By Innocent Anguyo
One of the reasons why buildings collapse is the use of substandard iron bars when constructing ring beams. But how can one know that the iron bars they are buying are substandard?
According to Sunday Onzima of Arab Contractors, a construction firm, the quantity and type of iron bars used in both rim and column beams is determined by safety, cost and beauty requirements of a building.
Onzima says the bending and collapse of iron bars is considered during their manufacture. In engineering mechanics, bending (also known as flexure) occurs when the beam is subjected to an external load applied vertically to a longitudinal axis of the beam. Usually, the column beam will fail when it is subjected to bending. So, a good column beam should not be bendable.
“A beam bends when weight from the roof rests on it. Therefore, the number of iron bars must be appropriate to prevent the beam from bending,” said Onzima.
Column beams can also fail due to crumpling. Therefore, corner columns must have more iron bars since they are subjected to immense load that comes from the several sides of the building and not centrically as is the case with centre columns.
To strengthen the iron bars, concrete is used to contain forces that would force the column beam to collapse.
Column beams are also subjected to wind load (pressure exerted by moving wind on a building).
According to Onzima, a column beam must have more iron bars than a ring beam because column beams are subjected to both dead and live load. Dead load comes from the beam and the roof, while live load is from items and occupants of upper floors of a house.
A minimum of three iron bars should be used at the ring beam, while column bars require four iron bars if safety requirements are to be met. More iron bars are also used at the base of the ring beam to resist tensile force, which is most felt at the base of the house.
For column beams, the weight of the roof and the ring beam is transferred to the column beams, which exert the weight on the foundation, and thereafter, the earth for circulation and subsequent neutralisation.
However, since tensile force aims at bending the iron bars, concrete is used to resist compressional force, hence preventing the iron bars from succumbing to bending.
The minimum safe number of iron bars should be considered in order to minimise costs. Onzima said if unnecessary iron bars are used in a beam, the extra bars become redundant and not only waste resources, but increase the weight of the house on the column beams.
The number and positioning of the iron bars should be in such a way that beauty is exhibited. However, this should be done by architects since they can harmonise beauty, safety and cost.
Different iron bars for different beams
Ring beams, Onzima says, use light tensile iron bars, depending on the size of the beam and the load it is expected to carry. Tensile iron bars are tailor-made to resist tensile forces that are exerted on the ream beam.
Therefore, the bigger the beam, the stronger the iron bars should be. If a beam carries a heavy load, then it should also be reinforced with bigger iron bars.
Column beams use high-yield iron bars. Also, to consider is that the bigger the column and the heavier the load, the bigger the size of iron bars used and the more the number of iron bars.
Another easy way to differentiate tensile iron bars from high-yield iron bars is by looking at their designs. Tensile iron bars are twisted while high-yield iron bars are engraved with dots.
Seek professional advice
Onzima advises people who want to build houses bearing iron bars to consult engineers. “For example, for a school, the structural engineer will know the kind of load that will be experienced by the building. They will also know how to strike a balance between safety, economy and beauty,” he says.
He noted that the safety of the building is determined by the structural engineer, the cost is determined by the quantity surveyor, while the architect will determine the beauty. Therefore they should all be consulted when building a house.
Stuart Jason Mwesigwa, the business development manager of Roofings, said the only way clients can access authentic iron bars is by buying from companies certified by the Uganda Bureau of Standards (UNBS).
Maggie Lukowe Senyonjo, the public relations officer of UNBS, urged people to buy construction materials from certified companies if they are to evade counterfeit iron b