Rafter Ties

[jimtheengineer10]

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.

 

[msquared48]

#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

 

[4thorns]

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.

 

[msquared48]

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

 

[BAretired]

See attached. Is tie B-C really a tie or a strut? Is it needed at all?

BA

 

[msquared48]

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

 

[jimtheengineer10]

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.

 

[jimtheengineer10]

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.

 

[BAretired]

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

BA

 

[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.

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.

 

[BAretired]

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

 

[msquared48]

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

 

[jimtheengineer10]

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.

 

[msquared48]

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

 

[BAretired]

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

 

[msquared48]

"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

 

[jimtheengineer10]

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.

 

[BAretired]

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:

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

 

[jimtheengineer10]

Attached is a picture that the owner gave me. I will post another picture in the next post. This is what he wanted to do. Hopefully after seeing the pics it will make more sense.

 

[jimtheengineer10]

Here is the plan view