Continuous beams – criterion for treating spans as approximately equal For design and analysis simplifications, spans of a continuous beam are considered approximately equal if the longest span does not exceed the shortest span by more than:

Introduction / Context:
In continuous beam analysis, many approximate methods and code tables assume “approximately equal spans.” This assumption allows redistribution patterns and coefficient-based bending moments to be applied with acceptable accuracy.

Given Data / Assumptions:

• Multi-span continuous beam in building or bridge work.
• Uniform load patterns and typical end conditions.

Concept / Approach:
Design handbooks often define approximately equal spans when variation is limited to about one-tenth. Keeping span lengths within ±10% supports the use of simplified moment and shear coefficients and avoids large shifts in internal force distribution.

Step-by-Step Solution:
Define tolerance → longest ≤ shortest × 1.10.Apply to coefficient tables/coarse analysis → valid if within this band.

Verification / Alternative check:
When spans differ by more than about 10%, exact analysis shows non-uniform moment envelopes; approximate equal-span coefficients become less reliable.

Why Other Options Are Wrong:

• 5% is overly restrictive and not necessary in practice.
• 15–25% permit significant differences that invalidate “equal span” simplifications.

Common Pitfalls:
Using equal-span tables without checking span variation; this can underestimate peak negative moments over shorter spans or supports.

Final Answer:
10%