lateral system alteration and code requiremets question

[maxwolf]

There's an existing, unreinforced masonry building, 4 stories, that was built ca. 1900. The perimeter masonry walls are load bearing, the interior is framed in steel with heavy cinder fill on terra cotta arched floors (155 lb/sf DL).

One of the exterior walls (5 wythe at 1st floor, 3 wythe at 4th) needs serious repointing (type O mortar, 350 psi, rated 'fair'). One proposal is to remove the exterior wythe of brick on this entire wall, parge and repair the backup remaining wythes, waterproof the wall, and then reinstall a wythe of veneer that is structurally disengaged (just cladding). Basically the wall looses a wythe of structural capacity. The wall on the other end of the building that this wall works in tandem with for lateral forces is left untouched.

The altered wall looks like it will pass dead and wind load combinations with the outer wythe removed per latest code, but seismic isn't even close to working. It never worked seismically according to modern code. I feel for grandfathered in buildings that one should never make them worse by altering them, even for load combinations that they were never meant to address when designed. Especially when you're talking removing 33% of shear capacity at the top of the wall, and about 20% at the bottom. Not to mention increasing the eccentricity between center of seismic force and center of resistance.

The counter argument is that the local code allows a designer to use 'old' code on old buildings, so removing a wythe of brick from a wall is 'ok' if the wall still works under the old code, which only considers wind.


Comments/corrections appreciated.

 

[JAE]

I've never heard of this "old code" used on old buildings. Is this something unique to this particular city/jurisdiction?

In most cases, if you invest more than about 20% of the building's worth, the code authority makes you upgrade the entire building to the current code. In other cases, doing moderate changes to a building requires the use of the current code applied to the work done on the building and not applicable to the entire system.

In unique cases, a meeting with the building official (and owner) to discuss options is warranted and this sometimes results in a reasonable approach, although for your case I'm not sure what that would be.

 

[wannabeSE]

I don't know your local code. However, chapter 34 of the IBC provides guidance on alterations that effect the lateral elements. In generel, the building may be altered if the demand to capacity ratio of any lateral element is not increased by more than 10% when compared to the original construction.

 

[maxwolf]

Thanks JAE and Wannabe,

The local code is New York City (2008 (current) vs 1968 (old code) vs 1938 (old, old code).

The 10% change limit rings a bell. I'll look it up in the am.

 

[hokie66]

Regardless of the code or law, replacing a wythe of load bearing brick with a brick veneer sounds like poor practice, and the structural capacity will certainly be reduced. Why not do the restoration work required to the existing?

 

[maxwolf]

Hokie66,

Sorry for the delayed response. It's two issues: the wall is right on the property line, and they have little money to work with.

 

[MiketheEngineer]

Money or not - your name will now be on it.

With the extensive changes proposed - it looks like a seismic upgrade is necessary. Maybe just reinforcing the existing steel will work??

BTW - I didn't think NY was in a seismic area?? Just not sure...

I would have thought wind would govern.

 

[SteelPE]

I the 10% limit is both in the IBC 2009 and IEBC 2009.

The IEBC also gives criteria for existing URM buildings. If I remember correctly, in certain cases they allow you to reduce the seismic loads applied to the building. They also allow you to test the masonry in place and figure out how strong the existing walls are. They even give the equations to get from the test data to allowable loads. Never done it, so I don't know how much of a PITA it is going to be.

If your code allows, you may want to look in the IEBC for further guidance.

 

[maxwolf]

Mike and Steel,
Thanks for the comments. I've advised them not to do it.
New York is a seismic zone, though rather mild. We just had a small quake a couple weeks ago that shocked a lot of people.

I found the 10% limits in lateral capacity reduction in the newer IBC but not the 2003 version, and not in the modified NYC version (which is based on IBC 2003).

A big thanks to all of you for the helpful input.