Seismic Requirements for Glazing

Archive of Older Topics Archive - Specifications Discussions #6
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Hello fellow specifiers, sorry for the long question, but I would appreciate your input on a confusing topic for a Wisconsinite specifying seismic requirements.

Our firm has done a number of projects in Illinois and Virginia that required seismic requirements be included in the specifications. It has been a frustrating process since we have yet to find an AHJ that is conversant in the topic and the AIA/Avitru Masterspecs dont seem to have any clear and consistent language about seismic requirements.

(FYI we have often borne the brunt of contractors irritated that we specified seismic requirements, insisting to the Owner that no one does that and therefore the Architect has no idea of what they are doing and we are just doing unnecessary things that add cost to the project).

We feel that we have a fair understanding of most of the nonstructural requirements except those for glazing. My question is about interpreting ASCE 7-10, particularly 13.5.4 and 13.5.9. The code mentions glass in glazed curtain walls, glazed storefronts, and glazed partitions. It does not mention glass in doors, windows, borrowed lites, and similar smaller openings. If I interpret this literally, then the requirements only pertain to glass in glazed curtain walls, glazed storefronts, and glazed partitions and therefore it does not pertain to other glazing applications.

But I do see a flaw in my interpretation logic:
There is no mention of skylights, which would of course need to be carefully designed for glazing displacement.
There is no exception or definition in ASCE that supports my interpretation. In other words, there is no definition of a glazing size limit that the code does or does not apply to.

I asked a commercial curtain wall rep in Virginia and he put me in touch with one of their engineers. The engineer disagrees with my interpretation, insisting that it applied to ALL glazing (but he was not very strong or clear about his line of reasoning).

But, there is also no indication in ASCE that suggests that the requirements broadly apply to all glazing. Why do they mention glazed curtain walls, glazed storefronts, and glazed partitions if they meant it to apply to ALL glazing?

So, what is your interpretation?

And ultimately I am trying to determine whether these requirements should be specified in the Glazing spec, or within individual glazing framing sections. I lean toward the Glazing spec since that is where edge clearances and installation is controlled through delegated design. But Avitru Masterspecs are surprisingly vague and confusing about all of this. I would appreciate any opinion on this.

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Edward -

Two basic seismic design items are loads and displacements. I think your confusion lies with displacements. The curtain wall doesn’t rack by itself, per se, it’s anchors are being tugged in different directions on different floors by drift of the building structure.

You could feasibly have the same scenario with a skylight, or even a punched window with its head anchors attached to the panel / floor above. A million different scenarios are possible.

I’d look into AAMA’s documents for more glazing system specific information (yes ASCE confusing / more generically minded). Curtain wall manual as well as their test standards 501.4 & 501.6 (the .4 test standard might be the most enlightening).

3

Brett, thank you for the response. It is helpful in understanding concerns related to large framed openings.

But our confusion is related to glass in general - ASCE 7 addresses “Glass in Glazed Curtain Walls, Glazed Storefronts, and Glazed Partitions”. If taken literally, seismic design requirements do not apply to any other forms of glass, including borrowed lites, transoms, and full-glass entrances.

But according to the curtain wall engineer I spoke with, this interpretation is incorrect, suggesting that it applies to ALL glazing. In that case we are inclined to add language to our Glazing spec to require delegated design to prevent relative displacement.

But if this is correct, then we cannot help but wonder why such language does not already exist in the AIA/Avitru Glazing masterspec.

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IBC Section 2404 states that glass in glazed curtain walls, glazed storefronts and glazed partitions shall meet the seismic requirements of ASCE 7, Section 13.5.9.

The way I read it, it doesn’t seem to assign seismic requirements for other glazed conditions, not even glass railings or skylights.

5

I have forwarded this string to our writer who handles these sections.
However based upon the above conversation. the code is the code and the code references ASCE-7 as Ken points out. if the code does not mention something then it is not in the code. the engineer you talked to needs to provide you the documentary chain that includes the glazing you mention…or it is not covered…
ALSO there are provisions in the code for overhead glazing , the skylights would fall into this category I would think (I do not have my code book open at this time) also the code has provisions for glass railings in chapter 24 especially when above a walking surface.
Then what about glass in doors. there is no seismic issue there, no story drift etc it’s just a door. the code also deals with glass in doors. no need to invoke ASCE-7, glass relites, or borrowed lites, same facts as for doors. no seismic issue there either. Here in seattle where we do have seismic categories that worry the AHJs, I’ve never had a AHJ call out a door or railing or relite or skylight beyond those provisions in the IBC. As for ASCE-7 if you go digging you can really scare yourself about the design of partitions…now that is a whole other story. be well my friends.

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The only thing that matters is what the code says. ASCE 7 is a reference standard and any interpretation of what that standard means is irrelevant. What’s relevant is the language that directs you to ASCE 7.

