Materials, Mechanics, and Structural Behavior
What the exam tests
Materials, mechanics, and structural behavior tests whether you can link material properties to structural response: stress and strain, elastic modulus, characteristic versus design strength, and the failure modes that govern concrete and steel elements.
Core rules and concepts
- Axial stress σ = P/A; axial strain ε = ΔL/L; within the elastic range E = σ/ε.
- Concrete class C25/30 denotes 25 MPa characteristic cylinder strength and 30 MPa characteristic cube strength; design values are lower after partial factors.
- Reinforcing steel commonly has a yield strength of 500 MPa; design checks compare applied stress with design resistance, not with the characteristic value directly.
- Deflection, crack width, and buckling (slenderness) can govern before strength does.
- Thermal expansion of steel is about 12 × 10⁻⁶ per °C; restraint converts temperature change into stress.
Worked example
A 20 mm diameter tie bar carries a 50 kN tensile force. Area A = πd²/4 = π(20)²/4 = 314 mm². Stress σ = 50,000 N / 314 mm² = 159 N/mm² = 159 MPa. With fy = 500 MPa, utilization is 159/500 = 0.32, so the bar remains elastic with a wide margin. The reasoning depends on converting kN to N and keeping area in mm² so the result is directly in MPa; using radius 20 mm instead of diameter would quadruple the area and understate stress fourfold.
Traps that cost marks
- Confusing characteristic strength with design strength.
- Using gross area where threads or holes reduce the net section.
- Mixing cube and cylinder strengths.
- Ignoring second-order or buckling effects in slender members.
- Reporting stress in kPa when MPa is expected.