ASCE 7-16 Use of Amplification factor Omega

[Insha]

ASCE 7-16 Table 15.4-2: Inverted pendulum type structure: Detailing Requirements are listed as "Sec. 12.2.5.3".
The only requirement listed in Section 12.2.5.3 is the moment at the base shall be calculated per procedures given in Section 12.8, which contains
the Equivalent Lateral Force Procedure.

For comparison, Section 12.2.5.2 (Cantilever Column Systems) clearly states that foundations and elements used to resist overturning
shall be designed for seismic effects including over-strength factors of Section 12.4.3.

Question: Do you agree with the conclusion that the amplified seismic force (applying Omega over-strength factor) does NOT apply to
the design of Inverted Pendulum structures or their components such as base plates and anchor bolts?

 

[HTURKAK]

Just for curious : where is written that the use of overstrength factor is code requirement for inverted pendulum structures ?

 

[Harbringer80]

Omega nought = 2 for inverted pendulum structures is listed in the table. I do not agree with your assessment.

Preventing a brittle failure mode to enforce ductility in a very non-redundant structure is basically the reason the concept of overstrength exists. IMO overstrength is required for the anchorage design.

Is your pendulum structure effectively a cantilever column?

 

[driftLimiter]

I agree special foundation detailing and overstrength requirements are applied to cantilever column structures, and no such provision is written for inverted pendulum structures. I would refer to section 11.2 to verify that your structure doesn't qualify as a cantilever column system. By my reading a structure could be both an inverted pendulum system and a cantilever column system if the overturning is solely resisted by cantilever columns in bending.

 

[Insha]

Commentary section C15.5.2 states; "Single-column pipe racks that resist lateral loads should be designed as inverted pendulums."
That leaves little to no room for interpretation.
The definition of Cantilevered Column System given in 11.2 seems to refer to a structural framing system in which multiple cantilevered columns are
part of the system. If that is the case, there is the potential for increased lateral load transfer to any given column.

 

[Insha]

Just for curious : where is written that the use of overstrength factor is code requirement for inverted pendulum structures ?

It's not, as far as I could find, which is the reason for the original question.

 

[HTURKAK]

It's not, as far as I could find, which is the reason for the original question.

You could say that the question is for the Single-column pipe rack at the original post.
As you said thta C15.5.2 states; "Single-column pipe racks that resist lateral loads should be designed as inverted pendulums."
Now the background for the inverted pendulums ;
- 12.2.5.3 Inverted Pendulum-Type Structures. Regardless of the structural system selected, inverted pendulums as defined in
Section 11.2 shall comply with this section. Supporting columns or piers of inverted pendulum-type structures shall be designed
for the bending moment calculated at the base determined using the procedures given in Section 12.8.
- Section 12.8 is 12.8 EQUIVALENT LATERAL FORCE (ELF) PROCEDURE.

Now your procedure ;
- Calculate the fundamental period of the structure (T)
- Calculate Cs =Sds/(R/Ie)
- If Cs is greater than the Cs value calculated with (12.8-3) or (12.8-4) take the upper limit calculated with (12.8-3) or (12.8-4)
- If Cs is less than the Cs value calculated with Cs =0.044SDSIe ≥ 0.01 , then use Cs =0.044SDSIe ≥ 0.01 as threshold .
- If Cs is in between the values calculated with (12.8-3) and (12.8-4),
Your design shear would be V= 0.50*SDS*Ie*W

EDIT: ENG Insha;
Please perform preliminary calculation for two alternatives ;

1- Assume the inverted pendulum is cantilever and apply 12.2.5.2 Cantilever Column Systems rules with overstrength applied ,
2- Perform the calculation as per the code 12.2.5.3 Inverted Pendulum-Type Structures. and use 12.8 Equivalent lateral force procedure and do not multiply with overstrength factor.
And pls share your findings. I bet the analysis as per 12.2.5.3 using 12.8 Equivalent lateral force procedure will be more stringent.

 

[driftLimiter]

Commentary section C15.5.2 states; "Single-column pipe racks that resist lateral loads should be designed as inverted pendulums."
That leaves little to no room for interpretation.
The definition of Cantilevered Column System given in 11.2 seems to refer to a structural framing system in which multiple cantilevered columns are
part of the system. If that is the case, there is the potential for increased lateral load transfer to any given column.

If the source of lateral resistance is single columns bending about a fixed base you should use special detailing for cantilevered columns in my opinion. I do not believe that inverted pendulum and cantilever column systems are mutually exclusive.

 

[straub46]

Commentary section C15.5.2 states; "Single-column pipe racks that resist lateral loads should be designed as inverted pendulums."
That leaves little to no room for interpretation.
The definition of Cantilevered Column System given in 11.2 seems to refer to a structural framing system in which multiple cantilevered columns are
part of the system. If that is the case, there is the potential for increased lateral load transfer to any given column.

That means there's also redundancy in the system. An inverted pendulum has no redundancy; if the anchorage fails, the entire thing comes down. I can't think of a more appropriate case to use overstrength than the anchorage of a system with zero redundancy. As driftLimiter said, I think an inverted pendulum is a special case of a cantilevered column system, and needs to follow the same detailing.

 

[driftLimiter]

That means there's also redundancy in the system. An inverted pendulum has no redundancy; if the anchorage fails, the entire thing comes down. I can't think of a more appropriate case to use overstrength than the anchorage of a system with zero redundancy. As driftLimiter said, I think an inverted pendulum is a special case of a cantilevered column system, and needs to follow the same detailing.

While I agree with your conclusion, I'd like to make a somewhat nuanced distinction on your response...

"I think an inverted pendulum is a special case of a cantilevered column system" - No its just a slender cantilever structure with more than 50% of the mass concentrated at the top.

Inverted pendulum doesnt need to have cantilever columns. It can be a trussed tower for example. Its about how the mass is distributed, and the the relatively slender support structure that cantilevers up. Not the same necessarily as a cantilever column system.

The 11.2 Definition for cantilever column system says: "the lateral forces resisted entirely by columns acting as cantilevers from the base"

 

[straub46]

True. The first thing that comes to my mind for an inverted pendulum is a water tower, and that was the image I had in mind, albeit with a pipe and single column.

 

[DynamicsMX]

No, I do not agree with that conclusion; engineering judgment and the fundamental intent of seismic design provisions suggest otherwise.

Inverted pendulum-type structures are inherently non-redundant, and any damage or inelastic deformation, especially at the base, can provide instability or collapse. We have to remember that the practice of reducing seismic forces is a modeling simplification that assumes the structure has adequate ductility or overstrength to redistribute and dissipate energy. However, in a system like an inverted pendulum, there is no margin for such redistribution or plastic behavior. Applying the overstrength factor is essential to ensure these elements remain elastic and preserve the overall stability of the system under seismic demands.

Basically, don’t reduce the forces. Just make sure everything — base plates, anchors, foundation — stays elastic.