Hello, doctor, I have a question about the special shear wall design section, ACI318-14 regulations:
According to clause 18.10.2.3 part a):
Longitudinal reinforcement shall extend beyond the point at which it is no longer required to resist exure by at least 0.8ℓw, except at the top of a wall
What exactly does this clause mean and how should this clause be controlled?
The length of 0.8ℓw is too much, why should we continue this amount of reinforcement from the section where we no longer need longitudinal reinforcement in terms of bending?
Does top of a wall mean the upper part of the wall in each floor or the end part of the whole wall (last floor)?
Where exactly is the section where longitudinal reinforcement is no longer needed in terms of bending? And how is it calculated?
This paragraph is related to the cutting of vertical reinforcements in the shear wall in the place where they are no longer needed. Cutting shear wall reinforcement is very similar to cutting reinforcement in beams (think of the wall as a vertical beam). In paragraph 9-21-3-1-3 of the ninth topic, this is also mentioned:
Clause 9-21-3-1-3 Topic IX: The rebars should be extended from the section where their presence is no longer necessary to withstand bending with a length at least equal to d or 12 times the diameter of the larger rebar.
In this paragraph, d is the effective depth of the section. In the shear wall, the effective depth is considered equal to 0.8lw. The value of 0.8lw is also reasonable and is for the expansion of diagonal shear cracks towards the top of the wall. In the figure below, the interpretation of clause 18.10.2.3 of the ACI Code 318-14 is shown. The important points of this clause, shown in the figure below, can be summarized as follows:
Longitudinal reinforcement shall extend beyond the point at which it is no longer required to resist exure by at least 0.8ℓw, except at the top of a wall
The meaning of top of a wall is on a wall above which a number of rebars are no longer needed. It does not mean the end part of the whole wall. Due to the fact that the amount of anchor decreases in the height of the wall, there is no need for longitudinal reinforcements in some sections of the wall and they can be cut. The amount of required reinforcements is determined based on the factored anchor (Mu) in each section of the wall and comparing that anchor with the bending capacity of the wall section.
Rebars a provide the required design resistance Phi*Mn for the Mu factored anchor.
If the rebars b are cut, because they are in a place where there is a possibility of flow (critical section), their length should be greater than Ld (considering 1.25 times Fy). Also, rebars b should be extended more than 0.8lw from the place where they are no longer needed for bending resistance.
At locations where yielding of longitudinal reinforcement is likely to occur as a result of lateral displacements, development lengths of longitudinal reinforcement shall be 1.25 times the values calculated for fy in tension
This process can be followed for the rest of the longitudinal reinforcements.
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This post is written by AminNajafgholizadeh