Live Load vs. Max. # of People

[n8structural]

I provide Structural Engineering for a local manufacturer that provided approximately 101 balconies for a new assisted living facility in Florida. Most balconies were designed for a 60 psf live load (1.5x40psf, per ASCE-7-2010), and used for individual resident balconies. We also engineered a large (29'-0"x6'-0") public balcony off a "community room". This large balcony was designed for a 100 psf live load.
Prior to occupancy, the Owner is requesting a "maximum allowed persons" (or similar term) for the public balcony so it can be posted. I have never been asked this before, and am having reservations about how to respond.
How should I go about "rating" this large balcony which was designed for 100 psf, as it pertains to a facilities standpoint (presuming the person asking/using this information is not an engineer). I have responded previously by simply saying it was designed for a 100 psf live load, but this question keeps coming back to me in a "maximum persons" number. I am a little more at ease that this is for an assisted living facility than a college dorm, as the likelihood of residents jumping/partying is slightly less.
Thank you in advance.

 

[SlideRuleEra]

To begin, I would calculate the upper bound for maximum number of people on the balcony similar to the way that the maximum occupancy of a building is determined. That is, how quickly the doors to the interior can evacuate the balcony. Of course, in this case the people are returning to the interior of the building... not evacuating the building.

This number may have to be revised downward. What answer did the above calc give?

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[jayrod12]

100PSF worth of people will be more than allowed by the architectural/life safety requirements indicated by SRE. They should talk with the architect and determine how many people are allowed on there for the number of exits/size of exits.

 

[WARose]

I've been pressed on this myself and what I've always wanted to ask the people who want to know this is: why do you want to know? (At least as far as the structural requirements go.) Do you think a herd of people are going to stop and say: "Hey, let's count how many people there are on this balcony [or whatever] and be sure no one else comes out here?" The odds of that happening are pretty much zilch. Yes, the fire Marshall will limit numbers to go into a building.....but every time I've seen them do that it's for a building/bar as a whole.

I just tell them it's designed for assembly and give a number (i.e. maybe a 240 # person for every X ft^2) that I still tell them not to hold me to it.

 

[n8structural]

SlideRuleEra,
Thank you for your response. I agree that the requested information goes beyond a structural response. I have deferred back to the Architect of Record for the project, and simply stated that the balcony was designed for a 100 psf live load. Not having been apart of the building design, I try to defer such questions. Thank you for your time.

 

[n8structural]

Thank you all for your response. Seems like we are all on the same page. Thank you again.

 

[JAE]

Using your 100 psf load and a 6 ft. cantilever you get a moment (per foot width of cantilever) of 1800 ft-lbs.

If you assume an individual human can take up a 1 ft. x 1.5 ft. area and weighs 160 lbs., you can place an outer row of people at the outer 1 ft. strip of your cantilever.
That puts 19 individuals along the outer width (29 ft. x 1 ft area) and it creates a moment at the support.

Do the same for row 2, 3, etc. and you find that the maximum number of rows of individuals would be 4 rows - totaling about 1677 ft-lbs (i.e. less than 1800)
Total number of people = 76

Or you can turn the people sideways so you have 29 people along the outer edge of balcony - with that we get max. 3 rows totaling about 1800 ft-lbs.
Total number of people = 67

So you could perhaps play around with a spreadsheet, the weight/person and area/person - calculating these numbers and perhaps go with maximum 60 or 65.




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[Shu Jiang PEng SE]

Normal weight for each person is 200 pounds and each one takes 2.0 square feet floor area.

 

[26sig]

Regarding the amount of space each person takes up, and weighs:
Are there any studies that verify these numbers? I know that everyone seems to have their favorite number; are these actually based
on research?


-CF

 

[JAE]

Tape measure across my hips and waist. Known weight as of this morning. Wild assumption that I'm typical.

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[Hokie93]

If the 'JAE Method' is not sufficiently rational or rigorous for you, take a look at "Reliability of Structures" by A.S. Nowak and K.R. Collins.

 

[JedClampett]

There have been several fatal accidents caused by overloading balconies (Chicago, San Francisco area). Probably neither was a structural issue, but I'm sure that it's interpreted that way. I'm guessing that the owner has heard of these. And they correctly sense that 60 psf means nothing to a group of people. But hopefully they can count.
I'd use an average of 200 lb per person, figure how many people that is for your footprint, then round way, way down. If your balcony is 8 ft. x 12 ft., that would be 29 people. Say five rows of three would be fifteen. Seems like fifteen is plenty.

 

[wannabeSE]

I agree with having the architect answer this question. IBC section 1004.3 has the posting requirements for occupant loads in assembly occupancies.

 

[kingnero]

I'd give them two numbers: one for sumo wrestlers and one for chinese people. and tell them also they have to interpolate linearly inbetween those two extremes accordingly.

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[LittleInch]

Check out airline passenger weight surveys. It's pretty critical for balconies but highly critical for aircraft, especially small one.

Average weight per passenger in the US appears to be circa 185 lbs.

Document in link quotes a Europe wide study and recommends 88 kg ( 193 lb) for a 70: 30 mix male / female.

https://www.easa.europa.eu/system/files/dfu/Weight Survey R20090095 Final.pdf

So a 200 lb average per person is justifiable and traceable.

The problem with baloneys is that the actual loading is not uniform. Tends to be concentrated at the edges (resting or leaning on the balcony posts) or standing in small groups.

So if max people for the area on a uniform basis is 87 ( 29 x 6 x 100 / 200) then 50% of that would seem to be a good number to start with - 44. I would revert with the answer you would not recommend more than 45 to 50 people.

At even 50 this is 3.5 ft^2 per person. Apparently an accepted value for maximum density of people in an elevator is 5 per m2 or 2.1 ft^2 each. But a balcony isn't an elevator and most people would find that too crowded for a balcony.

So 50 it is.

IMHO.

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