Plastic hinge length in Beam with hinge element

[marcellosabene]

Hy to everyone,
I have a problem with Scott and Fenves' finite element, which i am using to model beams/columns
members in a 2D Frame in OpenSees.
I've evaluated the Plastic Hinge Length "Lp" using Paulay and Priestley (1992) formulation,
but in this way i found a high axial deformability of the element even for gravitational loads only.

In Opensees wiki example: Pushover Analysis of 2-Story Moment Frame,
0.004*(Element Length) is used to evaluate the same Plastic Hinge Length.
In this way you obtain a smaller plastic hinge lenght (one order of magnitude) than that calculated with Paulay and Priestley (1992) formulation.

My questions are:

1) Which of the two formulations is more appropriate?
2) How can I neglect the axial stiffness using Scott and Fenves' finite element?

Thanks in advance for your reply and for your availability.

[selimgunay]

Plastic hinge length equations are all empirical, therefore it is difficult to comment on which one is the best option. But, with the beamwithhinges element, you can choose the larger value not to miss any inelastic effect around the plastic hinge length.

You can neglect the axial stiffness by using equaldof for the axial dof at the start and end nodes, however, if you are using reinforced concrete fiber sections, that is likely to artificially increase axial forces on your member.