Basics in Building Construction

Building Components And Types

Lesson 21 : Upper Floors

Ribbed Or Hollow Tiled Flooring

Concrete is incapable of resisting tension which is caused in the lower part of the thickness of the slab. This lower part does not partake in load bearing, and hence part of it can be replaced by hollow tiles so that weight of the slab is reduced. This results in a ribbed floor system, as shown in figure 6.

Unlike T- beam construction, the ribs of hollow tile construction are closely spaced. The clear spacing of ribs depends upon the size of hollow blocks available, but normally it should not exceed 50 cm. The width of ribs may vary between 6 to 10 cm. the span of ribs may be as much as 7 m. However, when the span exceeds 3m, lateral ribs of the same width as the main longitudinal ones are provided at intervals between 1 to 3 meters. In that case, longitudinal ribs are designed as continuous beams. Main reinforcement is provided at the bottom of the rib. To resist the support moment (negative) an additional bar is placed at the top of rib section. A minimum cover of 2.5 cm is provided. The depth of rib is calculated on the basis of bending moment as well as the cost ratio of steel and concrete. Depth of rib is usually kept as at least L/20 with free support and at least L/25 with fixed support, where L is the span of the ribs. (Figure6.)

Due to small span, the slab is normally not analyzed. Slab thickness of 4 to 5 cm is generally provided. To check its cracking and to distribute the load properly, shrinkage and temperature reinforcement is provided in the slab, in both the directions. Sometimes, a welded fabric is arranged approximately along the middle of the thickness of the slab.

Ribbed or hollow – tiled floors have the following advantages:

• They are light in weight, and hence the supporting system has to be designed for comparatively lighter loads.
• They provide better thermal insulation.
• They have better sound – proofing qualities.
• They have better fire resistance.
• Electrical, plumbing and other service installations can be conveniently installed through it, without affecting its appearance.