Engineering & Technology · Civil Structures & Structural Engineering

Concrete, steel, timber, masonry and composite construction

A construction material contributes through a member’s shape, loading direction, connections and environment. A high elastic modulus alone does not establish strength or durability. Combining materials also creates interfaces where force must transfer if the parts are to act together.

Use Statics & Solid Mechanics, Materials Selection & Degradation and Measurement, Units & Engineering Evidence. Floor pressures, member properties, oscillator inputs and remaining areas are hypothetical teaching assumptions. No site-specific hazards, current code values, construction permissions or qualified structural designs are supplied.

Back to the chapter

After reading, you can

Connect material behaviour, interfaces and compatible deformation without treating a material name as a structural performance guarantee.

Read material behaviour in the assembly

Concrete can resist substantial compression but commonly develops tensile cracking under structural loading. Reinforcement provides a tensile load path through bond and anchorage. Bonded reinforcing steel can share load before concrete cracks; cracking changes stiffness and force distribution rather than magically activating the steel for the first time. Actual behaviour depends on geometry, age and material condition.

Structural steel can carry tension and compression, but a slender compressed member may buckle before reaching its material strength. Yielding can provide ductile deformation in suitable details; connection fracture or local instability can instead limit the assembly. Elastic modulus, yield strength and toughness describe different properties. A nominal material grade does not prove the performance of a welded or bolted joint.

Account for direction, joints and moisture

Timber properties depend strongly on grain direction and defects. Engineered wood arranges smaller pieces to obtain useful larger components, while joints and moisture remain consequential. Moisture changes can cause swelling, shrinkage and distortion that differ with direction. These movements can affect fit and connections even when a simple vertical force balance still closes.

Masonry consists of units, mortar and interfaces. Compression-dominated action is often useful, while tension, shear transfer and out-of-plane stability require explicit attention to the actual reinforced or unreinforced system. Do not assign zero tensile resistance to every constituent as a universal fact. Material behaviour and the effective behaviour of a jointed wall are different levels of description.

For equal 1000 mm lengths and ideal common strain, component A has supplied E=200000 N/mm² and area 100 mm²; B has E=30000 N/mm² and area 900 mm². Axial rigidities EA are 20 and 27 MN, so stiffnesses are 20 and 27 kN/mm. Force lines are 20δ, 27δ and total 47δ kN with δ in millimetres. At total 40 kN, δ=40/47 mm and strain=0.0008511. These hypothetical elastic components are not a qualified steel/concrete member; strengths, slip, creep, cracking and buckling are untested.

Use compatibility to explain composite action

Composite construction uses different materials or components together so that their interaction contributes to structural behaviour. Bond, connectors or other interfaces must transfer the forces demanded by the model. If two equal-length axial components have the same shortening δ, they have the same strain magnitude ε=δ/L. In an assumed linear range, force is F=EAε, with E the elastic modulus and A area.

Equal strain therefore does not imply equal stress or force: stress is Eε and force also includes area. Slip, cracking, creep, shrinkage and temperature differences can change the result. Strength checks require separate evidence. Comparing embodied impacts likewise needs equivalent structural service, lifespan and maintenance assumptions rather than a universal ranking of material families.

A larger softer component can carry more force

Assume two perfectly compatible, uniform, 1000 mm long components under 40 kN total compression. Supply A with E=200000 N/mm² and area 100 mm², and B with E=30000 N/mm² and area 900 mm². Their axial rigidities are 20 and 27 MN. Force balance gives ε=40000/47000000=0.000851064, so shortening is 0.851064 mm.

A carries 17.0213 kN and B 22.9787 kN. Corresponding stress magnitudes are 170.213 and 25.532 MPa. B carries more force despite its lower modulus because its area is larger. The plot shows both component force lines and their sum. These are supplied hypothetical elastic properties; no cracking, yielding, bond or buckling limit has been verified.

Try the reasoning

Do the two components have equal stress because their strain is equal?
Show the answer and reasoning
No. Their moduli differ, so stress magnitudes differ by 200000/30000, about 6.67.
What changes if the components can slip freely and are not loaded through a common displacement constraint?
Show the answer and reasoning
The common-strain assumption is lost. Force introduction and interface conditions must determine a different model.
Does successful force sharing prove this composite member can carry 40 kN?
Show the answer and reasoning
No. The calculation assumes elastic sharing. Material strengths, interfaces, stability and other limit states remain unchecked.

What to carry forward

Explain what each material and interface contributes, then test the assumed interaction against the relevant behaviour.

References

Teaching resources and research supporting this lesson’s explanations and examples.