stiffness of plastic-hinge element

[golden_masood]

I have modeled a steel moment frame with inelastic plastic-hinge force-based frame element and bilinear steel model. I want to push the structure in a pushover analysis when I change the plastic hinge length the initial stiffness of pushover curve changes and also when I perform eigen value analysis changing plastic hinge length changes the elestic period of structure. I am completely confused. please help me what do these strange results mean?

[seismosoft]

Hi,

As usual, you should start with a simple model (e.g. a vertical cantilever) and check if the unexpected behaviour is still present. If it is, then send it to us, and we will have a quick look at it.

Seismosoft Support

[shahabnab]

Hi,,
I have same problem,, with change in the length of plastic hinge, the period of model will change. is this a bug , or the plastic hinge has effects on stiffness

[z.gronti]

Dear shahabnab,

The inelastic plastic hinge frame element is constituted of three parts, 2 plastic hinges at the edges of the element and an elastic part in the middle. As it is mentioned in the Help System of SeismoStruct "The use of inelastic elements in eigenvalue analysis features also the advantage of exempting the user from the onus of (manually) calculating the section mechanical properties of each element type, taking full account of the presence of longitudinal reinforcement bars within the section.". Hence, in the calculation of sections properties in the inelastic part, the longitudinal reinforcement bars are taken into account, whereas in the elastic part are not taken into account. I think that when changing the length of the plastic hinges, hence you change the proportion between the 3 parts and in this way the element's properties, you will get different results in the analysis.

[shahabnab]

Dear z.gronti

I am very thankful for your attention,,
But my study field is moment resisting STEEL frames( with IPE or W sections). I've implement several models, apparently changing the length of plastic hinge, affect the the stiffness of frames. also the ratio of 16.67% results accurately in a calibrated models...
thanks in advance Dear Gronti

[z.gronti]

Dear shahabnab,

In the plastic hinge parts of the element and due to the applied discretization, the section's properties are calculated with a higher level of accuracy than the elastic part of the element, where analytical expressions are applied. Again when changing the length of the plastic hinges, you change the proportion of the 3 parts and in this way the element's properties.