The IBC Approach To Smoke Control System Design
IBC Section 909This section includes requirements for active and passive smoke control systems with the objective of maintaining tenable conditions during occupant evacuation in the event of a fire. The active smoke control system requirements are broken down into three different methods for providing an active smoke control system. These methods are:
- Exhaust; and
- Opposed airflow.
Pressurization MethodThe pressurization method is considered to be the primary method for providing active smoke control, and it is most often used in high-rise buildings to pressurize stair enclosures and for providing zoned smoke control. The pressurization method can be applied in two different ways, both of which control smoke by providing pressure differences between two areas.
The pressurization method when providing zoned smoke control works by creating negative pressure in the zone of the fire's origin so that the smoke does not travel beyond that zone. Tenability of the zone of the fire's origin cannot be maintained, nor is it required to be when using zoned smoke control. The pressurization method can also be provided for stair enclosures. By maintaining positive pressure within the stair enclosure, smoke is kept from entering the enclosure and thus from compromising the stair safety.
Used either way, the minimum pressure difference for this configuration across a smoke barrier is 0.05-in. wg in a fully sprinklered building. When the code allows a building to be nonsprinklered and a smoke control system is provided, the minimum pressure difference should be two times the maximum pressure difference calculated for the design fire.
Exhaust MethodThe exhaust method is the method most commonly associated with smoke control, since it is often used for large, open spaces including atriums and malls. The exhaust method works by exhausting smoke at a specified rate that is greater than the rate at which it is produced, reducing the ability of the smoke layer to descend to a level in which it could create untenable conditions for occupants. The IBC sets this criterion at 10 ft above the highest occupiable walking surface for a minimum of 20 min.
This performance criterion is evaluated by calculating the smoke exhaust rate. The smoke exhaust rate of the design fire is calculated using calculation methods based on NFPA 92B. The calculations used are based upon the location of the design fire and how the smoke plume will develop. The different configurations are an:
- Axisymmetric plume;
- Balcony spill plume;
- Window plume; and
- Plume in contact with a wall.
The IBC specifies the minimum size of the design fire as 5,000 Btu/sec. This is prescribed to ensure that the designer uses a reasonably sized fire. However, as part of an engineering analysis, a larger or smaller design fire can be justified and utilized. To determine whether 5,000 Btu/sec is an adequate design fire for a specific scenario, the designer needs to consider the occupancy of the smoke zone, the use of the space, potential fuel sources, potential ignition sources, and the rate the fire increases in size.
Heat release rate data for the fuel sources can be used to determine an appropriate design fire size. The effect of sprinkler actuation on the design fire can also be factored in when determining a design fire size. There are many different factors that can be considered when selecting an appropriate design fire for a particular case, but it is critical that this process is well documented in the design report.
In addition to exhausting smoke from the smoke zone, the system needs to introduce makeup air into the smoke zone at a rate not exceeding 200 fpm (This is measured at the fire location. It could be very difficult to get all makeup air at outlets to be below 200 fpm.). Makeup air should be introduced below the level of the smoke interface. Makeup air is required to balance the airflow in the smoke zone and prevent it from becoming a vacuum. The IBC does not prescribe a calculation method for determining the amount of makeup air, but it should be developed by the design team. Makeup air is comprised of air introduced into the building naturally or mechanically, and air coming from building leakage, the proportions of which are determined based on the tightness of the building construction.
Opposed Airflow MethodThe seldom-used opposed airflow method is usually reserved for special situations. This method is most commonly used in conjunction with the pressurization or exhaust method. The opposed flow method works by exhausting smoke through permanently open, fixed openings located between smoke zones. This is done by slowing the movement of smoke into another smoke zone by using the horizontal velocity of air. The velocity of air is determined based on the design fire, the height of the opening, and the temperature of the smoke. The calculated required velocity will increase with larger openings or higher temperature smoke, but the velocity cannot exceed 200 fpm. If it does, the opposed airflow method cannot be used, which is why this method is best suited for smaller openings such as windows.
