Reinforced Concrete Design / module 3
Analysis of reinforced-concrete structures
Idealisation, effective spans, load patterns and moment redistribution — the decisions made before any section is designed, and the ones that quietly govern the answer.
What you will understand
- Measure a span correctly, and say why the choice matters
- Enumerate the load patterns that must be examined, and what each maximises
- Take a support moment at the face rather than the centreline, and justify it
- Explain moment redistribution as a trade of moment for rotation
- Apply the x/d restriction that redistribution imposes
- Judge when the simplified coefficients may and may not be used
Lessons
Idealisation, spans and load patterns
The decisions made before the analysis starts, which affect the answer more than the analysis does.
Moment redistribution
Moving moment away from a congested support — and the rotation you must pay for it.
Module summary
- Effective span is the clear span plus a small addition at each end — not centre to centre
- A moment overstated by a wrong span is overstated by the SQUARE of the error
- Alternate spans loaded maximises span moments; adjacent spans maximises the support between them
- A monolithic support lets the moment be taken at its face, which is a real saving
- Redistribution buys moment capacity with rotation capacity — and the code prices it in x/d
- Reducing a support moment RAISES the span moment: the free bending moment is unchanged
- The simplified coefficients are a convenience with conditions, not a general method
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Practice
A 300 mm deep beam frames into 500 mm columns with a clear span of 6.2 m. What is the effective span, in m?
Check yourself
Why must the span moment be increased when a support moment is redistributed down?
Practice
A support moment of 240 kNm is redistributed by 15%. What is the redistributed moment, in kNm?
Practice
A 350 kN reaction acts on a 450 mm wide monolithic column. By how much may the centreline support moment be reduced, in kNm?
Check yourself
The simplified moment coefficients for continuous beams may be used when:
Practice
A beam is designed with x/d = 0.28 at a support. What is the maximum redistribution permitted, expressed as a percentage?