Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Qualitative Design Review or QDR is defined in PD 7974: 2001 as the initial stage of any engineering design in which the basic design parameters for a project are established and the scope and objectives of the fire strategy can be defined. To undertake QDR effectively can seem to be an expensive luxury during the early stages of many projects, however; it can be a very valuable exercise which can identify problems early and potentially help to reduce project cost and risk. The following bulletin discusses the formal process of QDR and why it can be a very important part of any design process.
What is QDR?
Fires scenarios in buildings are unpredictable and there are a vast number of different scenarios that could occur, far too many to feasibly evaluate every case. For a Fire Engineer the Qualitative Design Review (QDR) is the process used to identify the significant hazards and most likely scenarios so that the required level of quantified design can be established. The process is essentially the qualitative assessment of the design problem calling upon the knowledge and experience of the Fire Engineer and other design team members and building operators.
Who Should Be Involved?
The size of the QDR team will depend upon the size and complexity of the project but will typically involve one or more fire engineers, other key members of the design team and wherever possible a member of operational management. A typical QDR team for a medium/large project would include:
· Fire Engineer
· Services Engineer
· Structural Engineer
· Operational Management
· Approvals Body and/or Insurer
The intention is then that this team work systematically through the design to identify possible fire hazards and a range of possible strategies which maintain the fire risk at an acceptable level. A structured approach is recommended to reduce the risk that an item is missed or overlooked.
Identify Design Parameters
One of the key objectives of the QDR is to review the early architectural design and identify the design parameters. It is important for the Fire Engineer to fully understand the proposed use and functional objectives of the building as well as any significant hazards or design aspirations that will need to be accounted for in the fire safety design. Some of the parameters that should be considered are:
· Building & Occupant Characteristics
· Fire Safety Objectives (Life Safety and/or Property/Business Protection)
· Architectural Design
Beyond these broader design objectives the QDR should also consider further details, such as:
· Fire Load Densities
· Site Conditions – Access for the Fire Service
· Construction Type
· Ongoing Management
· Potential Ignition Sources
· Consequences of a Hazard
Define Acceptance Criteria
It is important during the QDR to establish acceptance criteria for the design including any quantitative values such as tenability criteria for means of escape. This is required to allow any further assessment of a particular strategy to be completed and also allows opposing strategies to be directly compared against each other.
It is very useful at this stage if acceptance criteria can be agreed in principle with the approving authorities as this can reduce the risk of abortive works and approvals issues at a later stage in the project.
Fire Scenarios for Analysis
Once the design parameters are understood and any acceptance criteria have been established the QDR team will generate a number of trial designs which are intended to achieve the fire safety objectives. These could simply be different ways or configurations of systems to achieve the same basic acceptance criteria or could involve different strategies and systems to meet the functional objective. An example could be the protection of a balcony escape within an atrium void. One approach could be to provide tenable conditions for escape by providing active smoke control within the atrium and this smoke control could take many different forms. Alternative approaches may be to enclose the balconies, provide alternative means of escape or remove any risk from the atrium.
Design options should not just consider the addition of fire protection systems but should also look at other ways in which hazards can be reduced or eliminated through changes to the construction or layout of a building.
It should not automatically be assumed that a fire engineered solution will be best as it may be that a prescriptive approach is more appropriate to the design.
Methods of Analysis
Where a fire engineered solution is proposed it will invariably require some form of quantified assessment to be undertaken to show that the fire safety objectives can be achieved. There are three main forms of quantitative assessment:
· Comparative – typically used to show that the proposed design achieves a comparable level of fire safety to that of a building that complies with prescriptive codes. Comparisons can be on either a deterministic or probabilistic basis.
· Deterministic – Used to show that a pre-determined condition will or will not occur based upon worst case scenario assumptions.
· Probabilistic – Used to show that the probability of an event occurring is acceptably low.
In practice most quantitative assessments will use a deterministic approach to show that the agreed acceptance criteria can be met.
Benefits of QDR
A structured approach to QDR that involves a good mix of skills and knowledge as well as representation from key stake holders will lead to an optimised design solution. Knowing that a QDR process has been undertaken provides a client with confidence in the proposals knowing that all foreseeable fire safety issues have been considered. Also undertaking QDR at an early stage in the development of a building design can significantly reduce the risks to the project both in terms of budget and achieving statutory approvals.
This bulletin was written by Richard Sherwood BEng (Hons) Principal Fire Engineer.
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