Bent Beam at Ridge - Satisfying Moment, Thrust

[the Paper Owl]

Good Morning Everyone,
After a client meeting, it has been decided to 'open up' a space a bit more and utilize a vaulted ceiling for an area of the structure that does not have a continuous floor joist (rafter tie) and would not have a collar tie (conflicting with the ceiling desired).

I am in the process of detailing a bent beam for a small span of rafters that do not have floor joists (the area of the loft where the staircase is located / open to the first floor below) and would like to avoid the use of sandwiching steel in between the double rafter (proper flinch beam). I instead would like to rely on the strength of through bolts to satisfy the situation.

It is my understanding that by using a bent beam at ridge, the controlling factors in design are the moments that are created. The first moment that needs to be satisfied is from the rafters desire to be pushed into the interior of the house (primarily from the wind and snow load) at a fulcrum that would be the midpoint of the rafter. The bolt configuration (utilizing NDS Chapter 12) would then need to act together to satisfy this moment.

The second moment that is created is from the rafters desire to thrust away from the ridge. With the ridge acting as a rigid connection and routing the load path of thrust to be that at the connection of the wall, a combination of the connection at the wall as well as the rigidity of the bent beam (bolts strength in moment capacity) will resist the thrust. I have attached drawings for this situation.

My questions are as follows:
- In calculation for the first moment, I do not agree with the idea to run the total point load (from the wind pressure loading of the rafter) along a straight line (that is perpendicular to the rafter being loaded) until it intersects the adjacent rafter. This proves for a rather large moment arm that creates a moment that 'does not pass the sniff test' in regards to how large it is for the scenario.
- In practice, is it common to design the bent beam to withstand a large amount of the thrust now apparent from the lack of any rafter / collar tie, in lieu of detailing the connection for the rafter to the wall?

Thank you in advance for any help in the matter.

 

[jerseyshore]

This is one of the more common things talked about here in the wood-framed residential space and I think most people will recommend against trying to resist significant moments with wood.

Is there a reason you can't put a structural ridge in? Not only is it a much better detail structurally, it's also much easier to install.

 

[the Paper Owl]

Jerseyshore,
Thank you for the response. I have opted out of using a structural ridge, as the entire loft is open (under the ridge) and the structural ridge beam would need to span 28 feet (exterior wall to wall). Additionally, for a majority of the ridge (roughly 24 feet) I am able to utilize a continuous floor joist below and satisfy thrust through rafter ties. The scenario as described above is for a 4 foot section where I am unable to provide either a rafter tie or collar tie.

 

[jerseyshore]

Oh, just a 4' span, you can probably avoid all of this and just resist the thrust at the outside walls.

In reality, unless you have a high snow load/ low slope roof, you can probably do nothing and it would be fine. People rip out their ceiling joists all the time for much longer runs.

I think as long as the wall has a continuous double top plate I wouldn't sweat it, it's like 3 or 4 rafters.

 

[the Paper Owl]

Jerseyshore,
Thank you for the insight. Aspects of this design are potentially over thought. With the mountainous lot that it is, it is my goal for it to be a fortress. A fortress within budget.

Enjoy your day.

 

[KootK]

Additionally, for a majority of the ridge (roughly 24 feet) I am able to utilize a continuous floor joist below and satisfy thrust through rafter ties.

Let your ridge board in that area be a flush ridge beam spanning 4' between the nearest rafter set with a tension tie and the gable end wall which likely can provide vertical support.

 

[DoubleStud]

I am not sure I follow your post completely. You said you are utilizing floor joists below, did you mean ceiling joists?

What material is the ridge board? Will it be able to span 4 ft if we count it as a ridge beam? Perhaps you just have to stiffen the 2 "trusses" between the 4 ft.

 

[the Paper Owl]

KootK,
Great suggestion. I like the simplicity of the LTT connection and the load path that it creates, thank you for the response. I'm sure the contractor will be thankful that they do not have to saw cut a few continuous plywood sandwich pieces to exact dimensions.


Doublestud - I do not mean ceiling joists, the second floor loft floor joists act as my rafter ties for a majority of the small structure. Each rafter set (at the non-gable wall) will be indeed doubled. Thank you for reading through my prompt.