Avoid failure with smoke control commissioning
How to adjust when the installed system does not meet the performance criteria of the approved design or there are inconsistencies with the design concepts in the approved design documents.
Allyn J. Vaughn, PE, FSFPE, JBA Consulting Engineers Inc., Las Vegas
Special inspections are required for smoke control systems as outlined in Section 909 and Chapter 17 of the International Building Code (IBC). Special inspection agencies are required to have expertise in fire protection engineering and mechanical engineering, and certifications as air balancers. Often the agencies qualified to perform these special inspections are involved and experienced in the design of smoke control systems.
However, the special inspector’s role is to inspect and test the smoke control system per the approved design. While the special inspection agency can provide guidance to the design team, its role is to verify compliance with codes and standards as well as compliance with the approved design documents.
So what happens when the installed system does not meet the performance criteria of the approved design or there are inconsistencies with the design concepts in the approved design documents?
Generally, these issues arise near completion of the project when all efforts are geared toward opening the building and completing the project. There is no time to re-examine design concepts or replace major smoke control system equipment. The most viable solution is to perform minor adjustments to installed equipment and smoke control barriers, while maintaining a code-compliant and approved system.
What happens next often is based on how far apart the operation of the installed system is from the approved design criteria. Minor adjustments can be made in the field, but major changes are often impractical this late in construction. Can changes be made to the approved design criteria to match the installed system, or will wholesale changes be required?
The following sections address ways to limit the impact of smoke control system operations. Many of these pertain to coordination during design and construction as wholesale changes late in construction are not feasible.
Depending upon the size of the project, the design concepts for smoke control systems often are completed several years prior to final occupancy. Performance criteria are set early in the design phase so other disciplines can work toward providing design documents that conform to these performance criteria. Hopefully, any inconsistencies in the smoke control system design are identified during the design process. But sometimes changes are made to the building design that are not carried forward to all disciplines involved in the smoke control system design. These changes can occur during the design and construction phases. Proper coordination among members of the design team is crucial to ensure that design changes do not impact the smoke control system design.
Many disciplines influence smoke control systems. They include mechanical and electrical systems, such as fan systems, dampers, ductwork, controls, and monitoring systems. However, architectural features impact smoke control systems as well. These may include the location of walls, soffits, draft curtains, and openings in walls and floors. Such features can be affected by the architectural design or the interior design.
Knowing that your discipline can impact the performance of a smoke control system becomes more important as the design of a building is being finalized. One way to understand your impact on the overall performance of the system is to be involved in the design stages of smoke control systems and to flag an issue when you feel a change will affect that design. This can be communicated during design workshops that involve the various disciplines and trades impacting smoke control systems.
As the design of the building is being finalized, all stakeholders in the smoke control system should meet periodically to review the design features that could impact smoke control system performance. These include smoke zone boundaries, openings in smoke zone barriers, design of draft curtains, mechanical and electrical systems, and location of rated walls for dampers, as well as control system sequencing. A review of these features to confirm proper integration can reduce the impact on the installed system.
Changes to the building design also can occur during the construction phase and it is important to review these changes as well, because they are being constructed. Periodic review meetings with the design and construction team can help to determine if any changes have impacted the smoke control system design. Corrections can be made to either the smoke control system design or the building before it is too late in the construction process. In addition to the design team, personnel familiar with the smoke control system on the construction team should review potential field changes to flag potential impact on the performance of the smoke control system.
During construction, the special inspector also should be on board, performing inspections of smoke control systems and participating in the overall review process. The inspector should be able to flag any design issues that may be affected by the construction process or changes to the building design. The special inspector is required by code to be on-site prior to the concealment of system components, such as ductwork and devices. During these inspections, the special inspection agency should be able to identify any issues related to the installation of the system. Having periodic meetings with these groups can prove beneficial in confirming that the installed system meets design criteria.
Ongoing dialogue during the course of design and construction can help offset many system performance issues near the completion of the project. Making sure the systems are installed as designed and that the building is being constructed as designed is critical to the successful performance of the smoke control system. It is also just as important to ensure that the smoke control system design criteria are well suited to the building design. These issues can be addressed in both the design and construction phases of a project.
Some of the more difficult issues when testing and inspecting smoke control systems are related to system performance. The smoke control systems required by the IBC (Section 909) are performance-based and must meet certain criteria in the field during system operation. These criteria are dependent on the building design and construction.
