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Cable rafter collar ties

heysmith's picture

I am remodeling an old house which has a screened second story porch. I would like to remove the 2x4 rafter collar ties and replace with cables to open the room and ceiling. Room is approx 14 by 15 feet. Roof is 12/12 with 2x4 rafters on 2 ft centers set on a 4x6 which is equivalent to the upper plate. No snow load as in the desert! Ridge is a 1x6 which I plan to strengthen and add strong tie connectors at ridge as well as rafter on 4x6 plate. Goal is to have visually open so can see a stained glass window which will be set into the gable end. Rafters are about 12 feet long and have true 2x4 cut showing no obvious deflection,.

How best to substitute cable for existing 2x4 collar ties??? Thanks

(post #106469, reply #1 of 60)

Is this a shed style roof or a gabled roof over the porch?

If the former, you may not need any rafter ties, depending how the rafters are connected to the main roof

 

 


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(post #106469, reply #2 of 60)

Sorry, forgot the detail that is a gable roof.

(post #106469, reply #11 of 60)

Don't discount consideration of thin rod vs cable ties ... may look cleaner. Just thinkin out loud. Always consider many options, the one you choose will be the best fit.

There ain't NO free lunch. Not no how, not no where!

(post #106469, reply #3 of 60)

OK, then you do need something.

One option is to place a structural ridge beam, which can be set in under the existing.

The cable ties will work too. I've done that on a couple.

I see it more often on larger structures, like old churches and factory type buildings.

But you really should have an engineer to spec it all for you.

 

 


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(post #106469, reply #10 of 60)

Thanks, How did you attach cables to rafter and tensioner to make them asthetically pleasing?

(post #106469, reply #12 of 60)

I let the engineer spec that one.

Incidentally, yesteday i was inspecting a house with all high cathedral ceilings. Each room had one or more rod ties with turnbuckle connection in the middle. The way they were attached to the rafter system was hidden to me behind the ceiling finish surface

 

 


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Knowledge FHB Campus at Breaktime.
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(post #106469, reply #13 of 60)

Thanks, incidentally, I have already used hurricane ties for the existing rafters to the 4x6 top plate and plan to sister 2x6 next to the rafters for insulation space as well as for the increased structural strength.

(post #106469, reply #16 of 60)

We recently did a system similar to what you're looking for in a cathedral family room.  In this case, wall/rafter spread was mainly handled with a large ridge beam, but we also had steel tension rods with center turnbuckles as supplementary support. 


 The steel rods were threaded for the turnbuckle and had 1/4" steel plate welded to the ends for attachment to the rafters.  Each steel plate had (4) 3/8" holes drilled in them.  The plates were set at the geometric center of the rafter and 3/8" X 3 1/2" lag screws with washers completed the installation.


 



(post #106469, reply #17 of 60)

Diesel,

I'm going to have to do something like this soon too. Just curious, why did you feel the structural ridge was not enough?

Steve

(post #106469, reply #18 of 60)

It had nothing to do with how I felt..... just followed the architect and engineer's specs.  I'd say they ran the calcs and felt the that dormer wall needed more support against out thrust.  That ridge is a triple 16" LVL spanning 26' with no place for an intermediate post... so tension rods it was.

(post #106469, reply #4 of 60)

it should be i pretty simple job to do, cables are much stronger in tension than wood so just a tiny cable will do the job of a 2 x 6 etc.


the thing i would be most worried about is the attachment point. you want to make sure it doesn't come loose there. i would use a belt and suspenders. remember the rafters are usually tied to the joists or collar ties as well as the wall. make sure of the rafters connection at the wall also.


but once you have that figured out i would just make up the cables to the points at either end and then tighten the cables with a come-a-long untill you are relieving the tension on the joists. clamp down the cable at that point and then remove joist.


to do that you use the crosby clamps or cable clamps to make the "eye" on the ends of the cable. you have to make the cables long, and a temporary "eye" on the long end to be able to hook the come-a-long and apply tension. once you have tension you tighten cable clamps close to attachment point and cutt off the excess.

(post #106469, reply #14 of 60)

 "cables are much stronger in tension than wood"


By weight? By size?


Wood is very strong in tension. Arguably stronger than in compression. It never really gets used effectively that way because of the difficulties in attaching the ends.


By the way, this is just a niggling aside. Your post had excellent advice.

