Open Access The Current Situation and Development of Fire Resistance Design for Steel Structures in China

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1 Te Open Construction and Building Tecnology Journal, 010, 4, Open Access Te Current Situation and Development of Fire Resistance Design for Steel Structures in Cina Jinceng Zao* and Xiuying Yang Department of Civil Engineering, Sangai Jiao Tong University, Sangai, 0040, Cina Abstract: A review of te current situation and development of fire safety design for steel structures in Cina is given in tis paper. Several codes and specifications used for fire safety design in Cina are introduced, and te present design metods are summarized. Ten some researc acievements on te beavior of steel structures at elevated temperature are presented. Te acievements include material properties of steel at ig temperatures, finite element teoretical analysis and experimental researc on steel structures and structural members, ig-temperature beavior of beam-to-column joints, fire resistance of composite structures and structural members, and etc. From all of te content, te conclusion is drawn tat te fire safety researces of steel structures need to be furter carried out bot teoretically and experimentally, altoug obvious works ave been acieved. Keywords: Steel structures, fire resistance design, building codes, specifications, researc acievements. 1. INTRODUCTION Te steel structures ave been used widely in Cina, and te fire resistant beavior of steel structures as drawn more attention, more and more researces ave been carried out on fire safety design of steel structure, and many acievements ave been obtained. In Cina, tere are several national building codes currently in use for fire protection or prevention design of different kinds of buildings, for instance, Building code for fire protection design (GB ), Code for fire protection design of tall buildings (GB ), Tecnical specification for steel structure of tall buildings (JGJ 99-98), Tecnical specification for application of fire protection coating for steel structure (CECS4-90). Most of tem are mainly concerned wit te active metods for fire prevention, wile te tecnical code CECS 00: 006 was specially developed for fire safety design of steel structures by advanced metod, wic is put in force since BUILDING CODES AND SPECIFICATION.1. Building Code for Fire Protection Design (GB ) [1] For fire safety design of buildings, te most commonly used building code is te Building Code for fire protection design (GB ) approved by former Cina s State Development Planning Commission in In tis building code, buildings are divided into four groups wit different fire resistance grade according to te building s importance. For buildings wit different fire resistance grade, te fire duration requirement of structural or non-structural members is different. Te list of te fire Address correspondence to tis autor at te No. 800 Dong Cuan Road, Minang, Dept. of Civil Engineering, Sangai Jiao Tong University, Sangai, P. R. Cina (postalcode:0040); Tel: ; Fax: ; jczao@sjtu.edu.cn duration requirement for different kinds of building members is given in tis code, seen as Table 1. According to te test results under ISO Standard 834, te fire resistance limit of various kinds of building members wit different section sizes were rougly listed in appendix two of GB It sould be pointed out tat te Building Code for fire protection design (GB ) is only valid for: (1) residential buildings wit 9 or less stories and oter civil buildings wic are not taller tan 4 meters or one-storey public buildings wic are taller tan 4 meters. () onestorey, multi-storey and ig-rise industrial buildings... Code for Fire Protection Design of Tall Buildings (GB ) [] For residential buildings wit 10 stories or more tan 10 stories and oter civil buildings wit a eigt of more tan 4 meters, tere is anoter national standard Code for fire protection design of tall buildings (GB ) approved by Cina s Ministry of Construction in In tis building code, all of te tall buildings were divided into two general categories wit different fire resistance grade. For eac category, te code specified te fire duration requirement for different kinds of building members just being similar to tose listed in Table 1. For buildings wit a eigt over 50 meters, special measures approved by related experts sould be taken for fire safety design..3. Tecnical Specification for Steel Structure of Tall Buildings (JGJ 99-98) [3] In te national building code code for design of steel structures (GB ), tere are almost no items regarding te fire safety design. In 1998, te Tecnical specification for steel structure of tall buildings was issued. It / Bentam Open

