Considerations for Fire Engineering in Existing Buildings: A Closer Look

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Sustainable practice, both in terms of conscientious use of resources, and in terms of meeting the growing housing demands of our rapidly expanding communities, may be valid motivations behind the phrase, “The greenest building is the one that already exists.” Focussing on the repair and renovation of the vast stock of existing – yet ageing – structures in our cities, rather than on the addition of new buildings, is not only favourable for our economy, environment, and the preservation of historically valuable buildings, it might well become our primary option in the face of the climate crisis.

Considerations for Fire Engineering in Existing Buildings: A Closer Look

Today, we have one more reason for repurposing existing buildings: following the pandemic-induced shift to remote work and a mass exodus of the ‘city worker’, most major cities are now left with a surplus of office spaces in their cores. What to do with all this empty space? Most firms don’t expect to return to their offices in full capacity after the re-opening, while others intend to remain fully remote, even after “all of this” is over. As such, former office spaces will need to be repurposed.

Why Restore or Renovate?

Fire Safety in Existing Buildings: Why Do Fire Resistance Ratings Change?

Naturally, fire engineering for existing buildings deals with more constraints than when designing for new-build projects. Structural details and floor layouts are set in stone (figuratively and literally), and any refurbishment works or structural interventions become costly. After all, a finished building sits on the far end of the popular MacLeamy Curve, where any change to the design comes with high effort and costs. Therefore, it is favourable to find fire engineering solutions that provide safety without major changes to the existing structure.

A small, yet not to be overlooked part of refurbishment or upgrading works is to make sure the repurposed building still satisfies (or exceeds!) code requirements, in particular with respect to fire safety. Most commonly, designers may discover that existing conditions do not provide the necessary fire-resistance rating for a refurbished scheme. This can simply be due to a change in what is required by code updates, or because the use of the space itself has changed, for example where an office is being repurposed for residential use, or where an additional floor pushes the building above the height limit that it and its fire-ratings were originally designed for.

Fire Engineering in Action: Engineering Judgements & Analyses

Take a recent Entuitive project as an example. During the reconstruction of an administrative building consisting of a two-storey podium with four office towers, it was found that some parts of the building did not have adequate fire-resistance ratings. Several wall, beam, and column elements near the elevator and stair shafts needed attention. The unique challenge of this task was that these structural assemblies included an archaic material that is not commonly tested for fire nor studied extensively for fire performance, namely, terracotta. To evaluate an existing condition, typically an “Engineering Judgement” is carried out.

In this case an assessment of the fire performance of an existing condition was based on existing literature and engineering tools. Useful literature and tools included:

“Harmathy’s 10 rules for the determination of fire endurance rating of structural assemblies” Test records of structural assemblies, such as those by Underwriter Laboratories The component-additive-method found in the supplementary standard SB-2 to the Ontario Building Code Known fire performance of materials with similar or comparable properties (e.g., leaning on data for clay when assessing terracotta)

These are tools used to demonstrate that an existing condition has the same or better performance than a comparable “code-compliant” design, by showing that the intent of the Building Code (embodied in functional and objective statements) is still met. An important distinction to be made is that Engineering Judgments are not a means to simply justify an existing condition. Rather they assess the fire performance and trigger material changes where necessary. There is a fine line between reasoning the safety of an existing condition based on a solid foundation of known material and fire performance properties (with consideration for the intent of the Building Code), versus gaming the system and knowingly misinterpreting guidance to squeeze an existing condition into a code-approval process.

A different way of assessing an existing structural condition for its fire performance is to carry out a full, fire engineering analysis. This was the case in another Entuitive project, where, as part of the redevelopment of a 1970s academic tower building, it was discovered that some column rebars had insufficient concrete cover as per code requirements. A thermal analysis allowed the design team to determine whether the columns lost more of their load-bearing capacity under fire as a result of the misaligned reinforcement. For this, a finite element model representing the column was set up, in which the internal temperature developments could be simulated. Based on the depth of these internal temperatures, the axial load capacity of columns could be determined to analyse the column’s behaviour under combined fire and structural loading.

In the case of this tower building, the impact of the misaligned rebars in concrete columns causing the rebar cover to be less than that required by code did not have a significant impact on the residual load-bearing capacity, and no remedial works were found to be necessary. In the same building, steel plates were installed to support concrete slabs where they had been cored to accommodate mechanical and electrical services. These steel plates are attached on the soffit of the floor slab and are exposed from below, making it necessary to determine whether any fire protection, such as spray-applied fire resistive material or intumescent paint, was required. To do this, the slab’s capacity without considering any contribution from the steel plates was calculated using the Canadian concrete design standard CSA A23.3 (Annex B), and Eurocode 2 Part 1-2 to account for the heat-induced material degradation of concrete.

This demonstrated that the existing concrete waffle slab with cores had sufficient load-carrying capacity under fire without relying on the steel plates, and that the steel plates could hence remain unprotected in this configuration. Finally, however, if an existing condition is not safe when evaluated against current code, be that via Engineering Judgements or a fire engineering analysis, making material changes to the structure is still not the only solution. An alternative would be to test a mock-up of the existing condition in order to understand its fire performance. If satisfactory, this allows the existing condition to remain unchanged in the building. Otherwise, simple and practical solutions must be found for altering structural assemblies in a way that both satisfies the fire safety requirements and allows a refurbishment project to stay on time and within budget.

Fire Engineering Is a Key Component in Any Existing Building Project

While fire safety is not the only facet to be considered when it comes to the repair and renovation of existing buildings, it is a key component for delivering complete projects that provide life safety and protect the building asset. This is true for repurposing buildings as much as for new-build projects.

MacDonald Block

Ociciwan Arts Building

Mackimmie Block

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Authors: Matthew Smith & Hana Plathner

Interested in Learning More? Contact Matthew Smith