2nd Story Addition on two wythe masonry house

[mfstructural]

I wanted to get everyone's thoughts on adding a second story on the existing residence. It's a two wythe 8" masonry wall unreinforced on 10" concrete foundation walls. I understand we'll have to verify foundations but has anyone ever added a second story to a residence like this? In the past I've been on the fence and have generally recommended that a wood framed wall be added on the inside of the masonry to support the second story. this of course drives up costs. One option is to expoy a threaded rod through an 8" sill plate into the top of the existing wall. The shear will be transferred through that connection. I've attached some photos.

 

[SWComposites]

Seismic area?
High wind area?
What level of 2nd story loading?
Snow load?

 

[mfstructural]

It's in northern Illinois. Wind governs, snow load about 18psf. residential loading on second story

 

[mfstructural]

Just following up to see if anyone else has had experience with this.
Thanks

 

[driftLimiter]

I'm not really familiar with low seismic areas. But epoxy into unreinforced masonry isn't worth much. Frankly I would go with your first option and avoid using the unreinforced to support the second story.

 

[JAE]

Some things I'd do before committing to a second story:
1. Full visual investigation looking for any evidence of foundation settlement, cracking, damaged mortar spalling, efflorescence.
2. Survey or measure the building masonry walls to determine accurately if there's any wall bowing or other geometric displacements - both inside and outside wythes.
3. Possibly (but not mandatory if difficult) sample the masonry to get a lab to test for f'm masonry strength.
4. Verify basement walls are competent with no cracking, bowing, etc.

5. Finally, I'd do some initial calculations to see what sort of loading would be imposed on the masonry walls and compare that with a derived masonry wall capacity. The photo doesn't appear to indicate any header courses tying the two wythes together so it may be that these two 4" wythes are working independently of each other, which reduces the capacity of the wall compared with the wythes tied together. There may be cross wire ties but the age of that house suggests it might be before wire ties were commonly used. Another reason to take out a masonry prism for testing - see if there's any evidence of wire or ties.

Just some initial ideas.

 

[milkshakelake]

I think this is highly dependent on the standard practice and the building department in that area. In my area, you definitely can't just add another story. It needs to be designed for wind/seismic for modern codes. But there are other areas where the building code is more of a suggestion than anything else. For wind/seismic, unreinforced brick walls have minimal tension capacity for resisting overturning, and adding more on top would worsen the issue. Besides the gravity load issues mentioned above, the lateral will kill most of these "adding on top" ideas. I like the idea about adding a stud wall inside it.

The way we do it here might be overkill for you. But we add a bunch of shear walls to take 100% of the lateral load.

 

[Greenalleycat]

I'm with milkeshake on this one, though I live in a high seismicity country so you could never consider adding the second story without new structure

 

[milkshakelake]

I'm in a low-ish seismic area (usually seismic design category B) where a lot of these don't work out numbers-wise. But I think for very low seismic and a lenient regulatory environment, there's a chance that the numbers might actually work out with a simple addition on top. Though wind overturning is still an issue.

Talking realistically: There are much worse designed brick buildings that are much taller, and they've survived hurricanes. But the second you touch it and pass the 10% alteration limit in code, it triggers a whole bunch of calculations and design requirements.

 

[mfstructural]

Thanks for the input. From a practical standpoint, I agree with milkshake. I see a lot of two wythe buildings unreinforced that are two to three stories tall and still standing, but performing analysis with todays building codes is another issue. I don't personally like the idea of a second story on top of the brick without some sort of reinforcement or additional structure at the interior of the first story.

 

[Greenalleycat]

For us, this would just fail a whole bunch of more basic requirements such as no brittle failures etc - even if it theoretically calced out
There is also the big risk of degradation etc - bricks have durability concerns that are harder to QA than other materials
Also, the new structure has performance expectations in terms of life that the supporting structure has to meet
For us, that's 50 years basically - is there another 50 years in that brick? How do you maintain it if it needs it?

 

[RPGs]

I've done several of these 2nd story additions on multiwythe unreinforced masonry--but I am also in a very low seismic area.

It makes you no friends around here, but I usually add the interior wood-framed walls with a similar design intent that milkshakelake has. Chasing that load path down is always interesting.

 

[mfstructural]

RPGs...so you are saying you set the second story on the masonry or on the added interior wood framed walls? I'm in the Chicago area, low risk seismic. I just received a call today from someone looking to qualify a foundation for additional second story addition loads. I was sent the drawing and the architect has new second story floor trusses bearing on the interior wythe of the two wythe wall, which here is unreinforced 4" CMU. They are calling to replace the top course and anchor a "J" bolt to the top course. I'm not sure how they plan on doing that with 4" CMU with a very narrow cavity.

 

[Greenalleycat]

I don't know American codes but I'm assuming 'low risk seismic' just means you have a low PGA for your seismic calculations: the actual risk profile is building dependent
The existing construction is already high risk, let alone with adding a second story on this thing
Even if the numbers 'calc out' you want to consider what happens when you get a slightly larger than expected EQ
As a spoiler alert, I live in a city that used to have a lot of brick buildings...we don't have them any more

 

[RPGs]

I've only done them where I aligned the new 2nd floor platform and exterior wall with my new 1st floor interior wall. The 1st floor masonry then becomes a "veneer" (in a loose sense of the word), which is then tied to the interior wood frame wall with simpson helical wall ties and I detail a little roof eave over the masonry at the 2nd floor platform level. Essentially I specified a house within a house.

Yes, that sounds sketchy. I'd probably want that top course to be a bond beam (maybe up it to two courses) the full width of the wall, but I don't know how to ensure the two wythes are acting together below that. Sounds fun...

I saw a LinkedIn post once for a 2-3 story mass timber addition to an existing 5-6 story steel-framed building with multiwythe brick exterior walls where they installed a concrete cap perimeter beam on top of the masonry to build up from. I thought that was sexy and an ideal approach I'd like to use someday, but not sure it's in the budget for most small projects.