Movement joints and sliding/rotating bearings
Movement joints may require special connection details.
Purpose-made bearings are often used at movement/rotation structural joints. Nominal loads, particularly those which are temporary, can be carried by bolts in slotted holes.
Moderate loads (e.g. typical building beam reactions) can be taken on rubber bearings. These are rectangular pieces of solid or laminated rubber, typically 6 to 25mm thick, which can allow the supported beams to move (by shearing within the bearing depth) or rotate (by compressing or expanding in different parts of the bearing). The movements are limited by the thickness of the rubber, which cannot be too great or the bearing would compress too much under applied loads.
Large loads and large movements can be carried on PTFE (poly-tetro-fluoro-ethelyene) sliding bearings, and on roller/rocker bearings. PTFE is a synthetic material with a low coefficient of friction. Thin sheets of the material are used as a sliding surface over which stainless steel or anodised aluminium moves. Either flat sheets (for sliding bearings) or cylindrical/spherical contact surfaces (for rotational bearings) are used.
Rocker and roller bearings use steel-to-steel mating surfaces. In theory, line or point contact occurs between the parts of this type of bearing. It might therefore be supposed that the contact stresses must be infinite, and the contact zones must be squashed. In fact the material provides a complex system of elastic support to the contact points, and this type of bearing can take very large loads.
All bearings, and PTFE bearings in particular, require careful handling and protection to keep the contact surfaces free from debris which can damage the contact surfaces.
Where intimate contact between surfaces is required, machining may be used.
Machining is used to ensure intimate contact between surfaces which rely on such contact to transmit stresses (usually compressive stresses). The need for machining of contact surfaces is particularly likely if welding with a significant heat input has been carried out near to the joint.