Room Over Garage Rafter Thrust 3

[wayniac3]

I am working on a house located in the coastal area Virginia. Typical loads include: 110 mph wind, 20 psf roof live/snow. The attached sketches show the framing of the garage which includes 2x10 rafters, 2x8 ceiling joists adn 2x4 stud walls all at 16" o.c.

The room over the garage vaulted ceiling is going to cause rafter thrust. However, the room is approximately 27' deep. This is a bit long for a ridge beam scenario. Would it be possible to use double 2x10 rafters and reduce the thrust as the rafters would have twice the section modulus to increase stiffness? Also, could I make use of the knee walls to dissapate some of the thrust? I would appreciate constructive comments and suggestions. Thank you.

 

[wayniac3]

Additional Sketch

 

[yellerwhale]

Hi wanyiac3,

Simply doubling the rafters will not change the mechanics of the rafters spreading the walls.

Bearing on the short walls below the roof will probably overload & sag the garage ceiling.

I have done this type of job several times before.
It is a lot of work in both design and construction.

It will take the expertise of an experienced structural engineer and a patient builder.

Let me know if you are able to retain the expertise needed for this job.

 

[JAE]

If you tie the floor joists in with the sloping rafters, there will be no thrust on the walls right?

 

[yellerwhale]

JAE, that is a correct and clever way to avoid to wall thrust. It has constructability issues especially with the construction of the floor diaphragm, however I may have used that method once or twice. I've since found what I think is a better way, yet it has its issues also. Solving this puzzle takes some thought.

 

[BAretired]

I agree with JAE. There is no thrust on the walls if the rafters and floor joists are tied together.

Would it be possible to use double 2x10 rafters and reduce the thrust as the rafters would have twice the section modulus to increase stiffness?

Doubling the rafters does not reduce thrust. Tying the rafters to the floor joists produces a tension in the floor joists which removes thrust from the wall.

BA

 

[dhengr]

I would second JAE’s motion and run the floor diaphragm all the way out to the bearing walls, with, 13" long, shear blocking btwn. each set of jst./rafters, from the double top plate to the fl. sheathing. We’re all ears and open to hearing about constructability issues, yellerwhale, don’t hold your issues and good ideas so close to the vest, this isn’t a guessing game. You could also bear a horiz. seat cut on the rafters, on the floor sheathing, right over the jst. below. What size are these 2nd fl./garage clg. jsts., 22' is a pretty long fl. jst. span. The bigger problem might be getting the lateral loading down to the found. on the O.H. door elevation.

 

[msquared48]

The second floor joists can be detailed to function as collar ties. Then there will be no lateral spreading of the walls.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Side note - you will have to tie the floor joists together at the center bearing wall if they are not continuous rafter to rafter, in addition to connecting the rafters to the joists as JAE states.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[wayniac3]

Thanks for all the great responses. The ROG floor joists will span from side to side and are 2x12 @ 16"o.c. The joists will bear on a dropped steel beam spanning 21' front to back. This will cut the joist span to 11'.

 

[ishvaaag]

...and do not forget the added tension in the ROG floor joist design.

 

[wayniac3]

BARetired, thanks for pointing out my poor choice of words in my inital problem statement - maybe my understanding of the rafter thrust concept is flawed. My thought was an attempt to increase the resistance to the bending the rafters experience between the collar tie and their lower bearing point. Stiffer rafters should not deflect as much under the downward push of the roof load and induce any secondary order effects (this should not happen per the connection methods indicated above). Maybe this would be an issue if the rafter bearing exterior garage walls were balloon framed from the garage below to a 6'-0" tall height above the ROG floor and the ROG knee walls were not present. The connection from the rafter to the top plate would be critical to handle the thrust and the wall studs would need to be nailed to the floor joists and checked for bending as a cantilever from the floor framing to the top plate.

 

[BAretired]

wayniac3,

Sorry, I was not trying to point out a poor choice of words. I now believe I did not understand your original intentions. Let me try again.

I assumed you were originally intending to tie the rafters at the ROG floor level as well as at the collar ties. If that were the case, then beefing up the rafters would not help in reducing rafter spread.

If the rafters are not tied to the floor joists, then doubling the rafters would help to reduce rafter spread. In this case, the collar ties, being so high up, would have a large tensile force and would have to be adequately connected to the rafters.

If the load per rafter (projected on a horizontal plane) is w, then the bending moment is wL[sup]2[/sup]/8 where L is 22', i.e. M = 60.5w. If the height from the middle of the collar tie to the middle of the rafters at the ridge is 2', then the tension in the collar tie is 30.25w.

Bending moment in each rafter would be minimal at the wall, maximum at the collar tie and zero at the ridge. Bending deformation of the rafter would determine the spread of the rafters.

Tying the rafters at the ROG floor would seem to be a much better design concept.

BA

 

[msquared48]

I agree for the most part here, but not in the concept of reducing the rafter spread.

In the original case here with the collar ties set high with little leverage, when you model the collar ties, the lateral deflection will be reduced by doubling the rafters, and you will be able to employ double shear in the collar-tie to rafter connection. However, the collar tie in the original concept is set so high, the lateral kick, in all probability, would still be unacceptible even after doubling the rafters. Putting it another way, the high set of the original collar ties does not provide enough leverage to control the lateral spread.

Therefore, that is why I said to use the floor joists as collar ties, abandoning the higher ones structurally except to tie the ridge together.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dik]

Seal against CO and have CO detectors installed... 1/5 of 1% can kill you in a half hour...

Dik

 

[Stillerz]

I have built many houses just like this.

I always framed the entire floor above the garage the way you would on a typical second floor with, joists, rim joists and sheathed the entire floor and made sure to tie the floor framing to the walls.

On top of the sheathed floor framing I nailed 2x8 or 2x10 in the fat direction all the way around the perimeter (like a bottom plate of a wall). This gives a nice solid member to nail the bird's mouth of the rafter to, and also helps stiffen the floor framing laterally a little.

Any knee walls can be added after the rafters are in.

 

[wayniac3]

Thanks to everyone for the valuable information!