Evaluating Atrium Smoke Control System Design — A Checklist
Is the model geometry a close representation of the actual atrium geometry and does it include all obstructions that may impact smoke flow?
Does the designer provide an adequate technical basis and justification for all design inputs?
Is the proposed design fire sufficiently conservative for the space being modeled? Does the design fire anticipate worst-case temporary or seasonal fuel loads or is it limited by administrative controls?
Design fires at less than a fast-growing 5,000 Btu/s (5,275 kW) are rarely applicable for use in atria with large floor areas.
Design fires at less than a fast-growing 2,000 Btu/s (2,110 kW) are infrequently applicable for use in small or limited fuel loading atria, unless the floor area is limited by the installation of non-combustible features (e.g., fountains, stone benches) or must remain clear as part of the means of egress.
Are alternate design fire types and locations considered in the modeling (e.g., fires in low ceiling spaces adjacent to the atrium, including consideration of sprinkler effects)?
Are the fuel properties sufficiently conservative for the potential design fires?
The combination of a higher range soot yield and lower range heat of combustion results in the greatest mass of smoke particulate.
Given the prevalence of plastics in today’s society, use of a soot yield less than 0.05 g/g is rarely justifiable.
Are sufficiently conservative tenability criteria used in the evaluation of results? The following values are recommended as the basis for discussion (see endnotes 9-10):
• Minimum visibility distance of 30 ft (10 m)
• Maximum convective (direct) heat exposure
of 140°F (60°C)
• Maximum radiative (indirect) heat exposure of 2.5 kW/m2
• Maximum CO concentration of 800 ppm
What is the visibility constant used in the visibility distance calculation?
A visibility constant of 3 is appropriate for most occupancies, due to the need to negotiate around normal (not illuminated) objects during exiting.
A visibility constant of 8 for a backlit exit sign is appropriate only for confined egress routes containing little to no obstructions and where the exits are readily located (e.g., a hotel corridor with exit stairs at remote ends).
Are time to occupant notification, occupant travel delays due to impaired or slow-moving occupants, and response time delays sufficiently considered in the calculation of timed egress?
A minimum factor of safety of two is recommended for timed egress calculations.
An additional delay is recommended where there is the potential for sleeping occupants.
Require a third-party peer review when the design reviewer is unfamiliar with the model, the qualifications of the designer with respect to using the model are in question, or where there are significant concerns relating to the design assumptions used.
Fire Protection Engineering for Atria in 'Green Building' Designs
NFPA Journal®, January/February 2008
By John Stauder
As the need to design energy efficient buildings increases, fire protection engineers are becoming invaluable design team members, and their role on the team is continually evolving. Unique architectural features make even more challenging the fire protection engineer’s job to satisfy building code requirements while helping the design team to meet their green or Leadership in Energy and Engineering Design (LEED) goals.
Designers use atria to incorporate several green attributes into building design. They may feature an atrium to take advantage of the available natural light, use the openness that this large space creates, provide natural air movement through the space, and allow the occupants to connect with the outside through the abundant use of glass and windows. Sometimes, the conversion and reuse of an existing building will incorporate an atrium into the design.
While these aspects have architectural appeal and are important in green building design, the primary impact of the atrium on the design of the building is the code and standard requirement for a smoke control system. The 2005 NFPA 92B, Smoke Management Systems in Malls, Atria, and Large Spaces also has smoke control system requirements.
There are five design approaches for these large volume spaces identified in NFPA 92B; (1) natural smoke filling, (2) mechanical exhaust, (3) mechanical exhaust with smoke filling, (4) gravity smoke venting, and (5) gravity smoke venting with smoke filling. Several of these methods maintain the smoke layer at a specific level, while others allow the smoke layer to descend but still allow the occupants to egress from the space. Refer to NFPA 92B for more details on these design methods. The standard also requires a smoke development analysis with justification through calculations or modeling.
Using the mechanical smoke exhaust method of NFPA 92B and the algebraic equations of Chapter 6, the calculated exhaust and make-up air quantities may become a burden on the mechanical, electrical, and architectural designs in a green 57building project. The mechanical design may not include exhaust fans or if exhaust fans are included, the equipment may not have the capacity required by the calculations. Smoke control equipment is required to be provided with a secondary power supply, which can increase the size of a generator already provided or add one to the design. There may not be dedicated roof or mechanical space available for the smoke control equipment, depending on the architectural design of the project. Large skylights, glass windows, and the design for natural lighting or a green roof system are examples of this situation. If the requirements for the smoke control system design are not considered early enough in the design process, the project design may require significant changes to accommodate smoke control equipment.
For example, with the smoke layer maintained above the upper level and using a 2,000 Btu/sec design fire, the calculated exhaust quantity based on the axisymmetric plume calculation of NFPA 92B would be on the order of 181,000 cfm.
As an alternative to the mechanical smoke exhaust capacity determined by calculations, designers may use modeling as a part of the smoke development analysis and justification. This type of approach is typically considered a performance-based design (PBD) alternative. The main benefit of this approach for a green or LEED building is the alternative designs options that are available for consideration. The potential to reduce or eliminate mechanical exhaust fan capacity, the ability to analyze unique building geometry or architectural features, and the ability to consider alternative design approaches may be important considerations. For the most part, these types of designs require computer modeling. A common computer modeling tool for these types of analyses is the Fire Dynamics Simulator (FDS) and its companion visual output display program, “Smokeview.” These freely available programs were developed and are supported by the National Institute for Standards and Technology (NIST).
Unlike the prescriptive approach, which relies solely on calculations to determine the smoke exhaust quantity, the PBD approach may include a tenability analysis to reduce the required mechanical exhaust capacity. Passive approaches with smoke filling, natural ventilation, or limited mechanical exhaust systems can also be analyzed. Designers can incorporate into the analysis the impact of unique architectural designs and features. The prescriptive requirements of the exhaust method also typically limit the make-up air velocity to 200 feet per minute. As part of the PBD analysis, they can analyze velocity and evaluate the impact on the smoke plume and smoke production.
Designers may also include an egress with the design. Using an egress analysis, they may calculate the required safe egress time (RSET), which is the time for occupants to egress the atrium may be calculated, and available safe egress time (ASET), which is the time is takes for smoke layer to descend. Provided the RSET is greater than ASET, the smoke control system in this case would be designed to operate or maintain tenable conditions for the RSET, which may be less than 20 minutes.
Because PBD is technical in nature, not all jurisdictions have the capability to discuss and review the design criteria and analyze the results. For this reason, a third-party review is often required. Establishing the design goals and objectives as well as the pass/fail criteria of the proposed system with the authority early in the design process is important. If a third party is also to review the design, it also needs to be involved in these discussions.
As discussed previously, there are several different design methods available for atria in NFPA 92B. In some circumstances, using a PBD approach makes the most sense. Designers need to consider factors such as the architectural design of the atrium, the mechanical systems used in the building, or space available for smoke control equipment. If PBD is used, the local jurisdiction and third-party reviewer need to be involved early in the design process. Discussions that establish the parameters and aspects of the design and the pass/fail criteria are best done when changes have a limited impact on the design. Additionally, there are no guarantees when using this approach and designers may need to incorporate changes based on the results, which often requires additional modeling and analysis.
As green building concepts have become a normal part of the building design process, the use of technology to analyze smoke management systems for atria is a useful tool that can achieve the project’s design goals, while meeting the requirements of the building code.
John Stauder is a fire protection engineering consultant for Rolf Jensen & Associates working out of the RJA’s San Francisco office.