2 56 Te Open Construction and Building Tecnology Journal, 010, Volume 4 Zao and Yang Table 1. Combustion Beavior and Fire Duration Requirement for Different Kinds of Building Members Members Fire Resistance Grade Grade 1 Grade Grade 3 Grade 4 Fire wall Wall Load-bearing wall Hard-combustible 0.5 Non load - bearing wall 0.50 Hard-combustible 0.5 Partition wall Hard-combustible 0.50 Hard-combustible 0.5 Column For multi -stories Hard-combustible 0.5 For single story Combustible Beam Hard-combustible 0.5 Floor Hard-combustible 0.5 Load-bearing roof members Combustible Combustible Evacuation stairs Combustible 1.50 Ceiling 0.5 Hard-combustible 0.5 Hard-combustible 0.15 Combustible was edited by Cinese Institute of Building Tecnology and approved by Cina s Ministry of Construction. In tis specification, tere is a specific capter relating to fire protection of steel structures. Some recommendations are given in tis capter. First, te combustion beavior and fire duration requirement of building members are specified, tey are rougly as same as tose listed in Table 1 (only for buildings of grade one or two). Ten suggestions for te selection of fire protection materials and for te calculation of te material tickness are made. In te tird section of tis capter, construction details of fire protection metods for structural steel work columns, beams, floor, and roof systems are specified respectively..4. Tecnical Specification for Application of Fire Protection Coating for Steel Structure (CECS4-90) [4] Fire protection coatings are widely used for fire safety of steel structures in Cina. For tis reason, in 1990, Cina s Association of Standardization for Engineering Construction issued a national standard Tecnical specification for application of fire protection coating for steel structure. In te specification, te fire protection coatings are classified into two categories by te tickness necessary for a certain period of fire duration: (a) tin-type coatings, wit a tickness of not more tan 7mm; (b) tick-type coatings, wit a tickness from 8 to 50mm. Te application of tese two types of coatings sould refer to te following items: For internal steel works torougly exposed, ligtweigt roof steel structures and any oter kind of steel structures were decoration is needed, te tin-type fire protection coating is suitable if te fire duration requirement is 1.5 our or less; For internal steel works idden from view, ig-rise steel structures and multi-storey industrial buildings in steel structure, te tick-type fire protection coating sould be used if te fire duration requirement is more tan 1.5 our; For external steel work, appropriate fire protection coating sould be used. Tere are two ways to determine te tickness of te fire protection coating, one is test-based metod, and anoter is calculation-based metod. So te tickness of te fire protec-

3 Te Current Situation and Development of Fire Resistance Te Open Construction and Building Tecnology Journal, 010, Volume 4 57 tion coating can be determined eiter by ISO Standard 834 test results or by calculation using te metod specified in Appendix 3 of CECS4-90, seen as equation 1. ( / ) ( / ) T = W D W D T K (1) Equation 1 is valid wen W / D, T 9 mm and te fire duration t 60 minutes..5. Tecnical Code for Fire Safety of Steel Structure in Buildings (CECS 00:006) [5] It can been seen from various building codes or specifications introduced above tat te fire safety design of steel structures is mainly based upon ISO Standard 834 test of structural steel members. More clearly, some kind of fire protection materials (usually coatings, sometimes boards) must be applied to make te steel members meet te fire duration requirement specified by building codes or specifications. Te tickness of fire protection materials covering steel members is determined according to fire test results or tecnical parameters provided by manufacturers wic are also based on fire test results. Undoubtedly, tis design metod is relatively simple and convenient for engineers, but some inevitable drawbacks also exit in tis metod. Wit te rapid development of te steel structure and te researces on its fire resistance beavior, it is necessary and timely to develop an alternative to replace te customary fire test-based metod. Under tis background, in te year of 000, a local recommended specification for fire safety design of structural steel buildings, Tecnical code on fire safety of steel building structures (DG/TJ ), was approved and issued by Sangai Construction and Management Commission in Sangai. Altoug tis recommended specification as not been widely adopted for te design of practical engineering yet, te trend of advanced design rules for fire safety engineering as been sown clearly and te acievements made by Cinese researcers during te last decade of te twentiet century ave been reflected. Based on