Defining Plastic Hinges in built-up cold formed steel sections.

[g.alshamsi]

Hello all,

I have a cold-formed steel portal frame that I want to analyze under monotonic pushover and non-linear time history analysis.

The sections in my frame are made of built-up hat sections. I have successfully modeled the built-up studs using the section designer option, however I can't seem to define/assign hinges to the frame. For reference I want to define a plastic hinge (P-M2) from the material properties of the element. When I try to assign the hinge, I get the following error:
"Error in assigning hinges. Requested calculated properties may not be available for material or section type."

If I change the frame section to a built-in hot rolled section, I can define the hinge without any problem. I guess a work around is to change the section modifiers to get an equivalent section. But does anyone have any idea why I can't assign a hinge to my built-up sections?

Thanks,

 

[271828]

Are you sure these members can develop a plastic hinge?

For a plastic hinge to form, all compression elements in the cross-section need to have small width-to-thickness ratios so they would fail by yielding and not local buckling. I don't think I've ever seen a cold-formed steel section that would behave like that.

Otherwise, you just have something more like a hinge, not a plastic hinge.

 

[g.alshamsi]

Hmm that actually makes sense. The sections I'm using are built-up and relatively on the thicker side for cold-formed steel(2.5 mm), but they probably can't develop plastic hinges.

I was trying to enforce flexural buckling at the bottom of the columns to match the conditions of the experiment, any workaround to achieve that with CFS sections in sap?

Thanks for the reply anyways,,,

 

[271828]

I don't follow your second paragraph. A sketch might help.

 

[g.alshamsi]

I attached a print out of the shell model, as you can see in the image the columns are buckling and I'm trying to simulate this behavior on a frame model in SAP.

 

[271828]

Very cool model.

Would the following work?

Copy the shell model and delete everything except the column. Apply constant bending moment and increase it until local buckling occurs. The deflected shape will have regular elastic deformation and a lot larger deformation due to the hinge. Subtract the elastic deformation, leaving the contribution from hinge rotation. Use that moment-rotation to define a frame element hinge.

 

[g.alshamsi]

Hello, thanks again for replying,

In theory that might work, but I'd rather use a moment-rotation relation based on experiments rather than numerical simulations.

Assuming I can find something related in the literature, how do I go and define the hinge properties in SAP? Is it the same procedure as Define > Section Properties > Hinge Properties > Add new > Steel > Deformation Controlled > P-M3? I tried that but I keep on getting the same error. Maybe I'm missing something in the hinge definition?

 

[271828]

An experimental moment-rotation relationship would be even better, assuming one is available.

I haven't defined hinges in SAP2000, so you're ahead of me. Hopefully someone else will answer your second paragraph.

 

[g.alshamsi]

Damn it

Ah well, thanks for the input anyways!

 

[driftLimiter]

Cant you import custom hysteresis loops for these hinges? Its been almost 15 years since I have done this with sap. But for this you need the tested hysteresis because due to instabilities you dont have elastic-plastic behavior.