Study on Impact Between Adjacent Buildings: Comparison of Codal Provisions

Transcription

1 Study on Impact Between Adjacent Buldngs: Comparson of Codal Provsons Chenna Rajaram Graduate Student, Earthquake Engneerng Research Centre, Internatonal Insttute of Informaton Technology, Hyderabad, Inda Ramancharla Pradeep Kumar Assocate Professor, Earthquake Engneerng Research Centre, Internatonal Insttute of Informaton Technology, Hyderabad, Inda SUMMARY: Structures are beng bult very close to each other n metropoltan areas where the cost of land s very hgh. Due to closeness of the structures, they often collde wth each other when subjected to earthquakes. Many studes have been carred out on separaton dstance between adjacent structures to mtgate poundng. Although some modern codes ncluded sesmc separaton requrement for adjacent structures, some of them have faled n provdng the approprate mnmum separaton dstance. In ths paper, two lnear sngle degree of freedom oscllators are used to study the mpact force for fve dfferent ground motons rangng from 0. g to 0.8 g. The separaton dstance s calculated from the codal provsons of dfferent countres. The separaton dstance between the two structures decreases, the amount of mpact ncreases whch s applcable when the mpact tme s same. It may also decreases when separaton dstance decreases. For structures havng same perod, no need to provde separaton dstance. The amount of mpact depends on response of the structures at partcular tme, mnmum space between the structures and velocty of the structures. Keywords: Poundng, Separaton dstance, codal provsons. INTRODUCTION Poundng s the phenomena of collson between adjacent buldngs or dfferent parts of the same buldng durng strong vbratons. It may cause ether archtectural and or structural damage and may lead to partal or complete collapse of the structure. Reported case studes of poundng are as follows: Durng 985 Mexco Cty earthquake (J Agular et.al, 989), more than 0% of buldngs were damaged because of poundng. Durng 989 Loma Preta earthquake (Kazuhko Kasa et.al., 997) (M7.) over 00 structures were affected. These structures were located around 90 km away from the epcenter. A ten stored buldng experenced poundng wth an adjacent massve fve-storey buldng. The typcal floor mass of the fve-storey buldng s about eght tmes that of the ten-storey buldng and was separated by about 4cm. Poundng occurred at the 6 th level n the ten-storey buldng and at the roof level n the fve-storey buldng. The 999 Ch-Ch earthquake (Jeng Hsang Ln et.al., 00) n central Tawan, caused severe destructon to towns and vllages near the epcenter. Structural poundng events were also observed after the earthquake. Durng 004 Sumatra earthquake (Mw9.3), poundng damage at junctons was notced at the top ends of ples of the approach jetty. In 00 Dglpur earthquake (Durgesh C Ra et.al., 005) (Mw6.5), poundng damage was observed at the juncton of the approach segment and man berthng structure. Durng 006 Skkm earthquake (Hemanth B Kaushk er.al., 006) (Mw5.3), damage to a nne storey masonry nfll RC frame hostel buldng of Skkm Manpal Insttute of Medcal Scences (SMIMS) was observed. Poundng damages were observed between two long wngs n the buldng and corrdors connectng the wngs. From the above observaton t s evdent that poundng s usual phenomena between adjacent buldngs f the separaton dstance s nadequate. In the proposed study, frst revew the code provsons across the world s dscussed and later, the mpact force s evaluated between the structures whch followed the provsons.

