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Designing w/ long joists and timbers

Started by djkarl, January 03, 2010, 11:08:53 AM

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djkarl

Hi,

Are there unforseen problems with using very long (e.g. 19') joists?

I only ask because Steve Chappell's book has several warnings against designing with timbers over 16'.

I don't see why I can't just calculate the proper joists length for a 19' span.  Several species of 6x12 have a deflection of  L/480 (i.e. 0.43") at 19'... and that's with joists at 4' oc.  As required, I am having an engineer sign off on everything.

It doesn't seem too difficult to find 6x12 timbers.  I recognize they will weigh a lot and this is not the most efficient use of timber.

I've designed a hybrid which has a width of 40'.  I want to use 20' joists (about 19' span).  The house is a 1.5 story with a shed dormer.  Strucutrally, it can be thought of as having 3 beams right over top of each other: structural ridge beam on top, a large timber holding up the 2nd story joists and 8' below that is an I-beam holding up the 1st floor.  I only want one beam in the basement because I want to minimize posts in my shop; that is how I end up wanting 20' joists.

Thanks,

Don





beenthere

Might be the problems are "foreseen", not unforseen.  :)

One reaches a point that timbers can't support themselves, let alone any additional load.

Now they may not fail (fracture), but they will bend from being elastic, and not be stiff enough for joist material.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

witterbound

If you want the joists to be 6x12, what size will the plates and beams need to be that hold them up?  I'm used 3x6 rafters, and my plates and beams are 8x12's.  So, would you need 10x12, 12x14, or even larger plates/beams?   

djkarl

Good question about beam holding them up.

The center beam holding up the joists has a variety of spans as it goes along the house.  Good news is most of them are short (i.e. less than 9').

Another very nice thing about this design (steel beam in basement and wood beam above holding 2nd floor deck) is that getting tons of strength with steel is easy.  So, I don't truly have to line up all my posts.  If necessary, the wood posts can create point loads on the steel beam.  This makes it easy for me to keep most wood beam spans less than 9'.

I do have one span longer than 9', but it mostly holds up a small loft and small storage area, so the load is very light.

Another variable is the type of joint I ultimately choose to attach the joists to the beam.  The joints remove different amounts of wood in different places on the beam and therefore affect beam sizing.  I'm not sure if I'll ultimately end up with tusk tenons with diminished haunches or tusk splines.  My conservative approach of not counting any section of a beam with joinery in it makes my beams a bit oversized.  I think I'm to the point where I'll let my engineer weigh his opinion in on all this.  Since he has to sign off on the whole thing, I don't want to try to force my opinion on him.  BTW, my brain says tusk splines are the way to go, but I don't like the way they look as much.

Bottom line is I don't have an answer for beam sizing, but it won't be a problem with spans less than 9'.  I may go with an 8x12 just for aesthetics and ease of putting up partition walls.  I think when details are worked out, 8x12 will end up being overkill.

Don


witterbound

Yep, when I designed my plans I sent them to an engineer in NC.  He looked them over and made me change some things.  Sounds to me like you're at that point.  Good luck. 

Jim_Rogers

Quote from: djkarl on January 03, 2010, 11:08:53 AM
Are there unforeseen problems with using very long (e.g. 19') joists?

With joists this length the second floor may be too bouncy, you engineer will advise you about this.

QuoteSeveral species of 6x12 have a deflection of  L/480 (i.e. 0.43") at 19'... and that's with joists at 4' oc.

0.43" is around 7/16" deflection in the floor. It maybe noticeable on both the second floor and first floor. One the second floor things/furniture may lean. On the first floor the joists will look like they are sagging.

In the book "What not to build" they talk a lot about what a house "feels" like. And that the subconscious can feel a house and make you uncomfortable because of the way it feels.

Such things as posts that "look" too small to hold up a  beam. Beams that "look" too small to hold up a floor above. When your subconscious sees these things it gets scared about the structure that it is too weak and may collapse while you're in it. This is what makes a house "feel" unsafe.

Here is a classic example, while you're driving around look at front porches on houses and look for the one that is held up by thin posts. Something like a 4x4. Then compare it to one that is held up by 6x6's or even 8x8's; you'll soon see what I mean.

A porch being held up by a 4x4 post "looks" too weak. And when you enter this porch you may feel uncomfortable.

When designing a floor system you first start with the load and see what size timber will be needed to support that load. If you want a certain spacing then you have to make the timber deeper to hold up that load or change to a stronger type of wood to maintain the size you can easily get.

Then you check the timber hold up the floor system to make sure it is large enough to do that. Again you try to make it of the type of wood you want to use, and sizes that are readily available.

The entire frame and support system has to be carefully reviewed to make sure that it is safe and made out of timbers you can locally obtain. If you can't get them locally then you'll pay a premium to have them shipped in.

Good luck with your project.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Mad Professor

Barns have been done with at least a 40' free span span width , with a hay mall above.  I don't see why a 20' span between bents is not possible with proper support.