(post #106469, reply #19 of 60)

cables are stronger in tension than wood.


think about it like this, from a rigging perspective.


how much (what size of) cable would you need with a crane to pick up a load of 10 tons?


now on a straight vertical pull and not even worrying about the attachment points just the strength of the wood itself, how much (what size of) wood would you need?


i have the charts for the steel cable, but i am just guessing about the wood but i think it would have to be at least 6x6 to even come close to 10 tons.


so i think by both weight and size the cable is stronger, or just about any other way you could mention it, i think the statement steel is stronger than wood is true.


thanks for the niggle, i don't mind. 

(post #106469, reply #20 of 60)

In the small town museum near me, which is mostly filled with moldy looking stuffed cougars and rusty logging equipment, there is a picture of a huge Douglas fir. The caption goes on about "these magnificent trees" (which the early inhabitants immediately logged) and then states that pound for pound Douglas fir is stronger than steel. When I read it I started ranting on to my wife: "How can they say that? In shear? In compression? Anyway, a steel beam that weighed what that tree does would probably be a tenth of it's size..." They also repeat this canard every time they publish a picture of a good sized tree in our local paper.


That having been said, I have never seen a wood member fail in tension. The values must be pretty high.

(post #106469, reply #22 of 60)

i don't know what to say about that, but as a rigger i have a good practical knowledge of rigging and the forces tension exerts in picking loads.


rope is very strong, manilla made from hemp especially. wire rope the strongest. how can wire rope be stronger than fiber rope, but a piece of wood with its imperfections and made up of similar fibers be stronger than a wire rope?


that is not logical, it just doesn't make sense. i don't believe it is correct that a piece of wood is stronger than a piece of cable.

(post #106469, reply #25 of 60)


 

i don't know what to say about that, but as a rigger i have a good practical knowledge of rigging and the forces tension exerts in picking loads.


rope is very strong, manilla made from hemp especially. wire rope the strongest. how can wire rope be stronger than fiber rope, but a piece of wood with its imperfections and made up of similar fibers be stronger than a wire rope?


that is not logical, it just doesn't make sense. i don't believe it is correct that a piece of wood is stronger than a piece of cable.

 


Because you are viewing strength as a function of diameter.  To be equal "pound for pound"  fiber rope would have a cross-sectional area greater than ten times that of wire rope.  A fair comparison would be 3/8 wire rope versus 1 1/8" fiber rope.  Both (according to my quick numbers) should have a pound per foot roughly equal.

(post #106469, reply #26 of 60)

thats all well and good measuring "pound for pound" if it is true, and i don't want to have to look up or calculate the weights.


i would much rather just shoot from the hip, i still say steel is stronger than wood. you are not convincing me otherwise, also in this application we are discussing, i think 3/8 wire rope is plenty strong to replace the tension lost by removing a 2x8 joist.


so for our application, the practical matter we are discussing, the wire rope is much "smaller" than the wood, and again just shooting from the hip here, lighter than the 2x8.


you must be an engineer, quick to calculate, but with no bearing on the application. you guys do have your uses though, don't get mad.

(post #106469, reply #28 of 60)

Nope, I'm a carpenter.  Not my fault I went to college. 


Done a ton of rigging too, used to work for a rigging outfit


The whole premise of this argument was wether the claim that pound for pound wood is stronger than steel is in anyway valid.  Which is appears to arguably  be, or close to.  That does not in any way make wood or other natural fibers a better building material for skyscrapers, boilers, dams, planes, or rigging


Edited 6/28/2008 2:07 pm ET by McMark

(post #106469, reply #29 of 60)

Now you are reminding me of those astounding pictures of bamboo scaffold structures in Asia!

(post #106469, reply #27 of 60)

Looked it up


1 1/4" manilla  .42#/ft  SWL = 1.2tons


1/2" wire rope .42#/ft  SWL = 3.2tons


So at least as far as wire rope goes, steel is much stronger pound for pound than natural fiber.


But the strength of wood could be much different that manilla fiber

(post #106469, reply #23 of 60)

The caption goes on about "these magnificent trees" (which the early inhabitants immediately logged) and then states that pound for pound Douglas fir is stronger than steel.


 


Pretty un-scientific, and taken from Wikipedia.


 


Tensile strentgth for A36 steel  400Mpa, density 7.8g/cm*3


                              "pine wood"  40Mpa, density .8g/cm*3


Tensile strength as a function of density


    Steel (400Mpa/ 7.8) = 51 (units uncertain)


    Wood (40/.8) = 50


 


I do not know if this is valid scientifically, and only applies to tension


Edited 6/28/2008 1:32 pm ET by McMark

(post #106469, reply #32 of 60)

I think they can get away with it by saying "pound for pound," bla, bla, bla. Doug fir may be stronger for a given weight, though I would venture to say in tension and compression, but would wonder about shear. For example, a spider web, pound for pound is an order of magnitude stronger than steel, but I won't be using any cobwebs for collar ties any time soon.