2 . LITERATURE REVIEW Poundng s one of the recent topcs of nterest n the research communty. Many nvestgatons have been carred out on poundng damage durng prevous earthquake events. Stavros A Anagnostopoulos (987) studed the poundng of several adjacent buldngs n a block, due to strong earthquakes. Each structure s modeled as a sngle degree of freedom (SDOF) system and poundng s smulated usng mpact elements. The parametrc nvestgaton of ths problem showed that the end structures dsplace more than the nteror structures. Mason and Kasa (99) studed poundng between 5-storey and 8- storey buldngs. They assessed the nfluence of buldng separaton, relatve mass, and contact locaton on the mpact force. Van Jeng, Kazuhko Kasa and B F Mason (99) developed spectral dfference method (Double Dfference Combnaton rule) to estmate the requred separaton to preclude poundng. Ths was based on response spectrum approach. Ths method s useful not only for the assessment of poundng but also for studyng the problems nvolvng relatve dsplacement. Flatrault and Wagner (995) proposed poundng mtgaton technques. They suggested separaton dstance to deal wth poundng. Solutons were ether fllng the gaps between the buldngs wth a materal or by connectng them wth bumper walls. 3. REVIEW OF CODE PROVISIONS ON POUNDING Most of the world regulatons for sesmc desgn do not take nto account the poundng phenomenon. Among the exceptons are the codes of Argentna, Australa, Canada, France, Inda, Indonesa, Mexco, Tawan and USA. These codes specfy a mnmum separaton dstance between adjacent buldngs. However, the procedure to determne the separaton dstance vares from country to country. In UBC-997, t depends on the maxmum dsplacements of each buldng. In Canada and Israel, t s smple sum of the dsplacements of each buldng. In France t s a quadratc combnaton of the maxmum dsplacements. In Tawan t s depends on the buldng heght and n Argentna mnmum gap s.5 cm. Also, n some cases, these values depend on the type of sol and sesmc acton. The provsons on separaton dstance are very smlar n the 000 and 003 Internatonal Buldng (IBC, 003). In 006 verson there s no code provson on buldng separaton. Accordng to IBC-009 the separaton dstance between two adjacent buldngs s computed from equaton : C d max M (3.) I Where, δ max s the maxmum elastc dsplacement that occurs anywhere n a floor from the applcaton of the desgn base shear to the structure. C d s the deflecton amplfcaton factor and I s the mportance factor for sesmc loadng. Indan sesmc code (IS:893-00) recommends that the separaton between two adjacent unts or buldngs shall be a dstance equal to response reducton factor (R) tmes the sum of the calculated storey dsplacements. When the two buldngs are at the same elevaton levels, the factor R may be replaced by R/. Ths clause assumes only two dmensonal behavor of buldng.e., only translatonal poundng, and no torsonal poundng. But n realty torsonal poundng tends to be more realstc than un-drectonal poundng durng real ground motons. Accordng to Federal Emergency Management Agency (FEMA: ) the separaton dstance between adjacent structures shall be less than 4% of the buldng heght and above to avod poundng. FEMA states that buldngs ntended to meet enhanced objectves shall be adequately separated from adjacent structures to prevent poundng durng response to the desgn earthquakes, except as ndcated n secton..0.. Poundng may be presumed not to occur whenever the buldngs are separated at

3 any level by a dstance greater than or equal to s. The value of s need not exceed 0.04 tmes the heght of the buldngs above grade at the zone of potental mpacts. Peru code (NBC-PERU E030) states that every structure should be separated from other close structures a mnmum dstances to avod contact durng strong ground motons. Ths mnmum dstance not be lower than /3 of the sum of the maxmum dsplacement of adjacent blocks. ASCE 7-0 states that all portons of the structure shall be desgned and constructed to act as an ntegral unt n resstng sesmc forces unless separated structurally by a dstance suffcent to avod damagng contact under total deflecton as determned n secton..3. Separaton dstance between two structures depends on deflecton amplfcaton factor and mportance factor. From the observaton of all code provsons, the mnmum separaton dstance s not only depends on the response of the structure but also on varous factors lke mportance factor, amplfcaton factor etc. The detals of code provson for dfferent countres are as shown n table 3.. Ths case study deals wth the collson force of frst mpact of the structure by usng lnear mpact models. The response s consdered n translatonal drecton only and not consder n torsonal drecton. Table 3.. Buldng separaton dstance between two adjacent structures from dfferent country code provsons Country Formula INDIA (IS- 893:007 (Draft)) R tmes the sum of the calculated storey dsplacements usng desgn sesmc forces to avod damage of the two structures when the two unts deflect towards each other. When the two buldngs are at the same elevaton levels, the factor R may be replaced by R/. (Clause 7..3) IBC UBC FEMA: NBC Peru E M C d max I M M M... (Adjacent Buldngs located on the same property lne) (Clause 633..) Separaton dstance between adjacent structures shall be less than 4% of the buldng heght and above to avod poundng. Ths mnmum dstance not be lower than /3 of the sum of the maxmum dsplacement of adjacent blocks nor lower than S= (h-500). (Clause 3.8.) C d max (Clause..3) 6 ASCE:7-00 M I S = Separaton dstance (n cms) h = Heght of structure (n cms) R = Response reducton factor δ M = Separaton dstance between two structures δ M and δ M = Peak Dsplacement response of adjacent structures & C d = Total deflecton amplfcaton factor δ max = Maxmum elastc dsplacement that occurs anywhere n a floor from the applcaton of desgn base shear to the structure. I = Importance factor for sesmc loadng 4. MINIMUM SEPARATION BETWEEN BUILDINGS For the numercal study poundng between adjacent buldngs, two buldngs as shown n fgure 4. are consdered. These buldngs are dealzed as two equvalent lnear sngle degree of freedom (SDOF) systems. The two buldngs are referred hereafter as Buldng and Buldng and are separated by a dstance δ between them. The two buldngs have lumped masses m = 400kg, m = 640kg, equal stffnesses k = 45000kN/m and equal dampng

