Design and build contract

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The JCT Design and Build Contract 2005 replaces the standard Form of Building Contract With Contractor's Design 1998 Edition. Like the other JCT 05 Contracts, it is a significant reformatting exercise. However, the changes are more than cosmetic and organisational. some of the defining characteristics of the old With Contractor's Design Contract (as interpreted by the courts) have been fundamentally changed.

From its beginning in 1981, the With Contractor's Design form was seen as conferring single point responsibility on the contractor. The contractor not only took responsibility for the build but also for the design as well. The courts gave this view of the contract massive support in the case of Henry Boot v Co-op [2003 84 Con LR 164]. In that case the court decided that the contractor's obligation to complete the design started by the employer or his team required the contractor to take responsibility for that initial design and ratify it. The contractor's responsibility was not limited to completing the design but extended to overall responsibility for prior design work.


At a stroke the Design and Build 05 Contract changes all that. 2.11 states that (subject to clause 2.15 - which relates to divergences from statutory requirements), the contractor shall not be responsible for the contents of the Employer's Requirements or for verifying the adequacy of any design contained within them. so a contractor is no longer responsible for early design errors before he became involved. With that one change the whole concept of single point responsibility may collapse like a house of cards.

The payment provisions of the With Contractor's Design form of contract were widely interpreted to be favourable to the contractor. Clause 30.3 was interpreted to mean that the contractor drove the payment process by applying for payment and if the employer failed to put in place either or both of the 5 days payer's notice and/or the notice of withholding, then what the contractor applied for became his entitlement under the contract. There was some ambiguity within clause 30.3 but certainly many adjudicators read the contract in that way.

The payment regime is found at section 4 of the Design and Build 05 Contract at 4.7 to 4.10. It is still for the contractor to apply for payment but if the employer fails to put in his 5 days payer's notice the amount due to the contractor is not the amount stated in the contractor's application but the objective gross valuation of the works assessed under the contract at clause 4.8.

Therefore, no longer can a contractor go to an adjudicator and insist on being paid the value of his application. Instead the adjudicator will have to decide what is the proper gross valuation under 4.8.

Although the With Contractor's Design form of contract was regarded as innovative in allowing the contractor to develop the design after the contract had been made, the mechanisms of the contract for dealing with design detail in relation to a developing design were considered to be weak. The relationship between clause 5.3 and 8.1 was seen as the fault line. Goods were required to be of the standards described in the Employer's Requirements or if not described there, in the Contractor's Proposals, "or specifications referred to in clause 5.3". 5.3 merely required the contractor to provide the employer free of charge with specifications and details which the contractor prepared or used. In an effort to cure this weakness the Design and Build 05 Contract introduces at schedule 1 a design submission procedure under which documents are submitted for review and the employer marks them A, B or C status as appropriate. The contractor is not entitled to proceed with the design and is not entitled to payment unless given A or B status.

This paper presents the results of a detailed study of projects using the JCT With Contractor's Design (WCD) form of contract. Initially 49 projects were investigated to discover how the WCD contract is being used: for what kind of projects, by whom, in what circumstances, and what management approaches are being used. Fourteen projects were then selected to be investigated in more detail on their performance in terms of time, cost, quality and management. The variables were assessed in absolute terms and were then compared to what might have been expected if the contract had been managed in the conventional manner, i.e. using the JCT 80 standard form of building contract.

When we refer to something as ''structural'' in the Construction Industry, then we basically mean the parts that carry the loads, where ''load'' is the force acting on the building. The building therefore needs to accommodate the forces acting on them to prevent the building from collapsing.

similaritiesThe similarities between traditional timber framed construction and modern steel framed structures are the following:•In both traditional and steel framed structures, loads are carried on a framework of steel or timber whereby the walls don't usually carry any loads but only support weatherproofing infill walls.

  • Both have triangulated bracing between the roof trusses to add stiffness and stability.
  • They both have large columnsThese structures differ from modern timber framed structures for the following reasons:1. The loads on e.g. the internal walls, floors and roofs and work out the sizes of the components can be calculated with a computer. This differs in terms of structural principles as this will provide easier and probably better information.

2. With framed structures the loads are carried on a framework of steel or timber, but with modern framed structures, the vertical frame members are closer to each other with maximum spacing of 600mm studs and act more like a load bearing rather than a modern framed structure.

3. Unlike steel framed where steel performs well in both tension and compression, timber is only available in short lengths and cannot therefore achieve large spaces.

4. A very important element for the structure stability is bracing, this is to prevent collapse due to wind and gravity loads. For the framed structures bracings to walls and roofs are provided by the large triangulating members while bracing to for modern timber framed is provided by the layer of Oriented strand Board (OsB) which is an engineered wood panel that is formed by layering strands of wood in specific orientations and then bonding the layers with a resin.

