Reinforced Concrete Design / module 10
Columns and structural walls
Axial load and moment together, the interaction diagram that governs them, and what happens when the column is slender enough to notice its own deflection.
What you will understand
- Explain why axial load and moment interact rather than adding
- Read an M–N interaction diagram, and find the balance point on it
- Say why a modest axial compression increases the moment capacity
- Distinguish the ductile and brittle sides of the diagram
- Decide whether a column is short or slender, and why the answer depends on load
- Calculate a second-order moment by the nominal-curvature method
Lessons
Axial load and moment together
Why the two cannot be checked separately, and what the interaction diagram is really showing.
Slenderness and second-order effects
When a column starts to notice its own deflection — and why that depends on how hard it is being pushed.
Module summary
- N and M interact: the moment capacity depends on the axial load present
- A modest compression INCREASES moment capacity by deepening the compression zone
- The balance point is the peak, and the boundary between ductile and brittle failure
- Above the balance point, more axial load reduces moment capacity sharply
- Slenderness is not a property of the column alone — it depends on the axial load
- λlim ∝ 1/√n, so a heavily loaded column goes slender much sooner
- A minimum eccentricity always applies: no column is ever perfectly loaded
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
Check yourself
Why does a modest axial compression increase a column section's moment capacity?
Practice
A 400 × 400 column with 2454 mm² of steel, C30/37 (fcd = 17.0, fyd = 434.8 N/mm²). What is its squash load, in kN?
Practice
A 300 mm square column has an effective length of 2.7 m. What is λ?
Check yourself
Two identical columns, one at the top storey and one at the bottom of a ten-storey building. Which statement is correct?
Practice
A 450 mm deep column carries 2000 kN with Ac = 180,000 mm² and fcd = 17.0 N/mm². Give λlim, taking A = 0.7, B = 1.1, C = 0.7.
Practice
What is the minimum design eccentricity for a 600 mm deep column, in mm?