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Dont mesh membranes or you will get errors...Just leave them as it is (default mesh option for membranes). And do meshing only for shell elements. and makes sure you have maximum of four edges for either membrane or shell

Sir Umar, why partially fixed is not the right word to say? If we model a very flexible member to a member of infinite stiffness, that infinite stiff member will provide enough stiffness as close to that of a fixed connection.

You do not need to model 2 columns at one grid. You can use a built up or tapered custom section to model the equivalent stiffness. That would work!

use triangular or rectangular mesh and do it manually.

It means you need to increase, or decrease, reinforcement in beam, or column. But, You have to have a reinforcement which satisfies your elastic elastic analysis. You can't reduce the reinforcement, below the level required for resisting factored combos, to satisfy this provision. You need to add more, or reduce. But reduced moment capacity of that member should satisfy the forces of design combos. If you have provided more reinforcement than the requirement of elastic analysis, you will need to check the shear corresponding to probable moment strengths. Increase in moment capacity of member will lead to an increase in seismic-shear. I hope, I have answered your query.

It is good to see detailed discussion on a topic, and a very good way a modelling passive resistance of soil has been presented. However, I have reservations on some comments. In this case the support condition hinge (i.e zero horizontal displacement) cannot be assumed for basement walls at slab levels as diaphragms are undergoing considerable lateral displacement.Therefore, in order to evaluate flexure in basement walls it is advised to model a line element along the height of wall (meshed at wall area edges) to compute flexure in basement walls rather than using theoretical model of prop canti-lever assumption that seems to be invalid due to diaphragm's horizontal movement . Assuming the diaphragm as rigid, there will be negligible in plane displacement of diaphragm with respect to its supports,which are vertical elements of lateral load resisting system, at particular story-level. Lateral displacement of diaphragms is with respect to story-levels. Hence, the support provided by slab to basement wall, at a particular story-level, can be considered as hinge as it will resist translational movement due to in-plane stiffness of rigid diaphragm. Why would someone be worried about out of plane displacement of basement wall, when it will be proportioned for out of plane forces( earth pressure). As it has been correctly been pointed out that drift limits in building codes are intended to control NON/STRUCTURAL damage, and to limit secondary forces due to P-delta effect. The point is: one should be worried about drift of frame (in/plane relative displacement), rather than the out of plane displacement of basement wall.