## Question

**Idealized moment-curvature curves are not computed correctly.**

## Answer

The moment-curvature curve is calculated using the section geometry, reinforcement status, and inelastic material properties, and the idealized moment-curvature curve is obtained using the moment-curvature curve.

So, basically, the three items mentioned must be properly defined for the calculation to work.

First, for concrete materials, the Mander model simulates the effect of triaxial compression by cast reinforcement and transverse restraint bars to increase the strength of deep concrete. Therefore, it may be an inappropriate application if the steel is not in closed reinforcement, or if the steel is embedded in the column core without reinforcement, such as in SRC sections.

The second thing to note is that you must enter the appropriate strain rate for concrete in the inelastic material properties.

Input mistakes, or using some strain input values as default values, can result in yield points being calculated at inappropriate locations or failure to draw an idealized curve, as shown in the figure below.

If the idealized curve does not compute correctly, the input values of the inelastic material properties need to be rechecked.

[ Moment-curvature curves with incorrect yield strain values ]

Strain rates for concrete material properties can be determined by referring to the table below to determine extreme strain rates based on column cross-section geometry and rebar overlap.

[ Guidelines for Evaluating Seismic Performance of Existing Facilities (Bridges), <Table 4.2.1>. ]

Furthermore, the yield strain of concrete is suggested by the guidelines to be 70% of the strain at maximum strength.

In the Inelastic Material dialog, for example, you can see that the yield strain, ε_cy, is automatically calculated to be 0.0014 when the strain at maximum strength, ε_co, is 0.002.

[ Unconfined Concrete Dialog Box in Inelastic Material ]

However, the Constrained Concrete Input dialog box does not perform an automatic calculation, so you will need to manually enter the 70% strain by clicking the check box.

[ Confined Concrete Dialog Box in Inelastic Material ]