Fire in AS3600-2009
Fire in AS3600-2009
Data for axis depth of longitudinal bars in columns has been derived from Eurocode EN 1992-1-2:2004. Several errors in the conversion of this document have been noted. They include simple typographic errors as well as misinterpretation of the original document.
The effect on calculations is significant and not conservative. For example, axis depth for bars in a 400 column for 120 minute FRP is 53mm in EN1992-1-2:2004 and 38mm for the equivalent design using AS3600. 38mm gives a FRP of only 50 minutes according to EN1992-1-2.
1.1 Calculation of Load Level for Table 5.6.3
The load level is given as N*f / Nu in 5.6.3 of AS3600. The equivalent in 5.3.2 of EN1992-1-2 is Ned.fi / NRd. NRd is the "Design Value" which is the "Characteristic Value" reduced by a "Partial Factor". The load level is therefore more correctly represented as N*f/ФNu.
The effect of this is roughly increasing load level from 0.5 to 0.7.
1.2 Typographic Errors in Table 5.6.3
Table 5.6.3 is copied from Table 5.2a. The data for FRP values 120-240 are copied incorrectly to the row above. This creates the obvious abnormality that distances for FRP120 are less than FRP90.
1.3 Calculation of Load Level for Table 5.6.4
The value of η is calculated using characteristic values for steel and concrete. In EN1992-1-2:2004, the value of n is based on design values. The value of η is therefore underestimated.
1.4 Allowable Eccentricity for Slender Columns
Values tabulated in Table 5.6.4 are based on an eccentricity (e/b) of <=0.025. The values in the table, however, are derived from Tables C2, C5 & C8 in EN1992-1-2 (using Le/r of 30). These tables are for e/b <= 0.25.
Minimum e/b in AS3600 is 0.05, so the requirement, as written, is unusable.
1.5 Reinforcement Ratio
Table 5.6.4 uses the reinforcement ratio p, with values of 0.01, 0.05 and 0.1. The values used in EN1992-1-2 use a reinforcement ratio ω defined as Asfyd / Acfcd. The conversion ratio used is 0.1 to a get steel percentage. Allowing for the appropriate partial factors, this conversion is correct for f'c = 65MPa. As AS3600 is applicable for concrete ranging from 20MPa to 100MPa, the implied error in reinforcement ratio can be large.
A reinforcement ratio of 10% is very unlikely in a column, but the equivalent ω=0.1 is easily achieved with 3% steel in a 20MPa concrete column.
I don't suggest using this section of the new code.