Collar beam needed or not?

[mar2805]

Hi people.
See these pictures:
The system is the SAME in every picture

http://www.google.hr/imgres?imgurl=...DAJVdOrE8HKOaLzgdAK&tbm=isch&ved=0CFQQMygwMDA

http://www.google.hr/imgres?imgurl=...DAJVdOrE8HKOaLzgdAK&tbm=isch&ved=0CGcQMyhDMEM

http://www.google.hr/imgres?imgurl=...DAJVdOrE8HKOaLzgdAK&tbm=isch&ved=0CGMQMyg_MD8

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?

 

[robyengIT]

please find attached different types of old style wooden roofs

https://www.dropbox.com/s/hkguqz16lpcjlm5/roof old style.PDF?dl=0

 

[mar2805]

Thanx but not the answer that Im looking for
Anyone else maybe?

 

[jayrod12]

No it does not need collar ties in theory. This has been discussed many times but more often the discussion is a ridge beam as opposed to two knee beams (a lot of the time they're framed as walls where I am).

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.

 

[JAE]

For gravity loads the collar tie/beam would possibly not be required. For wind loads, however, with significant uplift on the leeward side, you might need some type of connection between the two rafters near the ridge to help tie the ridge together.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[msquared48]

This is standard attic framing in many older structures, and some new ones. No collar ties would be needed, but with proper detailing, the attic floor joists could serve as collar ties with little extra expense entailed. Just would make it a better structure.

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

I haven't really seen this configuration before, but I'm not a fan. It can work, but it also could lead to some problems.

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.

 

[mar2805]

@jayrod and JAE
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=...VfjrC87faO78gugO&tbm=isch&ved=0CBAQMygIMAg4ZA

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)

 

[mar2805]

Guys anyone?
Come on...

 

[msquared48]

What I mean is that extra, high collar ties are not needed here if the lateral kick from the rafters can be taken, through connections, into the floor joists. This is not rocket science... It's done all the time...

Mike McCann, PE, SE (WA)

 

[Tmoose]

What size are beams that "support" the middle of each rafter?
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.

 

[mar2805]

Supporting beams run all the way thru the builing and are supported by columns each 3-4m.
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

 

[mar2805]

Any more ideas or comments?

 

[ChiefInspectorJeff]

Code requires ties every other clear span rafter and has no exceptions except beam or wall supports.I think the wood frame construction manual has tables and requirements for that and for ridge straps.

 

[Backcheckrage]

If you are designing a rafter system like that then I would design some sort of collar ties to keep the system from doing the "splits". You need to consider what happens when there's wind or seismic load on the walls as well. It's just good engineering and detailing, forget calcs.

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.