Steel Appeal in Fire Safety

By Lee Coates
Wrightstyle

The fire regulations on which building safety depend are themselves based on an understanding of fire dynamics – the fundamental relationship between fuel, oxygen and heat – the so-called fire triangle on which all fires, intentional or otherwise, depend. Get those three elements together and the fire triangle is joined by a fourth element – the chemical chain reaction that is actually the fire. In technical jargon, the triangle
of combustion then becomes a tetrahedron.

It is a geometry that can either be friend or foe, as fuel and oxygen molecules gain energy and become active. This molecular energy is then transferred to other fuel and oxygen molecules to create and sustain the chain reaction. In an uncontrolled fire in a building, how it spreads of course depends on a whole range of factors – from the type of fuel (everything from ceiling tiles to furniture) to the building construction and ventilation.
Taming fire generally involves the removal of heat, in many cases using water or foam to soak up heat generated by the fire. Without energy in the form of heat, the fire cannot heat unburned fuel to ignition temperature and the fire will eventually go out. In addition, water and foam acts to smother the flames and suffocate the fire.

But what is really needed is containment – to prevent the fire spreading from its original location. Those protective barriers, often external curtain walling or internal glass screens, must also provide escape routes for the building’s occupants. That is where fire resistant glass and glazing systems are so important, because modern steel systems are so technically advanced that they have overcome the limitations inherent in the glass itself.
The biggest limitation is that glass softens over a range of 500°C to 1500°C. To put that in perspective, a candle flame burns at between 800°C and 1200°C. In a typical flashover fire inside a building, temperatures can reach between 1000°C and 1400°C.

These temperatures can disrupt the integrity of conventional panes of glass, which can crack and break because of thermal shock and temperature differentials across the exposed face. This will compromise the compartmentation of the building’s interior allowing fire to spread from room to room. That can, incidentally, be a problem that a sprinkler system actually causes. There have been several notable cases where cold water from a sprinkler system has come into contact with heated non-fire rated glass – causing the glass to break and allowing more oxygen to the seat of the fire.
As a fire escalates, the amount of heat produced can grow quickly, spreading like a predator from one fuel source to another – devouring materials that, in turn, will produce gases that are both highly toxic and flammable. To make things worse, due to thermal expansion, these flammable gases are usually under pressure and able to pass through relatively small holes and gaps in ducts and walls, spreading the fire to other parts of the building. Heat will also be transmitted by conduction through internal walls.

As the fire worsens, and when unburned flammable gases reach auto-ignition temperature, or are provided with an additional source of oxygen – for example, from a fractured window – an explosive effect called ‘flashover’ takes place. Flashover is the most feared phenomenon of any firefighter and signals several major changes in the fire and the response to it. First, it brings to an end all attempts at search and rescue in the area of the flashover. Simply, there will not be anybody alive to rescue. Second, it signals that the fire has reached the end of its growth stage and that it is now fully developed as an inferno. That then signals a change in firefighting response because it marks the start of a worse danger – the risk of structural collapse.

However, most fires start with only a minimum of real danger – a dropped cigarette, a spark from a faulty wire – and, if dealt with quickly or adequately contained, pose no real threat. However, an unchecked fire can spread with devastating speed, particularly in a large open space such as a supermarket, open-plan office or factory. And when it does get out of control, the best means of survival is escape.
Around the world, more stringent building and fire regulations have led to architectural and design teams taking a multi-disciplinary approach to assessing hazards – from power failure to cyber-attack, from civil disorder to fire and explosive detonation – and arriving at risk assessments that, hopefully, illuminate how that that building should be designed and built.
Designing in safety is nothing new, and starts with actively assessing the possible risks against that building’s occupants, structure, resources and continuity of operations. There are a number of assessment methodologies to understand the potential threats, identify the assets to be protected, and how best to mitigate against those risks. That assessment then guides the design team in determining acceptable risks and the cost-effectiveness of the measures proposed.
Assessing risk is the starting point, and in particular the need to build in compartmentation throughout the building, examining the whole building’s capacity to withstand a fire or other threats. For the glazed components, that should mean analysing the level of containment the glass will provide and its compatibility with its framing systems, because the safety of the glass cannot be assessed without its framing system. Put the right glass into the wrong frame, and you could be turning sixty minutes of fire-resistance into five minutes. In an evacuation situation where seconds count, getting the design wrong at the outset could be a costly – and deadly – mistake.
There are many types of fire-resistant glass currently on the market – and the ranges of products and sizes will continue to increase as the technology for combining glass and glazing systems develops. We have come a long way to meet the evolving design requirements of architects and the increasing stringency of building and fire regulations. Simply, the glass and framing technologies now on the market mean that the impossible is now possible.

