High Normal forces in beams

[E.Veliu]

Dear,

I have adopted the Inelastic plastic-hinge force-based frame element to model the beams and columns, and I am still realizing the axial forces in beams are very high. I was thinking that having the plastic-hinge approach would avoid the 'high axial forces in the elements. What type of element would you suggest using in order to avoid this issue?

Thank you,
Enes

[z.gronti]

Dear E.Veliu,

Perhaps you would like to select the "Do not consider the axial force contribution in the shear capacity of beams" check box, available in the Elements tab of the Advanced Settings. As it is written in SeismoBuild's User Manual "By activating this option the ability to carry out shear checks ignoring the actual axial force applied on the beam member is provided. This feature is particularly important to the shear capacity checks of beams, when the interaction between fibre modelled RC beams and the rigid diaphragm adopted to simulate the concrete slab (a very common configuration in RC buildings) may cause the development of unintended fictitious axial forces in them.".

[E.Veliu]

Thank you Zoi,

Hmm, it is tougher, I am worried about the stiffness and joints as well. A high N in beams means higher strength and stiffness in the structures, and higher shear forces in joints as well. Anyway, you are saying that high N in beams is not real, but it is there only because of a 'conflict' between the diaphragm constraint and the fibre-based element? Could you please share a reference for this, I would like to go through it.

I am somehow expecting the fibre-based elements would capture the 'beam elongation' effect that takes place in precast structures where the slabs and beams are not monolithically together. Is SB realistically capturing this effect in a case of a bare frame and without imposing the rigid diaphragm constraint? However, in the case of a cast-in-place structure, the 'beam elongation' phenomenon does not occur as the slab does not allow it.

Enes

[seismosoft]

These spurious axial forces is a well known problem of fibre modelling. For example see:
Barbagallo, F.; Bosco, M.; Marino, E.; Rossi, P. On the fibre modelling of beams in RC framed buildings with rigid diaphragm. Bull. Earthq. Eng. 2020, 18, 189–210.
Note however that both SeismoStruct and SeismoBuild have also a well tested displacement based plastic hinge model infrmDBPH with lumped plasticity, which does not have this problem (but has the other sources of inaccuracies that the displacement based plastic hinge models have). It is up to you to decide which frame element you want to employ.
Seismosoft Support

[E.Veliu]

When it comes to pushover analyses, would you recommend reducing the slab penalty function exponent to 1 in order to avoid imposing the rigid diaphragm assumption into the model, leaving the real beam axil stiffnesses to do the job?

[seismosoft]

Probably it is better to carry out the shear checks of the beams neglecting the axial forces applied on them, by selecting the corresponding project setting.
Seismosoft Support

[E.Veliu]

I am wondering what would be the drawback of not employing the rigid diaphragm constraint, but considering only the axial beam's real stiffnesses. Would you please be so kind as to share your insights?

[seismosoft]

It depends.
In most cases however the rigid diaphragm is an important structural characteristic that cannot be omitted.
Seismosoft Support