SEISMIC SAFETY EVALUATION FOR CONCRETE GRAVITY DAMS

Transcription

1 SEISMIC SAFETY EVALUATION FOR CONCRETE GRAVITY DAMS CHENBIN DU YONGWEN HONG 2 JUWEI YUAN 3 ZHIMING LIU 4 Professor,Dept. of Engineering Mehanis, Hohai University, Nanjing. China 2 Senior Engineer, Kunming Design an Researh Institute for Hyroeletri Projets, Kunming. China 3 Dotoral stuent, Dept. of Engineering Mehanis, Hohai University, Nanjing. China 4 Dotoral stuent, Dept. of Engineering Mehanis, Hohai University, Nanjing. China bu@hhu.eu.n, hong_yw@khii.om, yjw@hhu.eu.n, zhimingliu@hhu.eu.n ABSTRACT: The esign of aseismi reinforement an the seismi safety evaluation for Jinanqiao roller ompate onrete (RCC gravity am are isusse in the paper. The results iniate that it is not suitable to etermine the amount of reinforement by the elasti stress at key loations of am. For the esign of the steel reinforement it is require that the rak oes not amage the grout urtain in the am heel, an raks at the orners o not have any effet on the stability of the am. The stress that loate near stress onentration is hosen to etermine the initial amount of reinforement, an the final reinforement is etermine by the results of nonlinear finite element analysis of the am. The separate reinfore onrete nonlinear finite element moel is use to analysis the earthquake response of the am. The strain rate, amage variable an stiffness egraation are inlue in the onrete elasti-asti amage moel. The ieal elasti-asti onstitutive relationship is aopte in the analysis. Aiming at the possible slip moes of the powerhouse am setion an the am amage after onsiering the nonlinear earthquake response, aseismi stability evaluations of the am are analyze. The results show that the aseismi stability of the powerhouse am setion of Jinanqiao gravity am meets the requirements of China Coe. The reinforement sheme provies basis for the esign institute to etermine the final reinforement sheme. KEYWORDS: gravity am, onrete amage, aseismi reinforement, am heel. INTRODUCTION For the aseismi reinforement esign an seismi safety evaluation of the onrete gravity am, onventional linear-elasti response spetrum metho annot meet the requirement of engineering esign. More an more experts an sholars realize that nonlinear FEM proeures are neee for the analysis of the behaviour of ams uring strong earthquakes. The linear-elasti metho annot reflet the absorption of the earthquakes loaing energy ue to the asti of onrete, the elasti stress may be higher than that of the real situation. It is not only unpratial, but also unneessary that reinfores totally result from the stati loa an the analysis results of ynami Coe Spetrum. For the Xinfengjiang gravity am, esign earthquake loa was reue by ol China Coes (NHCE 978. That means the earthquake loa effet after multiy 0.25 as to the stati loa effet, then reinfore esign was arrie out aoring to the non-member system reinfore onrete struture. The am is still running well after suffere its esign earthquake (Huang 989. It is enough to show that aseismi reinforement esign of the gravity am aoring to this metho is feasible. Aoring to the seismi riterions of gravity ams, when suffering the rare earthquake, the am s rak is aeptable, as long as the am meets the emans of bearing apaity, whih means no am-failure ours (Lin 200, Chen Now, the problem is that there is lear exain to the aseismi reinforement esign of gravity am neither in the Seismi Design Coe of Hyrauli Strutures (DL (CIRWH 2000 nor in the Design Coe for Hyrauli Conrete Strutures (SL/T 9-96 (NHCE 997 in China. In the former, there is just general statement of shoul enhane the onrete strength or reinfore near the am s top without speifi aseismi reinforement formula. In the latter, the reinfore metho is not suitable for the onrete am is pointe out.

