What base shear actually represents, conceptually
A building's seismic base shear is the one number every other force in a static seismic analysis works back from — but it's easy to lose sight of what physical thing that number is actually standing in for.
Ground shaking doesn't apply a pressure or a static push to a building the way wind does — it accelerates the ground itself, and a building's own mass, connected to that moving ground, resists being accelerated along with it. That resistance to acceleration, inertia, is where seismic force actually comes from (F = m·a), generated at every level of the building wherever there's mass, not a force pushing in from outside the way wind pressure does. Base shear (Fb in EC8, V in ASCE 7) is the sum total of that inertial demand across the whole building, expressed as a single equivalent horizontal force acting at its base — the total lateral force the foundation and lateral system have to resist, even though physically it's really the accumulated effect of every floor's own mass trying to keep moving (or not moving) as the ground beneath it shifts.
Why it's expressed as a fraction of the building's own weight
Because seismic demand scales directly with mass, both codes express base shear as a design spectral acceleration — a fraction of gravity, g, read off the response spectrum at the building's own fundamental period — multiplied straight through by how much building there actually is: Fb = Sd(T1)·m·λ in EC8 (m being the building's total mass, per the standard's own notation), V = Cs·W in ASCE 7 (W being its total seismic weight), rather than computing an acceleration and a mass as two separate quantities by hand. This is worth sitting with, because it cuts against intuition built from gravity design: a heavier building doesn't automatically end up worse off from a seismic-force standpoint just because it carries more absolute force, since that demand scales together with roughly everything resisting it too — a genuinely different relationship than gravity load, which simply accumulates with no such offsetting effect.
A structure's own fundamental period — how fast it naturally wants to sway, driven mainly by height and lateral stiffness — decides which point on the response spectrum curve actually applies to it (see the separate entry on response spectra for what that curve represents). Seismic Base Shear computes this Fb/V directly for the code and ground/site parameters you choose, and that resulting value becomes the seismic (E) load case fed into Load Combination Generator's factored ULS combinations alongside gravity and wind loads (see the related tools below).