Structural fire protection - alternative approaches
Published: 01 January, 2007
Relative to other sectors, for example, commercial office developments, the basis for industrial design with respect to fire safety tends to be far more exacting. Here, severe fires occur in surroundings where continuous operations, and possibly a high level of cleanliness, is absolutely essential.
Relative to other sectors, for example, commercial office developments, the basis for industrial design with respect to fire safety tends to be far more exacting. Here, severe fires occur in surroundings where continuous operations, and possibly a high level of cleanliness, is absolutely essential.
For the industrial processes where cleanliness is of key importance, the selection of an appropriate passive fire protection (PFP) material could preclude the use of fibre-based products and instead favour thin film intumescents. These can be cleaned down effectively without producing fibres, but are more expensive than typical fibre based protection types. Therefore the fire rating required for the structure is of great importance, as very high ratings may require a thickness of intumescent that is not practical for the building use and would then add considerable cost to the project.
Vibration from industrial processes may also be an issue, and again could favour the use of a thin film intumescent instead of a lightweight spray (cementitious/ gypsum / fibre based). Again therefore the fire rating required of the protection material and its necessary thickness becomes critical.
Water, structure & robustness
Many industrial processes are termed hot - and steam escape is a problem. Even condensate drains issue soft steam. Many typical fire protection materials can be damaged therefore by repeated wetting and drying by steam. This can result in damage or loss of the protection material and therefore costly and continuous repairs.
The PFP has to be appropriate for the in-service conditions. The presence of liquid water is less common but if likely an epoxy intumescent or dense cementitious material would probably be the only options and these too are at the top end of the scale with respect to material cost.
In addition, the type of fires possible within the industrial structure are important. For example, if the process in the building involves hydrocarbons then a hydrocarbon-rated PFP may be necessary. This would then mean the use of epoxy based intumescents, boards and spray- applied cementitious materials, of a thickness appropriate to this more severe fire event.
It is important to note that when designing to comply with the Building Regulations, in the UK, as just one example, life safety needs only are addressed. The greater robustness required ensuring property protection and/or business continuity is not necessarily provided from these levels of protection. For this higher design performance, more localised structural deformations or localised areas of collapse become essential, rather than preventing collapse for a particular period of time only (the basis for life safety design).
Traditionally, fire protection materials used to protect structural elements have been tested in small-scale furnaces on single elements of structure, in order to comply with the hourly ratings needed for life safety purposes only.
This means the benefits of full structural frame response which is far more robust than a single element furnace test can determine, are not captured. This can lead to unnecessary application of protection material on elements of structure where it is not required, or even the over design of fire protection material thickness.
The importance of steel
As many industrial projects are on a tight timescale the use of steel structure is more prevalent than concrete. For the higher fire resistance ratings that can be required (these ratings are a function of use and/or structural frame height), the costs therefore start to increase.
The relevant question then becomes - is this additional cost genuinely providing greater safety and robustness in the event of fire? What is the structural response going to be in a fire, and is this appropriate to the needs of the particular business and its operations?
By determining the basis for design up front - life safety or business continuity - specific design techniques can be applied to develop a structure with a robust response in fire
The state of the art to provide this level of cost effectiveness for materials, as well as robust response of the structure, for a client, is to carry out a structural fire analysis. This means a specialist analysis of the proposed cold temperature structural in the likely design fire. By quantifying the fire response, the, strengths and weaknesses in the structural system can be identified.
This allows designers to determine then where fire protection is actually needed - rather than just apply protection throughout. It also means where a weakness in the structure has been identified, minor design alterations can be suggested to enhance the performance, as required.
In addition, the hourly fire rating appropriate to the level of response required of the structure can also be accurately quantified, rather than applying for example a uniform four hours throughout or complying with the ratings recommended in the life safety codes.
Key to the success of this approach is closely working with the various stakeholders involved on the project, particularly the Approving Authorities and building insurers.
Structural fire engineering
This form of analysis (structural fire engineering), developed over the last 15 years, means the importance of the structure itself and its response to fire is of critical importance, rather than sole reliance on passive fire protection and the application of high hourly fire-resistance ratings, as has been the tradition to recent years.
This structural approach is being used worldwide on a variety of building types and construction forms. Arup Fire has worked closely with Approvers and Insurers to develop an approach that is transparent, and acceptable for life safety, as well as business continuity.
The rapidly increasing use of this technique, is due to the fact that by providing a robust structure and relying less on passive fire protection where it is not needed, greater safety may be provided from the integrated system of structural design and fire protection.
The final design provided is therefore cost effective with a response clearly quantified for specific fire events. This provides greater surety for the various stakeholders involved in industrial projects, as well as in many other sectors.
