Rafter Thrust 2

[Lake06]

I am involved with a repair for a newly constructed home. The home has a valuated ceiling area and it was determined that a ridge board was utilized with collar ties located in the upper 3rd height. I realize that this should have been constructed with a ridge beam and not a ridge board. The home has all its finishes completed.

Owner would not like to utilize a ridge beam for the repair because this would involve ripping up the top of the ceiling and the height would make it difficult to install. I am thinking of utilizing 3 lvl boards (14”x1.75”) laid on their side and 2 lvl (9”x1.75”) running vertical to form an inverted T shape. This beam would run parallel to the top double plate of the house were the rafter meets the exterior wall. (I will post a picture of this detail later). I would install 2x12 joists which would tie the rafter into this lvl beam at 24” o.c. The double top plate and proposed beam would run parallel but would be aprox 1.5’ apart. There would be a proposed beam on each side of the home.

The home has a 12 on 12 roof slope with 2x12 rafters at 24” o.c. The valuated ceiling area is 28 feet wide by 20 feet long with the ridge running in the 20 foot dimension. The rafter is currently nailed into the double top plate with 5, 8d common nails and has a hurricane strap with provides 160 lbs of lateral resistance. I have calculated the lateral thrust due to dead load to be 210lbs per rafter. The lateral thrust due to dead and snow load was calculated to be 760 lbs per rafter. The exterior walls are 2x6 construction with ½” osb sheeting.

I have seen this detailed before to utilize this lvl beam but not sure of how to connect the beam at the ends. The lateral reaction would be very high at the ends of the beam. I was thinking of utilizing a 3/8” dia cable. This cable would be located at the ends of the beam and tie the beam into the opposite proposed beam located on the other side of the room. The cable can be hidden in the wall. Does anyone have any thoughts or comments on this? I guess the alternative would be installing rafter ties spanning the room located 4 feet above the top exterior plate spaced at 4 feet on center with ties nailed or bolted to both sides of the rafter.

 

[Ron]

Lake06...is there a collar tie at each rafter? If so, what's the issue?

Ridge Beam or ridge board with collar ties....same purpose.

Providing a beam effect at the top plate is good; however, what about rotation of the wall at the sill?

 

[IceNine]

In my experience, collar ties in the upper third do not resist much thrust. Thus, the ridge beam requirement.

 

[MiketheEngineer]

How about some nice 1'' diameter wrought iron rods about 4'-0'' oc across at the ceiling level - looks like old castle construction.

Ridge beam is probably best bet - but hard to install but you could drywall it and make look good??!!

 

[msquared48]

Have you considered the stiffness of the roof diaphragm to help prevent the spreading/thrust issue? At the 12:12 slope you quote, this will come into play, and it will be much stiffer considering the 20 foot X 14 foot sloping diaphragm than any horizontal LVL beam at the wall top plate.

Is there distress showing in any of the finish work yet?

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

 

[Ron]

Good point, Mike.

 

[ToadJones]

M^2-
I don't follow your last post?
Care to explain to dummy>?

 

[Lion06]

Mike-
That's slick. What do you use for the chord on the eave side of the diaphragm?

 

[msquared48]

Lion:

The wall double top plate can be the chord member, but has to be spliced properly to develop the tensile force.

Toad:

Think of the roof diaphragm as a deep beam and you'll understand. The more the diaphragm is elevated, the more it becomes a beam seeing gravity effects.

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

 

[Lion06]

Do you use blocking to take the chord forces from the diaphragm to the top plate?

 

[ToadJones]

Ah, I see.
Kind of embarrassed now

 

[msquared48]

Lion:

Yes. Same circumstance as using a wood ledger to transfer the diaphragm forces to the chord steel in a concrete wall.

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

 

[Lake06]

Msquared48

There does not appear to be any distress in the walls. I'm a little confused about how to utilizing the existing top plate to transfer the sheeting load into it. Perhaps you could show me through a detail or example. Are you recommending beefing up the double plate to resist the lateral thrust?

I have attached a detail of what I was originally proposing but I am open to your earlier comment concerning the diaphragm stiffness.

 

[msquared48]

As Lion06 stated, the sheeting (diaphragm) nailing load is transferred to the blocking, and the blocking load is transferred to the top plate of the double top plate by Simpson LPT or similar connectors, rated at 300 to 500 # per connector.

Since the top plates are only 8 to 12 feet long typically, these have to be connected to the bottom plate at the plate splice locations, I detail a 4 foot splice length, and enough connectors - nails, bolts, LPT's - are provided to develop the chord force at the splice location. This is done at each and every splice location of the double top plate over the length of the diaphragm edge. Commonly for residences, only nails are needed to develop this tensile force.

Hope this helps.

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

 

[csd72]

Lake06,

The rafters then become mainly axial members, transferring the axial force to the diaphragm by nails e.t.c.

Looking at the diapragm on each side separately it works rather like a truss with the rafters as the verticals, the ridge board and top plates acting as chords and the plywood acting as the tension chord/web. You than just need to make sure that the force at the ends can be taken into the wall.

 

[BAretired]

csd72,

That doesn't sound quite right. If gravity load on the roof is resolved into components parallel and normal to the deck, the deck carries the parallel component in diaphragm action while the rafters carry the normal component in bending.

Mike,

Would you have to remove some shingles over the walls in order to nail the deck to the blocking?

BA

 

[Lake06]

msquared48

I'm assuming the blocking would be installed between each rafter and the outside face (exterior)face of the blocking should be flush with the exterior face of the 2x6 wall. Should the blocking be the full height of the rafter ontop of the double top plate and if so should it be connected to the bottom of the diapragm or just the rafter? Also should the double top plate be spliced on the exterior face or interior vertical face?

 

[BAretired]

Wouldn't it be simpler to add a 20' long member under the deck in the overhang? Or maybe as a fascia board?

BA

 

[Lake06]

BAretired

There currently is a fascia board which is end nailed into the tail end of the rafters. The 2x12 rafters have a over hang of 1.5' from the exterior wall face.

What do you think of running the proposed beam as shown in my attached detail above?

 

[BAretired]

The current fascia board is likely not continuous and the deck is likely not nailed to it. A fascia board nailed only to the ends of the rafters does not serve as a chord to the diaphragm.

This can be corrected by (a) tying the fascia board together wherever it is spliced and (b) by nailing the deck to it (possibly through the shingles).

BA