Implications of the proposed BNBC code provisions on construction cost of RC buildings

Abstract

Bangladesh National Building Code (BNBC) was first published in 1993 to provide guidelines for design and construction of new buildings. Recently a thoroughly revised version of this code called BNBC-2020 has been published. The provisions of the new code is quite different from those of the previous one. Under such circumstances, engineers and designers are always inquisitive to know the effect of newer code provisions on the cost of construction. In the present study, an initiative is taken to conduct a controlled parametric and comparative study of RC frame structures based on two codes using FEM based structural analysis software to investigate the cost implication of the newer code. Structural analysis and design of five different multistoried (8, 12, 16, 20 and 24) RC building located in high seismic and high wind (Chattogram), moderate seismic and moderate wind (Dhaka), low seismic and high wind (Khulna), low seismic and low wind (Rajshahi) and high seismic and low wind (Sylhet) is conducted. Once the design of all the buildings is finalized, the detail bill of quantities (BoQ) and cost estimation of structural civil works for each of the buildings is prepared. Finally, a comparative study is conducted on the basis of detail analysis and cost estimation outcomes. This study reveals that the maximum lateral deflection and story drift under service condition in seismic loading is higher for all the studied buildings in BNBC-2020 than BNBC-1993 for all the districts under consideration. On the other hand, for wind loading condition, lateral deflection and drift are higher in BNBC-1993 compared to the same from BNBC-2020 loading. In terms of concrete volume, BNBC-2020 gives economic design than BNBC-1993. Besides, BNBC-1993 gives economic design than BNBC-2020 in terms of reinforcement requirements. When total cost of structural civil works is concerned, BNBC-2020 provisions generally results in a cost effective design for low seismic areas and produces costlier design for high seismic areas when compared with similar designs based on BNBC-1993. Though, in most cases the average cost difference is between -4.57% to +6.0%. If costs of other components such as electromechanical (lift, substation, generator, air conditioning etc.), plumbing and sanitary, finishing civil works (e.g. flooring, interior works etc.) are considered, then the cost difference would be even smaller. Therefore, the effects of recently published BNBC-2020 code on the construction cost of RC buildings may not be considered very significant.