Design Economics I

1
Design Economics I

• Decisions made in this stage in the project have
  major implication which is also fundamental to
  good cost management.
• A decision can be very expensive to correct at a
  later stage.
• The available information may be basic. The cost
  adviser is to establish the most cost effective
  solution such building shapes, heights,
  configurations etc for particular situation.

2
Design Economics I

• Smith (1998, p54) suggests that the project
  manager has to balance the following factors-
• functional
• technical
• aesthetic
• financial
• environmental
• or it is simply balancing (i) time (ii) and (iii)
  quality.
• An undue bias towards any one of these three will
  create an imbalance and lead to failure to
  achieve the client requirements.

3
Design Economics I

• Time
• Design length
• Start date
• Hand-over
• Completion

• Performance
• Appearance
• Quality
• Function
• Durability
• Maintenance

Clients Requirement

• Cost
• Budget
• Estimate
• Tender
• Final account
• Cost-in-use

Relationship between time, cost and quality
(Ashworth, 1994, p 103)
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Design Economics I

• Smith (1998, p56) simplifies the design
  considerations of these factors as follows
• Time usually increases with demands for higher
  quality
• Increase in time generally results in higher
  costs
• cost increases with greater quality
• The Project manager (and cost adviser) must
  understand the sensitivity of the relationship
  when discussing the factors
• the architect will traditionally favour better
  quality and higher cost,
• the builder may favour a lower cost and a timing
  of progress that optimises his resources.

5
Design Economics I

• The cost advisers role is to protect the
  clients building budget, making sure that
  designs do not, when costed exceed the clients
  predetermined financial limits.
• However quality and time should not be excluded
  for the sole sake of cost.

6
Design Economics I

• Types of design decisions that will influence
  cost
• total height
• storey height
• circulation areas
• plan shape (cost geometry)
• size
• configuration of buildings
• cellular or open plan
• mechanisation / prefabrication
• redundant performance
• buildability

7
Design Economics I

• TOTAL HEIGHT
• Ashworth (1994, p111) summaries the reasons why
  tall buildings are more expensive than low rise
  of comparable size
• the need to provide cranage for construction
  purposes
• the need to provide expensive services such as
  lifts throughout the building
• the higher cost of related elements such as the
  footings, staircases, etc
• the increase in fire regulation requirements
• the increase in requirements for circulation
  areas (ie lifts)
• there is less competition for high rise work
  therefore builders prices are higher.
• There are additional structural costs due to
  windloading factors (see Figure 1.2)

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Design Economics I

• TOTAL HEIGHT
• Ashworth (1994, p111) also pointed out that
  because of the above factors tall structures are
  preferred only where the land is either expensive
  or in scarce supply.

9
Design Economics I

• STOREY HEIGHT
• Largely determined by the needs of the user of
  the building.
• The reason for a greater storey height might
  include
• accommodating types of plants and machinery,
• for prestige reasons (ie a hotel room), or
• tradition (ie churches).
• These buildings also share the same problems as
  tall buildings (ie greater areas of circulation,
  vertical transportation costs are greater, etc).

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Design Economics I

• STOREY HEIGHT
• These buildings will also cost more per square
  metre because there is more wall required to
  enclose the same amount of space
• In multi storey building construction (noting the
  timsing factor on additional floor height), it
  is important to minimise the height of any
  vertical zone (ie floor to floor height, ceiling
  to underside of slab height, etc).

11
Design Economics I

• STOREY HEIGHT
• There are several effects that reduction in
  storey height will have on the components of a
  building
• it will make the footings less expensive
• it will allow shorter columns of smaller section
• it will require less treads and risers
• it will require less external wall area
• it will reduce the amount of internal walls and
  finishes required
• service distribution lines, pipes, etc., will all
  be reduced.

12
Design Economics I

• CIRCULATION AREAS
• Smith (1998, p60 - 61)
• Circulation areas can account for between 10 and
  50 of the gross floor areas of a building
  depending upon efficiency of layout and function.
• The amount of circulation space required affects
  the following factors
• Standards of usage (prestigious buildings require
  more circulation, ie banks)
• vertical circulation - higher quality building
  require more lifts and therefor more circulation
  space.
• Horizontal circulation - affected by usage of the
  building, ie schools where large volumes of
  occupants need to be moved quickly from points
  around a building, or hospitals where beds need
  to be move around.

13
Design Economics I

• PLAN SHAPE (COST GEOMETRY)
• (Smith, 1998, p61)
• The plan shape of a building has an important
  effect on its costs.
• The more a building retreats from a square (the
  circle is the most efficient enclosure of space,
  but the cost of building circular work makes it
  too expensive) the more expensive the external
  envelope (ie external walls and windows) will
  become in relation to the remaining building
  costs. The simpler the shape of the building the
  lower will be its unit cost.
• Longer, narrower, complicated and/or irregular
  outlines will increase the perimeter/floor area
  ratio which will be accompanied by higher cost.

14
Design Economics I

• PLAN SHAPE (COST GEOMETRY)
• (Smith, 1998, p61)
• The plan shape of a building has an important
  effect on its costs.
• The more a building retreats from a square (the
  circle is the most efficient enclosure of space,
  but the cost of building circular work makes it
  too expensive) the more expensive the external
  envelope (ie external walls and windows) will
  become in relation to the remaining building
  costs. The simpler the shape of the building the
  lower will be its unit cost.
• Longer, narrower, complicated and/or irregular
  outlines will increase the perimeter/floor area
  ratio which will be accompanied by higher cost.

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Design Economics I

• PLAN SHAPE (COST GEOMETRY)
• an irregular outline will also result in
  increased costs for other reasons setting out
  site works and drainage works are all likely to
  be more complicated and more expensive.
• Plan shapes also affect other elements e.g.
  internal partitions, wall finishes etc.

16
Design Economics I

• PLAN SHAPE (COST GEOMETRY)
• The two methods used for assessing the
• plan shape of a building
• The wall to floor ratio (or perimeter to floor
  area ratio)
• this measures the efficiency of enclosing space.
  A lower ratio implies that less wall is required
  to enclose the same floor area. As walls are
  extremely expensive, this reduction in wall area
  would represent a saving in the building cost.
• Computed by diving the face area of external
  surfaces of the building by the fully enclosed
  covered area (FECA)

17
Design Economics I

• PLAN SHAPE (COST GEOMETRY)
• The two methods used for assessing the
• plan shape of a building
• the perimeter over plan ratio (pop ratio)
• this measures the compactness of the plan shape.
  A circle is the most efficient way to enclose the
  most space but it is not cost effective to build
  curved walls.
• It computed by relating the perimeter of a
  buildings at a building at typical floor level to
  that of a circle enclosing the same area and
  expressing the result as a percentage.

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Design Economics I

• PLAN SHAPE (COST GEOMETRY)
• The two methods used for assessing the
• plan shape of a building
• the perimeter over plan ratio (pop ratio)
• the formula for POP ratio is
• POP 2??A x 100
• P
• Where
• A the FECA of a typical floor level
• P the perimeter enclosing that area, measured
  internally
• Note POP ratio for a circle 100
• POP ratio for a square 89