Adding on to an existing structure (wood frame construction)

[CEmonkee]

I’ve been contacted by a potential client about doing the engineering for an addition to their home. I have not yet made a site visit, but I know a few facts:

(1) The house is single story and built prior to 1950.
(2) The construction is wood, the foundation is post and block.
(3) The roof is stick framed.

Most of my experience is with new construction and small remodels (e.g. taking out walls, etc.).

If possible I would like to avoid getting into analyzing and retrofitting the existing structure. My thought was that the roof diaphragm could be treated as two separate diaphragms (one over the existing structure and one over the new addition). Please see the attached image file. The existing exterior wall would then become an “interior” shearwall (the red line in the sketch), and I would need to beef that wall up for the increased loading from the addition.

I would appreciate any comments on the above, as well as any advice anyone might have based on their experience with similar projects.

Thanks!

 

[msquared48]

Do not put any additional lateral or vertical load to the existing structure. Let the addition be self supporting. This should make the project simpler.

Mike McCann, PE, SE (WA)

 

[KootK]

I agree with your assumptions CEMonkee. While analysis would be simpler if the addition is isolated from the existing building, I feel that performance would be better if the buildings were tied together laterally. Your average homeowner isn't going to install a proper movement joint in their roofing and walls at the interface.

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.

 

[manstrom]

Just beef up the shear connection between the two structures.

If it is 1950s wood, you probably have 1x4 diagonal siding. Not bad in shear, but not great. Slap a layer of osb or thermoply over that and nail as required for shear.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[CEmonkee]

Hi All - Thanks for your replies and suggestions, I appreciate it. I will probably have addition questions once I make a site visit in a couple of weeks.

 

[msquared48]

Unless you separate the two structures, you may have to bring the original house up to current code. Check with the jurisdiction to see what they require. Seen this scenario way too often here.

Mike McCann, PE, SE (WA)

 

[CEmonkee]

Hi Mike - I don't think the client will want to go to the expense of bringing their entire house up to code...

Is there a good source for construction details for adding on to a home and keeping it structurally separate from the existing building? I can analyze the new addition for wind and seismic loading, but I'm not an architect or builder, and I'm not sure what the connection points to the existing structure would look like if it's going to be a completely separate structure.

Any advice or references you could provide would be most appreciated - thanks!