Structural design processes
Structral design requires the use of system planning i.e. the overal process and the sub-processes need to be optimised.
Figure 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 and justification for the design decisions.. A construction process then transforms the design output into the physical structure.
Models of the design process
A basic model
Figure 2 shows a model of the design process. The 'system model' is information about the stucture being designed and about its context.
The process is not normally linear. 'Review and revise' is at the heart of the process. Iterations may be needed and revisions are made based on continuous review activity,
At the the InceptionStage, requirements for the performance of the stucture and for the design process are established and information about, for example, the site is identified.
At the Conception Stage, investigations are carried leading to a decisoin about the general form of the structre to be adopted.
At the Production Stage, the chosen form of structure is developed to produce drawings and specifications to be passed on to the construction stage. Other outcomes include documents that justify the decisions taken.
Process mapping
The Institution of Structural Egineers publishes a Structural Plan of Work that sets out an overall process for structural engineers working on the design of a building.
The stages in the IStructE Plan of Work are shown in Figure 3.
A process map for structural design based on the IStructE Plan of Work is shown in Table 1.
The work starts at the top left of the map with the definition of requirements and moves across the stages and down the sub-processes to achieve the basic project outcomes In the case of stuctural design, the basic outcome is in the form of drawings and specifications that define what the structure will be. Other outcomes are required that, for example, justify the choices made or show how risk was controlled.
Table 1 only indicates some of the complexity of the system plan. It does not show iterations that are common, especially at the concept stage, nor does it show interactions among the sub-processes.
Controlling Risk
A structural collapse can have serious consequences and all structural engineering activities should be treated as being, to some degree safety critical.
It is important to pay special attention to the requirements of the client but such considerations should not over-ride duty of care to the publc.
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." Hundreds 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. Use of the 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 Programme
The work of the project is controlled by a programme where tasks are defined and allocated and timelines are established.
Items in such a programme include:
- Task descriptions
- Task schedules.
- Manpower schedules
- Reviews
Core actions in preparing a project programe include:
- Break down the work into tasks.
- Estimate the time it will take to complete each task.
- Define the precedence of the tasks, i.e. decide which tasks can be carried out in parallel and which need to be handled consecutively.
- Identify any lead times needed for starting the tasks - e.g. where one needs to wait for information to be delivered.
- Identify critical tasks, i.e. those which if not done on time will cause delay in the overall completion
- Draw up a schedule for the work, typically using a bar chart which shows when each task should start and finish. Leave some slack in the system to allow for unforeseen circumstances.
- Record progress and update the task times as the work proceeds.
Task Assignment
When assigning a task, take account of (a) the competence of the person who is to carry out the task and (b) the need to train people to do the task.
Reviews
Regular review meetings should be included in the schedule to address:
- Progress in relation to the programme.
- Progress in relation to meeting the requirements
Milestone reviews Two major reviews in the design plan are:
- 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.
- The final review assessesm the outcomes against the requirements.
Review and revise
Critical thinking is a key attribute in review work. Constantly asking reflective questions and responding to them is essential for ensuring successful outcomes. Key reflective questions are:
- Validation: Is the process suited to the context?
- Verification: Has the process been correctly implemented?
Analysis modelling
Use the strategies recommended in this document.
Code of practice conformance
Validation of code provisions
Ensure that the design context is within the scope of the codes that are specified in the Structural Design Brief. If it is not, seek to ensure that relevant issues that are not covered in the codes are addressed. This is especially important in innovative contexts. See, for example, the Ronan Point Collapse
Performing the calculations
Code of practice rules should be processed using software that has been subject to rigorous QA assessment.
Hand calculators should only be used in preliminary work and back-of-an-envelope checks. They should not be used for final calculations because of the increased risk of errors when values are being keyed-in.
See guidance on programming of calculations.
Input checking
Verify input to software
Results transfer
Verify that the results have been correctly transferred to drawings and schedules,
Inception Stage
Structural design brief
This is a collection of all requirements that may affect the structural design including information about the processes to be used. At this stage it should, if practical, not relate to a specific structural form so as not to limit the what might be achieved.
The brief starts with the client requirements and is developed at the Concept Stage based on information from the options analysis.
