Collar beam needed or not?
Collar beam needed or not?
(OP)
Hi people.
See these pictures:
The system is the SAME in every picture
http://www.google.hr/imgres?imgurl=http://www.dach...
http://www.google.hr/imgres?imgurl=http://www.dach...
http://www.google.hr/imgres?imgurl=http://www2.bau...
These are roof where the rafters on each side of the roof are resting on two supports.
SUpports are wooden bemas where the lowest one is resting on the floor and the higher one is resring on wooden columns.
SHould a system like this have an collar beam instaled?
See these pictures:
The system is the SAME in every picture
http://www.google.hr/imgres?imgurl=http://www.dach...
http://www.google.hr/imgres?imgurl=http://www.dach...
http://www.google.hr/imgres?imgurl=http://www2.bau...
These are roof where the rafters on each side of the roof are resting on two supports.
SUpports are wooden bemas where the lowest one is resting on the floor and the higher one is resring on wooden columns.
SHould a system like this have an collar beam instaled?






RE: Collar beam needed or not?
https://www.dropbox.com/s/hkguqz16lpcjlm5/roof%20o...
RE: Collar beam needed or not?
Anyone else maybe?
RE: Collar beam needed or not?
do a search for rafters and ridge beams on this site and I'm sure you'll have more than enough info to dig through.
RE: Collar beam needed or not?
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RE: Collar beam needed or not?
Mike McCann, PE, SE (WA)
RE: Collar beam needed or not?
To me it will depend mostly on the stiffness of the supporting beams. Same as if you had a ridge beam. When the ridge beam is stiff and prevents significant deflection, then a collar tie would not be needed. But, absent that level of rigidity, the ridge sags a lot and two things happen: 1) noticeable deflections in the roof, 2) the walls or end supports then experience significant outward thrust. So, you might have to detail the connection to the ceiling rafter (not the roof rafter) do resist that thrust.
RE: Collar beam needed or not?
OK undersootd, but Ill show a system later on thats almost the same a s the roof with an ridge beam and hopefully well agree on overdimensioning the elements.
@msquared48
Can you post some image or a link that would clear up what you mean with " the attic floor joists could serve as collar ties "
@JoshPlum
"To me it will depend mostly on the stiffness of the supporting beams"
To me this also seems to have the BIGEST IMPACT on the whole system!
If the supporting beam deflects (sags) the rafters will start to push on each other (in the ridge) and eventualy the whole system will start to experinece increase in horizontal forces on the lowest beams resting on the RC slab. Outwards thrust.
Heres a nice illustration of the problem that might happen
(http://www.google.hr/imgres?imgurl=http://www.nach...
But if there where collar beams instaled in this situation would there prevent the outward thrust?
As I understand they are instaled at every 2nd pair of rafters, right?
For this system with two knee beams, the two rafter will be joined in the ridge like this:
http://postimg.org/image/mqtqx88ch/
I dont think that this connection is weak (usualy an b srew bolt type connection) for uplift forces that might occour BUT also there is the SNOW loading that can couse unsymetrical loading of the roof (north side coverd with snow, south side snow melted due to sun)
RE: Collar beam needed or not?
Come on...
RE: Collar beam needed or not?
Mike McCann, PE, SE (WA)
RE: Collar beam needed or not?
Do those beams run the length of the building, and are they supported only at the ends/gables?
How deep are the birdsmouth cuts in the rafters at the intermediate beams?
Collar ties are normally installed too high to efficiently resist outward thrust.
In your example if the intermediate beams don't support the rafter, the thrust at the floor would be about 1/3 as much as if the collar ties had to resist it.
RE: Collar beam needed or not?
These beams are much bigger in size then rafters and are dimensioned based on the spacing between the coloumns and based on the load coming on them from the rafters.
The birdsmouth cuts on ratfers (rafters are usualy 14/14cm, 5/5 inches) are usualy around 3cm(thats around 1,25 inches).
In your example if the intermediate beams don't support the rafter, the thrust at the floor would be about 1/3 as much as if the collar ties had to resist it.
Intermediate beams in my images, always support the rafters!
My question is are these collare ties realy necessary in these kind of Systems (system 1 and system 2)?
Columns are usualy spaced 3-4m
System1
http://s29.postimg.org/cpej81yk7/roof_system1.jpg
system2
http://s18.postimg.org/u7wo3n9yx/roof_system2.jpg
RE: Collar beam needed or not?
RE: Collar beam needed or not?
RE: Collar beam needed or not?
Sometimes if your wall is CMU or a concrete wall, you can say that there's a "beam" at the top of the wall which spans horizontally, and supports the rafters from doing the "splits", just make sure your "beam" in the wall can span horizontally.
There were recent large earthquakes in my city and I have seen this system "open up" due to out of plane loads from walls.
If there is no over-riding reason to not add then, I would add them.