IBC Table 1604.5 Risk Category Residential with Child Care Center on Ground Floor 1

[sundiata83]

Hi All,

Looking at getting everyone's opinion on the appropriate Risk Category.

I have a mixed use building which has residential on all floors except for the ground level. The ground level will have a child care center that has around 70 occupants. From reading Table 1604.5, "Buildings and other structures containing Group E occupancies with an occupant load greater than 250" should be Risk Category III. Does the 250 occupants include the occupants for the whole building or just the occupants for Group E?

Would you use Risk Category II or Risk Category III to design the building? Ultimately looking to check whether to use I_e = 1.0 or 1.25.

Thanks!

 

[dauwerda]

I would say the 250 only includes the Group E occupants - keeping you in Risk Category II.

 

[BridgeSmith]

I don't know about the technicalities of the code, but if the worst were to happen and bury 60 kids, the excuse "the code wasn't clear, so I designed to the lower standard" will probably not get you off the hook, nor allow you to sleep again. If it was my stamp on it, I would definitely use the higher category. I doubt you'll get any pushback on that; most people will feel the same way. If it was an office for 70 adults, there might be a different answer...it's just the way of human nature.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

I'm guessing you're referring to the Seismic Importance factor since the wind I went the way of the dodo 10 years ago. In that case, the technical code interpretation is just as dauwerda said. But...I'd be inclined to follow Rod's line of thought as long as it won't be too punitive to the project. If you have 20 floors above your child care center - maybe stick with risk category 2 if the code will allow it since the impact of I_e=1.25 will be really expensive. If you have 2 floors, bumping it up will hardly be noticed.

 

[RWW0002]

phamENG, the importance factor may have "gone away" but a risk category structure would still have higher wind pressures via wind speed maps for Category III (go figure.. similar increase in wind pressure as previous code importance factor when everything shakes out..)

I agree that the question is does "occupant load greater than 250" apply to the building or the Group E. I tend to agree that it should only be Cat. III once the Group E exceeds 250, but not all engineers or code reviewers will agree.

We run into similar issues from time to time with "buildings whose primary occupancy is public assembly with an occupant load of greater than 300"

i.e. does this mean
(1)"Buildings whose primary occupancy is public assembly of over 300 people" or
(2) Buildings of over 300 people whose primary occupancy is public assembly"

After digging through past code editions and code committee proposals I believe that in both cases the occupant limit is intended to apply to the occupancy area noted (E or Assembly), but I have had other engineers argue with me on this one.

I am all for being conservative when merited, but keep in mind, a multi story apartment with 4999 occupants would be Category II. Let be consistent where we can.

 

[phamENG]

RWW0002 - I agree. I should have explained my point there a little better. I'll preface this by saying that where I am, the design wind forces are based on hurricanes. My rationale is likely flawed where design wind events are random, frequent, and/or difficult to forecast.

A "child care center" would not be open during a design wind event here. It would be evacuated - especially if it's on the ground floor. So I wouldn't be concerned about applying the risk category 3 wind loads to the structure. A seismic event, on the other hand, is random and impossible to predict in a way that would make a difference (even if they got out due to an early warning alarm, there's still the risk of the building collapsing on them). I've only had the opportunity to use that reasoning once, and it was accepted by the code official. Slightly different application - it was an outdoor stadium. I argued that it was foolish to design for assembly occupancy in a hurricane, but I ran my seismic analysis based on RC 3.

 

[RWW0002]

I don't know about the logic of using Cat. II wind for true Cat III structures for wind forces... Design wind forces do not necessarily prompt an evacuation of a building for much of the country and in fact may draw more occupants (a school would not necessarily be evacuated for design wind level storm in non-hurricane areas, and a church or community center near a mobile home park or similar may see more occupants trying to take shelter).

That being said, if you are using Cat III design criteria based on conservative interpretation of the above referenced sections and feel like dialing back to Cat. II for wind or other based on building use.. sure I could buy it. But I would note it as a Cat. II structure and factor up snow/wind/seismic as you decide may be appropriate, not the other way around.

Again, I think that Cat. II seems appropriate for your structure however just be forewarned that there may be some pushback from code official or reviewer.

 

[RWW0002]

Also, this area of the code should really be cleaned up. It has morphed over the years a bit and the wording has not been very clear or coherent. Extra 15-25% for wind/snow/seismic can have real effect in design and cost of structures and we are not very consistent with when we feel like the additional safety is appropriate. Attached is a proposal from the public hearings for changes to IBC in 2008 by Thomas Kinsman that was apparently not accepted (See Page 53). The discussion is a worthwhile read.

https://www.iccsafe.org/cs/codes/Documents/2007-08cycle/ProposedChanges/V1_S1-69.pdf

 

[phamENG]

That is an interesting read. Thanks for posting it. Any idea where we could find a transcript/minutes of the discussion around that proposal? I'm curious to hear the other side of the argument.

 

[r13]

I think the definition of Group E may help understand the vague language of the code.

A child care facility in which the number of occupants is greater than five but not more than 100 is permitted to be classified as Group E

Note that a child care with occupants more than 100 will be assigned to groups other than group E.

 

[RWW0002]

I would be interested in seeing the discussion as well, but I was unable to quickly locate it on the ICC website. In doing some quick Google work, it looks like some version of changes to this section have been suggested over the past few years, but few, if any, accepted..

 

[sundiata83]

Thanks all for the input. After discussing with colleagues, I'm leaning toward going with Category II. Even though the total occupant load will be above 250, the Group E portion will be less than 250.

Considering RWW0002's example of a multi unit apartment with occupant load of 4,999 which would still be Cat II.

If I take an average household size of 2.5 persons with 0.5 person average population under 18.*

I get 4,999 occupants / 2.5 occupant per household = 2,000 households. With 0.5 children/household. = 2,000 *0.5 = 1,0000 kids.

Let say there aren't many kids living in apartments. Even if we half that number to 500 kids. Are those 500 kids living in the apartment any more or less important than the kids at the child care center?

Now you could argue that kids in the child care center may need more help with egress during a fire, but during a seismic event, you would shelter in place. It's interesting that ASCE 7 and IBC are not in alignment with the Risk Category descriptions.

*

https://www.census.gov/data/tables/time-series/demo/families/households.html

 

[r13]

Note, Group E is the less critical designation in egress determination. I don't know how the code addresses other groups for structural design concerns.