IS 800 states its plasticity-spread cap on bending capacity explicitly, as its own numbered clause — where AISC 360 or Eurocode 3 cap a compact section's moment only implicitly through their classification limits, IS 800's 1.2Z_ef_y/γ_m0 term is a truly distinct, code-specific feature important to recognising by name.

Every check above runs automatically inside the Structyze Steel Beam module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.

The Example We'll Use

• Span 6 m, unbraced length 0.5 m
• Section 400 × 140 × 16 × 10 mm (rolled)
• Superimposed dead 2 kN/m, live 5 kN/m
• Material Fe410 (E250), f_y = 250 MPa, f_u = 410 MPa
• Deflection limit L/325 (Table 6, general)

Working Through It

Section classification finds both flange (4.375 against 8.4ϵ) and web (36.8 against 84ϵ) plastic. Factored load uses IS 800's single combined dead+live factor (1.5), not separate dead/live factors the way AISC or Eurocode 3 apply: w_u = 1.5(7.628) ≈ 11.44 kN/m, giving M_u ≈ 51.49 kN·m.

Laterally-supported bending capacity: β_bZ_pf_y/γ_m0 ≈ 272.4 kN·m, capped at 1.2Z_ef_y/γ_m0 ≈ 282.0 kN·m — the plain plastic term governs here (272.4 kN·m), since this section's major-axis shape factor sits comfortably below the 1.2 threshold where the cap would bite instead.

LTB capacity, checked separately at the 500 mm unbraced length, gives an enormous M_cr ≈ 11,240 kN·m — χ_LT comes out above 1.0 and gets capped there, so the LTB-governed capacity collapses to the same 272.4 kN·m as the laterally-supported value.

What Governs, and Why

Bending narrowly governs at utilisation 0.189, just ahead of deflection (0.169), with shear carrying wide margin throughout. Because χ_LT saturates at its 1.0 cap for this short unbraced length, the laterally-supported and LTB checks collapse to the identical capacity — a beam braced this closely is, mechanically, no different from one with continuous lateral support.

Where Structyze Takes Over

Every check above runs automatically inside the Structyze Steel Beam module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.

The free trial covers a curated set of flagship modules including this one. For the full catalogue — RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.

Quick FAQ

When does IS 800's 1.2Z_e cap actually govern instead of the plain plastic term?

It bites whenever the shape factor Z_p/Z_e exceeds 1.2 — for a typical rolled I-section bent about its major axis, the shape factor usually sits a little below 1.2, so the cap rarely governs there (as here). Bending about the minor axis is a different story: the minor-axis shape factor is often comfortably above 1.2. That's why a gantry girder's minor-axis check, elsewhere in this series, sees the cap become the binding term instead.

Why does IS 800 use one combined load factor instead of separate dead/live factors?

IS 800 Table 4 specifies w_u = 1.5(dead+live) as a single combined factor for this basic load combination — a actually different convention from AISC 360's 1.2D+1.6L or Eurocode's 1.35G+1.5Q, which apply different factors to dead and live load separately. All three approaches are code-legitimate; they simply weight the two load types differently.

Final Thoughts

The explicit 1.2Z_e cap is IS 800's own clearly-named safeguard against over-crediting plastic-zone spread — important to recognising by name rather than assuming every code handles compact-section bending capacity the same implicit way.