Reading the diagrams

FrameLab gives you two views of the same numbers, and both are worth using.

1 — Diagrams drawn on the structure

Each value is plotted perpendicular to its own member, offset from the axis in proportion to its magnitude. This is how engineers sketch frames by hand and how frame results are presented in every structural report. Its great advantage is that it is independent of the direction in which you happened to walk along the bar.

The bending moment is drawn on the tension side. Read it like this: the diagram sits on the face of the member that is being stretched, so that is the face that gets the tension reinforcement, the face where a crack would open, and the face where a weld root is in the worst position.

2 — Diagrams against the developed length

The line charts unroll the bar into a straight line. They are better for reading numbers off, for locating the exact position of an extreme, and for comparing several quantities at the same cross-section — the shared cursor follows your mouse across all charts at once.

The classic shapes

These are the same rules as for a straight beam — because within one segment a bent bar is a straight beam. The relationships dM/ds = V and dV/ds = −q hold inside every segment and break only at the corners.

Sanity checks you should always do

• Global equilibrium. Add up the reaction cards: the vertical ones must equal the total downward load, the horizontal ones must cancel if you applied no horizontal load.
• Free ends. M and V must be zero at a free end, and M must be zero at a pin (unless you applied a moment there).
• Continuity of M at corners. See the previous chapter.
• Does the deformed shape look believable? The Deformed view is the fastest way to catch a support you meant to place somewhere else.