Building Design and Construction

Glued Fastenings

Glued joints are generally made between two pieces of wood where the grain directions are parallel (as between the laminations of a beam or arch). Or such joints may be between solid-sawn or laminated timber and plywood, where the face grain of the plywood is either parallel or at right angles to the grain direction of the timber.
It is only in special cases that lumber may be glued with the grain direction of adjacent pieces at an angle. When the angle is large, dimensional changes caused by variations in wood moisture content set up large stresses in the glued joint.
Consequently, the strength of the joint may be considerably reduced over a period of time. Exact data are not available, however, on the magnitude of this expected strength reduction.
In joints connected with plywood gusset plates, this shrinkage differential is minimized, because plywood swells and shrinks much less than does solid wood.
Glued joints can be made between end-grain surfaces. They are seldom strong enough, however, to meet the requirements of even ordinary service. Seldom is it possible to develop more than 25% of the tensile strength of the wood in such butt joints. It is for this reason that plane sloping scarfs of relatively flat slope or finger  joints with thin tips and flat slope on the sides of the individual fingers are recommended to develop a high proportion of the strength of the wood.

Joints of end grain to side grain are also difficult to glue properly. When subjected to severe stresses as a result of unequal dimensional changes in the members due to changes in moisture content, joints suffer from severely reduced strength.
For the preceding reasons, joints between end-grain surfaces and between endgrain and side-grain surfaces should not be used if the joints are expected to carry load.
For joints made with wood of different species, the allowable shear stress for parallel-grain bonding is equal to the allowable shear stress parallel to the grain for the weaker species in the joint. This assumes uniform stress distribution in the joint.
When grain direction is not parallel, the allowable shear stress on the glued area between the two pieces may be estimated from Eq. (10.32).
(See K. F. Faherty and T. G. Williamson, ‘‘Wood Engineering and Construction Handbook,’’ McGraw-Hill Publishing Company, New York, for additional information.)

joints with thin tips and flat slope on the sides of the individual fingers are recommended to develop a high proportion of the strength of the wood.
Joints of end grain to side grain are also difficult to glue properly. When subjected to severe stresses as a result of unequal dimensional changes in the members due to changes in moisture content, joints suffer from severely reduced strength.
For the preceding reasons, joints between end-grain surfaces and between endgrain  and side-grain surfaces should not be used if the joints are expected to carry load.
For joints made with wood of different species, the allowable shear stress for parallel-grain bonding is equal to the allowable shear stress parallel to the grain for the weaker species in the joint. This assumes uniform stress distribution in the joint.
When grain direction is not parallel, the allowable shear stress on the glued area between the two pieces may be estimated from Eq. (10.32).
(See K. F. Faherty and T. G. Williamson, ‘‘Wood Engineering and Construction Handbook,’’ McGraw-Hill Publishing Company, New York, for additional information.)

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