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WILL1E
Moderator
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# Posted: 19 Jan 2021 09:11am
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I've been pouring over our (Wisconsin) building code documents as i enter the design phase of our cabin. I thought I'd start a post where people could post a question about a term or something they need clarification on.
I'll start!
House Width. When looking at span tables, lumber requirements, etc. they always refer to the House Width. Is the width always the longest dimension of the footprint of the structure? Or is there something else that determines it?
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gcrank1
Member
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# Posted: 19 Jan 2021 07:37pm
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Ive always taken 'Length' as the long side and 'width' as the other.
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Fanman
Member
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# Posted: 19 Jan 2021 09:17pm
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When looking at span tables, it's the width of the span parallel to the rafters and/or joists, from wall to wall.
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Brettny
Member
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# Posted: 20 Jan 2021 06:01am
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I believe that would be the span at the inside of the span. Not total beam length.
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ICC
Member
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# Posted: 20 Jan 2021 08:44am
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Quoting: WILL1E House Width. When looking at span tables, lumber requirements, etc. they always refer to the House Width. Is the width always the longest dimension of the footprint of the structure? Or is there something else that determines it?
Given a 20 x 30 ft structure
Width = 20 feet
Length = 30 feet
On questions regarding code it is always very helpful to reference or paste in the section as sometimes the wording can be confusing.
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WILL1E
Moderator
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# Posted: 20 Jan 2021 09:16am
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ICC Ok, so here's my example.
Say i'm doing a 16x20 cabin with an elevated platform that's resting on beams. So, my joist would run in the 16' direction and my built up beams would run in the 20' direction.
So if we say 16' dimension is the width and i'm using 1200psi lumber, then I need 3 2x8 beams with columns spaced a max of 9'4" apart right?
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Aklogcabin
Member
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# Posted: 20 Jan 2021 10:09am
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I believe that , length x width x height. No matter the number sequential. Is the standard. When looking at front of cabin. You can see how wide and high it is . From side length and height.
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WILL1E
Moderator
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# Posted: 20 Jan 2021 10:18am
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But what defines the "front" of the cabin?!
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Aklogcabin
Member
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# Posted: 20 Jan 2021 10:53am
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Whatever you want . I will say that when I was in college it was lxWxH.
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gcrank1
Member
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# Posted: 20 Jan 2021 10:57am
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I think that in your example the important part is the 'span'; ie, the unsupported 'length of the space' regardless of it being in the length or width of the structure.
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Brettny
Member
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# Posted: 20 Jan 2021 11:00am
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If you set up to 3 2x10s you would be well in spec and not have to worry about this.
For that link the width for you would be the 20ft side. Width should be the length of beam.
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WILL1E
Moderator
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# Posted: 20 Jan 2021 11:29am
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Quoting: gcrank1 regardless of it being in the length or width of the structure
Quoting: Brettny For that link the width for you would be the 20ft side. Width should be the length of beam.
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gcrank1
Member
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# Posted: 20 Jan 2021 12:50pm - Edited by: gcrank1
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They do that so you hire a contractor (lol)
I think that typically the span length is done on the 'width' side to reduce the length of joist required. If that is not possible there is a tipping point where it becomes necessary and/or more cost effective to put in a middle support beam.
My current prev owner built 16x24 has a pair of center posts with attached beams to provide a middle support for the 16' joists (think like under a typical outside deck). My posts continue up into the room to the roof ridge board (no truss raftering). They could have stopped under the floor deck just as well if all he had wanted to do was provide that middle joist bridge.
That effectively reduced the span to 8' per side rather than 16'. Upgrading to calling it a 10' span each side would have merit in choosing a joist size.
Btw, you dont just butt two 8' joists together at the center support to make a 16', you should have (probably a spec for it too) maybe 2' longer, so two 10' with a 2' overlap to nail together.
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WILL1E
Moderator
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# Posted: 20 Jan 2021 01:40pm
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Quoting: gcrank1 you should have (probably a spec for it too) maybe 2' longer,
d. Wood floor joists with ends that intersect over a beam shall
have the ends overlap at least 3 inches and be securely fastened
together with at least two 12d common nails or the ends shall be
butt−jointed or face−jointed and fastened with ties, straps, plates
or solid blocking.
