Sag Tie and Wind Bracing:

Purlin

Purlins are horizontal members spanning across top chord of trusses and support the roof cladding.

In that case of tiles and slates these are supported on secondary members called rafters which are laid over purlins.

The purlins are generally placed at the adjacent panel points of the top chord (or principal rafter), hence the distance between these points is also the 'spacing of the purlins'.

Sag Tie

A sag tie is a vertical member joining the apex of the truss to the mid-point of the bottom chord. It is given to decrease the deflection of the bottom chord member.

Sag Rods

These are round bar threaded at their ends (parallel to the roof slope) and secured to the purlin webs with nuts (often at their mid-points or one-third points of their span). This is used to decrease the stresses caused through biaxial bending of the purlins.

Wind Bracing

In case of roof trusses supported on steel columns, lateral bracing has to be given against horizontal forces due to wind or earthquake. These are known as wind bracings.

Bearing Plates

When a roof truss is supported on masonry (stone or brick) walls, the load is transferred to a suitable concrete base through steel plates which are connected to it by suitable anchor bolts. The size of the plate depends upon the support reaction and the allowable bearing pressure on the wall.