Author Topic: Collar ties needed?  (Read 5116 times)

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Offline randy j

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Collar ties needed?
« on: June 23, 2007, 12:38:15 AM »
Hi, Everyone,

I just found this site the other day, and I'm thrilled.  I've been interested in little houses for years, got tons of books on 'em.  Glad to be in the company of like minded folk.

The first strand I read when I came across this site was the one on the home page about the "Enchilada" house.  In it two people mentioned that they didn't care for the looks of the collar ties in the cottage.  The question came up, "Can you build without collar ties?"  This begs the question, "What do collar ties do?"  (I know most of you are old pro's, please bear with me.)  Answer: they keep the tops of the walls from spreading apart under the force of the gable roof above them.  The ties do just what their name says, they tie oppossing walls together so they can't be spread apart.  Triangular trusses have ties built right into them, but they are called "lower chord members".  In the "Enchilada" strand, John Raabe offered two methods of construction that will allow you to build without ties.  First, he suggested scissor trusses or trusses with ties located above the wall plate connection.  Second, he suggested using a structural ridge beam.  Both are good methods and are tried and true.  In  another strand, Joe Mann brought up "rigid frames".  These are related to cruck construction, which is very old.  In cruck construction you lean two bent members against each other at their tops, sorta like an A-frame.  The members are bent like a bananna, and you place them with the bends out so the shape approximates the profile of a gable structure more than an A-frame.  Using crucks, or rigid frames, along with purlins, you don't need collar ties.  These are some ideas I've seen on the site, I'd like to toss out a few more just to start some dialog.

First, hip roofs.  A hip roof on a suare building is a pyramid roof.  It doesn't need ties because the roof sheathing and the top wall plates act as ties.  In the '40s and '50s, pyramid roof houses were popular in the Northwest.  They would cut the peak off and put in a square skylight, a very nice design!  A hip roof on a rectangular building will require a ridge board to fill in but if the rectangle is not too long, the lateral loads are carried in the same way as the pyramid roof except for a slight amount of bending that is counter-acted by the stiffness in the walls, the roof sheathing and the ridge board.  If the rectangle gets very long, the center portion will spread but a steel tie rod would unobtrusively handle that.

Second, porch roofs.  They attach to the walls of a gable building near the top plate and under the right conditions, can counter-act spread.  The "right conditions", include: there being at least two porch roofs with one located on opposite sides of the building where the roof planes meet the porch roof planes.  Also, each porch roof must run the full length of the building.  The porch roofs must have a tensile attachment to the top plates of the adjacent, or gable end, walls.  And, all the members, including the porch roof ledgers, sheathing, must be sized to deal with the loads.  The porch roofs act as large beams that the walls push against, and they carry the forces to the top plates at the ends of the building. The end wall top plates will act as ties, but again they must be designed to handle the load.  By the way, the porch posts play no role in resisting the lateral forces, they just hold the outside edge of the porch up.  The posts could be made to act as buttresses with the porch rafters transfering the thrust to them (flying butresses!), but that is not the idea presented here.

Third, top plate beams.  The idea is similar to the porch roof idea, but instead of the porch roof carrying the forces to the end wall top plate, the top plates of the logitudinal walls are beefed up in the horizontal plane, and they do the job.  The "beams" could project into the interior of the building and used as shelves (with the appropriate support, of course)  They could also be made so they project to the exterior and form a soffit under the eaves.  There are a number of variations on this idea that could lead to some interesting design features.

Fourth, buttresses.  Yep, them things that have kept all those gothic churches from falling down.  They take up space, but if your design includes storage closets on the outside, you could have the closet walls designed to resist the spread.  Remember though, you need them on both sides of the house.  Also, partitions on the interior of the house, if located in the right spots, can act in the same way, again, if designed for the job.

Fifth, pole construction.  For those of you who have access to telephone poles for cheap.  The poles can be buried deep and they will resist quite a lot of lateral force.

Not counting center columns and skyhooks, there are still several other ways that have been devised to keep walls from spreading apart but these are a few that I thought might raise some interesting discussions.  Anyway, folks who want to build tiny houses are, generally, on a tight budget and John's suggestions are probably the cheapest and most conservative ways of getting it done.  Some of the things I've tossed out, shouldn't be attempted without a structural engineer.  That usually means some big bucks.

I've had fun thinking about this and hope it stirs up your gray cells too.  How many other ways of stopping spread can you folks think of?


Cheers,

Randy
« Last Edit: June 23, 2007, 12:33:27 PM by randy_j »

Offline MountainDon

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Re: Collar ties needed?
« Reply #1 on: June 23, 2007, 05:34:44 AM »
Build underground like Glenn.   ::)

Just because something has been done and has not failed, doesn’t mean it is good design.

Offline glenn kangiser

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Re: Collar ties needed?
« Reply #2 on: June 23, 2007, 05:52:40 AM »
Rods or cables as mentioned in the Dutch Hip ? roof above can also stop spreading.   I have seen them in old buildings.

Welcome to the forum, Randy.  Sounds like you have a pretty good background in building. :)
"Always work from the general to the specific." J. Raabe

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Offline JRR

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Re: Collar ties needed?
« Reply #3 on: June 23, 2007, 11:40:48 AM »
A cottage cheese and lettuce diet? (LOL)

Interesting stuff, Randy ... thanks!  I've seen tensile rods and nut-plates installed in old mansions in Charleston, SC.  (Like Glenn mentioned.)  The outside nut-plates often are of ornate designs.

The inverted catenary arch perhaps.  Somewhat like what you described early on.
« Last Edit: June 23, 2007, 11:56:48 AM by JRR »

Offline glenn kangiser

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Re: Collar ties needed?
« Reply #4 on: June 23, 2007, 12:00:05 PM »
Quote
A cottage cheese and lettuce diet? (LOL)


That should about do it for me--- probably  get the trots from that. :-/
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Offline randy j

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Re: Collar ties needed?
« Reply #5 on: June 23, 2007, 12:32:12 PM »
JRR,

That is way cool!  Thanks for the laugh.  Somtimes I forget that words have other associations.  I do have a bit of spread to deal with myself.  Too much sitting at the computer and no reduction in intake.

Randy

 

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