Sun Room - Moment frame

[TORCHMAN]

This seems like an example of a house extension that requires a moment frame. It is a small extension though and I am not sure if you think it is so small that it doesn't require one.

The way I analyze it is that it is braced to the existing structure on the north end so that wind from the east and west direction on that end are not a concern and wind from north and south directions are not a concern anywhere. What I am concerned with is wind from east and west direction on the south end.

Do you analyze it in the same way? Or do you find that the roof diaphragm would be strong enough to bring all lateral forces back to where it is braced back to the house? How would you check that?

Pictures attached:

 

[phamENG]

A few things:

-You're adding load to the existing structure. Have you checked to ensure that it can handle it?

-In the N-S direction, you still have to be concerned. With wind blowing S->N, your addition is in compression and as long as everything is properly braced, probably no issues. But you have to make sure you have a collector that can deliver that load to the existing house and the existing house has framing that can receive it and deliver it into the diaphragm. If wind is blowing N->S, you'll have suction on the end wall that will pull your addition away. Now you need a tension load path in your collector back to that framing in the existing house.

-In the E-W direction, you have a few options.

A) Assume a flexible diaphragm and put half the load on a new portal frame and half the load on the existing. Can the existing take it?​

B) Assume a rigid diaphragm with 3 sided support (check requirements in SDPWS to ensure you qualify) and dump it all into the new portal frame. I'd still suggest ensuring that there's sufficient capacity in the existing as it will be attached, and you'll probably find that the 3 sided deflections will create an unacceptable drift/displacement at the junction between new and existing. But at least there's a load path.​

C) similar to B, but forget the moment frame. Instead. put all of the shear into the existing and then cantilever the diaphragm using chords that tie back into the house. Might mean a little more invasive work in the house, but I've used it before with good success to give somebody that super open, freestanding sun room look that seems impossible.​

 

[TORCHMAN]

You're right phamENG, I will check the existing structure.

In the E/W direction, option A was how I originally thought of it.

I am interested in option C. What would that connection of the diaphragm chord to the existing structure look like? You say it may mean a little more invasive work, what have you done in the past?

 

[phamENG]

Depends on the existing construction. Trusses? Vaulted Ceiling? I've been able to grab a second floor diaphragm, sort of like the tension ties on a deck. Those are the easiest, but it involves opening either a floor or a ceiling and some modification to the diaphragm.

If it's roof on roof, then you have to get the load up into the rafter, I've doubled rafters before, but...if diaphragm nailing is the primary issue then you're pulling shingles off to nail off to your new chord. So there are ways to do it that will give the owner what they want, but they can be expensive.

 

[XR250]

Hard to say without seeing the existing structure. Bordering on moment frame for me.
Also, I would be careful about trying to pick up the ridge load on a transverse beam adjacent to the house without managing the differential deflection between the two. I avoid this detail if all possible.

 

[SE2607]

I would see no problem transferring the load back to the existing structure if there was room for a shear wall along the wall with double doors. You have to take out the moment via rotation. Also, neglecting wind load perpendicular to the existing structure is, IMO, unconservative. Think leeward wind component. Also, seismic loads perpendicular to the existing structure.

You might consider steel cantilever columns in lieu of frames.

My two cents.

 

[Tomfh]

What sort of deflection limits do you guys adopt for support of non structural glazed walling? Glazed walls can be so stiff, I wonder how we ensure they don’t actually carry all the shear? How much shear deflection can they take before they fail?

 

[phamENG]

I can't vouch for these numbers, but somebody in this thread (thread507-449680) mentions H/100 for water tightness and H/66 to prevent glazing from falling out. For a 10ft sunroom, that's 1.2 and 1.82 inches, respectively. So...the drywall will be powder before the seal on the window fails.

I usually try to keep my buildings to H/500 for wind serviceability (10 year MRI)...I'll let houses drift more if they are all 'soft' finishes and siding. But I try to put a hard limit at 0.4" as testing has shown that to be the threshold where drywall starts failing.