Fire Safety for Very Tall Buildings
Very tall buildings have unique fire safety design issues that are not experienced in other types of structures. For example, because the height of the structure is beyond the reach of ladders, tall buildings are equipped with more fire safety features as it is not possible for the fire department to initiate exterior rescues from ladders and suppress fires with outside hose streams.
In regards to fire safety, the performance history of very tall buildings while very successful, has not been without catastrophic incidents. Many of these incidents have resulted in 1) numerous deaths and injuries, 2) excessive property loss and 3) disruptions in business continuity. For example, the One Meridian Plaza high-rise fire in Philadelphia that occurred in 1991 resulted in the loss of three firefighters and building never being re-opened. In 1988, the fire in the Interstate Bank Building in Los Angeles experienced one fatality and resulted in the building being out of use for six months.
Based on research and lessons learned, the model building codes have made significant progress in addressing fire safety issues in very tall buildings. At the same time, the complexity and unique challenges of today’s very tall buildings have created an environment where comprehensive performance-based solutions have become a necessity.
To assist the design community with developing performance-based fire safety solutions for very tall buildings, in 2013, the Society of Fire Protection Engineers (SFPE) partnered with the International Code Council (ICC) to develop the Engineering Guide: Fire Safety in Very Tall Buildings.1 This publication is written as a guide to be used in conjunction with local codes and standards and serves as an added tool to those involved in the fire protection design of unique tall buildings. The guide focuses on design issues that affect the fire safety performance of tall buildings and how engineers can incorporate performance-based fire protection through hazard and risk analysis methodologies into the design of tall buildings. This article will discuss some of the unique fire safety design strategies/methodologies employed in the design of tall buildings that are referenced in the ICC/SFPE Guide.
Developing an effective evacuation strategy for a tall building is challenging as the time to complete a full building evacuation increases with building height. At the same time, above certain heights, the traditional method of requiring all occupants to simultaneous evacuate may not be practical as occupants become more vulnerable to additional risks when evacuating via stairways. That is why tall buildings often employ non-traditional or alternative evacuation strategies.
When designing an egress plan for a tall building, the primary goal should be to provide an appropriate means to allow occupants to move to a place of safety. To accomplish this goal, there are several evacuation methodologies that are available to the design team. These evacuation strategies can include but are not limited to 1) defend-in-place, 2) moving people to areas of refuge and 3) phased/progressive evacuation. It is also possible that a combination of these strategies can be this best solution. When deciding on an appropriate strategy, the design team should consider the required level of safety for the building occupants and the building performance objectives that are identified by the building’s stakeholders.
Using protected elevators has become another evacuation strategy that is becoming more prevalent in the design of tall buildings. In addition to assisting the fire department with operations and rescues, protected elevators are now being used for building evacuation, particularly for occupants with disabilities. When considering elevators in an evacuation strategy, there are a number of design considerations to consider: 1) safety and reliability of the elevators, 2) coordination of elevator controls and building safety systems, 3) education of building occupants and first responders and 4) communication to building occupants during the emergency.
The consequences of partial or global collapse of tall buildings due to a severe fire pose a significant risk to a large number of people, the fire service and surrounding buildings. At the same time, tall buildings often have unique design features whose role in the structure and fire response are not easily understood using traditional fire protection methods. These unique factors may warrant a need to adopt an advanced structural fire engineering analysis to demonstrate that the building’s performance objectives are met.
Performance-based design of structural fire resistance entails three steps: (1) determination of the thermal boundary conditions to a structure resulting from a fire; (2) calculation of the thermal response of the structure to the fire exposure, and (3) determination of the structural response of the structure. Guidance on performing this type of analysis can be found in the SFPE Engineering Standard on Calculating Fire Exposures to Structures2, and SFPE Engineering Standard on Calculation Methods to Predict the Thermal Performance of Structural and Fire Resistive Assemblies.3
Water-Based Fire Suppression Systems
In tall buildings, the water supply required for fire protection systems can be greater than the capability of the public water supply. As such, fire protection system water supplies for sprinkler systems and standpipes require the use of pumps and/or gravity water tanks to boost the water pressure. Reliability of this water supply is a key consideration. As such, redundant fire pumps, gravity-based storage supplies, or both may be needed to enhance system reliability.
