Inverted Pendulum vs Cantilever Column classification and application 1

[WiscEIT]

Hello All

I am writing to seek clarification in ASCE 7 Inverted Pendulum Structures vs cantilever column systems and discuss/seek answers to questions on (2) real world scenario I am facing. Inverted Pendulum type Structures are defined in chapter 12.2.5.3. The commentary C12.2.5.3 states it can be formed from many structural systems:

1) What kind of "many systems" is it referring to in the commentary C12.2.5.3 of ASCE 7?

2) If a cantilevered steel column in question is in a ordinary residential or commercial building structure (for occupants), then it would be classified as a steel special or ordinary cantilever column system per table 12.2.1 but C12.2.5.3 makes it sound like you can use categorize this as an inverted pendulum type structure (provided it meets the definition of inverted pendulum structure). If that is possible, you are then exempt from using over strength factor for base plate and foundation design that you would otherwise do for a special cantilever column or even meet the 15% axial load requirement in place for cantilever columns. This open ended provision does not make sense to me. Any thoughts? To supplement that why would the inverted pendulum structures not require overstrength factors for base and foundation as they are also non-redundant like cantilever columns?

Furthermore, seismic coefficients for inverted pendulum are specified in Table 15.4.2 (Non-Building Structures Not Similar to Buildings) but not in Table 15.4.1 ( Non-Building Structures similar to buildings)

3) Structure 1 (Real case): A few cantilevered steel columns are part of an industrial structure that is supporting an equipment (so Non-building Structure). This structure has a few steel braced frames in one direction, a couple of steel moment frames in the other direction. Now, the columns that are part of the moment frames or braced frames are cantilevered in the direction where frame action is absent. Additionally, I have a few cantilever columns that are standalone and not part of any frames ( no interaction with the frames so no dual system present at this time). Presence of all the moment frames and braced frames makes the structure a Non-building structure that is similar to buildings, correct? That means Table 15.4.2 and not Table 15.4.1? Does that mean I cannot use inverted pendulum structures for the cantilevered columns and I have to use steel special cantilever column per Table 12.2.1?

4) Structure 2 (Real Case): Similar to Structure 1 except no frames present. Only Steel beams and steel columns. Columns pinned at top and fixed at the base. Structure supports equipment (Non-Building Chapter 15). would this type of structure be classified as a "non-building structure not-similar to buildings" and consequently use inverted pendulum in Table 15.4.2?

5) Finally, what is worse: Cantilever column system or Inverted pendulum system design and costwise as required by ASCE-7. I think cantilever column system are worse because of AISC 341 detailing requirements, amplification of forces for base plate and foundation, and 15% axial stress limit requirement. What do you think?

Very sorry for the elaborate message. I would truly appreciate all the inputs I can get!

 

[KootK]

The key to this is recognizing the importance of the rotational comparability requirement for inverted pendulum structures. Almost nothing in the world of buildings adheres to that. It apies to rather exotic structures like elevated water towers, Olympic ski jump towers, and aircraft control towers. And some instances of equipment support.

 

[WiscEIT]

Thank you KootK for your quick response!! Thanks for providing a major distinction. What you said about instances of equipment supports makes me wonder if I can then use inverted-pendulumn type structure for my "Structure 2" as mentioned in 4) in the original post. ASCE "Guidelines for seismic evaluation and Design Petrochemical facilities document" shows types of equipment support structures that are classified inverted pendulums. One of them is shown below:

By rotational compatibility, I am guessing, you mean that the equipment rotates like a rigid body together with the pendulum. Could you elaborate a little bit on this rotational compatibility requirement and any other instances of equipment supports that you were thinking? I ll shorten my original questions as a result of your response:

a. Does my "structure 2" meet the cantilever stanchion classification in the image above and work as inverted-pendulum?
b. Should the seismic forces at the base be amplified by overstrength factor for inverted pendulum structures in general?
c. Any thoughts on 5) in the context of "structure 2"?

Thanks a lot!