MYTH AMONG LAYMAN,CONTRACTOR & ARCHITECT PREFERRING USING OF HIGHER REINFORCEMENT & LOWER SIZES OF COLUMN & BEAMS IN RCC BUILDINGS.
” This blog is dedicated to all clients, contractors ,architects and layman who think a RCC building can be make structurally sound using more reinforcement and lesser cross section of structural elements”.
Usually i come across various person like clients, contractors, architects and layman who say, can we reduce the size of column and beam by using higher reinforcement. Actually architects always want very light section which do not affect the beauty of the buildings. As per the structural requirement,when a structural engineer provide a big column or beam then architect says, why not we are using column and beams of lesser cross section by providing higher reinforcement. Occasionally, i have also heard an architect requesting for reducing the beam depth by increasing the depth of the slab or reducing the beam depth by increasing the reinforcement or reducing the depth of beam by increasing the width of beam.
This is an absolutely a wrong perception developed by an architect, contractor and layman.As they are not aware of the role of stiffness for deflection,durability and crack control,they are right for what their perception allow in this subject.Actually, a myth has developed in their mind that if higher reinforcement is used, then structural members will have higher strength.
Also, it was very strange when i came across a person who was suggesting me to put more reinforcement in slab. Actually, one of slab was having reinforcement of 8 mm diameter but our client was insisting me to provide 10 mm diameter reinforcement. Although i tried to convince him that using 8 mm diameter reinforcement is sufficient for that slab but he was not convinced. Then i tried to understand, what does he thinks about structure. After a quality discussion with him on the subject of structural analysis, i realized that he has developed a misconception that slab is one of the most important element in a building as everything is kept on slab. This is not the only case. I have also heard the same by few other people. Therefore, i realized that i have to educate more and more people and will have to bring all people out from this disastrous myth.
It pains me a lot and very unfortunate to state that how people misinterpret things on structural stability and strength on their own way.I am not exaggerating if many buildings have collapsed or severely damaged in recent past by taking very slender column considering only gravity loading. It is amazing to see builder proposes random column sizes as 230 x 230 mm,300 x 300 mm or 230 x 350 mm for three to four storey building without knowing the consequences of heavy gravity loading and earthquake loading. If somewhere , any building with these kind of proposed sizes are safe for gravity loading then it is taken granted same in other places too.I don’t know what will happen to these building for earthquake loading when it will exposed to.It is essential to keep in mind,during earthquake,severity of shaking in two building will be felt differently at the same site due to geometrical differences although both buildings have same number of floors. In the same site,a highly irregular building will feel higher intensity of shaking whereas a regular building will feel less intensity of shaking. Hence, column sizes can not be same in the two buildings however column spacing and floors are same.Actually, reason is that,a highly irregular building will have twisting/torsion in earthquake requiring bigger column sizes to counter twisting whereas highly regular and symmetrical building will have column of lesser cross section as it will not have torsion/twisting.Here, i am not going to elaborate the cause of torsion like difference in Center of rigidity(CR) & Centre of mass(CM).Please read other article related to the same for knowledge.
Actually architects,layman or builders in order to find large spaces for parking or hiding /concealing column in the masonry walls for aesthetics purposes or to get more free spaces for bed room /living room , slender columns are proposed ignoring large span.They can go for beam with more cross sectional area for large span than column.They do not know the concept of strong column and weak beam.Also, they do not know the failure mechanism.And of course, how they will know these very general and basic concept if we do not educate them.
And when they construct a building on their one way without consulting a structural engineer then something very unfortunate happens to human lives and economy.Therefore, our responsibility plays a important role to spread awareness and to stop building collapsing even under gravity loading.
I have noticed that layman, architects & builders think in unidirectional way. The way their perception allows. They think gravity loading is the only loading that is most likely to occur on a building ignoring a large part of seismic loading.They think if building seems safe in gravity loading then it looks fine to them.They seem relaxed and fully unaware of earthquake loading.They do not know column is weak for lateral loads(Seismic/Wind) and strong in compression.
As a structural engineer, it is our responsibility to guide and educate them.Although we will not be able to teach whole civil engineering to them but we should deliver a general concepts to stop tragedy in future. We should make them aware that however we should make building safe for gravity loading but at the same time we should not forget the bigger and serious concern of earthquake loading.Instead of creating havoc, fears and confusion to understand the complex mathematical modeling like numerical methods(differential equations) of physical phenomena of earthquake, we should come up with very simple examples to clear basic concepts of earthquake. We should relate the drifting or lateral deflection of the building when subjected to earthquake forces with the best and easiest example with newton first law of motion.As we know, when a car starts moving then passenger sitting in back seat feel backward push due to concept of newton’s first law of motion or law of inertia.In the same manner, when earthquake forces exerts in the building, then due to inertia,building will try to drift in opposite direction to earthquake force. We should tell them, column should be able to take huge lateral loads and it should be much stiffer than beam to follow the rules of strong column and weak beams.Also, in case of double and triple height column,column should have larger cross section or will have to be tied at different level to stop column failing in buckling.We should demonstrate them the simple concept of bucking failure with simple example by pushing two measuring scale. Say,one scale of length 10 cm and another one of 30 cm.If both of them is pushed by hand,undoubtedly scale of longer length will buckled more as compared with smaller one.These demonstration will help them the concept behind using stiffer column.After reading this article they will come out of the box like thinking that slab is most important part of building as everything supports on slab.They will really come out of this disastrous myth. This blog will help them a lot to understand basic concept of structural analysis.This will help them to understand , however building shall be safe for gravity loading but it is mono-maniacally designed for seismic loading.
Mohammad Sohel Akhtar(MSA)