Another issue to consider when designing water-based fire suppression systems is pressure control as it is possible for system components to be exposed to pressures that exceed its maximum working pressure. Consequently, it may be necessary to design vertical pressure zones to control pressures in the zone. Additionally, pressure regulating valves are often needed. When installed, care must be taken to ensure that these pressure regulating valves are installed properly and adequately maintained.
Fire Alarm and Communication Systems
Providing building occupants with accurate information during emergencies increases their ability to make appropriate decisions about their own safety. Fire alarm and communication systems are an important source of this information. Very tall buildings employ voice communication systems that are integrated into the fire alarm system. When designing voice communication systems it is important to ensure that the system provides reliable and credible information.
Fire alarm system survivability is another import factor to consider in fire alarm system design. For tall buildings, consideration should be given so that an attack by a fire in an evacuation zone does not impair the voice messaging outside the zone. Some of the design considerations to achieve survivability may include: 1) protection of control equipment from fire, 2) protection of circuits. 3) configuration of circuits and 4) shielding of panels.
Controlling the spread of smoke is more complicated in tall buildings. For example, tall buildings experience a phenomenon called stack effect. Stack effect occurs when a tall building experiences a pressure difference throughout its height as a result of temperature differentials between the outside air temperature and the inside building temperature. This causes air to move vertically, depending on the outside air temperature – either upward or downward in a building. It can also cause smoke from a building fire to spread throughout the building if not controlled. That is why tall buildings often employ smoke management systems that either vent, exhaust or limit the spread of smoke.
Other concerns in tall buildings included the air movement created by the piston effect of elevators and the effects of wind. Air movement caused by elevator cars ascending and descending in a shaft and the effects of wind can result in smoke movement in tall buildings. These impacts become more pronounced as the height of the building increase.
Because very tall buildings complicate smoke spread, effective smoke control is more difficult to achieve. The possible solutions are numerous and include a combination of active and passive features such as but not limited to: 1) smoke barrier walls and floors, 2) stairway pressurization systems, 3) pressurized zoned smoke control provided by the air-handling equipment, and 4) smoke dampers. The solution implemented into the design needs to address the building itself, its uses, relevant occupant characteristics and reliability.
First Service Issues
It goes without saying that tall buildings present unique challenges to the fire service. During the planning and design phases, it is important for the design team to work with the fire service to discuss the type of resources that are needed for an incident and the actions that will be needed to mitigate an incident. This includes developing construction and post-construction preplans. These preplans should include and not be limited to making provisions for 1) fire service access including transport to the highest level of the building, 2) establishing a water supply, 3) standpipe systems (temporary and permanent), 4) communication systems, and 5) understanding the operations of the fire protection systems in the building.
One of the challenges the fire service faces during incidents in tall buildings is the ability of firefighters to move equipment to the incident location. Designers should take into account how the fire service can transport its equipment from the response level to the highest level in a safe manner.
Additionally, care needs to be taken when designing the fire command center as it will provide the fire service command staff with essential information about the incident. The fire command center needs to be accessible and should include 1) controls for building systems, 2) contact information for building management, 3) current buildings plans, 4) emergency response and egress plans and 5) preplans.
1 International Code Council/SFPE. (2013). Engineering Guide: Fire Safety for Very Tall Buildings. Country Club Hills, IL.
2 SFPE. (2011). SFPE Standard S.01 2011, Engineering Standards on Calculating Fire Exposures to Structures. Gaithersburg, Maryland.
3 SFPE. 2015). SFPE Standard S.02 2015, SFPE Engineering Standard on Calculation Methods to Predict the Thermal Performance of Structural and Fire Resistive Assemblies. Gaithersburg, Maryland.