Structural design process

Figiure 1 shows how, when creating a structure, one starts with a set of requirements that define the performance of the structure. This is transformed by a design process into design output, i.e. into information about what the structure will be. A construction process then transforms the design output into the physical structure.

Figure 1 Engineering a structure

Figure 2 Structural design process

The structural design process (Figure 2) is an instance of the Top-down strategy.

Design activities

Main design activities include:

Resource allocation A budget for the design work needs to be established. This depends on:

• What the client is prepared to pay for
• Whether the design is innovative. Innovation increases the risk of unsatisfactory outcomes and requires more detailed attention to design process activiites.
• Whether the context is familiar to the members of the design team, Experience can allow some decisions to be made with less attention to detail.
• The degree to which the context is safety-critical. Most structures are safety-critical to some degree. The safety of the users of the structure should be treated as the responsibility of all involved in the design including the client. Safety criteria must be accepted as non-negotiable when making decisions. [Refer to case studies of failures: Tay Bridge, Edinburgh schools, Grenfell Tower Fire, Florida Bridge]

Plan  Work to a plan for the design activities.

Review and revise The process is not normally linear. There may be backtracking. It is essential (a) to shedule regular meetings to review progress and to check outcomes against requirements and (b) to be constantly alert for unforseen circumstances, faults, erors.

Gather information - about the site, about the regulatory framework, etc.

Establish the requirements. Based on a client brief and  a list of design issues (see diagram below), establish a requirements statement. Seek to ensure that the requirements are all identified at the start of the design process because adding requirement later can result in re-work and extra cost - but, if at a later stage important further requirements are identified, add them to the list.

Use the requirements  Make a checklist of the requirements and keep checking that you are satisfying them as the work proceeds.

Option analysis Develop a set of options to a degree of detail sufficient to assess them against the requirements. Compare them against the requirements and decide on the form of the structure to be used.

Refine the design Carry out technical assessment (mainly using codes of practice) and carry out further checks against all requirements.

Design output Produce drawings, specifications, etc.

Structural design issues

Figure 3 illustates some of the issues that may need to be considered in structural design.

Figure 3 Structual desing issues

Working with other disciplines

Adopt a holistic view (i.e. address all relevant issues) both in the structural context and in relation to the other disciplines that may be involved in the project, i.e. actively collaborate with contractors, architects, building services engineers, environmental specialists, etc.

Critical thinking in the structural design process

Naval architect, Stephen Payne, designer of the Queen Mary 2, said (at a talk given to the Institution of Engineers in Scotland) in 2020).  "When designing a cruise liner, the regulations represent the starting point for my safety assessment. The Titanic met the then current regulations." Thousands of people might drown if a passenger liner was lost at sea. There are equivalent risks in structural design e.g. for a long span bridge or the roof of a sports stadium. Most structures are to some degree safety critical due to the serious consequences of a structural collapse. Therefore Stephen Payne's principle that the starting point for design should be the regulations should be adopted in all strutural designs. This requires critical thinking by all participants.

Design reviews

Figiure 4 Some issues in design reviews

A main critical thinking strategy is that issues are kept under review throughout the design process. In structural design these include:

• At the start of the process It is especially important to focus on the requirements - to ensure that all requirements have been identified and are adequately addressed in the design. For examples of the use of requirements see Footbridge and Industrial bulding. The reasons for structural failures can often be led back to faults in the use of requirents - see, for example, the following case studies: Tay Rail Bridge Disaster, Ronan Point Collapse, Florida Bridge Failure, Hartford Civic Center Collapse. Processes for identifying requirements and for ensuring that they are addressed need to be used.

Review actiities include:

• Pervasive review activity Throughout the design process, there should be continual assessment of the inputs, the processes and the outputs  - see Figure 4. This involves asking questions such as:    Are important issues being missed?  Should I seek advice about this? Has this been properly assessed? Such questions need to answered and action taken when appropriate.
• Design reviews The design plan should include a schedule of formal meetings of the design team to review (a) progress and (b) the satisfaction of requirements.
• Milestone reviews  The two major reviews in the design plan are:

1. At the end of the concept stage the state of the design is reviewed to ensure that what is proposed has the potential to satisfy the requirements i.e. that the design is valid. This results in a Design Proposal for agreement by the client.
2. The final review verifies that the design will satisfy the requirements prior to a Design Verification report being issued.