In the IBC, Section 2404.1 applies to vertical glass, so the reference to ASCE 7 does not apply to sloped glazing or skylights–they are regulated by IBC Section 2404.2, which makes no mention of ASCE 7.

I have sent a request for a code opinion into ICC, but I’m pretty sure it will come back with the response that it only applies to “glazed curtain walls, glazed storefronts and glazed partitions” and that it does not apply to any other glass applications.

7

Now that I posted and reread what I wrote, the first sentence may not have come out the way I intended.

To explain what I meant by “what the standard means is irrelevant” is that you cannot look at the requirements in the standard alone–you have to look at what the code reference to the standard states as to how to interpret the standard. IBC Section 102.4 states “the…standards referenced in this code shall be considered part of the requirements of this code to the prescribed extent of each such reference…”

To explain the highlighted portion of IBC Section 102.4, if IBC Section 2404.1 just stated that glazed curtain walls shall comply with ASCE 7, Section 13.5.9, then only glazed curtain walls are subject to those ASCE 7 requirements, even though that section states that it applies to glazed curtain walls, glazed storefronts, and glazed partitions.

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Would IBC Section 1613 apply to direct you to ASCE 7?

“1613.1 Scope. Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports and attachments, shall be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7, excluding Chapter 14 and Appendix 11A. […],” 2015 IBC.

9

Hmmm, the ol’ catch-all phrase.

Yes, it would; but then it would need interpretation by the building official and would still likely lead to a literal interpretation of the application of ASCE 7, Section 13.5.9.

Since there are no IBC definitions provided for each of the glazed types mentioned, the IBC states that terms not defined “shall have their ordinarily accepted meanings as the context implies,” and I doubt transom lights, sidelights, borrowed lights, windows, skylights, etc. would be considered any one of those three. ASCE 7, Chapter 11 “Seismic Design Criteria,” has specific definitions for “glazed curtain wall,” “glazed storefront,” and “partition” (assuming a “glazed partition” would be a partition consisting of all-glazing and not a partition with glazed openings in it).

If the glazed element does not meet any of those definitions, then Section 13.5.9 would not be applicable. Where ASCE 7 uses the generic “glazing” term, then it applies to any type of glazing (limited to the context in which it was written).

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Perhaps this language inherently takes into account some differences between windows and curtain wall (and storefront). Generally, the glazed lites in windows are likely to be smaller than those in curtain wall. Windows would not span between floors. Windows are typically manufactured as a unit. Taken together, it would seem that they present less risk of glass coming out of the frame in an seismic event. I wonder if there is any testing around this concept. Maybe a manufacturer who makes all three products can weigh in.

11

Thanks Ron, I understand your comments and agree with the principle as it applies to those named glazed components. However, I’m going to disagree that those glazed components covered specifically in 13.5.9 are the only glazed components subject to the requirements in Chapter 13 in ASCE 7.

The IBC’s intent is clearly to direct you to ASCE 7 for resisting earthquake motions for basically everything attached to, and inclusive of, the building’s structure “including nonstructural components that are permanently attached […].” Chapter 13 of ASCE 7 addresses seismic design of nonstructural components. ASCE 7 addresses the scope of Chapter 13 as “[establishing] minimum design criteria for nonstructural components that are permanently attached to structures […],” which basically mimics the language in the IBC.

Section 13.5 in ASCE 7 covers architectural components and describes the general requirements including appropriate coefficients (13.5.1), and the forces and displacements (see 13.5.2) that all architectural components need to address in their design unless they are exempted from the requirements of the section by 13.5.1 (components supported by chains or otherwise suspended … meeting all the listed criteria). Once the section addresses those coefficients, forces, and displacements, the section goes further to define special requirements for certain architectural components (13.5.3 through 13.5.10; including 13.5.9 “Glass in Glazed Curtain Walls, Glazed Storefronts, and Glazed Partitions” as we’ve established). However, there is nothing that further exempts other applications of glass from being subject to the general requirements and scope of the chapter and section, nor does it exempt other nonstructural architectural components that don’t meet the exception in 13.5.1.

Note that Table 13.5-1 “Coefficients for Architectural Components” assigns coefficients to some specific components like “interior nonstructural walls and partitions,” “veneer,” “cabinets,” “laboratory equipment,” and, among other categories of components, there are some catch-all categories of “other rigid components,” and “other flexible components.” In this way they do address all architectural components. Also note that none of these components I’ve pulled from the table are addressed more specifically in the remainder of Section 13.5.

If I follow your implication that anything not addressed specifically in 13.5.3 through 13.5.10 is not addressed in the IBC’s reference to ASCE 7, and the scope of ASCE 7, then what about all those things listed in the table that don’t have corresponding headings later in the section? If the intent was that 13.5 only addressed the items with specific headings, then shouldn’t the list of coefficients in the table only correspond with those headings?

