Fire Protection Basics
Fire protection engineers look at many aspects of building design as they relate to life safety. Some prevalent concepts incorporated into residential occupancy design are building separation and compartmentation, exits that are clearly marked and maintained for use, smoke detectors and fire alarm systems to provide early warnings of hazardous conditions, and a monitored sprinkler system to control fires. These concepts are currently required for most multiunit residential occupancies.These concepts are incorporated by designers and confirmed by building officials. The systems noted address most life safety concerns in residential occupancies. Smoke control systems may also be required for residential occupancies in high-rise buildings, or in buildings involving unique features such as atria or other atypical designs. Atypical features may introduce hazards that warrant the additional complexity of automatic controls generally associated with smoke control systems.
In special cases, additional protection may be appropriate due to limited fire department access, longer exit times in larger buildings, and the increased exposure to those exit ways from fires or other hazardous conditions. When exit ways are threatened by other building features, or when the occupants may have a more extreme exposure due to longer required exit times, additional systems or features are appropriate to increase the level of protection to the occupants and compensate for the increased hazards.
Hospitality Occupants
The occupants found in residential, or hospitality occupancies are different than those found in other occupancies. By their very nature, hospitality facilities accommodate large groups of people, generally on a transient basis. The occupants are probably not familiar with their surroundings. To further complicate matters in hotels, the majority of people, unfamiliar with their surroundings, might be awakened from a deep sleep. When an emergency requires action on their part, they are likely to be disoriented and confused. Anything that can be done to simplify what is required of them can reduce the likelihood of a tragedy. This is why the code requires additional exits, exit lighting, exit signs, and sometimes, voice instructions.Smoke Control Requirements
In the case of a high-rise building, or a structure with an atrium, the physical construction features can be enhanced by the inclusion of a smoke control system. The Uniform Building Code (UBC), along with many jurisdictions, require mechanical smoke control systems in high-rise buildings. The recent model building codes (the IBC and NFPA 5000) rely upon the floor separations for passive smoke protection/control in high-rise buildings. Although the UBC requirements for mechanical floor smoke control systems in high-rise buildings will no longer be present in the International Building Code, or NFPA 5000, there may be occasions where it is required, or appropriate.Customized Approach
When developing smoke control system concepts, it is important to remember that there is no single approach that can be prescriptively applied to all situations. Smoke control approaches should be developed to meet the particular life safety requirements for a project, and to enhance the building features already included in the design.
Older editions of the model codes specified an "air change" design method to meet all smoke control needs. The codes have been updated since the mid 1990s to include general methods for providing smoke control that allow the designer to more appropriately protect the building occupants. These approaches are more "performance" oriented than those in previous editions of the code. It is important to remember that the smoke control approach selected for one job is probably not suitable for applying to the next job without significant modification or reconsideration.
Reliability Depends on Simplicity
After reviewing, testing, and specifying smoke control systems in various buildings, it has become clear that the best systems are those that employ the fewest devices and avoid unique programming sequences. A designer should be able to explain the smoke control system of the entire building easily in a few sentences.Older systems often employed pressure sandwiches in the vertical plane, or pressure donuts when viewed in 3-D. These approaches are complex and require many different programming sequences depending on which zone is in alarm. These become costly to design, negotiate, install, commission, and maintain; all too frequently, they don't operate appropriately when finally required to perform.
We often recommend simple approaches that involve only exhaust, negative pressure, or positive pressure in the smoke control zone to achieve the goals of the system. Single programming outputs are easier to understand and implement than scenarios that involve supply in some areas, exhaust in other areas, purge in some areas, and shut-off in others. To the extent possible, the building HVAC system should take into consideration the emergency smoke control approach that will be employed. This may affect the number, size or type of fans specified, or the duct layout.
We recently commissioned a smoke control system designed by another engineer for a large volume space that consisted of only two smoke zones. The HVAC system was laid out without regard for the smoke zones. With control dampers, the system worked fine, theoretically. However, it required more than 120 fire and smoke dampers to perform operations within the two-zone space. A system that could have been relatively simple, resulted in commissioning costs that exceeded six figures, delayed the project opening due to operational issues, may cause maintenance issues, and may have a reduced likelihood of operating properly when required. Consideration of the smoke control approach when laying out the HVAC system and controls could have simplified this project and reduced costs considerably.
