Seismic bands and vertical reinforcement in masonry

Horizontal seismic bands

A band is a continuous horizontal beam (RC or timber) running through all load-bearing walls at one level, forming a ring.

Purposes

• Tie the walls together so the building acts as a box.
• Reduce the out-of-plane span of the wall (supports it horizontally).
• Prevent cracks from growing by confining the masonry, and distribute the roof load.

Types and locations

1. Plinth band at the plinth level, also reduces differential settlement.
2. Lintel band at the lintel level of doors and windows (can double as lintel).
3. Roof / floor band below the roof or floor slab (not needed when an RC slab is cast over all walls).
4. Gable band along the top of gable walls.

Typical details (as per IS 4326 / NBC 202; check the code)

• RC band depth at least 75 mm, full wall thickness, concrete grade not less than M15 (1:2:4).
• At least 2 longitudinal bars of 8 mm diameter, for spans up to about 5 m, with 6 mm stirrups at about 150 mm; bars are lapped and anchored at the corners.
• Timber bands may be used in low-strength construction as two longitudinals connected by cross-members.

Vertical reinforcement

• Provided at corners, wall junctions and the sides of door and window openings.
• Bars (typically 10–12 mm diameter, one or two as per the number of storeys and zone) start in the foundation, pass through every band and extend into the roof band; they are embedded in concrete or mortar (in the hole made in the masonry).
• Purpose: increase flexural strength and ductility, tie walls at the corners, and prevent collapse of wall portions.

   roof band ================      <- tie everything
        |   |         |   |
        |   | window  |   |
        |   |         |   |  <- vertical bar at jamb
 lintel =======================
        | pier |     | pier |
 plinth band =================
        foundation (bars anchored)

Together, bands and vertical bars form a confined or reinforced masonry system that increases the ductility and avoids brittle collapse.