The ULS Design Functions spreadsheet (last posted here) has been updated to the new Australian Standard for concrete structures; AS 3600-2018. The new version can be downloaded from:
Note that the biaxial version included in the download zip file is not yet updated for the new code.
The interaction diagrams below compare the new code with results from the previous version, which generates the same results as the 2017 version of the bridge code (AS 5100):
With 32 MPa concrete the new code generates higher results where bending controls (because the reduction factor is increased from 0.8 to 0.85), but where compression controls the capacity is reduced, if the reduction factor of 0.6 is applied.
Under some conditions the new code allows the reduction factor for compression to be increased to 0.65. The higher reduction factor is applied in conjunction with the 100 MPa concrete in the screen shot below (click image for full size view):
In this case the new code gives significantly higher results over the full range of axial loads:
The new version also has a number of other recent updates as shown below:
I am suprised how modern codes introduce some odd coefficents which lead to non phisical solutions like artificial distrubance of interaction function around 4MNs. At least it could go from 0.85 to 0.6 linear to achive smooth transition.
The effect is engineers are not designing the most efficent solution, but solution to satisfy code figment.
The phi factor does have a linear transition from 0.85 to 0.6 (or 0.65 in the latest code when the ratio of live load to dead load is high enough). The peak in the moment capacity occurs because the unfactored capacity line is almost vertical, but the reduction factor is reducing. You then get a kink in the factored capacity curve when the reduction factor reaches its minimum value.
The Australian code line is actually smoother than the American ACI 318 results, at the cost of being significantly more conservative around the balance load. The Eurocode gives a smoother line, but uses a partial factor approach that would be a major change from the method used in the Australian and US codes.
The same basic approach has been used for over 35 years by the way, and is more efficient than the previous single “factor of safety” based methods.
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