NEW SEISMIC PARAMETERS FOR BUILDING CODE OF PAKISTAN AND THEIR EFFECT ON EXISTING REINFORCED CONCRETE BUILDINGS: A CASE STUDY.
ABSTRACT: The large scale devastation caused by the earthquake of October 08, 2005 in Northern Pakistan and Azad Jammu and Kashmir has raised several questions on the adequacy of the present design and construction practice in the country. Realizing the gravity of the situation, the Government of Pakistan appointed a Committee of technical experts and Government Officials to supervise and provide guidance for revision/updating of Building Code of Pakistan, to suggest modifications to the present codes of practice and to particularly incorporate the recommendations for earthquake resistant design of buildings. Therefore, It is important to check the adequacy of existing structures especially in high seismic risk zones according to revised seismic parameters. The National Insurance Complex Limited (NICL) Building, Jinnah Avenue, Blue Area Islamabad is selected for study. It was constructed in 1994.
In this research seismic analysis has been performed for National Insurance Complex Limited (NICL) building. These analyses include linear static analysis, response spectrum analysis, non-linear static analysis and linear time history analysis which are performed using ETABS. Different analysis approaches have been used to carry out the lateral analysis on the basis of BCOP 2007 and the time histroy of October 8, 2005 earthquake. The study revealed that base shear is increased by 7% due to the change in the seismic zoning for the current location of the building. It is concluded that the change in seismic zoning has not seriously threatened the stability of existing buildings. The buildings which are then designed according to some recogonized building codes when analyzed according to BCP-2007 should fall safe. However failure may be expected incase of the building is not preperly designed or poorly constructed.
Key words: Seismic zone, base shear, drift.
The large scale devastation caused by October 08, 2005 earthquake in Northern Pakistan and Azad Jammu and Kashmir has raised several questions on the adequacy of the present design and construction practice in the country(Ilyas, M and Rizwan,M. 2006). Realizing the gravity of the situation, the Government of Pakistan appointed a Committee of technical experts and Government Officials to supervise and provide guidance for revision/updating of Building Code of Pakistan, to suggest modifications to the present codes of practice and to incorporate the recommendations for earthquake resistant design of buildings.This committee resulted in the development of new building code of Pakistan which is abbreviated as BCOP-2007.It is expected that the structures constructed under any established earlier building codes may become nonconforming in relation to the revised seismic parameters present Building Code of Pakistan 2007.
Proper design and detailing considering realistic seismic parameters with good construction pract ces will reduce the devastation to a much lesser extent by reducing causalities.
It is essentially important to check the adequacy of existing structures specially in high seismic risk zones according to revised seismic parameters. The main purpose of the proposed research work is to review the design of existing reinforced concrete building considering revised seismic parameters and to ascertain the adequacy for resistance against adverse effects of earthquake induced forces. Moreover, to suggest remedy for different structural components in order to ensure their safety and stability. An urgent action is needed to avoid failure of vulnerable structures.
There is a lack of awareness for seismic protection in many parts of the country. In high seismic risk zones, time history analysis for high rise structures must be carried out to depict actual behavior. The compliance of the specified earthquake resistant design and construction practice must be ensured through appropriate legal, administrative and technical control. New buildings must be earthquake resistant in order to prevent constant addition to existing vulnerable structures that are already seriously threatened.
Gulten and Calim (2003) studied the torsionally unbanlanced multistorey RC structures. They found that by introducing 5% eccentricity an increase of about 10% bending moments in the columns and beams located at critical locations, occurred.
Erduran and Yakut (2004) studied the seismic performance for an RC frame system. According to them detailed assessment procedures generally require a full blown seismic analysis of the building to determine the forces and deformations experienced by its components under a presumed level of earthquake intensity. A number of widely used such procedures (FEMA 356, ATC-40 and EUROCODE 8) compare these demands with the recommended values of member capacities varying with the level of the performance objectives employed.
Yun et. al. (2006) presented a procedure for analyzing the building which was based on non linear and reliablilty theory. This procedure gives simplified method for the analysis of steel moment frames with fair accuracy specially during the non linear analysis. Hugo (2003) has devised various principles for engineers and architects for the seismic design of the buildings. It can be concluded from this report that any building which is designed using the principles mentioned in the report of the author will perform well during an earthquake.
Young et al. (1985) developed the damaged model for reinforced concrete. They investigated the full aapacity of member which may be used during an earthquake. Ahsan and Saif (2008) investigated the failure due to the kashmir hazara earthquake (2005) and concluded that although knowledgeable and competent structural engineers are avaialble in Pakistan but the execution of construction with respect to the design is very important. They also empahsized that the recommendations suggested by them are very significant for new construction. This will help in the reduction of property loss and human lives.
