Torsion at top of wall

[minorchord2000]

I have an existing wall about 15 feet high and about 30 feet long. The wall is 15 inches thick and any reinforcing is unknown. The owner wants to install a fall protection device at mid length along the wall which will provide a concentrated torsion of 90 in kips. What is a reasonable depth of internal beam section that I can assume to check whether the torsion can be neglected using the provisions of ACI 11.6.1 (threshold torsion). If I assume a depth of internal beam of 40 inches, the threshold torsion just exceeds the applied torsion. Is this assumption reasonable for a wall so described?

 

[JAE]

Is this torsion or a concentrated bending moment at a point on your wall?

In other words, when the fall impact force occurs, does the wall have fixity at its ends where the top portion twists in torsion? Or does the wall have flexural stiffness against bending from a moment applied at its top edge?

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[JAE]

Or do you have a combination of bending and torsion (more likely).


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[minorchord2000]

The torsion is a concentrated force at the top of the wall and is about the y-y axis (not the x-x axis as my attachment notes). Does this clarify your question?

 

[JAE]

I don't think this is a classic torsion condition per ACI 11.6.
If the torque (moment) is applied about the X-X axis, then this is more like a force couple applied to a wall where the attachment creates an upward tension force and a similar downward compression force in the wall in adjacent segments.

The wall will resist this though pure tension/compression behavior via vertical strips of wall section to the base. This all depends on the nature of the connection to the top of the wall of course.



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[minorchord2000]

JAE: did you see my correction about the axis about which the torsion is applied? It is about the y-y axis. The y-y axis is along the top of the wall so the torsion acts in the x-x direction.

 

[KootK]

- torsion and moment are pretty much the same thing just about different axes. JAE's speaking the truth here. This is a wall with bending applied to it. Torsion checks will not be appropriate.

- I would look at whether or not the load can be delivered to the wall via a connection that will, of necessity, rely on concrete breakout of an reinforced section. If that can be made to work, then I would be comfortable saying that the forces delivered to the wall will dissipate out into the wall such that further checks on the wall would not be necessary. Your wall is a beast. You just gotta get the load into it to begin with. As always, much depends on the specifics of the situation. But based on what I see of this so far, this is my opinion.

 

[JAE]

Yes agree...moment about the Y-Y axis is still not a torsional design situation - you are simply bending the wall at its top at once concentrated spot.

Even with the fixity you suggest at the end of the wall, I would not rely on "torsion" design to take the forces into the wall, but rather bending in the wall and the concerns over the attachment per KootK's statements.

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[minorchord2000]

Thanks gentlemen for your valued assistance.