(post #106469, reply #35 of 60)

There are too many variables involved to make a blanket statement that Douglas Fir is stronger than "steel" by weight.  Wood is orthotropic, which means it has different properties depending on the direction of the grain relative to the direction of measurement of a property such as compressive or tensile strength.  Steel is a generic term, and there are thousands of types of steel, plus different heat treatments that yield different properties for each one.


Steel often also has a grain direction, depending on how it is made, but properties are much more uniform and predictable for steel than for wood.


Wood works better in compression than in tension, mostly because we can utilize the entire end surface of the wood to carry the load, such as in a column.  In tension, only the longitudinal cross sectional area of the nails or bolts transfer load to the wood.   If you look at a Simpson Strong Tie specification, you will see that the load ratings of the brackets are only valid if all of the required fasteners are installed.   Likewise, notice how the steel plates that sandwich built-up trusses have hundreds of fingers that grab the wood in an attempt to distribute the load through the most area.


Another drawback of using wood in tension is that it can split at fasteners and lose all of its load carrying capability.


In most residential construction, wood can be used in tension and compression, because the loads are light and there are lots of pieces for redundancy. That's why collar ties and trusses are ok to be made of wood. 


If you replace lots of wooden collar ties (which were safe due to redundancy and load sharing) with fewer steel ones, be very careful with the end connections.  The steel ultimately connects back to wood rafters, and that wood needs to take the entire tensile load of the steel tie.  The best connection would be to make the steel bracket loop around the back of the rafter, so fasteners would only be needed to prevent the bracket from slipping up the rafter.  Alternatively, spread the load out over several bolts. 


The folks at Simpson Strong Tie have an architectural series of brackets, so the guy who asked the original question should check into that and might find what he needs. http://www.strongtie.com/products/apg/index.html


Paul Formisano, P.E.

(post #106469, reply #42 of 60)

Thanks, that's a pretty nice precis of the situation. This whole relative strength of wood-steel discussion has very little useful application, it's just interesting. Because the connections dictate the useful strength of wood in tension, it really doesn't matter how high the values are. There is no way to practically approach them. My understanding is that most tests of wood in tension produce only approximate numbers because of this.


Edited 7/1/2008 12:55 am ET by fingersandtoes

(post #106469, reply #5 of 60)

Needless to say it would have to be approved by the building inspector, which means it would have to be speced by an engineer. Certainly it could be done. The biggest issue, besides the strength of the cable, would be the connections. You couldn't just drill throug the joists because you would only be holding half the joist, from the hole drilled to the inside edge. You would have to use a bracket of some sort, probably something that would wrap around the joist. Connecting the cable to the bracket might also be an issue, would probably have to be welded or maybe connected with a hub, like a giant guitar string. Whatever the case, it would have to be a strong enough connection to withstand maximun strain, i.e. during heavy winds.


I don't know a lot about these things, just pointing out some issues that I see. But it would look nice and it certainly could be done. 


I was thinking something like this...



--------------------------------------------------------


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(post #106469, reply #6 of 60)

Mine have been eyebolt through top plates with heavy washers.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #106469, reply #7 of 60)

I'd have reservations about securing the top plate but not the rafter itself. What if a heavy wind grabs that roof and pulls it away from the top plate? I realize you're way beyond me in these matters, but i can't help but wonder about this. Even without collar ties, the rafters are tied to the ceiling joists. Maybe if there were Simson ties holding the rafters down, then I'd feel safer.


--------------------------------------------------------


Cheap Tools at MyToolbox.net
See some of my work at TedsCarpentry.com

~ Ted W ~

(post #106469, reply #8 of 60)

Good point. That is why I recommended the engineer. Think I did anyways. The PE wil spec all the attachment points and methods for the live loads in the area.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #106469, reply #9 of 60)

I thought we were the engineers.

--------------------------------------------------------


Cheap Tools at MyToolbox.net
See some of my work at TedsCarpentry.com

~ Ted W ~

(post #106469, reply #15 of 60)

Instead of cable, think about using 1/2' emt conduit.  At $2 for 10', it's a pretty cheap material, and easy to work with.  Use a vice to crush the ends and drill holes in the new flat surface.  Screw those into the rafters just over or under your existing, then knock out the 2x4 that currently exists.

Rebuilding my home in Cypress, CA


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