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2 story addition to existing single story masonry residence

2 story addition to existing single story masonry residence

2 story addition to existing single story masonry residence

Having reviewed a lot of posts in this forum I have not yet seen another example similar to this.

I have attached a very simple plan view of what the Owner wants to accomplish. This is in California, Seismic design cat D, existing one story masonry house, 8" cmu partially grouted cells(all of the info I know about the cmu), wood framed roof. Due to situational requirements on their property the owner wants to demo less than 1/3rd of the existing house and in that place build a new wood framed 2 story addition.

For purposes of lateral analysis, if I consider this as one single area then I do not see any way that this could be justified by current code. If though, I consider the existing 1 story house as Area A, the the addition as Area B then I think that I might be able to justify the addition. My thoughts are as follows:

1. The addition, Area B has independent lateral resisting elements in 3 of the 4 sides.
2. The existing house, Area A also has independent lateral resisting elements in 3 sides.
3. Both areas share a common line of lateral resistance with the new wood framed interior shear wall.

My questions are as follows:
1. Due to this design method would the existing exterior masonry walls be required to be tied into the roof diaphragm for out of plane forces? This would open a Can 'O' Worms.
2. Other thoughts.

Thanks in advance.


RE: 2 story addition to existing single story masonry residence

I would ask this in the structural forum to which gets more traffic

RE: 2 story addition to existing single story masonry residence

Yikes. That's a lot of new (lateral) load onto an existing wall. What is known about the existing building? What reinforcing?

Are you trying to combine the new timber wall and the existing CMU into one seismic system? You're probably better off introducing a new CMU wall.

Otherwise, this is doable. Just complex analysis and a pain to detail.

RE: 2 story addition to existing single story masonry residence

CEL, that is what I thought. I am about 90% sure that the existing masonry is partially grouted cells with #4's in the grouted cells. Not sure of the spacing, but since they are not fully grouted here in California in Seismic Design Cat D they can't be used as a lateral resisting element.

I think that the owner and Architect will have to re-design this addition as to basically be a separate structure right next to the existing.

RE: 2 story addition to existing single story masonry residence

I know most (all?) new masonry shear walls in California are fully grouted. But I think the only requirement for fully grouted special reinforced masonry shear walls is for stack bond. If the new addition is wood shear walls, most of the load in the y-direction may end up in the stiffer masonry walls. The first thing to find-out is whether or not the existing structure needs to meet current structural code if one shear wall is completely removed and replaced in a new location.

RE: 2 story addition to existing single story masonry residence

If you're trying to treat the new building as a separate entity you'll need to ensure that a proper structural separation is maintained. If there are only 2 stories the required separation will likely only be a few inches. If there are 4, you might be getting into a bigger challenge.

As noted above, don't mix SFRS with different stiffnesses. If you can separate the new building, use all wood. For the existing, add another strip footing and masonry wall.

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