Have you ever seen one of those signs indicating the occupant capacity of a room and thought,
how exactly do they get that number?
If you thought it was how many people could be physically packed into the room shoulder
to shoulder then, no that’s most likely not it.
Occupant loads are, for the most part, determined by: means of egress, which basically means
the design of exiting components throughout the space, as well as the number of available
sanitary fixtures such as toilets and lavatories.
So how are these components designed to meet the intended occupant load?
Egress is arguably one of the most important aspects of architectural and interior design.
The exiting requirements for proper and safe evacuation depends largely on
what the space is being used for.
For example a movie theatre will usually hold more people compared to a typical office of
the same size, and a storage room carrying flammable liquids is more dangerous than a
typical residential dwelling.
Building codes also differ depending on where you are.
Here in North America, most states, provinces and municipalities will adopt “model building
codes,” such as the International Building Code, or the National Building Code of Canada.
For this video I will refer mostly to the IBC.
In order to determine egress provisions.
We need to figure out how many people are expected to use a space.
In a space that only contains fixed seats, like a movie theatre,
it’s pretty straight forward to figure that out.
It can be done by counting the number of seats.
But what about spaces without fixed seating?
Well, another method is to use the “occupant load factor” table provided in the building code.
For example, the load factor of an exhibit gallery or museum is 30 sqft/occupant.
So in a 3000 sqft room the occupant load for an art gallery would be 100.
In Business Areas, such an office, the load factor is 150 sqft/occupant.
So in the same space the occupant load for an office would be 20.
The load factor table, however, does not restrict the space from having more occupants.
The 3,000 sqft gallery can be designed for more than 100 people, as long as the requirements
for egress, life safety, sanitary fixtures and other code mandates are satisfied.
The more difficult thing to do is designing the space for less occupants than what’s
specified by the load table.
So if you’re trying to argue what this 3000 sqft gallery will only be used by a maximum
of 25 people at any given time, then you need to make a good case to the local building
official and they would have to approve it.
So let’s talk exits.
Or more accurately - means of egress.
What exactly is a means of egress?
Well the IBC defines it as “a continuous and unobstructed path of vertical and horizontal
egress travel from any occupied portion of a building or structure to a public way.”
The reason we talk about it in this way is because exiting a building is not a one-step
event, it’s a process that involves multiple exiting components.
So, pretty obvious - every space needs at least one means of egress.
But two or more means of egress are required if the space exceeds a certain occupant load
under certain occupancy classifications.
Occupancy classifications are formal designations for the primary purpose of the building or
portion of building.
Since Art Galleries normally fall under the Assembly occupancy, we would look under the
requirements for Occupancy A, which indicates that if the space only has one means of egress,
the occupant load cannot exceed 49.
In our case of 100 occupants, we would have to provide two means of egress.
Exits, or access to exits, also have to be positioned and spaced accordingly.
Because, say there is a fire blocking the path to one doorway, there needs to be an
alternative way to get out.
You can’t just have two doors next to each other and call that two exits.
They have to be certain distances apart.
And the distances are based on ratios and formulas outlined by the code which may vary
depending on if the building is sprinklered or not.
The means of egress also have to be large enough to handle the occupant load.
For example, doorways and corridors along a means of egress must have a clear headroom
of 80” high, have a minimum clear width of 32” for doors, 36” for corridors
44" for corridors serving an occupant load of 50 or more, or 0.2” multiplied by the occupant load,
whichever is more.
These width capacities may be reduced in certain occupancies if the space is equipped with
an automatic sprinkler system.
Going back to our art gallery that require two means of egress.
Let’s say they are exit access doorways.
Since 0.2” x 100 is only 20”.
In this case, both doors have to be at least 32” wide each.
This is not necessarily the size of the door though, it’s actually the clear opening
between any protruding hardware when the door is opened at 90 degrees.
So often a 36” door needs to be used to achieve a 32” opening.
Let’s take a look at an example with a higher occupant load.
For instance a community meeting room with loose seating.
This usage, as per the load factor table is listed as 7 sqft/occupant.
So in our 3000 sqft room we could expect 429 people.
Which when multiplied by 0.2” would equal 85.8”.
Since we definitely need at least two exits or exit access doorways, one way to achieve
this is by having two doors, each having a clear width of a least 42.9”.
In High Hazard occupancies, and in all other occupancies where the occupant load of the
space is 50 or more, the door also must swing in the direction of exit travel.
Occupant load above certain capacities may also trigger life safety and fire protection
requirements, such as requirements for automatic sprinklers and fire alarm systems.
But while life safety and means of egress are very important building components, so
are personal hygiene and access to toilets.
Thus building codes mandate a minimum number of toilets, lavatories, drinking fountains,
service sinks, and sometimes also bathtubs and showers.
As per IBC, In the office area, we would need to provide 1 water closet for every 25 persons
of each sex for the first 50 people.
Since we only have 20 people, that’s 10 males and 10 females, we could provide
one restroom per sex.
However, there is an exception in the code where you do not need to provide a separate
facility for each sex in business occupancies of 25 or less.
So in this case, providing one restroom total is acceptable.
In addition, based on our occupant load, a minimum of one lavatory, one drinking fountain
and one service sink are also required.
In the community meeting room of 429 people, the requirement is one water closet for every
65 females and 1 for every 125 males.
Since we have about 215 of each sex, this means we need four water closets for women
and two for men.
In the female restroom this would be four toilets.
In the male restroom, up to half of toilets can be urinals.
In case you’re wondering why women get more toilets than men, this is due to the fact
that biologically it takes longer for women to use the restroom.
Additionally one lavatory is required for every 200 people of each sex, this means we
need two lavatories for both restrooms.
And, one drinking fountain and one service sink are also required.
Essentially, every built environment needs to be designed to allow its occupants to exit
safely in a timely manner in the event of an emergency, and provide adequate toilet
and personal hygiene facilities for its occupants.
While the examples I’ve given in this video are relatively simple, there are many more
complex requirements.
Plus, real world environments are generally not as straight forward, and often deal with
existing construction, mixed occupancies and municipal regulations.
Thus, most of the time, load calculations and code reviews must be carried out by
a licensed interior designer or architect.
I find that a lot of people perceive creative design to be dominated by form and aesthetics.
When in reality, form is often secondary to function, and design can be quite a constrained
exercise with a lot of limitations.
However, personally I think working with constrained parameters is even more interesting and,
as a matter of fact, requires even more creativity.
Thanks for watching.
This episode is a part of a series where I explore various aspects of
the interior design profession.
If you liked it, then you may enjoy some of these as well.
And please subscribe to keep watching.
Until next time! *snap!*