One way slab but with opposite reinforcements analogy

[planc]

For slabs with ratio greater than 2. It is considered one way slab with the reinforcements in the short directions.

But for sake of illustration. Supposed you put reinforcements in the long directions only, and no reinforcement in the short directions. Will it work if the reinforcements are four times greater?

This analogy is to analyze a concrete wall that is supported by the columns only without any top beams. If you put very big rebars or even angle bars between the columns every 39.37 inches (one meter) height of the concrete wall. How would it behave (analogy is just like slabs with ratio greater than 2 where you put reinforcements in the long directions only).

 

[HTURKAK]

It will work if the necessary reinf .for long direction is provided but the cost probably would be cracks along the long direction. I do not know the applicable standard in your region . In general , As/4 rebar is provided for distribution reinf.
But it will be absurd to design one way slab ( say L/B=2.0) since the design moment would be 4 times of short direction .If you provide 4As and min . dist. reinf As, the slab will work on short direction..


I could not see the analogy..If RC wall is supported on columns, the wall will be deep beam and still the main rebar will be at the bottom of the wall .. .. Will you post a sketch?

 

[Tomfh]

It will work but will crack heavily in the short direction.

 

[planc]

Attached is the sketch of the wall 3 meters (10 feet) high with 2 columns distanced 6 meters (19.68 feet) apart. There will be horizontal flat bar every 1 meter (3.2 feet) high with 12mm (0.46") rebar o.c. every 300mm (1 foot). There will also be vertical rebar 12mm (0.46") o.c. every 300mm (1 foot). There will be no beam above. What will be the behavior of this wall when it is subjected to lateral and wind load?

The analogy of the lateral and wind load is just like slabs when there is dead and live loading.

 

[le99]

The effect of change in reinforcing due to the different reinforcing schemes can be estimated as:

- Case 1: The short span Ls as the main direction, the moment in demand, M1_max = wLs²/8 (Assume simply supported)

- Case 2: The long span Ll as the main direction, the moment in demand, M2_max = wLl²/8 (assume simply supported)

- The ratio of M2_max/M1_max = Ll²/Ls² = As required for case 2/As required for case 1

Note, the edge support type will change the equation of the maximum moment. Equally important is the shear, which affects the thickness of the wall/slab.

 

[centondollar]

If you have discrete column supports instead of continuous (or almost continuous) wall supports for a slab, and the aspect ratio is small, the moment distribution will not be anything similar to the commonly designed "slab that has unit width and behaves like a beam". I suggest you use software or hand-calculations to solve the slab bending (Mxx, Myy) and twisting (Mxy=Myx) moments using the theory of elasticity, and then design reinforcement (do not forget to account for twisting!) accordingly.

Remember: slabs are not beams, and the effects of Poisson´s ratio and the plate kinematics and kinetics should therefore be accounted for in some manner. If the slab has low aspect ratio and odd support conditions (anything else than "long walls on two opposite edges", really), use the plate model!

 

[jayrod12]

Before we keep going down a path that isn't what I believe the OP intended.

I believe, and I hope they come back and clarify, they're asking about designing walls that would span horizontally 6m instead of 3m vertically and what sort of potential issues there are. I'm also fairly certain they're asking about designing it for out-of-plane lateral loads as opposed to gravity loads.

They only likened it to slabs because in a way, a wall is just a vertical slab.

To the OP, you can certainly analyze and reinforce the wall accordingly to span horizontally for out-of-plane loads. We aren't, however, going to design/check your reinforcement for you. But at a gut check, 12mm bars @ 1000 o/c is pretty light reinforcing for a 6m span. I'd be expecting closer to 15/20mm bars @ 300 and a 200 thick wall.Sorry, had your initial spec wrong, why are you putting flat bars at 1m spacing and then reinforcing at 300? Why not just provide reinforcing bars to do all of the tension work. I feel adding flat bars or angles will just complicate matters signficantly You also need to keep in mind that something has to support the vertical weight of this wall and transfer that to the columns. So if there isn't a beam below, the wall will have to do that and may require some additional considerations/reinforcement to address any potential problems.

If there is a beam below it, but just nothing above, you could look at it as a 3 sides supported plate to be as economical as possible, however in my mind it's not generally worth the time.

Why isn't there a support above the wall for the out of plane loads? Does it stop before reaching the slab/roof? Maybe a ribbon window or something?

 

[planc]

There is a support at the top of the column but it has poor anchoring (see actual attached photo). The previous contractor put very poor anchoring and during lateral loading, the anchors can breakaway by either cone failure, side breakout etc. It would be expensive for clients to remove the I-beam and replace with a concrete beam (because the rafter and thin metal roof were already put in place).

Without the competent top beam. That is why we planto put angle bar connected to the column for every 1 meter height in addition to 12mm rebar oc every 1 foot vertical and horizontal. You got other ideas? If we would continue with the main reinforcement from bottom beam to top I-beam. And the anchors at the support is poor. The anchors would just detached for any lateral loading.

 

[rapt]

Should be desi8gned as a slab supported on 3 sides, and that will provide crack control in both directions.

 

[planc]

We are designing for a concrete wall about 4" or 5" thick that is 3 meters high and 6 meters long, not a slab.

The reason I used the analogy of slab in wall is because in a lateral loading, the wall can act like slab sideways. See attached illustration.

So from now on, let's discuss about wall. You can support a wall supported on the bottom beam, side columns, without upper beam?

 

[rapt]

planc,

I realize that but if you are designing it for out of plane loads, the design is the same as a slab supported on 3 sides.

 

[planc]

For a slab supported on 3 sides.. you can use miss one reinforcement in the short span and put only reinforcements in the long directions (with 4 times the moment)?

But for the bottom beam in the wall. It is implanted dowel drilled 4 inches deep with low viscosity epoxy. In lateral loading, would the dowels epoxied 4 inches deep just pulled out?

What reinforcements are suggested for this wall, especially the reforcement between columns. An angle bar every 1 meter high in addition to 12mm rebar would be more rigid than just rebars, is it not?

 

[centondollar]

I suggest that you run the elastic analysis in FEA-software to receive dimensioning lateral (bending of a plate) effects (Qx, Qy, Mxx, Myy, Mxy) and the in-plane (thin slab, in-plane forces only) effects (Nx, Ny). Such a decoupled (linear-elastic) analysis can be performed if the lateral deflections are small (say, around "thickness/5,..., thickness/3") and if the in-plane loading of the wall is much smaller than the wall buckling load. After that, design rebar against the bending (Wood-Armer), shear and normal forces.

 

[rapt]

Planc,

Yes, but that will not give you crack control in the other direction. There is more to design than ultimate strength! Your ultimate load path only becomes possible when the elastic load path loses stiffness due to cracking and everything eventually redistributes to the ultimate load path.

I would then say the wall is not supported at the bottom, so it is only supported on 2 sides and your other dilemma is solved! But I would still have reinforcement in the other direction for crack control until your dowels fail.