However good you think your fire resistance is there are compelling reasons for never taking it for granted

Published:  31 January, 2014

Fire consultant Pat Cox outlines the many ways that fire resistance can be compromised.

We are all familiar with the idea that certain parts of a building are ‘protected’ by ‘fire resisting’ walls and doors. UK Building Regulations for example (England and Wales - Scotland has the Technical Standards) specify a minimum of 30 mins ‘fire resistance’ for such elements, but, in some instances a higher standard is required. Now, the assumption is that a fire rated board, used to create a ‘fire wall’, will give the specified resistance, but now we hit the first of several important ‘maybes’.

To be tested, such boards are placed over the open face of a vertical furnace, the actual test requiring that it be a ‘typical assembly’ as closely as possible mimicking the manner in which the boards will be used in a typical application. This means that the boards will be mounted on ‘studs’ of the correct material and having the same dimensions as would be required in a real structure. At least one ‘joint’ between boards must be included and exposed to the furnace.

So far, so good. The manufacturer will have run these tests on several configurations until they find one that works and that will give them the ‘fire test certificate’ they need to sell the product to architects, engineers and builders.

OK, now comes the crunch, they will specify the manner in which any wall should be constructed, but, once the boards are on a site and being used, these may not be followed by the men doing the constructing. Now throw in the electrician, the IT cabler, telephone engineer, the HVAC installer and the plumber, and suddenly we have a range of openings in the boards that probably weren’t included in the test assemblies. To be fair, most board manufacturers do take account of openings made to install light and power sockets and switches, but their solution may not be possible for the electrician, or he/she may not even be advised that they must make this opening ‘up’ in a certain way. Water pipes passing through such a wall structure also need to be protected, but probably the one thing almost always forgotten is the ventilation duct. Often, hidden above suspended ceilings, these large cross section ducts pass through several compartments and sometimes what appear to be compartment walls do not go very far above the suspended ceiling anyway. The snag here is that ceilings - especially suspended ceilings - rarely have any fire resistance. The exception is in the Marine Industry where ‘ceilings’ may be ‘rated’ as either A-30 or A-60 (all steel with mineral wool insulation) or as B-15 or B-30 (Non-combustible board with insulation) and Offshore where the ratings may be H-15, H-30 or H-60 (H stands for ‘Hydrocarbon’ and is a much more severe test than the Standard tests used ashore)

Of course, if we are talking about brick and concrete walls, the chances are we have a fairly solid basis for thinking we have at least the required minimum fire resistance in our walls and possibly even the floors. A fair enough assumption since a four-inch thick (single brick) wall should give a minimum of 60 minutes’ fire resistance and a double brick thickness (9 inches) at least 120 minutes, possibly more. Four inches of reinforced concrete is a nominal 120 minutes, but now we must throw in the ‘wobblies’. Is the wall a complete ‘slab-to-slab’ structure? Is it going to be stable if something collapses against it on the fire-exposed side? How much concrete covers the reinforcing iron in the floor slab? If it is less than an inch (25mm) the heat may reach the steel early, causing it to expand and break up the concrete. That can allow the fire to spread through cracks to combustible material resting on it. How good are the mortar joints between bricks or blocks? How is the joint between a brick or block wall closed where it meets the floor slab above it? Some might argue that this is the remit of the surveyor or inspector responsible for overseeing construction, but the reality is that it is impossible for these individuals to oversee everything happening on all the building sites in their area. Besides, the law actually only stipulates their ‘inspecting’ certain key elements such as the drains, the foundations, the structural stability and the roof … So they depend on the site manager of any construction site to a very large extent to ensure that everything is built according to plan. They, in their turn, have to rely on the integrity and honesty of the artisans actually doing the job as specified on the plans. Of course, that assumes that the plans have been prepared by someone who is aware of all these little problems and has addressed them.

In one 88-storey tower apartment block (not located in Europe), it was found that the stud and board internal walls in the apartment - rated at 30 mins’ fire resistance - were pierced between rooms and passages with back to back electrical outlets. Worse, however, was the gap between the edge of the floor slabs and the curtain walls. This was filled with sound insulation (combustible) material and hidden by a box construction which concealed air conditioning ducts and electrical wiring. In this instance it could be argued that with a full sprinkler system this should not be a problem. Except the sprinklers had been designed to a ‘residential’ standard not covering use in a high-rise tower. Someone had obviously realised this and made changes to cover (he hoped) the deficiencies, but I suspect we will never know whether it will be sufficient until there’s a fire.