12

Dear Guest, you are on the edge of the rabbit hole. please be careful. I’ve been down this one, and I keep going in circles…until i wake up and come back across the “looking glass.” ASCE-7 can get terribly frustrating, especially when combined with the IBC.

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Guest: I didn’t say the rest of ASCE 7 doesn’t apply, what I was saying is that 13.5.9 doesn’t apply, which obviously has very different requirements than those for other architectural components.

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Marc, Thank you for the cautionary words.

Ron, Fair enough. You’re right that you never stated that the rest of ASCE 7 doesn’t apply. I suppose my mistake was interpreting your comments in terms of the original question about glazing in general and whether ASCE 7 applied, and my statement that IBC Chapter 16 requires compliance with ASCE 7 broadly in response to earthquake motions. I glossed over the specificity of your reply.

Edward, my interpretation of the applicability of ASCE 7 as referenced by the IBC would generally side with the engineer you spoke with rather than with a more limited reading of ASCE 7 Chapter 13. Essentially, that the code requires compliance with ASCE 7 even for the other areas of glazing that are not specifically addressed in ASCE 7, Section 13.5.9. Marc had indicated earlier that the engineer should provide the documentary chain showing the requirements apply. I would think the chain I outlined above would do that. However, I also agree with Marc that there are probably some things that you likely shouldn’t worry about because the seismic effects on them probably wouldn’t be an issue or pose much risk to public HSW.

The sticking point for me is, where do you draw that line? I’m assuming this is what Marc was cautioning me about with the reference to Alice in Wonderland. Obviously, requiring engineering analysis and design of every nonstructural component would be cost prohibitive. Even ASCE 7 exempts some nonstructural components in 13.1.4. The commentary (C13.1.4) notes that, “The exemptions are made on the assumption that, either because of their inherent strength and stability or the lower level earthquake demand (accelerations and relative displacements), or both, these nonstructural components and systems can achieve the performance goals described earlier in this commentary without explicitly satisfying the requirements of this chapter.” I read this as ASCE 7 acknowledging that you can comply with “the spirit” of the code without perhaps complying with “the letter” of the code.

My advice would be to read the commentary for Chapter 13, especially C13.1, C13.1.1, C13.5, and since your question is specifically about glazing, C13.5.9. That last one has some good information about four glass performance categories and whether they satisfy immediate occupancy and life safety performance objectives that I think translate well to other glazed applications. When you feel like you have a good handle on it, make a professional judgement call on whether you should design (or require the design of) the glazing in question to meet ASCE 7 criteria. This will likely differ from one project to another based on the Owner’s goals for continued operation and property damage, the building’s Risk Category, the location of the components in the building in relation to egress pathways and occupied space, the expected seismic forces, etc. As an example, tempered glass in a door or borrowed light may pose minimal risk even in a path of egress if the seismic loads would cause it to break and fall. However, panes of annealed or heat strengthened glass falling out of their framing, or from their attachments, from higher heights may pose a significant risk. Obviously the AHJ may agree or disagree with your call(s) and require strict compliance with the requirements of the code.

15

Stepping back from specific code language let us look at the issue.

Buildings and portions of buildings distort in an earthquake and when glass is subjected to these distortions it cracks. This is a reality no matter what the code says.

The seismic code provisions are focused on life safety and not on preventing damage. Thus you need to be more sensitive to falling glass or other problems that put individuals at risk during and immediately after the shaking. Some clients may wish to go beyond this to try to minimize damage in moderate earthquakes but that is a client decision.

If the portion of the building containing the glass is not subject to these distortions you do not have a problem. Thus pieces of glass in a solid door are not expected to be a problem since doors are not considered as being subject to the distortion although a glass door may be a separate concern but I do not believe this is normally addressed.

Skylights located above the roof diaphragm will likely not see much if any distortion due to earthquake shaking. In some situations laminated glass can help you avoid problems with falling glass.

You will need input from your structural engineer since he or she will need to report the code drifts that must be accommodated. The structural engineer should be able to help you sort out the issues but recognize that any involvement beyond providing the drifts should be considered an extra service.

If your local engineer is not conversant hire a California Structural Engineer. Window wall manufacturers /contractors who deal with projects in earthquake country should be a valuable source of information and in some situations may know more than the structural engineer.

The reason that I mention west coast engineers is because they deal with these issues regularly.

For many windows these drifts can be accommodated by requiring the glass be installed in their frames with space to accommodate the racking of the frame. This also means that the spacers used to hold the glass in place needs to be located in the center of the edges and not at the corners.

Window walls and situations where the drifts cannot be accommodated in the frame require that structure around the frame be designed to accommodate the drifts. This may mean that movement joints being located in ways not contemplated by the designers. In addition the size of the joints may need to be larger than the designer would prefer. This is not just a specification issue. The joint material also needs to be able to accommodate the necessary movement without binding up, which may mean the physical joint being larger.

Once you have figured out what you should do then you need to look at the letter of the code to make sure you can say you are code compliant.