te local specification (DG/TJ ), anoter stylebook came into being in 005, wic was sent to be censored by national related department, and ten wit a quantity of modification, a new tecnical code was approved by Cina s Association of Standardization for Engineering Construction. Tis tecnical code is named Tecnical code for fire safety of steel structure in buildings, wic was put in force since Aug 006. Te remarkable igligt of te new tecnical code is tat it introduces te analytical and calculational approac to te fire resistance design of steel structures. In addition to te general rules suc as te fire resistance grade classification of structural steel buildings and te specifications of fire resistance requirements of steel members, wic are as same as tose provided in national building codes, tere are four entirely new sections regarding te fire resistance analysis of steel structures. In tis code, temperature-effect on member is equivalent to loads adding to bot ends of te member, sown as (Fig. 1), and ten te temperature-induced internal force of te member in fire is calculated as equations and 3. NT = NTe N. () f MTi = MTe M (3) fi Were ( ) NTe = set A T1+ T T0 Te T s ( ) M = E I T T 1 (a) temperature rising of member (b) equivalent loads Fig. (1). Te equivalent load of temperature-effect. It sould be noted tat only te simple calculation model aiming at structural steel members is presented in te specification, more advanced calculation model based on fire resistance analysis of wole structure as been not involved yet..6. Present Design Status According to Cinese building codes, all industrial and civil buildings in steel structure sould be protected from fire so as to meet te fire duration requirement specified by related codes or specifications. Generally, tere are several metods commonly in use for fire protection nowadays in Cina, respectively tey are: Coatings ( intumescent or non-intumescent ) protection Board protection Partially or completely encased in concrete Among tese metods, coatings protection is used most commonly for steel structures. Te procedure for fire safety design can be demonstrated by te flowing cart sown in Fig. (). It sould be noted tat te concept of performance-based fire safety design as been introduced to te practical engineering design, altoug tere are no items or parts relating to tis kind of design in te current building codes or specifications. For a long time in te past, fire safety design of building structures was prescription-based, te items or provisions in building codes or specifications must be strictly followed. Tis situation as become flexible in recent years, and te performance-based idea or concept as been reflected in te fire safety design of a series of large-span steel structures suc as Sangai Science and Tecnology Museum, Sangai Pudong International Airport, Sangai New International Exibition Center, mainly in te following aspects:

4 58 Te Open Construction and Building Tecnology Journal, 010, Volume 4 Zao and Yang Fig. (). Flowing cart for fire safety design of steel structures. Partition of fire zone: Te largest area of a fire zone is limited to 5000 square meters by Building Code for fire protection design (GB ), tis limitation as been widely surpassed were te fire action is not severe or te fire fatalness is not serious or some oter effective measures ave been taken. Some steel members are not protected against fire after deliberate consideration or calculation. Te application of advanced analytical models for fire safety design: in te fire design of steel work of Sangai New International Exibition Center, te fire modeling software package FPETOOL 3. developed by NIST in te US was adopted. Te design of sprinkler system referred to te NFPA (1996) standard as well. 3. RESEARCH ACTIVITIES Te researc works on fire resistance beavior of steel structures started about in te end of 1980s in Cina. Since ten, more tan ten years ad passed before considerable acievements were made in te following aspects: 3.1. Material Properties of Steel at Hig Temperatures A series of ig-temperature tests on structural steel Q35 and Q345, te most commonly used steel grades in Cina for building constructions, ave been conducted in te past several years to researc te mecanical and pysical beavior of structural steel under fire conditions. Several models of steel material properties ave been put forward for fire resistance analysis of steel structures [6-8]. A comparison among tese models and tat provided by Eurocode [9] is made as Figs. (3) and (4): In Fig. (3), f and yt f represent yield strengt of structural steel at elevated temperature and room temperature. In y0 Fig. (4), E and T E 0 respectively denote elastic modulus of structural steel at elevated temperature and room temperature.

