Redesign of fire-damaged structures
Most fire-damaged structures can be successfully repaired, this being the more cost-effective solution than demolition and reconstruction. Before carrying out any repair it is necessary to determine the extent of the damage to the concrete and reinforcement and hence their residual strengths. This is covered in a separate entry Defects and deterioration/Load related defects/Assessment of fire-damaged structures.
In general the design of the repaired sections of the building should comply with current codes of practice. However, the damaged structure may have been designed to codes of practice which are out of date; where this is the case it may be necessary to formulate a strategy which is compatible with the original design. In addition, limitations may be imposed on the restoration of listed buildings. Wherever possible the original structural design drawings for the building should be obtained as these will be of considerable help in assessing the ‘as built’ properties of the structure.
Structural analysis of the repaired structural frame should be based on methods of analysis and load arrangements as set out in current codes of practice. In addition to normal assumptions, the analysis should take due account of any dimensional changes, lack of verticality and residual forces which could have resulted from the elevated temperatures during the fire.
The design of the reinstated and repaired concrete elements should generally be based on design methods as set out in current codes of practice. Repaired concrete elements will comprise a combination of the remaining section of the existing member and the repair materials. Modified material properties should have been established in the fire damage assessment process and must be taken into account in the redesign of the repaired members. The strength properties of the repair material should be used where appropriate.
Further guidance is given in Concrete Society Technical Report 68, Assessment, design and repair of fire-damaged concrete structures, which includes design examples.