ETABS - Moment Frame Beam Type

Question

ETABS - Moment Frame Beam Type

Struct-007 (Structural)

(OP)

29 Dec 20 15:25

I am modeling a steel frame considering an intermediate moment frame as the lateral load resisting system.

All the beams are simply supported except the ones in the intermediate moment frame.

In ETABS, when I view the assignment of any of the simple beams, I find that they are assigned a Moment Frame Beam Type of "Standard Moment Connection". I want to remove this assignment as they are not continuous nor do they have fixed supports. How can I do that?

Below is a screenshot of the Assignment window for your reference.

Thank you.

fracture_point (Structural) · Answer 1 · 2020-12-29 20:34:02

These are special types of connections that are typically used for seismic design and where areas of plasticity may be expected. You should leave this as 'standard moment connection' unless it is your intent to change them for special connections. You should select the frame members you want to pin, and go to ASSIGN -> FRAME -> RELEASES/PARTIAL FIXITY and release any of the components that you wish (most likely M2 and M3 at each end). This will create simply-supported behaviour for the beams you assigned it to.

Struct-007 (Structural) · Answer 2 · 2020-12-30 05:02:47

Thank you fracture_point. This is what I did. However, I do not understand why ETABS forces me to define the connection as a Standard Moment Connection although the beam is pinned from both ends. This has consequences during the design check as ETABS is checking these beams considering the seismic limit states (AISC 341) although these members are not part of the moment-resisting frames I defined for the structure.

fracture_point (Structural) · Answer 3 · 2020-12-30 06:31:53

What seismic checks is it imposing? Can you share a screenshot?

Struct-007 (Structural) · Answer 4 · 2020-12-30 14:02:39

Thank you. Below are screenshots of one of the infill pinned-pinned beams in the structural frame. Please let me know if you require additional information.

sticksandtriangles (Structural) · Answer 5 · 2021-01-04 20:42:12

I have come across these flags before as well and I have also found it odd that there is not true explicit way to turn these off.

What I typically do when I get those silly seismic flags is change the framing type from IMF to OMF (even though these beams are not part of the lateral system). This will typically remove the odd seismic warnings.

S&T

Struct-007 (Structural) · Answer 6 · 2021-01-07 23:29:06

Thank you. This removed the seismic warning but I don't know why we need to define the beams as moment frames although they are pinned from both sides. I just want the beams in the Intermediate Moment Frames I defined in the Structure to be part of the moment frames (not all beams).

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ETABS - Moment Frame Beam Type