2 The esign of aseismi reinforement an the seismi safety evaluation for Jinanqiao roller ompate onrete (RCC gravity am are isusse in this paper. The RCC gravity am is 60 meters high, an the am site lies in the mile reahes of the Jinshajiang River in Yunnan provine in China. The reinfore onrete nonlinear finite element time-history metho is use to analyze the earthquake response of the am. The strain rate, amage variable an stiffness egraation are inlue in the onrete elasti-asti amage moel. The ieal elasti-asti onstitutive relation is aopte in analysis. For the esign of the steel reinforement it is require that the rak oes not amage the grout urtain in the am heel, an raks at the orners o not have any effet on the stability of the am. Final amount of reinforement is etermine by the nonlinear earthquake response results of am. The reinforement sheme provies the basis for the esign institute to etermine the final reinforement sheme. Aiming at the possible slip moes of the powerhouse am setion an the am amage after onsiering the nonlinear earthquake response, aseismi stability evaluations of the am are analyze. 2. PROJECT SURVEY The type of onrete am of Jinanqiao hyroeletri station uner onstrution is a roller ompate onrete gravity am, the rest length is 640 m. The lowest am founation elevation is 264 m, an the top elevation of am is 424 m. The maximum am height is 60 m. The projet lies in the Lijiang atform margin folbelt of Yangtze paraatform western rim, whih is strong ativity area of the northwest of Yunnan an the southeast of Qinghai-Tibetan ateau. The engineering geology environment onition of this area is very omiate. The river valleys are usually very eep utting valleys, an the bank slope is very steep. Regional earthquake is very ative. Aoring to the result of seismi safety evaluation finishe by the Institute of Geology, China Earthquake Aministration, the earthquake basi intensity at am site is Ⅷ, the fortifiation intensity is Ⅸ, the peak of aeleration at berok is 0.399g. The powerhouse am setion is hosen to be investigate in the paper. The length of transverse joints is 35 m. The berok of the am site is mainly basalt, others are blok-frature, frature hloritization rok mass an weak seams t a an t b mae from tuff. The material of am is mainly roller ompate onrete (C There is a little normal onrete (C use in the upstream an ownstream surfaes an the am base. In orer to alulate stress of the intake, three-imensional moel is hosen. The epth of am base is 480 m, whih is three times of height of am. The onstraints aroun the base are all normal onstraints. The base is onsiere to be a massless founation. The am boy an the founation are ivie by 8-noe hexaheron. Total number of noes of the powerhouse am setion is 3264, an the number of elements is The finite element moel of am boy an founation is shown in Fig.. Typial am setion is presente in Fig.2. The loas onsiere in the alulation inlue self-weight, hyrostati pressure, seiment pressure, uift pressure, wave pressure, an the earthquake effet. The hyroynami pressure is etermine by Westergarr metho that takes the extra mass in the upstream am fae into onsieration @ Figure Finite element moel of powerhouse am setion Figure 2 Typial am setion

3 3. ELASTIC-PLASTIC DYNAMICAL DAMAGE MODEL OF CONCRETE MATERIAL On the base of the asti amage moel presente by Lubliner(989 an the onrete asti amage moel uner yle repeat loa propose by Lee an Fenves(998, the effet of strain rate on the asti eformation is taken into onsieration, onrete elasti-asti amage moel with rate-relate is obtaine. The onrete elasti-asti amage moel an be summarize by the following: σ = ( σ (3. el ( σ = D 0 ε ε (3.2 &% ε = h( σε, % & ε (3.3 G( σ & ε = & λ (3.4 σ the Eqn.3. efines the effetive stress with amage, Eqn. 3.2 efines the onnetion of effetive stress an elasti strain, the Eqns. 3.3 an 3.4 efine the asti behavior. ε% is equivalent asti strain, & % ε is equivalent asti strain rate, λ is asti multiier, an G is asti potential funtion. The onrete stress-strain relationship with the repetitive loa is shown in Fig.3. In this moel, the onrete elasti-asti yiel surfae (shown in Fig.4 aopts the formula presente by Lee an Fenves (998: ~ [ 3 ( ~ F( σ, ε = q αp + β ε σ max γ σ max α σ t ] σ ( ~ ε (3.9 σ t0 -S 2 -S K =2/3 (- t (- E 0 w t = w t =0 E 0 (- t E 0 O K = (- E 0 w =0 w = ε E 0 Figure 3 Stress-strain relationship of onrete Figure 4 yiel surfae in the eviatori ane with uner yle loas ifferent values of K σ( ε where β ( ε = % % ( α ( α σt( % εt +, σ an σ t are the effetive ohesive stress in ompression an in σ b0 σ 0 tension, respetively. α =, σ b0 an σ 0 are the initial yiel stress of biaxial an uniaxial 2σ b0 σ 0 3( K ompressive loa, respetively. α is in λ = 2K, for onrete material, parameter K an be taken as 2/3, that means λ = 3. -S 3

