High rise buildings have several unique fire hazards due to the height of the building. A high rise building, as defined in the Uniform Fire Code, is “any building or structure having floors used for human occupancy located either more than six stories or more than 75 feet above the lowest level accessible to a fire department vehicle.” If protected properly, there are several issues that can minimize the fire hazards associated with high-rise buildings
Construction
The exterior vertical fire spread can increase the spread of fire from the fire floor. The (H) distance between any floor and the bottom of the window on the next story above the floor and the (h) distance of the window height is a determining factor. If the H/h ratio is less than 2.8 for permanently closed windows then exterior fire spread can be minimized. The same is true for a 3.8 ratio for windows that can be opened.
Internal vertical fire spread, such as poke throughs from utilities, should be protected with approved fire stops with a 2-hour fire resistance. Open interior stairwells also offer a fire spread hazard. The door openings should be protected with approved fire rated doors of at least two hours that are operated by automatic detection systems. Protecting the spread of fire and smoke in atriums and other large open areas that extend more than one floor is another area of concern. Approved smoke tight fire partition barriers and 1-1/2 hour fire resistant rated windows and doors should be provided between the open floors.
Occupancy
Office buildings can have various occupancies. A typical office configuration will involve perimeter offices with a cubicle type open landscape arrangement on the interior of the floor. Combustible loading can vary, but is usually defined as light hazard. In some cases, excessive paper and plastics are prevalent. Plastic office furniture is commonly used now which increases the overall fire load. Reams of paper are often found in hallways as well as other office supplies. Electrical closets are sometimes used for storage of office or janitorial supplies due to the lack of storage space on the office floor. Housekeeping is critical in minimizing the fire load and maintaining the light hazards occupancy classification.
Mechanical floors in high rise buildings pose a different exposure. Most mechanical floors contain generators, compressors and elevator machine rooms. In each case oil, is used as either a fuel or lubricant. Safety shutoff valves, leak detection and containment should be provided on oil under continuous pressure to avoid an undetected oil leak that can spread throughout the floor. Mechanical floors are classified as an Ordinary Hazard occupancy.
Protection
Most building codes throughout the United States require new high rise buildings to be protected by automatic sprinkler protection. Building codes and local laws are being revised throughout the United States in order to provide sprinkler protection in existing high rise buildings. For example, in New York City the building code has been revised so that by 2019 all high rise office buildings that are not listed as historical should be protected by automatic sprinkler protection. This is a dramatic change. In the past compartmentation under New York City’s Local Law 5 was viewed as an acceptable alternative to providing automatic sprinkler protection.
A bill enacted before Congress is trying to change the depreciation methodology for automatic sprinkler systems. Instead of depreciating the cost of the labor and materials for a sprinkler system over the life of the building, which in most cases in 28 years, the sprinkler system would be able to be depreciated over a 5-year period. This would offer greater financial incentive for building owners.
In office or residential areas, sprinklers should provide a minimum of 0.10 gpm/ft2 over the most remote 1500 ft2. The hose stream allowance is 250 gal/min, the duration is 60 minutes and the sprinkler temperature rating is 165° F. Stock rooms, computer centers, mechanical floors and other occupancies would require a more stringent design criteria.
One of the difficulties in designing the water supply for a High-Rise Building is the high pressures the system is under. Usually Pressure Reducing Valves (PRV) are installed when pressures exceed 175 psi. This is the maximum working pressure of the fittings. Whenever possible, design the water supply and fire protection systems so that pressure reducing valves are not necessary. Difficulties in maintaining and testing the PRV’s are quite common. A dedicated 3 inch riser is needed to handle the water supply during testing procedures. In most cases the 3 inch riser is not installed and testing is not performed. Please see the May 2001 article, “Pressure Reducing Valves” and June 2003 article, “Solving the Problem of Pressure Reducing Valve Testing” for additional information.
If you would like additional information about providing adequate protection for fire hazards in a high-rise building, please contact us.