Lateral requirements for shrinking a building 2

[milkshakelake]

I have a 100' long building that's being shrunken down to 60' due to zoning issues. Generally when I extend a building, I know that ASCE 7 requirements must be met for the entire building for 10% or more addition. But if I shrink it, does that still apply? The goal is to keep the existing front facade system and not have to check or reinforce it, since the loading is decreasing. I am 99% sure the existing facade will not pass whether I keep the building or reduce the load, but we are making it better by reducing the load.

Reference: ASCE 7-10. The wording of the code makes it pretty unclear about how to approach this:

 

[Harbringer]

You need to demonstrate compliance with Exception 2 AND 3.

I assume you are removing elements of the LFRS to do this. If your LFRS elements are equally spaced this may not be an issue, if they are more concentrated at one end you may be increasing loading to certain elements. A plan layout would help.

 

[milkshakelake]

Sketch below. It does comply with the exceptions, but that whole section is about "additions," which is confusing me.

 

[Greenalleycat]

I'm assuming that the system you are concerned about is the Brick LFRS on the left side?
I'm in New Zealand so obviously completely different jurisdiction etc, so my thoughts are more academic than anything on this one
Under our Building AcI would be required to check that the alterations comply with the following:

A building consent authority must not grant a building consent for the alteration of an existing building, or part of an existing building, unless the building consent authority is satisfied that, after the alteration the building will,—
(i)
if it complied with the other provisions of the building code immediately before the building work began, continue to comply with those provisions; or
(ii)
if it did not comply with the other provisions of the building code immediately before the building work began, continue to comply at least to the same extent as it did then comply.

So (ii) would be operative here as we are assuming it doesn't currently comply
There is guidance that clarifies this provision to be a holistic review - e.g if a single bolt gets more loading that is fine as long as the overall performance of the building isn't impacted

Applying this to your case, I agree from the info provided that it's unlikely that this makes the facade perform any worse than it does currently
Hence, it would pass a 'Section 112 check' under our Act and I would be satisfied to carry on

 

[KootK]

I can get behind the use of 11B.3 as, effectively, a negative addition so long as the exceptions are met.

On balance, however, this approach doesn't leave a great taste in my mouth. Assuming that this is masonry of some sort, the work proposed will involve:

a) Disassembling and reassembling the masonry per your specifications.

b) Disconnecting the diaphragm(s) and reconnecting them per your specifications.

c) Reconnecting the wall to whatever foundation it will now rest upon per your specifications.

That just feels like you exerting a whole lot of design and construction control over an element that you ultimately intend to punt on as "existing with a history of satisfactory performance".

 

[Harbringer]

Sketch below. It does comply with the exceptions, but that whole section is about "additions," which is confusing me.

It's referencing additions AND alterations. Removing 40' of building would be an alteration so the section still applies.

You are removing LRFS in the longitudinal direction also right? Your capacity in that direction may decrease faster than your demand decreases (especially if you have few openings in a shear wall on the 40' segment). You need to actually run some numbers.

I'd look to show that the lateral capacity to any one line (longitudinal or transverse) has not decreased more than the demand has decreased. If this is not true then you need to perform upgrades.

 

[HTURKAK]

When you look commentary ; C11.1.2 Scope.
..
ASCE 7 is not retroactive and usually applies to existing structures only when there is an addition, change of use, or alteration. Minimum acceptable seismic resistance of existing buildings is a policy issue normally set by the authority having jurisdiction. Appendix 11B of the standard contains rules of application for basic conditions. ASCE 41 (2014) provides technical guidance but does not contain policy recommendations. A chapter in the International
Building Code (IBC) applies to alteration, repair, addition, and change of occupancy of existing buildings, and the International Code Council maintains the International Existing Building
Code (IEBC) and associated commentary.
.....
I agree with Harbringer (Structural) and a magenta star for his post.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[milkshakelake]

if it did not comply with the other provisions of the building code immediately before the building work began, continue to comply at least to the same extent as it did then comply.

This is exactly what I was thinking, but the code you mentioned puts it more eloquently. Thanks! I can actually use this code to justify it, even if it doesn't apply to my area. As much as I complain sometimes to my colleagues, the regulators here do look at precedents set by others.

That just feels like you exerting a whole lot of design and construction control over an element that you ultimately intend to punt on as "existing with a history of satisfactory performance".

I would make the new LFRS in the rear out of CMU, concrete, or some kind of modern material. Reconnect the existing flexible diaphragm to it. Design it for half the seismic/wind load and call it a day. I've done this kind of thing before with additions, so I think it has merit. With additions, the only difference is that I add extra shear walls to reduce the load on existing LFRS, which I'm not doing here.

You are removing LRFS in the longitudinal direction also right? Your capacity in that direction may decrease faster than your demand decreases

I didn't think of that. Need to upgrade my brain. Thanks for the thought! Will definitely check it.


@HTURKAK

Thanks for the response. In my jurisdiction, they use the ASCE 11B.3, so it has been adopted into policy.

 

[KootK]

I would make the new LFRS in the rear out of CMU, concrete, or some kind of modern material.

Ah. I mistakenly thought that you intended to port over the existing backup / shear wall in addition to the facade and were attempting to avoid a real design of the VLFRS there via that strategy. If the the relocated facade is getting a newly designed VLFRS behind it, that certainly works for me.