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gcrank1
Member
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# Posted: 20 Jan 2021 02:33pm
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Well, there ya go 
Just buy a pair for each 16' span of 8'3" long, or a pair of 10' and cut 9" off of each 
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ICC
Member
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# Posted: 20 Jan 2021 05:27pm
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It is bad practice to overlap a large amount more than what is necessary. IF there was say 3 feet of overlap over a central beam, and IF those joists were slightly flexy, there is a possibility that when the joists are loaded as when some people walk across the floor, the overlapping end will be pushing up while the center of the joist is deflecting down. That MAY result in subfloor nails being pushed up and loosened in the area of the overlap. Not good. So, the unneeded overlap is not only perhaps wasteful but could create problems. Best to avoid.
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ICC
Member
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# Posted: 20 Jan 2021 05:33pm
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Quoting: gcrank1 My posts continue up into the room to the roof ridge board (no truss raftering). They could have stopped under the floor deck just as well if all he had wanted to do was provide that middle joist bridge.
I interpret that to mean that the central beam consists of 2x material nailed together and nailed to the sides of those posts. Is that correct? Or is the beam on top of the supports that extend down to the ground and then other posts sit on top of it and continue up to the roof ridge?
The first way is not good as the load is being carried by nails. Or bolts; still not a good method.
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gcrank1
Member
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# Posted: 20 Jan 2021 07:29pm - Edited by: gcrank1
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Considering the 'quality' of the rest of the visible 'workmanship' I can only assume the first.....
There is a lot that is not done the way it should be, and this forum is a great resource to sort this stuff out.
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Brettny
Member
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# Posted: 21 Jan 2021 05:52am
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Will your building a 16x20 with the runners going the 20ft direction. The last table you posted is for runner size and how far apart your columns should be...maybe I'm not understanding the confusion.
I wouldnt use anything but 20' lumber for those runners. Its available at lowes and HD and the cost per foot over 16' is pretty minimal. You dont want these beams to all "break" on the same post and you dont want to add a 4th set of posts. Use 20' lumber.
Any small lumber yard or even small sawmill may have 20' lumber too. It's not as common at 16' but available.
"Break" is when you say use 10' lumber end to end to span 20'.
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WILL1E
Moderator
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# Posted: 21 Jan 2021 09:15am
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Brettny Sorry, this post has gotten confusing. In the WI building code, they toss around width loosely and sometimes in ways that make it sound opposite of how it was used someplace else. However, nowhere do they provide a true definition of width.
When they say building width for joists and rafters, my assumption is width is the dimension in which the joist are running.
For building width in regards to beams, my assumption is it's actually the length or depth of a building, aka perpendicular to the floor joist.
My goal is to use single piece lumber and not break it over columns. But, if i need to do another set of posts to stagger the joints it won't be the end of the world.
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ICC
Member
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# Posted: 21 Jan 2021 09:17am - Edited by: ICC
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Quoting: WILL1E So if we say 16' dimension is the width and i'm using 1200psi lumber, then I need 3 2x8 beams with columns spaced a max of 9'4" apart right? Beams.jpg
When a table like this states "supporting one floor only" that probably means supporting only the floor directly above, not any exterior walls or a roof. That would be like a beam that runs centrally down the length of a house to support the middle of the floor above, not a beam situated out at the perimeter exterior wall where there is the weight of the floor and the wall above plus the loads from the roof assembly. The spans are too long for the beam sizes plus there is no adjustment for snow loads. In the IRC girder tables for exterior walls make note of the roof, number of floors, etc as well as having sections for different snow loads.
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ICC
Member
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# Posted: 21 Jan 2021 09:20am - Edited by: ICC
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Look at the table in this IRC codebook. Scroll down to table R602.7(1)
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WILL1E
Moderator
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# Posted: 21 Jan 2021 12:22pm
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Ok, so i'm looking at that table.
Our snow load requirement here is 40, so i'm looking under the 50psf.
If i do a single story cabin with a loft, i believe that puts me under "roof, ceiling and one clear span floor".
So if i do a 16x24 cabin, my building "width" would be 24 as we are talking about the beams. Right?
If i use 3-2x10's, i have to put a pier every 5'11"...right?
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Brettny
Member
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# Posted: 21 Jan 2021 02:06pm
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That chart is pretty difficult to see on my phone but I'm going to save this to view on my computer.
If your spacing does need to be 5'11 I would look at useing 3 2x12. Taking away just one set of posts and adding some thickness in lumber could save you some money and it's far easier to do than adding more posts.
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ICC
Member
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# Posted: 21 Jan 2021 02:56pm - Edited by: ICC
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Quoting: WILL1E So if i do a 16x24 cabin, my building "width" would be 24 as we are talking about the beams. Right?
