Concrete structures are generally considered to resist well the effect of a fire, due to the thermal properties of concrete and the protection it offers to the reinforcement. However, this is often a misconception, as also witnessed by several fire-induced collapses of building and bridges in the past. The WTC Building 7 of New York in 2001, the Windsor Tower of Madrid in 2005, and the Architecture Faculty of Delft University in 2008 are just few example of composite and reinforced concrete building that suffered major structural damages as a consequence of an accidental fire. These collapses highlight a shortcoming of current methods for the design of concrete structures and motivate the need of an advance of research and practice in the field.
In this respect, a correct representation of the temperature distribution in concrete section and of the degradation model of the materials is of outmost importance. With respect to the degradation of concrete in particular, a critical aspect of concrete as building material is that, contrarily to steel, they experience permanent damages when heated to 300°C. At this temperature micro-cracks develop as a consequence of material dehydration and thermal expansion of the aggregates (Hertz, 2005) and the strength loss is permanent, leading to high costs of repairing even in case the structure has survived the fire without any collapse.
Other critical aspects of the response of reinforced concrete elements to fire are anchorage failure of the reinforcement and spalling of concrete (Hertz, 2005). An example of spalling failure of a concrete deck is given by the Montebello freeway incident, where the deck of a viaduct was reported to have burst from the extreme heat coming from a truck on fire underneath the viaduct.
Finally, a major problem in the current design of concrete structure is the fact that verifications are required only during the fire, when the steel is hot but the inner concrete is still cold. This situation does not always represent the most severe condition of the structure during the fire. For example, columns and high slender beams typically show the lowest resistance several hours after the fire, due to the time required by the heat to penetrate the concrete core. The lack of consideration of this situation put in great danger the firemen and other people that may want to reenter the building when the fire is over and has been the cause of several collapses. An example is the case of the fire in a concrete building in Cairo, Egypt, which collapse 3 hours after the beginning of the fire; another example is provided by the fire of a car park in Gretzenbach, Switzerland, where the concrete roof collapsed ca. 6 hours after the beginning of the fire. In both cases, the collapse occurred during the cooling phase of the fire and the casualties were mostly firemen or former occupants, who re-entered the building.
Publication
Hertz, K.D., Campeanu, B.M., Giraudo, M., Giuliani, L. (2013):
Modeling and assessment of the response of super-light elements to fire
Proceedings of the 5th international conference on Structural Engineering, Mechanichs and Computation (SEMC 2013), Cape Town, South Africa, 2013
Carstensen J., Jomaas G., Pankaj P. (2013):
Element size and other restrictions in finite-element modeling of reinforced concrete at elevated temperatures
Journal of Engineering Mechanics, vol: 139, issue: 10, pg. 1325-1333, 2013
Giuliani L., Budny I.:
Different design approaches to structural fire safety
Int. J. of Lifecycle Performance Engineering (IJLCPE), vol.1, (2), pg.135-158, 2013
Hertz, K.D. (2006):
Quenched Reinforcement Exposed to Fire
Magazine of Concrete Research, vol: 58, issue: 1, pg: 43-48, 2006
Hertz, K.D., Sørensen L.S. (2005):
Test Method for Spalling of Fire Exposed Concrete
Fire Safety Journal, vol: 40, issue: 5, pages: 466-476, 2005
Hertz, K.D. (2005):
Concrete Strength for Fire Safety Design
Magazine of Concrete Research, vol: 57, issue: 8, pages: 445-453., 2005
Hertz, K.D (2004):
Reinforcement Data for Fire Safety Design
Magazine of Concrete Research, vol: 56, issue: 8, pages: 453-459, 2004
Contact
Kristian Hertz
Luisa Giuliani