At what height do we splice the vertical bars in columns and shear walls is it at the middle of the element or at the bottom ( Slab level) and why ?
In theory, a splice behaves exactly like a single continuous bar, so there should be no effect as to where you actually do the splice.
However, in the real world it depends.
If you're dealing with a compression-only column or wall, then I'd suggest splicing near (but not at) the slab level.
One reason is simple ease of construction. For example, if you must splice between floors 2 and 3 of a building and you do so just above floor 2, then the construction will look like this:
- Pouring of column from floor 1 to 2 without the splicing rebar (but with protruding rebar awaiting the splice).
- Pouring of floor 2's slab
- Working over floor 2's slab, the workers now have easy access to the splicing area, without needing to use scaffolding or anything of the sort to reach it.
Obviously, the splicing shouldn't occur too close to the slab otherwise you might end up with a jumble of rebar: you have to fit twice the column reinforcement due to the splice and you have to deal with the anchorage of the slab's reinforcement. So make sure the splicing doesn't affect interfere with anything else.
Now, if you're dealing with a moment-resisting column, then the best splicing location is wherever the moment is minimal, which frequently is near the midspan.
If, for some odd reason it is near one of the slabs, remember to check for interferences.
An important exception is if you have to deal with earthquake loads. In this case, one should always try to splice column reinforcement at the midspan. This is because the inertial loads at the nodes are huge, and you don't want to rely on your splice in this delicate area.
I also found this discussion on the subject useful.