4 The relation of K an the shape of yiel surfae in the eviatori ane is shown in Fig.4. The non-assoiate flow rule base on the Druker-prager flow surfae is onsiere in the alulation. 4. THE PRINCIPLE OF ASEISMIC REINFORCEMENT DESIGN The prinie for the am reinforement is that it is require that the rak oes not amage the grout urtain in the am heel, an raks at the orners o not have any effet on the stability of the am. If the steel amount is etermine by the non-member system reinfore onrete struture reinforement prinie aoring to the Design Coe for Hyrauli Conrete Strutures, we have: T (0.6 T + fyas (4. γ where T is the elasti total tension fore alulate by the esign loa that inluing stati loa an earthquake effet multiie 0.35, T is the tension resultant fore that onrete bears, T = A t b, where, At is resultant fore of the setion whose tensile stress less than the onrete ynami tensile strength, an b is the esign setion thikness. f is the ynami esign strength of steel bar. y Aoring to the NHCE (997, the struture oeffiientγ =.2. Conrete bears 30% of the total tension fore, that ist = 0.3T. Then we have:.02t As fy (4.2 The maximum tensile stress is 3.5 MPa at the 5 m far from the am heel, the maximum tensile stress at the 6.46 m is 3.5MPa. The number of steel bar an be alulate by the Eqn. 4.2 is mm 2, nee 282 steel bars of Ⅱ grae with the iameter 36 mm are neee here. Obviously, the number of steel bar is very large. It is not only unreasonable to nee so many steel bars, but also is unneessary. The large tension stress aroun am heel an orners of the am is ause by stress onentration. The result of fiel test iniates that the stress aroun these orners is far less than the result of linear-elasti alulation. The large tension stress relate to assumption that the rok an onrete aroun the am heel or the hange slopes are ieal elasti soli. In fat, the rok an onrete all have some miro raks, the large tensile stress will be release ue to these raks. Tensile stress is not so large at these aes in real ase. It is not suitable to etermine the amount of reinforement by the elasti stress at key loations of the am. Maximam Tensile Stress(MPa At Along Base of Dam (m Horizontal Aeleration (g Figure 5 Maximum tension stress along am base Figure 6 Earthquake waves aopte in alulation t (s

5 The stress shoul hoose from the ae away from the orners. Taking the am heel for exame, the elasti stress at m that is 5 m away from am heel is hosen. Maximum tensile stress along the base is shown in Fig.5. Here, the onrete is roller ompate onrete C 20, ynami tensile strength f = 3.5MPa. The struture oeffiientγ =.2, the steel bar esign strength f y is 335MPa. The number of steel bar an be alulate by the Eqn. 4.2 is 262.9mm 2, nee 5 steel bars of Ⅱ grae with the iameter 28 are neee here. At the same time, the nonlinear time-history response analysis with the above moel, in whih the steel bars elements an onrete elements are ometely separate, is arrie out. The earthquake aeleration in alulation is shown in Fig.6. The vertial earthquake aeleration peak value is taken 2/3 of that of horizon aeleration. The reinforement steel is simulate by the ieal elasti-asti moel. Conrete is simulate by previous the elasti-asti amage moel. The results show that when reinforement aoring to 5B28 or 5B25 on the am heel an the upstream an ownstream hange slopes, the rak amage epth at the upstream hange slope is.3m to.5m, the rak amage epth at the ownstream hange slope is 2.5m to 3.0m, there is no breakthrough rak. The rak amage epth at the am heel is 3.8m to 5.0m, it oes not amage the grout urtain in the am heel. The maximum tensile stress of steel appears at the am heel, it is about 265 MPa to 288MPa, whih is less than the yiel strength of steel. Comparison results with ifferent reinfore sheme are shown in table 4.. Comparing with without reinforement, the area of tensile stress rops from 20m to 2m. The rak amage epth of the am heel an the hange slopes obviously reue after reinfore. Figs.7 (a, (b are the amage raking profiles for ases without reinforement an with 5B28 reinforement, respetively. Figs.8 (a, (b are the time-history urves of steel tensile stress with reinfore 5B25 at ifferent elevations. reinforement sheme Table 4. omparison of ifferent reinforement sheme Dam heel epth/m (Damage egree D=0.9 Upstream hange slope Downstream hange slope The maximum steel tensile stress /MPa Without reinforement / 5B B B (a without reinforement Figure 7 Damage raking epth of am (b 5B28@72

