Shear Wall Structural detail for Seismic design (special Boundary Element detail)

[M.IDR]

There is a confusion about the designing and detailing about shear walls
A 21 Storied Building has shear walls and moment resisting frame. Structural system selected as "Dual System", while analyzing the shear wall in a FEM software it requires "Special boundary elements" because the stress level in Earthquake load combination which is about 1900psi in the pier which exceeds the limit of 0.2fc' (e.g. fc' =4000psi, 0.2*4000psi=800psi) as per the section of ACI 18.10.6 Reference code: ACI 318-14

As per the section ACI 18.10.6 "Boundary elements of special structural walls" Special Boundary elements are required as shown in FEM results of the shear wall pier as per the ACI 18.10.6.2 or 18.10.6.3. As per ACI 18.10.6.3 if compressive stress exceed 0.2fc' in a pier, it will require special boundary element which is applicable in this case, and also the neutral axis depth exceeds the limit to require boundary element. Results are shown in the figure below.

Now as per the relevant ACI 18.10.6.4, The boundary element should extend upto greater of (c-0.1lw and c/2) which is almost about 80 inches as shown in the FEM results of pier.

Now , While designing the shear wall it will have transverse reinforcement about over total length having transverse reinforcements as per "ACI Table 18.10.6.4(f)—Transverse reinforcement for special boundary elements" e.g. typical detail will be as shown

So, Dear Engineers am I right or wrong? am I interpreting the codal requirements in a right way?

does any one can share a typical detail of a shear wall of building having stories about 21 stories including shear walls, or more specifically having structural system of Dual system? If material properties are also included it would much appreciated, thank you.

 

[sticksandtriangles]

Let me caveat this by saying I have not designed anything over 6 stories tall, so take everything I am saying with a grain of salt. I am interested in tall building concrete core wall design though and I want to explore in much more depth on my blog in the near future.

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[li]The boundary element should extend upto greater of (c-0.1lw and c/2) which is almost about 80 inches[/li]
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This is telling me that you have essentially a coupled wall with large axial loads due to earthquake loading. I think it makes sense to apply special boundary element detailing for the full length of the wall pier and to provide the transverse reinforcement required in table 18.10.6.4(f)​

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[li]So, Dear Engineers am I right or wrong? am I interpreting the codal requirements in a right way?[/li]
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I think so​

No onto picking at your design because I don't know enough about tall building design:

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[li]f'c of 4000psi? I would have thought we would have been speccing higher strength concrete on these tall buildings?[/li]
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[li]What sort of wall thicknesses are we dealing with here? Just based on the layout, the x direction feels quite weak. My gut reaction is that the small 6-8 foot long shearwalls likely do not add much stiffness, how is drift looking on the building?[/li]
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[li]Have you done any fancy modelling for the wall column intersections that I see near your core walls? I have always wondered about the implications of having the wall corners and the columns intersect as you have shown[/li]
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[li]How have you modelled your floor slab in etabs? Does the floor slab resist bending for seismic loading, increasing the core, frame coupling effect?[/li]
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I plan on diving into all these questions in more depth on my blog but I just wanted to get your thoughts.

does any one can share a typical detail of a shear wall of building having stories about 21 stories including shear walls, or more specifically having structural system of Dual system?

I will do some research tonight hopefully and let you know what I find.


S&T -

http://www.re-tug.com

 

[ggcdn]

A few thoughts:

What is the seismicity and drift like for that building? It looks very flexible and torsionally irregular. With such short walls it doesn't surprise me that the whole cross section needs to be confined, at least coming from a California seismicity context.

I would be concerned that ETABS may be be checking your elements incorrectly when you model a line element connected to a wall element like that. The confinement calculation is directly related to the axial load in the wall pier, so if the frame is stealing load away from the wall pier, its going to be off. Likewise, the stresses will be off because the length isn't correct. Personally, I don't mix frame elements in wall elements like that. I will just discretize the shell and add different thickness to the pieces. This way you also will get the correct wall length for shear calculations, and the correct span for beam/slabs.

You can also do a hand check of the confinement requirements using the displacement-based approach in ACI318. This is actually the preferred way of doing it in my opinion.

Finally, you will need to check your confinement reinforcing ratio meets the requirements for special boundary elements. I am surprised #3 ties will provide enough capacity (Ash/(bc*s)), but then we are usually using 6+ ksi concrete in a building this size, which does affect the confinement.

-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[sticksandtriangles]

does any one can share a typical detail of a shear wall of building having stories about 21 stories including shear walls, or more specifically having structural system of Dual system? If material properties are also included it would much appreciated, thank you.

I found this online,

https://peer.berkeley.edu/sites/def...report_for_the_tall_buildings_initiative.pdf,

a 42 story building with a dual system concrete core wall and special moment frames and it looks like they have (2) different building designs presented for the same building, starting on page 129.

A few notes from my review, building A:

- f`c = 6000psi for the first 20 stories, 5000psi above
- wall thicknesses = 24" first 20 stories, 18" above

I am surprised #3 ties will provide enough capacity (Ash/(bc*s)),

I am surprised by this as well.



S&T -

http://www.re-tug.com

 

[dold]

Quote (sonofatiks)
I am surprised #3 ties will provide enough capacity (Ash/(bc*s)),

I am surprised by this as well.

Make that three of us. I've done a bunch of special shearwalls for rather small (3-ish story) podium type jobs. Boundary elements would have absolute gobs of steel in them. like 2 #4 ties at every pair of bars. Essentially closed hoops around every set of 4 bars.

But 1900psi is pretty dang high. And if i'm reading "elevation 5" correctly, those are suuuuper skinny shearwalls to not have them coupled somehow. But the force output looks like they might be behaving that way? Can we see a screenshot of the deflected shape, OP? 12" thick x 8.8' wide and 21 stories tall! Wow. What are your shear stresses? 12" thick seems a bit light. And i'd be using a minimum of f'c = 5ksi for the wall.

 

[rapt]

Lw is the horizontal length of the wall which you have said is 8.8'.

.1Lw would then be about .88'.

Why do you need to read a number from an FEM output to work out the length?

 

[MotorCity]

Its not clear where the moment frames are located, but mixing moment frames and shear walls has the potential to introduce several horizontal and vertical irregularities, even if it is permitted in the dual system. If the shear walls and moment frames are in the same story, the forces will be attracted to the shear walls because they are inherently stiffer than the moment frames. The moment frames would see little if any of the story shear. If the moment frames and shear walls are in different stories, you will probably inevitably have a soft story.