 02, 2016, 08:44:03 PM

Quote from: VannL on November 02, 2016, 11:48:45 AM
from ChugiakTinkerer, which I found after posting my question last night, my calculations are coming out to:
20X40, equalling a 19X39 for 741sf.
Live load (@ 40) = 29640 lbs
Dead load = 7410 lbs
Total load 37050 lbs
Divide that by the number of supporting joist (39 at 12" on center) = 950 lbs per joist.
Divide that by the supported length of the joists = 50 pounds per linear foot.

And that is the number I take to my lumber yard when shopping for LVL or other engineered lumber?

That is correct, there is another way to think through it.
At 12" on center spacing the joist is supporting 6" of floor width on each side of the joist... the tributary width of floor supported by the joist is 12". At 12" oc spacing every linear foot along the beam is thus 1 square foot, Pounds per Lineal Foot on the joist = Pounds per Square Foot design load.

At 16" spacing... 16/12=1.33... 50 PSF x 1.33 = 66.66 PLF. We have a 1.33' trib width @16"o.c.
66.66 PLF x 19' LF span= 1267 lbs/joist @ 16" oc and 19' span.

Or,
The tributary Area, which is the trib width of 1.33' X the tributary length 19' = a tributary area of 25.27' delivering load to the joist at 50 lbs per square foot. 50psf x 25.27sf=1264, within rounding error

Anyway, hopefully more ways to make it click.

I guess that was more for education when it gets to engineered products. I simply take the plans to the supplier and let them do the design. I'd have them look at doing the roof as well. I'd be happy to explain the printout they provide you with, but, I've learned a lot asking those guys questions and listening.