Live Load & Dead Load, still don’t fully understand
Hi guys,
I still don’t fully understand the whole “Live load”, “dead Load” thing although I do sort of get the basics. So I’m coming from the product design world, just recently starting to dabble in architectural design. I’m designing a 20×40′ small home, two stories, 10/12 roof, will be in snow country. I’m trying to spec the appropriate I-joists, so I see a local company makes a variety of I-joists for different spans, their NI-80 looks like it would do the trick, it allows a 20′ span, 16″ spacing, 5/8″ plywood, shows LL=40, DL=15…
Now, what I’m trying to figure out is, do those LL, DL numbers that are listed by the manufacture mean that’s the load of the actual product, let’s say in this case, if it’s the main floor, of the joists on the sill/foundation….. OR do those numbers mean it’s what it can SUPPORT, ie the walls, 2nd floor joists, 2nd floor walls and roof?? and in that case, how would I do the calculations to see what the actual load I’m getting onto those lower joists to see if I’m within spec.
So I’m trying to figure out if the I-joists I’m choosing are ok for this two story house design, I want to get this right before I move forward with the design. Any help with this is appreciated.
The jist of the design is 20×40, half two story, half cathedral ceiling. So in essence half of it is lofted.
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Ts,
" how would I do the calculations to see what the actual load I’m getting onto those lower joists to see if I’m within spec."
The short answer is you don't. Unlike dimensional lumber which can often be sized using either tables provided in many building codes, or calculated by the loads imposed, I-joists need to be engineered by the supplier to conform to your design. Take it in to your lumberyard. The only choice you will have to make is whether y0u want stiffer then code minimum deflection.
That said, in your situation the main floor joists don't pick up any of the other loads you cite. The walls, second floor, and roof all bear on the exterior walls. You may need to provide blocking in addition to the rim-joists, but that is independent of the size of the I-joists, and will be detailed in the installation instructions provided by the manufacturer. The I-joists can be sized entirely based on their clear span.
Edit: To just enlarge on what I said about deflection. Dimensional lumber joists or rafters were typically specified so that they had a 1:360 allowable deflection. That meant with a typical span of say 10 feet, they would only sag 1/3". With the long clear spans of I-joists that ratio of deflection is often inadequate. It's worth moving up to a deeper joist to stiffen the floor.
Yeah thanks.
I had these options selected... they're quote at L/480 for LL and L/240 total Load. That's about 1" deflection on a 20' floor span under total load?? lol, seems like a lot.
https://www.nordic.ca/data/files/datasheet/file/NS-NT306-CA-en_Maximum_Floor_Spans_2020-09-24.pdf
S6.1 LL = 40, DL = 15, 5/8” plywood
14” joist
12” spacing = 20’2” = NI-60
16” spacing = 20’0’ = NI-80
S7.1 LL = 40, DL = 15, 3/4” plywood
11’7/8” joist
12” spacing = 20’-11” = NI-80
14” joist
16” spacing = 20’1’ = NI-60
But I'm trying to understand what the LL and DL numbers mean, is this something pre-calculated so that as long as you pick the right I-joist for the span you're looking for, it will automatically be ok for the load of a 2 story house? is that how it works....?
I notice if you move down the chart, they have tables for larger Dead loads, it goes 15, 20, 35...
just trying to get a good grasp on what these numbers mean so I can just plug and play while I design this.
Edit: I'm reading some of the documention on the Nordic website and there is a lot of information on construction details using their product, hopefully I can get some answers there. BTW, seems like for this design, blocking + rims board would probably be the best option.
Dead load is everything attached to the building: framing, sheathing, drywall, insulation, etc.. It is static, meaning it doesn't change over time. Live loads change over time--primarily occupants, but also furniture, snow, wind, etc.. When there is a load from above, it will be partly live load and partly dead load. For the floors in most homes, a 40 lb live load and a 10 lb dead load per square foot will cover most situations, as long as there is not additional loading from above. Bedrooms can be reduced to a 30 lb live load but I usually stick with 40 lbs--nobody likes bouncy floors, even if they are structurally sound.
Deflection for floors is traditionally L/360, but as you note, for longer spans that can get extreme. I spec L/480 for any floor spans over 12', and sometimes for shorter spans if extra stiffness is desired. A tip I've learned from licensed engineers is to also limit total deflection to 0.5" or 0.6" regardless of the length of the span.
More info here: https://codes.iccsafe.org/content/IRC2015/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR301.4.
I have four years of engineering school and 25 years of building and designing houses, and I am still careful about my calculations. I would not recommend trying to engineer your own framing on your first project. Mistakes can be catastrophic.
Hi Michael,
Ok, so that explanation of LL and DL is great, that's sort of what I figured from reading around. The 40/10 is like an average estimate for most cases. But I guess one of my questions would be, shouldn't the ground floor get a higher load rating as it could potentially experience more load due to a 2nd, 3rd floor?
"A tip I've learned from licensed engineers is to also limit total deflection to 0.5" or 0.6" regardless of the length of the span."
