Moment Frame Analysis Calculator

The portal method is a classic approximate technique for analyzing how a moment frame resists lateral (sideways) load from wind or seismic forces, without solving the full statically-indeterminate structure. It rests on three assumptions taught in every structural analysis course: a point of zero moment (an "inflection point") occurs at the mid-height of every column and the mid-span of every beam, and the horizontal shear at any story is split among that story's columns so each interior column carries twice the shear of each exterior column -- because an interior column is shared between two bays and an exterior column belongs to only one. This calculator applies those three assumptions to the BASE story of a multi-story frame, the governing (highest-demand) location, because story shear in the portal method is cumulative: the shear a story's columns must resist equals the sum of every lateral force applied at that story and every story above it, not just the force at that one level. With a uniform Lateral Force per Story entered, that cumulative sum at the base equals Total Base Shear (lateral force x number of stories), which is what feeds the column shear, column moment, and beam moment outputs here. Column moment follows directly from the inflection-point assumption: with zero moment at column mid-height, the moment at the column's top or bottom equals its shear times half the story height (note that column moment, above, is reported for the EXTERIOR column).

Beam moment at a representative interior joint sums the INTERIOR-column moments framing in from the story below and the story above that joint -- the story below carries the full base-story cumulative shear, while the story above carries one story's less (since the base level's own force doesn't act above it); a single-story frame has no story above its roof beam, so only the below-joint term applies there. Beam shear at that same interior bay follows the standard companion result for a beam in double curvature: twice the beam moment divided by the bay width, which is why Bay Width feeds only that one output. The Estimated Lateral Drift figure applies the companion δ = Vh³/(12EI × number of columns) racking formula using an assumed structural-steel modulus and a fixed placeholder column moment of inertia, because this calculator takes no member-section input; it is illustrative only; a real drift check (and comparison to a code drift limit) needs the actual selected column's section properties. None of this replaces a full lateral analysis: real frames rarely have identical force at every story, and a licensed engineer's model accounts for that, along with second-order (P-delta) effects this approximate method ignores entirely.