ASCE 7 inverted pendulum and non-building classification

[engjg]

I am designing a steel braced framed structure to support a rooftop equipment penthouse and exhaust stacks, structure is 65'overall in height in a SDC D. Believe structure will meet inverted pendulum definition. I am trying to decide most appropriate non-building structure type. Considering inverted pendulum R=2 from table ASCE 7-10 15.4-2 and would like to avoid special seismic detailing if possible. Height to width ratio of structure probably 1.5 to 2. Trying to envision how I will satisfy inverted pendulum moment analysis requirement for steel braced structure. More easy for me to envision satisfying this for say a single shaft support water tower but not a braced frame. Perhaps one could someone increase the applied shear using the in a ELF analysis to produce the desired moment envelope? Further complicating...believe I will be forced into modal analysis by a weight irregularity not sure at all how to tackle moment envelope there?

 

[WARose]

Trying to envision how I will satisfy inverted pendulum moment analysis requirement for steel braced structure.

I don't see how you can classify a braced steel frame as a inverted pendulum. That's not how it is going to act.

 

[azcats]

I don't do a lot of this so forgive me if I'm out of line...

Could you use non-building structure similar to building table 15.4-1 and the Steel OCBF with unlimited height that references AISC 360? R goes down to 1.5 but you appear to be relieved of the AISC 341 detailing requirements.

 

[SAIL3]

sounds like the OP has 2 different type structures....lower a typ braced frame supporting a penthouse/stack which one might be able to consider as a rigid structure when analyzing the braced frame somewhat similar to supporting a tank....the stack/penthouse then could also be designed for seismic loads imparted by the braced frame.....
a sketch would help.....

 

[Deker]

For the structure to be considered an inverted pendulum, the mass at the top must be rigid enough that the rotation of the mass matches the rotation at the top of the supporting structure. Your braced frame acts as a vertical truss and resists moment through axial load in the columns. To satisfy the moment envelope requirement, you can take the vertical reactions at the base of the columns due to seismic load and decrease them linearly to half that value at the top of the columns.

 

[wannabeSE]

Since 65' tall would be an extremely tall for a penthouse, I am guessing this is the overall height of the building and penthouse combined. It is unlikely that a penthouse can be considered a non-building structure covered by chapter 15. If the penthouse meets the requirements in 2015 IBC §1510.2 (height limits and area not exceeding 1/3 the supporting roof area), I would treat it as a non-structural component on top of the roof an use ASCE 7 chapter 13. Otherwise, I would treat it as another floor and use chapter 12.

 

[engjg]

Thanks all for the input, see attached crude sketch.

WARose: I agree and although I would meet the 50% mass at the top now re-reading asce-7's definition of inverted pendulum I think there is more wiggle room to not to fall in that bucket as they mention and "relies on rotational restraint to the the top of the cantilevered element"..as Deker has mentioned.

azcats: Good idea...still in prelim design so don't know if an R=1.5 would overaly demanding...but certainly appears an option for me. Al thought I was planning on a knee braced portion (see sketch) which may not work with that category.

SAIL3: 1st statement was exactly my thought on how to approach although I don't know if the relative rigidity is that different. Regarding the second statement the top portion is designed by another engineer could you expand on your thoughts and a design flow path between the two engineers.

Deker: Good point and believe this is my out; although without anlayzing the top portion (which I was not planning on) hard to judge relative rigidities.

wannabeSE: Your right 65' is not the overall height (see sketch)...and I think my penthouse terminology may be in error as this is a self supported structure (see sketch).

 

[SAIL3]

I doubt if one could consider the penthouse itself as rigid..probably consists of cladding and braced frames in one direction and moment frames in the other....the stack itself could probably be considered as a rigid structure...that said, one would need to incorporate the preliminary structural members sizes and loads of the penthouse into the model and approximate the stack as a rigid structure with a lumped mass ,say, @ mid-height of stack and perform a dynamic model analysis....the preliminary seismic loads of this first iteration would then be sent to the designers of the penthouse/stack for any redesign of penthouse/stack in order to complete final interaction between penthouse and support structure....in other words a feed-back loop....