Apparently keeping everything all right, still a building can collapse, if it is erected on lowland. It has been noticed that the owner has purchased a plot on the brink of a jheel and he wants to make a 6-storied house. He hires a mistri and his workers start driving 15 to 20 ft. long, untreated sal balli piles. Even sometimes bamboo poles of the similar length are used. In course of time, the sal balli piles may vanish, eaten up by termites or borers and the building then rest entirely on the soft silt. It is bound to settle and then to collapse. Or the owner may get advice from some of his friends and fills the ditch with sand over which he starts his construction as usual. At the bottom of these wetlands below the water lies a layer of fine silt which makes it water tight. This silt layer can't take any load or in other words lacks bearing capacity. This layer is usually underlain by a layer of silty clay, clay or sand. A building made on this type of soil is bound to tilt or settle down. Settlement of a building will not necessarily be uniform or symmetrical. One corner may settle more than the others and the building will be subject to excessive bending or twisting and the building will ultimately collapse.

Building foundations must be laid on a firm ground. There are some definite methods of determining the bearing capacity of various categories of soils and also methods of piling or consolidation of the filling soil. If one doesn't care to follow those set rules then his building is liable to collapse even without an earthquake. Recent collapse of the 10-storied Spectrum Sweater factory at Savar is that. Hence, it's always better to select a high ground for any building avoiding all sorts of filled up land. If at all the lowland can't be avoided then proper investigation of the sub-soil is to be done and foundation is to be designed by a qualified and experienced Foundation Engineer.

Next comes the question of proper planning and design of the building. There are two distinctly different types of buildings -- brick-walled concrete-roof building and concrete framed building. In brick-walled building the roof load is carried by the brick walls and then the walls transmit this load to the ground through the foundation. Here we need a firm ground to carry the weight of the building. In older buildings, the roofs were made of lime concrete on 1" thick, 12" x 12" burned clay tiles supported by small wooden rafters or steel t-beams (eM©v). These are again supported by wooden beams or steel I-sections (Kwo). The lime concrete made of hydrated lime, surki (burnt brick dust) and khoa (brick chips) is placed in 6" to 8" layers on clay tiles and is hammered down to 3" to 4" thickness. The old houses of Bengal (Bangladesh and West Bengal) dating back to the Sultanate era are made with these type of terraced roof. Later at the beginning of the twentieth century during the British Raj cement concrete roof was introduced in Bengal. Still lime was profusely used in building construction in mortar and in plastering works. The old railway station buildings, the British government offices are usually of this type and many of them are still usable with little repair and maintenance.

Mosques and culverts made during the Sultanate Period are still remaining in tact in several places in Bengal where only lime and surki were used instead of any cement. In Bajra Jogini village of Sonargaon area, the present writer noticed the three-span road culvert made of small bricks and lime mortar in the sixteenth century. It was surprising to notice that the small bricks in lime mortar formed a solid stony element without showing the trace of a joint. It's more interesting that the artisans who made this culvert had profound experience based knowledge in foundation works of bridges and buildings. The bridge did not tilt or settle as it did not collapse for long five hundred years but it was only corroded due to weathering on the southern face. Many old buildings of this type have been wilfully destroyed during and immediately after the liberation war. The Bhowmic Bari, a palatial house in Kushtia district was demolished by the greedy people to collect bricks, floor-stones and timber (Burma teak) of the doors, windows, staircases and sunshades. The Lahiri House of Kushtia town suffered the same fate. Surely, there may be thousands of old houses meeting the same fate all over the country.

Usually old houses of this type of construction were limited to two-storey-high with similar floor areas in the top and bottom stories with the staircase placed at the centre. Walls are usually thick, 25 to 30 in. for ground floor, and 15 to 25 in. for the top floor with thick lime mortar plaster inside. The roof height is also not less than 15ft. floor to floor with wide ventilators just below the roof level. All these provided a natural air-conditioning effect for these old buildings keeping them cool in summer and warm in winter. For wide brick foundations, usually over-burnt bricks to offset saline effects were used over an 8 to 10 inch thick layer of lime concrete. Many of these types of buildings could withstand the great earthquakes of 1897 and 1935. The profusely ornamental Kanta Mandir of Dinajpur escaped any damage but the nine minarets (chura) were destroyed in 1897 earthquake. It also destroyed the "Deul" (tower) of Jaipurhat commemorating the Great Journey of Shree Chaitannya on foot from his birthplace, Nabadwipa to his ancestral home in Shreehatta (Sylhet). No damage of any mosque was, however, recorded. In fact, brick buildings with 15 to 25 in. thick wall cemented in lime mortar and plastered in lime make them not only cool and cosy but also stable.

After partition of India in 1947, the lime stone quarries were placed outside the boundaries of the newly independent Bangladesh (then East Bengal and later East Pakiatan). Consequently, use of hydrated lime in building works was abruptly replaced by Portland cement in mortar, plaster and also in roof casting. The International Earthquake Association with its head office in Tokyo but research activities limited in California specifies that in a moderate earthquake zone, height of a masonry building should be restricted to 9 metres i.e. up to three stories. There should be a concrete tie on the plinth, over the door and window openings and also below the roof. Moreover, total area of opening in any wall will not exceed 1/3 of the vertical area of the wall itself. There shouldn't be any overhang or cantilever and protruded Cornish or chimney over the roof. Moreover the floors should be symmetrical that is the top floor should be similar as the bottom one with the staircase at the middle of the floors.

