hokie, the Philippines is so poor, we don't develope our own codes. We just copy everything from the USA ACI. And in the ACI (see below illustration and text) it is stated that in seismic design, the shear and moment derived from factored load of 1.2 DL + 1.6 LL have to be multiply further (let's say 1.3) so the effect is like 1.3 (1.2 DL + 1.6 LL) to come up with probable moment and probable shear strength. Etabs used this. Since they all use etabs. They just follow the output.
5.3.1 Beam Design Shear
The beam design shear is determined using the capacity design
approach as outlined in Section 3.2. Figure 5-10 illustrates this
approach applied to a beam. A free body diagram of the beam is
isolated from the frame, and is loaded by factored gravity loads
(using the appropriate load combinations defined by ASCE 7)
as well as the moments and shears acting at the ends of the
beam. Assuming the beam is yielding in flexure, the beam end
moments are set equal to the probable moment strengths Mpr
described in Section 5.1. The design shears are then calculated
as the shears required to maintain moment equilibrium of the
free body (that is, summing moments about one end to obtain
the shear at the opposite end).
This approach is intended to result in a conservatively high
estimate of the design shears. For a typical beam in a special
moment frame, the resulting beam shears do not trend to zero
near mid-span, as they typically would in a gravity-only beam.
Instead, most beams in a special moment frame will have nonreversing
shear demand along their length. If the shear does
reverse along the span, it is likely that non-reversing beam
plastic hinges will occur (see Section 5.1).
