基于無應力狀態(tài)法的分段成形梁橋合攏技術的研究(中 英文0811)

1、基于無應力狀態(tài)法的分段成形梁橋合攏技術的研究 張師定(青島華城市政工程設計院，青島266071）摘要：本文從橋梁體系轉換入手，應用無應力狀態(tài)控制法理論，剖析現行橋梁合攏體系轉換做法的不周；提出成橋設計內力分布應當選擇連續(xù)結構、橋梁合攏方案必須采取專門技術等新理念；對于連續(xù)梁橋，建議采用“頂升-降落”支點法；對于連續(xù)剛構橋，筆者建議采用“頂升合攏梁端”法；最后，給出了簡化的數字例子。關鍵詞：橋梁體系轉換；橋梁合攏專門技術；無應力狀態(tài)控制法理論；成橋內力分布中國已建成許多座跨度超過百米的預應力混凝土梁橋，然而，經多年運營發(fā)現，跨中下撓是最突出的問題。許多學者對此問題進行了深入研究12345，發(fā)現的

2、主要原因有：(1) 混凝土材料的長期收縮徐變的影響；(2) 混凝土材料長期收縮徐變導致預應力損失；(3) 混凝土開裂導致截面剛度變小等。然而，更為深奧的道理尚未被認識。筆者從橋梁體系轉換入手，應用無應力狀態(tài)控制理論，透徹剖析現行體系轉換做法的不周，提出新的設計理念。1 現行混凝土梁橋合攏方案以懸臂澆筑節(jié)段施工的三跨預應力混凝土連續(xù)箱梁橋為例?？傮w合攏順序：先邊跨合攏，解除臨時固結落梁后，再中跨合攏。邊跨合攏方案：通過外施勁性骨架鎖口，并預張拉合攏束，懸吊架法澆筑合攏段砼。 中跨合攏方案：邊跨合攏完成后，通過外施勁性骨架鎖口中跨合攏段，并預張拉合攏束，懸吊架法澆筑合攏段砼。1.1邊跨合攏段施工方

3、案1.1.1 施工準備首先完成掛籃懸澆段施工和邊跨支架現澆段施工。然后將邊跨端掛籃推進至邊跨合攏段，中跨端掛籃位置不變。施工時預先在邊跨懸澆塊和支架現澆段箱梁處，預留底模和側模錨固孔，利用掛籃吊帶將掛籃底模和側模錨固在邊跨合攏段兩側已完成的箱梁上，作為合攏段施工的底模和側模。1.1.2 配重邊跨合攏段施工配重僅在T構端上進行，配重重量為合攏段砼重量的一半，而邊跨現澆段不予配重。配重可以為沙包或水箱等。1.1.3 合攏段鎖口合攏段鎖口采用外施（或內施）勁性骨架，勁性骨架設置在箱梁底板和頂板上，共四組，每組有兩根型鋼組成。按照規(guī)范規(guī)定，在當天溫度最低時將型鋼焊接在合攏箱梁的預埋鋼板上，完成鎖定。1

4、.1.4 合攏束預張拉(1)精確固定邊跨合攏段底模板和外側模板；(2) 綁扎底、腹板鋼筋，安裝底、腹板波紋管；(3) 立合攏段內模；(4) 綁扎頂板鋼筋和安裝波紋管；(5) 穿合攏束。邊跨預張拉合攏束,完成后.開始進行合攏段砼澆筑工作。1.1.5 合攏段砼施工合攏段砼澆筑時間選在一天氣溫較低時進行。砼施工中配重逐級卸除，且澆注砼的重量與卸除配重的重量相等，使合攏段始終處于相對穩(wěn)定狀態(tài)。1.1.6 預應力筋張拉與落梁預應力筋張拉需滿足一下先決條件：（1） 合攏段強度達到設計強度的95%；（2） 砼彈性模量達到設計的100%；（3） 砼齡期不少于5天。開始進行預應力束張拉，張拉前拆除合攏段勁性骨架

5、。張拉順序為：先底板束，后頂板束；先長束，后短束，并將合攏束補拉到設計噸位。張拉完畢后進行孔道壓漿。當張拉并錨固縱向預應力束及橫向和豎向預應力筋后，拆除兩墩上臨時固結并落梁，完成體系轉換。張拉完成后拆除邊跨合攏段及現澆段的吊架、模板及支架。1.2中跨合攏段施工方案1.2.1 施工準備預先在中跨兩個梁塊（與合攏段相鄰）施工時預留底模和側模錨固孔，利用掛籃吊帶將掛籃底模和側模錨固在兩側已完成的箱梁上，作為合攏段施工的底模和側模。1.2.2 配重中跨合攏段施工配重在兩個懸臂端上同時對稱進行，各端配重重量為合攏段砼（包括橫隔梁，如果有）重量的一半。1.2.3 合攏段鎖口要求同1.1.3.1.2.4 合

