How Contractors Design Portion (CDP) Fills in the Gap

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

Why MEP Contractors Change MEP Design Models

In the MEP environment, a building’s MEP designs are initially developed at high level and then detailed to make them clash free and installation ready. MEP designers/consultants play a significant role in design decisions, construction planning, cost estimation and documentation. While design development is typically the role of the consultant and design detailing is done by the MEP contractor, when using MEP (M&E) BIM models and Revit BIM libraries, contractors invariably need to make changes to the MEP design model created by the consultant. 

To make a design installation ready, contractors may have to make several changes to the design-intent such as resizing of ducts, re-routing of pipework, adding wall penetrations, bolt locations and datum points for hangars and changing equipment. Once these changes are made by the contractor, the design would be installation ready and will need to be approved by the MEP consultant. The question that this article seeks to answer is why do MEP contractors need to make these and other changes to MEP design models?

• To adjust invert elevations – During the installation of plumbing or drainage pipes, MEP contractors deal with the point of the bottom inside of the pipe, this is known as the invert elevations. To guide pipe design and match the invert elevation height, elevation information can be vertically adjusted at the centre of the pipe using Revit. However, if you are unaware that the elevation information is in centre of the pipe, it could cause confusion in adjusting invert elevations and create discrepancies while coordinating with other disciplines. This is the reason why MEP contractors need to manually adjust invert levels, create spot elevations for the inside bottom of the pipe and change the design models to install pipes which are coordinated with other disciplines. 
• To retrofit MEP systems into a prefabricated module environment – Planning for prefabrication of MEP components into the 3D model would not be considered by designers and therefore the contractor is the party who will make adjustments to services to allow them to fit into prefabricated modules to maximise the advantages that are gained from off-site fabrication. In several MEP projects which require prefabrication of risers, ceilings and plant room areas, MEP services drawings and modules specifically for fabricators and installers is necessary to facilitate proper installation. MEP contractors make changes in the MEP design model to ensure that services fit within modules within the ceiling or riser space to allow prefabrication of MEP components off-site allowing faster installation on-site.
• To facilitate efficient spatial coordination – When installing MEP systems, effective spatial coordination with other building services and disciplines is imperative. A consultant may leave clashes in a model as his focus will be on getting a design issue by a due date. A contractor is more concerned with actual fitting so after conducting a clash test on the 3d model, MEP contractors will invariably change design models to ensure that all services are not clashing. The installation programme of the MEP system depends on clash free layouts and MEP contractors must make sure that MEP systems are spatially coordinated with other disciplines in the 3D model before creating M&E (MEP) Coordinated Drawings.
• To deal with constructability issues – There are several factors that influence the constructability or sequence in which MEP systems are installed. Some of the conflicts that require a change in MEP design include routing, fitting and sequencing of large equipment within a given space, conflicting piping network and installation of MEP systems within a crowded space. As MEP contractors need to tackle several conflicts and constructability issues before installing MEP systems, a change in MEP design usually occurs. 
• To install MEP systems economically and efficiently – While the MEP design intent may seem to be perfectly coordinated, it need not necessarily be economic or efficient when it involves installation. There are several costs involved when changes need to be made after installation such as re-routing pipes to reduce bends, re-positioning ducts to allow supply and extract in the correct locations, changing equipment or adding wall penetrations, bolt locations and datum points for hangars. To make sure that MEP systems are installed economically and efficiently, MEP contractors must make changes to MEP design models.
• Changes in materials and components – In some cases the MEP model from a consultant is accurately modelled with specified parts, materials and components. However, in some cases projects do not have specified parts and a consultant may use library elements from Revit leaving the contractor to update the model using his planned procurement schedule. This will result in changes due to sizes and access requirements for new components such as a change from copper to plastic pipe which is thicker or a change from one set of pumps to another that may be larger and may have different valve arrangements. The knock-on effect of such component changes can mean that other systems also need to be changed. 

Given the many reasons why MEP contractors need to change MEP designs and with the adoption of MEP (M&E) BIM practices, there is an overlap in the scope of MEP contractors and consultants during the planning phase. To know more about how you can reduce the duplication of efforts, additional costs, manage project schedules and reduce scope overlap, read more in this post to find out the possible routes that can be taken.

To ensure MEP design models are installation ready for MEP contractors to use on site, a viable solution would be to work with a 3D BIM coordination specialist or MEP engineering design service provider. At XS CAD, our experienced team of MEP designers in India provide BIM support and spatially coordinated building services drawings for key stakeholders in the MEP (M&E) industry, from MEP (M&E) consulting engineers and MEP (M&E) building services contractors. In our spatially coordinated MEP building services models, we use the latest 3D MEP (M&E) modelling software (Revit MEP) and clash detection technology (Autodesk Navisworks) to provide 3D M&E (MEP) coordinated drawings which adhere to engineering standards, the structural and architectural elements within a building.