Beam Load & Bending Moment Calculator
Calculate bending moments and maximum deflection for simply supported beams under point loads or uniform distributed loads. Used for lintels, floor joists, and structural planning.
Structural Beam Guide
Bending Moments and Shear Forces
A beam supporting a load experiences bending moments (tend to curve the beam) and shear forces (tend to slice through it). For a simply supported beam (resting on two supports) with a central point load W: maximum bending moment = WL/4 (at centre). Maximum shear force = W/2 (at the supports). For a uniformly distributed load (UDL) of w kN/m: maximum bending moment = wL²/8 (at centre). Maximum shear force = wL/2 (at supports). These are the most-used structural formulas in building design.
Lintels and Floor Joists
Common applications: lintels over door/window openings carry the weight of masonry above. A typical domestic lintel over a 1.2m window opening carries approximately 3-5 kN/m of load (depending on wall height above and roof). Standard steel lintels (Catnic, Birtley) have pre-tested load capacities — always check the manufacturer's span/load tables rather than calculating independently. Floor joists in C24 timber: a typical 50×200mm joist at 400mm centres over a 3.6m span can carry approximately 1
Deflection Limits
Structural codes limit beam deflection to protect finishes and serviceability. Typical limits: floor joists: L/360 of span (a 3.6m span → maximum 10mm deflection). Lintels and beams: L/250 for most applications. For a 4.8m span: limit = 4800/250 = 19.2mm. Deflection = 5wL⁴/(384EI) for UDL on simply supported beam, where E = Young's modulus, I = second moment of area. Steel has much higher stiffness (E = 200 GPa) than timber (E = 11 GPa), which is why steel can span further for the same depth.
When You Need a Structural Engineer
Always engage a structural engineer for: removing or modifying load-bearing walls, openings in excess of 1.2m in load-bearing walls, beams spanning more than 4m, any work near foundations, loft conversions that change the roof structure, any situation where the load path is unclear. This calculator provides educational guidance only — it does not replace structural engineering calculations. Building Regulations require structural calculations for many alterations. An RICS or ICE-registered struc
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