‘Quality of construction materials should be maintained to build earthquake-resilient structures’
In conversation with Professor Dr M Shamim Z Bosunia, Chairman and Managing Director, Abode of Consultants (Pvt) Ltd. and Former President, Institution of Engineers, Bangladesh
The fear of earthquakes—often referred to as “seismophobia”—is likely to increase with the expansion of construction projects and rapid urbanisation. Therefore, assessing earthquake risks before undertaking construction is of utmost importance.
While talking about safe construction practices in light of earthquakes, Professor Bosunia opined that earlier beliefs concerning earthquake were different than those of now. It was widely believed that the force of earthquakes is proportionally connected to the mass of buildings. However, scientific findings have now begun to reshape these beliefs from a rational point of view. According to him, the seismic zones are divided based on several variables, such as the soil condition, geographical formation of the land, regional fault, etc. Earthquake risks differ from zone to zone. For example, Sylhet falls in the zone of severe damage. Again, the earthquake risks in Dhaka are different than those of Khulna because of the zone difference.
Professor Bosunia asserted that there are no concrete design criteria to build an earthquake-resilient structure. However, BNBC has provided some recommendations for earthquake-resistant design. Earthquake-resistant design is required to protect a structure or a building from the effects of earthquakes. The structures must comply with some requirements related to things such as the thickness of the column, the thickness of the beam and slab systems.
Earthquake load, often known as dead load, refers to the intrinsic weights of the immovable structure of any building, such as column weight, beam weight, slab weight, partition wall, etc. Live load refers to the loads which depend on the occupancy type and change over time with situations, such as people moving around a building or the presence of movable objects in the buildings. Generally, in a 20-storey building, the dead load is around 80-85 percent while the live load is 15-20 percent, informed Dr Bosunia.
The effects of an earthquake hugely depend on the dead load. It is proposed in the code that a designer must comply with dead load, live load and wind load to design a building or a structure. The code offers some recommendations, particularly on the thickness of column, beam and slabs. For instance, it is common practice to use 8 percent rods in the column during construction. But if one wants to design an earthquake-resistant building, one must reduce the use of rods to 6 percent in each column. The latter installation will enlarge the column. Simply put, the larger the column, the better it works for earthquake resistance.
Usually, the tremor jolts the ground when an earthquake takes place, and the seismic wave gradually transmits through a building vertically. The building starts to sway because of the transmission of the wave. Therefore, according to Professor Bosunia, for precautionary purposes, we should take the horizontal deflection of a building into account and calculate the measurement of sway. The extent to which a building can sway must be kept within the recommended guidelines for that particular building. It is also important to note that the transmission of seismic waves passes through the joints of columns, beams and slabs. Therefore, the proper construction of joint details of column, beam and slab is important to build an earthquake-resistant structure. Poor construction of joints makes buildings more vulnerable to earthquake damage.
The strength of concrete is also very important. The buildings in Dhaka city are constructed in such a way that the load on the column is shared between concrete and steel. 60 percent load is supported by concrete, whereas steel can support as much as 40 percent. Therefore, the quality of concrete should be a major concern. Concrete is the most widely used construction material all over the world; yet the most unpredictable one. In order to be earthquake-resistant, the strength of the concrete should be a minimum of 3000 PSI. Another point to note is that people with no training are often involved in concrete production. We should investigate this issue and ensure proper training to maintain the quality of concrete as much as possible.
To build an earthquake-resistant structure, Professor Bosunia pointed out three steps. First, the quality of the material should be ensured. Second, good designers should be assigned to design the buildings. Third, the practice of ductile detailing of concrete structures should be put in place. Ductile detailing is a process of providing reinforcement to concrete to sustain earthquakes.
If the beam column junction shakes because of the seismic wave, it may lead to a catastrophic outcome. Therefore, the material strength is of paramount importance when it comes to constructing a building. The structure should be designed by qualified designers as well. Follow-up strategies should be adopted to check whether the structure is built as per the design. Professor Bosunia emphasised that people with little or no knowledge should not be involved with construction. At the early stages of construction, we see engineers visiting the sites to monitor the process. However, when the construction turns into a multi-storey project, they no longer scrutinise the sites fully due to the absence of lift. This is a major problem and some solutions must be found to tackle it. The construction workers should also be trained properly to build earthquake-resilient structures.
Professor Bosunia concluded by highlighting the significance of the foundation during construction. Foundation can be vulnerable to seismic effects as well. Several factors come into play (soil interaction, wind, etc.) when we take earthquake load into consideration. For instance, sand affects the pile foundation through soil liquefaction, which may make the building sink into the mire. Experts should check the soil characteristics to prevent buildings from tilting. Also, pile foundation should not be installed anywhere close to water bodies.
The interview was taken by Maisha Zaman and Labiba Faiaz Bari of The Daily Star.