Obesity and the structural limits of architecture

Dec 10 '12 15 Last Comment
J. James R.J. James R.
Dec 10, 12 11:52 am

Is it time to update our codes to reflect the waistlines?

We talk about the interplay of design, policy, planning and personal responsibility— but what about the technical ramifications? Most fire and structural calculation tables date back decades, even centuries, making basic assumptions about occupants, people and objects, in them.

For instance, the weight of an average American has gone up one pound per year over the last twenty years.

In 1900, the average weight of a young, in-his-20s, male was 133 lbs; a difference of 63 lbs between then and now. That means an elevator with a gross weight limit of one-ton can only hold 10 21st-century men versus 15 early-20th-century men. Fewer people people being able to circulate means more money on increasing capacity leading directly to higher building costs.

In comparison, an airplane has a gross maximum weight of about 700 lbs per square yard. That means the three people who occupy them between then and now have amounted to 60 extra pounds in just two decades. This obviously plays into their cost-profit calculations as six average adults are now essentially forcing someone else off the plane and cuts into the available commercial cargo space an airline can rent out.


Here's a basic scenario:

We have 300 people on a single floor of an office tower. If we use 30 feet of gross floor space per person, the floor space is 9,000 square feet. The average man-woman weighs 178 lbs in 2012 and 161 in 1900. 

In 1990, the people exerted a load of 0.25 kilonewtons per square meter.

In 2012, the people exerted a load of 0.28 kilonewtons per square meter.

In 1900, the people exerted a load of 0.21 kilonewtons per square meter.

Assume a basic office floor maximum is 2.394 kilonetwens per square meter... 

... By year 3950, if average weight continues to grow at a constant level, these 300 office workers would lead to structural failure at around Year 3950 CE.

This, of course, clearly makes the assumption of 300 people in an unfinished and unfurnished room with a bare-bones structure with no wind, snow or seismic loading. 

The staircases and elevators would long give out before the floor does retrospectively.


But at what point do these extra kilonewtons we're packing on become mathematically significant enough to change our design philosophies?


Dec 10, 12 1:42 pm

also, we often cram more people in tighter with smaller workstations instead of offices.  i'm not really going to worry too much about it until a floor slab actually caves in due to the weight of the occupants.  then, when that happens, i will say "gee, i wish i had listened to J. James R."

Dec 10, 12 1:43 pm

Well, I for one have long said that if they are going to weigh baggage on airlines and charge for excess then they had ought to do the same with passengers.

Actually, many of the smaller niche airlines do weight passengers, but I'm guessing it's illegal to charge fat people extra.


Dec 10, 12 2:18 pm

not much of an issue. dead loads far outweigh live ones. Try lifting two drywall sheets for a simple test. 

Dec 10, 12 2:24 pm

JJR, listen to Rusty.

Dec 11, 12 11:08 am

Rusty's on it.  I'm just more concerned about having to upgrade to code to accommodate wider wastelines.  Last I checked the ANSI standards just kept getting bigger and wider.  Reminds me of a recent episode of South Park where they had to redesign the boy's room to handle Cartman's Hover Round.

Dec 11, 12 9:22 pm

yawn. i'm less concerned with fat bodies in offices and more concerned with tubby getting stuck in a tank turret, or too fat to outrun an rpg.

today in the nytimes there is an article about cities actually seeing small measures of success when it comes to efforts at stemming the obesity epidemic.

here's the other thing about fat people, the more you embarrass them, the more the stay home, and the fatter they get. so, kudos to you.

Isaac GaetzIsaac Gaetz
Dec 13, 12 5:02 pm

Here is my structural engineers perspective on this.

In general, Live loads have quite a bit of leeway and rarely exceeded. Imagine what 100psf live loading really means, A person who is 200# and standing, is going to occupy at least 2square feet, and in general another person isn't going stand immediately next to them. the only way to achieve 100psf density over a large area is to cram people in like a packed elevator, except over the entire floor. This is pretty unusual. The other thing to keep in mind is that as people get larger, they take up more space... so they make it harder to fit in more people.

Now there are some cases that very heavy people should potentially be concerned about. A very person walking up or down stairs will at some point have all of their weight  carried on a a single stair tread. In much the same way, this very heavy person may exceed hand railing design loads if they throw their weight hard into the railing. OSHA type limits are usually based on a 200lb or 250lb load representing a person, so If this very large person exceeds that load, theoretically it could be a problem, but in practice, it rarely seems to be an issue. 

Dec 13, 12 5:35 pm

If this is a concern, for the stairs this rule:

whatever is the heaviest person that can still walk, design for that.

Dec 13, 12 5:44 pm

whatever is the heaviest person that can still walk, design for that.

what if that person is carrying a box?  do you need to account for the box weight?

Dec 13, 12 6:49 pm

what about a very heavy person carrying a box and wearing stilettos?  that's like, major psi from the heel there *crush*  do you need to account for that?

Dec 14, 12 2:17 pm

Okay, finally, this thread is interesting, thanks to Frac.

Next cover of Architecture magazine: photo of a 350-lb bearded guy in 6-inch stilletos carrying a piano up a stairway.

"Is Obesity Killing Our Floors?"

Dec 14, 12 10:45 pm

as muscle weighs more than fat per volume, I would actually think that the fatter people get the less weight there is per sq ft in an occupied space

Isaac GaetzIsaac Gaetz
Dec 14, 12 10:49 pm

Mdler, This same thought occurred to me earlier today and I agree. a room full of body builders holding pianos is the most dangerous scenario. 

Dec 15, 12 3:30 pm

body builders in stilettos.  that should be a requirement for body-building contests

J. James R.J. James R.
Dec 15, 12 11:07 pm

You all obviously don't know about Pavel Patel.

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