A slab is a flat, horizontal reinforced concrete element that is used to form flooring, ceiling, and roofs of a structure. These are one of the necessary components of the building that provide a solid, stable base.
There are numerous types of slabs in the building process, and one of the most architecturally pleasing ones is the one-way ribbed slab. Here, we take a look at what a one-way ribbed slab is, its design, construction, applications, pros, cons, and more.
What is a One-Way Ribbed Slab?
The one-way ribbed slab, or joist slab, is a thin floor slab supported by tapered reinforced concrete ribs (joists). The ribs are tapered and placed at a uniform spacing along a straight line. These ribs are supported by the girders that rest on columns. The spaces between the ribs are formed using removable pans or permanent fillers to provide a horizontal surface.
Applications of One-Way Ribbed Slabs
- The one-way ribbed slabs are generally used for flooring.
- They are used for places where the structure needs to withhold medium to heavy loads.
- The ribbed slabs are used for parking structures.
- They are used for smaller-span residential buildings in corridors and small rooms.
- They are ideal for the construction of small office spaces and warehouses.
Advantages and Disadvantages of One-Way Ribbed Slab
Advantages
- Lightweight and saves the usage of materials.
- Attractive in appearance when exposed.
- Economical if reusable formwork pans are used.
- Vertical penetrations between ribs are easy.
- Medium to long spans.
- Large holes in the topping can be easily accommodated.
- Used for heat transfer in passive cooling.
- Fast to install.
- Highly durable.
Disadvantages
- The depth of the slab between the ribs may limit the fire-resisting capacity.
- Requires a proprietary formwork.
- Greater floor-to-floor height.
- Large vertical penetrations are more difficult to handle.
- Higher formwork costs compared to other slab systems.
- Slightly greater floor thickness.
One-Way Ribbed Slabs vs Two-Way Ribbed Slabs
| One-Way Ribbed Slab | Two-Way Ribbed Slab |
| Spans in one direction. | Spans in two directions. |
| Supported by beams on two opposite sides. | Supported by beams on all four sides. |
| Transfers load to the two supporting beams. | Transfers load to the columns and supporting walls. |
| The one-way ribbed slab is comparatively thicker than the two-way ribbed slab. | The two-way ribbed slab is thinner than the one-way ribbed slab. |
| The one-way ribbed slab requires less reinforcement. | The two-way ribbed slab requires more reinforcement due to two-directional spanning. |
| The one-way ribbed slabs are suitable for short spans. | The two-way ribbed slabs are suitable for longer spans. |
| They are applicable for long and narrow structures. | They are applicable for structures that are square or rectangular. |
Design and Construction of One-Way Ribbed Slabs
Designing of One-Way Ribbed Slabs
- The minimum thickness of the one-way ribbed slabs depends on the type of support.
- For a simply supported structure, the minimum thickness should be 1/16 times the length of the slab (l/16).
- For one end continuous structure, the minimum thickness should be 1/18.5 times the length of the slab (l/16).
- For both ends continuous, the minimum thickness should be 1/21 times the length of the slab.
- For a cantilever, the minimum thickness should be ⅛ times the length of the slab.
- The ribs are generally tapered and uniformly spaced between 400 to 750 mm. The voids formed using molds (pans) should be 500 mm wide and 150 to 500 mm deep (as per the ACI Code, Section 9.8.1).
- The shear strength (Vc) provided by the concrete for the ribs should not be more than 10% of that of the beam.
- The minimum width of the rib should be one-third (1/3rd) of the total depth or 100 mm (whichever is greater).
- The secondary reinforcement in the slab in the transverse direction should not be less than the shrinkage reinforcement or 1/5th of the area of the main reinforcement in the ribs.
- The secondary reinforcement parallel to the ribs should be spaced at a distance of not more than half the spacing between the ribs.
- If the live load on the ribbed slab is less than 3 kN/m² and the span of the ribs exceeds 5 m, a secondary transverse rib should be placed at the midspan.
Construction of One-Way Ribbed Slabs
Formwork Preparation
The initial step in constructing one-way ribbed slabs is placing the formwork in the correct shape and dimensions. The formwork is usually made from timber or steel and is used to create the voids for the ribs and the thickness of the slab.
Placing the Reinforcement
Steel reinforcement in the form of a mesh or bars is placed within the formwork at the appropriate position, covering both the ribs and the solid parts of the slab.
Concrete Pouring
After the formwork and the reinforcement are placed, the prepared concrete mortar is poured into the voids, covering all the areas of the ribbed slab. The poured concrete is compacted using a vibrator to remove the air gaps, ensuring that the reinforcement and the cement mortar form a strong bond.
Curing
After the concrete is poured, the concrete slab is kept for hardening with formwork to make it a solid block. After they are properly finished, the slab is cured using water or another wet curing process to avoid premature cracking and drying.
Removing the framework
After the slab achieves sufficient strength to stand on its own, the formwork is removed and the final results of the finished one-way ribbed slabs are revealed.
Finishing
The constructed one-way ribbed slab can be further altered based on the architectural requirements by polishing or applying coatings to make it aesthetically pleasing and more functional.
Conclusion
One-way ribbed slabs are among the aesthetically pleasing and lightweight slabs that are applicable for several smaller-spanned buildings or structures. They are widely used in places where the structure needs to hold a minimum of heavy loads. They can be used as precast structures to attain the necessary shape, size, and other physical properties based on the requirements.