5. Load-bearing construction is usually more effective for small cellular buildings as the strength depends on the thickness of the wall whereas modern steel framed can have spans of approximately 12 meters and can therefore be used for large buildings such as B&Q.

6. Unlike the modern timber framed structure, the modern steel framed structures have a shallow pitch to minimise spread and where there also be extra depth at the knee joint for strength and rigidity.

Modern timber framed wallThe insulation is fitted between the timber studs such as the vertical members to prevent cold air penetrating through joints between the insulation and the timber.

The rafters have to be measured before cutting the insulation boards as spacing's can vary. The insulation boards must be trimmed precisely to achieve continuity of insulation and they must be fitted tightly between the timber studs. Gaps are filled with expanding sealant. Battens are nailed to the studs to prevent the insulation moving within the timber stud cavity. Thermal sheathing boards should also be fixed to the external surface of the timber frame structure to avoid thermal bridging.

This method differs from a new build masonry cavity wall construction as the masonry wall is made of brick and blocks with a gap in the middle where the insulation is fitted in the cavity. For the partial fill of insulation, the insulation is fixed tightly to the inner leaf of the wall secured by wall ties, maintaining a clear cavity.

One needs to ensure that the timber that is being used in the specific construction process is of a high durability. To ensure durability of the timber a careful assessment of the environment should be done in which the timber would be placed. When doing this assessment one needs to take into consideration the possible risks at hand. Essentially one needs to establish whether or not the timber will be kept dry to know what treatment would be more advantageous to the prescribed situation. Other things that should be taken into consideration is the economic cost of using the specified timber as opposed to some other construction materials, the likely health risk associated with using the specified timber, the consequences of failure and also the cost of remedial work or replacement. With these risk carefully analyzed it is possible to decide on the type of treatment needed to be done. Essentially the preservative that is applied should be permanent, penetrate the wood, be safe to apply and handle, be economical and not destroy the wood. No one preservative will protect the wood against all possible conditions, but they are several means of preserving the wood in various situations. These may be in the form of mechanical protection or preservative treatment. Mechanical protection involves erecting barriers to attacks in the form of paints, ventilation and water drainage; soil poisoning is also a means of repelling insect attack. Preservatives on the other hand are used to deter rather than to bar insects or fungi.

Timber or lumber that is treated with a preservative generally have it applied through combined vacuum and pressure treatment. The preservatives used to pressure-treat lumber are classified as pesticides. Treating lumber provides long-term resistance to organisms that cause deterioration. If it is applied correctly, it extends the productive life of lumber by five to ten times. If left untreated, wood that is exposed to moisture or soil for sustained periods of time will become weakened by various types of fungi, bacteria or insects.

Type of preservativesPreservatives can be classified into three broad categories:Water-based preservtivesWater is the most common solvent carrier in preservative formulations due to its availability and low cost. Water based preservative- these are used on timbers that are used in above ground situations, it is ideal for decorative purposes and for use in the interior of buildings as it is generally odorless and non-staining. Due to the fact that these preservatives are water-soluble they tend to leach out when exposed to moisture over a period of time. When this type of preservative is applied it may cause the wood to swell, it is with this in mind the wood may have to be re-dried to acceptable moisture content. Water based preservatives reduce the flammability of the timber. The most commonly used types are copper chrome and arsenic; these penetrate the timber after a few days and have the advantage of allowing the timber to resist leaching. Water-based systems do however have the drawback that they swell timber, leading to increased twisting, splitting and checking than alternatives.

Oil-bourne preservatives,Oil bourne preservities includes pentachlorophenol, copper naphthenate, and creosote. All of them are toxic and are generally not used in consumer products. The chemical composition of these preservatives varies in strength and composition according to application. When oil bourne preservatives are used they tend to gives the timber a green color. They are used for treating post, plywood and siding. Creosote and creosote solution are used for protection against decay and organism; they are used on timbers that are located on the exterior of buildings or on timber that is susceptible to the elements. They produce an unfavourable odour and therefore cannot be used on the interior of buildings. There are three types of creosote solutions, namely; creosote, creosote-coal tar mixtures and creosote petroleum mixtures. Creosote is used for protection against organisms, creosote-coal tar mixtures are used for protection against marine organisms and salt water while creosote petroleum mixtures are used to reduce cost as the mixture is mixed to a 50%-50% ratio, they have the tendency to leave an oily film on the treated surface.

Light Organic solvent PreservaivesLight organic solvent preservative have good penetration capabilities and are generally non-corrosive, the treated timber can be decorated with the use of paints and it is not necessary to re-dry the timber after it has been treated. Due to the effect of the solvent emission on the atmosphere, a number of organic solvent preservative are now formulated as emulsions with water. When this is done the penetration capabilities becomes less effective, therefore the solvent is still required in some cases.


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