PLINTH BEAMS AND GRADE BEAMS

Two types of beams, viz., plinth beam and grade beam are constructed in walls for load distribution which are discussed below.

Plinth Beams

Plinth beams are constructed at plinth level in load-bearing wall for uniform load distribution and to even out settlement of buildings. They also allow to provide Damp Proof course at this level. Such beams are very much helpful to buildings in seismic areas. Similar types of beams are provided at lintel level and roof level. Through beams provided at lintel level and roof level are very much helpful for any alternations to be made in future. Further, they are also helpful in any underpinning work to be taken at a later date (Fig. 19.13).

Figure 19.13 Details of plinth beam

The minimum depth of plinth beams is 10–15 cm and should be provided to the full width of the wall above the plinth beam. The minimum reinforcement recommended is three numbers of 6 mm MS bars at top and bottom and longitudinal bars with 6 mm stirrups at 23 cm spacing.

Grade Beams

Grade beams are provided in between isolated footings. Thus under reamed piles and column on isolated footing are connected by grade beams and then the wall is constructed over it. These beams are stronger than the plinth beams. The recommended minimum depth of grade beam is 150 mm and minimum three numbers of 8 mm bars of Fe415 should be provided at top and bottom (Fig. 19.14).

Figure 19.14 Details of grade beam

SALIENT POINTS

1. An arch is a mechanical arrangement of wedge-shaped blocks of stones or bricks mutually supporting each other and in turn supported at the ends by piers or abutments.
2. Basically the stability of an arch depends on the friction developed between the wedge-shaped blocks and the cohesion of the mortar.
3. Stability of an arch is disturbed by one of the following reasons:(i) Crushing of arch materials(ii) Sliding of wedge-shaped blocks(iii) Rotation about an edge(iv) Differential settlement of supports.
4. Arches are classified based on number of centres.
5. One-centred arches are: Semi-circular, segmental, horse shoe and stilted.
6. Two-centred arches are: blunt or drop, equilateral and acute.
7. Materials used for arches are stone, brick and concrete.
8. Stone arches are ashlar masonry arch and rubble masonry arch.
9. Brick arches are: gaughed arches, axed or rough cut arches, rough brick arches and brick flat arches.
10. Methods of construction of arches are made under three operations, namely installation of centring, laying of arch and removal of centring.
11. A lintel is a horizontal structural member placed to span openings for doors, windows and corridors.
12. Lintels may be made out of several materials such as wood, stone, brick, reinforced brickwork, RCC or rolled steel sections embedded in cement concrete.
13. The purpose of providing a lintel is just the same as that of an arch. Lintels take the load of the wall above the openings and transfer to the walls.
14. Types of lintels are: wooden lintels, stone lintels, brick lintels, steel lintels, RCC lintels and Reinforced brick lintels.
15. Plinth beam are constructed at plinth level in load-bearing walls for uniform load distribution and to even out settlements.
16. Grade beams are provided in between isolated foundation.