The photographs from the recent Haitian 7.0 devastating earthquake of January 12 2010 followed by the magnitude 8.8 earthquake off Chile on February 27, 2010 have driven this basic primmer to sensitize all concerned about an essential watchpoint when constructing reinforced concrete buildings.

Apologies to the initiated and experts in this area are in order for the oversimplification but the intent here is to ensure understanding by the broadest audience.

The prime consideration for structural integrity is to “tie” the building together. This means that the foundation (or substructure) is firmly connected to the walls, columns, beams and roofing (collectively called superstructure). The creation of the situation where the entire structure can behave as one, so to speak, permits stress at one point to be distributed throughout the structure rather than being concentrated only at the point of application.

It is generally known that it is the steel in a reinforced concrete building that provides the strength but, what may not be such common knowledge, is that what is critical is not the presence of steel but where and how the steel is placed and, of course, the quality of the concrete. The very simplistic sketch below demonstrates how the connection process is developed between structural members (i.e. for example, between a roof slab and a beam).

What is shown is that if there is shallow embedment of the connecting steel, forces applied to the structure could extract the steel from its anchor point whereas with deep embedment the steel would be forced to break before extraction thereby developing the full potential of both the steel and the concrete.

Many Jamaicans utilize the services of a builder (as compared with a professional contractor or engineer) who is not au fait with what is termed in the trade “steel detailing”. Thus the correct amounts of steel and concrete may be put together but the connecting details are lacking and, under earth movements, structural members could separate and collapse as whole independent units – the roof could separate from the support beams and crash to the ground.

This very fundamental article would not be complete without a word about concrete. Concrete is referred to in the trade as a very “forgiving” material because this material, often subject to great abuse, will still perform. For concrete to develop its maximum strength three factors are essential. These are mixing, placing and curing.

The common problem with site mixes (i.e. not from a concrete truck) is that too much water is used and the result is that the plant mix (or concrete truck) will get perhaps 25%-50% more strength using the same amount of materials (cement, sand and gravel) as the site mix.

Placing is the act pouring and settling the concrete to form the structural element. Here, the object is to densify the concrete to achieve maximum strength. Frequently builders use a short steel rod in vertical strokes and can thereby achieve some increase in density. Generally speaking the proper method is the use of a vibrator which will significantly enhance strength. On common pitiful attempt at densification is the mason filling concrete block pockets and shaking the steel in the pocket as a means of vibration. This process inevitably shakes the steel away from bonding with the concrete and the block wall is almost as if there were no steel included.

Curing is a method of permitting the concrete to set or harden. If exposed to sunlight water in the mix will evaporate and in essence deprive surface cement of water and the surface eventually spalls off – as we have seen in many sidewalks in Jamaica.

So, if we break the chain of mixing, placing and curing at any link the result will be weaker concrete to interact with your steel. So, once again, you could pay for the correct amount of cement, sand, gravel and water and yet be given concrete of significantly less strength than if proper techniques were used.

Just think – you could have the correct amount of all the building materials for steel and concrete and yet take over a substandard building that is an accident waiting to happen! Oh! By the way earthquakes are measured on a Logarithmic scale – i.e. a Force 7 is not 2 times more than a Force 5. The shaking is one hundred times as great – and about 80 times as great from a force 7 to a force 8.8! By the way, in 1960 Chile experienced a force 9.5 Earthquake and the resulting tsunami killed 201 people as far away as Hawaii (61) and Japan (140).

(This article was reviewed by Engineer David Goldson.)