Applying NFPA 13 in high-rise buildings

Designing and installing fire standpipes and sprinkler systems in modern high-rise buildings properly requires in-depth analysis to ensure occupant safety.

10/18/2011


Nearly 3,700 years ago, the responsibility of the stability of a structure was first bestowed upon building inspectors. The inspectors made sure that building materials were properly cut and made from unflawed components. When the inspectors deemed a stone to be of sufficient quality, they were authorized to affix the king’s seal to the stone. If subsequently a building collapsed and a sealed stone was found to be at fault, the inspector was at fault. This king was, of course, Hammurabi, the inspectors were the progenitors of today’s testing laboratories, and the rationales they used to determine the stone’s quality were the original standards.

Today, we have a plethora of testing agencies, nongovernmental standards writing organizations, and governmental standards writing organizations, as well as an armed host of lawyers, all seeking to make sense of the ever-increasing complexity of the building environment.

The primary design and installation standard used in most of the U.S. for commercial fire sprinkler systems is NFPA 13: Standard for the Installation of Sprinkler Systems. To properly use this standard to design or install a system, one must be familiar with the standards upon which it is based and those which it references. It is within these auxiliary standards that lurk the minefields that can explode with devastating effect. For example, one of the commonly misunderstood provisions of NFPA 13 is Section 6.1.1., which many believe requires that all materials and devices be listed. It does not.

NFPA 13 recognizes there are eminently acceptable materials, such as pipe, fittings, and valves, manufactured to other long-respected standards promulgated by the American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), and the Manufacturer’s Standardization Society (MSS) of the Valve and Fittings Industry that, when properly selected and applied, will result in perfectly satisfactory strength, durability, and performance. In fact, the materials manufactured to these standards typically have ranges much more appropriate to today’s taller structures than do those listed to the arbitrary and archaic limit of 175 psi found in NFPA 13.

It is therefore important that the engineer designing the fire standpipes and sprinkler systems in modern high-rise buildings properly analyze the needs of the systems, determine the pressures to which the systems will be subject, and, through judicious application of the principles of the ASME Boiler and Pressure Vessel Code provisions found in ASME B31.1 and B31.9, select the pipe, fittings, and valves most suitable for the installation.

An equally important factor in materials selection is the system’s life expectancy. It must be determined, through conversations with the owner, user, and regulatory agencies, what the expectations are for the service life of the systems and their components as well as the building life they are protecting. This, coupled with the engineer’s knowledge of the environment into which they will be placed, allows for selection of materials and corrosion mitigation strategies that will assure an adequate service life, as not all materials are created equal and not all materials used in proximity to each other are compatible.

Blind reliance on listed materials bearing a mark from Underwriters Laboratories (UL) or Factory Mutual Global (FM Global) does not come close to satisfying the standard of care to which an engineer is held, nor does it assure there will not be legal arguments at some future date as to the adequacy of the materials selected. A simple example is the standard commercial riser manifold produced by most of the manufacturers that bears both the FM Global and UL marks and a molded or engraved pressure limit identifier of 175 psi. Upon detailed examination, it can be determined that many of these devices are clearly acceptable for pressures well in excess of 175 psi. However, they are only listed to 175 psi because the UL and FM Global standards under which they are listed arbitrarily chose this number from an oft-repeated pressure in NFPA 13. And, in fact, it is not possible in modern high-rise buildings to limit the pressure of the principal piping systems to 175 psi. Even though recent times have seen the inclusion of products listed at pressures up to 300 psi, such listings do not cover the full catalogue of needs, nor is 300 psi a magic bullet number any more than is 175 psi.

There is simply no substitute for doing an appropriate engineering analysis of system needs and specifying materials and devices that will meet the demands of that analysis. Mere reliance on the obvious provisions within NPFA 13, the listing directories of UL and FM Global and manufacturer’s catalogues written to satisfy the requirements of those documents is not adequate.

