Guidelines for a one-way concrete flooring system

Words by Basheer Bava, Sunday 09 April 2017

An overview and guide to one-way slab systems, written by Mohamed Basheer Bava, Lead Structural Engineer with BSBG.

The floor system is the major part of a building structure. 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 one-way flooring system.

Overview of the one-way slab system

A one-way slab is supported on two opposite sides, so structural action is only ever in one direction. Total load is carried in the direction perpendicular to the supporting beam. Main reinforcement is provided in only one direction. Nominal distribution reinforcement is provided in cross direction. If a slab is supported on all the four sides but the ratio of longer span to shorter span is greater than two, the slab will be considered as a one-way slab.

Due to the huge difference in length, load is not transferred along the longer direction. The various forms and types of the one-way systems are tabulated below:

One-way system

1. Solid slab on RC beams/bands
2. Pre-cast Hollow core on RC/pre-cast beams
3. One-way joist system
4. Composite metal deck slab on steel beams
5. Double-‘T’ on RC/pre-cast beams

One-way slab systems
1. Solid slab on RC beams/bands

One-way in-situ solid slabs are the most basic form of slab. Deflection usually governs the design, and steel content is usually increased to reduce service stress and increase span capacity.

Usage:

Office buildings - Low rise
Shopping malls
Warehouses

Economic span range: 4-7m
Economic slab thickness range: 150-250mm

Advantages:

Simple construction.
Penetrations in the slabs are easily manageable.

Disadvantages:

Associated downstand beams may require greater storey height.
Deter fast formwork cycles.
Compromise flexibility of partition location and horizontal service distribution.

2. Pre-cast hollow core on RC/pre-cast beams

Pre-cast floor systems will typically consist of factory pre-tensioned pre-cast planks, tied together with a structural topping to provide a robust and efficient floor system to resist the imposed vertical and horizontal loading. The most commonly used pre-cast slabs in the UAE market are the pre-stressed, pre-cast hollow core planks.

Usage:

Office buildings - Low rise
Residential buildings - Low rise
Educational
Parking structures
Warehouses

Economic span range: 6-16m
Typical plank thickness range: 150-400mm

Advantages:

Heavy load carrying capacity.
Exceptional fire resistance.
Lower self-weight.
Superior acoustic insulation and thermal properties.
Rapid speed of erection.
Factory made with highest quality control.

Disadvantages:

Irregular shaped hollow cores are difficult to cast and expensive.
The segments cannot be cut on site, must be carefully designed.

3. One-way joist system

Introducing voids to the soffit of a slab reduces dead weight and increases the efficiency of the concrete section. A slightly deeper section is required, but these stiffer floors facilitate longer spans and provision of holes.

Usage:

Office buildings - Low rise
Parking structures
Warehouses
Economic Span Range: 8-12m

Typical joist depth range: 350-600mm

Advantages:

Medium to long spans.
Lightweight.
Holes in topping easily accommodated.
Large holes can be accommodated.
Profile may be expressed architecturally.

Disadvantages:

Higher formwork costs than for other slab systems.
Slightly greater floor thicknesses.
Slower floor to floor cycle.

4. Composite metal deck slab on steel beams

Composite slabs consists of profiled steel decking with an in-situ RC topping. The decking not only acts as a permanent formwork, but also provides sufficient shear bond with the concrete. Concrete is good in compression and steel is good in tension. By joining the two materials together structurally these strengths can be exploited to result in a highly efficient and lightweight design.

Usage:

Office buildings - Low rise/high rise
Residential buildings - Low rise/high rise
Educational
Hospitals
Cinemas & Theatres
Hotels

Economic span range: 2.5-4.0m

Advantages:

Speed of construction.
Elimination of formwork.
Safe method of construction.
Saving in weight.
Shallower depth.
Sustainability.

Disadvantages:

Limited spans and capacities.
Propping may be required.
Fire proofing.

5. Double-T on RC/Pre-cast beams

Double-T’s are used for long spans. They are relatively lightweight with a high load capacity. The units are pre-stressed and can be left exposed. TT2 units are intended for up to two hours fire resistance; TT4 for up to four hours. Double ‘T’s are used in conjunction with a structural topping where enhanced performance is required. The units act compositely with the in-situ structural topping to create a robust composite floor.

Usage:

Carparks.
Office buildings.
Warehouses, roofing, bridges and industrial buildings.

Economic span range: 8-20m
Typical double-T depth range : 500-1200mm

Advantages:

Speed of construction.
Longer spans.
Elimination of formwork or propping.

Disadvantages:

Crane may prove critical.

Basheer Bava

Associate Director

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