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The Eighth East Asia-Pacific Conference on Structural Engineering and Construction 5-7 December 2001, Nanyang Technological University, Singapore

Paper No.: 1443

A COMPARISON OF BNBC-93 WITH OTHER BUILDING CODES WITH RESPECT TO EARTHQUAKE AND WIND ANALYSIS

F. Atique 1 and Z. Wadud1

ABSTRACT: 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. This paper aims at the comparison of various provisions for earthquake and wind analysis as given in building codes of different countries. Primarily Bangladesh National Building Code, 1993 (BNBC-93) has been studied and compared with Uniform Building Codes, 1991 and 1997 (UBC-91 and UBC-97), National Building Code of India, 1983 (NBCIndia-83), and Outline Code of Bangladesh, 1979. The study revealed that the developed countries have increased the factor of safety against earthquake by suggesting higher values of base shear. But BNBC is the least conservative as compared to other codes and practices. This may have serious implications in case of a major earthquake in the country. Wind analysis of BNBC is quite similar to those of other codes.

KEYWORDS: Earthquake forces, wind analysis, static analysis, base shear, base moment, building codes,

structural systems

1. INTRODUCTION Earthquake hazards have been the prime concern of structural engineers for many years. Advanced research has been and is being carried out throughout the world to make structures earthquake resistant. Bangladesh lies in the vicinity of world's loftiest mountain range, the Himalayas and is well within an active tectonic zone. Therefore the country is prone to major earthquakes. Because of lack of advanced modeling and computational facilities in Bangladesh and other developing countries, earthquake and wind analysis are carried out by static analysis method, within the bounds of building codes and practices of respective countries. As the number of high rise buildings is increasing, the code to be followed for building design, detailing and construction is becoming an important aspect. This paper is aimed to review and compare some of the current seismic design provisions dealing with the specification of seismic design forces. In addition, building codes for wind analysis have been compared. Also, seismic and wind forces on a typical building have been compared. 2. EARTHQUAKE HAZARDS IN BANGLADESH Due to the lack of sophisticated earthquake monitoring equipment and facilities in Bangladesh, the level of earthquake activities is poorly defined. As a result, data on micro-seismicity is totally missing, although such activities are quite frequent. The distribution of all major earthquakes recorded and reported in and around Bangladesh is presented in table 1. The table shows that there has not been any major earthquake in Bangladesh for the last 50 years. The return period of a major earthquake with a magnitude of 6.8 is 50 years, while that of a magnitude 7.4 is 100 years. The probabilities of occurrence of such tremors are 98 and 99 per cent respectively. An earthquake of such magnitude is forecast in Bangladesh by 2003. Major seismic sources for the region are Assam fault zone, Tripura fault zone, Sub-Dauki fault zone and Bogra fault zone. These faults are capable of producing earthquakes of magnitudes 8.0, 7.0, 7.3 and 7.0 Richter respectively.

1

Bangladesh University of Engineering and Technology, Bangladesh, B.Sc.

Table 1. List of major earthquakes affecting Bangladesh

Date January 10, 1869 July 14, 1885 June 12, 1897 July 8, 1918 July 3, 1930 January 15, 1934 August 15, 1950 Name of the earthquake Cachar Earthquake Bengal Earthquake Great Indian Earthquake Srimongol Earthquake Dhubri Earthquake Bihar-Nepal Earthquake Assam earthquake Magnitude (Richter) 7.5 7.0 8.7 7.6 7.1 8.3 8.5 Epicentral distance from Dhaka (km) 250 170 230 150 250 510 780

