Height, Area, and Use
In order for a building to be classified
under Part 9 of the Ontario Building Code there are three conditions that must
be satisfied:
- The building must be no more than 600 m2
in building area
- The building must be no more than 3
Storeys in building height
- The building must be of C, D, E, F2 or F3
occupancy
Items 1 and 2 are easy to determine if one
has a clear understanding of the terms ‘Building Area’ and ‘Building Height’.
Item 3, occupancy, is determined by identifying the use of the building (or
part of the building) and assigning a letter designation according to the code.
A description of occupancies and their corresponding designations can be found
in Part 3 on OBC Table 3.1.2.1.
A more detailed description of occupancies and
their letter designations can be found on the
Major Occupancy
Classifications Page. If any of
the uses are classified as Assembly ‘A’, Industrial ‘F1’ or Care and Detention
‘B’, then the building must be designed according to Part 3 of the
Ontario Building Code.
The description of major occupancies
permitted under Part 9 can be found on OBC Table 9.10.2.1.
Building Area
Building Area does not refer to Gross Building
Area which is the addition of all floor areas above grade. The calculation of
Gross Building Area is made for other reasons and does not apply to the
determination between Part 9 and Part 3 buildings.
Gross Building Area is used for purposes such as:
·
Determining the need for an Architect or Engineer.
Generally, any building exceeding 600 m2 (6460 ft2) in
gross building area or any building exceeding 3 storeys in building height
requires the services of an Architect or Engineer. Refer to Table 2.3.1.1.
·
Determining the ratio of floor space to lot
area. This is a zoning requirement and is used to determine what is referred to
as GFA (Gross Floor Area). In some jurisdictions it is also referred to as FSI
(Floor Space Index). Whatever the term, the calculation is the same – a ratio
of the Lot Area to the Gross Building Area. Note here how words and syntax are
used in interchanging ways. In the building Code, the term is ‘Gross Building
Area’, for zoning purposes it is ‘Gross Floor Area (GFA)’. When the
building code refers to 'Floor Area', the definition is different.
Figure 1 showing a building with 600
m2 of Gross Building Area (200m2 per floor)
The building Area in the example above is
200 m2 and the Gross Building Area is 600 m2. The best
way to think of building area is to imagine a bird’s eye view of the building.
For instance, when the second floor is overhanging the first floor, it needs to
be counted as part of the overall building area. Consider the following
example:
Figure 2 Building Area Calculation
In the diagram above, the building area is
240 m2 and the Gross building area is 600 m2. Note that
this building is the same size as the building shown in Figure 1 but the
‘Building Area’ is larger.
.
.
Consider the same building in Figure 2
above with a wall thickness. For the purpose of making the calculations easier,
say the wall thickness is 500 mm. This would create interior dimensions of 19 m
x 9 m for a total floor area of 171 m2. This value of Floor Area
would be used for various calculations such as occupant load, and allowable
mezzanine area. The building in Figure 2 then has three very different numbers associated
with area descriptions:
Gross Building Area 600 m2
Building Area 240 m2
Floor Area 171 m2 (for
one floor with wall thickness of 500 mm)
Note these calculations and how they are
derived because they will need to be used for any number of different
situations and for different purposes. When reading a question or trying to
solve a problem, check the definitions in the building code for ‘keywords’ that
form the question. In this case, an important distinction is made between floor
area, building area, and gross building area.
Floor area in the examples above is
essentially the area of the floor to the inside face of exterior walls.
Building area is essentially the largest
floor area (including all walls)
Gross building area is essentially all
floors combined.
Consider the following building descriptions
below and indicate which are classified as Part 9 and which are classified as
Part 3
A
An unsprinklered 3 storey Warehouse (F3) with a
building area of 500 m2
______________________
B
A restaurant with a building area of 400 m2
and seating capacity of 46
______________________
C
A 3 storey Concrete Apartment building with an
area of 400 m2 per floor
______________________
D
An unsprinklered wood frame Restaurant with seating
for 25 people
______________________
Building Height
Building Height means the number of storeys
contained between the roof and the floor of the first storey.
First Storey means the storey with its floor closest to grade and having its
ceiling more than 1.8 m above grade.
Grade means the average level of proposed or finished ground adjoining a
building at all exterior walls.
These three terms – building height, first
storey, and grade are defined terms in the building code (OBC 1.1.3.2.).
Determining building height requires that
the first storey and grade are properly identified. Consider the following
example:
Figure 3 Building Height in relation
to Grade
Both of these buildings seem identical in
size. The 3 storey building on the right however is lowered slightly further
into the ground than the 4 storey building. This minor adjustment has a
significant impact on how the building needs to be considered and built. The 4
storey building would need to be considered under Part 3 and the 3 storey
building could be considered under Part 9. The implications in terms of cost
are significant. This is why an
accurate determination of grade and height is important to
identify early on in the design process.