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. ss5

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. ss5

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) ss5

4. 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. ss5

5. Design Economics I • The cost adviser’s role is to “protect” the client’s 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. ss5

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 ss5

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 builder’s prices are higher. • There are additional structural costs due to windloading factors (see Figure 1.2) ss5

8. 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”. ss5

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). ss5

10. 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). ss5

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. ss5

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. ss5

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. ss5

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. ss5

15. 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. ss5

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) ss5

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. ss5

18. 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% ss5