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Fire allowing for safety a surface layer will exceed 200°C. When this hap- factors and mate- pens differential expansion eventually causes failure rial over-strength, of the bond between concrete and reinforcement. the weakened con- It goes on to show that in the Madrid fire this criti- crete columns were cal temperature was reached to a depth of 100mm no longer strong over more than half the floors and columns, and con- enough to with- cludes that the risk of damage to reinforcement-con- stand the load shed crete bonds due to the intense heat was inadmissibly onto them from the high throughout the structure, with the exception of weakened mullions a few individual mem- together with any Everywhere the bers on the 5th storey. whole frame expan- fire spread the In other words, inde- sion effects. pendent of whether the concrete was then The final clue steel mullions were fire supporting this condemned protected or not, every- conclusion is in where the fire spread the East façade as northern this picture of the east façade as the northern end concrete was then condemned. Thus, whilst the steel end collapses collapses. Whilst much of the façade glazing has and concrete materials regain nearly all their strength gone, the steel mullion columns are intact. It seems on cooling, the two components are no longer able that a “soft storey” is the reason why collapse is to act as reinforced concrete. Widespread exposure taking place at the right hand (northern) end, with to fire such as that experienced by the concrete ele- References 1 INTEMAC Report NIT intact floors above dropping as an integral assembly. ments of the Windsor Building would thus mean that 2-05, December 2005 If so, it was only load shedding at the floor where the so much of the structure would be condemned that “Fire in the Windsor concrete columns failed that mattered together with re-instatement would be impractical. building, Madrid. Survey loss of column capacity from prolonged exposure of the fire resistance and to the fire at that level. Had the steel mullions been So what are the major lessons from Madrid? residual bearing capacity properly fire protected, the multiple load paths 1. Don’t jump to conclusions. of the structure after the they provided would almost certainly have helped Get the facts and carefully review them. fire” redistribute the loads shed by internal concrete 2. Effective compartmentation is the primary 2 BS 5950-8: 2003 columns weakened by the fire. fire design consideration. Structural use of steel- Finally, the assessment in the report condemned 3. Structural robustness mitigates the extent work in building – Code of practice for fire resistant the remaining structure in a way that has wide im- of damage in the contingency that the com- design plications for re-instatement of concrete structures partmentation fails. 3 SCI Publication P-113, in comparison with fire-damaged steelwork3. The 4. Even parts of the structure that are still June 1991 “Investigation report states that concrete exposed to fires in excess standing may be damaged beyond repair. of Broadgate Phase 8 fire” of 500°C will heat up such that temperatures within NSC March 2006 29


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