ONE-WAY SLAB

One-Way Ribbed SlabsOne-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.

There are two main types of ribbed floors (a) hollow block floors and (b) moulded floors as shown in Figure. Each type of floor consists of number of reinforced concrete ribs spanning in the short directions supporting a thin topping slab above. 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.

(a )Hollow block floor; )b( moulded floor

Key Components

The ribbed slab consists of the following key components:

Key components

a. Topping slab:

b. Regularly spaced ribs:

Minimum dimensions:According to ACI Code 8.11.2, ribs are not to be less than 10 cm in width, and a depth of not more than 3.5 times the minimum web width. Clear spacing between ribs is not to exceed 75.0 cm as specified by ACI Code 8.11.3.Loads:The load on ribs consists of dead and live loads. The dead load includes own weight of the slab, weight of the surface finish, and equivalent partition load. The live load is dependant on the intended use of the building.

Arrangement of ribs: )a( ribs are arranged in the shorterdirection; )b( ribs are arranged in the longer direction

The designer has to make up his mind regarding the option he prefers.Shear strength:According to ACI Code 8.11.8, shear strength provided by rib concrete Vc may be taken 10 % greater than those for beams. It is permitted to increase shear strength using shear reinforcement or by widening the ends of ribs.Although shear reinforcement is not often required, it is recommended to use 4φ 6 mm stirrups per meter run,

Flexural strength:Ribs are designed as rectangular beams in the regions of negative moment at the supports and as T-shaped beams in the regions of positive moments between the supports. Effective flange width be is taken as half the distance between ribs, center-to-center.

c. Hollow blocks:

Hollow blocks are made of lightweight concrete or other lightweight materials. They are commercially available in standard sizes and shapes.The most common concrete hollow block sizes are 40 × 25 cm in plan and heights of 14, 17, 20, and 24 cm.

Minimum Thickness

To control deflection, ACI Code 9.5.2.1 specifies minimum thickness values for one-way ribbed slabs, shown in Table.

Minimum thickness of one-way ribbed slabs

Summary of One-way Ribbed Slab Design Procedure

Once design compressive strength of concrete and yield stress ofreinforcement are specified, the next steps are followed:1. The direction of ribs is chosen. 2. The overall slab thickness h is determined based on deflection control requirement. Also, thickness of topping slab t, rib width b, and hollow block size, if any, are to be determined based on code requirements.3. The factored load on each of the ribs is computed.4. The shear force and bending moment diagrams using the load evaluated in step 3 are drawn.5. The strength of web in shear is checked.6. Design positive and negative moment reinforcement. Clear distance between bars is to be checked to guarantee a free flow of concrete.7. Neat sketches showing arrangement of ribs and details of thereinforcement are to be prepared.

Example (3.3)Example (3.3)::

SolutionSolution: :

Ribbed slab

1 .The direction of ribs is chosen:

Ribs are arranged in the short direction as shown.

2 .The overall slab thickness h is determined:

3 .The factored load on each of the ribs is to be computed:

Load per rib, as shown in Figure 8.9.b:

Factored load per rib

4 .Critical shear forces and bending moments are determined:

5 .Check rib strength for beam shear:

6 .Design flexural reinforcement:

Section size at maximum positive moment

7. Neat sketches showing arrangement of ribs and details of thereinforcement are to be prepared:

Reinforcement details at section A-A

Example (8.4)Example (8.4)::

Plan of ribbed slab

SolutionSolution: :

1- The direction of ribs is chosen:Ribs are arranged in the short direction as shown in Figure. 2- The overall slab thickness h is determined:

3- The factored load on each of the ribs is to be computed:

Factored load per rib

4- Critical shear forces and bending moments are determined:Analyzing the strip using the BENARI structural software, one gets the shown shear force and bending moment diagrams.

Shearing force and bending moment diagrams

5 -Check rib width for beam shear:

Since critical shear section can be taken at distance d from faces of beam, the rib shear resistance will be considered adequate and the assumed web width will be kept unchanged.Though shear reinforcement is not required, 4φ 6 mm U-stirrups per meter run are to be used to carry the bottom flexural reinforcement.

6- Design flexural reinforcement:a. Positive moment reinforcement:Since factored positive moments create compression in the flange, the sections at their corresponding locations are to be designed as T-sections.Since the positive moment values are relatively small, the largest of the three values will be considered here and same reinforcement will be used at the two other locations.

Section at maximum positive moment

b. Negative moment reinforcement:Since factored negative moments create compression in the web, the sections at their corresponding locations are to be designed as rectangular sections.

c. Neat sketches showing arrangement of ribs and details of the reinforcement are to be prepared:

(continued( ;)e )Slab reinforcement; )f( sect ion A-A