Fire Performance for Complex Structures
Leveraging Performance-Based Fire Engineering
When designing buildings, fire and life safety is typically achieved by following prescriptive codes. The National Building Code of Canada and the International Building Code outline the fire safety measures to incorporate in a building which, if followed, are expected to provide the required level of performance in the event of a fire.
However, there are cases when the prescriptive guidance may not be adequate due to the complexity of the material or the structural system. In these cases, we should leverage performance-based fire engineering to demonstrate that the building performs as required. This is not unexpected for complex buildings or materials since the codes we use are intended to cover conventional buildings and may not adequately cover every unique scenario.
Below we discuss how Entuitive leverages our structural engineering capabilities and our in-house fire engineering expertise to ensure that complex structures provide the necessary level of performance in the event of fire.
Mass timber is an example of a material that is seeing rapid development and implementation. In some cases, timber buildings can be outside of prescriptive guidance.
An underlying assumption in tall building design is that the fire-resistance rating determined on the basis of standardized testing is a proxy for whether a structural element will survive the whole duration of a fire. This typically assumes that the fire will burn itself out once all combustible contents are consumed by the fire, should sprinklers or firefighters not be able to suppress it.
In the case of tall timber buildings, if the timber structure itself continues to burn, this underlying assumption is invalid. For that reason, we need to ensure an alternative way for tall timber buildings to achieve burnout.
One such alternative might be through compartment testing. This is why the recent update to NBCC 2020 allows a certain portion of a timber structure to be exposed in mass timber buildings above 6 stories tall, since this amount of exposed timber is what demonstrated decay of the fire during representative compartment fire testing. The recent updates to NBCC 2020 reference testing from 2013¹ and 2014² as the rationale for partially exposed mass timber.
In tall timber buildings, performance-based fire engineering is already being used to expose more timber by relying on more recent testing³. And there is more testing underway that will provide much-needed data for fire safety engineers to be able to model different spaces and quantitatively demonstrate that burnout is achieved.
Aside from burnout, other phenomenon in tall mass timber buildings not currently captured in building codes include the fact that heat may continue to penetrate cross-sections even after a fire is extinguished or is decaying. This can potentially compromise the structural elements⁴. Even below the temperature threshold of charring, timber loses much of its strength.
For tall timber buildings it is imperative that competent structural engineers collaborate with similarly competent fire engineers to understand if all aspects of the fire performance have been captured, or if performance-based fire engineering should be leveraged to demonstrate that life safety requirements are met.
While tall mass timber buildings may seem the more obvious application of performance-based fire engineering because of the “combustible” material and the rapid development of codes, complex structural steel buildings can also benefit from a performance-based approach in certain circumstances.
In standard fire testing, structural elements are tested individually. The building codes require a fire-resistance rating determined on the basis of ULC ratings, meaning the architect or code consultant will specify ULC Designs for floor assemblies, beams, and columns that have been tested independently and in isolation.
Complex structural steel framing should be assessed to determine if this assumption of independent element behaviour is adequate.
On a current project, bilinear steel columns are present due to the architectural layout of the building. These columns change slope at several locations, and at these locations the floor beams must provide large compressive or tensile restraining forces to restrain the columns.
In the event of a fire, the floor first expands, imposing a large thermal expansion into the columns. As the fire continues to heat the floor, the structural elements lose their strength and stiffness, compromising the necessary column restraint. At the same time, increased bending forces (p-delta effects) are imposed in both the floor elements and the columns, which is a loading scenario not covered by standardized fire testing.
On this project, it was found that using prescriptive ULC Designs for individual elements and spraying the structural elements for two hours did not provide adequate performance for the overall assembly. When modelling the structure with exposure to a standard fire, the complex behaviour described above caused the structure to fail anywhere from 100 minutes to 110 minutes, thus not meeting the minimum two-hour fire-resistance rating.
Heat transfer calculations suggested that an increased fire protection thickness would be required for this design. In this case, the fire protection of key elements was increased based on a three-hour, fire-resistance-rated UL assembly, the next-higher rating in ULC Designs. This way, an off-the-shelf ULC Design can be used to determine the required fire protection thickness and the existing UL tested assembly ensures that the proposed structure will provide the required two-hour fire-resistance rating, taking into account the complex loading.
Another example of where performance may need to be demonstrated beyond the code is in transfer structures. Transfer elements are designed with stringent deflection criteria to ensure floors above meet serviceability criteria.
However, in the event of a fire, the structural steel is still heating up. Typical standard fire testing allows the steel to heat up to approximately 550C, at which point it still has 50% of its load carrying capacity. What is not considered in the design of transfer elements is that the stiffness of the steel has severely reduced at this point, and despite being able to support the required load, the transfer element may have severely deflected.
In this case, performance-based design can be leveraged to understand what the required performance of a transfer element is so that the serviceability criteria of all upper floors is still maintained during a fire. By doing so, asset owners can ensure that interior finishes or glazing are not damaged by excessive movement when a floor below deflects during a fire, and thus more of the building can remain occupied after a severe fire event.
The above has shown the importance of demonstrating performance when designing buildings that incorporate complex structural arrangements or innovative materials that may be outside current prescriptive codes. In these cases, a performance-based approach may be required to ensure the overall building performs as required for a range of fire events, ultimately improving the resilience of the asset and those businesses or operations using it.
To learn more about Fire Engineering at Entuitive, reach out to Matt Smith.