In preparing specifications, the engineer must be careful to specify only that which he needs and intends to enforce. Clauses such as “Comply with all applicable federal, state, and local codes and standards” don’t cut it. First, few engineers know what constitutes “all” of those applicable federal, state, and local codes and standards as they each have internal references to and adoptions of yet more codes and standards. Second, even fewer engineers have “all” of those codes and standards on their shelves; most of those who do have them are not aware of what is in them. And what is in Code A is not always compatible with the requirement of Standard B, though both are applicable and the more stringent will apply.

For example, NPFA 13 requires that to fully equip a building, sprinklers must be installed in all places that a fire could occur. An elevator machine room is such a space. However, ASME A17.1 Rule 102.2: Safety Code for Elevators and Escalators requires that electrical power to the equipment be shut off prior to the application of water. There is no direct connection between the response time and temperature rating of a heat detector and that of a sprinkler. The response time index (RTI) of a sprinkler is determined by what is known as the plunge test; there is no similar or equivalent test for a heat detector. Therefore, there is no way to reasonably comply with Section 21.4.1 of NPFA 72: National Fire Alarm and Signaling Code, which requires a heat detector be provided to respond appropriately prior to sprinkler activation. Feasibility requires a much more complex solution, especially when the intent of the code is taken into consideration.

As pointed out by the California State Fire Marshal, “Without completing the recall to the designated floor or alternate floor, the elevator car is prone to ‘suddenly stop dead’ wherever it may be in the hoistway upon activation of the elevator power shunt-trip. This life safety dilemma is amplified in high rise buildings, where most of the occupants may be not aware of a fire on another floor until they are inside the elevator car. Most elevators will continue to function normally above and below the fire floor until recalled by a smoke detector located at the fire floor elevator lobby, hoistway or machine room, or through manual intervention.”

To resolve this, a programmed time delay of elevator power interruption is necessary. The length of this delay must be determined so as to be long enough to permit completion of recall. The provisions and advice given in NFPA 72 and its companion document, NFPA 13, are in conflict with themselves, ASME A17.1, and the physical laws of nature. Furthermore, they would appear to be in direct conflict with the provisions in the International Building Code (IBC) and International Fire Code with regard to exempt locations, as some building and fire officials have determined that sprinklers within elevator machine rooms either constitute a serious life safety risk or fire hazard or are undesirable because of the nature of the room. Simply placing a heat detector of nominally lower temperature rating than the sprinkler an arbitrary maximum distance away from the sprinkler does not in any way ensure the desired sequence of events.

One method of accomplishing the intent of these seemingly incompatible codes and standards would be to install a double interlock pre-action system within the elevator machine room and to require that one of the events necessary for the double interlock to release be the completion of the timing sequence for elevator recall. As elevator manufacturers will not permit direct monitoring of elevator circuitry to determine the initiation and status of recall, one must rely on a predetermined time and initiate that timer based on either smoke or heat detection within the elevator machine room. In the event of an actual fire, the other portion of the double interlock, the fusing of the sprinkler and consequent reduction in air pressure within the air-supervised pre-action system, will assure the presence of water in a timely manner when the electrically held valve is opened by the timing sequence.

Given that a consulting or specifying engineer has a statutory first duty to preserve the public safety and a fiduciary duty to his client, normally the owner, it is an abrogation of those duties for the engineer to delegate responsibility to the design/build contractor, who is only required to comply with the lawful mandate of the local inspection authorities and has a fiduciary duty only to its ownership, by means of that all-too-commonly used bailout, “Contractor to provide and install a system complying with all applicable codes and standards.”

Dillon is president of Dillon Consulting Engineers. He is very active in codes and standards throughout the U.S., and with ASHRAE and NFPA. Dillon is internationally regarded for his analysis and direction of engineering issues in complex buildings, especially tall buildings. Copeland is manager of field services at Dillon Consulting Engineers. Her expertise is primarily in codes and inspections, and she has more than 30 years in all aspects of building including design, construction, inspections, and post-construction litigation.



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