Section 909 of the IBC outlines four basic methods for use in smoke control system design, with performance standards as the criteria for each method of design. These methods are tied to National Fire Protection Assn. (NFPA) standards for smoke control systems. Three of the methods require performance design criteria to maintain the smoke in the zone of origin (pressurization, passive, and airflow methods), thereby preventing the spread of smoke throughout the building. The fourth option is to maintain the smoke layer above the highest walking surface (exhaust method) in an effort to maintain a tenable environment. The exhaust method is often used in large spaces such as atria and covered malls.
Pressurization method systems
Many of the smoke control systems designed for confined spaces use the pressurization method. This method of design creates a negative pressure in the zone of origin to prevent the spread of smoke beyond that zone. It requires a confined space and mechanical equipment to achieve the performance criteria. Walls and floors are used to create the physical envelope, with openings protected by smoke and draft control assemblies (fire doors and shutters, as well as dampers). Mechanical equipment includes exhaust or supply fans and dampers to exhaust the space and/or pressurize adjacent spaces. The typical design standard is to achieve a minimum pressure difference of -0.05 in. WC within the zone, relative to adjacent zones and stairwells. The design can exceed these minimum pressure differences, but an upper limit is imposed by door opening forces, which is typically 30 lbs.
Review of the design for a pressurization system can include confirmation of proper layout, and the required exhaust rate also can be calculated based on typical leakage rates. However, until the system is installed and the building is ready to be tested, you cannot confirm compliance with the performance criteria.
Many things can come into play when this testing is performed. Walls that are not sufficiently sealed can leak more than the calculated rates, or walls can be tighter than expected creating overpressurization of the zone and excessive door opening forces. Often the construction is looser than calculated, and you must either increase the exhaust capacity or find the leaks and seal them to meet the performance criteria. Obvious holes are easier to find than many of the smaller holes or cracks that add up. A ¼-in. crack at the joint between the wall and floor above does not seem like much, but when present along the entire length of the zone, that quarter-inch gap can add up to a significant area, resulting in excessive leakage impacting system performance. If the exhaust fans are not capable of overcoming this increased leakage, the pressure differences cannot be met.
As noted, part of the special inspector’s responsibility is to verify the integrity of smoke barriers. Holes in smoke barriers can be difficult to identify, particularly above ceilings where access is limited near curtain walls. Smoke generators and/or door fans can be useful in determining were the boundary walls are compromised.
Many of the systems designed to the pressurization method employ variable frequency drives (VFDs) on the fans. This allows the system to adjust the exhaust rate to meet the performance criteria. If the zone does not meet the minimum design pressures, the drive can be adjusted to allow more exhaust for the space. If the zone is tighter than expected, the drive can be adjusted to provide less exhaust, thereby reducing the pressure in the zone. Of course the fan capacity needs to be within the range required for the zone in order for the variable drive to be effective. In some cases, the fan needs to be increased in capacity, which requires a new fan or motor and costly replacement. Section 909.10.5 requires fans to be selected for stable performance. Slowing down the fan may result in unstable performance, and the fan curve must be verified when changes such as these are made. With experience and review of calculations, the engineer can reduce these more drastic measures in commissioning smoke control systems.
Proper sizing of the supply or exhaust fan for expected conditions is critical to the successful operation of pressurization method systems. The fan must be within reasonable limits to overcome field conditions that do not specifically correlate to design values. The use of VFDs can help offset these impacts, primarily when the building construction is tighter than calculated. When construction is looser than calculated, often variable drives alone cannot overcome the additional exhaust rates needed and revisions to the fan may be necessary.
Exhaust method systems
Exhaust method systems can be easier to commission since they use specific exhaust- and supply-air rates that are measured in the field. They do not rely on pressure differences and zone boundaries to meet their performance criteria. The measurements taken for performance on exhaust method systems are exhaust rates for the exhaust fan(s), as well as confirming make-up air rates. The exhaust method is intended to maintain the smoke layer at a certain height above the highest walking surface. The minimum height is 6 ft in the 2006 edition of the IBC. Previous editions of the IBC and Uniform Building Codes, as well as NFPA Standards, required a minimum height of 10 ft. The smoke layer height also can be increased to match draft curtains at smoke zone boundaries or to accommodate make-up air supply locations. Make-up air must be introduced below the smoke layer so as to not interfere with the plume dynamics.
