Response modification factor for vertical vessel on foundation or skirt supported per ASCE 7-16 1

[quasiblu]

Good day,
I am trying not much successfully to convince myself that those vertical vessels will perform happily when the following are applied:

** Table 15.4-2, "All steel [...] distributed mass cantilever structures not otherwise covered herein, including [...] skirt-supported vertical vessels [...] -- Welded steel with special detailing(f)" -- R=3.
** Section 15.7.10.5, which says that shell buckling checks be performed with I/R=1.
** The commentary C15.7.10.5, telling that R=3 is applied to cross section of anchor bolts and foundation sizing.

I have doubts that the cross section of the anchor bolts and the interface soil-foundation is enough to achieve R=3 in most of the cases, while everything above is designed to stay in the elastic domain, and I believe an estimation of the ductile capacity should be performed if one wants to choose R=3.

As a comparison, if I open EN 1998-4(2006)(Seismic design for silos), I see in Section 3.5.2.3:
** Anchoring systems shall generally be designed to remain elastic in seismic design [...]
** If the anchoring system is part of the dissipative mechanism, then it should be verified that it possesses the necessary ductility capacity.

I feel like R=3 just comes directly from buildings, and it is very much loved because it limits the foundation size to a feasible size.

Enjoy your lunch.

 

[human909]

I've always used R=3 for silos as that is what is prescribed in my local code. That said, I can't claim to have great seismic knowledge beyond straight forward code application, so I won't say much more on that. I do deal with silos regularly so I have a few comments:

I would add a couple things:
-Silos are not structures you want to skimp on. As one experience engineer said to me recently, unlike most structures, silos generally DO see their design loads REGULARLY during their lifetime and exceeding them isn't uncommon either!
-What seismic PGA do you need to consider? In my experience seismic uplift is always cancelled out by gravitational loads. Though obviously this depends on the PGA you are designing for.
-In my experience wind on an empty silo dominates and determines the foundation size. Again, this depends on the design PGA.
-I've regularly seen designers design silo foundations for the full seismic moment of a full silo while not including the restorative moment of a full silo. Doing so you end up with absurd foundation sizes.

 

[quasiblu]

thanks

 

[quasiblu]

I will add ref to ASCE "Anchorage design for petrochemical facilities", 1st paragraph of Section 3.11.5, that reads:
Historically, the foundation anchors for tall vertical vessels and stacks have tended to stretch beyond yield when subjected to strong ground motion, which probably prevented collapse of these vessels. Based on this experience, it is recommended that these anchors be designed with ductile embedment into the foundation. (Special care should be taken not to significantly oversize the anchors.) Oversizing could cause the anchors to not yield during a seismic event, thus increasing the load on the foundation and creating overturning moments in the foundation beyond those assumed in the design.

 

[HTURKAK]

** If the anchoring system is part of the dissipative mechanism, then it should be verified that it possesses the necessary ductility capacity.]

The full clause of EC-1998-4 3.5.2.3 Anchors

(1)P Anchoring systems shall generally be designed to remain elastic in the seismic design
situation. However, they shall also be provided with sufficient ductility, so as to avoid brittle
failures. The connection of anchoring elements to the structure and to its foundation shall have
an overstrength factor of not less than 1,25 with respect to the resistance of the anchoring
elements.
(2) If the anchoring system is part of the dissipative mechanisms, then it should be
verified that it possesses the necessary ductility capacity.

Literally , if cast in anchors used, it shall be ductile and less strong than other attachments ( concrete failure, base plate, chair ...)


I would suggest you ;

- Pls look ACI-318-19 CHAPTER 17—ANCHORING TO CONCRETE,

- If this is a real question, pls provide more details to get better responds.







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[quasiblu]

Thanks Hturkak.
This is not really a query about a specific application. More a discussion about understanding the correct approach.
My believing is that a seismic design where:
* vertical vessel R/I=1
* anchor bolts R=3
* foundation R=3
Will not have sufficient dissipation capacity to achieve a dissipation R=3 and may be subject to collapse as observed by ASCE "Anchorage design for petrochemical facilities", 1st paragraph of Section 3.11.5 (quoted in the post above).

To have R=3 both of the following have to be true:
* anchors yield for forces close to those calculated with R=3
* anchors will have sufficient ductility capacity to cut the forces along the load path

For what concerns EC-1998-4 3.5.2.3, (1) and (2), per my understanding:
(1) means that in general anchors are not to yield, and even in that case, an overstrength to their connection is be be given, so that in the case for some reason forces greater than the design forces will come, collapse will be announced by visible deformations.
(2) means that where something allows/compels not to follow paragraph (1), then please at least backcheck the ductility capacity.

The point is that we should not just say that as long as I have picked a specific structural system, I can therefore adopt its R, Omega, Cd as per a table in a Standard (the Standard gives max values of R).
R depends on the actual dissipation capacity, i.e., it is a behavior resulting from design and not an input. Compatibility between initial assumption and final design should be somehow backchecked.
Example, if I start assuming that I dissipate to R=3 with the anchors only, and in the end I choose anchors having a unity check UC=0.5<1 for seismic, I should then observe that I am bringing to foundation (at least) seismic loads coming from an R equal to 0.5*3=1.5 -- and therefore if I adopt those anchors, I should check foundations and overturning with R=1.5 (at least -- because the ductility capacity should also be checked).

 

[bones206]

I have doubts that the cross section of the anchor bolts and the interface soil-foundation is enough to achieve R=3 in most of the cases, while everything above is designed to stay in the elastic domain, and I believe an estimation of the ductile capacity should be performed if one wants to choose R=3.

This is the crucial design task for the engineer to ensure good seismic performance. You want the vessel "rocking and stretching", meaning the anchors are designed as the ductile stretch element converting kinetic energy to strain energy, while other elements in the load path are designed (with amplified loads) to remain elastic. ACI Chapter 17 anchorage provisions for seismic design promote this behavior by prescribing that ductile anchor failure modes govern over non-ductile modes. It's important to provide adequate stretch length as well so the anchors don't fail in tension.