MWFRS vs. C&C loads for thick concrete roof (Storm Shelter)?

[MidwestStructE]

Due to IBC 2018 (and later) requirement, storm shelters are being built across the US when cities expand schools. The shelter typically has a concrete or precast roof.
There is a discussion about whether the uplift loads on this roof needs to be calculated as MWFRS or C&C, when the roof size exceeds 700 sq ft (ASCE 7-16 Section 30.2.3). I wonder what opinions there are on this on this forum?

The story so far:

IBC 2021 implicates ICC 500-2020 which implicates ASCE 7-16, which increases C&C loads relative to ASCE 7-10.

The difference in a typical uplift load is from ~170psf MWFRS to ~270psf average C&C (adding up the zone loads). (using the typical 250mph wind speed for tornado)

The minimum requirement for MWFRS in this case would be is a single space of > 700 sq ft with a solid concrete or precast roof.

As engineers, we are used to applying C&C loads to edges of metal deck roofs. These could bend upwards at the corners and edges, within the duration of a gust, and is a known factor. The same is not true for a 12" thick concrete roof.

The main issue is the connections, from the roof to the walls, and the walls to the foundations. No one is expecting a 12" reinforced concrete roof to flip up at the corner like a metal deck might, even under tornado loads. We can show that it won't. When we calculate the uplift, we are combining the zone loads to form an aggregate load in these connections. I think this factor alone proves that the localized C&C loads are not the issue for these roofs.

To me, this type of roof meets the MWFRS definition of ASCE 7-16 Section 26.2 - "An assemblage of structural elements assigned to provide support and stability for the overall building or other structure. The system generally receives wind loading from more than one surface." It receives wind loading from the wall surfaces as a diaphragm, and also vertical direct loads.

Opinions differ on this, and I think the ASCE maybe should clarify this, because this question clearly comes up a lot.

So, would the answer differ for a 2-way concrete slab vs a 1-way? If so, why? Or for a precast reinforced plank joined to an assemblage of planks by a topping, should the panel size be used, and again if so why?

 

[JLNJ]

If the trib area for the section which acts as one piece exceeds 400 sq ft, I would use the MWRFS number.

 

[271828]

The easy answer is to go with the 700 sq ft limit. If the effective wind area is bigger than that, then go MWFRS.

As a practical matter, the GCp charts usually flatten out at 500 sq ft anyway, so that sets a demarcation at about JLNJ's 400 ft2.

Is there an article or example somewhere that talks about using the reasoning in your paragraphs, quoted below, as a criterion for deciding between MWFRS and C&C? If not, then I don't think I'd go with that argument. To me, it's not obviously a good argument. I'm not sure it's wrong, though. Ha.

The main issue is the connections, from the roof to the walls, and the walls to the foundations. No one is expecting a 12" reinforced concrete roof to flip up at the corner like a metal deck might, even under tornado loads. We can show that it won't. When we calculate the uplift, we are combining the zone loads to form an aggregate load in these connections. I think this factor alone proves that the localized C&C loads are not the issue for these roofs.

To me, this type of roof meets the MWFRS definition of ASCE 7-16 Section 26.2 - "An assemblage of structural elements assigned to provide support and stability for the overall building or other structure. The system generally receives wind loading from more than one surface." It receives wind loading from the wall surfaces as a diaphragm, and also vertical direct loads.

 

[MidwestStructE]

Thanks for the replies!

I think it's all about showing that a 700 sq. ft section "acts as one piece" as JLNJ indicated.

We have 30.2.3: Tributary Areas Greater than 700 ft2 (65 m2).

C&C elements with tributary areas greater than 700 ft2 (65 m2) shall be permitted to be designed using the provisions for main wind force resisting systems (MWFRS).

Would it be fair to argue that if C&C loads applied to areas smaller than 700 sq. ft do not cause failure or significant structural deformation of the element without engaging the entire element (e.g. uplift), then the element is permitted to be taken as having a tributary area of 700 sq. ft

Maybe this is a better way of expressing my "deck flips up at the corner" argument?

This would involve engineering knowledge of the system, for example if dealing with something with different strengths in different directions like precast planks, you would have to think about applying the load in the least favorable way.

section which acts as one piece

700 sq ft limit. If the effective wind area is bigger than that, then go MWFRS

it's not obviously a good argument. I'm not sure it's wrong, though. Ha

 

[JLNJ]

On the plus side, the chance of your structure ever seeing any sort of a tornado is almost nil! Just kidding, well, maybe 50% kidding. Don’t fret over a few psf of suction.

Make the design simple and stout and give it a strong and smart load path to the ground. And hope it never sees a tornado. All bets are off in a direct tornado strike.

 

[NotDeadYet]

Take this with a grain of salt - I am still an EI

I am under the impression that C&C is for local conditions of individual components (is this 4x8 piece of sheathing going to split?) and MWFRS was for global conditions of the entire structure (is my wall going to separate from my roof?). Based on this and the sf limit, the direction from code would dictate that at a minimum you use MWFRS.

But, considering ASCE 7-22 updated the tornado loads section to produce more conservative values, which are based on historical evidence in a time period where we are seeing more and more record breaking extreme weather, you could certainly make an argument for using higher wind loads. I doubt any future occupants will complain.

Minimum building requirements aside, what is the construction cost difference of a design based on C&C vs MWFRS? Does it blow the clients budget? Does it exceed the estimated value of a school-age child's life ($10M)?

This very quickly can turn into a conversation about what the client thinks is best for them (morally, ethically, or risk-management wise) as opposed to what the minimum building requirements are...