Residential Smoke Control
The following sections will describe typical smoke control considerations for high-rise residential occupancies. They describe common considerations, but each should be evaluated before applying to any given project.
A common design assumption for smoke control systems is that the fire will be controlled by sprinklers. Using the smoke control system for an uncontrolled fire is not typical. A standard smoke control system should not be offered in lieu of sprinklers, but rather in support of sprinklers to enhance other building features already in place. Smoke production in a non-sprinkler protected building can quickly overcome the capabilities of a smoke control system.
Pressurized Exits
High-rise stairs are usually separated with rated fire construction. They are often required to be pressurized. The positive pressure serves to prevent smoke from entering the stair. It is an added feature to enhance the rated separation already required. Often these pressurization systems are only activated when smoke reaches a smoke detector at an opening into the stair. However, it is hard to envision a scenario when pressurizing the exit enclosure is an incorrect response. Therefore, common practice is to pressurize, or to initiate the pressurization systems on receiving any alarm in the building where the stair is located. This simple approach makes the overall system operation easier to understand and implement. This helps the designers, the reviewing authorities, the installing contractors, and anyone else trying to understand the system.Floor Protection
The code allows mechanical smoke control on high-rise floors to be done through either a pressurization or an exhaust scheme. The exhaust criteria is to maintain a smoke layer 10 ft above the walking surface on the affected floor. In view of the fact that residential floors are subdivided into compartments and typically have less than 10 ft of clear height, it becomes difficult to employ an exhaust scheme on a high-rise residential floor. Therefore, the pressurization approach is usually employed.Residential floors are often designed with a 1-hr rated corridor with guestrooms on both sides. Often a central air-handling system is provided for the corridors, but the air conditioning in the guestrooms is provided through independent units in each room. Centralized control of the pressure in the guestrooms is difficult. Pressurization of the central corridor is more easily accommodated. Therefore, the corridor walls are often used as a "passive smoke barrier" and the pressure is controlled within the corridor space.
The approach most typically employed for hotels is to negatively pressurize the common corridor with respect to non-alarm floors. The intent is to prevent smoke reaching the corridor on the floor of alarm from escaping to noninvolved floors. This is consistent with the overall philosophy in hotel occupancies. A low-rise hotel could be built without any smoke control provisions whatsoever. The differences between a high-rise hotel floor and a low-rise hotel floor are mainly associated with the exposure to and from the other floors of the building. Preventing contamination of noninvolved floors is a logical design goal in that case.
An alternate design might provide positive pressure in the corridor to prevent smoke in a guestroom from entering the common corridor area. This would protect the common corridor exit access. However, as previously stated, a low-rise hotel could be built without any active smoke control system. Therefore, protecting the exit access on any floor does not address the hazards associated with high-rise buildings. On the other hand, if the smoke control system was being provided as an alternate to (or to compensate for) reduced construction in the corridor, or to adjust for increased exit distances, the positive pressurization of the corridor may be appropriate.
The smoke control system should enhance the natural building features provided. It is important that the system does not counteract them. A situation where an exhaust system pulls smoke into a clear zone from a fire in an adjacent area should be avoided at all costs.
System Initiation
When designing the control sequence for systems, it is important that the alarm inputs provide reliable information about fire zones. For this reason, manual fire alarm stations are rarely used to initiate automatic control functions. An occupant from the fire zone might travel to another zone before initiating an alarm.In a hotel, the smoke control system would actually reduce life safety if it were to pull smoke into the corridor from a fire in a guest room. To address this concern, the corridor exhaust system should be initiated only for a fire or smoke detected in the corridor. The system should not operate from a sprinkler waterflow switch that protects the entire floor (corridor and guestrooms).
Often, the corridor smoke exhaust system on a hotel floor is operated based upon activation of smoke detectors within the common corridor area. The design intent is to wait for smoke to migrate from a guestroom fire to the corridor before operating the corridor exhaust. Sometimes this concept is further emphasized by using alarm verification or considering activation of two corridor smoke detectors as a signal to start the corridor exhaust.
