Guidelines for a two-way concrete flooring system

Words by Basheer Bava, Monday 10 July 2017

For structural engineers and architects, Basheer Bava, BSBG Lead Structural Engineer, provides an indispensable guide on the two-way concrete flooring system.

The floor system is the major part of a building structure, and the selection of an appropriate system is vital in achieving an overall economic building. This brief will provide guidance to architects and structural engineers during the concept phase of the design for selecting a suitable flooring system.

An overview of two-way slab systems

Two-way slabs are slabs that are supported on four sides. In two-way slabs, the load will be carried in both directions, thus main reinforcement is provided in both directions for two-way slabs. The slabs are considered as spanning two-way when the longer to shorter span length is less than a ratio of two. The bending of these slabs takes the shape of a dish-like form when loaded uniformly. The various forms and types of two-way slab systems are tabulated below:

Two-way slab systems

1. Flat plate (Conventional RC or PT)

A flat plate is a two-way system usually supported directly on columns or load-bearing walls. The main feature of the flat plate floor is a uniform thickness with a flat soffit which requires only simple formwork and is easy to construct. The floor allows great flexibility for locating horizontal services above a suspended ceiling or in a bulkhead. A flat plate with pre-stressing tendons (PT) results in longer spans and thinner slabs.

Usage:

Office buildings – Low Rise & High Rise
Residential buildings – Low Rise & High Rise
Parking
Hotels

Economic Span Range:

5-8m (Conventional RC)
6-10m ( Post-Tensioned)

Advantages:

Typically has the lowest floor to floor cycle time of the cast in-situ options due to the most simplified formwork and reinforcement detailing.
No beams - simplifying under-floor services.
Minimum structural depth and reduced floor-to-floor height.

Disadvantages:

Long-term deflection may be a controlling factor.
May not be suitable for heavy loads.
High concentration of reinforcement around the columns in order to ensure the slab has adequate punching shear capacity.

2. Flat slab with drop panels (Conventional RC or PT)

Drop panels, formed by thickening the bottom of the slab around columns, increase shear capacity and the stiffness of the slab, allowing thinner slabs to be used. A flat slab with prestressing tendons (PT) results in longer spans and thinner slabs.

Usage:

Office buildings - Low Rise & High Rise
Residential buildings - Low Rise & High Rise
Parking
Hotels

Economic Span Range:

6-9m (Conventional RC)
7-11m (Post-tensioned)

Advantages:

More efficient structural system than a flat plate, typically with lower stress concentration at column locations.
Slabs are generally thinner in comparison to the flat plate solution.
Absence of beams allows lower storey heights.
Flexibility of partition location and horizontal service distribution.

Disadvantages:

Formwork is more complicated than with a flat slab system, which can increase floor to floor cycle time.
Drop panels require a higher level of coordination with the services in the ceiling space than flat plates and may not be architecturally acceptable for areas where suspended ceiling is not envisaged.

3. Flat slab with beams in two directions (Conventional RC)

A two-way slab with beams is a type of economical flooring system, often used as it costs less than flat plates or flat slabs. In other words, when the loads or spans or both become quite large, the slab thickness and column sizes required for flat plates or flat slabs are of such magnitude that it is more economical to use two-way slabs with beams, despite the higher form work costs.

Usage:

Office buildings - Low Rise & High Rise
Residential buildings - Low Rise & High Rise
Parking
Warehouses
Supermarkets

Economic Span Range:

9-12m (Conventional RC)

Advantages:

Economical for longer spans and high loads.

Disadvantages:

Presence of beams may require greater storey height.
Requires a regular column layout.
Slow floor cycle.
Flexibility of partition location and horizontal service distribution.

4. Flat slab with edge beams/bands (Conventional RC or PT)

Introducing edge beams to flat slabs overcomes many of the problems associated with shear at perimeter columns and edge deflection. A flat slab with prestressing tendons (PT) results in longer spans and thinner slabs.

Usage:

Office buildings - Low Rise & High Rise
Residential buildings - Low Rise & High Rise
Parking
Hotels

Economic Span Range:

5-9m (Conventional RC)
7-11m (Post-Tensioned)

Advantages:

Similar to the flat slab option, with the benefit being a slightly reduced slab thickness leading to lighter floor construction via the introduction of perimeter stiffening beams or bands.

Disadvantages:

Similar to the flat slab option, but with added formwork complexity around the building perimeter and potentially adverse effects on the facade design/architecture.

5. Waffle Slabs (Conventional RC)

Introducing voids to the soffit of a flat slab reduces deadweight. These slabs are economical in spans up to 14m in square panels. Thickness is governed by deflection, punching shear around columns and shear in ribs.

Usage:

Carparks
Office buildings
Roofing

Economic Span Range:

8-14m

Advantages:

Slab soffit profile may be expressed architecturally.
Longer spans possible.
Lightweight in nature.

Disadvantages:

Higher formwork costs than for other slab systems.
Slightly greater floor thicknesses.
Slower floor-to-floor cycle.
Requires square or rectangular column/grid arrangements.

Basheer Bava

Associate Director

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