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  • Writer's pictureSi Shen

Conceptual design (1)

There is no strict definition of conceptual design. It can generally be understood as an early stage of design development, after the project gets official funding confirmed, but before ‘detailed design’. It is sometimes also called ‘scheme design’. At conceptual design stage, nothing is in production yet and the most important deliverables are usually conceptual design statement and a set of general arrangement drawings, which nowadays usually take the form of BIM model. This blog post explains some of the underlying core concepts of conceptual design.

Uncertainty is usually high at the conceptual design stage. Naturally, when we face uncertainties, we plan for the worst and make allowance for potential changes. Also, approach all decisions from the risk point of view. Think more about what happens if things go wrong rather than what benefits to be gained when all stars line up. These principles should be born in mind at all times.

Looking at the project internally, what we typically look to achieve by conceptual design stage are:

1) Spaceproofing. 2) Inter-disciplinary coordination 3) Cost confirmation/update.

Inter-disciplinary coordination

At the conceptual design stage, to ensure coordination and the final product meets client requirements, a key concept is called progressive assurance. It basically means that each party goes off on its own but comes back at certain coordination points set out in the project plan. These coordination points are typically called ‘stage gates’, which takes the form of staged submission from each discipline and representatives from each discipline get together for a series of coordination meetings. This is like marching a large army – soldiers have to stop and line up once in a while in order to maintain formation. Typically around 2-3 stage gates are set for the conceptual design period. With the development of BIM nowadays, these meetings can take place much more regularly (say weekly), as submission is made live on a shared system.

When each party goes off in its own direction, they have to make some assumptions about what others will do. This can be based on past experience of similar projects and/or early coordination meetings. These assumptions must be recorded and validated at a later stage, hence the purpose of an assumptions register.


To ensure spaceproofing the following must be done:

· Structural member sizing. The structural engineer is required to come up with nominal dimensions for key structural members. The engineer should be mindful of the existence of uncertainties at this stage so as not to communicate with others the bear minimum requirement. Due to existence of uncertainty, all parties would wish to have a bit of margin on top of their minimum requirement when negotiating with others. Having the margin is important so that when some assumptions turn out to be worse off than previously assumed, there is still wriggle room for changes, without adversely impacting on others. In other words, the structural sizing will be able to largely ‘absorb’ impacts from others without having to change.

· Critical details. Some critical design such as support connection may fundamentally affect how a structure behaves. This is something structural engineers tend to overlook. For example, if a simply supported support details is required for a beam supported by a wall, a corbel detail could potentially be required, which means extra space provision that may clash with services.

· Construction methodology and sequence. To determine these we need to know the particular constraints to the site. For example, if we were to prefabricate or precast something off-site, these elements need to be brought to the site; if it is done by lorries, the access road to the site must be able to accommodate the potentially large and heavy lorries; crane capacity must also be sufficient to lift the elements into place. Construction methodology and sequence can generate transient loadings, affecting structural member sizing to a significant extent.

Cost confirmation/update

The project sponsor (the party who pays for the project) would wish to do a ‘reality check’ once the design has been developed further, to compare the latest cost estimation against what was initially thought to be. Therefore a cost update is typically done at multiple points along the development path of the project to keep the cost in check. All parties involved are required to provide information to the project sponsor.

The conceptual design stage is typically designed in such a way that the project sponsor has multiple points of intervention, in order to steer the project development in the desired direction. The project sponsor typically has the choice to make the following decisions:

· Opportunity to stop: The project sponsor may wish to cancel the project before too much cost is sunk, when he starts to see ridiculous hikes in cost, as a sensible way to cut loss.

· Opportunity to re-think: The project sponsor may wish to send the project back to an earlier stage, to take another look at a completely different project concept. Two or more options can be developed to the same or similar degree of details in order to better compare which one is the best to go forward with.

· Opportunity to alter: The sponsor may also wish to alter the project on a relatively small scale, without completely changing the nature of the project, in order to control risk, or to embark on attractive opportunities, or to bring better value for money.

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