We intend to provide in this article the most practical ways to reduce the cost of building construction. This is necessary because, in every human development activity that involves cost, it is always anticipated to achieve the best with the least. This anticipated benefit forms the three basic things that clients expect from every building project they are executing. The three basic things include:
- Low cost of construction
- Short construction times
- Excellent functional performance and quality.
Building constructions are usually capital intensive projects, especially for multi-storey buildings. Having the core duty of structural engineers in view, beginning from the time of conceptualization, efforts are made to ensure that a structurally sound and efficient building is achieved using the minimum possible cost. This is the factor on which the limit state principles applied at the design stage of a project are based. At the construction stage, the focus is based on the prices of concrete, reinforcement, formwork, and labour.
The effort to achieve economy in construction begins from the conceptual stage. It then proceeds to the design stage and finally the construction stage and the imperceptible use stage. More care should be taken during the conceptual stage because the outcome at this stage determines the outcome at the design stage and finally the outcome at the construction stage.
Processes of Project Cost Optimisation
At the conceptual stage, key considerations are aesthetics and service integration. This is usually the duty of the Architect. Though architectural requirements usually dictate the arrangement of floor layouts and positioning of supporting walls and columns, it is necessary that the design Structural Engineer liaise with the Architect to ensure that the most economical arrangements are achieved without grossly compromising the aesthetics and service requirements of the structure.
Design Stage (Structural Design)
At the design stage, spans and layouts, maximum spans, loads, intended use, fire resistance, and stability of the structure play key roles. The sections of key structural members to bear the loads are assigned. Efforts should be geared towards achieving the most economical of these options where feasible because the outcome here determines the cost of construction of a reinforced concrete or steel –framed structure which is obviously affected by the prices of concrete, reinforcement, formwork, and labour. There are also wider aspects of the economy, such as whether the anticipated life and use of a proposed structure warrant the use of higher or lower factors of safety than usual, or whether the use of a more expensive form of construction is warranted by improvements in the integrity and appearance of the structure. The logical processes outlined below are often following in arriving at a decision:
- Establish layout while considering spans, loads, intended use, stability, aesthetics, service integration, programme, etc. Identify worst case(s) of span and load.
- Envisage the structure as a whole. With rough sketches of typical structural bays, consider, and whenever possible, discuss likely alternative forms of construction. Identify preferred structural solutions.
- Determine the most economical sections of the slab by interpolation while considering the maximum slab span and the relevant characteristics imposed loads – these can be determined from suitable charts.
- Determine the most economical sections of beams by interpolation while considering maximum beam span and estimated ultimate applied uniformly distributed loads obtained from slabs, claddings, partitions, and other line loads that the beams support – these can be determined from suitable charts.
- Determine the most economical sections of columns by interpolation while considering the estimated total ultimate loads and the number of storeys – these can be determined from suitable charts.
- Using engineering judgment, compare and select the option(s) which appear(s) to be the best balance between structural and aesthetic requirements, buildability, and economic constraints.
Once the design satisfies service requirements, the construction stage forms the most important stage where savings in cost can be achieved. Before bidding for the project, the project cost is estimated. The costs are estimated by multiplying quantities of concrete, formwork, and reinforcement, by appropriate rates. In the estimation, due allowances should be made for differences in self-weight (cost of support), overall thickness (cost of perimeter cladding), and time. The most economical proportions of these materials (concrete, formwork, and reinforcement) and labour requirement to fix them will depend on the current relationship between the unit prices while more focus should be on the floor plates from where the bulk of the costs emanate. Finally, it is necessary to visualize the construction process as a whole and the resultant impact on programme and cost.
Attempts to determine the most economical proportions for a particular member based only on inclusive prices of concrete, reinforcement, and formwork may be misleading. It is nevertheless possible to lay down certain principles. In broad terms, the price of concrete increases with the cement content as does its durability and strength. Concrete grades are often determined by durability requirements with different grades used for foundations and superstructures. Strength is an important factor in the design of columns and beams but rarely so in the case of slabs. Nevertheless, the same grade is often used for all parts of a superstructure, except that higher strength concrete may sometimes be used to reduce the size of heavily loaded columns.