How about shouldered M/T bents that support the flooring? 

Waste of wood? maybe but you'll get what you want.



beenthere

Quote from: Mad Professor on January 05, 2010, 12:21:21 PM
Barns have been done with at least a 40' free span span width , with a hay mall above.  I don't see why a 20' span between bents is not possible with proper support....

I'd sure like to see some pics of the barns with 40' free span. Any chance? 
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

moonhill

The 40' free span should be come kind of truss.  I don't see a single beam in a barn simply spanning 40'. I want more info.   

I am spanning 20' in the addition I am now forming at my own house.  There is really only one of five tie beams that is spanning the 20' the other four have a post or wall under them at some point.  The one which spans the 20' is a naturally curved spruce it is roughly 12"x12", it has about a 7" crank in it.  I don't think I will see it deflect to a flat beam.  There is drop-in 2x joist let into the sides spanning between tie beams.   It is hard to find straight tree at times around here. ;D

You could also put keys between two beams to make a deeper beam.   

Tim
This is a test, please stand by...

GaryinMississippi

Quote from: Mad Professor on January 05, 2010, 12:21:21 PM
Barns have been done with at least a 40' free span span width , with a hay mall above.  I don't see why a 20' span between bents is not possible with proper support.

How about shouldered M/T bents that support the flooring? 

Waste of wood? maybe but you'll get what you want.




Apples and oranges.  The 40' free span width is across a bent (which has larger members and support bracing) not between bents.

moonhill

This is a test, please stand by...

Brad_bb

From being in Steve's workshop, it was clear that the stresses rose very quickly after about 16 foot between bents.  There may be a distinction here between bent spacing and floor joists.  The thing is with longer floor joists, you'd need bigger wood with close spacings. 
   Just two days ago I was going through a building from what we believe to be about 1850-1880 my family owns.  It will be demo'd in the near future.  It is a post and beam and brick structure.   It is 40 feet wide, with with supporting beams down the middle(12"X12"'s).  That makes a 20' floor span on either side on 2 floors.  They used 10"X12"'s (although they could be 10"X14"'s as I was just eyeballing), on 4 foot centers for floor joists.  They needed to be big like that because of the 20' span, and because it was an industrial building that would have decent floor loads.  These floor joists were attached to 12"X16"s, which were 10 feet long (I assume because anything longer would be too heavy to assemble at the time), and supported on each end(every 10 feet) with a 12"X12" post.  Here's some pics:





  Incidentally, the grain looks like some type of pine.  I'm hoping the demo guy will salvage these as there are 76 of these huge floor joists and  20 of the 10foot 12"X16"'s and 6 of the 12"X12"'s, all in excellent condition with only 1 layer of paint to remove.  But it will be sold before demo, so who knows. 
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Mad Professor

Quote from: beenthere on January 05, 2010, 02:16:35 PM
Quote from: Mad Professor on January 05, 2010, 12:21:21 PM
Barns have been done with at least a 40' free span span width , with a hay mall above.  I don't see why a 20' span between bents is not possible with proper support....

I'd sure like to see some pics of the barns with 40' free span. Any chance? 

From one of Richard Babcock's books, he's raising the bent that contains the "swing beam", it allowed a free span in which a team of horses could enter and turn.  How far you think it is between bents???

Reprinted with proper atribution and permission of Richard (Barns at Wolfttap, and Barns whose roots run deep ).

I've have to check another of Richard's books to check how far between bents. 


moonhill

My program to open that doc. is out of date and I can't open it. 

But I did go to the book case and pull out Babcocks book The Barns at Wolf Trap, in which I see on the front cover the swing beam truss leaning on some cribbing with two additional bents up right.  The swing beam is comprised of two timbers a bottom cord and a top cord, not a single beam spanning the 40'.  The two bent which are raised up are a single timber spanning the 40'.  These bents seem to have missing timber via the open mortices in the post, suggesting the barn has been tampered with?  I am guessing the timber was removed to allow room for seating and no obstructions for a clear view.  This barn is no longer carrying the loads it did as a working barn. 

Tim 
This is a test, please stand by...

djkarl

Hi All,

Thanks for all the great replies.  I'm glad I asked the questions and the replies did help.

One thing becoming more evident is that white oak's strength to weight ratio may not be appropriate for long joists.  SYP (i.e. loblolly) actually performs much better for my needs and is more easily available.  Since I'm not opposed to mixing and matching species, I may just do that.

Jim, thanks for tip on book.  It is quite inexpensive, so I got it.  The total was less than $10 including shipping from hamiltonbooks.com.

Brad, sounds like some nice wood in the building you've found.  I've never carefully torn down large structures.  I've only carefully torn down small structures and demolished one large structure.  Thanks for the pics, too!

I'll just have to have another conversation with my supplier about availability... including straightness of logs.

Thanks again,

Don


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