How to use the Bolt and Weld Strength Calculator
- 1. Pick the jointBolts in tension, bolts in shear, fillet weld or butt weld.
- 2. Enter load and sizesLoad, number and size of bolts, or weld size and length.
- 3. Check itStress against allowable, joint capacity and the size or length needed.
Joint formulas
- Bolts in tension: σ = P / (n·As), where As is the tensile stress area (157 mm² for M16).
- Bolts in shear: τ = P / (n × planes × πd²/4). Bearing on the plate: σb = P / (n·d·t).
- Fillet weld: the throat is 0.707 × leg size s, and the throat shear is τ = P / (0.707·s·L).
- Butt weld: σ = P / (t·L), where t is the plate thickness.
The joint capacity is the smallest of these strengths. Allowable stresses depend on the material and the code, so they are inputs here.
Worked example and exam tips
A plate is joined by two 100 mm transverse fillet welds with a 10 mm leg. The allowable stress is 70 MPa. Capacity = 0.707 × 10 × 200 × 70 = 98.98 kN. If the load is 120 kN, the weld length needed is 120 000 / (0.707 × 10 × 70) = 242 mm, so about 121 mm per side plus 12.5 mm at each end for starting and stopping.
- Use the stress area As for threaded bolts, not the nominal diameter.
- In double shear each bolt is cut on two planes, so its shear capacity doubles.
- Some books design transverse fillet welds on tensile stress. The formula has the same form.
Frequently asked questions
Why is the throat 0.707 times the leg?
A 45° fillet weld fails across its smallest section, the throat. For equal legs s, that is s × cos 45° = 0.707 s.
What is the tensile stress area of a bolt?
An area between the root and pitch diameters that gives the real tensile strength of a threaded rod. ISO 898-1 lists it for each size.
Does the tool handle eccentric loads?
No. It assumes the load is shared equally by all bolts or along the whole weld. Eccentric joints need extra torsion or bending terms.
Is this calculator free?
Yes. You fill in a short form once to see your result, and the other Uncodemy tools then open without it. Every calculation runs in your browser and nothing you type is sent anywhere.