4 ratos ξ=0.05. Let u (t) and u (t) are ndependent responses of Buldng and Buldng. The governng dfferental equaton of moton for SDOF system s expressed as follows: m u ( t) c u ( t) k u ( t) m u ( t) (4.) g Fgure 4.. Modelng of poundng between two adjacent structures Where, denotes the buldng under consderaton. For the purpose of studyng the collson between the buldngs, SE component of El-Centro ground moton (see Fgure 4. (a)) whose PGA s g s consdered. Also for fndng the response of buldng to earthquake ground moton, Newmark s approach s used. Typcal response of buldng to El-Centro ground moton s shown n Fgure 4. (b) & (c). Now f another buldng (say Buldng ) s placed adjacent to Buldng, mnmum dstance between the buldngs can be checked by the followng condton: u t) u ( ) (4.) ( t If the above condton satsfes then collson occurs. For the purpose of fndng the mnmum gap between two buldngs, dfferent tme perods for Buldng.e., 0.075, 0.0, 0.5, 0.5, 0.75, 0.0, 0.5 and 0.5 sec are consdered. The peak of relatve response of adjacent buldngs gves the mnmum separaton dstance between them. The mnmum separaton dstance between two adjacent structures s as shown n Fgure 4.3. (a)

5 (b) (c) Fgure 4.. Elcentro ground moton and response (a) Elcentro earthquake ground moton, (b) Response of structure to El-centro ground moton (T=0.075 sec, ξ=0.05) (c) Response of structure to El-centro ground moton (T=0.0 sec, ξ=0.05) From ths fgure t can be observed that as the tme perod of the structure ncreases mnmum dstance s ncreases. And for the two structures wth same natural perod, there s no need to provde any separaton dstance because these buldngs wll vbrate n phase and does not collde at any pont of tme. However, ths stuaton s not realstc because t s very dffcult to construct two structures wth same natural perod. Also, t can be observed from the fgure that the mnmum separaton dstance s gettng saturated when natural perod of buldng s ncreasng say beyond sec. To study ths case, nonlnear analyss s necessary. As most of the code provsons are based on lnear analyss, and hence lnear analyss s used n ths study. Fgure 4.3. Mnmum space requred between two structures 5. CASE STUDY Impact force s evaluated between two buldngs by provdng mnmum separaton dstance between buldngs as per the code recommendatons. For ths purpose, Buldng wth tme perod 0. sec natural perod and natural perod of Buldng.e, 0.075, 0., 0.5, 0. sec are consdered. Fve

