Reinforced Concrete Design
Core concepts
Master flexural design (singly and doubly reinforced sections), the equivalent rectangular stress block, tension- and compression-controlled behavior, and the strain limits that separate them. Understand shear design (concrete contribution plus stirrups), development length and anchorage, and serviceability checks (deflection, crack control). Know column behavior: tied vs. spiral, slenderness and moment magnification, and the interaction diagram.
Common traps
- Under- vs. over-reinforced: the balanced ratio is a boundary, not a design target. Questions test whether a section is tension-controlled (ductile) at the given reinforcement.
- φ factors: they depend on the tensile strain in the extreme steel - do not assume a single value.
- Effective depth d vs. total depth h - a classic distractor.
- Minimum reinforcement: the smaller of the cracking-moment-based value and a code-prescribed fraction of the gross section.
Worked rule of thumb
For a rectangular section, compute the reinforcement index and check the strain at the extreme tension steel. If the net tensile strain is at or above the tension-controlled threshold, use the high φ; below the compression-controlled threshold, use the low φ; interpolate between. A quick check: for typical Grade 60 steel, a reinforcement ratio near 0.01–0.015 usually lands in the tension-controlled zone for beams of normal proportions.
Always confirm that the assumed steel yields before the concrete crushes - otherwise the section is brittle and the design is invalid.