6 Maximum Tensile 主拉应力 Stress (MPa (MPa 时间 (s t (s (aelevation at 270m Maximum 主拉应力 Tensile Stress (MPa (MPa 时间 (s t (s (b Elevation at 335m Figure 8 Steel tensile stress versus time of reinforement at ifferent elevation with 5B25 Figs.9 (a an (b are the time-history urves of steel tensile stress at elevations of 270m an 335m, respetively. When reinforement aoring to 72 at the am heel an the upstream an ownstream hange slopes, the maximum tensile stress of steel is 265MPa at the elevation of 270 m at 7.0s, while the maximum tensile stress is 36MPa at the elevation of 280 m at 0.6s, an the maximum tensile stress is 225MPa at the elevation of 335 m at.8s. Comparing with the sheme of 5B25@ 72, the steel maximum tensile stress reue obviously by the sheme of 5B28@ 72, whih provies more safety for the am. Maximum 主拉应力 Tensile (MPa Stress (MPa 时间 t (s Maximum 主拉应力 Tensile (MPa Stress (MPa (aelevation at 270m (b Elevation at 335m Figure 9 Steel Tensile stress versus time of reinforement at ifferent elevation with 5B28 时间 t (s Besies, the ase for reinforement with is also alulate. Though at this moment the steel stress reue obviously, the amage rak at key loations of the am is not improve, an the number of steel inreases largely. Hene, we suggest that reinforement aoring to at the am heel an the upstream an ownstream hange, an reinforement aoring to on the other ae of am. The final sheme of Institute of Design is single row B28@200 at the am heel, ouble row B28@200 at the upstream an ownstream hange slopes. 5. SEISMIC SAFETY ANALYSIS OF THE DAM 5. Analysis of the whole safety of the am Anti-sliing stability analysis is performe by using rigi boy limit equilibrium metho base on Chinese Coes. Then, the ases without an with onsiering powerhouse at the am toe are analyze, respetively. Calulation is performe through the ombination of am an ant when powerhouse at the am toe is onsiere. The eep sliing moes of the powerhouse am setion are shown in Figs.0 (a, (b. Sliing moe :The near horizontal surfae in the rak ane in whih hloritization rok mass abunant region uner am founation is taken as the left sliing ane, an the right sliing ane is sheare from Ⅳa rok mass.

