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May 12, 2021
You have identified that your building is required to have a smoke control system – what’s next?
A smoke control system is subject to the many requirements of Section 909 in the International Building Code. One of those requirements is that a qualified design professional must prepare a rational analysis (Section 909.9). Each type of smoke control system (exhaust, airflow, or pressurization) has unique design criteria, design options, and system requirements that are addressed in the smoke control rational analysis.
Our fire protection engineers at Performance Based Fire Protection Engineering have prepared hundreds of smoke control rational analyses. Through this experience, we identified the six key components of a smoke control rational analysis and why each is invaluable to successful smoke control system implementation:
One key component of a smoke control rational analysis is to identify which codes apply and determine any local code amendments or AHJ-specific requirements.
For example, when designing a stairwell pressurization system, some authorities require that pressure differentials be maintained with one or multiple open stairwell doors. This varies from the International Building Code, which states all doors must be closed. This could lead to an undersized smoke control system if not accounted for during the design.
Another AHJ we have worked with has a very specific preference for how results are presented and requires specific modeling tools and inputs to be used. Again, understanding and documenting these requirements will prevent costly change orders and delays during permitting and construction.
Here is where the magic happens. These are the programs where all the input data becomes useful results.
Through a series of computer simulations, thousands of first-order equations are simultaneously solved and converge on a solution. While certain simplified algebraic equations can provide useful initial estimates, only a computer model can incorporate and analyze the effects of unique geometries and the various design conditions for the most accurate results. At Performance Based Fire Protection Engineering, we have the most advanced numerical tools and simulation software available for our use.
One of the key elements of selecting an appropriate modeling tool is ensuring its applicability to the analysis being performed. CONTAM is a building airflow and contaminant dispersal model developed by the National Institute of Standards and Technology (NIST) and is most often utilized to evaluate pressurization systems such as stairways and elevators. Fire Dynamics Simulator (FDS) is a computational fluid dynamics modeling tool and is most often utilized to analyze atriums and other large space smoke control systems.
Believe it or not, there are still some firms that rely on these algebraic equations and “rules of thumb” as the basis for smoke control design. Unfortunately, these simplified approaches often result in inadequately designed systems leading to delays and overruns during commissioning. Conversely, the advanced modeling approaches utilized by our fire protection engineers often reduce the required mechanical capacities, electrical demand, architectural impact and last-minute headaches typically associated with smoke control systems.
The performance criteria component of the smoke control rational analysis is a statement of design goals that the smoke control system must satisfy to be considered an appropriate system. Compliance with the established performance criteria must then be demonstrated through the appropriate calculation method, including computer modeling or algebraic hand calculations.
For example, the performance criterion for an atrium exhaust system is to maintain tenable conditions six feet above the highest walking surface. For stairwell and elevator shaft pressurization systems, the performance criteria consist of minimum and maximum pressure differentials that must be maintained to prevent smoke intrusion.
The International Building Code defines the performance criteria for the examples above and may vary through a performance-based design approach.
Section 909 of the building code specifies that sensitivity design conditions, such as wind and extreme winter and summer temperatures, are evaluated in the design of any smoke control system. We have found ASHRAE climate design conditions to provide the most reliable, up-to-date, and referenceable data. Extreme ambient temperatures lead to a phenomenon known as the “stack effect,” which can have a significant impact on the design of smoke control systems.
The computer modeling programs used by Performance Based Fire Protection Engineering to design smoke control systems can accurately predict the impact of these environmental conditions on the performance of the smoke control systems during the design phase.
If you are reading this blog like most would read an actual smoke control rational analysis, then you skipped straight to the good stuff – the results!
Within a smoke control rational analysis, the results of interest are relative depending on the project goals and may vary depending on the type of system being designed. For exhaust systems, we have found that fan capacities and locations and make-up air arrangements are the primary result of a smoke control rational analysis and are generally of most interest to the mechanical engineer of record and the owner. This is understandable since fan capacities drive the mechanical design and directly influence construction costs.
In contrast, with a pressurization type system, the AHJ is often primarily concerned with the pressure differentials calculated by the modeling software to confirm that the system satisfies the performance criteria. It is important that the smoke control rational analysis clearly draws the connection between system requirements (capacity, locations) to expected performance (smoke layer height, pressure differentials). Results from an egress analysis may also be included in the smoke control rational analysis if a time-based egress approach is utilized.
The detailed system, commissioning and testing requirements ensure that the components of the system that are specified and installed will meet the prescriptive requirements and work together to satisfy the established performance criteria.
Commissioning is the initial validation and documentation that all system components are provided with the correct status monitoring and perform the required functions individually and together as a system.
After initial commissioning, periodic testing is performed to ensure this remains true throughout the life of the system. The smoke control rational analysis should include the requirements for the following:
Every smoke control rational analysis has six key components necessary for successfully implementing a smoke control system. And since Section 909 of the International Building Code says a qualified design professional must prepare a rational analysis, you need an expert you can trust to get the job done right.
Performance Based Fire Protection Engineering provides comprehensive smoke control rational analysis services for public and private projects. Get custom solutions that meet or exceed prescriptive code, deliver your vision, decrease project cost and increase the value of your asset when you contact us.