Rafter on ridge beam design

[Mixtli]

Hello, in the image below, we can see the model with support conditions. At its right you can see the result for self-weight analysis. We can see that Rafter001, as expected, only has vertical reactions. Therefore, the design forces will be V and M only, right? but for Rafter002 to 011, where we have now a ridge beam, the reactions changed, and now they also have Normal force, causing a reaccion outwards pushing the below support outwards.
Is there something wrong with the model? why, when we add a ridge beam there is horizontal reaction, and the ridge beam is bent outwards (towards -Y) ?
Thanks a lot for any input.
M.

 

[phamENG]

"Rafter 1" is modeled with idealized supports, whereas all the others are designed with more realistic supports. A ridge beam is not infinitely rigid. It deflects. When it deflects, that translates into compression in the rafter and thrust at the base. This is one of the reasons it's important to control ridge beam deflection. For most houses, the roof diaphragm will end up preventing noticeable displacement under service loads. But it's important to remember that this is taking place so as the scale gets larger you can recognize when you need to start accounting for it in discrete elements and connections.

 

[Mixtli]

Thanks Pham,
I remember reading BAretired and KootK posts explaining the there would be only vertical reactions because there was only vertical forces, gravitational, but can’t remember what they said about the ridge beam deflecting, wondering if the can hear me right now, haha.

 

[JAE]

Geometry!

 

[BAretired]

Thanks Pham,
I remember reading BAretired and KootK posts explaining the there would be only vertical reactions because there was only vertical forces, gravitational, but can’t remember what they said about the ridge beam deflecting, wondering if the can hear me right now, haha.

For a doubly sloped roof, the ridge beam deflects down and the eaves deflects outward.
For a singly sloped roof, the ridge beam deflects down while the eave deflects north.

There can be no horizontal reactions at the top of wall in most cases, because the wall is hinged top and bottom. This makes it equivalent to a roller support. If the bottom of rafter bears on a pinned support, there would be a horizontal reaction at each rafter.

Horizontal resistance is provided by the roof diaphragm and the minor axis of the ridge beam. Together, they limit the horizontal deflection by spanning from endwall to endwall.