A beam is a horizontal structural member that carries loads across a span. Floor joists, roof rafters, and bridge girders are all beams. They work primarily in bending: the top of a loaded beam is in compression, the bottom in tension, and the middle resists shear.

Material matters. A timber beam is simple to cut and fasten but limited in span. Steel beams can carry heavier loads over longer distances. Reinforced concrete beams combine concrete's compressive strength with steel's tensile strength. Engineered wood products like glulam and LVL allow longer spans than solid sawn lumber.

Beam design is a balance of strength, stiffness, and cost. Deflection — how much a beam sags under load — often governs the size more than pure strength. A floor that feels bouncy may be structurally adequate but unpleasant to live on.

Connections are where beams meet columns, walls, or other beams. A weak connection can bring down an otherwise sound structure. Engineers detail these joints carefully, using plates, bolts, welds, and hangers to transfer forces safely.