Feature Article of Friday, 9 November 2012
Columnist: Boadi-Danquah, Eugene
As a background, when buildings are designed by structural engineers, our primary task is to ensure that we give our client a solution that is robust enough to withstand the loads (forces) anticipated on the building. The loads we design the buildings for vary vastly; dependent on the intended use of the building. For example, a five story building intended for residential use is totally different from a design point of view to a similar structure intended for commercial or industrial use.
In arriving at the loads, a competent structural engineer will primarily rely on the client to advise on what the structure is intended to be used for. This could also be inferred from a complete set of the client’s architect’s drawings, clearly identifying what each section/segment of the layout of the structure is to be used for.
With knowledge of the intended use of the building, it remains incumbent on a competent structural engineer to deliver on just one task; provide a structural solution that will not fail under reasonable anticipated conditions within the bounds of the intended use of the structure.
In delivering the structural solution, the engineer is responsible for obtaining any information on the condition of the site, which he foresees as contributing to him arriving at a structural solution that will not fail. This information will generally include, but not limited to, the nature of the ground (for his foundation); the wind pattern (for lateral stability), seismic (earthquake) data etc.
With all the relevant information handy, a client requires from his engineer; a complete set of structural drawings, showing the sizes of structural elements(beams, columns, etc), a specification of what materials to be used, and commonly for a reinforced concrete solution, details of what sizes/ amount of steel(iron rods) to place and where to place them.
WHY WILL A REINFORCED CONCRETE STRUCTURE FAIL?
A reinforced concrete structure fails for one of two reasons only: The engineering solution provided or delivered for the structure is not adequate or structure is not robust enough to withstand the loads imposed on the structure from its use, hence the structural elements (Beams, columns, foundations etc) are overstressed, and they collapse or give up.
OR: The solution was initially adequate, however years of continuous use and lack of maintenance or degradation might have compromised the structural integrity of the structure.
HOW WILL A REINFORCED CONCRETE STRUCTURE FAIL?
There are dozens of ways how a concrete structure will collapse, but the two broad categories are;
With a warning and without warning.
Generally, an overstressed concrete structure will show it signs by developing tensile cracks in structural elements. Generally, this could be concealed if the interior of the structure is clad or drylined. This could cause a structure that has been showing years of structural fatigue to collapse given a false indication of a collapse without ‘warning’.
There are also cases where a concrete structure can collapse totally without warning, and this type is mostly associated with shear failures. These occur when adequate reinforcement or alternative detail has not been provided for shear. Also in multi-storey structures, where a suitable foundation solution has not been provided, bases of the most overloaded columns(pillars) will spend their life trying to punch through the ground or pads when they are overloaded, and when they succeed, pull the whole structure down.
WHO IS RESPONSIBLE FOR A STRUCTURAL FAILURE?
Client/Owner: Where the owner has not advised his Architects or Engineer accurately on the intended use of his structure, he risks putting his Engineer in a position where he could under-estimate the loads coming down on the structure. This could lead to an engineering solution that is not robust enough for the intended use. Other times, clients change the original intended purpose of the building without employing the services of an Engineer for an assessment of whether the original solution will suffice for the revised use of the structure.
Engineer: Even with full knowledge of the intended use of a structure; some Engineers may not have full understanding of the task they are handed, or may not be competent enough for the task given to them; this often leads to them producing engineering solutions which fall short of what is required to prevent a structural collapse. It therefore rests on a client to employ the services of engineers with a proven track record on the projects similar to what they intend to undertake. Clients also have a duty to employ the services of an engineer’s representative on site, to ensure that the proposed engineering solution is what is delivered.
Architect: Some architects do not engage a structural engineer early enough in their delivery of projects to clients, and most often end up producing architectural drawings that are difficult, if not impossible to sensibly engineer. In less developed countries, some unprofessional architects even go to the extent of usurping the duties of an engineer, by providing structural details to clients. This is very bad practise and can lead to collapses, as architects are not educated to provide foundation or reinforcement details on drawings.
Contractor: I have decided to put the contractor last, because most times, the contractor is the worst culprit when it comes to collapsed buildings. The contribution of a contractor to structural collapse could take different forms. The most common ways however are through non-compliance with engineering drawings through the either the use of inferior materials to what has been specified on engineering drawings, or less material than noted on an engineering drawings.
Especially with reinforced concrete, most contractors are of the view that; once concrete is placed around steel (iron rods), no one would know how much steel was put in, or what type of steel was used. Contractors often get away with this, because structures might have been over-designed by a non-professional, hence underspecifying only brings it back to optimal; or in the life of the structure, the expected loads might not be encountered, so structure may not collapse.
In all these, everyone along the chain of the delivery of structural projects has a moral and a professional duty to ensure that the final product has the capacity to satisfy its intended purpose. If everyone will be honest, and professional where desired; lives would be preserved through the delivery of proper functioning structures wherever they are to be cited.
Eugene Boadi-Danquah (firstname.lastname@example.org)
Eugene is a Ghanaian structural engineer based in London with a major interest in reinforced concrete structures, having contributed technical expertise to major reinforced concrete schemes in Europe, Middle East and Africa.