Tensile Strength
Part of Structural Engineering
How materials resist being pulled apart — the property that governs chain links, tension ties, rope loads, and the bottom chord of every beam and truss.
Why This Matters
When you pull on a chain, the links are in tension. When a bridge hangs from cables, the cables are in tension. When a roof beam sags under load, the bottom of the beam is stretched — it is in tension. The tensile strength of a material determines how hard it can be pulled before it snaps or yields permanently.
Understanding tensile strength is essential for a different reason than compressive strength. Masonry (stone, brick, concrete) is weak in tension — typically 5–15 times weaker in tension than in compression. This means masonry cannot be used as tension members. A chain made of stone would fail at a tiny fraction of the load a chain of equivalent weight in iron could carry. Recognizing which parts of a structure are in tension, and ensuring those parts are made from materials with adequate tensile strength, is a core structural principle.
Tensile strength also governs the design of mechanical connections — bolts, pins, hooks, and links. Every iron fitting that holds something together under pull force must be designed for the tensile forces it will experience, and tested against the tensile strength of its material.
Tensile Failure Modes
Necking and ductile fracture (metals): When a metal rod is pulled in tension, it first deforms elastically (returns to original length when load is removed). As stress increases beyond the yield point, plastic deformation begins. The rod necks — the cross-section shrinks at one location — and ultimately fractures at the neck. Ductile metals (wrought iron, mild steel) fail this way. The fracture is visible and slow; the material gives significant warning before final failure.
Brittle fracture (cast iron, stone, concrete): Brittle materials fail suddenly with no plastic deformation. The fracture occurs at the material’s tensile strength with no warning necking. Cast iron in tension can fail at stresses well below what a test of otherwise identical specimens might suggest — internal flaws or surface notches become stress concentration points that initiate fracture at much lower average stresses.
Fiber pullout (timber, rope): Timber and rope do not “break” as cleanly as metal in tension. Timber fails by fibers pulling out along the grain at the failure point — a brushy fracture rather than a clean break. Rope fibers slide past each other. The failure mode depends on how well the fibers are bonded and interlocked.
Tensile Strength Values
| Material | Tensile strength (PSI) | Notes |
|---|---|---|
| Wrought iron | 35,000–50,000 | Reliable, ductile |
| Mild steel | 50,000–70,000 | Strong, ductile |
| Cast iron | 15,000–30,000 | Brittle, unreliable in tension |
| Copper (annealed) | 25,000–35,000 | Ductile |
| Bronze | 30,000–60,000 | Varies with alloy |
| Oak (along grain) | 8,000–15,000 | Highly variable, knots reduce |
| Pine (along grain) | 6,000–12,000 | Variable |
| Hemp rope (per sq in cross-section) | 3,000–8,000 | Reduce by 50% for long-term use |
| Leather (good quality) | 2,000–6,000 | Variable |
| Stone/granite | 1,000–3,000 | Unreliable, do not design for |
| Limestone | 500–1,500 | Unreliable, do not design for |
| Common brick | 150–400 | Very low, effectively zero for design |
| Plain concrete | 250–400 | Very low, effectively zero for design |
Tensile Testing
You can measure tensile strength with minimal equipment. The principle: grip both ends of a sample and apply increasing force until it breaks.
Simple tensile test setup:
- Prepare a test specimen — for metal, a rod or strip of known cross-section; for timber, a smooth piece avoiding knots; for rope, use a full section
- Attach the specimen at both ends to a loading frame (a heavy timber or iron frame with gripping jaws)
- Apply load by hanging calibrated weights from the lower grip, or by using a hand-operated screw press with a load cell (a short, stiff spring whose compression you measure)
- Increase load until the specimen fails
- Tensile strength = failure load / original cross-sectional area
Gripping the specimen: Getting the specimen into the grip without it slipping or being damaged by the grip itself is the main technical challenge. For round rods: drill holes through thick iron blocks and tighten iron wedges around the rod. For flat strips: sandwich between serrated iron plates and bolt together. For rope: use a bowline knot termination or splice into a thimble.
Test multiple specimens: Any single specimen may be unrepresentative due to a hidden flaw. Test at least 5 specimens from the same material and use the average of the lower three (rather than the single minimum or the average) as your design tensile strength.
Tensile Member Design
Step 1: Identify the tensile force. From load analysis, determine the maximum tension force in the member (in pounds).
Step 2: Choose material. Select a ductile material — wrought iron, mild steel, or tough timber along the grain. Avoid brittle materials in tension.
Step 3: Calculate required area. Required area = Tensile force / Allowable tensile stress Allowable tensile stress = Material tensile strength / Safety factor (use 4–6 for chains and ties in public structures)
For a tension rod carrying 20,000 lb in a roof truss, using wrought iron (allowable stress = 40,000/5 = 8,000 PSI): Required area = 20,000 / 8,000 = 2.5 sq in → 1 3/4-inch diameter rod (area = 2.4 sq in — close enough with a slight margin)
Step 4: Check connections. The connection between the tension member and the rest of the structure is often more critical than the member itself.
Clevis pin connection: The rod end has a hole through it; a pin passes through the hole and through a bracket. The pin must resist shear (cutting) and the bracket must resist bearing (crushing at the pin contact).
Shear stress in pin = Force / (2 × pin cross-section area) [double shear] Required pin area = Force / (2 × allowable shear stress)
For 20,000 lb force, wrought iron pin, allowable shear = 6,000 PSI: Required area = 20,000 / (2 × 6,000) = 1.67 sq in → 1.5-inch diameter pin (area = 1.77 sq in)
Thread capacity for threaded ends: A threaded rod has a reduced cross-section at the root of the thread. Calculate using the root diameter (smaller than nominal), not the nominal rod diameter.
Tension in Masonry Structures
Although masonry cannot carry tension as a structural material, tension forces appear in masonry structures and must be handled:
Arch base tension: The base of an arch exerts horizontal thrust on the abutments. If this thrust is not contained, the abutment slides outward and the arch falls. Iron tie rods connecting the two abutment bases contain this horizontal thrust in tension, preventing outward movement without requiring massive abutments.
Dome hoop tension: The lower portion of a dome develops circumferential tension (trying to split the dome into petals). An iron chain or rod around the dome base at the point of maximum tension resists this.
Frame tie beams: In a timber-framed building where the roof rafters push the tops of the walls outward, a horizontal tie beam at the base of the rafters carries the outward thrust in tension. The tie beam must be properly connected to both rafters at each end.
Tension rods and chains in masonry buildings are invisible from outside but are often the critical members preventing collapse. When inspecting old masonry structures, look for iron clamps, tie rods, and wall plates (the iron bearing plates where tie rods terminate in the exterior wall face) — these are the tensile skeleton that holds the masonry together.