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|FPE Extra Issue 18, June 2017|
Stair and Elevator Pressurization Design Considerations
By Steve Strege, P.E.
Under the current International Building Code (IBC), high-rise and underground buildings of more than 30 feet below grade require smoke-proof enclosures. In addition, elevator lobbies are required for any hoistway connecting four or more floors. The primary purpose of these requirements is to protect vertical shafts from smoke infiltration, thus mitigating floor-to-floor smoke spread in the event of a fire.
A common method used for smoke-proofing enclosures is stairwell pressurization. Similarly, elevator pressurization may be used in lieu of elevator lobbies to protect the elevator hoistways from smoke infiltration. Elevator pressurization is often the preferred method by architects, since lobbies take up valuable floor space and may detract from desired building circulation schemes. Buildings that use both elevator and stairwell pressurization can be challenging because the two systems may have competing airflow paths.
Know the Codes
It is important for the designing engineer to know the building code of record, including any local amendments. Franchises such as hotel chains may also have special smoke control requirements for buildings of their franchisees.
Performance objectives can vary widely between codes and local amendments. For example, the minimum stair-to-building differential pressure for satisfactory pressurization in sprinklered buildings is 0.05 and 0.10 in. w.g. under the NFPA and IBC codes, respectively. Both codes require that these minimum differential pressures be maintained with all doors closed, under maximum anticipated stack and wind effects. Some local amendments also require satisfactory stair pressurization with a specified number of doors open (typically a minimum of two doors, with one being the exit discharge door).
For elevator pressurization systems, it is important for the designing engineer to consider both elevator recall and non-recall conditions. The elevator recall condition is particularly challenging and may create a significant back pressure on the stair doors located on the recall level.
Use Appropriate Design Tools
Two main types of tools are available for stair and elevator pressurization design: algebraic hand calculations and computer models. CONTAM, developed by the U.S. National Institute of Standards and Technology (NIST), is one type of computer model that is well recognized by the fire protection community for evaluating smoke control systems using the Pressurization Method.
The level of effort involved with using these different tools can be significant, with hand calculations requiring hours and computer models taking days. Considering the level of effort involved, using solely algebraic hand calculations for stair and elevator pressurization design can be tempting, but in many cases, this is not advisable, given the architectural complexity of many buildings.
Algebraic hand calculations, found in the Handbook of Smoke Control Engineering, assume a simple building geometry consisting of two internal stairways within an open floor layout. The geometry assumes the layout of every floor is identical and that there are no elevator hoistways. Changes in floor-to-floor layout can have a significantly impact on shaft-to-building differential pressures because of the changes in airflow resistance. For example, underground levels tend to have a tight building envelope, resulting in high airflow resistance and, thus, lower shaft-to-building pressure differentials. Conversely, the ground level, mechanical room floors, and floors with direct access to a parking garage can have a relatively loose envelope, resulting in low airflow resistance and higher differential pressures.
The building acts as a “system” or “network” of airflow paths. What happens on one floor affects all the other floors. In addition to the issue of geometry simplifications, hand calculations lack the consideration of other active forces, such as HVAC, elevator pressurization and wind, all of which can be significant.
Based on these shortcomings, algebraic hand calculations are recommended only as a first-order approximation or as confirmation for a more-detailed computer modeling analysis. In some less-common cases, using only hand calculations for pressurization design may be suitable for simple buildings with uncomplicated floor-to-floor layouts, mild climate, no other active systems (i.e., elevator pressurization, corridor purge, laboratories, etc.), no stair doors open, and low wind conditions. In most cases, the designing engineer should take the time to use a computer model to conduct a proper Rational Smoke Control Analysis, to account for the uniqueness of the building geometry, other active mechanical systems, and weather.Steve Strege is with JENSEN HUGHES