Another difficulty often encountered is trying to design for all permutations of multiple alarms. We have typically considered the first automatic alarm to be the most reliable predictor of fire origin location. Responding to subsequent alarms may be counterproductive and defeats the purpose of the system's intent.
Take an example of a multiple zone pressurization system. The system will respond to the first smoke detector and initiate exhaust in that zone. However, occupants may exit from one zone to another and allow smoke to cross the smoke barrier and set off another detector. A designer can either leave the system configuration based upon the original alarm, or activate exhaust for the second zone. It is difficult to predict whether the second alarm is due to a fire in the second zone, or whether the smoke is due to trace amounts trailing exiting occupants. If it is the latter, and the exhaust for the second zone is initiated, the system may not function as originally intended, and may impact the level of protection for the occupants of the building.
Bathroom Exhaust
Hotel residential floors are separated from each other using the compartmentation concepts described previously. Most of the openings between floors are in the vicinity of the central corridor. Pressurizing this corridor to prevent smoke spread from one floor to the other is an appropriate way to prevent contamination of non-involved floors.
One possible opening between floors is the bathroom exhaust system in the guestroom bathrooms. These systems are frequently connected on vertical risers for groups of guestrooms. The building control system and smoke control system should be designed to address the importance of any openings into the exhaust shaft, and include the exhaust fans operating as part of the subduct system. It is important that those fans continue to operate to prevent smoke spread between floors. These fans should be in continuous operation and operate on standby power. Otherwise, dampers should be used to protect the openings in these shafts.
Operable Windows
Frequently, hospitality and hotel buildings utilize operable windows. This seems like a simple adjustment to the building design. However, a building design, based upon pressure differentials between floors, could be profoundly impacted when that building has an unpredictable quantity of openings to the exterior. On windy days, design pressures on a floor with closed windows on the windward side and open windows on the leeward side are going to be overcome by the environmental conditions. In this situation, additional computer modeling, and sometimes additional controls, are required to predict and accommodate for the exterior pressure conditions.Mixed-use Protection
The lower floors of hospitality occupancies are often occupied by uses considerably different than a hotel guestroom floor. These spaces might be occupied by meeting rooms, retail areas, or casinos. Many of the considerations described above might not hold true in these atypical floors. A different smoke control approach may be more appropriate.Typically, the floor to ceiling height is greater and individual compartments are larger on the lower floors of the building, and an exhaust system may be more helpful. Alternatively, lower floors of hotel buildings may have interconnected levels. The design intent for a smoke control system in these areas might be to keep smoke on one floor from traveling to an adjacent floor through the floor opening. This also would be done with an exhaust approach.
Some of the lower floors may also use the pressurization approach described above to prevent smoke on these floors from traveling to the noninvolved hotel floors.
The main point to acknowledge for these floors is that there is no "one-size fits all" approach. The smoke control concepts must be developed to address the particular life safety needs for the project. Those life safety needs should be discussed with the designers and the project reviewers to make sure that the overall life safety rationale is properly implemented in the smoke control system design.
Building Controls
The HVAC system and environmental management system on hotel floors are often very simple. A main AHU can provide modest quantities of conditioned air in the corridor space, and separate units condition guestroom air. Frequently, a complicated BMS is not present for these situations. Direct control of the fan and damper system by the fire alarm system can be more appropriate in residential occupancies. This is different than in business occupancies, or in the large mixed-use spaces frequently at the base of hospitality occupancies, where a BMS may be the more appropriate avenue to accomplish fan and damper control. In either case, the control system should be listed for the intended application.Summary
Smoke control systems are one element of the overall life safety approach of projects involving high-rise buildings, atria, underground buildings, and other atypical designs. Current code editions include a performance-based approach for smoke control systems. This allows designers to identify the system objective and design a system to enhance the level of overall building safety. The general objective of these systems is to maintain a tenable environment in exits and to minimize the transfer of smoke from one zone to another. The reliability of these systems depends on the simplicity of their design.For example, exhausting a fire zone is much simpler than combining the exhaust with positively pressurizing adjacent zones to achieve the same pressure differential goals. The newer model building codes have accepted the importance of system reliability, and therefore, they provide a better opportunity to use passive barriers and simplify the smoke control systems. ES