Documentation of the Smoke Control DesignWhichever method is applied for your particular IBC smoke control design, documentation of the approach is critical. Unlike traditional prescriptive requirements, the IBC only provides a methodology for meeting the smoke control requirements, and to meet those requirements, a detailed analysis is needed that goes beyond entering values into one equation to calculate an exhaust rate. Recognizing the complexity of a smoke control system design, the IBC not only requires detailed construction documents, but also requires a detailed engineering analysis report justifying the design. The analysis report should contain the following items:
- A discussion regarding the selection of the type of smoke control method used;
- How the method to be used operates;
- Accompanying systems required to operate the smoke control system;
- How the system is to be constructed;
- The impact of stack effect;
- The impact of the temperature of the smoke;
- The impact of wind;
- How the system interfaces with the building HVAC system;
- How the air inlets and outlets will be maintained in low temperature climates where snow or ice could block them;
- If smoke barriers are used, allowable leakage and opening protection criteria;
- A description of the design fire including consideration of the use of the space, potential fuel and ignition sources, and the size of the design fire;
- The impact of sprinkler actuation on the design fire's growth;
- Air inlet and outlet locations with consideration to plug holing;
- Verification that equipment requirements are met for exhaust fans and dampers, which are required to be listed; and fans, which should be equipped with 1.5 times the number of belts required by the design;
- Sequence of operations for activating the smoke control system and shutting down other systems;
- Special inspection test reports;
- Plans indicating the location of all the system equipment;
- Documentation on the system equipment's maintenance procedures and schedule; and
- Testing and maintenance log.
An extensive amount of information needs to be considered. The engineering analysis goes all the way from concepts to O&M, but in doing so a more thorough design is developed. The report serves not only as a document to gain approval of the system design, but also as an operational guide to be maintained within the building by the owner. This requires a qualified engineer to prepare the design. This is a requirement that could be a challenge to those unfamiliar with the IBC or UBC.
To prepare an engineered smoke control design as required by the IBC, it is recommended that a fire protection engineer be part of the design team. A fire protection engineer has the skill set required to perform the smoke control analysis including an understanding of fire growth and development; the dynamics of smoke; the interaction of sprinklers, wind, smoke temperature; and the interaction with other fire protection systems such as smoke detection and fire alarm.
These skills are also valuable on the enforcement side of the design process. Once the system is designed and the analysis prepared, the system needs to be reviewed and approved by the authority having jurisdiction (AHJ).
Section 909.3 of the IBC requires special inspection and test requirements, similar in concept to special inspections for structural engineering designs. Where the AHJ is not accustomed to reviewing an engineered smoke control system and the testing and commissioning process, a third-party reviewer, such as a fire protection engineer, can be included either as part of the design team or retained directly by the AHJ to assist the AHJ in the review process. The third-party reviewer should not be the mechanical engineer of record. However, the fire protection engineers on design teams are excellent choices to serve as third-party reviewers and inspectors since they are familiar with the building and smoke control concepts, but are not the mechanical engineer of record for the system.
Although not a simple process of meeting cookbook prescriptive requirements, the reward is that the IBC allows for a comprehensively designed smoke control system, which equates to better performance in critical fire scenarios. In addition, the IBC also creates an opportunity for architectural features and configurations that may not have been feasible under BOCA or SBCCI to be reviewed and analyzed under an engineering analysis by a fire protection engineer for unique building configurations.
The IBC also goes into great depth in providing direction for items such as:
- Fire fighter smoke control panel;
- System response time;
- Acceptance testing;
- Equipment requirements;
- Power requirements;
- Detection and control systems;
- Duct material; and
- Control diagram requirements.
In conclusion, the IBC smoke control requirements are performance-based and require an engineering analysis. This approach allows the system to be designed to perform in a real world fire event, as opposed to an arbitrary prescriptive requirement. As buildings become more complex, it also opens up opportunities for more unique building features and configurations. ES