Understanding One Way Slab ,structural action of one way slab

Slabs comprise flat or floor surfaces of a building. It is very important as a civil engineer to know some of basic of working in every structural element. We will discuss one-way slab today. Irrespective of the code you follow and minimum reinforcement implemented, it is essential to know why is the reinforcement provided in so and so pattern & where you have to provide.
First, we need to understand what makes a slab one-way. The criteria are
  • Longer span to shorter span ratio
  • Load transfers direction
That is if      lx/ly > 2  it is a one-way slab

Here ly = longer side length
         lx = shorter side length

As mentioned above,  load transferring direction also distinguishes one-way slab. In one-way slab, the load transfer happens only along one direction. This can be explained clearly from the figure below

Load Distribution and Laying Reinforcement in a One Way Slab

As shown in the figure the load is transferred in the direction perpendicular to the direction of support provided or it is parallel to the span ( center to center distance between supports). A slab that has only two opposite support can also be designated as the one-way slab. But slabs which are supported on four sides and ly/lx ratio is greater than 2 also comes under the category of one - way slab. When a one-way slab is supported on four sides, the load transfer is along the longer direction than shorter direction.


Fig. Plan view and 3D view of load transferring in a one-way slab supported in two longer directions(ly)

Now talking about reinforcement, one-way slab incorporates main reinforcement and distribution reinforcement. Main reinforcement in one way slab is provided parallel to the span (from the figure it is provided along AD). Main reinforcement is involved in load carrying. We also provide distribution reinforcement perpendicular to the main reinforcement. While arranging the reinforcement the main reinforcement comes below distribution reinforcement. This is to ensure that the main reinforcement occupies greater effective depth.

Fig. A typical one-way slab reinforcement details

The above reinforcement is laid based on Indian Standard codes and specifications.