Shear walls on small three seasons room addition w/ all windows

[mfstructural]

I'm designed a small three seasons room addition to a house and had a question regarding shear walls. The addition is 20'x9'x9'(H), see attached files. There are windows around the perimeter of the room. At first I was thinking of running headers below and over the windows(between the posts, so having posts go from foundation to underside of roof, so there would be a double 2x at top of the wall as well) and using a Simpson corner post cap to provide rigidity at the corners. If it's a rigid structure I was thinking it would want to rotate like a couple (see last image attached). For wind load of 20psf this is a T=C=(20psf*9'*4.5'(H))/20'=40 lbs. Does this seem like a reasonable approach? for a 40 pound load, some lags or anchors between the addition and existing structure will be adequate.

The other option is to provide a shear wall or steel frame at the 20' long wall. The architect is asking what is the minimum length of wall we need at the corners for shear. Right now it's approximately 1'-5" from corner to window. I have not run any numbers but there are many windows. From a practical standpoint, there is a similar three seasons room there now with all windows with no evidence of movement.

Just trying to figure out the best most economical way to approach this.

Thanks,

 

[phamENG]

I've done cantilevered wood columns on a covered screened porch, but that's it. I wouldn't be too fond of it for something more closed in like this. Screens can handle a little more sway than a window can if they don't get the install just right.

Eng16080 - OP mentioned the owner wants the flexibility to add a second floor to this in the future.

 

[XR250]

What I have done on a few previous jobs is provide shearwalls under the windows and cantilever columns through to provide the fixity.

 

[Eng16080]

Thanks phamENG. I did in fact miss the part about a potential 2nd floor being added later. Considering that, I suppose I would want some form of reliable lateral resistance at the outside wall. As much as I hate to say it for something so minimal, I would probably recommend a steel moment frame. I would explain to the owner that it's only necessary because of the potential 2nd floor.

I would at least run the numbers on the cantilevered column approach, accounting for potential future loads from the 2nd floor. It probably won't work, but is worth checking.

 

[Eng16080]

If seismic loads are a concern here, be sure to check the associated deflection of any cantilevered columns.