In all the construction projects, the designs of the buildings to be constructed are planned in the pre-construction phase, and based on these designs further activities are carried on. There are direct linkages and relationships between the design (shape) and the cost incurred by the project in the construction sector. In this article, we will analyse this relationship and try to understand different aspects related to it.
Any design of a construction project, which is finalized, can be expressed in terms of many variables. These variables are known as design variables and they have direct cost implications. The relationship between these variables and cost can be understood and a direct forecast of the tentative costs can be done based on these variables.
Some of the common design variables that have direct cost implications are as follows:
1. Plane Shape
2. Complexity of Shape
3. Building’s Size
4. The average height of Storey
5. Number of storeys
6. Building envelope
7. Circulation space
8. Building’s grouping
9. Glazed wall percentage
10. Span of floor
11. Electrical and Mechanical Services
Now we will have a look at all these factors one by one.
By plane shape, we mean the shape of the plan view. In simple cases, the plan shapes can be square or rectangular. The cost of shapes with a lower perimeter to area ratio will be lower. For example, the shape of a square plan will have a low cost than the shape of the rectangular plan.
The shape of the building
The shape or the design of the building is directly linked with the cost incurred in constructing this building. The simpler the shape of the building the low will be the project cost. On the other hand, the complex is the shape or design, the higher will be the project cost.
For example, we can consider two projects with different plans A and B. Both the plans A and B have the same area but there are many irregularities and complex shapes that exist in plan B rather than Plan A. Hence it can be concluded here that the total cost will be higher for plan B as compared to plan A. This is due to the simplicity of Plan A, which is lacking in Plan B.
There are two common types of shapes, on which the buildings can be made:
1. Regular shapes
2. Irregular shapes
The regular shapes are simple shapes like square or rectangle and the irregular shapes are comprised of other complex shapes. The cost implication of constructing the building on these shapes is simple. For regular shapes, we will find that the cost incurred is less as compared to the irregular shapes. This is due to the fact that the irregular shapes have higher perimeters than the regular shapes and it is difficult to carve irregular shapes as compared to the regular shapes. Hence, it can be concluded that the cost of construction is low for regular shapes and is higher for irregular shapes.
Size of the building
The size of the building is another important parameter that has a direct relationship with the cost of construction. Buildings of the same shapes and different sizes can be taken into consideration for this. The overall cost per unit area is smaller for larger buildings than the smaller ones. This is due to the fact that the cost and other parameters don’t increase proportionally with the size. We are bound to incur smaller costs in case of the buildings of larger sizes when we calculate the cost per unit area.
For example, we have two rectangular buildings, A and B. Building B is having a 4 times plan area as compared to building A. In this case, when we calculate the per-unit cost of the two buildings, we will find that building B has a lower cost per unit area than building A. Hence, we have found that size has a direct relationship with the cost of the building.
The average height of Storey
The cost implication of the building to be constructed is also related to the height of the Storey of the buildings. The higher is the height the higher will be the cost to construct it. If we compare two buildings with different storey heights, we will find that the cost to construct the building with higher storey height is more as compared to the lower height one.
Number of storeys
The number of storeys is also directly linked with the cost. The more is the number of storeys the more will be the cost. This is due to the fact that more efforts and labour cost is required to make more storeys, especially on height. IF we have two buildings one 4 storey and the other with 8 storey, we will find that the cost of an 8 storey building is much higher than the cost of 4 storey building.
The envelope of the building
The envelope of the building is for example an important design variable. The simple will be the envelope of the building, the low will be the cost. For example, the cost of the square envelope will be the minimum, when we compare it to the other shapes.
There are many other design variables having an impact on the cost of the building. For example, the circulation cost is linked to the overall cost. The lower is the circulation space of the building, the lower will be the overall cost. The mechanical and electrical services are also related to the cost of building getting construction. The extent of usage of these services determines the actual cost of the building. The percentage of the glazed wall is another design variable that is important and determines the overall cost of the building. The lower will be the percentage of the glazed wall, the lower will be the cost.
Apart from all the factors discussed above, the constructability of the building is also a big determinant in the overall cost of construction. The higher the constructability of the building, the easier it is to construct it and the lower will be the cost.
Indices to represent complex shapes of buildings
In scientific studies, there are many indexes that are used to describe the shape complexities of various buildings. Some of these are as follows:
• W/F index: Wall/Floor index.
• LBI index: Length/Breadth index.
• PSI index: Plan/Shape index.
• Building planning “m” index.
• VOLM (volume/block compactness) index.
• Cook’s JC (shape-effectiveness) index.
• POP (plan compactness ratio) index.
• Optimum envelope area
All the above indexes are used to describe the shape or design of the building as well as its design features. These indexes are represented as numerical values and their exact measure will define the Complex features of the design of the building to be constructed.
Thus, we have seen in detail the different design variables which are direct determinants in the construction cost of the buildings. These factors or design of the building is directly related to the estimated cost of the building. We have also seen different aspects related to the design of the buildings and indices to represent the complexity of the building design.
All these aspects should be kept in mind at the time of planning in the preconstruction phase. They should be taken care of in the project management part. The design software like CCIS or Construction Cost Information System can be used to take care of these design variables in real-time.