5 Te Current Situation and Development of Fire Resistance Te Open Construction and Building Tecnology Journal, 010, Volume 4 59 From te Fig. (3), it can be seen tat tere is visible difference among different researc results about te yield strengt of structural steel at elevated temperatures altoug te results of elastic modulus are similar, te reason is tat tere is no uniform standard to define te ig-temperature yield strengt internationally for no obvious yield limit and yield platform on te stress-strain curve of steel at ig temperature. For example, EUROCODE 3 takes te stress wic strain value.0% corresponds as yield strengt, wile reference [8] selected te yield strengt according to te strain value 0.%. Fig. (3). Comparison of yield strengt of structural steel at ig temperatures. Fig. (4). Comparison of elastic modulus of structural steel at ig temperatures. Te ig-temperature tests mentioned above are all based on single pat, wic is oter transient state test or steady state test. Transient state test is te test in wic te load applied to te specimen is kept constant wile eating, wile in steady state test te load is added wit constant temperature. To take te effect of loading and eating pats into consideration, in 00, a total of 5 tests about mecanical properties of structural steel at elevated temperatures were performed in Sangai Jiao Tong University [10]. Treedimensional material models (a parametric equation of stress, strain and temperature) at different combinations of loading or unloading, eating or cooling were proposed. Two pats are taken as examples ere, one is eating and loading pat, te oter is cooling and constant-load pat. Teir parametric equations of stress-strain-temperature relationsip are expressed as equations 4 and 5 respectively, and te following (Figs. 5 and 6) sow te simulative surface patc of te two equations in bot pats. = f.357 k (4) kt y ( ) t Were = T T 0 ( ) 5 ( T ) ( + ( T )) = f y Te material models of different pats can be used to simulate te beavior of structures under real fire conditions. Because bot te eating and cooling pase exist in a real fire, and meanwile, an arbitrary point of a structural member may be at te state of loading or unloading, it is apparently unreasonable to make use of te same model for different situations. 3.. Fire Tests on Steel Structures and Structural Members In 1995, four tests on large-scale models of steel frames under different load levels and eating processes were performed in Cina for te first time [11]. All frames being tested were two-dimensional, one-storey and one-span unbraced portal frames and tey were not protected against fire. During every test, te load applied on te frame kept constant wile te temperature was increased by a speciallybuilt gas furnace, te canges in temperature and deflection of te frame were ten constantly recorded from te beginning of every test. For more detailed information about tese tests, reference [11] can be referred to. Anoter tree tests on one-storey and two-span steel frames wit several beams protected by fire coatings were carried out in te following year [1]. In 1997, to investigate te beavior of plane steel frames wit damage cumulation trougout te wole period of fire, tree totally fireprotected plane frame models were tested [13]. As for fire test on structural members, several steel columns and steel beams were tested under fire conditions [14, 15]. Te material properties of fire-resistant steel at ig temperatures were tested in reference [16], wic also carried out experiments to study te beaviors of fire-resistant steel beams and columns under ISO Standard (5)

6 60 Te Open Construction and Building Tecnology Journal, 010, Volume 4 Zao and Yang during last decade. In te analysis, finite element metod is widely adopted, and tis pattern is likely to be continued. In reference [0], a direct iteration metod for nonlinear analysis of plane steel frames in fire was presented. References [1] and [13] all dealt wit finite element analysis of plane steel frames in fire conditions, and te latter took te effects of damage cumulation into consideration. And te fire resistance beaviors of large space steel building were researced in reference [1]. Fig. (5). Te simulative surface patc of eating and loading pat. Fig. (6). Te simulative surface patc of cooling and constant-load pat. To investigate te beavior and failure mode of restrained steel beams under fire conditions, reference [17] carried out tests in 006. Tis reference studied te influence of axial restriction on steel beam and te effect of internal force on connections at bot ends of te beam. Te conclusion was drawn tat te fire resistance property of restrained steel beams is stronger tan tat of generic simply supported beams. Besides tese acievements, reference [18] and reference [19] carried out tests respectively to researc te fire resistance beavior of H-Sape steel beams and columns. Altoug fire tests are time consuming and expensive, and sometimes, because of te difficulty of test control and data collection, test results may not be satisfactory, undoubtedly, te furter experimental researc is very important and necessary, it is greatly significant for investigation on te performance of steel structures under fire conditions. Only troug tests, te real beavior of structures can be observed and te validity of a proposed analytical metod can be cecked Finite Element Analysis of Steel Structures and Structural Members A lot of teoretical researc works on fire resistance beavior