| Product(s): | RAM Structural System; Concrete Column | ||
| Version(s): | Any | ||
| Area: | Design | ||
| Original Author: | Bentley Technical Support Group |
Why is the reported moment larger than the reported member forces?
The concrete column design report indicates the ultimate factored moments after accounting for moment magnification (Mc). See the program manual (RamConcreteColumn.pdf) for details on how this calculation is performed.
Furthermore, in concrete column design per the ACI code, for certain slenderness ranges, even when there are small or zero moments from the analysis, the column must be designed for a minimum moment due to a minimum expected eccentricity. For example, ACI 318-08 Section 10.10.6.5, equation 10-17 (ACI 318-02 Section 10.12.3.2 Equation 10-14) indicates that:
M2,min = Pu ( 0.6 + 0.03h )
about each axis separately, where 0.6 and h are in inches (but M2 is in kip-ft).
M2, min is multiplied by the moment magnifier to calculate the design moment (see ACI 318-08 Eq 10-11). As Pu approaches 0.75Pc, the moment magnifier and the calculated design moment can become very large.