Support moment where two unequal spans (or unequally loaded spans) meet For design, the negative moment at the common support is generally taken as which value relative to the two span-based values obtained?

Introduction:
In continuous beams and slabs, standard coefficients or analysis yield different negative support moments when adjacent spans are unequal in length or have different loading. A practical design convention reconciles these to produce a single support moment for detailing.

Given Data / Assumptions:

• Two spans meet at an interior support.
• Spans may be unequal and/or carry different loads.
• Support moment values are computed for each adjacent span.

Concept / Approach:

Design practice commonly adopts the average of the two calculated support moments at the junction of unequal or unequally loaded spans. This approach balances distribution and avoids over-reliance on one span’s stiffness or loading in the detailing of reinforcement.

Step-by-Step Solution:

Verification / Alternative check:

Comparisons with rigorous frame analysis show that the average provides a reasonable, conservative basis for practical detailing over a range of common stiffness and load ratios.

Why Other Options Are Wrong:

Lesser value may under-reinforce; greater value may be overly conservative for economy; summation is not meaningful for a single section; “none of these” is unnecessary when a clear convention exists.

Common Pitfalls:

Failing to check redistribution limits, insufficient anchorage beyond the support, and neglecting torsion compatibility if edge or corner conditions exist.

Final Answer:

average of two values