Reinforced and unreinforced masonry; rat-trap bond

Unreinforced masonry (URM)

A masonry structure built of units and mortar only, with no steel bars in the walls. Its strength comes from the compressive strength of the masonry; its tensile and shear capacity is small. Load-bearing brick or stone buildings that have, at most, RCC bands (as in IS 1905 / NBC 109) are of this type.

Reinforced masonry (RM)

Masonry in which steel bars (vertical bars in the cores or pockets of blocks, horizontal bars in bed joints, or RCC bands and tie columns) are provided to carry tension, shear and flexure and to give ductility. It resists earthquakes and wind better and allows larger openings and taller walls.

Rat-trap bond

Bricks are laid on edge, with stretchers on the faces and headers across the wall at regular intervals, leaving a cavity inside the wall. The wall is usually 230 mm thick, with a cavity about 110 to 120 mm wide.

Plan, alternate courses        Elevation
1: |##|  |##|  |##|           ____ ____ ____ ____
   |##############|           |____|____|____|____|
   |##############|           |_|__|_|__|_|__|_|__|
2: |##############|           |____|____|____|____|
   |   cavity     |
   |##############|

Advantages

• Saves about 25 percent of bricks and 10 to 15 percent of mortar, so it is economical.
• Good thermal insulation and dampness resistance because of the cavity.
• Lighter wall, so a smaller load on the foundation.
• Pleasing face; plaster is not essential.