6 earthquake records, vz., Loma-Preta earthquake, Elcentro earthquake, Parkfeld earthquake, Petrola earthquake and Northrdge earthquake were selected. Characterstcs of the selected ground motons are gven n Table 5.. Table 5.. Detals of ground moton data Earthquake Name Lomapreta Elcentro 3 Parkfeld 4 Petrola 5 Northrdge Locaton Year M w PGA, (g) Lomapreta, Calforna, USA Imperal Valley, Calforna, USA Parkfeld, Calforna, USA Cape Mendocno, Calforna, USA Northrdge, Calforna, USA Trfunac Duraton (sec) Predomnant Tme Perod Range, sec Energy, ergs x x x x x0 When both the buldngs are subjected to ground moton, collson may take place and durng collson usually energy transfer from one buldng to another buldng s a natural phenomenon. Due to ths energy transfer, both the structures behave dfferently due to ether loss of energy or ganng energy. There are dfferent mpact models avalable for calculaton of mpact. For example lnear sprng model, Kelvn model (Susender Muthukumar et.al., 004) are lnear models. Hertz model and hertz damp model are nonlnear models. In lnear sprng model, energy loss durng mpact s not consdered for calculatng the mpact force. The contact force durng mpact s taken as, F c k ( u u k 0; ); u u u u 0 0 (5.) Kelvn approach takes nto account dampng also. The calculaton of collson force accordng to Kelvn model s as follows, F c k ( u u k 0; ) c ( u u k ); u u u u 0 0 (5.) The dampng co-effcent c k can be related to the coeffcent resttuton e by equatng energy loss durng mpact. c k m m ln e k k ; m m (5.3) ln e In ths study, Kelvn model s used. For the calculaton of mpact force between two structures stffness of the sprng, k k s assumed as 4378 MN/m (Ref. Susender et al.). The co-effcent of resttuton, e = 0.6 s assumed and t s defned as the rato of the relatve veloctes of the bodes after collson to the relatve veloctes of the bodes before collson.

7 6. RESULTS & DISCUSSIONS In ths study structures havng natural perod range from sec to 0. sec wth an nterval of 0.05 sec are taken. Structure havng natural perod 0. sec s kept constant and other buldng perod s kept varyng and the mnmum separaton dstances are calculated from above code provsons (see table 6.). As the structure s natural perod ncreases, the response of the structure also ncreases for a gven ground moton and dampng. The structures are subjected to Lomapreta ground moton. The predomnant frequences range present n the ground moton s sec, whch s far away from the fundamental perod of the structures. All ground moton records whch are consdered n ths analyss are shown from fgure 6. (a) to (d). (a) (b) (c) (d) Fgure 6.. Ground motons consdered n ths analyss (a) Lomapreta, (b) Parkfeld (c) Petrola and (d) Northrdge As per code provsons, the separaton dstances are kept between the structures and ntal mpact forces are calculated usng Kelvn model approach (see table 6.). Accordng to UBC-997, ASCE and IBC, the ntal collson force generated between T and T =0.075 s s 37 kn when structures subjected to Lomapreta ground moton. Because the separaton dstance s very less compared to all other codal provsons. The summary of mpact forces for all structures and codes are lsted n table 4. For structures T =0. s and T =0.5 s, the mpact force s 800 kn as per ASCE and IBC codal provsons. The mpact force between the buldngs (T =0.0 and T =0.075 s) s 389 kn as per IS:893-00(see table 6.3) when subjected to Elcentro ground moton (Refer fg 4.(a)). For other buldngs (T =0.,

