Bangladesh lies well within an active seismic zone and is prone to earthquakes. To determine earthquake forces on a structure, static analysis has gained popularity in the country and also in many other countries because of the simplicity of the method. This calls for the use of an established and tested building code so as to ensure the safety of the structure and its occupants against the natural hazard. Bangladesh National Building Code (BNBC) was first organized in the year of 1993 to fulfill the purpose [1]. As the number of high rise buildings is increasing, the international codes followed for building design, detailing and construction is revised quite frequently to adopt the new practices. Initiative has already been taken to update BNBC 1993 and a draft copy has already been prepared [2] called BNBC 2010 (draft). A total change at wind load and earthquake provisions in the proposed code can be noticed [1, 2]. This paper is aimed to review and compare the current and proposed seismic design provisions dealing with the specification of seismic design forces among the existing and recently proposed BNBC codes as well as other codes of different countries. The researcher have made a similar study [3] where they compared the BNBC 1993 code with contemporary codes like Uniform Building Code (UBC) 91 and UBC 97, National Building Code of India, 1983 (NBC India-83), and Outline Code of Bangladesh, 1979.
National Building Code Of India.pdf
The method of calculation of seismic loading is more or less same in BNBC 2010 (draft) and BNBC 1993. Both these codes consider the earthquake force as a lateral force. The forces are determined on the basis of a base shear by Equivalent Lateral Force procedure. Base shear is calculated on the basis of seismic zone factor, structural importance factor and response reduction factor which is a function of structural system. Time period and soil type as a function of acceleration spectrum (Cs) defined by BNBC 2010 and as a function of numerical coefficient (C) defined by BNBC 1993 are used in the expression of base shear (Table 1). The base shear/weight ratios have been compared graphically with respect to the height of the building. NBC-India 2005 code also follows somewhat similar approach [3]. Base shear/weight ratios are computed from the given formula and hence plotted against height.
Seismic weight is the total dead load of building or structure, including partition walls, and applicable portions of other imposed loads. In BNBC 2010 (draft) a minimum of 25 % of live load is applicable for live load less than equal 3 KN/m2, otherwise 50 %. But in BNBC 1993 a minimum of 25 % of the floor live load shall be applicable irrespective of live load. Total weights of permanent equipments are considered in both codes. Allowance for partition wall is considered in BNBC 93 but it is not considered in BNBC 2010 (Draft). In Indian code seismic weight is the total dead load plus appropriate amounts of specified imposed load.
Figure 3 is a plot of factored base shear for the maximum seismic loading that is governed by each of the respective codes and it shows proposed BNBC code exceeds the Indian code by some margins. Another graph (Fig. 4) is plotted below showing the seismic base shear for Jessore and Kolkata having similar tectonic and geological features but defined as low and moderate seismic intensity zone respectively in the respective codes. Due to the absence of Intermediate Moment Resisting Frame (IMRF) in the building system of the Indian code, Special Moment Resisting Frame (SMRF) as a lateral load resisting system is considered for moderate seismic risk. Soil type is assumed as before. Kolkata is found higher in terms of factored base shear.
According to the ASCE 7 05, each building is assigned to one of the six structural design categories (SDC) depending on risk category and the values of Ss and S1. On such basis, Sylhet is found to be under SDC D. The base shear/weight ratios are plotted for Sylhet following Equivalent Lateral Procedure which is applicable to a SDC D category building with no certain vertical or horizontal irregularities unless T > 3.5 Ts. Since BNBC 2010 and ASCE 7 05 have similar load factors, graph is plotted on the basis of nominal base shear. Existing BNBC 1993 code provides less base shear values compared to ASCE 7 05 standard. But the revised base shears in the proposed BNBC 2010 code will definitely be much closer to that of ASCE 7 05 standard (Fig. 5).
Therefore, this increase in factor of safety against the earthquake imposed by the proposed BNBC 2010 code by suggesting higher values of base shear is appreciable. But remarkably higher reinforcement requirement in ground floor column of low storied buildings than before might be a concern for building design in Bangladesh by the proposed code. Further studies need to be made in this aspect. 2ff7e9595c
Comments