Using ASCE 41-13 to evaluate Unreinforced Masonry

[JIMEY]

Hello,

I am using ASCE 41-13 to evaluate unreinforced masonry, and I am a bit confused about the performance objectives of it. Section 15.2 contains a "Special Procedure for Unreinforced Masonry", so that was where I started. The very first paragraph explains that this procedure only satisfies a Reduced Performance Objective. Specifically, it can be used for a Collapse Prevention Performance Level (S-5) at a BSE-1E Seismic Hazard. So to me this implies that if the goal is to meet the Basic Performance Objective for Existing Buildings (BPOE), then this procedure is not applicable. Right?

But then I checked out the Tier 1 evaluation, and there is a checklist (section 16.16) for unreinforced masonry which apparently can be used to meet a Life Safety Performance Level, and another checklist that can be used to meet an Immediate Occupancy Performance Level. So I would just like to know why Section 15.2 says that URM can't be relied upon for Life Safety, but then the Tier 1 evaluation appears to totally disregard that. Doesn't make any sense to me, but perhaps I'm missing something...

 

[MrHershey]

Correct. As you note, 15.2 is trying to meet a 'Limited Performance Objective', which is less than BPOE. Take a look at 2.2.3.

This is not to say that URM can't meet BPOE (Life Safety performance level), just that 15.2 cannot be used to qualify it as meeting this performance level. To do this you'd have to use the tiered process instead.

15.2 is an alternate procedure to the tiered process, they're totally separate from each other. You'll note that Chapters 4-6 and the checklists in Chapter 16 aren't referenced in 15.2, other than a quick reference for evaluating the condition of the materials. It's intended to be a wholly separate process.

 

[JIMEY]

Oh, I think I understand. So let's say I wanted to evaluate a URM building to a Life Safety performance level. The only route available to me would be to do a tier 1 evaluation, and if there are any deficiencies, proceed to a tier 2 evaluation (and possibly a tier 2 retrofit). But if deficiencies still exist, I am out of options because there is no tier 3 method for evaluating URM. Correct?

Also, what if I want to check it for a Seismic Hazard level higher than BSE-1E? Is this impossible because both tier 1 & 2 only evaluate to BSE-1E? It just seems strange to me that every other material has the option of a tier 3 evaluation except for URM. but maybe they just haven't gotten around to developing one yet.

 

[MrHershey]

Not sure why you'd think you can't go Tier 3 for URM. Chapter 11 is mostly Tier 3 modeling criteria and acceptance criteria for masonry and there's tables in there for both URM and reinforced masonry. Have never really dug in to doing it myself, but certainly seems it's possible just like any other system.

Tier 1 and Tier 2 procedures are by definition at BSE-1E level. See 4.1.2 and then 5.2.1 (which just references 4.1.2). I believe the only way to really look at something higher would be to go straight to Tier 3.

 

[JIMEY]

Haha. For some reason I failed to realize that URM was also covered in chapter 11. I went straight for chapter 15. Thanks for the help! I think this makes sense now.