3. COMPARISON OF SEISMIC CODES Analysis of a structure for earthquake can follow two methods: firstly dynamic analysis through simulation of the structure in computer or static analysis assuming the earthquake forces to be static forces. Dynamic analysis is more accurate but involves the cumbersome modeling of the structure and earthquake forces. On the other hand, static analysis is rather simple and easier to perform. Because of its simplicity, static analysis is preferred for high-rise buildings in Bangladesh. Most of the current building codes have provisions for dynamic analysis, but they also allow equivalent static force procedure to determine the earthquake forces on a building. All these codes consider the earthquake force as a lateral force. The forces are determined on the basis of a base shear. It is the total design lateral force acting at the bottom of a structure. The base shear is assumed to depend on all or some of the following factors: 1. Seismic activity of the region 2. Importance of the structure 3. Type of structural system employed 4. Soil profile 5. Weight of the structure 6. Time period The base shear is then reallocated to various floor levels on the basis of the load on that floor and the height of the floor from the base. In addition, a concentrated force is assumed to act at the roof level. During analysis phase these lateral forces are considered to be live loads. The following codes have been reviewed and compared in this work: 1. The Uniform Building Code (UBC) 1997 2. The Uniform Building Code (UBC) 1991 3. National Building Code of India (NBC-India) 1983 4. Outline Code of Bangladesh 1979 5. Bangladesh National Building Code (BNBC) 1993 In general, the lateral force provisions for earthquake in various codes have been presented in table 2. It may be mentioned that UBC-97 specifies a minimum and a maximum value of base shear to be considered for earthquake, but no other code specifies such a closed spectrum. The seismic design provisions of these codes can be related to one another because of the same approach followed in all codes. BNBC-93 is similar to UBC-91 except that the coefficient expressing seismic zone are based on local seismological information. NBC-India-83 has been included in the study because India is the closest neighbor to Bangladesh and shares the same tectonic zone. Earthquake Code of Bangladesh-79 has been included to determine the differences with the BNBC-93. A comparison of base shear is the simplest way to compare the final result. Only RC structures have been dealt with, because of the wide use of RC structures in Bangladesh. Two types of RC structures have been considered: RC ductile moment resisting framed building and frame shear wall building. An office building of 51 ft×51 ft (15.6m×15.6m) plan has been considered for all computations. The

Table 2: Various earthquake codes in a nutshell

Base shear Seismic factor Importance of structure BNBC-93 ZICW/Rw Z = seismic zone factor I = importance factor f(occupancy) Rw = coefficient = f (structural system) W = total dead load + specified portions of other loads C = 1.25S/T2/3 S = f(soil type) T = time period =f (structure type) Bangladesh-79 ZIKSCW Z = seismic zone factor I = importance factor f(occupancy) K= horizontal force factor = f (structural system) W = total dead load + specified portions of other loads S = f(soil type) C = 1/(15T 1/2 ) T = time period = f (structure type) NBC-India-83 KCI0 W 0 = basic horizontal seismic coefficient K= structural performance factor C = f (T) = f (structure type) W = total dead load + appropriate live load ,I=coefficients = f (soilfoundation system) UBC-91 ZICW/Rw Z = seismic zone factor I = importance factor f(occupancy) Rw = coefficient = f (structural system) W = total dead load + specified portions of other loads C = 1.25S/T2/3 S = f(soil type) T = time period =f (structure type) UBC-97 Cv IW/RT Cv = seismic coefficient f(zone factor Z & soil type) I = importance factor f(occupancy) R= coefficient = f(structural system)

Structural system factor

Effective weight of structure Soil structure interaction

W = total seismic dead load T = time period = f (structure type)