6、攏束預張拉 要求同1.1.4.1.2.5 合攏段砼施工合攏段砼澆筑時間選在一天氣溫較低時進行。砼施工中配重逐級卸除，且澆注砼的重量與卸除配重的重量相等，使合攏段始終處于相對穩(wěn)定狀態(tài)。1.2.6 預應力張拉要求同1.1.6. 2 無應力狀態(tài)法理論對合攏方案的要求橋梁結構往往不是一次形成的，施工過程中往往發(fā)生多次體系轉換，其成橋行為不一定與一次落架成橋相同4。 這樣以來，最優(yōu)受力、目標線形及施工控制（措施）三者間相互影響，使問題復雜化。秦順全先生提出無應力狀態(tài)控制理論與方法6來解決此類問題，是橋梁發(fā)展的一個里程碑。【定理1】對于給定的結構，包括其外荷載、結構體系、支承邊界條件、單元的無應力長度及無

7、應力曲率，無論其形成過程如何，其最終對應的結構內力和位移是唯一的?！就普?】結構單元的內力和位移隨著結構的加載、體系轉換、或斜拉索的張拉而變化，但單元的無應力長度只有人為地調整才會發(fā)生變化。當結構體系和荷載一定時，單元的無應力長度的變化必然唯一地對應一個單元軸力的變化，單元的無應力曲率的變化必然唯一地對應一個單元彎矩的變化?！径ɡ?】對于上一階段形成的結構，安裝無應力狀態(tài)下的非合攏性構件后，形成的本階段結構的結構內力和位移是唯一的，無論非合攏性構件是否一次形成、是否用支架或懸臂拼裝。2.1 合攏方案應遵循的基本原則預應力混凝土連續(xù)梁（剛架）橋設計時，雖然考慮其梁體分段形成，但期望其一期恒載下結

8、構內力狀態(tài)仍呈連續(xù)梁狀態(tài)，而非懸臂梁（受力不利）狀態(tài)。結合無應力狀態(tài)法的要求，梁體施工過程中必須滿足如下條件：（1） 在任一施工階段，結構各構件受力安全；（2） 安裝任一結構構件時，必須保證該構件無應力；（3） 必須采用專門技術措施，保證合攏處彈性曲線連接，使成橋內力狀態(tài)為設計目標狀態(tài)；（4） 必須合理設置施工預拱度，保證成橋線形符合設計目標。注：預拱度設置改變不了結構內力分布，代替不了合理的體系轉換措施。（5） 施工方案在安全的前提下，必須經濟合理。2.2 合攏方案中的專門技術措施依據無應力法理論，要求合攏前的兩側梁端轉角必須連續(xù)。其專門技術措施包括：方案一：跨中合攏前，在兩側梁端施加 一對

9、力偶，待跨中合攏后，卸除該力偶。方案二：跨中合攏前，在兩側梁端各施加一個集中力，使兩側梁端轉角連續(xù)（消除轉角差），待跨中合攏后，卸除該集中力。方案三：跨中合攏前，分別轉動兩個支點處的梁體，從而消除兩側梁端轉角差，待跨中合攏后，再卸除該扭矩。方案四：跨中合攏前，分別頂升兩個支點處的梁體，從而消除兩側梁端轉角差，待跨中合攏后，再卸除該頂力。2.3 數字例圖2-1 連續(xù)梁計算模型與目標成橋線形圖2-2 目標成橋彎矩分布圖2-3 中跨合攏前懸臂狀態(tài)以（0.7l+l+0.7l）三跨等截面連續(xù)梁（見圖2-1）為簡單例，比較了不同施工方案典型截面的恒載內力及變形情況，成果見表2-1.方案一未發(fā)生體系轉換，其

10、恒載內力狀態(tài)完全是連續(xù)梁（見圖2-2）。方案二類似現行方案，恒載內力為懸臂梁狀態(tài)，筆者不推薦該方案。方案三對于合攏前懸臂狀態(tài)（見圖2-3）采用專門技術措施，即“頂升-降落”支點。對于連續(xù)梁橋，筆者建議采用此方案。方案四對于合攏前懸臂狀態(tài)（見圖2-3）專門技術措施，即頂升合攏梁端。對于連續(xù)剛構橋，筆者建議采用此方案。表2-1 (0.7l+l+0.7l)三跨連續(xù)梁橋的施工方案比較方案編號項目各方案施工方案中跨跨中截面彎矩轉角撓度方案一滿堂支架整體現澆方案滿堂支架現澆，一次成形。0.0487ql200.00521ql4/EI方案二現行懸臂施工方案支架上現澆懸臂梁，拆支架后，合攏中跨。00.0357q