The reinforced concrete floor design is taught in the undergraduate level. Foolproof guidance is available in Building Code Requirements for Reinforced Concrete by the American Concrete Institute, which is revised in every four years, supported by discussion and comparison of simultaneous laboratory research at the University of Illinois at Urbana-Champaign, Illinois and Lehigh University at Bethlehem, Pennsylvania. It's simple to understand but easy to follow. It provides all details of RCC floor supported by brick walls or in a RCC framed building of medium height. We have also a Building Code of our own which provides similar guidance as the ACI Code. Economic design and durable construction, however, greatly depend on the academic performance and field experience of the qualified Engineer. Proper selection of span ratios and selection of floor system not only adds to the economic design but also provides stability of the structure. For example, a floor space of 40 ft. x 60 ft. can be covered up in more than one way and it's the engineer's job to find out system that will lead to an economic design ultimately yielding a saving in materials, cost of shuttering and labour. But an ordinary beam-girder arrangement with columns dividing the floor space in 20ft. x 20ft. will definitely add to the safety of the building in earthquake shaking. Again excessive reinforcing steel provided over and above the actual requirement will reduce the ductility and make the concrete structure brittle, being easily vulnerable to collapse during an earthquake. Now-a-days, designs software are available in the market but they are not foolproof. Adequate knowledge and training are necessary to use these programmes to design any structure.

A medium high (six storied) building can collapse even without an earthquake if the quality of materials and quality of construction are not rigidly controlled. For a building of reinforced concrete quality of the construction material and quality of the construction itself are very important. First class bricks, may be a little bit over-burnt will insure against saline corrosion at the slight extra cost of increased quantity of mortar used. Bricks should be soaked in clean potable water for at least 24 hours before their use. This will wash out any un-burnt salt in them. Durability of the cement mortar depends on the quality of the cement and quality of the sand. The Chhatak cement was once found to be the best cement in the country confirming the ASTM (American Society of Testing Materials) standard (also BSS). The present writer while engaged in the Structures & Concrete Laboratory of the Ahsanullah Engg College and later BUET got very good result of the Chhatak Cement. Later many other cement companies have been established in Bangladesh and many of them produce better quality cement than imported brands.

However, the dusty sand used in mortar mars the quality of the cement. According to the ACI (mentioned earlier) Code coarse (Ottawa) sand is to be used in the mortar. We have coarse Sylhet sand of FM (Fineness Modulus) 2.2 to 2.8 but for mortar we use fine Kaliakur sand (FM 1.5 to2.0) and for concrete we use a mixture of Sylhet and Kaliakur sand. This practice is followed by our Public Works Department (PWD). The present writer with his close association with the BUET laboratory couldn't find any justification of this practice. Building technology being an Applied Science, all the clauses must be supported by research and experiment instead of personal opinion. It was found that if the sand was screened and washed (i.e. the dust was removed), the strength of the mortar as well as the concrete was increased. Again if only Sylhet sand is used in concrete instead of a mixture of Sylhet and Kaliakur, the crushing strength of concrete is remarkably increased. Proper mixing, casting and curing of concrete add to the strength and durability of the concrete. And the higher is the strength of concrete the less is the creep.

Creep is the permanent deformation and deflection of concrete structural members. A concrete slab or beam is permanently bent (deflected) and excessive deflection makes it cracked. This is due to the creep factor of concrete. The crushing strength of a concrete block made of certain brand of cement, sand of a particular variety and chips of a kind can be determined within a month's time but creep factor of the same concrete takes many years to determine. A column can be dangerously shortened due to creep when the entire load will be carried by the reinforcing bars. The bars will buckle and the column will fail leading to the collapse of the building. This will happen in 10 to 15 years after construction for the sole reason keeping other factors ideal. Concrete made of brick chips instead of stone chips may have a satisfactory crushing strength but has a higher creep factor as observed by Prof. Adam Neville and confirmed by Dr. Edward Cohen, a former President of the ACI. Hence a six-storied framed building made of brick-chip concrete is bound to fail in about 10 years' time even without an earthquake.

In fine it can be said that to make a safe house with hard earned money, one must go for a good high land. Filled up wetlands or drains (wetlands and drainage channels are mostly khas lands) are not suitable for a safe and healthy home. It always proves economic to take advice of a qualified engineer with sufficient field experience to plan, design and supervise the construction of the house strictly according to the Building Codes at a small fraction of the total cost of the building. Never use inferior materials in construction of a house. Best building materials will ultimately pay off the extra material cost as these will provide the owner a durable and healthy building which will not only provide safety in an earthquake but also invariably reduce the maintenance cost.

Dr MH Rashid, FASCE, FIEAust is a consulting structural engineer; formerly a member of the Faculty of the RUET and the BUET.