Changes to the brief should be avoided, if practical, after the Concept Stage
This guidance about the project brief in the RIBA plan of work states that: "The project brief is likely to be presented as a report, however, where possible, information and requirements should be scheduled in a database or spreadsheet format that will be easy to expand and will be easy to use to test whether proposals satisfy requirements later in the project." Such a schedule can be viewed as a design process map or an extended plan of work.
Establishing the design brief
To establish the requirments, design issues should be identified and expressed in the form of requirements. Figure 4 shows generic structural design issues.
Principles
- Work to a process for establishing the brief
- Ensure that all requirements have been identified and adequately stated in the Structural Design Brief
Recommended verbal forms to be used in the brief (from GG 101-page 4)
- The verb ’must’ indicates a statutory or legislative requirement.
- The verb ’shall’ indicates a requirement of the Overseeing Organisation.
- The verb ’should’ indicates advice expressed as a recommendation.
- The verb ’may’ indicates advice expressed as a permissible approach.
- The verb ’can’ or verbs expressed in the present tense other than ’must’, ’shall’, ’should’ and ’may’ are used to introduce notes, which provide a short clarification of a concept or statement of fact.
Using the design brief
- Work to a process for ensuring that the reqirements will be satisfied
- Be vigilant throughout the design process about requirements conformance
- At design review meetings have requirements conformance on the agenda.
- Ensure that that the mandatory requirements have been satisfied.
- Explain why any non-mandatory requirements have not been fully implemented.
Site information
This document may include information about: topography, existing site features, former usage, services, ground conditions, etc.
Conception Stage
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.
Principles for the options analsis
- At the outset, all options should be 'on the table'.
- Select a set of 'candidate options' that have potential to meet the requirements
- Carry out partial designs for these options to provide information adequate for assessing their relative merits.
- Assess the options using proposal testing principles.
- Consider using a multi-criteria assessment method
Design brief - Stage 2
The design brief should be updated taking account of the decisions made as a result of the options analysis. For example a detailed list of codes of the relevatn codes of practice should be added to the brief.
Design proposal - Stage 2
This document is similar to the Approval in Principle document for bridges and to the Basis of Structural Design document recommended in the IStructE Plan of Work.
It is recommended that the Design Proposal - stage 2 includes:
1. A project description
2. Reference to documents:
- Site Information
- Options Analysis Report
- Structural Design Brief (Stage 2)
3. Information about the proposed structure - what has been decided at the end of the Concept Stage (this may part of the Options Analysis Report)
4. Statements in relation to:
- The requirements that have been addressed at the end of Stage 2.
- Actions/processes needed at the Production Stage to ensure that all requirements will be properly addressed.
- Issues to be addressed that are not included in codes of practice.
Production Stage
Technical assessment
Technical assessment is the use of analysis modelling and performance criteria to seek to ensure that the structure will satisfy the technical requirements set out in the Design Brief - Stage 2. The main structural engineering standards used in the UK are the Eurocodes [[1, 2 ]. A main requirement of technical assessment is to ensure that the strucutre will be stable i.e. that it will not be close to collapse.
Analysis modelling
Analysis modelling (i.e. structural analysis) is the use of mathematical models to predict structural behaviour.
There is significant risk that errors in the modelling will cause the structure to be unsafe and therefore a fit for purpose analysis modelling process must be used.
Conformace to technical criteria
The code of practice
Validation of code provisions
Ensure that the design context is within the scope of the codes that are specified in the Structural Design Brief. If it is not, seek to ensure that relevant issues that are not covered in the codes are addressed. This is especially important in innovative contexts. See, for example, the Ronan Point Collapse
Performing the calculations
Code of practice rules should be processed using software that has been subject to rigorous QA assessment.
Hand calculators should only be used in preliminary work and back-of-an-envelope checks. They should not be used for final calculations because of the increased risk of errors when values are being keyed-in.
Input checking
Verify the input to software
Results transfer
Verify that the results have been correctly transferred to drawings and schedules,
Drawings and specifications
Design verification report
This report demonstrates that the mandatory requirements have been satisfied and shows how the non-mandatory requirements have been addressed.
It records validation of the processes used and how the processes were verified.
It serves as a record for archiving purposes of what has been done .
Typical contents of a verification report:
1. Modelling
- Description of the models used.
- Modelling reviews: model validation, results verification
2. Code of practice calculations.
- Record of calculations
- Record of verification processes
3. Stability and Robustness report.
4. Sustainability Report
5. Access and Maintenance strategy
6. Cost analysis
7. Construction Methods statement