A slab is structural element whose thickness is small compared to its own length and width. Slabs are usually used in floor and roof construction. According to the way loads are transferred to supporting beams and columns, slabs are classified into two types; one-way and two-way.
When the ratio of the longer to the shorter side (L/ S) of the slab is at least equal to 2.0, it is called one-way slab, shown in Figure 8.1.a. Under the action of loads, it is deflected in the short direction only, in a cylindrical form. Therefore, main reinforcement is placed in the shorter direction, while the longer direction is provided with shrinkage reinforcement to limit cracking, as shown in Figure 8.1.b. When the slab is supported on two sides only, the load will be transferred to these sides regardless of its longer span to shorter span ratio, and it will be classified as one-way slab.
One-Way Ribbed Slabs
Since resistance of concrete in tension is too small compared with that in compression, concrete in the tension zone may be gathered in regularly spaced ribs cast monolithically with topping slab on top of these ribs. Hollow blocks made of lightweight concrete or other materials are arranged between the ribs or the voids between ribs are left out without any filling material. The use of these blocks makes it possible to have smooth ceiling which is often required for architectural or hygienic considerations and have good sound and temperature insulating properties besides reducing the dead load of the slab greatly. Ribbed slab floors are most economical for buildings such as hospitals, schools, hotels, and apartment construction where loads are somewhat small and the spans are relatively large.
There are two main types of ribbed floors (a) hollow block floors and (b) moulded floors as shown in Figure 8.5. Each type of floor consists of number of reinforced concrete ribs spanning in the short directions supporting a thin topping slab above. The hollow block floor is most suitable for small irregularly shaped floors as the hollow blocks can be easily fitted into the irregular shapes. On the other hand, for large symmetrically supported floors, the moulded type of floor is most suitable and economical. Steel or fiberglass moulds can be used for temporary formwork. Ribbed slabs can be used in one-way and two-way edge supported slabs. Either drop or hidden beams can be used to support these slabs depending on the span. If the ribs are provided in one direction only, the slab is classified as being one-way, regardless of the ratio of longer to shorter panel dimensions.
This Video lecture shows how to design a Ribbed slab using SAP2000
Duration: 35 Min