A Case in Point – Fire Safety and the House of Pain
They call it the House of Pain, and the fire-fighters of Engine Company 10 and Truck Company 13 experience quite a lot of it. Theirs is the busiest fire station in the United States, serving a large residential area of northeast Washington DC. In April and May this year, they were called out 1,587 times.
Early on May 30th 1999, the District of Columbia Fire and Emergency Medical Services Communications Centre received a 911 telephone call reporting a fire at an address in Cherry Road. The first units were on the scene within minutes, and windows on the first and second floors were opened to provide ventilation.
Another fire team positioned alongside sliding glass doors at the basement level reported that the basement was full of smoke. Despite confusion over the location of the firefighters upstairs, a decision was taken to break out the basement’s sliding glass, after which firefighters entered the basement to conduct a search.
 

They reported that there were a number of small fires on the floor of the basement. However, these rapidly increased in size after the sliding glass door was opened. The firefighters were ordered out of the basement as the fire quickly intensified, successfully escaping just before it became engulfed in a fully-fledged inferno. Seconds later, from upstairs, came the first report of a firefighter down. Firefighter Anthony Phillips was pronounced dead on arrival at hospital; another firefighter, Louis Mathews, died the following day as a result of his injuries.

It was the very routine nature of the fire and its tragic outcome that prompted the District of Columbia Fire and Emergency Medical Services Department Reconstruction Committee to request a full investigation into the fire dynamics of the incident. This was carried out by the Building and Fire Research Laboratory (BFRL) at the National Institute of Standards and Technology (NIST).
The investigation made use of NIST’s Fire Dynamics Simulator (FDS), a computer modelling programme that looked at all available data. Specifically, the investigators wanted to know how the opening of windows and doors had affected the dynamics of the fire. The investigating team then identified the fuel package that was involved in the fire, and NIST’s simulator identified the heat release rates of different types of furniture and furnishings, expressed as British Thermal Units (BTUs) or Kilowatts (kW) per second.

The model divides the space involved in the fire into thousands of “cells.” In the Cherry Road simulations, the cells measured just 200 millimetres by 100 millimetres high. Once the physical data was entered into the computer, it was able to model the conditions for each cell, and then combine all of them together to provide an overall simulation of the fire. Investigators determined that the fire started near an electrical fixture in the ceiling of the basement, developing quickly but then depleting the supply of oxygen necessary for combustion.

It was at this point, when the fire’s heat release rate was being constrained, that firefighters made their entry on the first floor of the building. However, and against some expectations, opening windows on the front of the townhouse on the first and second floors seemed to have had no noticeable impact on the fire’s development. It was the breaking open of the basement door that created the firestorm. The FDS calculations were that the opening of the basement sliding glass doors provided outside air into a pre-heated but under ventilated fire compartment, which then developed into a post-flashover fire within 60 seconds.
Some of the resulting fire gases flowed up the basement stairwell with a high velocity and collected in a pre-heated, oxygen depleted first floor living room with limited ventilation. More precisely, the model showed that the superheated gases moved up the stairs at approximately 18 miles an hour. As the townhouse was only ten metres high, it meant that the extremely hot gases moved through the building in less than two seconds.
This is at the heart of compartmentation; to provide effective barriers against the passage of fire, heat and toxic gas and, by preventing oxygen from reaching the seat of the fire, inhibiting its progress. This allows people to escape and, by containing the fire, minimises fire damage.
As the Cherry Road fire showed, failure to contain fire can have very human consequences, even in a residential home. In larger multi-storey buildings, the stakes are very much higher, and the responsibility on the designer that much greater.

Lee Coates leads research, development and worldwide testing at Wrightstyle Limited

For further information, go to www.wrightstyle.co.uk