Common performance issues for exhaust method systems pertain to supply- and exhaust-air rates. Often the calculated exhaust rate is a finite number, whereas exhaust rates for fans are dependent upon the selected unit and the static pressure losses within the duct. It is recommended to select a fan that has a higher rate than calculated while also taking into account any loss that may be attributed to pressure losses in the duct. While this concept may seem straightforward for single units, systems employing multiple fans have the same issues but multiplied. If an exhaust rate of 120,000 cfm is required and four units are provided, losses in the duct need to be considered for each unit. If one unit is used and the rate is less than specified due to field conditions, it still may be higher than the calculated rate. However, if four units have rates lower than specified, the entire system will likely be under the calculated rate due to the multiple losses. Employing VFDs and sizing the fans conservatively higher will overcome many of these issues.
Other performance issues related to the exhaust method deal with make-up air and are sometimes harder to resolve in the field. As noted above, make-up air is required to be delivered to the zone beneath the smoke layer. This needs to be addressed during the design stage as it is too difficult to change during construction. If make-up air delivery issues arise after construction is completed, often the best remedy is to find alternative sources, such as natural air from doors or selecting other fans if possible. However, the most common problem is when the make-up air has a delivery rate greater than 200 fpm.
NFPA 92B, The Standard for Smoke Management Systems in Malls, Atria, and Large Spaces, limits make-up air to no more than 200 fpm where the make-up air could come into contact with the plume, unless an engineering analysis supports higher rates. Many of the exhaust method systems being designed do not incorporate this type of engineering analysis and therefore limit the make-up air to 200 fpm. A simple calculation of make-up air vent sizing will take into account the required exhaust rate divided by 200 fpm to determine the area of the vents. However, this is the free area and does not account for any reductions for louvers or grilles. These need to be considered during the design phase so that make-up air vents are properly sized.
Addressing excessive make-up air velocity issues in the field can be difficult. The code does allow some subjectivity in where the measurement must be taken. The 200 fpm rate is at the fire or more specifically at the plume. Most of the time, this can be a reasonable distance from the vent or louver. If a fire is not expected to be immediately in front of the vent, the measurements should be taken at a location where the plume may reasonably be expected. This would allow for the velocity to decrease as it disperses past the vent. If a fire is expected to be near the vent, additional make-up air locations are needed, which will impact the operation and control of the system.
Sometimes the installed system cannot perform as the design criteria intended. At these times the design team is often asked to re-evaluate the design parameters to see if an alternative design will meet the performance of the installed system. This is a difficult task because the design is already approved and the installed system may not perform to any of the design methods allowed by code.
In these cases, it is important to consider the design intent to see if alternative design methods can be used. If the intent of the system is to provide a tenable environment, fire modeling can be performed to see if the installed system meets the accepted criteria for tenable environments. This might work for exhaust method systems whose prescribed formulas are often conservative. However, the cost of fire modeling can be extensive, so this type if analysis must be evaluated for overall cost impact.
Design changes for systems using the pressurization method may not be feasible if a prescribed pressure difference is not being met in the field. It may be possible to obtain approval of a revised design method if it can be demonstrated that the smoke will remain in the zone as intended by code. However, this may not be practical or possible, and changes to the fans may need to be made.
The main thing to consider when exploring design changes at this stage of construction is that the system must perform as intended by code either to prevent the spread of smoke within the building or to provide a tenable environment. Any designs that do not meet these objectives are likely to be rejected, since they do not meet the performance objective for smoke management systems.
One of the most frustrating things for the design, construction, or inspection team to discover is that the smoke control system does not perform as required and the system will delay occupancy of the building. This usually occurs near the end of the project when all of the efforts are geared toward opening the building. Without an approved system, final occupancy permits cannot be given and the building may not open on time. In fact many building-opening delays stem from life safety system testing issues.
Keeping in mind some of the items noted above may help prevent or reduce the impact of smoke control commissioning on opening a building. The main thing to remember is that it is very difficult to make changes to the systems late in the construction phase, so up front coordination is crucial to proper system operation.
Vaughn is the vice president of growth markets for JBA Consulting Engineers. He has more than 30 years of experience in the design, installation, and commissioning of smoke control systems. He has been a special inspector for smoke control system for more than 15 years and has supervised commissioning efforts for several high-rise buildings throughout the world.
Download Atrium Smoke Exhaust Calculations (PDF)
Download Corridor Smoke Exhaust Calculations (PDF)
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