One of Bangalore's five-storey buildings — at Shantipura Grama near Electronics City Phase II — under construction tilted and some of the floors dangerously got a slant. Let alone the wobble of buildings due to inadequate safety materials and plan, people too shiver to note the callousness prevalent in the building industry. Apart from non-adherence to building specifications, what are the reasons on the ground that make a building get shaken up, and eventually lose ground?
It is understood that there are two types of building failures. The first one, ultimate failure, is characterised by collapse, severe damage, instability and buckling. The second type, known as serviceability failure, is marked by deflections, vibrations, cracking in slabs, beams and walls, and permanent deformation, amongst several other things. Modern structures are increasingly prone to serviceability failure because they are comparatively taller and have longer spans, slender members, thinner slabs and walls, and are built at a faster pace. But old structures inherently had lower height and thicker members, with longer construction period.
Good, safe construction calls for greater attention to planning, design, detailing and fabrication. In design, the integrity of the overall structural system is the key to minimising the likelihood of progressive collapse.
What is it that involves this understanding to ensure robust and stable design?
PropertyPlus spoke to Chandramouleeswar S., Secretary, Association of Consulting Civil Engineers (India), and civil engineering practitioner, to know about the nitty-gritty at the site that leads to disasters.
Excerpts…
The recent Shantigrama building episode…
It is not the fifth floor only but the entire building was in distress. The building has not swayed, it has tilted to one side due to either settlement of the foundation or failure of the foundation due to “two-way shear failure” or “punching shear failure” as it is called technically.
Some of the reasons for this failure are:
Weak bearing soils
Some soils are simply not capable of supporting the weight or bearing pressure exerted by a building's foundation. The footings press or sink into the soft soils, similar in theory to how a person standing in the mud sinks into soft, wet clay.In such cases, footings may be designed to spread the load over the weak soils, thereby reducing potential foundation settlement. However, the majority of settlement problems caused by weak bearing soils occur in residential construction, where the footings are designed based upon general guidelines and not site-specific soil information.
Poor compaction
Placement of ‘fill soils' is common practice in the development of both commercial and residential sub-divisions.
In general, before a foundation can be constructed on a plot, hilltops are cut down and valleys are filled in order to create buildable lots.
Properly placed and compacted fill soils can provide adequate support for foundations, and are sometimes brought in from off-site locations.
When filled-up soils are not adequately compacted, they can compress under a foundation load, resulting in settlement of the structure.
Changes in moisture content
Extreme changes in moisture content within foundation soils can result in damaging settlement. Excess moisture can saturate foundation soils, which often leads to softening or weakening of clays and silts.
The reduced ability of the soil to support the load results in foundation settlement. Increased moisture within foundation soils is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing, or a raised groundwater table.
Soils with high clay content also have a tendency to shrink with loss of moisture. As clay soils dry out, they shrink or contract, resulting in a general decrease in soil volume.
Therefore, settlement damage is often observed in a structure supported on dried-out soil.
Maturing trees and vegetation
Maturing trees, bush and other vegetation in close proximity to a home or building are a common cause of settlement. As trees and other vegetation mature, their demand for water also grows.
The root systems continually expand and can draw moisture from the soil beneath the foundation. Again, clay-rich soils shrink as they lose moisture, resulting in settlement of overlying structures. Many home and building owners often state that they did not have a settlement problem until decades after the structure was built.
This time-frame coincides with the maturation and growth of the trees and vegetation.
Soil consolidation
Consolidation occurs when the weight of a structure or newly-placed fill soils compress lower, weak clayey soils. The applied load forces water out of the clay soils, allowing the individual soil particles to become more densely spaced.
Consolidation results in downward movement or settlement of overlying structures. Settlement caused by consolidation of foundation soils may take weeks, months, or years to be considered ‘complete.' As this occurs, the foundation will experience downward movement, sometimes at an uneven rate. This leads to cracks and structural damage.
(Some of the other reasons like faulty design, improper soil investigation and foundation factors would be discussed in our article next week)
