Is threaded rod as strong as a bolt? Stress area, grade and where it differs

Buyers often ask whether all-thread rod can replace a bolt. In straight tension the answer is usually yes, for the same size and grade. Elsewhere it depends on how the joint is loaded.

What "strength" means on a fastener

• Tensile strength: the stress at which the fastener breaks. The minimum breaking load is the minimum tensile strength multiplied by the stress area.
• Proof load: the load the fastener must carry without permanent stretch. Under ISO 898-1 it is the proof stress multiplied by the stress area.
• Stress area: the effective cross-section of the threaded part. ISO 898-1 gives it as As = π/4 × (d − 0.9382 P)², where d is the nominal diameter and P the pitch.

Why rod and bolt carry the same tensile load

A bolt in tension is limited by its thread, not its shank, because the stress area is smaller than the shank. Threaded rod simply has that stress area along its whole length. ISO 898-1 applies the same requirements to bolts, screws and studs, and treats a fully threaded stud as a full-size fastener. So an M12 class 8.8 rod and an M12 class 8.8 bolt have the same specified minimum loads.

Worked example: M12 coarse thread (calculated)

M12 has a 1.75 mm pitch, so the stress area is 84.3 mm², against 113.1 mm² for a plain 12 mm shank. Using the ISO 898-1 proof stress and minimum tensile strength for each class:

• Class 4.8: proof load 26.1 kN, minimum breaking load 35.4 kN
• Class 8.8: proof load 48.9 kN, minimum breaking load 67.4 kN
• Class 10.9: proof load 69.9 kN, minimum breaking load 87.6 kN

These are calculated from the standard's nominal values. They are not test results or design resistances, which apply safety factors under your design code.

The grade sets the strength

• Metric rod: order by ISO 898-1 property class. Class 4.8 is low-carbon steel; 8.8 and 10.9 are quenched and tempered.
• ASTM A307 grade A: bolts, studs and threaded rod for general use, minimum tensile strength 60 ksi.
• ASTM A193 grade B7: alloy steel bolting for high-temperature or high-pressure service, 125 ksi minimum tensile and 105 ksi minimum yield up to 2½ in.
• ASTM F1554: anchor rods for concrete foundations. The grade number is the minimum yield in ksi; grades 36, 55 and 105 have minimum tensile strengths of 58, 75 and 125 ksi.

Where rod is not a drop-in substitute

1. A nut used as a head. Portland Bolt, a US anchor bolt maker, reports that in its own tests rod with a nut developed up to 12% less strength than headed bolts from the same steel lot, often breaking at the nut. It advises getting the engineer of record's approval before substituting.
2. Shear through the threads. Rod always has threads in the shear plane. Design rules such as AISC 360 give a lower shear strength when threads are not excluded from the shear plane.
3. Bending and angled loads. Portland Bolt notes that headed bolts perform better under angled or moment loads, so a substitution may need redesign.
4. The nut. Match the nut to the rod: under ISO 898-2, class 8 nuts for class 8.8, class 10 for 10.9. Portland Bolt pairs A193 B7 with A194 2H heavy hex nuts, and A307 grade A with A563 grade A nuts.

Writing the order line

Threaded rod DIN 976-1 – M12 × 1000 – 8.8 – zinc plated, with ISO 4032 nuts class 8

For inch rod, name the ASTM specification and grade, the thread, the length and the nut, for example A193 B7 rod with A194 2H nuts.

What to send

1. Size, length and property class or ASTM grade.
2. How the rod is loaded: tension, shear or bending, and whether it replaces a headed bolt.
3. Finish, and the nuts and washers needed.
4. Quantity and the inspection document you need with the shipment.