Question
What exactly is the meaning of the P-M calculation methods available in GEN, and how do the "Keep P constant" and "Keep M/P constant" options differ?
Answer
Both "Keep P constant" and "Keep M/P constant" methods do not affect the computation of the P-M curve, which represents the capacity of a column.
The key difference lies in how the demand-to-capacity ratio is computed.
- Keep P Constant: This method displays the ratio (Rat. = Member Force / Ultimate Strength), where the ultimate strength is determined based on the same axial force (P) as specified in the load combination.
- Keep M/P Constant: In this method, the ratio (Rat. = Member Force / Ultimate Strength) is calculated based on the same eccentricity ratio (M/P) as in the specified load combination (P, My, and Mz).
Summary:
The choice of method does not change how the P-M or PMM curves are generated. The difference comes into play when computing the demand-to-capacity ratio.