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Different Kinds of Frames

Different Kinds of Frames

Different Kinds of Frames

Hello everyone:

I have a quick question. Does anyone know of any sources/references that goes through the differences between different kinds of frames? i.e. ordinary moment frame vs special moment frame, in particular? What's the difference between these two (and others)?

RE: Different Kinds of Frames

This is a question regarding classification of seismic force resisting frames, right?

If so, then it is a matter of expected ductility and seismic detailing requirements. Essentially, there are numerous seismic detailing requirements that must be met to call a frame a "special" moment resisting frame. These detailing requirements are there to ensure greater ductility to justify the higher R value (response modification factor in US codes) that allows us to design for a lower seismic force.

RE: Different Kinds of Frames

Best places to start are ASCE 7, chapter 14 (which has detailing requirements for SFRS elements and references), AISC 341 which includes detailed requirements for steel SFRS frames, and ACI 318 for concrete.

RE: Different Kinds of Frames

Yes, it's a question about seismic (and wind, i guess, if it governs) force resisting frames.

Thank you for confirming this stuff.

Your responses make me think I'm overthinking it and freaked myself out. It's just the detailing requirements, right? Like, you don't have to calculate the deflection and make sure it deflects enough too, right?

RE: Different Kinds of Frames

Quote (333OnlyHalfEvil)

(and wind, i guess, if it governs)

Be careful with this statement. It's a trap I used to fall into a lot, and I think it's prevalent in a lot of East Coast engineers. You're in California, so I probably don't need to tell you, but I will for posterity's sake: just because the wind load is higher doesn't mean you ignore seismic loading and detailing. Depending on your seismic design category, you may be required to follow prescriptive detailing requirements regardless of which load is higher.

RE: Different Kinds of Frames

To elaborate on phamENG's excellent post.

Say that you have a special moment frame, where the Seismic Base shear is 100 kips, but the wind base shear is 120 kips. The engineer who isn't experienced with seismic design might be tempted to say, "Since wind controls I don't have to worry about some of the SMF detailing requirements."

However, that's not a valid argument. If you used an OMF or IMF then your R value would be lower (i.e. lower ductility) and seismic WOULD control. Therefore, you do have to follow all the SMF detailing requirements. If you start to neglect some SMF requirements, then your expected ductility goes down and so does your R value.

In addition, the building code may have restrictions on the height of the structure and require that it be an SMF. If so, then you are still obligated to follow those extra detailing provisions for SMF.

RE: Different Kinds of Frames

2016 CBC 1604.10 Wind and seismic detailing:

Lateral force-resisting systems shall meet seismic detailing requirements and limitations prescribed in this code and ASCE 7, excluding Chapter 14 and Appendix 11A, even when wind load effects are greater than seismic load effects.

RE: Different Kinds of Frames

ACI318 makes the difference pretty plain.

It is related to everything, design loading, strength, detailing, deflections.

RE: Different Kinds of Frames

what software do you use in order to analyze simple 2d frames?

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