Seismic Design of Mid-Rise Buildings with Heavy Rigid Floors and Light-Frame Walls

[bones206]

I read this document the other day: NEHRP Seismic Design Technical Brief No. 12 - Seismic Design of Cold-Formed Steel Lateral Load-Resisting Systems - A Guide for Practicing Engineers.

The following paragraph from that document seemed to suggest that concrete slabs/planks supported on light gauge framing may be fundamentally not complying with the IBC:

Concrete-filled deck and concrete plank floor systems are also used in CFS construction and may be supported directly by CFS load-bearing walls without repetitive joist or truss framing. The weight of these systems increases seismic demands considerably. In addition, this type of system does not comply with the definition of light-frame construction provided in Chapter 2 of the2015 International Building Code (ICC 2015). Therefore, careful consideration should be given to the selection of the SFRS for this type of structure. See IBC SEAOC Structural/Seismic Design Manual (SEAOC 2012),Volume 2, Example 3 for additional discussion.

For reference, this is the definition of light-frame construction in IBC 2015:

LIGHT-FRAME CONSTRUCTION: A type of construction whose vertical and horizontal structural elements are primarily formed by a system of repetitive wood or cold-formed steel framing members.

Considering how prevalent light-frame bearing/shear wall with heavy concrete plank or slab-on-metal deck buildings are in North America (i.e., hotels), I was surprised to see this typology may not have a solid code-basis.

I don't really have a specific question - just wanted to bring it up for discussion. It seems like the rigid wall/flexible diaphragm (RWFD) building type has been getting special attention in recent years related to seismic behavior (FEMA P-1026), which lead to new detailing requirements in ASCE 7-22. Maybe the inverse (heavy rigid diaphragm/light-framed wall) type deserves more attention as well. This is probably applicable to CLT floors on light-frame wood walls as well, which is becoming more popular.

Any thoughts here? Is this something where the codes need to catch up on building trends and do some testing/research to ensure proper seismic performance? Or is it just a matter of slightly editing the code definition of light-frame construction to include concrete diaphragms so we can continue merrily on our way?

 

[Deker]

Can you elaborate on why you think this type of construction doesn’t comply with IBC? I’d think the only issue is that you wouldn’t qualify for some of the exceptions in the code that are allowed for “light-frame construction.”

 

[bones206]

I'm not sure that I have a fully formed opinion on this, but the NEHRP authors seemed to go out of their way to make a point about this. They didn't really come out and say it's non-compliant with code, but they seemed to imply that maybe it wasn't in the spirit of the code? Maybe I'm misreading the whole thing, but it sparked my curiosity when I read it.

ASCE 7 Chapter 14 does reference AISI S400 for seismic design of CFM systems, and that document explicitly allows for concrete floor diaphragms, so there doesn't seem to be an issue there. In theory that completes the code path.

Perhaps ASCE 7 seismic provisions written with the unspoken assumption that the light-frame systems are 100% light-framed - both walls and diaphragms. Intuitively, the difference between a massive, rigid floor and a light, flexible floor seems pretty significant from a seismic response point of view. In the former, there is a large concentration of mass at the floor levels and probably all the energy dissipation is going to occur in the light-framed shear wall elements. In the latter, the structural mass is uniformly distributed between floors and walls, and the floors and walls will likely both contribute to the energy dissipation. I can imagine those two scenarios could have quite different R-values.

 

[dold]

I don't think the authors are saying that it doesn't comply with the IBC, just that this system might not qualify for certain R-values, just as you've explained in your most recent post.

Perhaps some of the structures that have been designed using this construction and an improperly selected R-value would be "non-compliant" with the IBC though.

 

[Craig_H]

A little over a year ago, I came across a 4-storey residential building composed of concrete hollow-core plank on light wood framed walls. Many things about the building struck me as odd. It is a combination of materials that I feel is fundamentally flawed.

In my case, the building had significant lateral load resisting deficiencies. That said, it was apparent that the building had not been designed with an explicit lateral load resisting system. None of the interior load bearing walls were shear walls, and as such, all that was left were the little pieces of exterior walls between walls and windows.

I imagine that it may be possible to make these systems work with careful design and beefy corridor shear walls, but I do have reservations. Progressive collapse under a fire situation seems more likely when compared to other systems. I also imagine challenges resulting from trades being unfamiliar with the detailing that is required due to the dead load being a multiple of what they are accustomed to.

In the end, why the desire to combine a heavy floor system with a light wall? I just don't see an advantage.

 

[KootK]

Intuitively, the difference between a massive, rigid floor and a light, flexible floor seems pretty significant from a seismic response point of view.

