Ridge beam and Design?

[addicted]

Hey Guys
 I've been looking around for a short time on information about ridge beams. My question is... How much does a ridge beam reduce the outward thrust at the rafter plate?

Thanks in advance.
Rusty
beware that answers will be followed by more questions.

[beenthere]

Theoretically, I would say all of it.

If the ridge beam deflects under load, then there would be outward thrust.

[addicted]

So.............does that mean you need a larger ridge beam as the pitch decreases? If you go from a 10/12 to a 6/12 does the amount of force on the ridge increase?
Rusty

[beenthere]

Let's hear more details of the question.

[Jim_Rogers]

We need to first clarify what the first posted means by a ridge beam.

A ridge beam that connects rafters together at the top doesn't do much other than connect them together, unless it is supported by a post. Then it is a "supported ridge" or a "structural ridge".

Think of it like this.

Let's say you lean a ladder up against a house, on an angle. The ladder is sitting on a puddle of water, but the puddle is frozen so it's ice. Very slippery. At nearly any angle the ladder is going to slide out away from the house as you climb the ladder. Faster when it's at a low angle.

Now, let's hook the top of the ladder to house.

It can't slide out very much at all, even at a low angle, unless the point where it is hooked to the house is lowered.

A ridge beam would/could be easily lowered with a lot of load, such as snow.

A structural ridge or supported ridge, can't be lowered unless the foundation or some joint between the ridge and the ground/foundation fails, and the ridge beam is lowered.

My point is we need to understand is this just a ridge beam? or a supported/structural ridge beam?

Jim Rogers

[addicted]

Thanks guys
You're right... I should have been more specific.  I'm in the beginning stages of designing a frame and was thinking of using a structural ridge beam, supported by posts, on which the rafters will be attached on top thus reducing the outward thrust at the rafter sill plate. (high post cape) So with what I've learned from most of the books mentioned on this sight and a few TF courses, if you have the rafters unsupported at the ridge, the outward thrust at the rafter sill plate increases dramatically as you lower the pitch of the roof. IE. 12/12 to an 8/12 pitch. So if you put in a supported ridge beam it will remove all outward thrust. But let's say that your local saw mill can't cut a post long enough to reach all the way up to your ridge beam at a 12/12 pitch, hence you reduce the length of the post, which reduces the pitch of the roof.

Here's the question. If you reduce the pitch of the roof do you have to change the depth of your supported ridge beam. My thinking is the lower the pitch of the roof the more force you will have at the supported ridge. Is this correct?
Rusty

[Jim_Rogers]

If your sawmill can't make a post long enough to go from the foundation up to the ridge beam, then change the design by adding a scarf into the post to make two beam into one.... vertical scarfs can be done very nicely and they are strong, if done correctly.

The depth of the ridge beam should be calculated to support the load that is on it. That is a vertical load of holding up half the rafter span, on each side of the ridge.

Here is a cross section to show what I mean:



Keep asking questions.....

Jim Rogers

[witterbound]

Seems like one would typically have a tie beam that ties the two ouside posts together, and then you'd have a king post sitting on top of the tie beam running to the ridge.  If you needed, you could add a third post under the king post. 

[icolquhoun]

just to rule out the obvious, but if the sawmill can't cut a long enough post, what about the sills, plates, and tie beams, which in most traditional capes are all over 20' I understand these can be scarfed
typical new england cape was 24widex36deep or thereabouts
not that I approve of this frame, but it's a supported ridge using a 2 piece post:

(http:timberframedesign.net/files/ridge_beam.jpg)
{Off forum pictures are not allowed, please place them into your gallery and post a link from there, moderator}

[addicted]

The limitation on the saw was just hypothetical. I was just trying to imagine the forces on a supported ridge beam at different pitches. But the idea of having a post on top of a tie beam brings up more questions. The post on top of the tie beam would now be a king post right? So aren't king posts in tension? Or does the load from the supported ridge beam nulify the tension and put it in compression?
Rusty

[Jim_Rogers]

The post on top of the tie beam would now be a king post right? So aren't king posts in tension?

If the post on top of a tie beam is hung between two rafters then yes it is a king post and it is in tension.
But we are talking about supporting a ridge with a post above a tie beam.
The rafters will not "hang" the post.
The rafters would look something like this:



The rafters would be secured to the ridge beam with some long timber lock screws or something like that.
The gap between the rafters is because the ridge beam will shrink and if they touch before it shrinks then the will not be supported when it does.

Your design has to be right to convert the outward thrust of the rafter to a vertical load on the frame. (very important)

Jim Rogers

[routestep]

If you lower the pitch of the roof, the ridge beam would have to support more load and hence you would increase its depth to keep it from sagging. If you had a flat roof in the extreme, the ridge beam would carry half the weight of the roof and the plate would support the other half. There would be no outward thrust.

If the ridge beam is well supported by posts down its length (it doesn't sag) then the rafters should not put any outward thrust on the plate. Or at least I don't see how it could. If the ridge sags you could get a lot of thrust force, but the movement out might not be too great.

If the rafter is bolted to the ridge and plate and it (the rafter) sags there could be an inward thrust (or the bolts might tear out in an extreme case).

