Rajshahi Science And Technology University (RSTU)

BSc. THESIS PROPOSAL

Title : A Comparison of BNBC-93 with other building codes with respect to earthquake and wind analysis.

Analysis of a structure for earthquake can follow two methods:firstly dynamic analysis throughsimulation of the structure in computer or static analysisassuming the earthquake forces to be static forces. Dynamicanalysis is more accurate but involves the cumbersome modeling ofthe structure and earthquake forces. On the other hand, staticanalysis is rather simple and easier to perform. Because of itssimplicity, static analysis is preferred for high-rise buildingsin Bangladesh. Most of the current building codes have provisionsfor dynamic analysis, but they also allow equivalent static forceprocedure to determine the earthquake forces on a building. Allthese codes consider the earthquake force as a lateral force. Theforces are determined on the basis of a base shear. It is thetotal design lateral force acting at the bottom of a structure.The base shear is assumed to depend on all or some of thefollowing factors:

1. Seismic activity of the region2. Importance of the structure3. Type of structural system employed4. Soil profile5. Weight of the structure6. Time period

The base shear is then reallocated to various floor levels on thebasis of the load on that floor and the height of the floor fromthe base. In addition, a concentrated force is assumed to act atthe roof level. During analysis phase these lateral forces areconsidered to be live loads. The following codes have beenreviewed and compared in this work:

1. The Uniform Building Code (UBC) 19972. The Uniform Building Code (UBC) 19913. National Building Code of India (NBC-India) 19834. Outline Code of Bangladesh 19795. Bangladesh National Building Code (BNBC) 1993

In general, the lateral force provisions for earthquake invarious codes have been presented in table . It may be mentionedthat UBC-97 specifies a minimum and a maximum value of base shearto be considered for earthquake, but no other code specifies sucha closed spectrum. The seismic design provisions of these codescan be related to one another because of the same approachfollowed in all codes. BNBC-93 is similar to UBC-91 except thatthe coefficient expressing seismic zone are based on localseismological information. NBC-India-83 has been included in thestudy because India is the closest neighbor to Bangladesh andshares the same tectonic zone. Earthquake Code of Bangladesh-79has been included to determine the differences with the BNBC-93.A comparison of base shear is the simplest way to compare thefinal result. Only RC structures have been dealt with, because ofthe wide use of RC structures in Bangladesh. Two types of RCstructures have been considered: RC ductile moment resistingframed building and frame shear wall building. An office buildingof 51 ft51 ft (15.6m15.6m) plan has been considered for allcomputations.

The building is assumed to be located in seismic zone-3 (UBC) ofUSA, zone V (NBC-India-83) of India and zone 3 (BNBC-93) ofBangladesh. These zones share same seismic activity. Dense soilconditions have been assumed. Building height has been varied and

consequently 10, 15, 20 and 25 storied structures have beenconsidered for the purpose of comparison. Live load per floor hasbeen taken as 160 kip and dead load 500 kip. The base shearvalues are presented as a graphical plot. Figure 1 refers to RCductile moment resisting frame and figure 2 depicts frame shearwall building. It is seen that the Outline Code of Bangladesh-79gives the lowest value for base shears for both type ofbuildings, which is why it was later revised to incorporate moresafety. However, despite the revision of the code, BNBC-93 isstill the least conservative for RC ductile moment resistingframes. For a 10 story building BNBC-93 is even less conservativethan its predecessor, Outline Code of Bangladesh-79. The NBC-India-83 is more conservative at lower building heights than theUBC-91. Although the BNBC-93 follows UBC-91 in many aspects, thedifference in base shear values between UBC-91 and BNBC-93 is dueto the difference in the zonal factor, Z. For the same level ofseismicity, the zonal factor in BNBC-93 is lower than UBC-91.After the Kobe earthquake in 1995, UBC was made more conservativeand hence UBC-97 is the most conservative of all, providingalmost double safety than the UBC-91. UBC-97 is also 2.23 timesmore conservative than the BNBC-93 for RC ductile momentresisting frames. Barring Outline code of Bangladesh-79, BNBC-93and NBC-India-83 both are least conservative, giving almost thesame base shear for frame shear wall buildings. BNBC-93, though,gives more conservative values for higher stories. UBC-91 is moreconservative than both these codes. UBC-97 is, as usual, the mostconservative. On an average, UBC-97 is 2.61 times moreconservative than the BNBC-93 for framed shear wall buildings.The higher factor of safety as compared to RC ductile momentresisting frames can be attributed to the fact that frame shearwall buildings are more rigid and their collapse is morecritical. Therefore, BNBC-93, suggests the least conservativebase shear values and is designed for only 38% to 45% seismicload specified by the UBC-97. While developed countries are goingfor more conservative design, this contradiction of BNBC-93 couldbe suicidal. Some modifications need be made in this respect.