Building description: The National Insurance Complex Limited (NICL) Building, Jinnah Avenue, Blue Area Islamabad is selected for study. It was constructed in 1994. The building mainly comprises of two blocks, Tower block and low-rise block which are separated from each other by an expansion joint. The Tower block is mainly used for offices and the low-rise block is used for parking of vehicles. Tower block has sixteen floors with two basements and low-rise block has two basements and two floors. After the earthquake of October 08, 2005 some cracking was observed in the masonry walls which are non-structural in nature and does not impair strength of the structural system. Some cracks were also observed in the construction joints of parapet wall and retaining wall which do not cause structural instability. The material properties considered in the design were verified using non destructive methods.
Structural system: The structural system for the NICL building is an essentially space frame providing support for gravity loads and resistance to lateral load is mostly provided by concrete shear walls and lift-well walls. Slab system consists of two-way slab with shallow wide Ribs acting as column strips along the column centre lines. Flexural effective width of the slab also acts as a wide shallow beam to transfer gravity and other unbalanced forces to the columns and shear walls. Reinforcement has been laid out as for two-way slab system with concentration of reinforcement in the column strips.
Concrete shear walls and lift-well walls provide lateral resistance in two orthogonal directions. Structural members (columns shear walls and lift-well walls) resist the total seismic lateral loads in proportion to their relative rigidities with shear walls resisting almost 76 percent of total seismic base shear loads due to their greater stiffness in the two principal directions. Shear walls at the periphery of the building are located at each corner of the building whereas lift-well walls are located eccentrically with respect to the building center. Foundation system of the Tower Block building employs cast-in-place piles with thick pile caps providing load transfer mechanism from columns and shear walls to the deep foundations. Low-rise block uses raft foundation.
Seismic parameters: All structures and their components are analyzed to determine their adequacy to withstand lateral forces caused by seismic loads. The original design of the building was based on BCP-1986 with following seismic parameters:
Z. =###Seismic Zone factor###=###0.30
I. =###Importance factor =###=###1.00
Ct. =###Numerical coefficient###=###0.03
S. =###Soil factor###=###1.50
Rw. =###Numerical Coefficient###=###12.0
Above parameters have been revised in the Building Code of Pakistan 2007. New seismic provisions, which are based on revised seismic zoning map of Pakistan which is shown in Figure-1.
Few parameters which have been revised by the BCOP-2007 are shown below.
Z. =###Seismic Zone factor###=###0.20
I. =###Importance factor =###=###1.00
Cv. =###Seismic coefficient###=###0.32
Ca. =###Seismic coefficient###=###0.28
R. =###Numerical Coefficient###=###5.50
S. =###Soil profile type =###=###SD
Material properties: Reinforcement yield strength considered as per original design criteria of National Insurance Complex Limited building is 60,000 psi. Following concrete strengths taken from original design criteria:
Shear walls###5000 psi
Dead loads: Dead loads are the vertical loads due to the weight of all permanent structural and non-structural components of a building, such as walls, built-in and moveable partitions, floors, roofs and finishes including all other permanent construction. Following loads are used in analysis as taken from original design criteria
Live loads: Live loads include loads due to intended use and of an area onnel, moveable equinments lateral earth nressures ehicle and impact loadings. For National Insurance Complex Limited building, floor live loads are taken as per occupancy and intended use requirements, from original design criteria as given below.
Parking Floor###100 psf
Ground Floor###100 psf
Mezzanine Floor###60 psf
First Floor###60 psf
Typical Floor###60 psf
Roof Floor###30 psf
Wind loads: Lateral loads due to wind were imposed on the building using wind velocity of 80 mph and Exposure Category-C as per BCP-2007. Wind loads were combined with other applicable loads as per BCP-2007 load combinations.
Analytical modeling: The analysis for the study is carried out using Extended Three Dimensional Analysis of Building System ETABS Nonlinear Version. The building is modeled in 3-D with spatial distribution of masses and stiffness of the structural system. Tower Block of the building is modeled separately as there is 1 inch wide expansion joint separating the two blocks. Superstructure is modeled by using discrete frame elements for columns, beams, shell elements, slabs, shear walls and other concrete lift-well walls. Concrete dimensions of frame elements were based on sizes of the structural members provided in the structural drawings. Materials properties used in the review are as provided in the original design criteria of the building. In order to carry out the study different types of computer models are prepared in software ETABS which are given in Table-1.
Table-1 Types of analysis and their nomenclature
LS-A 86###Building is modeled with 3D space frame, with un-cracked sections considering provisions of BCP 1986. LS-B 86 3D interior frame of building is modeled with cracked sections considering provisions of BCP 1986.
LS-A 07###Building is modeled with 3D space frame, with un-cracked sections considering provisions of BCP 2007. LS-B 07 3D interior frame of building is modeled with cracked sections considering provisions of BCP 2007.