[addicted]

Thanks guys
 So now I've been looking for a way to calculate that load on the supported ridge beam, but all the books that I have don't include the pitch. If the load on the beam is greater with a lower pitch then wouldn't the pitch be included in the calculation? And if the pitch must be included calculating the load on the ridge beam, then shouldn't it also be included in calculating the load on the rafters? This is starting to make my head hurt. So what is the standard way to calculate the load on a supported ridge beam and rafters if the pitch is directly related to the load?
Rusty

[meddins]

... If the load on the beam is greater with a lower pitch then wouldn't the pitch be included in the calculation? And if the pitch must be included calculating the load on the ridge beam, then shouldn't it also be included in calculating the load on the rafters? This is starting to make my head hurt. So what is the standard way to calculate the load on a supported ridge beam and rafters if the pitch is directly related to the load?

My understanding of this situation...

I don't think the load on the ridge beam is increased by the lower pitch. The load is simply the weight of the structure the ridge beam supports plus forces like wind.

However, a lower roof pitch does increase the rafter's horizontal thrust at the wall plate/post, which is different from saying it increases the load..

Controlling or eliminating that horizontal thrust is a function of truss design, that is, inserting the right members at the right spot in the truss to transfer loads vertically down and away from the wall plate/post.

My understanding is that when you calculate load for the ridge beam, you can draw your roof in plan view and treat the ridge beam as a uniformly loaded beam (assuming rafters are spaced at regular intervals along it.)

Hopefully Jim or someone will correct this if I'm wrong. I'm going round and round with roof design too at the moment..

[Jim_Rogers]

The post on top of the tie beam would now be a king post right? .....
Rusty

As every timber in a timber frame is given a name based on it's location, the name of the post that supports a ridge beam may not be called a king post. I'll have to do some research on that post's name and see if I can find out what it is called.

If a post supports a common purlin, it is called a purlin post. When a post is on the first floor and supports the middle of a continuous tie beam, I've heard it called a prick post, but I don't know how or why it's called that.

The post that holds up a ridge beam, I think is called a crown post, but I'll have to look into one of my books to see if that is correct. This post would not touch the rafters, where a king post is hung between the rafters and holds up the tie beam.

Jim Rogers

[addicted]

Thanks guys. I guess I'll start sizing the rafters and supported ridge beam based on horizontal distance and 50% shared load between the rafter sill and the ridge beam.  This is what I get for over thinking things.
However this does not mean that the questions will stop.

Enjoy the snow
Rusty

[Jim_Rogers]

According to my research:

"Crown post ends at a collar or collar purlin, king post goes to ridge beam or apex of roof."

Source: Jack Sobon.

[Thehardway]

Jim,

I agree with you and Jack on the crownpost/kingpost however it seems as though the confusion here on the term "king post" stems form the fact that a post historically was only referred to as "kingpost" when used in a truss. That truss may have had a "ridge beam" or "ridge plate" or "Ridge Piece" which it supported but the thrust was directed through the truss rafters/top chords to the wall plates, not straight down to the ground through the post.

We have crossed the thin line here between timber frame construction which never really used a central post to support a "ridge beam", and modern "post and beam" construction which commonly uses tall posts and transfers loads directly to the ground and would use typically steel plate joinery.  Therefore this member is an orphan doesn't have a proper name and the joinery at the apex is questionable.

I believe the traditional way of framing this structure would probably have been purlin to a purlin plate.  It could also have been done with crown posts.  I suspect the structural "ridge beam" with central post was not used because the number of intersections and size of the joinery that would have been required in the apex at the post connection would have compromised its integrity, therefore, it evolved later in modern post and beam when steel plate connections and glu-lams became the norm.   This is not to say it was never done but that there are probably not many examples of it that are left standing for documentation and perhaps there is a reason for this?
What do you think?  Is there any good reason to depart from traditional design parameters here?

[addicted]

Thehardway
Thanks for your input on this ridge beam subject.  Being very new to timber framing, I was not aware of the short comings of this type of design. Would you mind expanding on the weakness in the joinery at the apex and any other areas in this design that might pose a problem?
Thanks
Rusty

[Left Coast Chris]

One more thought to consider:  The International Building Code and the Uniform Building Code allow for snow load reduction for steeper pitches of the roof.   So...... for the steeper pitches the ridge beam size could be reduced depending on your local snow load.   Also keep in mind that the International Building Code requires more load combinations to be considered so if you are trying to meet code you should have an Engineer determine the load and size the beam(wind, rain, live, snow are now combined).

Here is another tricky thing to consider:  for dead load only, the steeper pitched roof has the same tributary area for load analysis as the flat roof but the rafters, sheathing length and roofing material need to be longer to span from plate to ridge.  Since they are longer they have more dead load than the flat scenairo.   Engineers adjust for a little more dead load for the steeper pitches.    More common though is for steeper roofs to be used in heavy snow load conditions so snow controls more than the small increase in dead load and the reduction in snow load for slope is often significant and almost always more than offsets the increase in dead load so you can reduce the size of the ridge beam due to the steeper pitch.