8 s) the mpact force s zero. For the structures havng same tme perod, no need to provde mnmum space between them. Because the response for both structures s same. The mpact force for structures havng tme perod 0. and s s 6.57 kn accordng to UBC-997 even though the separaton dstance s less. The amount of mpact depends on response of the structures at partcular tme, mnmum space between the structures and velocty of the structures. In case of UBC-997, the velocty of structures (0. and s) s less compared to IS: code durng mpact. As the mnmum space between structures decreases the amount of mpact ncreases, but ths mpact occurs at the same tme even the separaton dstance decreases. The separaton dstance and mpact forces are same as per ASCE:07-00 and IBC-009. For structures havng perod of 0. and 0.5 s, the mpact force s 70 kn as per UBC-997. But for the same structures (0. and 0.5 s), the mpact force s 605 kn as per ASCE:07-00 and IBC-009. Here the separaton dstance decreases from 0.00 m to 0.00 m and the mpact occurs at the same tme. For structures T =0.0 and T =0. s, the mpact forces are 460 and 576 kn as per UBC and ASCE/IBC respectvely. In ths case the mpact force reduces as the separaton dstance ncreases. Because the mpact occurs at the same tme. Now the structures are subjected to Parkfeld ground moton. As per IS:893-00, the mpact force between structures T =0. s & T =0.075 s s 50 kn(see table 6.4). But the mpact force s 48 kn as per UBC and ASCE/IBC. Here the mpact occurs at the same tme. For structures havng same perod no need to provde separaton dstance. For structures havng perods T =0. and T =0.5 s, the mpact force s 44 kn as per UBC-997. But as per ASCE/IBC, the mpact force s less even though the separaton dstance s less. It means the mpact has not occurred at the same tme. Ths s also happened wth structure havng perod T =0. s. Even though the ground moton s same, the mpact occur at same tme for structure s and not occurred at the same tme for structures 0.5 and 0. s. It s clearly showed that the mpact s dependent on velocty of structure also. The structure s not effected by ts ampltude of ground moton. It s effected by frequency of ground moton. Now the structures are subjected to Petrola ground moton. For structures 0.0 s and s, the ntal mpact force s 36 kn as per IS: (see table 6.5). The mpact force between the same structures s 7 kn as per UBC and ASCE/IBC even though the separaton dstance decreases from m to m. It means that the mpact forces are not occurred at the same tme. For structures havng same perod, no need to provde separaton dstance. The mpact force between structures 0. s and 0.5 s s 03 kn as per IS: But the mpact values are less as per UBC and ASCE/IBC even though the separaton dstances are small. Here, the mpact values are ntally ncreases and then decreases. For structures 0. s and 0. s, the mpact values are 9 kn and 3466 kn as per UBC and ASCE/IBC respectvely. Here, the amount of mpact decreases as the separaton dstance ncreases. Now the structures are subjected to Northrdge ground moton. The structure havng perod 0. s s matched wth ground moton frequency. At predomnant frequences, the response of structure wll be more and may lead to hgh mpact force. Ths effect can be clearly seen n the table 6.6 for structures T =0.0 s and T =0.0 s. The mpact force ntally ncreases and then decreases wth ncrease of separaton dstance. Wth the same separaton dstance between two structures, the ntal mpact force wll be same n both lnear and nonlnear analyss before yeld starts. The mpact values between structures havng perod 0. s and 0. s, are 0878 kn, 88 kn, 0700 kn and 45 kn as per IS:893-00, UBC-997, NBC-PERU and ASCE/IBC respectvely. From the comparson of all codal provsons, FEMA: and NBC-PERU codes have no collsons n all the ground motons (except Northrdge ground moton). The calculated separaton dstances are hgh for these (FEMA and NBC-PERU) code provsons. Also the clauses n the codes on poundng are based on heght of the structure only. Table 6.. Mnmum gap requred between adjacent structures havng tme perod T and T wth respect to dfferent codal provsons T =0.0 sec T =0.075 sec T =0.0 sec T =0.5 sec T =0.0 sec Gap(m) Gap(m) Gap(m) Gap(m) IS:

9 UBC FEMA: NBC-Peru:E ASCE:07-00 and IBC Table 6.. Intal mpact forces when structures subjected to Lomapreta ground moton T =0.0 sec T =0.075 sec T =0.0 sec T =0.5 sec T =0.0 sec Force(kN) Force(kN) Force(kN) Force(kN) IS: UBC FEMA: NBC-Peru:E ASCE:07-00 and IBC Table 6.3. Intal mpact forces when structures subjected to Elcentro ground moton T =0.0 sec T =0.075 sec T =0.0 sec T =0.5 sec T =0.0 sec Force(kN) Force(kN) Force(kN) Force(kN) IS: UBC FEMA: NBC-Peru:E ASCE:07-00 and IBC Table 6.4. Intal mpact forces when structures subjected to Parkfeld ground moton T =0.0 sec T =0.075 sec T =0.0 sec T =0.5 sec T =0.0 sec Force(kN) Force(kN) Force(kN) Force(kN) IS: UBC FEMA: NBC-Peru:E ASCE:07-00 and IBC Table 6.5. Intal mpact forces when structures subjected to Petrola ground moton T =0.0 sec T =0.075 sec T =0.0 sec T =0.5 sec T =0.0 sec Force(kN) Force(kN) Force(kN) Force(kN) IS: UBC FEMA: NBC-Peru:E ASCE:07-00 and IBC Table 6.6. Intal mpact forces when structures subjected to Northrdge ground moton T =0.0 sec T =0.075 sec T =0.0 sec T =0.5 sec T =0.0 sec Force(kN) Force(kN) Force(kN) Force(kN) IS: UBC FEMA: NBC-Peru:E

