Concrete Wall Slenderness

[J1D]

There is a question from a fellow structural engineer about a concrete wall slenderness ratio. It is an end wall supporting gable roof, 14m long. The height is 12m to ridge and 8m to eave. The wall is 8” thick with double reinforcement. Further, the wall is simply supported at top and bottom, also there is a concrete floor (lateral support) at the 3m height. It is apparently a slender wall and exceeds the limit without deflection check. He check and strength and deflection. They are okay.

The question is whether the wall is too slender? Can he use the mean value as the wall height? any other things he should check?

 

[JAE]

Is this a tilt-up wall or cast-in-place? With individual panels, I'd still use the actual panel height - but adjusted based on the individual panel width using an average over that width.

For most wall panel systems, we use h/50 as a good starting point for design.

Also, using two layers of steel in an 8" wall is a bit overkill. You would probably be more economical with a thicker wall and one layer of steel.

 

[SacreBleu]

Your maximum ratio is about 54. That seems OK. However, as JAE mentions, if you have reinforcing both faces, horizontal and vertical. That seems excessive. Since you have a floor at 3m, doesn't that appreciably reduce your effective height? It seems to me that you are OK even with 8" width...and doesn't just reinforcement in center of wall work OK?
Question - why the mixed units?

 

[J1D]

Thank for the input, particularly about the maximun ratio (I am not very familar with the wall design now since off the commercial projects for a quite a while). I believe it is c.i.p. concrete wall. The double reinforcement seems a bit too excesive to me as well. With single reinforcement in wall center, do you use half of the wall thickness as the effective depth in the bending calculation against wind?

The wall is 200mm thick.

 

[UcfSE]

There isn't a hard and fast absolute limit on slenderness of tilt walls. There are typical industry limits of L/h < 50 or 60 depending on where you are from. It also depends on what code you are using. If using the ACI 318 there are approximate methods in the code that are acceptable to take into account P-delta effects. If you L/r > 100 (10.11), you have to do a second order analysis per 10.10. If your wall is tension-controlled then the alternate design method in 14.8 is acceptable as a substitue for a second-order analysis provided you meet all the criteria. Thickening the walls was also a suggestion to avoid double layers of steel. Note that if you have a wall 10" thick or thicker double layers are required by the ACI. There is also an ACI publication, 551 I believe, that specifically covers tilt walls. If you are not governed by the ACI then this is all just info but maybe the code you do use has similar requirements.