How Buildings Stand Up
Every structure on earth must transfer loads — gravity, wind, occupants, snow — safely to the ground. The beam is the most fundamental structural element: a horizontal member that spans between supports and resists loads through internal shear forces and bending moments. Understanding how loads flow through a beam is the first step in structural engineering, whether you're designing a bridge or a bookshelf.
Reactions, Shear, and Moment
When a load pushes down on a beam, the supports push back with reaction forces. For a simply supported beam, these reactions follow a simple lever rule: the closer the load is to a support, the more force that support carries. Between the supports, the beam develops internal shear forces (vertical sliding tendency) and bending moments (rotational stress) that vary along its length. This simulation draws those diagrams in real time as you move the load.
Support Conditions Matter
How a beam is attached to its supports dramatically changes the force distribution. A simply supported beam is free to rotate at both ends — like a plank resting on two sawhorses. A cantilever is rigidly fixed at one end, concentrating all the moment there. A fixed-fixed beam clamps both ends, reducing the midspan moment but creating moments at the supports. Each condition suits different architectural situations.
From Diagrams to Design
Shear and moment diagrams are not abstract exercises — they directly determine the size of every beam in a building. The maximum bending moment tells the engineer the minimum cross-section needed to prevent failure. The maximum shear force determines whether the beam needs web stiffeners. Every skyscraper, bridge, and house is designed by computing these diagrams for every load combination the structure might face.