COMPARISON OF WIND CODESApart from the small hilly region in the south east and the northeast, Bangladesh is a vast plain land adjacent to the Bay ofBengal. The estimated basic wind velocity in the capital Dhaka,located at the geographic center of the country, is as high as210 km. Beside this high natural wind, the country is alsoaffected by adverse cyclonic action. Specially, the coastal areasare prone to severe cyclonic weather. This is why wind forceanalysis is also very important for high rise buildings in thecountry. For wind pressure, BNBC-93 has been compared with UBC-97

and NBC-India-83. Basic features of these codes are presented intable 3. In BNBC-93, calculation of design wind pressure is atwo-step process. In the first step, the sustained wind pressureis calculated on the basis of importance of structure, height andexposure condition and basic wind speed, which in turn depends onthe region the structure is located in. The exposure of thestructure to wind forces is a function of terrain type,vegetation and built up environment in the surrounding. Thesustained wind pressure is then converted to design wind pressureby multiplication with the gust coefficient and pressurecoefficient for the structure. Pressure coefficient considers thedirection of wind relative to the structure and roof slope. InNBC-India-83, the design wind speed at various heights aredetermined first on the basis of risk level, terrain roughness,height and size of structure and local topography. The terrainfactor refers to exposure category. In addition another factordescribes the local topography e.g. hills, valleys, cliffs,

ridges etc. In the second step, design wind speed is converted topressure by a simple conversion

factor. In UBC-97, the calculations have been made simpler,giving the design wind pressure in one direct step. Design windpressures for buildings and structures is determined for anyheight on the basis of height, exposure and gust, direction ofwind relative to structure, roof slope, importance ofthestructure and wind stagnation pressure. Wind stagnationpressure is again a function of basic wind speed.The definitionof basic wind speed is same for all these codes: it is thefastest-mile wind speed associated with an annual probability of0.02 measured at a point 33 ft above the mean ground level inaflat and open terrain. The exposure category has been definedslightly differently in UBC-97 and BNBC-93. For same condition,the comparison of codes reveals that BNBC-93 is more conservativethan NBC-India-83. There is not much difference between BNBC-93and UBC-97.

COMPARISON OF WIND AND SEISMIC LOAD AS PER BNBC-93

It is a common practice among design engineers in Bangladesh touse earthquake forces for designing buildings ranging from 8 to20 stories, and wind for buildings higher than 20-stories.Engineers neglect the combination of earthquake and wind load onaccount of the assumption that the earthquake and severe windwill not act simultaneously on the structure. To check thevalidity of the assumption, 2 types of buildings, ie. RC momentresisting frames (R=12) and framed shear wall (R=8) buildingshave been considered. It may be mentioned that the type ofstructural system affects only the earthquake forces, wind forcesremain unaffected by the structural system of the building,unless the structure exhibits a wind-breaking mechanism. Thelocation of the building is capital Dhaka. The result of thestudy is presented in figure 3 for base shear and figure 4 forbase moment. Base shear computations indicates that seismicforces govern the design of the rigid frame shear wall structureupto 23 stories, wind load taking over beyond that. For RC momentresisting frames, wind forces govern at 10 stories and above.Moment at base also suggests the same trend, only difference isthat the wind forces start governing at a higher altitude.