Structural response of reinforced concrete columns during and after exposure to non-uniform heating and cooling regimes
Maclean, Jamie Wall
In addition to the immediate life safety concerns during building fires, uncontrolled fires within buildings have the potential to cause extensive structural damage. Current design guidance for structures in fire focuses exclusively on the life safety of the occupants within buildings. With regard to the structure this is generally achieved by specifying a defined fire resistance period during which structural integrity must be maintained and fire spread must be prevented. This is to ensure that the building's egress routes are not compromised until all occupants have escaped from the building and fire-fighting operations have been completed. Designers are not typically required to explicitly consider the residual post-fire effects on structures. Particularly in relation to concrete structures which tend to perform well in fire and can often be reinstated, this raises questions about whether the post-fire effects are important from a life safety perspective. This thesis explores the applicability of some of some simple models to reflect the complex behaviour observed when symmetrically reinforced concrete columns are subjected to non-uniform heating regimes. An experimental test series was designed to provide an extensive data bank of the performance of a single reinforced concrete column subjected to many combinations of different loading and heating conditions. To this end 46 geometrically identical reinforced concrete columns were subjected to a combination of loading and heating conditions and both the thermal and mechanical response were monitored during both the heating and cooling phases of the experiments. All surviving columns were destructively tested to determine their residual performance 24 hours after cooling back to ambient temperature conditions. A sectional analysis model is presented to determine the load-moment interaction of a reinforced concrete column after exposure to elevated temperatures. This has been aided with the use of non-destructive testing of each of the columns with the aim of helping practitioners determine the post-fire properties and to aid in the residual analysis of a concrete structure exposed to elevated temperatures. In comparing the results to the current design guidance available it would be expected that each experiment would react in an identical fashion as the design of each of the columns and the external heat source is identical in each case. This experimental test series has concluded that this is not necessary the case. From a post-fire residual structural performance standpoint, this raises a number of questions regarding how practitioners can approach the assessment and analysis of reinforced concrete structures in the future. Given the findings of this work, and the one of a kind data bank that has been created as regards to the performance of reinforced concrete columns subjected to non-uniform heating and cooling regimes, fire engineering modellers now have the capability to validate Finite Element models against a series of 46 reinforced concrete columns through the full range of heating, cooling and post-cooling structural performance. The results of which have profound implications for the current design methodologies recommended. This data bank can now be used to validate such models and inform future design methodologies.