Risk Category for a 30 story residential building 4

[Kovan A.]

Hi
In IBC 2018 for this kind of building i only find that if its occupant load >5000 it will be Risk category III otherwise it will be Risk category II, am i right?
Occupant load of this building is around 2000 should i choose Risk Category II for this building

Base04 – Base01 (Storage, Parking) = 1078m2 / story
Ground floor (Shops) = 900m2
First floor - Residential = 600m2
- shower, sauna, etc. (for resident) = 375m2
Second floor – 25TH floor (residential) = 975m2 /story

thanks

 

[Aesur]

Be sure to check the other uses of the building:
1. I assume the building may have commercial space on the lower levels, are there any implications from this? I am thinking from a standpoint that would reclassify the group identification to something healthcare or education related.
2. Are there assembly rooms, like a gym, movie theater, pool area, etc. with occupancy greater than 300? Note that it does say "primary occupancy" however I have had some jurisdictions argue that it was still a III instead of a II.

In my experience most of the time a project as described would be a Risk Category II structure, however it never hurts to check with the jurisdiction/plan reviewer and architect first to avoid any rework.

 

[Kovan A.]

Thanks Aesur

no it just a 30 story residential building, only at ground level there is some multi propose shops, with 4 basements of parking garage
there is only a small area for shower and sauna, there is no assembly rooms,movie theater, pool area or any occupancy greater than 300 occupant load
So i think it is safe to say it is in Risk Category II.

 

[marinaman]

Just my 2 cents, but.....If I were designing the project, it would be a Category III. ASCE 7-10 lists III as "substantial risk to human life". The way I look at that is that, if the building were to catch on fire, or you were to have an earthquake, or a high wind event....and something were to happen where the building was damaged or collapsed, what is the risk here.....that's my thought.

Loading up a 30 story building and setting the first couple floors on fire would likely be a catastrophe. Similarly, if a gas leak occurred nearby and an explosion happened....or any number of other things were to happen, and the building had an issue, the number of folks in the building could result in a large single event loss.

Looking at IBC 2015, their risk listing is a little more complete. For me, even though one could argue that the building doesn't meet any of the specific line items for risk cat III, I believe that an engineer has to apply some of his own judgement in a case like this....and for me, this building would be designed as Cat III.

If you are on the fence, schedule a meeting with the local building official. Tell him the number of people that could be in this building. Tell him how slow or fast they could get out if something were to happen. Think about how you would feel about it if you lived on the 30th floor. I'm a conservative person..and I feel as though a 30 story building is a substantial structure.

Others might would argue that by using Risk Cat III, the SDC could move up, and as a result, I'm driving money into the job, not only in the structure, but also PME and other areas. With that thought in mind, maybe work with the Geotechnical Engineer and the code and look at the building as II compared to III and see what difference it makes in the structure. I'd probably discuss this with the design team and get their input too. Collectively, between the local building official and the design team....I'd want everybody to be on the same page. Who knows, PME or others may want III for other reasons.....I'd talk it out.

 

[Kovan A.]

thanks marinaman

If it is me, i would count a 10 person occupant load to be the risk category III,however ASCE code definitely after consider lots of things they come up with those conditions for each Risk Category.As long as engineer follow the code i believe we should get acceptable result in critical situation.
Just by choosing category III over category II for this building, beside many other factors(drift limits, SDC,etc), the important factor become 1=1.25 in another word 25% percent additional lateral load for seismic, after designing the building for category II the owner hardly accepted the size of columns and walls sections.So it is not easy just consider every factor as its critical value, otherwise there weren't be so many variable and types in codes.

I agree with how you think but this is our reality.

 

[bones206]

Here's a good thread to review: thread176-458198

I agree with the general sentiment to come to a mutual agreement on Risk Category with the project team and AHJ as early on in the project as possible. I don't believe it is the structural engineer's sole responsibility to assign Risk Category.

 

[Rabbit12]

Have you talked to the building official? Are they ok going with a risk category II? If you haven't talked to them I would ASAP. Ultimately, if they decide to think like marinaman (who I tend to agree with) you'll be designing to category III.

 

[r13]

The fire safety for tall building is governed by fire code, it shouldn't be in the mix of building design category determination, as the "category" is mainly used to determine the strength level of a building needs to be design to.

 

[r13]

I addition, determine/settle the proper design category internally. A question like this, will only raise flag in the owner's mind: "if I know this, why I need you for?"

 

[bones206]

An owner should be interested in having a say over Risk Category as it effectively sets limits to what the structure can be used for. A municipality might have interest if perhaps they want buildings of certain sizes to have an additional robustness against natural disasters than the code minimum. I’m working on a water treatment plant that was designed for Risk Category III until a year into design when someone had a conversation with the city water dept who decided they’d prefer Cat IV. I don’t think they were happy with our team when we gave them the change order for that.

Also, there can be interfaces with other disciplines that complicate things. What if the 30 story building had a fire pump room that is classified as Risk Cat III since it’s relied on for life safety? How does that dovetail into a Cat II mixed-use building?

I definitely think a Risk Category pow-wow with all the stakeholders in the beginning of the project is valuable and shows the client that you know what you’re doing and are providing valuable guidance and foresight to them, not that you are leaning on them to make the decision for you.

 

[r13]

bones206,

We are talking about two different clienteles - public vs private. Note that most of public entities, such as water, electricity services, have their own engineering and technical staffs, they wrote their own specs, and often times, through IBC out of the window, as discussed in another thread. So they are capable of understand the stake at hand, and make their own calls to theirs like.

However, project like this is most likely privately owned/developed, and handled by outside architects and consulting companies. The fire safety and many other related issues, are to be considered in the feasibility and preliminary layout stage by code search, usually includes the governing building code, OSHA regulations, and NFPA (national fire code). The structural engineer then is entered to suggest the support system, which depends the strength level required per code specified "design category", for the preliminary layout, note that the layout has incorporated the fire safety concern already prior to entering the design stage, not after.

I wouldn't say all private owners (usually are non-technical business persons) lack knowledge of code issues, but usually they are more concerned with budget, and un-necessary spending, in here - double the fire concerns.

 

[Aesur]

I agree with @retired13; fire safety is tied to occupancy category, building structure is risk category. These are two separate classifications, ie. occupancy category determines fire ratings and related items whereas risk category determines the forces applied to a building structural system. The risk category should typically not be determined based on mechanical, electrical, plumbing or fire requirements, but solely on the use of the structure. However please note that risk category can play a part in the MEP design of connections to the structure.

When questions arise for the risk category, it is best to consult the team, which includes the owner and building official. Jurisdictions all have their own interpretations of the code and it can occasionally be extremely difficult to change their minds. When it comes to the owner, they will most definitely want a say, but you should be prepared to back your recommendations if you recommend a higher risk category, because they rarely are willing to spend the money and they definitely aren't willing to spend extra money for a structure to make the engineer "feel better" when it still meets code.