117 Saint-Venant’s Principle

Local effects of loads on structures have negligible global effects.

• Proposed by French mathematician and engineer Adhémar Barré de Saint-Venant.

• Load effects at one point of concentration on a structure become negligible a short distance away from that point. For example, overtightening a bolt deforms just the region that is near the hole.

• Locate loads closer than 3–5 characteristic lengths to merge load effects. For example, spacing bolts less than 3–5 bolt diameters apart creates a weld-like join. Separate loads more than 3–5 characteristic lengths to keep load effects isolated.

• Secure structural and mechanical systems by controlling 3–5 characteristic aspects of a system. For example, the anchored section of a cantilevered arm should be 3–5 times the length of the cantilever.

See Also Factor of Safety • Redundancy • Satisficing

Bolts spaced within 3-5 bolt diameters apart (top) have overlapping stress cones, forming a weld-like connection. Bolts spaced farther apart (bottom) have negligible impact on one another. Both effects are due to Saint-Venant’s principle.