of steel structures and steel members as been done To analyze te mecanical performance and distortion rule of te plane steel frames under fire conditions, a finite element routing process was compiled in reference []. Based on te virtual work principle, tis reference developed te termal-inelastic stiffness equation and te termal load equation of plane steel frame under different stresstemperature pat. Ten nonlinear finite element analysis was carried out according to increment iteration metod. During te analysis, te corresponding material beavior was used according to different stress-temperature pats, and te transition principle among different pats was taken into account. Te element termal-inelastic stiffness equation being presented by reference [] is sown as equation 6. [ k ] d{} d d{} f { R } ine = + (6) T From mentioned above we can know tat bot equations supplied by te tecnical code (CECS 00: 006) and nonlinear finite element metods can analyze te fire resistance beavior of steel structures. To study te difference among te results of different metod s, a comparison was carried out in reference [3]. In tis reference, te fire resistance beavior of a tree-storey and tree-span plane steel structure was analyzed using tree different metods: calculative metod according to CECS 00: 006, finite element routing process presented by reference [], and ANSYS finite element analytical program. Fig. (7) sows te computational model of te steel frame, te fire region can be seen in Fig. (7), tree sides of te beam, except te top side, are in fire. Te canging curves of mid-span deflection and internal axial force corresponding to temperatures of te eated beam are given in Figs. (8) and (9). From te comparison of tree computational results, te conclusion can be drawn tat te mid-span deflections of te beam in fire are pretty muc te same ting, especially wen te temperature is below 500 C, wile as to internal axial forces, metod of tecnical code CECS is a little more conservative tan te oter two metods. But on te wole, te trend of frame beavior canging wit temperatures is uniform. Besides te researces on ig-temperature beaviors of steel material and steel structures, te fire resistance of beam-to-column joints and steel-concrete composite structures as also been investigated and some acievements ave been obtained in Cina Fire Resistance of Beam-to-Column Joints Reference [4] analyzed te temperature elevating of steel connections using finite element metod, meanwile te tests were carried out to validate te teoretical results, and te experience equation to compute te temperature of steel connections according to test results was acquired. Te

7 Te Current Situation and Development of Fire Resistance Te Open Construction and Building Tecnology Journal, 010, Volume 4 61 fire resistance beavior of ig-strengt bolted connections for steel structures was studied in reference [5], during te study, te finite element metod was used and brick elements were adopted. A program was developed considering bot te material and te geometrical nonlinearity. And based on a large number of parameters analysis, te practical equations for calculating te limit temperature of te igstrengt bolted connection under fire were proposed in tis reference. To investigate te beavior of H-sape steel beams (end-plate connection) under fire, reference [6] carried out tests in 006. Except te steel connections, te ig temperature performance of concrete filled steel tubular beam-to-column connections ad also been researced [7]. Fig. (7). Te computational model of te steel frame. Fig. (8). Curves of mid-span deflection wit temperature of beam in fire. Fig. (9). Curves of internal axial force wit temperature of beam in fire Fire Resistance of Steel-Concrete Composite Structures Te researces on fire resistance of composite structures are not as many as tose of steel structures, but tere are still some productions being acquired. Te termal properties and te stress-strain relations of steel and concrete in steel reinforced concrete columns under ig temperature were determined in reference [8], finite element metod was applied to calculate te temperature fields of te section. A numerical metod was developed to calculate te fire resistance and te load-deformation curves of steel reinforced concrete columns, and te teoretical metod was proved by tests. Based on te teoretical model, influences of te parameters on fire resistance beavior and bearing capacity were analyzed. A simplified model was developed for calculating te fire resistance of steel reinforced concrete columns. Reference [9] studied te beavior of ligtweigt steelconcrete composite beams subjected to ISO Standard 834. Te fire region and te full curve of te members were calculated, te load resistance ratio was studied and te related factors about tose subjects were also derived in tis reference. To investigate te structural caracteristics of steelconcrete composite floor system subjected to fire, teoretical analysis and experimental researc ad been carried out respectively in reference [30] and reference [31], te former establised a nonlinear finite element equation and corresponding program to analyze te structural beavior of steelconcrete composite floors subjected to fire, and te latter tested four specimens wic were composed of steel beams and steel profile deck-concrete composite slab, te results from teoretical analysis and experimental researc were compared and tey agreed wit eac oter very well. Wit te finite element