building is assumed to be located in seismic zone-3 (UBC) of USA, zone V (NBC-India-83) of India and zone 3 (BNBC-93) of Bangladesh. These zones share same seismic activity. Dense soil conditions have been assumed. Building height has been varied and consequently 10, 15, 20 and 25 storied structures have been considered for the purpose of comparison. Live load per floor has been taken as 160 kip and dead load 500 kip. The base shear values are presented as a graphical plot. Figure 1 refers to RC ductile moment resisting frame and figure 2 depicts frame shear wall building. It is seen that the Outline Code of Bangladesh-79 gives the lowest value for base shears for both type of buildings, which is why it was later revised to incorporate more safety. However, despite the revision of the code, BNBC-93 is still the least conservative for RC ductile moment resisting frames. For a 10 story building BNBC-93 is even less conservative than its predecessor, Outline Code of Bangladesh-79. The NBC-India-83 is more conservative at lower building heights than the UBC-91. Although the BNBC-93 follows UBC-91 in many aspects, the difference in base shear values between UBC-91 and BNBC-93 is due to the difference in the zonal factor, Z. For the same level of seismicity, the zonal factor in BNBC-93 is lower than UBC-91. After the Kobe earthquake in 1995, UBC was made more conservative and hence UBC-97 is the most conservative of all, providing almost double safety than the UBC-91. UBC-97 is also 2.23 times more conservative than the BNBC-93 for RC ductile moment resisting frames. Barring Outline code of Bangladesh-79, BNBC-93 and NBC-India-83 both are least conservative, giving almost the same base shear for frame shear wall buildings. BNBC-93, though, gives more conservative values for higher stories. UBC-91 is more conservative than both these codes. UBC-97 is, as usual, the most conservative. On an average, UBC-97 is 2.61 times more conservative than the BNBC-93 for framed shear wall buildings. The higher factor of safety as compared to RC ductile moment resisting frames can be attributed to the fact that frame shear wall buildings are more rigid and their collapse is more critical. Therefore, BNBC-93, suggests the least conservative base shear values and is designed for only 38% to 45% seismic load specified by the UBC-97. While developed countries are going for more conservative design, this contradiction of BNBC-93 could be suicidal. Some modifications need be made in this respect.

0.07

UBC-91

Base Shear/Weight of Building

0.06

Bangladesh-79 NBC-India-83 BNBC-93

0.05

UBC-97

0.04

0.03

0.02

0.01 10 15 20 25

Number of Stories

Figure 1. Seismic base shear comparison for RC ductile moment resisting frames

0.16

UBC-91 Bangladesh-79 NBC-India-83 BNBC-93 UBC-97

Base Shear/Weight of Building

0.14

0.12

0.10

0.08

0.06

0.04

0.02 10 15 20 25

Number of Stories

Figure 2. Seismic base shear comparison for shear wall frame buildings 4. COMPARISON OF WIND CODES Apart from the small hilly region in the south east and the north east, Bangladesh is a vast plain land adjacent to the Bay of Bengal. The estimated basic wind velocity in the capital Dhaka, located at the geographic center of the country, is as high as 210 km. Beside this high natural wind, the country is also affected by adverse cyclonic action. Specially, the coastal areas are prone to severe cyclonic weather. This is why wind force analysis is also very important for high rise buildings in the country. For wind pressure, BNBC-93 has been compared with UBC-97 and NBC-India-83. Basic features of these codes are presented in table 3. In BNBC-93, calculation of design wind pressure is a two-step process. In the first step, the sustained wind pressure is calculated on the basis of importance of structure, height and exposure condition and basic wind speed, which in turn depends on the region the structure is located in. The exposure of the structure to wind forces is a function of terrain type, vegetation and built up environment in the surrounding. The sustained wind pressure is then converted to design wind pressure by multiplication with the gust coefficient and pressure coefficient for the structure. Pressure coefficient considers the direction of wind relative to the structure and roof slope. In NBC-India-83, the design wind speed at various heights are determined first on the basis of risk level, terrain roughness, height and size of structure and local topography. The terrain factor refers to exposure category. In addition another factor describes the local topography e.g. hills, valleys, cliffs,

Table 3: Comparison of building codes with respect to wind force determination

BNBC-93 Qz =Cc CI Cz Vb 2 q z = sustained wind pressure at 2 height z, kN/m CI = structure importance coefficient Cc = velocity to pressure conversion = 47.2 x 10-6 Cz = combined height and exposure coefficient Vb = basic wind speed in km/h Pz = CG Cp q z Pz = design wind pressure at 2 height z,kN/m CG = gust coefficient Cp = pressure coefficient Vb = 180 km/hr CI = 1.00 Cz = 1.539 (at 45 m) Pz = CG Cp q z CG = 1.133 Cp = 0.8

2 Pz= 2.13 kN/m

NBC-India-83 Vz = Vb k1 k2 k3 Vz = design wind speed at any height z in m/s Vb = basic wind speed in m/s k1 = probability factor k2 = terrain height and structure size factor k3 = topography factor

