Basic Connections
Lap Joints - Double Shear
The videos below show how a lap joint acts in a situation of Double Shear. When the joint is pulled apart due to tensile forces acting on the primary member, the joint can fail in one of two ways. Either the bolts can shear in two planes (indicated where they glow red during the animation) TOP VIDEO, or the plate material can pull through if the amount of steel between the bolts and the edge of the plates is insufficient to resist the tensile forces BOTTOM VIDEO. Resisting this sort of pull through shear will require that there is adequate thickness of plate material as well as distance from the edge of the plate. In this sort of lap joint, although the numbers of plates on either side of the bolt are different, the line of action of the force runs through the middle.
It is possible to further reinforce the connection or distribute the load by varying the connection through the addition of more plates or more bolts. The addition of plates increases the number of shear planes. This in turn engages more area of plate thickness at each intersection of bolt and plate.
Typical instances where you will find variations of this type of joint would be in a pin connection.
In order to prevent the joint from rotating, as in a typical hinge connection, more bolts are added. These are distributed in rows so that there is sufficient plate area between the bolts and the end of the plate to prevent pull through forces. This also adds more shear area to resist the forces.
LAP JOINT IN DOUBLE SHEAR - SHEAR PLANES WHERE THE BOLT FAILS SHOWN IN RED.
LAP JOINT IN DOUBLE SHEAR - PLATE PULL OUT DUE TO TENSION LOADS SHOWN IN RED.
