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Rafter Ties

Rafter Ties

Rafter Ties

(OP)
Attached is a picture that the homeowner had in mind.

Question 1:
My question is if a ridge beam is not installed then the rafter ties need to be installed near the bottom of the rafter and not in the top portion right?

Question 2:
Another option I was thinking of was installing a 2x4 from the rafter back to the 2x4 stud wall? If this was done could a regular ridge board be installed with collar ties in the top portion?

Thanks for the help.

RE: Rafter Ties

#1. Not necessarily. Collar ties are set this way a lot, but more lateral spreading of the walls is seen. A way to limit that is to use larger rafters. In your situation though, if you balloon frame the outside walls, you may be able to develop the floor joists as a collar tie with strategic tension connections, engaging the thrust from the rafters in the bending of the outside walls. Just a thought.

#2. I would need to see a sketch of what you are thinking to comment further.

Mike McCann
MMC Engineering

RE: Rafter Ties

Is this what you had in mind for part 2 of you question Jim? (see attached) I've seen this design in post and beam construction. I'd imagine that over open areas this would equate to a truss and as Mike said larger rafters (top chords) may be required. Connections may also be an issue and require more than just 2x4 diagonals.

RE: Rafter Ties

If that's what you are thnking, I woold attach the diagonal directly to the rafters and floor joists where they hopefully align every four feet or so. You will also need a tension tie where the floor joists butt at the center wall if they are not continuous.

Mike McCann
MMC Engineering

RE: Rafter Ties

It is a collar tie, just set high, and not very effective, but I would still leave it as it does help keep the rafters together at the ridge.

Mike McCann
MMC Engineering

RE: Rafter Ties

(OP)
Thanks for all the replies.

After reading the replies it sounds like I should leave the rafter tie (B-C member), install diagonal 2x4's fastening them to the rafter and floor joists @ 4' o.c., increase the rafter size to 2x10's, and install a tension tie where the floor joists meet.

What is typically used for a tension tie?

Thanks.

RE: Rafter Ties

Calculate the tension force and use a strap tie each side to carry that force.

BA

RE: Rafter Ties

The lower the collar tie is, the higher the tension. A quick calc for collar tie tension "T" is T=wl^2/8d where d is the distance from the top of the roof to the collar tie.

Don't forget that the roof joists would need to be designed as beam-columns. The calculation gets pretty hairy. Another reason I don't like rafter roofs.

RE: Rafter Ties

Quote (manstrom)

The lower the collar tie is, the higher the tension. A quick calc for collar tie tension "T" is T=wl^2/8d where d is the distance from the top of the roof to the collar tie.

Not true. Ordinarily, the lower the collar tie, the lower the tension. But in this case, we have two ties, B-C and F-G-H (see attached).

Tie B-C is actually a redundant member which holds the roof together during wind uplift. But under snow load, B-C is in compression, not tension. If snow is equal on each side of the ridge, the walls at points D and E carry all of the gravity load. Members D-F and E-H are tension members and the wood stud walls at F and H will be in compression. Member F-G-H is a tension member and requires a metal strap tie or a plywood gusset each side to carry the calculated tension force.

BA

RE: Rafter Ties

I agree with BA here totally - the lower the collar tie, the lower the tension in the tie due to the deeper moment arm from the ridge to the tie to resist the moment of the rafters.

Mike McCann
MMC Engineering

RE: Rafter Ties

(OP)
The owner would like to avoid using a strap tie or gusset as the beams will be visible. Will spikes work or is a strap tie/gusset absolutely necessary? In most post above (post #8) I included a picture of what the homeowner would like to do? Thanks.

RE: Rafter Ties

If I read this posted detail correctly, the owner wants to develop the tension to the tension tie through end grain nailing? No way!

You need to develop the tension through side grain nailing that imparts a shear to the nails, ie, side plates or knife plates with through bolts, or metal straps.

Rather than connecting to the side of the joists, it might be possible to connect to the top of the joists with strapping to develop the tension force. Should be less visible there.

Mike McCann
MMC Engineering

RE: Rafter Ties

Not end grain nailing, Mike. Take another look. He has two spikes, one attaching the lower lap to the wall and another attaching the upper lap to the lower lap.

@jim, What beam? I thought they were ceiling joists and would be covered by ceiling. Are you using beams at 4' o/c? If so, your elevation should say so. I agree with Mike that you need a more positive connection than one or two spikes per lap. A strap tie on top of the beams would be okay, but why are you asking us? Isn't this pretty elementary stuff? What is the magnitude of the tension you are carrying?