11、l3/EI0.01525ql4/EI方案三頂升-降落支點方案支架上現澆懸臂梁，拆支架后，頂升中支點位移0.025ql4/EI，然后合攏中跨，最后支點高程復位。0.0487ql200.00521ql4/EI方案四頂升合攏端方案支架上現澆懸臂梁，拆支架后，頂升合攏端梁P=0.1477ql，然后合攏中跨，撤銷頂升力。0.0487ql200.00521ql4/EI注1：EI-連續(xù)梁抗彎剛度；注2：q-滿跨均布荷載集度；注3：l-中跨跨度；注4：表中數值由結構力學求得；注5：方案四中P值由下式求得。 0.2417Pl2/EI=0.0357ql3/EI （2-1） 3 現行合攏方案之工藝缺陷及后果3.1

12、設計成橋狀態(tài)內力分布的缺陷 現行發(fā)生體系轉換的預應力混凝土梁橋，在設計過程中，往往對成橋內力分布的預想不清楚、不科學，往往形成“偽連續(xù)”。例如，本應成橋目標狀態(tài)為連續(xù)梁，而誤設計為若干個簡支懸臂梁；本應成橋目標狀態(tài)為連續(xù)剛構，而誤設計為若干個T構。3.2設計對體系轉換過程考慮不周對比第1節(jié)及第2節(jié)內容，發(fā)現：現行施工方案的最大缺點是缺失第2節(jié)第（3）條的要求，即缺失2.2節(jié)專門技術措施，另外，合攏時掛籃自重及配重使梁跨結構受力惡化，并持久影響；后果則是成橋內力狀態(tài)非連續(xù)梁狀態(tài)，而傾向于懸臂梁狀態(tài)；跨度越大，則內力差別越明顯，跨中下撓度就越大。悲觀的是，發(fā)生體系轉換的預應力混凝土連續(xù)梁（剛構）橋

13、，幾乎都沒有按第2節(jié)的方案去做。4 結束語筆者確信，大跨預應力混凝土梁橋存在的設計誤區(qū)已找到。從現在起，我們應當創(chuàng)新設計思路：（1） 優(yōu)化成橋設計內力分布，以真正的連續(xù)梁（剛構）內力分布為內力設計目標；（2） 優(yōu)化橋梁合攏方案，必須頂升中支點或采取其他措施，使梁合攏端圓順連接。（3） 確保實施合理的橋梁合攏方案后，就可以達到優(yōu)化的成橋目標設計內力分布。筆者現在認識到這點，橋梁界必須改良現有的理論與做法，否則，橋梁建成之日，就是危橋誕生之時。本文的觀點對其他形式橋梁的設計與施工有借鑒作用。 參考文獻 1許震.大跨預應力混凝土連續(xù)剛構橋下撓分析及對策研究D,重慶交通大學碩士學位論文，2007.2孫

14、劍川.預應力混凝土連續(xù)剛構橋下撓問題研究D,西南交通大學碩士學位論文，2010.3劉亞軍.預應力混凝土連續(xù)剛構橋下撓問題的研究D,武漢理工大學碩士學位論文，2010.4張師定.橋梁建筑的結構構思與設計技巧M,人民交通出版社，2002.5北京市政工程設計院.大跨徑預應力混凝土連續(xù)剛構橋設計指南S, 2013.6秦順全.橋梁施工控制無應力狀態(tài)理論與實踐M,人民交通出版社，2007.張師定，男，工學學士，高級工程師，陜西大荔人，主要研究方向：橋梁與房屋結構的優(yōu)化設計及體系轉換分析。青島華城市政工程設計院總工程師，E-mail：zhangshiding 電話通訊地址：（266

15、071）青島市市北區(qū)勁松三路277號依山半島1-1-803# Study on Closure Technique of Sectional Forming Beam Bridge Based on Unstressed State Control MethodZHANG Shi-ding（Qingdao Achene Municipal Engineering Design Co.,Ltd.,Qingdao 266071,China） Abstract:Based on unstressed state control method, proceed with bridge system c

16、hange, shortcoming of present plan of bridge system change is ana-lysed. It is suggested to regard continuous beam/girder as target of internal force distribution under completion stage of bridge, and use special technique of bridge closure, for example, lift-up/drop-down supports method for continu

17、ous beam bridge;lift-up adjacent ends method for continuous girder bridge. Finally,a digital simple example is presented.Keywords:bridge system change; technique of bridge closure; unstressed state control method; internal force distribution under completion stage of bridgeThere are many pre-stresse

18、d concrete beam bridges with span of over 100m ,which built in China.But after running for many years,a large problem has found that deflection at the center of each span is heavy.Many scholar studied12345 deeply on it,and main reasons are as follows:(1) Influence of concrete shrinkage and creep;(2)

