Spatial Separation: The Finite Size of Window Openings

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Technical Note (National Research Council of Canada. Division of Building Research), 1960-02-01

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Spatial Separation: The Finite Size of Window Openings

McGuire, J. H.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

'fE

C

lHI N ][ CAlL

NOT FOR PUBLICATION

No.

NOTJE

FOR INTERNAL USE

PREPARED BY J.H. McGuire CHECKED _{APPROVED BY} N.B.H.

DATEFebruary

1960

PREPARED FOR Re IIt%\ioted Circulation.

SUBJECT Spatial Separation: The Finite Size of Window Openings

In DBR Internal Report No.

187,

Tables

5

and

6

contain recommended minimum separations between a building and its lot line. One of the most important considerations involved in the preparation of such a table is the number, size, and distribution of the window

openings in the bUilding and these factors have been grouped together as the variable "percentage window opening". By appeal to the funda-mentals of radiative heat transfer such an action is valid where (a)

the window distribution is uniform and (b) the separation between windows is small compared with the distance specified as the minimum

separation which should be required between a building and its lot line. These two conditions can also be expressed by the statement that the windows must approximate an infinite number of uniformly distributed,

infinitely small, openings.

Where the condition of uniformity of distribution is not met, some simple remedy can be thought of ｾｮ､ Report-No.

187

recommends that the value adopted for the percentage window opening should relate to a localized area where the window density is high.

Where the second requirement is not met, however, and the

dimensions between windows are significant compared with the recommended distance of spatial separation, no simple remedial measure has so far been suggested. It is not unlikely that this situation will remain as yet another of the minor inconsistencies inherent in this approach to the problem. It is possibly some consolation that these inconsistencies, together with others, are also inherent in the alternative approaches known as at the time of writing.

｟Ｏｾ

r"

The discrepancies will be greatest where very low percentage window openings are involved and the orders of magnitude to be

expected in such cases are illustrated by the following ･ｸ｡ｭｰｬ･ｳｾ

1) A bUilding 200 ft wide with fire-resistant floors so that only one story at a time need be considered. Suppose the story height be 10 ft and. that there be two windows 10 ft ｨｩｾｨ by 40 ft wide on one ｂｴｾｲｹｯ This would only classify as 40% window opening and One might thus say 10 ft by 200 ft by

40%

gives

l7

ft (from Taole

6).

The one window ｡ｬｯｮ･ｾ howeverg gives 10 ft by 40 ft by ＱＰｾ

=

2102 fto The "normal" table has been used as this gives the sm811er sPace separations and hence greater discrepancieso

2) A DD1Jd1ng with a wall 100 ft square including one window 40 ft by 50 ft. Here it should be appreciated that the most appro-priate result is 40 ft by 50 ft by ＱＰｾ ｧｬｶ･ｾ ｡ｾ fie It might, however, be looked up as 100 ft by 100 ft by 2 g ves 4005 fto

The next example includes a case with percentage opening of less than 20% which is the lower limit covered in the tables given in Internal Report Noo 1870 It has been specially calculated for this note and for convenience the geometrical form considered is a circular ､ｩ･ｊ｣ｾＮ The results given below all refer to a circular window of radius 9.48 fto

Disc

9048

ft radius ＱＰｾ opening =

19.8

ft Disc 21.2 ft radius 20% opening - 17 ft

Ｈｩｾ･ｯ could be a window

9.48

ft in radius)

Disc 30 ft radius 10% opening - 1203 ft

Ｈｩｾ･ｯ could be 8 window

9048

ft in radius)

Disc 35.8 ft radius 7% opening - zero + 2.5 ft (flame factor) (ioeo could be a window

9.48

ft in radius)

From the above it can be seen that for values of window opening between 100% and 20% the order of the discrepancies is not intolerable0 As the window opening tends to 7%, however» the

simple approach breaks down eompletelyo The reason for the break-down is fundamentalo The object of the separation is to reduce the

3

-ｯｰ･ｮＱｮｾ this is achieved by reducing the configuration factor to

0.07 ＨｯｲＭＷｾＩＮ Ifg however, the window opening is ｯｮｾｹ Ｗｾ then the

theoretical mean intensity at the radiating surface is already

reduced by the required factor. No separation is therefore needed. Where Table

5

of Internal Report No.

187

is considered the cor-responding oritical window opening is ＳＮＵｾＮ

It will be noticed that no appreciable problem arises in the use of the tables themselves, only when an attempt is made to extend the fundamental principles to the case of very low percentage window openings. In fact the problem can be resolved quite readily for where percentage window openings of less than ｬｏｾ are considered oalculations should be based on localized areas. Where the windows are scattered uniformly each individual opening should constitute the basis of the calculations. Where they are grouped together each group should be considered individually and an appropriate value adopted for the percentage window area.