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Flat plate slabs and their risks in use:
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Two types of concrete slab systems are being used in the construction of multi-storied buildings in Bangladesh.
- Beam column slab system.
- Flat plate slab system.
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Beam column slab system consists of beams with floors or slabs. Then transfer of load takes place from slab to beam and then from beam to column.
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But in flat plate slab system the slab is usually placed directly on top of the column where beams are not used.
This is not a new idea from a structural point of view. In the past, flat plate slab systems were commonly used in some buildings. But now this system has gained great popularity among construction firms and buyers.
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Reasons behind customer attraction to this system are noted:
⬛ The inner surface of the slab is very smooth because there are no beams on the inner side
⬛ Architectural view
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Due to these reasons, the business demand of this slab system is increasing day by day. For this reason, the construction company is also matching the needs of the buyers. But it must be understood that the possibility of damage and collapse in case of earthquake is much higher in flat plate slab system than in beam slab system.
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In Bangladesh, six to ten-story apartment buildings are common in Dhaka and other major cities. In these buildings, beam column slab systems typically have slab thicknesses of 100 mm to 125 mm (4”-5”).
Again, in case of similar construction in flat plate slab system, the thickness of the slab will generally be 175mm to 225mm or more.
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Generally in beam column slab system 70% of the weight of the building comes from the slab. Since the thickness of the flat plate is much higher, the weight of this slab will be more than that. The heavier a structure is, the more energy it will generate during an earthquake. This helps us understand why Japan, where the earthquake rate is highest, uses very light materials such as straw, wood, paper, etc. to build houses.
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In fact, in the case of flat plates, the reason for the possibility of building collapse is the increase in seismic load on the structure. Seismic load due to inertia force when the building starts swaying during earthquake is considered as lateral load of the building. Due to this, the maximum stress in the building is generated at the joint of column and slab. So proper structural design is required for this joint. Engineers follow a particular principle in designing earthquake resistant buildings. It is known as “Strong Column Wick Beam”. “It is a concept that is expected to dissipate the incoming energy through yielding and damage of the beam, excluding the column as a result of the earthquake”.
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That means the column has to be designed with more strength. In fact the strength and stability of the entire structure actually depends on the columns. Therefore, the damage of the beam without the column significantly reduces the possibility of collapse of the entire structure. It also reduces the risk of human death.
In addition, if the opposite happens, i.e. if the column fails, there is nothing to support the weight of the building. In that case buildings collapse and property is damaged and human life is destroyed.
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This “Strong Column Weak Beam” concept is very simple for engineers to apply in building beam column slab systems. Engineers are faced with a complex situation in flat plate system design. Since there is no beam in this case, a portion of the slab along the column is treated as a beam. Due to the complexity of such problems it is avoided at B.C. level. Also, what is more surprising is that most design codes do not address this in detail.
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If the column is designed to be too strong then slab damage occurs around the column. This causes high stress to the column which leads to punching shear failure. The slab then breaks from the column and subsequently causes a cascading effect and the entire building collapses. This failure is most critical in determining the slab thickness of flat plate slab joints. If the slab thickness is too high, the column will weaken which is not desirable. There is no such problem in beam slab system. Even if the beam ever fails, the probability of slab failure is very low due to the inherent properties of the slab.
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The design of flat plate slabs is very complex and requires a lot of technical knowledge and excessive supervision in its construction. Most of the buildings that have suffered structural failure due to earthquakes and construction errors in the past in our country have seen flat plate systems. So it can easily be said that beam column slab system is much safer than flat plate system. Another major aspect of this system is that it requires a large amount of construction materials, which increases the cost of construction.
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The bridge that I have built, if properly maintained, this bridge will last forever.’ – Chief Engineer Sir Robert William said this at the time of inauguration of Hardinge Bridge….
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Today there is no William Gales, no Lord Hardinge. But their immortal achievements remain. The bridge still bears witness to its eternal youth – said engineer Gail Satya. 2015 marked the centenary of Hardinge Bridge. But even today it looks like a newly rejuvenated bridge. March 4, 1915, is a unique day in the history of British Bengal.
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His Excellency the Viceroy of India ‘Baron Hardinge Pensurst’ came to Pakshi. He opened a closed door of communication between north and south of Bengal. As a beauty on one side, a witness of history on the other side, this bridge carries the tradition and significance of Bengal. Hardinge Bridge is one of the longest railway bridges in Asia.
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The location of this bridge is in Pakshi of Ishwardi Upazila of Pabna. To the south of Pakshi station, this red colored bridge stands over the river Padma. On the other side is Bheramara Upazila of Kushtia. In 1889, the then-British Government felt the need to establish uninterrupted communication between Assam, Nagaland, Tripura and North Bengal with Calcutta. That is why the bridge was first proposed to be built in 1889. Later, after 19 long years, the construction of the bridge was approved. British engineer Sir Robert William Gales was commissioned to construct the bridge.
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The bridge was designed by the famous British architect Alexander Meadows Rendell. British construction company Braithwaite & Kirk undertook the construction of the bridge. Padma was then full of youth. Robert William Gales understood that the construction of the bridge would be very challenging for him. That is why he decided to build a river protection dam before starting the construction of the main bridge. Only dams were constructed throughout the period 1910-11. In this case, Gales uses an extraordinary method. He started mixing large stones and earth together and throwing them on the river bank. He dammed like this for about fifteen kilometers on both banks. It is believed that some more bridges could have been built with the stones used to block the river. Gales’ method really worked. A hundred years have passed, but the dam is still completely intact. As if no stone had yet fallen.
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In February 1912, when the work was in full swing, there were a total of 24,400 workers. is The British used the highest technology of the time in the construction of the bridge? On New Year’s Day 1915 (1 January 1915) a line was opened with the first goods train. At that time, its construction cost 3 crore 51 lakh 32 thousand 164 rupees or 4 crore 75 lakh taka. In the construction of the bridge, no damage to the navigability of the river has been taken into consideration very seriously. As a result, there was no impact on the navigability of Padma for this bridge. Although much of the north side of the bridge is now overgrown. But it’s not for Hardinge. If there is any loss, it is for the ‘Lalon Shah Bridge’.
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At that time bungalow houses and cottages were built in Pakshi for many British citizens. The British had free movement in Pakistan at that time. Robert William Gales had a large bungalow, which still stands. It is said that from this bungalow, which is about a kilometer away, he used to observe the progress of the construction work through binoculars. During construction, many small factories, shops and bazaars were built in and around Pakshi. British rule had an impact on public life. The standard of living of the local people changed.
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Construction went on for five long years. Finally, in 1915, the construction work was completed. 3 crore 51 lakh 32 thousand 164 Indian rupees were spent on the construction of the bridge. On January 1, 1915, the first experimental train from Ishwardi to Khulna over the Hardinge Bridge. Then on March 4th of the same year, Lord Baron Hardinge himself cut the ribbon and inaugurated the bridge.
However, one point to be noted here is that this discussion will generally apply to those multi-storied buildings with flat plate systems where the load goes directly to the foundation through the columns. Where columns do the main load transfer work. Reinforced concrete wall columns are often used in building construction to transfer lateral loads from exterior walls and loads from floors. This type of technical wall is called “shear wall”. Buildings that have pile or mat foundations along with shear walls are more resistant during earthquakes. In this case the risk of corrosion damage is greatly reduced even if it is a flat plate slab.
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However, it must be ensured that piles, mats or similar deep foundations are placed under these buildings. Only if shear walls are used with shallow foundations in each column will the risk not be reduced.
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A lot of research has been done on this topic in various developing countries and it has been proved that the building acts as less strong during earthquakes.
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