Material Properties

Part of Structural Engineering

Understanding the mechanical behavior of stone, timber, masonry, iron, and concrete so structures can be designed to match material strengths to applied loads.

Why This Matters

Every structural decision depends on material properties. A stone that is strong in compression but weak in tension cannot be used as a lintel carrying bending loads unless you understand this and design accordingly. A timber beam strong along the grain but weak across it must be oriented correctly to carry loads efficiently. Iron that is strong in tension is ideal for chains and rods, but brittle cast iron will shatter under impact.

Material properties are not just numbers in a book — they are the fundamental rules governing whether a structure stands or fails. For a rebuilding civilization, these properties must be understood from first principles and measured directly from available materials, because the exact properties of locally quarried stone or locally cast iron will differ from any reference value.

Knowing material properties also teaches you the logic behind structural forms. Masonry is used in compression (arches, columns, walls), not in tension (beams spanning freely). Timber is used in bending (floor beams, roof rafters), where its high tensile strength along the grain provides excellent performance. Iron and steel are used in tension (chains, tie rods) and bending (beams, columns). Matching the right material to the right structural role is the foundation of efficient construction.

Stress and Strain

Stress: Force per unit area. When a force is applied to a structural member, the material inside the member resists by developing internal stress.

• Stress (σ or τ) = Force (F) / Area (A)
• Units: PSI (pounds per square inch) or MPa

Strain: The proportional deformation — how much the material shortens, elongates, or shears relative to its original dimension.

• Strain (ε) = Change in length / Original length (dimensionless)

Elastic vs plastic behavior: Up to the elastic limit (yield point), stress and strain are proportional — remove the load and the deformation disappears. Beyond the yield point, permanent deformation occurs. Materials designed for structural use should always be kept well below yield stress.

Modulus of elasticity (E): The ratio of stress to strain in the elastic range. A high modulus means the material is stiff — it deflects little under load. A low modulus means it is flexible.

• E = Stress / Strain

Stone and Masonry Properties

Behavior: Stone is strong in compression, weak in tension (typically 10–20 times stronger in compression than tension). Shear strength is intermediate. Masonry assemblies (stone plus mortar) have similar characteristics but may be weaker due to mortar joint quality.

Key properties:

Property	Granite	Limestone	Sandstone	Common brick
Compressive strength (PSI)	15,000–30,000	5,000–15,000	1,000–10,000	1,500–4,000
Tensile strength (PSI)	1,000–3,000	500–1,500	200–1,000	150–400
Modulus of elasticity (× 10⁶ PSI)	7–10	4–8	2–5	1–2
Density (lb/cu ft)	165–175	140–170	120–145	110–120

Testing masonry compressive strength: See Compression Strength article for test methods. For masonry assemblies, test prisms (stacked units with mortar) rather than individual units to capture the mortar’s contribution.

Anisotropy: Many stones are stronger in one direction than another due to bedding planes or grain. Limestone and sandstone should be laid in construction with bedding planes horizontal (the same orientation as they formed in nature). A stone laid with bedding planes vertical may delaminate under load.

Timber Properties

Behavior: Timber is strong along the grain in both tension and compression, and relatively weak across the grain. The ratio of along-grain to across-grain strength is typically 10:1 to 20:1. Bending resistance depends primarily on the along-grain tensile and compressive strength, which is why deep beams oriented to put the fibers along the span direction are so efficient.

Key properties:

Property	Oak	Douglas Fir	Pine	Elm
Bending strength (PSI)	7,000–12,000	7,500–12,000	5,500–9,000	6,000–8,000
Tensile strength along grain (PSI)	8,000–15,000	8,000–14,000	6,000–12,000	7,000–11,000
Compressive strength along grain (PSI)	5,000–8,000	5,000–9,000	4,000–7,000	4,500–6,000
Modulus of elasticity (× 10⁶ PSI)	1.5–2.2	1.6–2.0	1.1–1.8	1.1–1.6
Shear strength (PSI)	1,000–1,500	800–1,300	700–1,000	800–1,200

Moisture content effects: Timber strength decreases significantly with increasing moisture content. Green timber (just cut) may be 50% weaker than the same timber at 15% moisture content. Design with dry timber or apply a reduction factor (typically multiply design strength by 0.75 for green timber).

Defects: Knots reduce timber strength, particularly in the tension zone of a beam. A knot at the bottom of a floor beam at midspan (where bending tension is maximum) can reduce the beam’s capacity by 30–50%. Inspect timber for knot size and location; position large knots in the compression zone (top of beam) where they are less critical.

Cast Iron and Wrought Iron Properties

Cast iron: Hard, brittle, very strong in compression, weak in tension (ratio approximately 3:1 to 5:1). Fails without warning in tension — no plastic deformation, just sudden brittle fracture. Good for columns, compression chords, machine bases. Bad for chains, tie rods, or any element subjected to impact or tension.

Wrought iron: Tougher and more ductile than cast iron. Better in tension. Can be bent and formed without fracturing. The fibrous grain structure gives good tensile strength along the grain direction. Standard material for chains, tie rods, bolts, and structural tension members before steel became available.

Mild steel (once available): Superior to both in nearly every way — high strength, ductile, good in both tension and compression. Allow for 25–30% additional capacity compared to wrought iron in tension applications.

Properties:

Property	Cast iron	Wrought iron	Mild steel
Tensile strength (PSI)	15,000–30,000	35,000–50,000	50,000–70,000
Compressive strength (PSI)	80,000–120,000	35,000–50,000	50,000–70,000
Modulus of elasticity (× 10⁶ PSI)	14–17	27–29	29–30
Ductility (elongation at failure)	<1% (brittle)	15–25%	20–30%

Concrete Properties

Behavior: Similar to masonry — strong in compression, weak in tension (ratio approximately 10:1). Unlike stone, concrete can be formed in any shape and poured around reinforcement to improve tensile performance.

Key variables: Mix proportions (water:cement:aggregate ratio) and curing conditions determine strength. Typical values for a 1:2:4 (cement:sand:aggregate) mix cured 28 days:

• Compressive strength: 2,500–4,000 PSI
• Tensile strength: 250–400 PSI (roughly 10% of compressive)
• Modulus of elasticity: 2–3 × 10⁶ PSI

Curing effect: Concrete gains approximately 65% of its 28-day strength by 7 days, and continues gaining strength slowly for years. Do not apply full structural loads for at least 14–21 days after pouring.

Matching Materials to Structural Roles

Structural role	Best material	Avoid
Column (compression)	Stone, brick, concrete, timber, cast iron	Cast iron under impact or tension
Beam (bending)	Timber, wrought iron, steel	Stone, plain concrete, cast iron
Arch (compression)	Stone, brick, concrete	Timber (deflects too much)
Tension tie/rod	Wrought iron, steel	Cast iron, stone, brick
Chain/rope	Wrought iron, hemp, leather	Cast iron, wood
Foundation (bearing)	Concrete, stone	Timber (rots underground)