BA

RE: Rafter Ties

"The owner planned on installing a couple spikes to hold the joists together and to fasten them to the column. Is a tension tie required in addition to the spikes? See picture to see what I am talking about."

I guess I am confused here too. Ususlly you attach a joist to a beam and a beam to a column, not a joist to a column. Is this detail at the splice of the lower chord which consists of floor joists?

Mike McCann
MMC Engineering

RE: Rafter Ties

(OP)
Sorry for the confusion. The picture is of two 8"x8" beams bearing on top of an 8"x8" wood column. These are spaced at 10' o.c. and there will be 4x6 floor joists running perpendicular to the beams and bearing on top of the beams.

Can a strap tie could be installed on top of the beams along with the spikes? This would make it so it isn't visible.

RE: Rafter Ties

There is definitely confusion, more so now than before. You have changed the problem completely with beams at 10' centers and ceiling joists parallel to the ridge. Here is what you said earlier:

Quote (Jim)

After reading the replies it sounds like I should leave the rafter tie (B-C member), install diagonal 2x4's fastening them to the rafter and floor joists @ 4' o.c., increase the rafter size to 2x10's, and install a tension tie where the floor joists meet.

If you are going to use beams at 10' centers, why don't you provide posts above the ceiling at 10' centers and a ridge beam? Then you don't have to worry about tie forces.

Another option is to span the beams parallel to the ridge, then use joists perpendicular to the ridge with ties and diagonals spaced at 4' centers as originally intended.

A third option is to space diagonal members D-F and E-H at 10' instead of 4', but the attachment would be visible on the sides of the 8x8 beam, so the client isn't going to like it. Also 10' is probably too far apart for the roof to work as intended. So this is not really an option.

BA

RE: Rafter Ties

It makes even less sense now than it made before. What is left of the 8x8 beam after the joist pockets have been cut? Have you checked the capacity of the joists and beams for floor loading?

It would be more sensible to use 2x8 joists on top of the beams to avoid notching or alternatively, to use joist hangers and flush framing.

I doubt whether an 8x8 beam is adequate and, in any event it is not an economic size for a beam. The lap connection over the central column will tend to promote cracking in one of the beams and the notched connection to the exterior column is a poor detail.

The 2x4 diagonal brace fastened to rafters @ 4'-0" centers makes no sense at all with the framing system proposed. If you don't see this jimtheengineer10, I think you should be working under the supervision of a more experienced engineer.


BA

RE: Rafter Ties

(OP)
BAretired,
I was skeptical of the plans and that is why I am asking the questions before I take on the project. The owner says he has built something just like this before and that it is still standing. Due to this he thinks he should be able to build the exact same thing even if engineering calcs show otherwise.

I think I will talk to the owner and recommend the following:
- install an lvl ridge beam and a couple posts on the second floor
- use 2x8 joists to sit on top of the beams instead of the 4x6 joists
- use hangers to connect the 8x8 beams to the posts

Would you recommend using hangers on each side of the post to support the two beams instead of the lap connection or is there a better option?

Thanks for all the help.

RE: Rafter Ties

Clients who have built it before and it is still standing are the hardest to deal with. I always tell them that the design must be in accordance with the code or I won't get involved.

My preference would be to butt the beams together at the middle of the column. A two span continuous beam is another option, but it may be difficult to buy one that long. In either case, I would use a suitable post to beam attachment as made by Simpson Strong Tie or similar.

The attachment to the exterior column should be a suitable hanger if the column is continuous as shown, but it might be easier to cut the column off at the underside of beam and let the beam run through. The upper length of column can easily be braced back to the beam.

BA

RE: Rafter Ties

To me, this looks like timber frame construction, post and beam. From everything I have seen over the years, ridge beams are usually employed, not collar ties. The loft floor joists connections are just not designed to take the lateral kick of collar ties and hide the connection.

I have seen hammer trusses employed, but that's an entirely different animal.

I am not saying that is cannot be done, but it is definitely not the norm.

Mike McCann
MMC Engineering

RE: Rafter Ties

@Mike: Probably the owner doesn't want posts at 10' centers on the second floor which he would need if using a ridge beam.

@jim: Why is the owner using 8x8 posts on the North and South walls? It would make more sense to span the second floor joists perpendicular to the ridge. They would bear on two exterior walls and a beam in the middle with a ten foot span between columns.

The roof rafters could be supported on stub walls bearing on second floor joists and rim boards. Ties at 4' centers could be provided as originally described in your earlier post.

BA

RE: Rafter Ties

(OP)
Thanks for all the help with this. I decided to give the project to an architect who has done quite a few of these.

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