Contractors have many roles to play and many services to take care of during building construction, and one of the seldom acknowledged services they provide is that of the contractor’s design portion, or CDP. The CDP is critically important to the smooth functioning of the building’s services, as it fills in the gap between the functionality of installation drawings and as-built drawings. It provides additional design drawings for AV, security and sound masking in a building. In short, it mops up the work of the remaining design services.

Just what are installation and as-built drawings?

Installation drawings typically include the data required by trades to install most parts of the MEP systems, such as plant rooms, data centres, ventilation systems, underfloor heating, etc. These drawings are created by consultants, contractors or subcontractors from coordinated drawings and are then submitted for approvals. They generally consist of:

• Detailed plans
• Sections
• Elevations
• 3D BIM models with components, installation information

Data in installation drawings includes:

• Precise positioning
• Supports and fixings
• Manufacturers’ shop drawing data
• Space allowances for installation
• Builders work in connection, eg. cutting and sealing holes, chasing block and brickwork for conduits or pipes, lifting and replacing floors, constructing plinths, etc.
• Plant or equipment requirements
• Service connection requirements
• Access space for operation/maintenance requirements
• Access requirements for access panels, decking, platforms, ladders, handrails

As-built Drawings

Changes, both minor or significant, are inevitable during construction, due to changing circumstances on site. Clients may ask for updated drawings, created from as-built surveys. Thus, as-built drawings (also known as record or ‘as constructed’ drawings) are developed, during or after construction, to record what has actually been built. As-built drawings are also required for the Health and Safety file and the operations/maintenance file presented to clients.

Contractor use red ink to mark-up changes to the ‘final construction issue’ drawings on-site, which can be used to create record drawings for the completed project. One of the specific details that MEP contractors record is under-floor cabling. Tenants tend to cut off and leave in earlier cables and then add their own cabling. In the absence of cabling records, later tenants will find the situation quite challenging.

Record drawings must be updated by facilities management teams regularly, including any modifications made to the building. If a BIM (Building Information Modelling) model was developed, it must be appropriately updated with changes before handing it over to clients.

What is CDP and how does it fill the gap?

Contractor’s design portion, or CDP, is a contract, assurance or agreement by the main contractor to take responsibility to design certain parts, or portions, of the building. Either using in-house talent or outsourcing design work to trade subcontractors, the main contractor must ensure that all the designs are coordinated.

Typically, the CDP is required when consultants cannot or have agreed not to provide BIM models for audiovisual (AV) systems, security, sound masking, etc. Main contractors need to fill the gap themselves or have specialist consultants proved electrical design services or other services and then coordinate them with existing MEP systems design.

General CDP Process

• Main contractor acquires building regulations approval for subcontractor designs
• Design consultants decide when and how much of their design is entrusted to subcontractors for completion
• Clear communication of requirements from subcontractors, including function, form and quality
• Main contractors to include trade contractor design (scope, program, cost) as part of tender bid
• Subcontractors provide BIM models of audiovisual (AV) systems, security, sound masking, etc.

What Main Contractors Need from Subcontractors

Different subcontractors need to provide different services to the main contractor, who then incorporates these services into the final as-built drawings. The common services are:

AV (audio-visual) Design Services

• Develop AV functional capabilities, designs and budgets
• Define project requirements, with written specifications and bid form
• Coordinate AV system design with the project team
• Develop complete system design package, with system drawings, specifications, equipment lists, etc.
• Create AV room layout and elevation drawings with dimensions
• Create connection-level drawings for video, audio, control, LAN
• Configure sound reinforcement systems
• Design cabinet layouts, equipment rack elevations, jack field layouts
• Test and commission AV systems for system functionality
• Prepare and follow up on punch list documents
• Provide and record client/user training assistance
• Coordinate networked AV devices with the client’s IT team
• Identify electrical circuiting, conduit and architectural work requirements as part of the final design

Details of equipment, engineering, project management and AV Integrator installation services must be given to the main contractor.

Using BIM technology can enhance the feedback of security system functioning. The BIM models can be used to locate specific system devices and coordinate them with other devices, so that every system device is properly placed, connected and can be analysed for performance. Security systems can be integrated with the building’s other operating systems for the main contractor to analyse how the security system functions in its space and with relation to the people in that space.

Security (CCTV/Access Control) Design Services

• Provide bid documents, supporting drawings, a bid form
• Develop proposals evaluations, with comparative bid analysis, recommendations
• Create shop drawing submission reviews

Sound Masking Design Services

• Determine sound masking system requirements with the project team
• Develop sound masking floor plans and drawings
• Create bid documents, supporting drawings, with bid form
• Create shop drawing submission reviews

In addition to coordinating with each subcontractor individually, the main contractor must ensure that the subcontractors coordinate with each other. For example, the MEP subcontractor must be aware of the grid system and a suspended ceiling’s fixings positions, while the ceiling subcontractor must be aware of the plant details above the ceiling for access purposes.

To conclude, the contractor’s design portion ties up all possible loose ends nice and tight. Technically certified and experienced electrical design services providers can enhance and ease the work of main contractors by providing accurate and timely design and drafting services. From moving seamlessly to fill the gap between installation drawings and as-built drawings to providing AV, security and sound masking services design, CDP makes a crucial contribution to the longevity and effectiveness of a building and its services.

XS CAD has valuable experience providing BIM MEP services, mechanical CAD drafting services and electrical design and drafting services for global firms. Our range of services for building services contractors across the world include MEP drafting, electrical drafting and public health system drafting.

For further details, contact info@xscad.com