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Timber Connections – Steel

So there are really two types of joinery for heavy timber frames, traditional and steel.  There is a hybrid of the two so maybe that could make a third type but typically as soon as you start adding steel plates it’s considered a steel joint, even if it is a concealed plate.

The use of steel plates for timber joints is a fairly straight forward concept.  The force transferred from the timber (in compression or tension) to the steel plate by steel pins.  These pins can be bolts, lags, nails, etc.  The steel plates then carry the load along the plate to the next timber member.  The steel pins in this member then transfer the force back into the timber frame.

The plate configuration and the number of steel pin connectors all depend on the forces that need to be transferred from one member to the next.  The simplest configuration is a plate on one side of the member.  This is also the weakest version because the bolt is in single shear.  In single shear there are two members trying to slide past each other in opposite directions.  So the two members create a shear plane.  Sometime a simple connection is all that is needed and it will be cheaper.

Then you can also place a piece of steel in the middle of the timber member.  This is called a knife plate because it cuts into the beam.  This is a stronger connection because the bolts are in double shear.  In single shear there were two members sliding past each other.  In double shear there are three members that create two shear planes.  Knife plate connections are a concealed connection.  The bolts can be countersunk and plugged to hide the connection.  This is also a more expensive connection because more time is required in the fabrication of the timber to cut out the space for the steel.

The most common configuration for steel plate connection in trusses is having a steel plate on each side of the timber.  This is also a double shear connection but it is stronger than the knife plate because you have two surfaces of steel and one of wood where the knife plate is vice versa.

There are other ways to strengthen a steel connection.  One way is to make the steel plate thicker.  Larger diameter bolts can be used or shear plate can be added to the connection.  The exact configuration of the connection needs to be designed for each individual case.  Depending on the configuration and the loads at the connection a number of possibilities are available.

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