Yesterday we covered the bolt itself — preload, clamping, friction. Today: the pattern. A joint with the right number of correctly-sized bolts can still fail catastrophically if those bolts are arranged poorly. Load distribution across a bolt group is rarely uniform, and the geometry of the pattern decides which bolt yields first.

Three loading modes you'll encounter:

• Direct shear — load passes through the centroid of the bolt group. Each bolt carries an equal share: F_bolt = F_total / n. Simple and friendly.
• Eccentric shear — load is offset from the centroid, creating a moment. Bolts farthest from the centroid carry the most load. This is where bracket connections fail.
• Tension from prying — bending moment on the joint pulls bolts on one side into tension while the opposite edge pivots. Pry forces can amplify bolt tension by 30-50%.

The eccentric load rule of thumb: for a force F applied at eccentricity e from a bolt group's centroid, the moment is M = F·e. The shear on each bolt from that moment is proportional to its distance r from the centroid: F_i = M·r_i / Σr². Add this vectorially to the direct shear share. The corner bolt almost always governs.

Real example — a wall-mounted shelf bracket: a 4-bolt rectangular pattern (200mm wide × 100mm tall) supports a 500 N load hanging 300mm out from the wall. Direct shear per bolt: 125 N. But the 150 N·m moment from the offset load tries to peel the top bolts out in tension. With r ≈ 112mm to each corner and Σr² ≈ 50,000 mm², the top bolts see roughly 335 N of pull-out tension on top of the shear. The bottom bolts get compression (the bracket pivots against the wall there). Designing all four bolts the same is fine; pretending they share load equally is how shelves fall down.

Practical pattern guidelines:

• Spread bolts wide in the direction of the moment — doubling the lever arm halves the bolt force.
• Edge distance ≥ 1.5× bolt diameter, spacing ≥ 3× diameter — too close to an edge and the material tears out before the bolt yields.
• Symmetric patterns simplify analysis and prevent surprises during installation (workers can't put it on backward).
• Avoid single-row patterns under moment loads — you have no leverage to resist rotation.
• Match pattern stiffness to joint stiffness — a flexible bracket on stiff bolts will pry; a rigid bracket distributes load more evenly.

Aerospace engineers obsess over bolt patterns because a wing-to-fuselage joint sees combined shear, tension, and moment cycling millions of times. Your shelf bracket isn't that critical, but the math is identical.