7 Sliing moe 2:Take t b weak seam as the left sliing ane, an the right sliing ane is sheare from Ⅳa rok of powerhouse en. powerhouse Chloritization Rok ivie line of resist boy #slie moe 2#slie moe (a Dam-ant separate struture right slie surfae tb Chloritization Rok #slie moe 2#slie moe (b Dam-ant jointe struture Figure 0 Deep sliing moes of the powerhouse am setion ivie line of resist boy tb right slie surfae The stability heking formula is given by the following equation (CIWRH 2000: γϕs 0 R (5. γ whereγ 0 -strutural importane fator, γ 0 =., ϕ -Design situation oeffiient, ϕ = 0.85 oeffiient of ultimate Limit State, S -struture effet, R -struture resistane., γ -struture Taking the rigi boy limit equilibrium metho, anti-sliing stability along founation ane an sliing moel an 2 is evaluate for ases without an with powerhouse. Struture oeffiient is obtaine by inverse alulation base on Eqn. 5.. The results are presente in table 5., where γ is greater than 0.65, an it meets China Coe requirements uner earthquake ase. Struture oeffiient of Ultimate Limit State γ uner earthquake ase is less than that of the normal storage water level an the hek floo level. It is the ontrol ase of anti-sliing stability of shallow an eep layers in am founation. From table 5., anti-sliing stability of shallow an eep layers in am founation in the ase of ombination of am-ant is greater than that in the ase of am-ant separate. That means ombination of am-ant an improve anti-sliing stability of am. Table 5. Anti-sliing stability results of shallow an eep layers in am founation am-ant separate struture am-ant jointe struture Sliing moe Effet Resistane Struture Effet γ γ 0 ϕ S /kn 0 ϕs Resistane Struture Rγ /kn oeffiient γ /kn Rγ /kn oeffiient γ Founation ane Moe Moe Calulation ase: normal ase + earthquake 5.2 stability analysis onsiering loal raking As a speial ase without reinforement, three rak epths are 9m, 3m an 7m at the am heel, upstream hange slope, an ownstream hange slope, respetively. Three layers are shown in Fig.2. Struture oeffiient of Ultimate Limit State of three sliing surfaes are alulate as following:

8 Founation ane (elevation is 264 m: Effet (normal impoun + earthquake γ 0 ϕs =20696.kN, resistane Rγ = kN, struture oeffiient γ by inverse alulation is 0.9. Horizontal sliing surfae at upstream hange slope (elevation is 345 m: Effet (normal impoun + earthquake γ 0 ϕs =6427.2kN, resistane Rγ = kN, struture oeffiient γ by inverse alulation is Horizontal sliing surfae at ownstream hange slope (elevation is 395 m: Effet (normal impoun + earthquake γ 0 ϕs = kN, resistane Rγ = 476.0kN, struture oeffiient γ by inverse alulation is The results show that even without reinforement, the anti-sliing stability in the above three anes where the amage rak is serious an also satisfy the requirement of the China Coe. Struture oeffiient γ at the ownstream hange slope is less than that of upstream slope an am heel. As long as the epth of rak in the founation ane oes not amage the grout urtain in the am heel, the am is safety. 6. CONCLUSION The am seismi reinforement sheme shoul obey following prinies: it is require that the rak oes not amage the grout urtain in the am heel, an raks at the orners o not have any effet on the stability of the am. Final amount of reinforement shoul be etermine by non-linear finite element metho. The rak amage epth at the key loations is obviously reue after reinforement. Consiering the possible sliing moes of the powerhouse am setion an weak surfaes after the nonlinear earthquake response, aseismi stability evaluation of the am show that the aseismi stability of the powerhouse am setion of Jinanqiao gravity am an satisfy the requirements of China Coe. ACKNOWLEDGEMENTS The authors gratefully aknowlege the supports of this researh by National Basi Researh Program of China (973 Program, Grant No. 2007CB7404, an innovative projet for grauate stuent of Jiangsu Provine. REFERENCES Northwest Hyro Consulting Engineers(NHCE(978. Design oe for hyrauli onrete strutures(sdj Beijing, China Water Power Press. Huang He-sheng (989. The stuy of the earthquake of Ms=4.5 on Sep.5, 687 in Xinfengjiang Reservoir area. South China Journal of Seismology 9:3, Lin, G., Chen, J. Y. (200. Seismi safety evaluation of large onrete ams. Journal of Hyrauli Engineering 2, 8-5. Chen, H.Q. (2005. Seismi fortifiation levels an performane objetives for large ams. Earthquake Resistant Engineering 27,s -6 China Institute of Water Resoures an Hyropower Researh (CIWRH(2000. Speifiations for seismi esign of hyrauli strutures(dl Beijing, China Power Press. Northwest Hyro Consulting Engineers (NHCE(997. Design oe for hyrauli onrete strutures (SL/T Beijing, China Water Power Press. Lubliner, J., Oliver J., Oller S., an Oñate,E.(989. A asti-amage moel for onrete. International Journal of Solis an Strutures,25:3, Lee, J., an Fenves, G. L.(998. Plasti-amage moel for yli loaing of onrete strutures. Journal of Engineering Mehanis 24:8,