computation, teoretical analysis and experimental researc, te fire-resistance beavior and bearingcapability mecanism of restrained composite beams were investigated in reference [3], te analysis metod used in te wole temperature elevating process and te simplified calculation metod being applied in practical design were also establised in tis reference. It must be mentioned tat te researc acievements being referred to in tis paper are only typical representations in te corresponding fields, tere are still many oter studies being carried out by many oter researcers wit regard to fire resistance of steel and composite structures in Cina. 4. CONCLUSIONS From te above content, we can see tat many works ave been done on fire resistance design of steel structures in Cina, and te design metod gets obvious development wit te researces being carried out. Wit te continuing increase of steel production and rapid development of economy in Cina, more and more structural steel buildings in various types will be constructed predicatively. For te fire resistance design of steel structures at te present time, traditional metod according to ISO Standard 834 test on single member is widely used, altoug

8 6 Te Open Construction and Building Tecnology Journal, 010, Volume 4 Zao and Yang te concept of advanced metod or performance-based metod based on structural analysis as been introduced to practical design, a systematical calculation metod for fire safety design of structural steel buildings as not been establised. Bot te researc works and building codes or specifications regarding fire safety design ave fallen beind te demands of practical engineering, furter systematical researces still need to be carried out bot teoretically and experimentally. ACKNOWLEDGEMENT Te autors are grateful to te National Natural Science Foundation of Cina for te financial support. NOTATION T = Coating tickness to be calculated, in mm 1 T = Coating tickness by ISO Standard 834 test, in mm W = Weigt of te structural steel member to be 1 insulated by coating, in kg/m W = Weigt of te tested steel beam, in kg/m D = Perimeter of te section covered by coating, for 1 te structural steel member to be insulated D = Perimeter of te section covered by coating, for te tested steel beam K N T = A factor. For beams, K =1; for columns, K =1.5 = Axial temperature-induced internal force of member in fire M = Temperature-induced bending moment at bot Ti end of member N f 1 = Axial force of member in fire according to equivalent load analysis M = Bending moment at bot end of member in fire fi according to equivalent load analysis T T = Temperatures of bot sides of te member T = Temperature of te member before affected by 0 fire E T = Elastic modulus of structural steel at te tem- T + T perature of ( ) 1 A = Te sectional area of member in fire I = Te sectional inertial moment of member in fire = Te sectional eigt of member in fire = Stress = Strain T = Temperature f = Yield point of structural steel at room temperature y [ k ine] = Termal-inelastic stiffness matrix of an element, [ ] = [ ] + [ ] + [ ] kine K0 KL K [ k 0] = Stiffness matrix for analysis witout considering geometrical nonlinearity [ k L] = Stiffness matrix aroused by initial displacement [ k ] = Stiffness matrix aroused by initial stress { R T } = Equivalent termal load REFERENCES [1] GB , Building Code for Fire Protection Design. 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9 Te Current Situation and Development of Fire Resistance Te Open Construction and Building Tecnology Journal, 010, Volume 4 63 [4] W. Y. Wang, Y. L. Dong and K. Q. Yu, Experimental Study and Analysis of te Temperature on Steel Connections in Fire, Journal of Qingdao Tecnology University, vol. 7, pp , 006. [5] G. Q. Li and Y. Z. Yin, Fire-resistant beavior of ig-strengt bolted connections for steel structures, Cina Civil Engineering Journal, vol. 36, pp. 18-5, 003. [6] X. D. Li, Y. L. Dong and S. P. Cong, Experimental investigation of beavior of H-section steel beams (End-Plate Connection) under fire, Journal of Harbin Institute of Tecnology, vol. 38, pp , 006. [7] Z. Tao, L. H. Han and Y. C. Wang, Several key issues in te performance of connections to concrete filled steel tubular beamcolumns in fire condition, Steel Construction, vol. 0, pp , 005. [8] Y. Q. Zeng and L. H. Han, Fire performance and design metod of steel reinforced concrete (SRC) columns, Progress in Steel Building Structures, vol. 8, pp. -9, 006. [9] X. Y. Mao and Y. Xiao, Beavior of ligtweigt steel-concrete composite beams subjected to standard fire, Journal of Hunan University (Natural Sciences), vol. 3, pp , 005. [30] S. C. Jiang, G. Q. Li, G. B. Lou and Y. J. Sun, Numerical approac of beavior of steel-concrete composite slabs subjected to fire, Cina Journal of Building Structure, vol. 5, pp , 004. [31] S. C. Jiang, G. Q. Li, H. Y. Zou and Q. Wang, Experimental study of beavior of steel-concrete composite slabs subjected to fire, Cina Journal of Building Structure, vol. 5, pp , 004. [3] Y. Z. Wang, Beavior and Design of Composite Beam in Fire wit Considering Global Structure Effect, P. D. tesis, Tongji University, Sangai, P. R. Cina, 006. Received: June 0, 009 Revised: June 4, 009 Accepted: June 5, 009 Zao and Yang; Licensee Bentam Open. Tis is an open access article licensed under te terms of te Creative Commons Attribution Non-Commercial License (ttp://creativecommons.org/- licenses/by-nc/3.0/) wic permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided te work is properly cited.