Background Knowledge for Definition of Effective Slab Width Used for T-beams

Tips & Tricks

When you define the effective slab width of floor beams, the following three options are available for selection in RFEM (L = member length):

  • L/6
  • L/8
  • effective width per side

Factor 1/6 can be derived from the rules valid for reinforced concrete construction according to EN 1992‑1‑1 or DIN 1045‑1. The effective width beff,i for each side must be determined as follows:

beff,i = 0.2 bi + 0.1 l0 < 0.2 l0 → max. beff,i = 0.2 l0

The equivalent span length l0 (distance of zero point of moments) depends on the structural system and is

  • the span length L for a single‑span beam,
  • 0.85 L for a final span of a continuous beam,
  • 0.7 L for a middle span of a continuous beam.

This results in a maximum effective width for a final span of a continuous beam with
max. beff,i = 0.2 l0 = 0.2 ∙ 0.85 L = circa L/6

Factor 1/8 is based on the rules for determining the effective width bei of chord parts on both sides of the girder web in composite construction according to EN 1994‑1‑1, Section (5). We calculate bei in composite construction as follows:

bei = l0/8

Provided that the member length L corresponds to the span length of the member, L/8 is to be used for one‑span composite beams (l0 = L).


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