Live Load Means How Many People? 4

[marinaman]

If analysis of an existing structure reveals a live load capacity of 20psf, but the code mandates a live load for this structure of 60psf (balcony fixed seating), how many people is 20psf?

I know ASCE uses 60psf to include dynamic forces due to people moving around on an elevated floor. I believe that I'll have to take the number of fixed seats that are existing, ratio them by 20/60, count the number of seats left, and make that the restriction. Any other ideas?

 

[frv]

That seems like a bad idea. I don't think you can get around that 60 psf. People can still go on to your balcony, regardless of what a sign says. Do you really want to take the risk that a bunch of teenagers will think to themselves... "let's test that limit"?

 

[racookpe1978]

And that "live load" could literally be just that: jumping, people crowding literally right next to each other right at the railing's edge, jostling and perhaps even carrying another.

We've seen the shenanigans going on at parties and athletic events.

Are you going to gamble they won't happen on your balcony just because of a "sign" on the wall?

So you have "fixed seats" - how are people going to get to and from the seats, how many other people are going to crowd around without ever sitting down in those fixed seats?

 

[civilperson]

60 psf will be exceeded but the reserve strength, (safety factor), should suffice on the rare occasions when overloaded. Building to the code is considered good judgement.

 

[trainguy]

My former boss one mentioned that a movie theatre lineup is about 100psf, for what it's worth.

Crush loading in a passenger rail vehicle is often taken as 6 persons per square meter, each at 95th percentile male (220lb) - sorry about mixing units...

tg

 

[DaveAtkins]

I tried something similar to this once. I explained to a Code official that a client would limit storage live load in a room by creating wide aisles between shelving.

The Code official would not accept this. On my project, he said a room originally designed for 60 psf live load simply cannot be used for storage without reinforcing the floor members.

DaveAtkins

 

[Ron]

I know of no code-acceptable loading for people in the 20psf range (not stairs, not balconies, not observation platforms...anything). If an average person of say 175 lbs, wearing size 10 shoes, puts both feet closely together, his loading on that (roughly) square foot is 175psf. Obviously you won't pack that many people of that size so closely together, but you could give each of them say, three square feet of space and that wouldn't be unreasonable for a gathering. Then you have roughly 58psf. That's without any dynamic consideration.

Where people are involved, and can gather in close spaces such as a balcony, a crosswalk, or mezzanine, a high unit load is necessary. Go back and read a few of the articles about the Hyatt-Regency walkway failure.

People will ignore posted signs as racookepe1978 noted.

If you are considering rating an area that you know will hold people and it's capacity is less than the code-allowable (in this case much less), you are being irresponsible and you are not meeting an acceptable standard of care. If anything ever happened and this came out through investigation...and it would, your actions would likely constitute professional negligence.

Tell the owner it won't work for the application. If you're the owner, it won't work for the application.

 

[JedClampett]

There was a whole bunch of people killed in Chicago a few years back when a balcony collapsed. They were having a party on it. As I remember the party overflowed to a balcony below and some of the people killed were on that, when the structure and partiers landed on them.
My point is, if it's available for occupancy, you need to design for that load. That's your best legal protection against litigation, plus it's the law.

 

[asixth]

I was doing some renovation supervisions at a university recently when I noticed that all the rooms had a capacity sign out the front. I didn't check the existing 1970's design but I hope the person limit wasn't for structural reasons.

 

[rowingengineer]

Even though i would agree with Ron, I believe you need to open a dialogue with your code official. There opinion as per Dave's post would be very important.

When in doubt, just take the next small step.

 

[Ron]

asixth...most occupancy limitations are for fire egress purposes.

 

[csd72]

Ron beat me to it.

If there is ever a fire in this area do you think people will evacuate and spread out in an orderly fashion - no, there will be a lote of people crushing into a small area near the exit door trying to get out.

Use the code limits as it is often impossible to justify using anything less.

 

[NomLaser]

There was recently a balcony that came off of a 2nd story apartment building in our city. There were 2 kids near their 20's and some free weights involved. Estimates put it nearly 750lbs on a 8'x8' balcony. People do crazy things on balconies. Luckily no one was seriously injured but they had to inspect all of the apartments built by this builder and several balconies needed repaired and were condemned until repairs were made.

http://www.indy.com/posts/lafayette-balcony-collapses-2-hurt

 

[Teguci]

If you have space for people to stand you need to have the structural capacity to support them. There are lots of decks that fail every year during parties.

http://www.chicagobreakingnews.com/2009/07/pedestrian-bridge-collapses-in-indiana.html

I would argue that the above linked failure should never have happened. If you can fit 150 people on a structure, then you need to design for 150 people.

 

[marinaman]

I appreciate the responses. I've written to the client that the balcony must be retrofitted to increase the live load capacity.

Its been argued to me that the structure is 90 years old, served well for 90 years, and why stiffen it now? I've stated that that may be true, but its probably never been fully loaded, and I can only report the numerical findings and make a recommendation.

It is to be stiffened.

 

[JAE]

I hate those "its been OK for 90 years" arguments.

A good engineer will respond:

"For 90 years the structure has stood because of a combination of luck and statistics. For whatever reason, the smaller than required safety margin has not been exceeded. Today, under the current code, and as your engineer, I am liable for the future performance of the structure and I can tell you definitively that it does not provide the required safety margin as is. If you want to leave it as is two things will happen. I will have to step away from the project and report the lack of safety margin to the building official, and I will write you a letter documenting this opinion and all liability will fall on you. I urge you to accept what I'm saying. If you want a second opinion to confirm my opinion, feel free to do so."

....OK....I live in an ideal world and been smoking something - but you have to admit it sounds good, eh?

 

[hokie66]

JAE,
Can you pass some of that stuff around?