Sunroom Addition

[grogannc]

I have a client that wants to install a sunroom on the back of an existing house. Its three sided with the back side being the house wall. Dimensions are 12'x 20'. They obviously want to maximize the glass and are showing the front wall with 6 sections of 12" width wall approximately evenly spaced. The sunroom would be supported on existing deck framing with kickers. They have proposed 6x6 timber columns with the outer corners continuous from the foundation to the roof. Its a low sloped roof, sloping from the rear existing house wall to the new sunroom front wall.

Its seems crazy to make them put in steel columns instead. Why can't I just secure(bolt) the roof to the existing house (hopefully the floor diaphragm), block the new roof diaphragm so that it acts as a cantilevered diaphragm and somewhat ignore whats going on laterally, nailing the sheathing like a portal frame knowing some credit will be gained from the 12" walls?

 

[KootK]

Why can't I just secure(bolt) the roof to the existing house (hopefully the floor diaphragm), block the new roof diaphragm so that it acts as a cantilevered diaphragm a

This sounds fine so long as you've got some place to dump the diaphragm tension and compression chord forces. Any chance the side walls are solid?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[grogannc]

One side actually does have a small storage room which replaces 7 ft out of the 12f. Theo other side is the same as the front wall, mostly glass. The storage room is about 4 ft wide and sits on a continuous footing. Not sure if its anchored but it is tied into the main house. They aren't opposed to removing drywall inside to tie things in so I was planning on coming up with some sort of connection to transfer the chord forces into the main home.

 

[KootK]

One trick that I've used in the past is to say that all of the roof framing members are "chords" rather than just the two boundary members. That gets the connection force per member nice and low. Maybe low enough that you don't need to do anything invasive on the inside.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Another trick I have used is that some of these sunrooms actually have a continuous 4 foot high wall running the 20 foot dimension on the outside wall that is sheathed with plywood, creating a low shear wall.

I cantilever 4X6 or 6X6 members off the 4' high wall taking the load to the low shear wall, and reducing the additional load seen to the existing structure.

Obviously will not work without the 4' high wall.

Mike McCann, PE, SE (WA)

 

[grogannc]

Those are both good ideas.

KootK - Do you linearly distribute the chord forces for the roof members, switching from tension/compression at midway (10 ft in this case)?

Mike - This one only has a 1 ft wall but I will at least tie the 6x6's into it. Should be worth something.

I've just started running numbers and the loads are so small on this, 65 plf at the roof or about a 400# point load at the top corner of the addition. Thanks for the help.

 

[KootK]

KootK - Do you linearly distribute the chord forces for the roof members, switching from tension/compression at midway (10 ft in this case)?

No. I would call that a plastic distribution (M/Zx). I go with an elastic distribution (M/Sx). In plan, the chord force connection diagram would look kinda like the shear diagram on a uniformly loaded, simply supported beam (two triangles). That was a mouthful. Let me know if I need to clarify.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[grogannc]

"No. I would call that a plastic distribution (M/Zx). I go with an elastic distribution (M/Sx). In plan, the chord force connection diagram would look kinda like the shear diagram on a uniformly loaded, simply supported beam (two triangles). That was a mouthful. Let me know if I need to clarify. "

This is what I was trying to say. We are on the same page. I just didn't do a good job wording it.

 

[KootK]

Nah, upon read, you worded it just fine.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.