That's good to know thanks. But if you noticed the chart to I-joists linked above gives a L/480 and L/240 total load rating, which means pretty heavy deflection under total load even when using the right speced joist, but I guess those are just industry standard #'s so I'll just trust them and go with it..
Seems like people are suggesting to find the right joist for intended span and then bump up one level on depth. So if the NI-80 14" will work for a 20' span, then I might just bump to a NI-80 16", deflection concerns solved :)
Definitely not trying to do these calcs myself but the whole idea here is that (since I don't have direct access to an engineer for calls/feedback throughout the design process), I want to atleast have a "rough" idea of how to build this in CAD, selecting all the proper components, so I have no major surprises when it comes time to have it approved. I'm realizing it looks like this is sort of a product specific thing, the documentation on the joist companies website (Nordic) has a lot of good information on how to use their products in design. I need to dive into it more.
TS,
"shouldn't the ground floor get a higher load rating as it could potentially experience more load due to a 2nd, 3rd floor?"
If you follow the load paths for the roof, and floors above they don't affect the first floor. There could be situations where they did, and that would have to be accounted for, but if the roof and the floors are clear-spanning from exterior to exterior, the only loads that get transmitted to the floors below are through the exterior walls. So they don't affect bending or deflection, you only need to worry about the ends of the I-joists getting crushed - and that's where the blocking comes in.
Increasing the depth makes a huge difference in horizontal structural members like beams and joists. Doubling the depth of a joist means it can carry four times as much load, and will deflect 1/8 it's original amount.
Yep, that makes complete sense in this case... thanks.
Now, just to throw a wrench in the works though, what if you had a load bearing wall/column etc. on the first floor, that helped support beam/joists on the second floor. Then I'm assuming this have to be factored into the equations. That would be a point load on the main floor joists? so would you consider that as part of the dead load? I'm assuming this is where things get complicated and maybe too theoretical for this convo....
Dead load is the weight of the structure itself, live load is the weight of whatever that structure is holding up (furniture, people, etc — all the stuff you put in the building yourself). That’s pretty much all there is to it.
Every floor gets the same load rating because that rating is for how much stuff per square foot you might put on the floor. Basically floor live loads are only for the floor in question, not anywhere else. Stuff you put on the second floor doesn’t put any load on the first floor, because the second floor is already supporting any stuff you put up there.
Vertical loads from upper levels transfer down to the foundation through a combination of load bearing walls, where the studs carry the load, and vertical columns. Architects generally try to minimize the number of vertical columns in a building because rule #1 for columns is that they’re ALWAYS in the wrong place. Always. Furniture layout is always an issue when you have to work around columns. This is why “clear span” is a big deal in large commercial buildings — it means there are big “clear” spans with NO columns to get in your way.
The basics are like this:
Dead load is the weight of the building.
Live load is the weight of the stuff you put in the building after it’s built.
All of that load transfers down through vertical structural members to the foundation that ultimately supports everything — the sum of ALL live AND dead loads. Each vertical piece of structure supports its own weight plus some fraction of all the loads (live and dead) above it. That “some fraction” part is what engineering determines.
Horizontal structural members generally only support the dead load of their immediate area (their own weight plus anything built directly on them such as subfloor), and the live load in their immediate area. Horizontal structural members only support loads on other levels if a column ties into them somewhere other than where another column is directly underneath, which is a situation that designers usually try to avoid.
Bill
Bill,
Seriously, VERY good explanation, really appreciate that. I've searched all over the internet and it wasn't quite clear on what affects what, but I totally get the jist of it now. So in essence, my design is fairly simple then, I only really need to worry about choosing a pretty thick joist and probably adding some joist blocking at the end bearings to help with the deadload coming down from the verticle walls/roof... cool...I'll post up my progress as I go. Really appreciate all the great feedback here. So helpful.
Forces are vectors, arrows, basically. A force coming down through a wall should be able to "point" straight down to the foundation. That means the joist isn't really loaded as a "joist" where it's between an upper floor and lower floor stud wall. The end of the joist between the studs just acts in compression here, so the bending moments that cause floor deflection don't apply. You do want to line things up (joist sits right over a stud, not between two studs, and the stud on the upper floor sits right over the joist). If you're worried about the joist cupping, blocking will help.
Things get complicated if you have loads coming down into a header. Imagine the header, a horizontal piece, with a force vector (arrow) coming into the middle. That BIG arrow (force) splits to two smaller forces going down through the studs on either end of the header. You can run into concentrated loads with things like this, and deflection in the header. If you cantilever out past the support under a joist, things also get more complex.
I always point out that you do NOT want to do anything with a structure without knowing what you're doing. Structural failures can kill people. If you have ANY doubts here, contact an architect or engineer for help. Internet forums are no substitute for proper engineering.
Bill
" If you have ANY doubts here, contact an architect or engineer for help. Internet forums are no substitute for proper engineering."
Ofcourse, the whole idea here though is to be able to CAD a rough model to then present to an architect or engineer, just so it's clear to everyone helping here, I WILL NOT BE BUILDING THIS WITHOUT ALL APPROPRIRATE certs, so no one worry about that. And as it sits, this is just a personal CAD project to gain some exposure to architectural design. BUT I can't move forward with the design without knowing the fundamentals first, which forums are a great help for this.
