I am asking for your advice/help on resolving this technical issue.
An existing building has been earthquake strengthened. The structural design required that continuous steel columns be installed through the full height of the building internally with cross bracing at each floor level. Holes were made in each floor level of the building to erect the continuous steel columns.
Each floor level of the building is a separate fire cell.
The floor penetrations were fire-stopped around columns to maintain integrity rating for the floor fire separations.
The steel columns are not required to be fire rated for fire stability therefore no passive fire protection were installed to columns between floor levels
The Local Council has stated the ‘insulation’ component of the floor fire separation has been compromised because the steel columns will conduct heat through to the fire cell above in event of a fire below the floor. This may be a valid point given that the criteria for insulation failure is quite modest (140C average or 180C any point on non-fire side). It is not hard to imagine a fully involved fire below the floor resulting in steel column temperature above the floor >>180C.
Encapsulate the exposed columns on each floor to achieve FRR - / - / XX?
I investigated thermal breaks that could be installed between bolted column joints coinciding at floor levels. However these tend to be for low temperature applications to prevent thermal bridging and condensation in building.
I am not sure if intumescent coating system would be suitable unless specifically designed to keep the steel below the ‘critical temperature’ for insulation rather than stability. I suspect the resulting coating system would be very expensive.
Has any other Fire Engineers come across this situation and if so I would be interested to know how you have dealt with this?.
It is possible to undertake a thermal transfer calculation longitudinally within the column. The idea is to determine the input area, the cross sectional area of the column, the length of the column where it penetrates the floor, and then determine the residual temperature above the floor. Intumescent will not help, as most intumescents only start to activate at 200/250C, which is above the cold side maximum temperature. Essentially, you will need to shield the lower portion of the column above the floor level to reduce the level of radiation to combustible material - say 1.0m height. The material chosen for the shield will determine the emissivity, which will probably require ventilation input at the lower end of the shield. A single metal shield will substantially change the radiant temperature from the column to the outer surface of the shield. Aesthetics will be another problem.
If the floor is a fire separation and the floor requires all three stability/integrity/insulation ratings then any elements penetrating the fire separation must prevent fire spread through the fire separation irrespective of their function. Hence the radiation shields required on some metal pipe penetrations. So while the column may not need to meet the stability requirement, as its failure in fire must not compromise the floor’s integrity, the column must meet the integrity and insulation requirements where it is part of the floor fire separation.
One would think that meeting Acceptable Solutions as a minimum requirements is not adequate sometimes. One size does not fit all. When you penetrate fire separations, especially floors, issue is not only fire spread but structural stability at takes.
I can only suggest to use one-way fire rated GIB to “encapsulate” unrated steel structure to x/x/x rating of firecells in which it is located. Fire rated GIB is the most viable option at reasonable cost. But intumescent coating will provide insulation rating to steel, if industrial appearance is desirable.
If the floor has an insulation rating, you will need to maintain this. Most metal pipe firestopping systems that have an insulation rating require metal,or kaowool radiation shields on the cold side.
The other thing to consider is structural failure. While it may not be taking gravity loads itself, failure of the steel can’t affect a fire rated element. In this case, will failure or deflection of this steel make the floor Gib fail, or the firestopping system, or other structure?
Similarly, will a piece of red hot steel through the floor cause issues with the framing and joists? You usually have to line the hole through the floor with Gib.