Mild steel and high yield reinforcements have been widely used over the years, but recently, probably following the development of Eurocodes, grade 500 is now produced as standard, available in three ductility classes A, B, and C. It is always uneconomical in material terms to use compression reinforcement in beams and columns, but the advantages gained by being able to reduce member sizes and maintain the same column size over several storeys generally offset the additional material costs. For equal weights of reinforcement, the combined material and fixing costs of small diameter bars are greater than those of large diameter bars. Therefore, it is generally more sensible to use the largest diameter bars consistent with the requirements for crack control. Fabric (welded mesh) is more expensive than bar reinforcement in material terms, but the saving in fixing time will often result in an overall economy, particularly in slabs and walls.
Formwork is obviously cheaper if surfaces are plane and at right angles to each other and if there is the repetition of use. The simplest form of floor construction is a solid slab of constant thickness. Beam and slab construction are more efficient structurally but less economical in formwork costs. Two-way beam systems complicate both formwork and reinforcement details with consequent delays in the construction programme. Increased slab efficiency and economy over longer spans may be obtained by using a ribbed form of construction. Standard types of troughs and waffle moulds are available in a range of depths.
Precasting usually reduces considerably the amount of formwork, labour, and erection time. Individual moulds are more expensive but can be used many more times than site formwork. Structural connections are normally more expensive than monolithic construction. The economic advantage of precasting and the structural advantage of in situ casting may be combined in composite forms of construction.
In summary, it could be opined that economy in the use of formwork is generally achieved by the uniformity of member size and the avoidance of complex shapes and intersections. In particular cases, the use of available formwork of standard sizes may determine the structural arrangement. Fast-track construction which saves cost requires the repetitive use of a rapid formwork system and careful attention to both reinforcement details and concreting methods.
Personnel (skilled and unskilled) engaged in the construction activities should be knowledgeable, qualified, and fit for the duties. Specifications for personnel should be within acceptable limits because overspecification is quite costly.
At the use stage, a number of costs are also incurred. If a structure has more cooling requirements, more expenses are incurred on use. In terms of thermal mass, concrete’s thermal mass tends to reduce excessive diurnal temperature fluctuations and causes a useful delay between peak external and peak internal temperatures. It can, therefore, reduce cooling requirements in buildings, thereby reducing both initial and running costs of services. Concrete can be formed into appropriate shapes to aid the transfer of heat from circulating air to the structure.
Other Measures to Consider When Using Alternative Systems of Construction
a. Use of reinforced concrete frames instead of steel frames can save up to 24% in frame costs and 5.5% in overall construction costs.
b. ‘Pay as your pour’ method of reinforced concrete construction can save up to 0.3% of overall construction cost when compared to structural steel-framed buildings.
c. Foundations for concrete-framed buildings may cost up to 30% more than those for steel-framed buildings. However, this is more than compensated by up to 24% savings in superstructure costs. Superstructures cost 5 to 15 times as much as foundations.
d. Time builds on cost. There should be an effort to reduce time wastage by comparing the time of insitu-concrete framed buildings against steel-framed buildings, increasing buildability through design discipline, repetition, integration, simplification, standardization of design details, rationalising reinforcement, designing and detailing for prefabrication, precasting or part-precasting. Time can also be saved by preparing against delays caused by extreme hot and cold weather works, reducing striking times and propping times as well as avoiding late changes to concrete.
e. Accuracy in construction should be kept within the following limits to ensure that costs are not adversely affected:
Variation in the plane for beams: concrete +22 mm, steel +20 mm
Position in plan: concrete +12 mm, steel +10 mm
In many cases, the most economical solution can only be determined by comparing the approximate costs of different designs. This may be necessary to decide, say, when a simple cantilever retaining wall ceases to be more economical than one with counterforts or when a beam and slab bridge is more economical than a voided slab.
The application of whole-life costing which focuses attention on whether the initial cost of a construction of high quality, with little or no subsequent maintenance, is likely to be more economical than a cheaper construction, combined with the expense of maintenance is often necessary to choose the most feasible option. The experience and method of working of the contractor, the position of the site and the nature of the available materials, and even the method of measuring the quantities, together with numerous other points, all have their effect, consciously or not, on cost. So many and varied are factors involved that only experience and a continuing study of design trends can give reliable guidance.
Goodchild, C.H. (1997). Economic Concrete Frame Elements. British Cement Association Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS