Clipped hip dormer roof 1

[COEngineeer]

So I have this project with a lot of dormers. The dormers ridge beams are clipped at the front so I must use bent ridge steel beam. Do I have any other option? Do you think I can do wood moment connection? What shall I use? Let me try to draw it with the keyboad of the section of the roof.


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[structuralaggie]

No wood moment connection (always a bad idea), steel would be the best for what you have shown. Usually, though you can support the dormers on the rafters on either side of them. There should be a better solution. Isn't there a window where you are showing the steel?

 

[COEngineeer]

Here is the elevation. The thing is, if I use bent steel (W 10x12), I will have to connect this bent steel beam to wood. I hate Steel to wood connection. Any idea?

 

[structuralaggie]

Ok, much better picture. This thing is so small it would really be a shame to use steel. Does the ceiling follow the profile of the facia? If so, the hips can support the roof, and be supported by a tail bearing truss composed of rafters and ceiling joist located at the apex of the hips. Analyze with RISA or equiv. for stresses.

 

[COEngineeer]

I dont know if I follow you structuralaggie. But I have simmilar dormers around the house (13 total). Some are a lot larger than pictured. As usual, I didnt get Reflected Ceiling Plan but I am pretty sure it will follow the profile if possible.

 

[structuralaggie]

The one you showed looks pretty small, so if there is a ceiling you should be able to get it to work using wood only. If there is no ceiling, you will need steel. I would use a bent C5x9 (min.) located at the apex of the roof (same place as the wood truss indicated above).

I could post a picture, but I'm not sure how to do that.

 

[COEngineeer]

I think i got it.. I think steel is the way to go like my original plan but I will use W10x12 just because I have rafters hanging from it.

 

[shepherd]

I'd try to provide a post at the ridge/hip work point and support the post down to the ceiling joist or a beam in the ceiling space. That does assume you have a ceiling space.

 

[COEngineeer]

Yeah, cant do that shepherd. I swear I almost break my laptop. This architect is crazy. I am pretty sure none of the interior walls line up between upper, main and lower level. I cant put the columns straight down to foundation!

 

[dik]

One thing of concern is the ventilation of the space and maintaining your thermal envelope to prevent 'ice damming'.

Dik

 

[structuralaggie]

W10x12 will certainly work, but it won't fit in the ceiling/roof cavity unless you are using 2x12 rafters. The way you are proposing the bent beam will work too, but 90 deg. to how you are proposing is how I would do it, with a simpson hrc connector at the hips to the bent steel beam.

 

[COEngineeer]

Ahhh, I think I understand what you are saying now struct, So you use the bent channel matching the profile of the roof and then you put HRC connector at the top to do the clip hip? But how am I suppose to attach the HRC on to te bent steel? Pack the web?

 

[structuralaggie]

Yes, now I think we are on the same page. Bolt rafters w/ 3/4" diameter thru bolts @ 12" o.c. to the flat face of the channel - nice clean connection. You can pack the other side if you want, but you only really need to do one side. The HRC is good for small loads. If you get large loads, a sloped and welded contraption to the bent beam should be used.

 

[NKT]

Another way to approach this is to design the dormer roof as a self-supporting shell: analyze it as a 3-D truss (ridge and hip beams) and then design the diaphragm to carry the reactions. You may end up with blocked diaphragms at the dormers with this approach, but it would allow for a raised or vaulted ceiling.

 

[structuralaggie]

I agree that would work, at least for a little while. I don't ever count on a diaphram taking long term loading because of creep (unless it is really deep). Also, I have hard enough time getting residential contractors to block shear walls, let alone the roof.

COEngineer - Architects usually don't like steel. If you threaten steel or a flat portion of the ceiling, I bet he will take making the ceiling flat.

 

[COEngineeer]

structuraggie, I told my boss and he likes the idea, but if we do this, we are putting a couple more interior point load. Thanks!

 

[CJJS]

Why not just specify a pre manufactured truss? Looks like it would have to be a top chord bearing truss.

 

[COEngineeer]

CJ, based on the section cut, it looks like they want the ceiling follow the roof.

 

[NKT]

structuralaggie:
It does have to built properly and with a steep roof pitch to perform well. I always go with a redundant system for these designs: diaphragm design and strap connections at the corners each designed to handle the loads independently. I also insist on structural observation before going with this approach, to verify the blocking and nailing.

 

[COEngineeer]

How about NKT's idea and add collar tie on each side(3 total) and thru bolt them together?