UNPROTECTED AREAS
(D6.3) The requirements of D6.3 will be met where the external wall or side of the building is separated from the boundary by at least the distance, related to the extent of the relevant unprotected areas, when determined -
a. in a building of purpose group 1, 2 or 3 or purpose sub-group 5B or 6B, by Method 1 below; or
b. in a building of any purpose group, by Method 2 below; or
c. in accordance with the Building Research Establishment Report (BR187), "External fire spread: building separation and boundary distances": (BRE, 1991).
METHOD 1 - SIMPLE GEOMETRY
This method applies only to a building of purpose group 1, 2 or 3 or purpose sub-group 5B or 6B which is not more than 9m in height and not less than 1m from the boundary. (The method is simple but restrictive and if failure is indicated Method 2 or the BRE Report methods should be applied.) The boundary distance will be acceptable if the extent of the unprotected area, the length of the side of the building and the minimum distance of any unprotected area from the boundary are within the limits shown in the following table -
Table to Method 1: Distance of sides of buildings from boundaries by simple geometry
|
Maximum total unprotected area of compartment (m2) |
Minimum distance between side of building and boundary (m) |
|
5.6 |
1.0 |
|
12.0 |
2.0 |
|
18.0 |
3.0 |
|
24.0 |
4.0 |
|
30.0 |
5.0 |
|
no limit |
6.0 |
Note: Maximum length of side 24m in all cases.
When calculating the unprotected area take -
a. the whole area including the frame of any door or window;
b. the whole area of any other opening or part of the external wall which has less than the required fire resistance; and
c. for those parts of the external wall clad in combustible material more than 1mm thick, the whole area if the wall behind does not have the required fire resistance, or half the area where it does.
METHOD 2 - ENCLOSING RECTANGLES (GEOMETRIC METHOD)
This method applies to a building or compartment 1m or more from any point on the relevant boundary.
The method is based on the concept of enclosing rectangles (see below) and the use of Tables A and B to this method, in which the distances given relate to different purpose groups, or sub-groups.
Diagram 1 to Method 2: Establishing length of baseline of enclosing rectangle
Diagram
Step 1 - Establish a plane of reference which is the most favourable
for the side of the building
under consideration and which -
a. touches all or part of the side;
b. however far extended, does not pass within the building (but may pass through projections such as a balcony or coping); and
c. does not cross the relevant boundary.
Normally it will be best for the plane of reference to be roughly parallel to the relevant boundary. (Where the boundary distance has not been set, an assumed relationship with the relevant boundary should be made.)
Step 2 - On the plane of reference, and at right angles to it, project lines marking those proposed unprotected areas which are 80° or less to the plane of reference.
Step 3 - Construct a rectangle which encloses all the unprotected areas projected on to the plane of reference and which complies with Diagram 2 to this method:
Diagram 2 to Method 2: Establishing enclosing rectangle
Diagram
Step 4 - From Tables A or B, according to the use of the building or compartment, select an enclosing rectangle which is equal to, or next greater in both height and width to, the rectangle constructed under step 3. (The tables provide for enclosing rectangles of 3, 6, 9, 12, or 15m high, with widths in steps from 3m to no limit.)