40meters wall slenderness

[geo321]

Hello

I am asking your opinions regarding the below:
2 separate walls are being extended 40 meters up and they are carrying a staircase from one side and an elevator from the other side.
I am planning to brave vertically these 2 walls by 2 isolated beams very 3.5 to 4 meters.
If these beams are located at that vertical grid, what verifications should i perform in order to conclude if these beams are bracing the walls or not ?? In other words, what is the embraced height of the wall ? Is it the vertical spacing between the beams?
To be mentioned that a slab is bracing the top of the 2 walls at the 40 meters height. A quick sketch is attached.
Thank you for the help

 

[BAretired]

The sketch is not to scale, so we don't know the plan location of the two beams or the thickness of the walls. These could be important parameters.

I don't see much help from the Elevator shaft. If the stair is neglected, even though it does contribute some stiffness to the assembly, the walls and beams together constitute a rigid frame. The top and bottom beam reinforcement should be developed into each wall.

I think it would be more conservative to consider the unbraced height of wall to be the center to center distance between beams rather than the clear distance.

BA

 

[geo321]

hello bAretired. thank you for your answer.
"the walls and beams together constitute a rigid frame". This is exactly what I would like to discuss, in particular the rigid thing.
What are the lists of verifications which you should conduct before concluding that it is a rigid frame or a flexible one ?

 

[BAretired]

In analyzing a rigid frame, deformations must be sufficiently small such that the geometry of the structure is not significantly altered. If deformations are large, the frame may be considered flexible; further analysis is iterative with appropriate corrections applied to the geometry with each iteration. After several iterations, deformations may become negligible or increase without limit in which case, the frame is deemed unstable.

BA

 

[geo321]

Noted BAretired. So you are talking about second order versus first order limits I guess. The code gives some limitations on the above.
However a system can buckle due to gravity loads only. Let's say you have a wall of 10 meters width and slender enough and u introduced one single beamn in the middle of that element. Do you consider that wall a braced one?? And based on what ??
I guess it has a lot to do with the engineering judgment of each one and better be conservative in such cases.

 

[BAretired]

No, I would not consider that to be a braced wall. Would you?

BA

 

[geo321]

Of course not. This is the purpose of my post:
To.put limits and to quantify it.
I didn't find much infos in the code. Rather the code treats the sway and non sway stories.

 

[BAretired]

I don't know if there is a simple answer to that. A single brace in the middle of a long wall braces the portion of wall immediately adjacent to the brace but does not prevent the extremities from buckling in opposite directions.

A double beam braces the wall in two locations, but if the space between braces is large, the middle section of wall is not well braced. A horizontal strip of wall, acting as a beam spanning horizontally between braces must be sufficiently stiff to prevent excessive movement of the mid-height of the wall.

The concrete code may not cover this situation, but maybe you could borrow an idea from the steel code where a brace is required to have a resistance equivalent to, say 2% of the compressive load in the wall with specified limits on deflection. For a long wall, this may provide a guide to the maximum spacing of braces.

And as you suggested earlier, a little engineering judgment wouldn't hurt.

BA

 

[geo321]

Thank u baretired.

 

[BAretired]

You are welcome geo321.

BA

 

[KootK]

Yeah, that would be a tough thing to evaluate analytically. Ideas:

- for east west global stability, imagine short widths of the wall acting as the columns in a moment frame that includes the beams. Then use conventional methods to evaluate the stability of that.

- For north south local stability (vertical wall edge buckling) model the wall edges as vertical columns braced by horizontal beam strips in the wall at the elevations of the real beams. Be conservative with your stiffness assumptions and see if you can expend just enough effort to demonstrate that it's not a critical failure mode.

Is this shaft not part of a building that you can tie back to for the stability of the shaft?

Any chance your beams could just become continuous vertical walls? That would simplify a number of things.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Robbiee]

Here are my thoughts on this interesting question:
1- Agree with BAretired that the assembly of the two walls and the beams will act as a frame if connected properly.
2- The minimum thickness of the wall can be controlled by the length of the wall from the beam to the edge of the wall. This can be thought of similar to the flange of a W-steel section, where bucking under compression will happen if the ratio (bf over t), the length of the flange from the web to the edge divided by the thickness of the flange is greater than about 10.
3- If you choose smaller wall thickness than the one noted in (2), then, and borrowing from masonry wall design,you need to limit the compression in the wall to 10% of its factored compression capacity.
4- The connections of the beams to the wall needs to resist 2% of the axial force in the wall at the level in consideration. (Required per Canadian CSA A23.3 to be capable of bracing the walls).
5- I also would check the connection of the beams to the walls for the forces resulting from actual lateral deflection due to wind and seismic. Seismic deflection calculated using reduced force for ductility needs to be multiplied by the ductility factor used. Punching shear will be one of the checks in addition to beam shear and bending. `