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E N E R G Y E F F I C I E N C Y P R O J E C T

C O M M E R C I A L B U I L D I N G S

DISCUSSION PAPER:

DEFINITION OF BASIC FORMS FOR REPRESENTATIVE BUILDINGS

This paper follows on from the “Representative Buildings” paper, issued in October 2000. It

considers whether a small number of building forms may be enough to define the geometry of

Representative Buildings used in computational modelling to prepare the BCA Energy

Specification.

Uses of Representative Buildings

The energy efficiency measures of the BCA will define performance requirements for annual

energy consumption and identify specific features of building fabric and services that will be

deemed to satisfy the performance criteria. Computational modelling, tempered by practical

experience, will be used to set annual energy budgets and evaluate possible energy saving

features.

The modelling will analyse a set of Representative Buildings, selected to reasonably reflect

prevailing standards in the diverse stock of buildings identified as Classes 2-9 under the BCA.

The buildings will be particular combinations of different sizes and shapes (termed “forms” in

this paper), envelope construction, services systems and patterns of usage. A building form

may be common to a number of BCA Classes, while the construction, services and usage

profile will change to suit the classification. The chosen combinations will be illustrated to

sketch plan level and costed on an elemental basis. To demonstrate their real-world relevance,

they will also be related to comparable built examples.

The Representative Buildings will be tested at nominated locations throughout the defined

climate zones, with varying solar orientations. Modelling will first establish baseline energy

consumption rates and then assess the relative consumption of similar buildings enhanced by

specific energy saving features. Features demonstrated to be cost effective will form the basis

of deemed-to-satisfy provisions of the Code and the simulated performance of enhanced

buildings will be considered in defining performance criteria.

Selecting forms for Representative Buildings

To keep modelling effort within practical bounds, the total number of combinations analysed

should be limited. Initial sensitivity studies on a pilot range of buildings and locations will

determine which fabric, services or usage options may be removed from further consideration.

Defining even the pilot buildings, however, requires the selection of a small sample from the

wide range of building forms found in the built environment.

To test how this spectrum may best be represented, the effects of size and shape on building

envelope area has been briefly but systematically examined. (In this context, building

envelope area is the total surface area of the lowest floor, the external walls and roof.) By

itself, surface area is a potentially misleading indicator of environmental interaction, as it does

not take account of the varying exposures of external surfaces to sun and wind or of their

different performance characteristics. In this analysis, however, it has been used simply to

compare the geometry of buildings in which the other variables are kept constant. The aim of

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E N E R G Y E F F I C I E N C Y P R O J E C T

C O M M E R C I A L B U I L D I N G S

the investigation was to identify which building sizes show most change in envelope area

when their three dimensional form is altered.

The analysis looked at rectangular buildings with total floor areas ranging from 100m 2 to

100,000m 2 . The aspect ratios of their floorplates were varied from 1:1 through to 20:1 and

the number of storeys from 1 to 20. (Floor-to-floor height was held at a constant 3.6m and the

variations were applied whether or not the resulting building forms might be considered

realistic.) The total envelope area generated by these variations was divided by the total floor

area in each case (giving an envelope:floor area ratio) so that buildings of different sizes could

be readily compared. Figure 1 below summarises results within the range where trends were

clearly established.

absolute change in envelope:floor area ratio

5.0:1

4.5:1

4.0:1

3.5:1

3.0:1

2.5:1

2.0:1

1.5:1

1.0:1

0.5:1

Influence of shape effects

on envelope:floor area ratio

aspect ratio effect

(on single storey)

storey effect

(with 1:1 aspect ratio)

combined effect

(aspect ratios 1:1 - 1:20,

1-10 storeys)

0.0:1

0 2,000 4,000 6,000 8,000 10,000

total floor area

Figure 1

Two observations from the plotted results are that, in isolation, increasing the number of

storeys has a greater potential effect than varying the shape of the floorplate and these

changes have most impact on buildings with less than 2,000m 2 total floor area. Buildings

smaller than 1,000m 2 are particularly sensitive to changes in form. The results suggest that

representative forms should be concentrated below 2,000m 2 and that a single form may serve

for the whole range of larger buildings.

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E N E R G Y E F F I C I E N C Y P R O J E C T

C O M M E R C I A L B U I L D I N G S

Large buildings

Figure 2 below shows the sensitivity of a 10,000m 2 building to variations in form. While

increasing the number of storeys initially has the greatest effect, its impact rapidly diminishes

as the number of storeys increases. After 5 storeys, the worst case aspect ratio has overtaken

it as the dominant influence on the envelope:floor area ratio. Given the rapid fall off in storey

effect, there seems little point in examining building forms of more than 10 storeys.