This is an area the average risk assessment doesn’t look at, largely because there is an assumption that the designed fire resistance will, in the first instance, be adequate and correct, and secondly, be maintained for the life of the building. Neither should ever be taken for granted for several reasons. Chief among these being that every change, every repair, ever addition, to the plumbing, wiring, heating or ventilation introduces new hazards. Some may well be insignificant in themselves, but their impact on the passive fire resistance of the building may be huge. Every penetration introduces a path for smoke and fire spread, and these need to be monitored regularly.

Every time a workman repairs, installs or replaces something that affects a penetration through any fire rated wall or floor slab, it needs to be checked, and if it hasn’t been properly resealed, it must be done immediately. Lives, livelihoods and even the continuation of the business itself may depend on something as simple as closing the opening made to pass a new television or computer cable through a wall. Openings in fire-rated structures do need to be provided, the most obvious being doors. We’ve all seen them, and probably many have cursed them, especially when trying to drag your luggage through multiple doors in an older hotel to get from a staircase to a corridor, to your room. We’ve all seen them wedged open as well where they obstruct movement, or ventilation of spaces on hot days. Why are they there? They are, of course, there to prevent the rapid spread of smoke and flame along escape routes or its penetration into stairs. In hotels they separate the bedrooms from the supposedly ‘sterile’ corridor, and they are fitted with small smoke or flame and heat ‘seals’ around the edge of the door. So far, so good, but there is a catch to them. They are heavy, and the higher the rating, the heavier the door will be. Secondly, they are tested and certified as a ‘set’, that is the door, its frame and the ‘furniture’ it is fitted with. That means each door is only certified if it is fitted in the same frame it was supplied with, the locks, hinges and closers are exactly the same as was tested, and the door seals are as tested on the original test samples. You can’t change the hardware, or modify the frame, or for that matter replace a damaged door by installing a new one in an old frame without invalidating the fire certificate.

And now we come to the major drawback with doors. They wear out. Remember, they’re heavy, so there are mechanical forces in play on the frame, and if a door is held open (even with an approved hold open device) you need to keep the holding device and the closer on the same line. A hold open device on the floor and a closer on the top of the door will, between them, warp the door. Eventually, though it closes, the door will not ‘seat’ properly in the frame. That means you no longer have a fire barrier. Similarly, if a door is not properly ‘hung’ at installation the frame can eventually be warped and allow the door to drag on the floor and that can cause it to close too slowly, or fail to close.

Many people don’t appreciate that changing the floor covering can affect their doors. Taking out a deep pile carpet and replacing it with a harder wearing and thinner one may leave a large gap, and going the other way round may cause the door to drag on the new carpet. No! Don’t even think about ‘trimming’ the bottom of the door! You’ve just invalidated its fire certificate. Better is to have a simple - and preferably non-combustible - mat on either side of the door so it swings across a section of floor or carpet that doesn’t affect it.

Now, the last thing that impacts on the fire resistance of your structure is the type of fire load you expose it to. In the opening paragraphs above, I mentioned that the offshore tests for fire rated divisions are designated with an ‘H’ in front of the number. The reason is simple. Hydrocarbon fires (essentially Classes B and C) tend to reach peak heat output very rapidly - far more rapidly than you will get from a ‘standard’ fire load of Class A materials. No worries, you may say, my office, shop or factory only has Class A risks. Are you sure? Are you aware that plastics, expanded foams and many synthetics behave like hydrocarbons in a fire? How about all the plastic furniture, the plastic casings for all those computers? The foam filled chairs? The several tons of plastic packaging and wrapping materials around all the items in your store, showroom, or warehouse? The sprinkler design codes have been changed in recent years to state that, especially in storage facilities, the plastic packaging material must be taken into account for the design. Why? Quite simple, that 30-minute fire wall or door in your office, if tested on an Offshore test rig and to the hydrocarbon test, only survives about six minutes. Now, about that risk assessment of yours; how big is your fire load? And how good is your fire resistance protecting that means of escape again?

  • Operation Florian

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