Shear strength of basement walls 1

A shear wall with high compression force is in a beam-column situation, so you will need to provide vertical reinforcement to resist the rotation produced by the resultant couple, and horizontal bars/ties to resist shear.

1. According to the code you need to ensure the same reinforcement in the 1st basement level as you have in the 1st storey above the ground. The reason for this, as I see it is the fact that the connection between those two levels is the most critical and yielding may go a bit under. Also, you're not allowed to splice the bars inside the critical area. Long story short, everything between "Base" and "GF" is a critical area (or maybe even up to F1, I actually think that for 7 storeys it's 2 storeys high, check it in the code. 2 storeys up = 2 storeys below, 1 storey up = 1 storey below). This means that the red area alone should be able to sustain the shear on its own, right (if it was good in the 1st storey it'll be good below)?

2. No, the code clearly states that concrete contribution may not be considered. It's highly variable and also reduces significantly with inelastic deformations.

Another thing, I don't like to mess with shear, it's fragile. If you're going to overdesign, do it for shear, foundations and diaphragms. You have 2 out of 3, so I'd go with a safer design, no doubt.

Just my opinion, hope it helps.

EDIT: However, thinking about it (not considering the code), I think you could get away with using some of the horizontal reinforcement, BUT I'd design to the full elastic force in that case (without any q-factor). Also, this reminds me, I hope you increased your shear force accordingly (1,5 for DCM) and don't go too close to VRd,max.

OP said:

It seems unreasonable to take into account only vertical reinforcement, neglecting the contribution of horizontal bars for this type of walls.

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I disagree. Per the sketches below, which I find more salient turned sideways, I suspect that shear cracks must be considered for which the horizontal reinforcing would be ineffective.

OP said:

2. Would it be reasonable to take into account the contribution of concrete (Vc) to the shear strength, due to the fact that the plastic hinges should develop above these walls?

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I don't know the eurocodes well so I can't speak the rules in your area of practice. In many locales it would be deemed acceptable to include the concrete contribution to shear strength for the base structure so long as the plastic hinge(s) were designed to occur above the base structure and the base structure was capacity designed for the forces imposed upon it by the plastic hinge(s).

Jack Moehle's seismic concrete books is one of the few texts that deals with shear walls over basement walls. I've included a few salient snippets from that publication below.