Introduction to Flat Slabs
Image Source: Horizon Engineers
Reinforced concrete flat slabs are commonly used in construction as they provide a number of benefits to the designer including:
However, flat slabs have a lower stiffness in comparison to a beam-column floor plan which can lead to relatively large deflections. In addition to this, the shear capacity can also be reduced in particular around the column head where large shear forces can develop.
There are two main failure modes of flat slabs:
Slabs are designed to fail by flexural failure, the failure mode is ductile therefore giving relatively large deflections under excessive loading, also cracks will appear on the bottom surface before failure occurs. These signs allow the problem to be addressed before failure occurs.
Punching shear failure by comparison is a brittle failure mode when shear reinforcement is not added, meaning failure will occur before significant deflections take place, in addition to this any cracks that will develop before failure will propagate from the top surface. Since this surface is typically covered, it is unlikely that there will be sufficient warning available before failure occurs.
However, Thornsteinsson noted that it can be difficult to classify a failure mode to be an ideal representation of either flexural or punching shear failure and instead these modes often interact.
- Thin sections allowing for greater roof heights and lighter floors.
- Exposed ceilings
- Flexible column arrangements, this is more difficult to achieve for a beam-column design
- Fast and cheap construction using simple formwork.
However, flat slabs have a lower stiffness in comparison to a beam-column floor plan which can lead to relatively large deflections. In addition to this, the shear capacity can also be reduced in particular around the column head where large shear forces can develop.
There are two main failure modes of flat slabs:
- Flexural Failure
- Punching Shear Failure
Slabs are designed to fail by flexural failure, the failure mode is ductile therefore giving relatively large deflections under excessive loading, also cracks will appear on the bottom surface before failure occurs. These signs allow the problem to be addressed before failure occurs.
Punching shear failure by comparison is a brittle failure mode when shear reinforcement is not added, meaning failure will occur before significant deflections take place, in addition to this any cracks that will develop before failure will propagate from the top surface. Since this surface is typically covered, it is unlikely that there will be sufficient warning available before failure occurs.
However, Thornsteinsson noted that it can be difficult to classify a failure mode to be an ideal representation of either flexural or punching shear failure and instead these modes often interact.