UBC-97 P = Ce Cq q s Iw P = design wind pressure Ce = combined height , exposure and gust coefficient Cq = pressure coefficient for the structure Iw = importance factor q s = wind stagnation pressure at the standard height of 33 feet.

p z = 0.6 Vz2 2 p z = design wind pressure in N/m at height z Vb = 50 m/s K1 = 1.0 K2 = 1.075 K3 = 1.0 Vz = 53.75 m/s Pz= 0.6 Vz2 Pz = 1.73 kN/ m2

2 q s = 1.535 kN/m [Vb =180 km/hr] Ce = 1.76 Cq = 0.8 Iw =1.0

P = 2.16 kN/m2

ridges etc. In the second step, design wind speed is converted to pressure by a simple conversion factor. In UBC-97, the calculations have been made simpler, giving the design wind pressure in one direct step. Design wind pressures for buildings and structures is determined for any height on the basis of height, exposure and gust, direction of wind relative to structure, roof slope, importance of the structure and wind stagnation pressure. Wind stagnation pressure is again a function of basic wind speed. The definition of basic wind speed is same for all these codes: it is the fastest-mile wind speed associated with an annual probability of 0.02 measured at a point 33 ft above the mean ground level in a flat and open terrain. The exposure category has been defined slightly differently in UBC-97 and BNBC-93. For same condition, the comparison of codes reveals that BNBC-93 is more conservative than NBC-India-83. There is not much difference between BNBC-93 and UBC-97. 5. COMPARISON OF WIND AND SEISMIC LOAD AS PER BNBC-93 It is a common practice among design engineers in Bangladesh to use earthquake forces for designing buildings ranging from 8 to 20 stories, and wind for buildings higher than 20-stories. Engineers neglect the combination of earthquake and wind load on account of the assumption that the earthquake and severe wind will not act simultaneously on the structure. To check the validity of the assumption, 2 types of buildings, ie. RC moment resisting frames (R=12) and framed shear wall (R=8) buildings have been considered. It may be mentioned that the type of structural system affects only the earthquake forces, wind forces remain unaffected by the structural system of the building, unless the structure exhibits a wind-breaking mechanism. The location of the building is capital Dhaka. The result of the study is presented in figure 3 for base shear and figure 4 for base moment. Base shear computations indicates that seismic forces govern the design of the rigid frame shear wall structure upto 23 stories, wind load taking over beyond that. For RC moment resisting frames, wind forces govern at 10 stories and above. Moment at base also suggests the same trend, only difference is that the wind forces start governing at a higher altitude.

460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 8

seismic (RC moment resisting frame) seismic (frame shear wall) wind

Base shear (k)

10

12

14

16

18

20

22

24

No. of stories in building

Figure 3. Comparison of base shear for wind and seismic force in a typical building at Dhaka

75000 70000 65000 60000 55000 50000 45000 40000 35000 30000 25000 20000 15000 10000 5000 0 8 10 12 14 16 18 20 22 24 seismic (RC moment resisting frame) seismic (frame shear wall) wind

Base moment (k-ft)

No. of stories in building

Figure 4. Comparison of base moment for wind and seismic force in a typical building at Dhaka 6. CONCLUSION Bangladesh lies on an active seismic zone and is prone to major earthquakes. But the earthquake design provisions in BNBC-93 is the least conservative among the current codes compared in this paper. This may hamper the integrity of the structure and cause serious loss of life and properties in case of a major earthquake. This calls for a more conservative approach in the seismic design of the buildings in Bangladesh. Also, wind loads should not be ignored and should to be properly catered for in the design of medium to high-rise structures. 7. REFERENCES [1] [2] [3] [4] [5] ICBO, Uniform Building Code, California, 1991 ICBO, Uniform Building Code, California, 1997 ISI, National Building Code of India, 1983 HBRI and BSTI, Bangladesh National Building Code, Dhaka, 1993 Bari, M.S. and Khondoker, J.U., " Seismic forces on buildings: A comparative study of different codes", Journal of Civil Engineering, Institution of Engineers, Bangladesh, Vol. CE 27, No. 2,1999

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