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 building code. One of those requirements is that a rational analysis be prepared by a qualified design professional (Section 909.9). Each type of smoke control system (exhaust, air flow, 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, and through this experience have identified the 6 key components of a smoke control rational analysis and why each is invaluable to successful smoke control system implementation.
- Summary of Applicable Codes, Standards and Local AHJ Requirements
- Calculation Method
- Establish Performance Criteria
- Sensitivity Design Conditions
- The Results
- Detailed System, Commissioning, and Testing Requirements
Key Component Number 1 – Summary of Applicable Codes, Standards and Local AHJ Requirements
Nothing too groundbreaking or earth-shattering here. Nonetheless, a 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 a single or multiple open stairwell door. This varies from the International Building Code which states all doors closed. If not accounted for during the design, this could lead to an undersized smoke control system. 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. Understanding and documenting these requirements will prevent costly change orders and delays during permitting and construction.
Key Component Number 2 – Calculation Method
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 there are certain simplified algebraic equations which 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 for analysis of atrium and other large space smoke control systems. Believe it or not, there are still some firms that rely on these algebraic equations and/or ‘rules of thumb’ as the basis for smoke control design. It is often seen that these simplified approaches result in inadequately designed systems leading to delays and overruns during commissioning. The advanced modeling approaches utilized by our fire protection engineers often reduces the required mechanical capacities, electrical demand, architectural impact, and last-minute headaches typically associated with smoke control systems.
Key Component Number 3 – Establish Performance Criteria
The performance criteria component of the smoke control rational analysis is a statement of design goals which 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 6 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. For the examples above, the performance criteria are defined by the International Building Code, but also may be varied through a performance-based design approach.
Key Component Number 4 – Sensitivity Design Conditions
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 “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.
Key Component Number 5 – The Results
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 as well as make-up air arrangements are the primary result of a smoke control rational analysis, and 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 cost. 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.
Key Component Number 6 – Detailed System, Commissioning, and Testing Requirements
These 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 remain true throughout the life of the system. The smoke control rational analysis should include the requirements for the following: sequence of operations, special inspection(s), construction inspections, firefighter’s smoke control panel, power supply, power/status monitoring, fire and smoke dampers, and detection/activation.
Atrium Smoke Control and Rational Analysis Projects:
Check out our past work with atrium smoke control systems here!
If you want to learn more about atrium smoke control systems and testing, as provided by the SFPE, check it out here!
Reference code sections based on the 2018 International Building Code:
International Building Code Section 909 – Smoke Control Systems
International Building Code Section 909.3 – Special Inspections and Test Requirements
International Building Code Section 909.4 – Rational Analysis