10 5 ASCE:07-00 and IBC CONCLUSIONS From the above observatons, the duraton of strong moton ncreases wth an ncrease of magntude of ground moton. The conclusons are drawn from ths study are as follows: In general when the separaton dstance between the two structures decreases, the amount of mpact s ncreases, whch s not n all cases. At predomnant frequences, the response of the structure s more and may lead to collapse of the whole structure. In ths case, structure havng perod of 0. s s matched wth the frequency of ground moton. The amount of mpact s also hgh when subjected to Northrdge ground moton. Among all the codal provsons, the calculated separaton dstance s hgh for FEMA: and NBC PeruE Because the clauses for these codes depends on heght of the structure. For structures havng same perod, no need to provde separaton dstance. The amount of mpact depends on response of the structures at partcular tme, mnmum space between the structures and velocty of the structures. REFERENCES J. Agular, H. Juárez, R. Ortega, and J. Iglesas (989), The Mexco Earthquake of September 9, 985 Statstcs of Damage and of Retrofttng Technques n Renforced Concrete Buldngs Affected by the 985 Earthquake, Earthquake Spectra, Vol. 5: Issue, pp Kazuhko kasa and B.F Mason (997), Buldng poundng damage durng the 989 Lomapreta earthquake, Engneerng Structures, Vol. 9: No. 3, pp Jeng-Hsang Ln and Cheng Chang Weng (00), A study on sesmc poundng probablty of buldngs n Tape metropoltan area, Journal of the Chnese Insttute of Engneers, Vol. 5: No., pp Durgesh C Ra and C V R Murthy (005), Engneerng lessons not learnt from 00 Dglpur earthquake-a revew after 004 sumatra earthquake, Current Scence, Vol. 89: No. 0, pp Hemant B Kaushk, Kastubh Dasgupta, Dpt R Sahoo and Gayatr Kharel (006), Performance of structures durng the Skkm earthquake of 4 feb.06, Current Scence, Vol. 9: No. 4, pp Stavros A. Anagnostopoulos (988), Poundng of buldngs n seres durng earthquakes, Earthquake Engneerng and Structural Dynamcs, Vol. 6, pp Bruce F. Mason and Kazuhko Kasa (99), Dynamcs of poundng when two buldngs collde, Earthquake Engneerng and Structural Dynamcs, Vol. : Issue 9, pp Van Jeng, Kazuhko Kasa and Bruce F. Mason (99), A Spectral Dfference Method to Estmate Buldng Separatons to Avod Poundng, Earthquake Spectra, Vol. 8: Issue, pp Andre Flatrault and Perre Wagner (995), Analytcal predcton of expermental buldng poundng, Earthquake Engneerng and Structural Dynamcs, Vol. 4: Issue 8, pp Internatonal Buldng, IBC-003, Internatonal Councl. INC. Indan standard crtera for earthquake resstant desgn of structures, part- general provsons and buldngs, IS:893-00, Bureau of Indan standards, New Delh. Federal Emergency Management Agency (FEMA), NEHRP Gudelnes for the sesmc rehabltaton of buldngs, FEMA:73-997, Washngton D.C., USA. Unform Buldng, UBC-997, Volume-, Structural Engneerng Desgn Provsons, Internatonal Conference of Buldng Offcals, Calforna. Natonal Buldng - PERU, Techncal Standard of Buldng E.030, Earthquake Resstant Desgn. Amercan Socety of Cvl Engneers for Mnmum Desgn Loads for Buldngs and Other Structures, ASCE/SEI 7-05, USA Susender Muthukumar and Regnald Desroches (004), Evaluaton of mpact models for sesmc poundng, Proceedngs of Thrteenth World Conference on Earthquake Engneerng, paper No.35.