Risk Category for Outdoor Assembly space 1

[RickyTickyTavi]

I have an odd project. Client has an existing parking garage and they are converting the top level of the garage to an open air "Assembly Space". We've designed strengthening for the deck, and a myriad of other modifications. However the city official has indicated that this is a "theatre" with occupancy of more than 300 people, therefore we have to modify the building for the increased wind-loads, going from Risk Category II to III (Seismic does not govern here, ever). My rationale is that this is an outdoor theater and that the occupancy will be 0 during a wind-code event. The Architect agrees with me but wants to push back on the city some.

Does the code (IBC) have any carve out for this case? I can't seem to find anything that would be good code rationale for the permitting official. Anyone ever run across this?

 

[JStephen]

Would those 300 people run to the next level down in a wind-code event? I'm visualizing a sudden thunderstorm where everyone scrambles for shelter.

 

[RickyTickyTavi]

No, its an outdoor movie theatre. City wants the building upgraded to 150 mph (ultimate) wind velocity. I did recommend to the architect that an easy way to get around this is lower the amount of people to 299, but they werent having it.

 

[EZBuilding]

The primary occupancy of the building will still be a parking garage, not public occupancy. I think this takes you away from a 300 person public occupancy and instead looking at the total occupancy of the building.

From FBC 2017 Ch. 16:
"Buildings and other structures whose primary occupancy is public assembly with an occupant load greater than 300.

Any other occupancy with an occupant load greater than 5,000"

 

[jittles]

Open air and outdoor seem to belie the fact that this is on the roof of a building. Rooftop movie theater might be a more apt description.

I suppose I agree with you that the rooftop theater most likely isn't going to be fully occupied during a 150 mph wind event. Not sure how that would be or could be considered as an exception to the code. Not a fun argument to have with a building official, no doubt.

 

[r13]

You can argue, but IBC says,

1604.5.1 Multiple Occupancies

Where a building or structure is occupied by two or more occupancies not included in the same risk category, it shall be assigned the classification of the highest risk category corresponding to the various occupancies.

 

[JLNJ]

You might have to drill down into the derivation of the code to see what they intended by PRIMARY occupancy. Primary, as in what it’s usually used for, or primary, as in which use has the greatest number of occupants? By square footage? Maybe they OK it for the top story of a 20 story deck but the top of a three story deck is a no-go.

If I were the official I would not buy into the zero occupancy during a wind event argument. Who is going to administer THAT policy? You going to cover it with a note on the drawing? A posted placard that says “leave if it’s windy”?

I imagine you are already doubling or tripling the live load. A little extra wind might be a drop in the bucket.

 

[azcats]

Isn't 150 mph going to be hurricane event? Hard to imagine being occupancy during that.

Could you argue that the 140 mph would cover Risk Category 3 wind speeds for unpredictable wind events and that there's no way the rooftop would be occupied during your regularly scheduled hurricane?

 

[EZBuilding]

Building category is one of the worst defined elements in the building code. I wish it had bigger impacts upon Architectural requirements - occupancy is primarily determined by them through Life-Safety, it seems like it would make more sense for the Architect to define the risk category.

Risk category III is prefaced by:
Buildings and other structures that represent a substantial hazard to human life in the event of failure, including but not limited to:

I believe that the odds of an individual being at the top of this building, whether it is an exterior movie theater or a parking garage is approximately the same during a design wind event. I believe the classification Buildings and other structures whose primary occupancy is public assembly with an occupant load greater than 300 is attempting to encompass buildings which could be construed as a make shift shelter in an emergency situation. Several hundred people in a time of need may choose to run into a large auditorium during a hurricane, it is unlikely they would decide to shelter in an exposed area.

I may have been a little bit hasty in my previous categorization, as there needs to be an understanding of what the primary occupancy. I believe primary should be determined by the number of occupants. If your occupancy for the movie theater is 400 and your occupancy for the parking garage is 500, it is my opinion that the primary occupancy is parking.

The unpredictability of earthquakes, however, does require a different lens through which to view this problem.

 

[r13]

I think this structure should be categorized by IBC 1604.5.1 (mixed uses) rather than determined by the primary use, especially the under covered subjects are very different, one more for properties, the other more focus on human. If severe wind event occurs during service, the people tend to take shelter at the lower floors, thus, it is arguable the building will be a shelter at some point of time.

 

[phamENG]

I agree with where you're going with that EZBuilding. I know this conversation has played out before, and I think I posted the same anecdote, but here it goes...

I designed a really fancy set of football bleachers (private school) and the occupancy load pushed us into the >300 category and the building official demanding RCIII. Well...I successfully argued that, during a design wind event, the max occupancy would probably be no more than 10...in case somebody fell asleep in the broom closet or really wanted to make sure the concessions equipment was secure. I'm in hurricane country (essentially...official hurricane prone region ends about 50 miles south of me), so that's reasonable argument to make. Even a severe thunderstorm is predictable in this region with enough warning to get people out. Besides, even at RCII, it's still essentially capable of resisting a category 2 hurricane if Table C26.5-4 in ASCE 7-10 is to believed. For seismic, I did analyze it as a RCIII since there's no telling when an earthquake will strike (no matter how unlikely).

This reasoning does NOT apply to places like the mid-west, where design wind speeds are based on shorter duration storms that are more difficult to predict. I think that's one reason it's not as well defined in the model codes - carving out the sources of the weather data used to build the maps is more than they really want to do. I know with 7-16 they've improved on that some, but from what I understand the available information is still a long way from making that a practical step in design - and there's probably not enough demand for it anyway. It's not the most conservative thing to do. I only did it in the case above because the architect insisted on a bunch of elegant cantilevered canopies and accents, and they couldn't reasonably be designed within the geometric constraints they wanted.

Seismic does not govern here, ever

Be careful with this one. Just because the ultimate wind load is higher than the ultimate seismic load doesn't mean you're exempt from detailing and ductility requirements. Even here on the east coast, where we get one noticeable earthquake every 2 to 3 generations and our wind loads are driven by hurricanes, we have to pay homage to the seismic gods. (Whether people actually do or not is another thing, I suppose).

 

[RickyTickyTavi]

Fair point PhamENG on the detailing, I agree. Project site is located near the Gulf-Coast, Hurricane country. I like the argument posited by EZbuilding and yourself. I may present that argument to the code official and Architect, indicating that the code qualifies "primary" a certain way.

I understand your point Retired13, but I guess I have trouble reconciling the fact that the garage and adjacent mid-rise both likely have more than 300 people during normal business hours and both are traditionally classified as Cat II structures. To further clarify my OP, the conversion of the roof of the garage is only occurring for a portion of the roof top square footage (ball park 25% of the top level).

 

[r13]

I can only wish you the best luck in this case. As the code language is very clear, it is not the footage area, nor the primary use the determinant, it is the official's guts to shoulder the decision shall something bad happens. But I would agree, if the event occurs infrequently, thus does not constitute a regular use.