ACI minimum reinforcement for walls 1

[ramihabchi]

Does the minimum reinforcement given for walls in codes,example in aci 318-11 21.9.2 0.25%,guarantee a ductile behavior of wall?

 

[Agent666]

Not really, recent experience here in NZ during the 2011 Christchurch earthquakes showed our minimum which is 0.32% was insufficient to prevent fracture of the reinforcement and in some cases maintain the stability of walls (walls buckling). Generally due to the concrete strength being up to 2 times the specified value, essentially a single crack forms at lower reinforcement ratios because the reinforcement capacity is lower than the concrete cracking capacity. You do not get the distributed cracking required to dissipate the seismic energy. Instead you yield the bars over a very short region until they fail due to low cycle fatigue.

As a result of our experiences, our minimum reinforcement ratios have doubled to 0.63% in the end regions of the wall (end region being specified at 0.15 times the length of the wall). Also while the middle region minimum is still set at 0.32%, it is also required to be at least 30% of the ratio in the end regions for more highly reinforced walls.

I note in ACI318-19 they have halved the minimum to 0.12% for lower levels of shear..... pure madness...........
I note they have added some further refinement to walls with lower aspect ratios and its somewhat based on the required tensile reinforcement ratio for strength, resulting in higher limits being required throughout when you require more tension reinforcement for certain wall configurations. Perhaps investigate these, as the intent might be similar to our updated requirements.

 

[Agent666]

I'd point out as well for good ductile performance you also need to adequate confinement and prevention of bar buckling of the vertical reinforcement. So it's not about simply providing minimum levels of longitudinal reinforcement, there are a lot of other considerations as well.

 

[KootK]

I believe that the minimums are usually meant merely to target a modicum of crack control.

 

[r13]

Agree with KootK. In order to use the minimum reinforcement, the wall must not be a shear wall (which require design), nor slender wall (satisfy L/r ratio), and the resultant compressive force from all loads must be located in the middle third of the wall (compression control). Thus, it is more of service criteria rather than strength criteria.

[ADD] However, the wall does have some flexural strength, though negligible, as it needs to be evaluated using the "empirical method", and the minimum reinforcing applies when the resulting demand of reinforcing is less than the minimum.