In engineering design, properly assessing the stress in a bolted joint is critical to ensuring safety and reliability. This guide walks you through a practical approach to calculating bolt joint stresses, step by step.
Step 1 — Bolt Pattern Geometry
Start by defining the locations of all bolts relative to the plate's reference point or centroid. Measure the x and y coordinates of each bolt, and note the number of bolts n.
Step 2 — Direct Tensile Load
If the total tensile load on the joint is Ft, and the load is evenly distributed, each bolt carries:
Ft,b = Ft / n
Step 3 — Overturning Moment
If there’s an applied moment M, it produces additional tension in bolts farther from the neutral axis. Calculate the moment-induced force in each bolt as:
Fm,b = (M × rb) / Σ(r²)
where rb is the distance from the bolt to the centroid and Σ(r²) is the sum of the squares of all bolt distances.
Step 4 — Shear
Shear is assumed to be shared equally between all bolts if the connection is symmetrical. Otherwise, resolve forces and moments appropriately.
Step 5 — Stress Calculation
To calculate tensile stress, use the bolt's tensile stress area As (not nominal diameter):
σ = F / As
Compare calculated stress to the allowable stress for the bolt material to ensure a sufficient factor of safety.
Pro Tip
Using tools like my Bolt Joint Calculator automates these calculations and accounts for factors of safety, eccentricities, and more.