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Limitations of Traditional Solutions
Every fire protection system has its place, with the selection process normally coming down to the following factors:
- duration required (30/60/90/120/180 minutes)
- type of steel member to be protected
- location—internal or external
- design and aesthetic considerations
- weight restrictions
- construction schedules
- cost and budget factors
While it is recognised that cast epoxy intumescent is only a practicable and affordable solution for structures like feature columns—or architectural details like suspension rods, nodes and triangular hollow sections—the limitations of alternative methods of passive fire protection need to be pointed out.
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| Interactive Columns provide fire protection with a slimmer profile than alternative methods, ideal where floor space is at a premium. |
Thin-film Intumescent
As opposed to the niche market of cast epoxy intumescent, thin-film intumescent is a utilitarian solution for the majority of fire protection requirements. It does however have certain drawbacks:
- the successful application of thin-film intumescent paints on site depends entirely on the quality of the subcontractors’ workmanship (and is subject to the time pressures of completing a job on schedule)
- the thicker the coating, the less decorative the finish (resulting in an ‘orange
peel’ texture which can seriously undermine the aesthetic value of the
system)
not only is skilled application required, quality and primer compatibility checks are also imperative - application also requires precautions to be taken against over-spraying
even if the original application is carried out diligently, the finish is difficult to keep consistent when repairing damage over time - repair is difficult to hide and normally requires a major part of any column to be re-sprayed
- thin-film intumescent is not particularly suitable for exterior applications (especially very exposed ones)
Off-site application of thin-film intumescent
While the application of thin-film intumescent in factory conditions off-site addresses many of the problems detailed above, it can add significantly to the overall cost of the fire protection process.
However account must be taken of the additional work required to ensure system integrity and finish:
- abutments and junction and may require remedial work to reinstate the fire protection
- bolt connections require coating on site following erection (alternatively see Bolt Caps)
- a top seal is usually required for transportation purposes to protect against standing water
- repair to transit and installation damage is a certainty
- the repair of thin film sprayed material will affect the surface finish aesthetics leaving a witness of the damage incurred and affect the overall aesthetic value of any feature columns.
Secondary Cladding
As a highly decorative coating to a non-fire rated structure (e.g. a single-storey building), secondary cladding can look very striking. For other applications, like circular hollow sections, it can look cumbersome and unattractive.
However, there are some basic inherent problems with it. The design joints can be used as a chewing gum shelf in public areas. The cladding is also susceptible to damage due to vandalism.
The cost of repair is considerably higher than for most other systems. This is especially true of stainless steel, in recent years a very fashionable form of secondary cladding—for feature columns in particular. At about £500 per square metre to install, it can cost twice that amount to repair.
Other questions that need to be raised about secondary cladding are:
- do the design joints compromise the architect’s aesthetic design concept?
- will the integrity of the system be breached by using inappropriate fixing details?
- who exactly is liable should there be a failure of the system in a fire?
- what is the floor space design criteria—and how does secondary cladding affect the amount of area that is rentable?
- secondary cladding normally requires a ‘register plate’ to be fastened to a column. The plate will prevent that part of the intumescent coating from expanding even if, and when, it falls away,
- all intumescents cannot work if the material cannot expand under heat, to its designed char thickness.
There is no pressure of expansion in the intumescent char and therefore the material will not force the space required under a secondary cladding to insulate the steel. Failure to allow enough room for expansion will seriously compromise the integrity of the fire protection.
Descriptions of specific secondary cladding systems—and their shortcomings—are summarised here:
GRP covered
A fairly versatile system, manufactured from polyester resin and fibre glass. It normally has a sprayed thin-film polyester surface (Gel coat) to give the required finish. The fire protection is applied to the column surface prior to installation.
However, it is not durable or scratch-resistant and joints are visible. It has a sophisticated support structure that may, in fact, compromise the fire protection system underneath.
Metal Clip system:
This system often incorporates a fibre board (Conlit or similar). The product is rolled from sheet steel and is generally galvanised, but can also come powder-coated. It is easy to fit though only to uncomplicated structures.
The system is not particularly secure or durable. It is difficult to cut for cleats and other details. Once damaged, complete replacement is the only solution.
Stainless Steel:
Stainless steel is the top-of-the-range cladding system for columns. It is similar to the metal clip system except that the design element built into the finish makes it more secure. It can be used for concrete as well as rectangular and CHS columns.
This system is also difficult to cut and in need of complete replacement if damaged (at very considerable cost). Its joints may allow water ingress with danger of unseen corrosion.
Concrete-constructed Columns
Concrete columns are generally thicker than an equivalent steel option, and so use up valuable space around columns. They increase building weight and are time-consuming to construct (thus detrimental to on-site scheduling).
In order to prevent an outdated appearance, concrete columns can incur excessive costs achieving an aesthetically pleasing finish.
Concrete-filled Columns
The basic concept of concrete-filled columns is to increase the thermal capacity of the columns—and to reduce the fire protection requirement for the steel surface (so increasing the usable space in a building).
The main problem with this principle is that sufficient fire resistance is only really achieved if a small amount of steel fibre is added (about 2% by weight) to help prevent premature cracking of the concrete core.
Even so, vent holes are needed every 4 m. (one at each floor level) to ensure that the steel shell does not split in the event of a fire. This can happen with a build-up of steam within the columns, generated by water encapsulated in the concrete itself. The look of these holes can detract from the desired design effect.
Also, it is impossible to hide any significant defect in the steelwork (like weld marks) that might occur during the manufacturing process.
Note: Circular hollow sections under 169 mm nominal diameter cannot use concrete filling as a way of enhancing the fire protection to reduce the specification.