Uniform Distributed Load (UDL)

Uniform Distributed Load (UDL) is a load spread evenly along the length of a beam or over an area of a slab, expressed as kN/m (line UDL) or kN/m² (area UDL). It represents idealisations of self-weight, live load, and many superimposed dead loads in structural design. Per IS 875 Part 2:1987, occupancy live loads are tabulated as area UDLs (e.g., 2 kN/m² for residential, 4 kN/m² for office corridors), and these are the most common load type structural engineers handle.

For a simply-supported beam of span L carrying UDL w (kN/m), the maximum bending moment at mid-span is M = wL²/8 and the maximum shear at support is V = wL/2. The deflection at mid-span is δ = 5wL⁴/(384EI). For a continuous beam with same UDL across multiple equal spans, end-span moment is approximately wL²/10 and interior-span moment wL²/14 — substantially less than the simply-supported case. These standard formulas, found in any structural handbook including IS 456 SP-24, are the engineer's first-pass sizing tools and should be memorised.

Real loads are rarely perfectly uniform — partition walls produce nearly-line loads, equipment produces concentrated loads, and live loads vary with usage pattern. Per IS 456 Cl. 22.6, the engineer must apply live load in the most adverse pattern (alternate spans loaded for maximum positive moment, adjacent spans loaded for maximum negative moment) — software does this automatically via pattern-loading. For preliminary hand calculation, treating live load as full-UDL on all spans is conservative and usually within 5-10% of the rigorous pattern-loaded result, adequate for sizing.