Gabled Roof Thrust

[DCBII]

I'm working on a steel gabled roof design (see attached layout). At the ridge line there are steel ridge beams supported by steel columns. I did that to eliminate/reduce the thrust applied by the roof beams to the exterior CMU walls. I have an attic that acts as a rigid diaphragm (concrete over metal deck) immediately below the roof (also shown). This is a high snow load region, and under gravity loads (1.2D + 1.6S), my finite element model shows that there is a very large axial force in the beam (92 kips) caused by the inability of the walls to spread out as the ridge beam deflects. From the model's perspective, the walls are effectively tied together by the rigid diaphragm in the attic. I'm not sure how much I believe the model. It seems to me there could be some slip in the connection to the wall, and that the model is assuming too rigid of a diaphragm. I also have a flexible diaphragm in the roof itself which I think would help resolve some of this load.

How would other engineers deal with this? Would you design "horizontal slip" connections at the walls since the attic diaphragm can anchor the walls instead of the roof? Or would you recognize that this probably is a limitation of finite element modeling? Any other approaches?

Roof Layout

 

[driftLimiter]

Seems to me that the diaphragm is pretty stiff axially. In this case the hand calc would match the FEA in my opinion. If you detail the connection of the Steel to the wall to allow for slip (model one side as a roller and see the maximum displacement required and accommodate that) then you can justify this behavior. Otherwise your stuffing those beams between two CMU walls that are tied together rigidly I don't know how else you can get around the thrust. I have worked with some engineers who 'don't believe' the thrust exists but its pretty simple statics.

 

[LuK13]

What's the max vertical deflection of the ridge beam? What's the spacing on the columns that support the ridge beam?

Most of my experience in gable roofs is wood-framed. Deflection of the ridge beam is going to result in some rafter thrust, but it is usually negligible in the cases I deal with. Your attic floor sounds pretty rigid to me. What's the detailing where the rafter tails, attic floor, and CMU walls all meet? Are the CMU walls really going to be able to give a little bit? How much give do you have compared to the deflection of the ridge beam?

You can quantify a lot of this and then make a judgement call. Otherwise we are just kinda shooting from the hip.

 

[XR250]

The short CMU "parapets" may crack due to their lack of flexibility. I think trying to make a slip connection is pointless as it likely won't work or won't get constructed properly. Best to make the ridge mega-stiff to limit the spread or somehow tie the rafters into the floor diaphragm to reist the thrust directly.