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### Wood Lap Joint Glue Strength Design Equations and Calculator

Civil Engineering and Design

Wood Lap Joint Glue Adhesive Strength Equations and Calculator

Wood adhesive joints play an important role in modern timber engineering. In order to add value to the raw material several highly engineered wood based products have been developed. Often these involve the use of adhesive joints. Typical adhesive are distributed over contact area, assembled and then pressure is applied while adhesive cures.

τA = SUS / ks

Joint shear stress equation:

τ = F / ( b · L )

Minimum overlap length of adhesive glue surface formula:

Lmin = F / ( b · τA )

Strength check of adhesive glue line:

τ ≤ τA

Optimum overlap length formula:

Anticipates that any breakage occurs in the joint itself, and not in the basic lumber material equation:

Lopt = max {Lmin, L1 }

where:

L1 = ( SUb · s ) / SUS

where:

SU = joint strength in shear of glue or adhesive [psi, MPa],
kS = shear safety coefficient factor (typically 2 or greater),
F = transferred force [lb, N],
b = width (minimum) of connected lumber [in, mm],
s = thickness of connected lumber (in, mm],
SUb = Tensile strength of basic wood / lumber material in axial loading [psi, MPa],
L = Length of overlap (in, mm).,
L1 = Minimum length of over lap on wood strength (in, mm), glue surface should to be stronger than wood.

Reference:

Kent's Machine Engineers Handbook, 12th Edition Design and Production, 1950 Colin Carmichael

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