That is my suspicion as well. And I do feel that additional research on this building typology is probably warranted.

Frankly, I do not have high expectations for these buildings as far as seismic resistance goes. In the developing world, a block and plank building has proven to be one of the last places that one wants to be during an earthquake. In many respects, I have a hard time seeing a CFM and plank doing all that much better. In both cases, you get monster inertial diaphragm forces and marginal continuity throughout the lateral load path unless the designer is really on the ball and willing to upset some of the other intere$ted parties.

In the hands of a skilled designer, CFM and plank will kind of morph into CFM and pseudo hot rolled steel. That, in the sense that you usually have to start using strap braced shear walls, often with HSS as the chord elements. The trouble that I see with this, in practice, is that CFM still tends to beget light duty CFM connections in a lot of places throughout the lateral load path where you would otherwise tend to have gobs of natural capacity if it was truly a hot rolled steel system.

I came across a 4-storey residential building composed of concrete hollow-core plank on light wood framed walls. Many things about the building struck me as odd. It is a combination of materials that I feel is fundamentally flawed.

I feel the same. On an intuitive level, I feel that one ought to generally move from weaker to stronger systems as one moves down the load path and collects more load. This is kind of the reverse of what we do at the foundations where we often transition into earth materials softer and weaker than our construction materials.

Yes, anything can be designed with the right amount of design attention. It's just that, when you violate this principle, you tend to wind up needing a lot more design attention to get it all buttoned up.

 

[dold]

In the end, why the desire to combine a heavy floor system with a light wall? I just don't see an advantage.

Reduced ceiling depth is the main reason when I've seen these types of schemes proposed. Not a huge fan.

 

[bones206]

I'm guessing this type of construction is a spin-off of the traditional block-and-plank building. So many chain hotels were built that way, and those projects tend to be developer/contractor driven (i.e., race to the bottom), so it's not surprising the "block" side of the equation got VE'd into lighter/cheaper/faster wall types. Perhaps leaving the "plank" side in place lead to where we are now, as dold mentioned floor heights are important for the value proposition of these buildings.

I'm a big fan of light-gauge in general, but like KootK said we have to really step up our details for lateral load path if there's a heavy concrete floor system. Some of the hotels I see are a house of cards.

I'm a little concerned that the CLT of on wood stud walls building type is heading down a similar path, although the wood crowd is generally good about R&D in support of of code development. https://www.woodworks.org/resources...-and-roof-panels-over-light-frame-wood-walls/

 

[XR250]

After being in the light gage space for a 15 years (and choosing to leave) , I share y'all's concerns. KootK said it perfectly.

 

[KootK]

but like KootK said we have to really step up our details for lateral load path if there's a heavy concrete floor system.

A good example of this is the recent thread where a member was asking about viable diaphragm chord options for such systems. And much consideration was given to the use of CFM track as chords. I went easy on that because that seemed like a "solutions focused" thread that I didn't want to bog down with my arcane theoretical concerns.

That said, CFM track as a chord for a heavy precast plank diaphragm? Seriously?!? Don't we have some licorice or old shoelaces lying around that might be used instead?

I'm a big fan of light-gauge in general

Ditto. Horses for courses though, right? That was rhetorical. Clearly you feel the same way.

so it's not surprising the "block" side of the equation got VE'd into lighter/cheaper/faster wall types.

For sure. As a profession, I feel that we are kind of trapped in a logical inconsistency. Consider:

1) I think that we, and the public, feel that it would best if structural engineers started with robust conceptual schemes and used calculation to validate those schemes.

2) In practice, what we often do is start with weak conceptual schemes and use calculations to fail to invalidate those schemes.

Both of those are "engineering" after a fashion but the second one is clearly much worse from a reliability perspective as it relies upon designers:

a) Being perfect with respect to the technical skill that they bring to bear on the invalidation and;

b) Not giving in to the temptations of market pressure (promotions, mortgage, etc).

Fat bloody chance of that.

We like the phrase "race to the bottom" around here. To some degree, I'm hesitant to use that phrase because it implies a malicious intent that I feel mostly does not exist. Everybody is just responding rationally-ish to the market pressures of their own space.

Unfortunately, what does exist is probably worse: a deterministic process that will inevitably lead to the "bottom" even if nobody consciously pursues that (no racing, just one domino falling after the next). It's like being chased around your toilet by entropy. Eventually things get ugly.

 

[XR250]

That said, CFM track as a chord for a heavy precast plank diaphragm? Seriously?!? Don't we have some licorice or old shoelaces lying around that might be used instead?

Thanks for making me laugh this morning, KootK.