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Challenge
Rebar cages were found to be misaligned in some of the columns in MacKimmie tower, leading to a reduced concrete cover, potentially exposing the re-bar steel to higher temperatures in fire scenarios. Any fire engineering interventions are limited by the boundaries of the existing structure. Ideally, it can be proven that the existing condition is safe in fire without any interventions. Naturally, eliminating the need for repair/upgrading work entirely is the least invasive/easiest solution for the contractor.
Solution
A thermal analysis of the column sections with misaligned rebar cages allowed us to determine the impact on material degradation, and hence load-bearing capacity under fire that the reduced concrete cover had. The impact of misaligned rebars was found to not have a significant impact on the residual load-bearing capacity, and no remedial works were necessary.
Challenge
To strengthen waffle slab floors after coring these to accommodate mechanical and electrical services, steel plates were attached to the soffit of the floor slabs. Since the steel plates would be exposed to fire from below, it was necessary to determine whether they required fire protection to maintain the load-bearing capacity of the overall floor in case of fire.
Solution
Calculation of the residual load-bearing capacity of the existing concrete waffle floor slab, including cores, based on the Canadian concrete design standard CSA A23.3 Annex B and Eurocode 2 Part 1-2, as well as using the load combination for extreme events in NBCC 2015, demonstrated that the floor slab maintained sufficient load-bearing capacity in fire without relying on any contribution from the steel plates. This meant that fire protection of the steel plates was redundant in the event of a fire, and resulted in a significant cost savings relative to the prescriptive guidance which said to apply spray-applied fire protection to the exposed steel.
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