The decreasing curvature of each storey line from the top of the chart, highlights that changes

to floorplate shape have their greatest absolute effect on low rise buildings. Since aspect ratio

is relatively unimportant in controlling envelope area in a large, high building (in this case

10,000m 2 and 10 storeys), the representative building form could reasonably adopt a 1:1

floorplate aspect ratio.

Envelope:floor area ratio variation with

changes in storeys and aspect ratio

envelope:floor area ratio

2.50

2.00

1.50

1.00

1 storey

2 storey

3 storey

4 storey

5 storey

6 storey

7 storey

8 storey

9 storey

10 storey

0.50

0.00

0.01:1 0.1:1 1:1

aspect ratio

10:1 100:1

Figure 2

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E N E R G Y E F F I C I E N C Y P R O J E C T

C O M M E R C I A L B U I L D I N G S

Small buildings

A plot, in Figure 3 below, for a building of 100m 2 total floor area shows a similarly

diminishing storey effect but indicates the relatively greater influence of floorplate shape on

the envelope area of a small building. For this reason, smaller forms selected for

Representative Buildings need to include significant aspect ratio variations.

Envelope:floor area ratio variation with

changes in storeys and aspect ratio

envelope:floor area ratio

6.00

5.00

4.00

3.00

2.00

1 storey

2 storey

3 storey

4 storey

5 storey

6 storey

7 storey

8 storey

9 storey

10 storey

1.00

0.00

0.01:1 0.1:1 1:1

aspect ratio

10:1 100:1

Figure 3

Building forms for development

In light of the observations summarised above, five building forms have been suggested for

use in pilot modelling and refinement by the Working Groups to better match real-world

examples of the type intended (the building forms are detailed in the attached table). Because

computational modelling will not be particularly sensitive to core area dimensions, core sizes

have been set at the low end of likely ranges. In the case of the smaller buildings in

particular, the areas are simply nominal allowances for utility space. It is anticipated that the

Working Groups may adjust the dimensions of the nominated forms to more closely reflect

illustrative examples. Pilot modelling will also explore variations in size and shape to

confirm that the responses are in line with conclusions drawn from the analysis summarised in

this paper.

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C O M M E R C I A L B U I L D I N G S

Proposals for Representative Building forms

The discussion paper on Representative Buildings considered ABS data for the total cost of buildings approved for construction in Australia in the

years 95/96 to 99/00. Table 3 in the paper identified five building cost ranges reported in the ABS data and, using typical cost rates, deduced

average areas per building for each range. The cost ranges and the average area per building in each of them are shown below.

• $5m……………9,930 m 2

The paper also made preliminary suggestions for six Representative Buildings (two residential and four for Classes 3-9). The forms of these

buildings have since been developed into the five listed in the Table below, with total floor areas falling into the ranges indicated above. Total area

in each case is given as Fully Enclosed Covered Area (FECA). Each building form has a notional core whose area is deducted from the FECA to

establish a total Net Lettable Area (NLA) or functional area free of stairs, lifts, ducts and the like.

Proposed building forms:

Building Floorplate Height

ID

A

B

typical locations

CBD of capital city or

major regional town

CBD edge, major

regional towns or

resort centres

C Commercial and

industrial zones in

cities and towns,

university or hospital

campuses

D

City suburbs, regional

and smaller towns

BCA

Classes

building usage

total

(FECA)

(m 2 )

total

NLA

aspect FECA

(m 2 ) storeys ratio (m 2 ) length depth

zone depth

efficiency

core

area

(m 2 )

NLA

(m 2 ) flr-flr

2,3,5 2 = apartments

3 = hotel

5 = office tower 10,000 9,000 10 1:1 1,000 31.6 31.6 3.6 90% 100 900 3.6 0.20 2.7 0.7 0.9

2,3,5,9 2 = apartments

3 = hotel

5 = office block

9 = health care building

2,000 1,800 3 2:1 667 36.5 18.3 3.6 90% 67 600 3.6 0.20 2.7 0.7 0.9

6,7,8,9 6 = sales showroom

7 = controlled

environment storage

8 = factory, workshop

9 = auditorium

3,5,6,

8,9

3 = motel

5 = offices

6 = shops

8 = small laboratories

and workshops

9 = hospital ward block

flr thickness

ceiling height

plenum height

plenum

wall height

1,000 950 1 1:1 1,000 31.6 31.6 3.6 95% 50 950 6 0.20 4.8 1.0 1.2

500 475 1 5:1 500 50.0 10.0 5.0 95% 25 475 3.3 0.15 2.4 0.8 0.9

E All cities and towns 2,3 2 = home units (duplex)

3 = hotel or motel villas 200 190 1 2:1 200 20.0 10.0 5.0 95% 10 190 3.3 0.15 2.4 0.8 0.9

April 2001 Page 5 of 5

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