RS 86###Building is modeled with 3D space frame, shear walls, lift wells and slab system considering provisions of BCP 1986. RS 07###Building is modeled with 3D space frame, shear walls, lift wells and slab system considering provisions of BCP 2007. NLS 86###3D interior frame of building is modeled with beam and column elements considering provisions of BCP 1986.
NLS 07###3D interior frame of building is modeled with beam and column elements considering provisions of BCP 2007. NLS 01 3D interior frame of building is modeled with beam and column elements.
NLS 02###3D exterior frame of building is modeled with beams, columns and shear wall elements. TH 01 Building is modeled with 3D space frame considering actual time history record.
TH 02###Building is modeled with 3D space frame considering scaled time history record.
Discussion on results: The different analysis options which are shown in Table-1 were analyzed on separate computer model based on the parameters which have also been mentioned in the previous sections. The main considerations were given on the story drifts and drift ratios because the divergence of these values during different analysis options can comment on the status of the structure.
The linear static analysis and Response spectra analysis based on BCOP-86 and BCOP-07 is shown in figure -2
The linear static analysis of building due to revised seismic zoning of Pakistan increases the base shear by 7%. Due to the increase in base shear maximum lateral displacement at top of building increases by 32%. It was also observed that the maximum storey drift for eigth storey increases to 0.42 The structural system of building shows unsymmetrical distribution of stiffness which result in additional torsion, due to this reason the lateral displacements for elastic response spectrum analysis are on higher side to that of linear static analysis.
It was observed that the maximum storey drift for storey number seven is having a storey drift ratio of 0.6. Shear wall and lift wells attract 76% of total base shear and 24% of total base shear is taken by reinforced concrete frame.
The next set of analysis was carried out for non linear behavior of building. The behaviour of the building for lateral displacements and storey drift ratios is shown in Figure-3.
Non-linear static analysis of building has shown that maximum storey drift is experienced by storey number three with storey drift ratio of 1.1 which is due to the presence of mezzanine floor in the building. Plastic hinges are formed at beams only showing strong column weak beam structure. For revised seismic parameters the building remains in state of immediate occupancy. Push over analysis reveal that the beam at third floor will yield to collapse at base shear of 0.547 times the weight of structure.
The results of NLS-01 and NLS-02 is shown in figure -4 and 5. The two analysis differ in a sense that NLS-01 was without considering shear wall and NLS-02 was with shear wall.
The later displacement for both the cases remains the same, however the collapse load occur for beams at story-3 at 0.345W ("W= Total weight of structure") where as in case of NLS-02 the collapse load occurs at fourth storey at 0.547W as eminent from figure 4 and 5 as well.
Time history record of October 08, 2005 at Islamabad is collected and TH-01 is prepared in ETABS. Time history record is shown in Figure -6. Time history analysis of NICL building is carried out using ETABS and buildings response is studied.
TH-01 analysis of NICL building reveals that the structure suffered maximum displacement at roof with magnitude of 3.40 inches at time of 71 second and global drift of 0.13%. At this displacement magnitude and global drift ratio the structure should not suffer any crack in non structural walls whereas NICL building actually suffered cracks in its non structural walls.
Further more the floor acceleration at roof after TH-01 analysis comes to be 0.033g. At this magnitude of floor displacement the occupants should not experience any sever shaking whereas the occupants at top floor experienced violent shaking. It means that the time history record is probably scaled down. The displacement response of NICL building at roof is shown in figure-7 and acceleration response is shown in figure-8. As the time history record at Nilor for October 08, 2005 earthquake in Kashmir and Hazara seems to be scaled down so TH-02 is prepared in ETABS. In order to perform time history analysis of NICL building for scaled up time history, Original E-W component is scaled up to achieve the value of global drift of 1.0% and to the reach peak acceleration of 0.2g to make it compatible with seismic zone factor of seismic zone 2b. The Scaled up E- W Component of October 08, 2005 earthquake at Nilor is shown in figure-9.
TH-02 analysis of NICL building reveals that the structure suffered maximum displacement of 30.76 inches at time of 71 second and global drift of 1.13%. The displacement response at roof of NICL building is shown in figure 4.21. Further more the peak acceleration observed is 0.296g at roof. Acceleration response of NICL building is shown in figure -10.
Conclusions: It is concluded that the change in seismic zoning has not threatened the stability of NICL building which is because of its proper design, detailing and construction. According to the previous building code of Pakistan, it is concluded that the buildings which are designed according to previous building codes when analyzed according to present building code of Pakistan with revised parameters should satisfy seismic requirements. However, if it is not the case then they were not either properly designed or were poorly constructed.
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|Author:||Sharif, M. B.; Qazi, A. U.; Ilyas, M.; Mohsin, N.|
|Publication:||Pakistan Journal of Science|
|Date:||Jun 30, 2011|
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