The following shows the principal design options commonly adopted.
(a) Design by promoter or a consultant The whole of the design, including all drawings and specifications, is completed before construction tenders are sought except for drawings not needed for tendering purposes, such as for concrete reinforcement.
A promoter may have sufficient staff to undertake design work in house
or he may put all design out to a consultant, or divide the design work between them.
On schemes involving different types of engineering, design may be let out in separate packages to different specialist consultants. For instance the design of an industrial estate may be packaged into roads and drainage; water supply and sewerage; power supplies, and landscaping. For large schemes the promoter may appoint an overall consultant with wide experience to co-ordinate the inputs of the specialist design consultants.
Some elements of design may be left for the construction contractor or his sub-contractors to undertake, such as the design of heating and ventilating systems, or the cladding for a building. Specialist suppliers may need to design their product or services to suit the project. Advantages are:
The promoter can check all aspects of the design to ensure they meet his
requirements before construction starts.
Competitive tenders for construction are obtained on a clearly defined basis encouraging construction contractors to submit lowest prices.
The risk of having to make alterations to the work during construction is minimized, giving a better chance of the project cost not exceeding the tendered price.
The promoter is not committed to proceed with construction until he sees tendered prices and accepts a tender.
(b) Outline designs provided with detailed design by others The promoter draws up outline designs and a specification of his requirements.
He appoints a firm or firms of specialist designers to carry out detailed design, and then engages a management contractor to co-ordinate both the detailed design and the construction. This type of arrangement can be seen in some management contracts (see Section 2.5).
(c) Layout design by promoter; detailed design by contractor The promoter specifies functions and design standards, and supplies layout plans. The contractor then undertakes the detailed design before proceeding with construction. The works may be relatively small, such as the design of a retaining wall; or fairly extensive such as the design of an intake and drainage pumping station, or the structural and reinforced concrete design for a water tower.
Advantages are:
The contractor can adopt designs suiting his constructional equipment and his usual construction methods, enabling him to tender his lowest price.
The cost of making design alterations during construction do not fall on the promoter.
Disadvantages are:
The design may tend to suit the contractor more than the promoter.
Control over design details is lost to the promoter.
The contractor must increase his price to cover design risks.
(d) Functional specification by promoter: design by contractor
The promoter specifies the functions the project is to perform, for example
the size, quality and performance criteria for the intended works. He also provides drawings showing the location of the intended works and draft layouts for them, and may specify standards for design. The works required may be extensive, such as design of a road, or the civil works and plant for sewage treatment works. The contractor undertakes the layout and detailed designs to the standards required.
This is the basic set-up for design and build (D&B) contracts where most of the design responsibility is held by the contractor. The advantages, disadvantages and complexities of such contracts are dealt with in Section 2.6.