Canadian Consulting Engineer

when rules don’t make sense

The first evidence of building regulations is in the Codes of Hammurabi of Babylon in the third century B.C. They were made to deal with the hazards and punishments that result from faulty constructio...

May 1, 2000   By Demir Delen, P.Eng., Morrison Hershfield

The first evidence of building regulations is in the Codes of Hammurabi of Babylon in the third century B.C. They were made to deal with the hazards and punishments that result from faulty construction. Their basic objectives — to safeguard life and property from the effects of fire and other hazards — have not changed throughout the centuries. What has changed is the level of safety societies expect.

Building codes provide for safety measures such as ensuring there is adequate structural fire resistance, so that buildings do not collapse in the early stages of a fire; adequate containment, so that fires do not spread; adequate means of egress, so that people can quickly evacuate; and adequate fire detection and suppression systems, so that fires can be detected and controlled in the early stages.

Committees of people representing a variety of experiences, opinions and areas of economic self-interest write the codes. Codes are made over a period of many years, do not follow a consistent set of assumptions, and contain many redundancies since changes are usually made in reaction to major fires. Moreover, they are not explicit about what levels of safety they prescribe.

Design professionals, property owners and the regulating authorities all agree that buildings should be safe. They only disagree on the meaning of the term “safe” and the degree of safety. Since building codes do not offer definitive guidance on safety levels, there is an ongoing debate about whether the codes are too restrictive or not restrictive enough.

Building codes prescribe, for example, a relationship between the height and area of a building and the structural fire resistance needed to protect the occupants and the firefighters. The concern with height is understandable considering the difficulties related to fire fighting, evacuation and smoke spread. A building’s area, however, has no direct relationship to life safety. Since the required number of exits increases in direct proportion to the number of occupants or “occupant load,” and since the maximum travel distance to an exit is limited by the building code, a building that is 100,000 square metres in area could have a shorter evacuation time than a building which is 1,000 square metres in area.

It seems, therefore, that there is no need to increase the fire resistance of structural elements in a building based only on its floor area. A high-rise building may be designed with a large attached low-rise portion. Unless there is a firewall separating the two portions, building codes require that the structural fire resistance requirements of the low portion be based on the fire resistance requirements of the tower portion. There simply is no logical reason for this restrictive approach.

The relationship between structural fire resistance and life safety is weak. Providing a fire resistance rating for the structural elements which is much more than the calculated evacuation time (with a safety factor) for a building will not achieve a significant increase in life safety. This statement is especially true when the maximum attainable temperature during a fire is limited by a sprinkler system. Therefore, relaxing the requirements for structural fire resistance, particularly for sprinklered buildings and in areas where the fire departments have a low response time, will not increase fire fatalities, but will reduce construction costs.

The Canadian fire loss statistics consistently show that the highest number of fatalities occur in our smallest buildings: the single family dwelling. It is also clear that other types of buildings where occupants are awake and can take action to protect themselves from smoke and fire are inherently safe. Hence, to reduce the life safety requirements that apply to certain types of non-residential buildings is justifiable.

Another extremely conservative area in the codes is the exit requirements, particularly in buildings such as arenas and convention centres, which have high occupant loads and contain large open areas. Methods exist to design an egress system by calculating the amount of time available to safely evacuate a building and comparing it to the time it takes for smoke and toxicity levels to exceed a certain level. This approach usually justifies allowing travel distances to exits to be increased from those prescribed by the code, and reducing the exit capacity without lowering the level of safety. Architects can then design the spaces to be more functional.

Building codes today are very conservative in the occupant load factors they prescribe. Canadian codes still specify a factor of one person for every 100 square feet of floor area for office buildings, for example. This factor is based on a 1934 study in the U.S. All subsequent studies, including the latest in 1996, have confirmed that the actual occupant load factor is substantially greater, that in fact most office buildings hold much fewer people than one per 100 square feet. A conservative occupant load factor increases the cost of office buildings because it means providing additional egress capacity and exits to accommodate the over-estimated population. Having to provide that exit capacity also reduces the rentable space.

Take another example, the requirements for hoses in fire hose stations. Building codes today require that cabinets equipped with a rack of 30 metres of hose be near every exit in buildings over three storeys in height. Building inspectors ensure that the required number of 30 metre fire hoses is provided before permitting the building to be occupied. However, most fire departments do not allow the occupants of a building to use the fire hose, and make this clear when they approve the building’s fire safety plan which is mandated under the fire code. The firefighters, on the other hand, carry their own fire hose when fighting a fire since they do not trust that the building’s hose is properly maintained. One can easily calculate the length of fire hose sitting in buildings, say in downtown Toronto, that had to be provided in order for the building to be occupied, yet which is not allowed to be used. It is long enough to go across Canada! This is not cost-effective fire safety.

Put away the tape measure

When codes are prescriptive and there is a dispute between building officials and design professionals about means of egress, out comes the tape measure or scale and the discussion revolves around dimensions such as width of corridors or stairs, travel distances etc. If codes were performance-based, the discussion would not be about doors and stairs, but about whether people can evacuate safely in a fire according to stated goals.

Fortunately, the Canadian Commission on Building and Fire Codes targeted year 2003 to introduce objective-based (performance) approaches in the National Building Code of Canada and the National Fire Code of Canada.

Research during the past 30 years has given us a good scientific understanding of how fire develops and spreads. Solving fire problems by merely complying with a prescriptive code does not give an indication of the safety factors involved. If codes focused more on defining the acceptable level of safety instead of prescribing dimensions, then it would be the fire safety consultants’ responsibility to meet the safety criteria by considering variables such as ignition sources, time of detection, fire growth rates, smoke control, building use, occupant behaviour and egress time.

Architects and engineers would no longer try to comply with the prescriptive requirements of the building code. Instead, they would design for possible fire scenarios and to assure safety. By permitting the use of any design that will meet performance objectives and functional requirements, performance-based codes will encourage designers and builders to seek cost-effective solutions without reducing safety. Cost-effectiveness will occur, in part, when fewer redundancies are required.

By not stating fixed technical solutions, performance-based codes would also allow for the use of non-technical solutions such as fire safety traini
ng and human factors. People should be counted along with the building design components such as building layout, materials and fire safety equipment since the fire protection design’s overall performance often depends on the occupants’ decisions and actions. Contrary to behavioural research findings, building officials and designers usually take the worst case approach and assume the lowest level of performance from people.

The design of active and passive fire and life safety systems in buildings spans across all disciplines, electrical, mechanical and structural engineering, as well as architecture. What separates a good fire safety design from a bad one are the choices and assumptions made. Fire protection engineers and building code consultants should be included in the design team starting at the conceptual stage in order to bring a holistic approach. They will set the parameters to ensure the building has an effective fire safety design at a reduced construction cost.CCE

Demir Delen, P.Eng., has over 25 years of experience in fire and life safety issues in buildings. He is the director of fire protection engineering at Morrison Hershfield Limited in Toronto. E-mail: ddelen@mhgroup.ca


Print this page

Related Stories

Leave a Reply

Your email address will not be published. Required fields are marked *

*