So I think my next question would be, in general, what is common practice for a two story building that will experience a snow load? Again, just general advice here so I can atleast start modeling some components.
1. some extra blocking at the joist ends
2. actually using an I-joist+rim board
3. Double rim board
TS,
I joists always have a micro-lam rim joist. You also need to put squash-blocks under any point loads. Squash Blocks are 2"x4's or 2"x6" cut to the same length as the I-joists height, and sistered on the sides. What is required is spelled out in the manufacturers installation instructions - which are usually available on-line.
Again - unlike dimensional lumber, I-joists are part of an engineered system. You don't think up the details, you just follow the ones given to you.
OK, sounds good. I've seen many projects where someone tried to DIY stuff they didn't really understand and got themselves in trouble. At work, I work with large critical facility power systems (and occasionally industrial stuff too), and I've seen some MAJOR damage when people did stuff they shouldn't there. Plugging in bus plugs with bad prongs has caused serious injuries to some electricians too. I always try to be safe with things, and try to encourage others to do the same. Too many are tempted to cut corners to save a few bucks. I've walked away from projects like that before.
I think what you'll find useful to get you started here is to get yourself a copy of the IRC. You can get an online version here: https://codes.iccsafe.org/content/IRC2018
There are lots of drawings and details in there to help you understand how the pieces need to go together. The drawings especially are good when you're getting started since it's hard to explain all that stuff with words.
You'll also find the tables at southerpine.com for joists useful. There is another lumber industry association website that has useful info too that I can't think of at the moment.
Even though they like to stuff our engineer brains full of fancy math in school, most actual design work is done from tables. Many/most of those tables are in the code books. From those tables, you can size a LOT of your structural elements. You can also see the general rules for how things like cantilevers have to be built, snow load for different areas, stuff like that.
What usually ends up happening as you gain experience with this stuff is that you'll find you want to do more interesting design work, then you run into the exceptions to the rules, obscure rules, and all the footnotes to the regular rules. That's when thing get interesting (and where we consulting engineers make our money :-). Usually these fancy things get involved when you're trying to make an "interesting" structure, push something to the limits (really big open indoor spaces, etc.), or do something differently for cost savings without compromising your design somewhere else. Those are the questions that get big discussions going with industry people (including here), and you can get a lot of great ideas.
Go through the code design info to get an idea how things go together. You'll get the basics pretty quick from the example drawings. The tables will help you with sizing a lot of the structural elements.
Bill
"I think what you'll find useful to get you started here is to get yourself a copy of the IRC. You can get an online version here: https://codes.iccsafe.org/content/IRC2018"
Yeah I stumbled onto those a few days ago, ALOT of useful info for sure. Southernpine looks good also. Thanks!
I'm going to keep researching a bit and I'll start modeling things out and throw my progress up here now and then for some feedback. Super helpful feedback so far. Learning so much. Much appreciated.
If you are planning to buy Nordic ijoist, I suppose you are in Canada. If your construction is pretty basic, ie no point load and no crazy free span, you can figure out pretty much everything from their tables and the building code. Of course, if you have any doubt always check with an engineer.
You already got plenty of good answers when it comes to the floor loads.
As for the roof, you calculate the dead load the same way as the floors. The live load is a combination of the rain and snow weight. There is an equation for small structures in the Canadian building code (section 9) to calculate that load :
S = Cb * Ss + Sr
Where S is the specified snow load, Cb is a factor based on the width of your roof, Ss is the 1-in-50-year ground snow load and Sr = associated 1-in-50-year rain load. You can find the values of Ss and Sr for the region where you are building in tables in the code appendices. So check out your code.
For sure, Like I mentioned above, Nordic has some serious documentation to go along with the products, how they can be used, different scenarios etc. with drawings etc. I've been spending some time combing through them, really helpful.
Looking into getting a copy of the Canadian building code, NRC store is down right now lol.
That calc makes sense, just figuring out the total load of worst case rain and snow.
I want to give a plug for two pieces of online software:
Weyerhaeuser's ForteWEB: https://www.forteweb.com
Boise Cascade's BCCalc: https://www.bccalc.com
Each of these is a tool to size various framing components (both commodity dimensional lumber and engineered products) to a given set of loads and constraints. The sizing takes into account strength, deflection, and qualitative factors like how 'good' a floor system feels.
Once you get the hang of them, they are excellent learning tools. They also generate 'official' documentation of a member's compliance that is useful for handing to your licensed design professional to help back up your desired design. Of course, garbage in, garbage out, so you have to understand how to use them appropriately.
If you really want to get an intuitive sense of what happens to things under load, I'm sure there are YouTube and other videos out there about how you draw shear and moment diagrams and other statics basics. Just depends on how much you want to learn!
Thanks, BCCalc looks pretty good. Gonna play around with it.
I'm using Solidworks for modeling, so I can always run simulations and FEA analysis with that also.