Shear walls 3

[Gus14]

I was reading reinforced concrete mechanics by MacGregor about shear walls. The book says that a possible solution to decrease foundation size is to " Attach the base of the wall to the ground-floor diaphragm and basement floor to provide a horizontal force couple to react to the moment at the base of the wall ".

Does this mean there will be a lateral force running through the ground-floor diaphragm we need to design for?

Isn't this the case in every building? so this should be considered with every high-rise building with a basement, regardless of whether we need to reduce the footing size or not. Right?

I attached a snip of the book page,
Your file's link is:

https://files.engineering.com/getfi...b8a1-4dc3-a01d-af353b007a0d&file=Question.pdf

 

[Ingenuity]

Take a read of this: thread507-502282 and the references it links.

 

[ggcdn]

Yes, its often called a shear reversal or backstay effect. Lateral load will migrate out of the tower LFRS and into the stiff perimeter basement walls.

Some codes are concerned about the reliability of this load path and make you jump through all sorts of hoops. For example, CSA A23.3-14 clause 21 related to walls and foundations, and LATBSDC (Performance-based design guidelines) both require you to do upper and lower bound analysis of the loadpath and design for the worst case.

In ASCE7 they dont require bounding the problem, but the system is a lateral offset of LFRS elements, so the transfer forces need to be amplified by overstrength factor in high seismic areas.



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

 

[Gus14]

Thank you, Ingenuity and Ggcdn, for replying. This effect on the podium diaphragm can't be ignored and is tricky to evaluate. I wonder if early skyscrapers before the age of modelling were this sophisticated.

 

[KootK]

I wonder if early skyscrapers before the age of modelling were this sophisticated.

Certainly not. Until fairly recently, it was fairly ubiquitous to utilize the model shown below. I saw it start to fall out of favor around 2005 for the low seismic regions in which I was working (middle US/Canada). The timing was surely different in other markets.

The model was clearly chosen for it's convenience for the engineer and the client friendly results that it produced rather than for rigorous accuracy.

 

[KootK]

For the common situation where there is a lateral offset between the core walls and the basement walls, I feel that this is often a more accurate model. That, because the basement walls parallel to load can lack the dead load to resist overturning without engaging the orthogonal walls. This creates some interesting considerations at the vertical joints between orthogonal walls which, if I recall correctly, has been studied a bit in Park & Paulay's book son seismic design of concrete and, perhaps, Jack Mohle's.

 

[Gus14]

Thank you kootk for replying,

This creates some interesting considerations at the vertical joints between orthogonal walls which, if I recall correctly, has been studied a bit in Park & Paulay's book son seismic design of concrete and, perhaps, Jack Mohle's.

Very interesting, I will look into it.

 

[TRPAM]

actually there is no couple action on the basement wall, the slab on the ground floor don't have shear capacity to create that action ( its not stiff enough like outrigger).

 

[KootK]

actually there is no couple action on the basement wall, the slab on the ground floor don't have shear capacity to create that action ( its not stiff enough like outrigger).

I disagree. The concept that is usually employed for this is the virtual outrigger as described in this article: Link

It becomes the ground slab's in plane shear capacity that is relevant, not it's out of plane shear capacity. And a ground slab's in plane shear capacity is -- or can be made to be -- significant. Of more concern at a slab on grade is usually the connections between the slab on grade and the basement walls as well as the distribution of any local, concentrated loads being delivered into the slab on grade by the core wall group.