No. A 16 x 24 is always considered to be 16 feet wide and 24 feet long. When building width is mentioned, no matter if we are talking about joists, rafters or beams, we are meaning the 16-foot dimension.
You are correct that we are sizing a beam that runs in the long 24-foot direction. The reason the building width enters into the conversation is that a wider building has more roof and therefore more weight. All the roof load, the materials, lots of rain, wind, snow is transferred down the two long exterior walls and to the support girders/beams. In the example table it can be seen that when keeping the girder size the same the support posts have to be closer together as the building width increases from 12 ft to 24 ft to 36 ft.
A loft also increases the load a little as the floor and even simple bedding will weigh more than a simple no use attic. So shift a bit towards larger members or slightly closer pier spacing.
There are footnotes to the tables in the IRC. Read those whenver using one. They can be important.
There may be a mention someplace in the chapter that when a building width falls in between the tabulated data it if okay to extrapolate the sizes. So with the table using 12 feet and 24 feet widths and the building being designed is 16 feet wide we can figure what would work for us without so much guesswork.
16 is 1/3 of the ways from 12 to 24. Follow that? So the pier spacing could be shortened a bout 1/3 from the 12 foot value. Or the girder size could be increased keeping the spacing about the same. I'll have a look a little later and see how I might approach the problem. Meanwhile you see if you can wrap your thinking around what needs to be determined.
I don't like digging holes for piers as they really need to have big bottoms in order to have a large enough concrete footing. So I tend to try to go up to using 2x10's or 2x12's to build a layered girder, but I also like to have at least three layers as that reduces the ease that a girder can rotate or tip sideways.
Also keep in mind that if you are using PT piers they should be at least 6x6, not 4x4's. And just a FYI, the girders are built in place on top of the piers once they are installed and leveled. No building on the ground and trying to lift.
I'll probably consider using 4-2x10 when I have a look at the numbers.
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toyota_mdt_tech
Member
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# Posted: 21 Jan 2021 03:24pm
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ICC, I always enjoy reading your post, especially anything on code. Very insightful. 
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ICC
Member
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# Posted: 21 Jan 2021 09:35pm
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If we look at the "roof, ceiling and one clear span floor" part of Table R602.7(1) here are my thoughts on what I would do...
As stated I like tall and thick girders because that can reduce the number of piers. So looking at the row for a 4-2x10 built up we go over to the 50 PSF snow column and find that for a 12 foot wide building, piers can be spaced up to 8-10 (8'10") apart. Jumping over to the column for the same snow load but a 24-foot width the spacing can be 6-10.
Two feet closer spacing for the 24-foot width instead of 12-foot width. Two feet closer spacing for a 12-foot difference on overall width. Depending on one's aptitude for math this can be looked at in different ways.
In my head, I "see" that if we look at that 12 foot and 24-foot width, a 16-foot width is 1/3 of the difference between 12 and 24, longer than 12 feet. So we can easily extrapolate that the pier spacing should be 1/3 less of the difference bewteen the 6-10 and 8-10 spacing. One-third of 2 feet (24 inches) is 8 inches. Therefore we can space the piers at 8-10 minus 8" = 8 ft 2 inches for a 16 foot wide building.
There are other methods of logic to solve the problem. I can think of at least one other thought process that provides the same result. And it makes sense; building gets wider which means more roof load, which means closer spacing for the piers if the girder size remains the same.
Does that make sense?
Looking at other girder size options one could alternatively select a 3-2x12 built-up size. There is also a 2 foot difference in the pier spacing and only different by two inches; no meaningful difference.
One thing to consider though when striving for fewer holes is the question of what the soil is capable of bearing. If the soil was very sandy it can not support as high a PSI so in a case like that more posts would be better. More posts, more area of footer support would equal less PSI loading.
All code books are just rule books. They are not a how-to-do guide. They were never meant to be a how-to book. Similarly the basketball rule book is just that. It tells you how points are made, what moves can and cannot be made. But it does not actually tell you how to play a winnings game.
It should also be kept in mind that no codebook, neither IRC nor WI has any prescriptive rules about pier and beam foundations. Pier and beam foundations require an engineer to make calculations and design something to suit the local conditions. A full perimeter concrete footing is a prescribed method in the codebooks.
If there are requirements for a building permit and an inspection the pier and beam may be refused. That comes down to local enforcement. Some areas are more by-the-book than others.
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WILL1E
Moderator
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# Posted: 22 Jan 2021 10:01am
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ICC All makes sense now!! Thank you for elaborating!
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