eBusiness Weekly
Engineering and Construction Matters with Innocent Chatikobo
Introduction
Reinforced concrete is the most commonly used structural material in engineering construction.
Although concrete is strong in resisting compressive stress, it is weak in tension. To withstand tensional stresses, steel is needed in concrete. The reinforcement may be simple bars or rods bend and tied with stirrups. The steel used is normally mild steel and the common nominal diameters used are Y8, Y10, Y12, Y14, Y16, Y20, Y25, and Y32.
Indication of reinforcement in drawings
Engineering drawing is a language to communicate with details. There is a standard to indicate reinforcement in drawings e.g.
5Y10 — 001 – 150 which means
5 — number of steel members
Y10 — 10mm diameter of steel
001 — bar mark / number
150 — distance between centres for steel members
The notation can also vary e.g. notation for slab
Cover to reinforcement
Reinforcement steel should be well covered to protect the steel against corrosion (rusting) and to provide resistance against fire. Some of the average required covers are the following
RCC slab and staircases — 20mm
RCC columns cover to stirrups — 30mm
Underground structures — 50mm
Beams in superstructure (to stirrups) — 25mm
Ground resting floor slab(top surface) and retaining walls — 50mm
Cutting and bending of bars
Reinforcement bars are cut into required lengths and bent into required shapes as shown on schedule normally included on drawings.
The cutting and bending operations can be done manually or by means of machinery. Manual bending is normally ideal for smaller diameter bars while larger diameter bars usually needs a machine.
Large diameter bars can also be bent manually by first heating to red hot , though this has a possibility of creating weak points on the steel bar. After cutting and bending, the bars should be bundled and clearly numbered according to the bar mark to make work easier for steel fixers
Lapping
When a bar is not long enough or a joint is required, lapping is required. Bars may be intentionally left short for constructability and ease of transportation. We have two main methods of lapping
Where two bars overlap each other for some minimum distance e.g. 750mm. This is called lap length.
These two bars are in physical contact and wired together.
Where another bar is used to provide lap to the two bars that are being joined. The minimum lap length (usually 750mm) is still required.
Stirrups
A stirrup is a piece of steel bent into a rectangular shape. These are required at areas of high shear, such as bearing points and below large point loads. Use of steel stirrups helps increase concrete beam spans and reduction in the need for additional piers.
The size of stirrups varies with depth of beam. The deeper the beam, the more shear capacity. When depth is not adequate, steel stirrups are added to increase the shear capacity of the beam.
The spacing of stirrups also determines the shear capacity, therefore the more the stirrups, the greater the shear capacity. A stirrup should be fabricated from one piece of steel and should have adequate lap at each end
Stools
They are used to separate the top reinforcement mesh and bottom reinforcement mesh. They are used in slabs and decks to separate the bottom and top reinforcement and maintain the mesh positions. The stools should be strong enough to bear loads without changing the gap of 2 layers. 12mm, 14mm or 16mm bars are normally used to make stools. Dimension of the stool depends on the slab specifications.
Other material used in reinforcement works
When carrying out reinforcement works, we use other various materials depending with the nature of concrete work
a) Binding / tying wires: re-bars are joined using tying wires. Joining using wire enables flexibility other than if they were to be welded. To bind these wires hackers are used though others may use pliers
b) Cover blocks: they are made of highly concentrated cement. These cover blocks must be immersed in water for 28 days to get maximum strength. These blocks are placed at the bottom and sides of beam or deck reinforcement.
The blocks ensure the covering requirements of re-bars are met before covering. People should avoid use of wood blocks and brick or concrete rubble as cover blocks as this can create weak points in the concrete
Stay tuned for part 2 where we shall look at beams and slabs reinforcement as well as how to prepare a
Bar cutting and bending schedule
Innocent Chatikobo is an angineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in structural engineering and construction. For your comments, views, questions and feedback he can be contacted on the following platforms. Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732; Email: [email protected]; Facebook: AtroServe Engineering Zimbabwe; Whatsapp Channel: AtroServe Engineering Zimbabwe