19、 Loss of prestress due to concrete shrinkage and creep;(3) Decrease of cross section stiffness due to concrete cracking,and etc.In the authors opinion , there is an important field unfounded now. Starting with bridge system change,based on unstressed state control theory,a new idea with respect to s

20、hortcoming of current system change is presented. 1 Present plan of concrete beam bridge closureTake it as an ideal example,a pre-stressed concrete boxed-beam bridges with 3 spans, using cast-in-place cantilever method, is as belows.General sequence of closures:first,close side span .then remove the

21、 temporary consolidation device and drop down beam,finally close mid-span.Plan of side span closure:erect stiff skeleton outside to lock adjacent ends,pre-tension cables for closure,then cast concrete of closure segment with movable suspended scaffolding. Plan of mid span closure:after side span is

22、closed,erect stiff skeleton outside to lock adjacent ends,pre-tension cables for closure,then cast concrete of closure segment with movable suspended scaffolding.1.1 Construction plan of side span closure1.1.1 Construction preparationFirst,cast cantilever segments with movable suspended scaffolding,

23、as well as cast segments of side span on falsework.Then move the form traveler of side span ahead to closure sector,and hold place of the form traveler of mid span . At the ends of adjacent beams(i.e.both besides of closure segment of side span),have holes reserved for anchoring bottom and vertical

24、form for casting closure segment to fix on already completed portion of the bridge,use suspended steel belts of form traveler.1.1.2 Additional weightAt the end of side T girder,apply additional weight which value equals to half of closure segment weight,but no additional weight is there at the end o

25、f segments of side span on falsework.Additional weight can be bags filled sands or tanks filled water.1.1.3 Lock adjacent ends for closureErect outside(or inside) stiff skeleton which consists of 4 sets with each set has 2 standard steel tie members ,placed at bottom slab and top slab of box-girder,

26、to lock adjacent ends.According to the requirement of codes,when erecting stiff skeleton on steel plates reserved at completed portion ,air temperature around structure must be lowest during a day.1.1.4 Pretension of closure tendons(1) Actually fix bottom form and vertical form of side span closure

27、segment;(2) Place rebars and corrugated pipes in webs and bottom slab;(3) Place inside formwork;(4) Place rebars and corrugated pipes in top slab;(5) Place closure tendons in corrugated pipes.Pretension tendons necessary,then pour closure concrete.1.1.5 Pour closure concreteWhen pouring closure conc

28、rete ,air temperature around structure must be lower during a day.Additional weight is reduced grade by grade,keep casting concrete weight equal reduced weight,to maintain closure segment under steady state.1.1.6 Tension of tendons and drop down beamTension closure tendons under some conditions:(1)

29、Closure concrete strength is up 95% of design value;(2) Elastic modulus of closure concrete is up 100% of design value;(3) Age of concrete is more than 5d.Before tensioning,remove stiff skeleton.Sequence of tension:first,tendons in bottom slab ,second,tendons in top slab;first,long tendons,second,sh

30、ort tendons.Tension closure tendons pretensioned to design stress.After have tensioned,grout ducts.After longitudinal ,lateral and vertical tendons have tensioned,grout ducts,remove the temporary consolidation device of two mid piers and drop down beams,finally close mid-span,so structural system ch

31、ange is finished.Remove movable traveler,form and falsework.1.2 Construction plan of mid span closure1.2.1 Construction preparationAt the ends of adjacent cantilevers(i.e.both sides of closure segment ),have holes reserved for anchoring bottom and vertical form for casting closure segment to fix on

32、already completed portion of the bridge,use suspended steel belts of form traveler.1.2.2 Additional weightAt each end of adjacent cantilevers,apply additional weight which value equals to half of closure segment weight,include cross-girder if designed.1.2.3 Lock adjacent ends for closureSame as 1.1.

33、3.1.2.4 Pretension of closure tendonsSame as 1.1.4.1.2.5 Pour closure concreteWhen pouring closure concrete ,air temperature around structure must be lower during a day.Additional weight is reduced grade by grade,keep casting concrete weight equal reduced weight,to maintain closure segment under ste

34、ady state.1.2.6 Tension of tendons Same as 1.1.6. 2 Requirement for closure plan based on theory of unstressed state controlBridge structure is not usually one-step forming.During construction,there are many times of changing structural system.The bridge built in such way does not behave exactly the

35、 same way as a bridge that is built totally on falsework.So,these aspects,for example,optimum internal fore,objective geometric configuration,monitoring and control of construction,influence with each other,and make it complex.To solve the problem,Mr.Ain-shushuan presented theory and technique6 of u

36、nstressed state control ,which became a landmark in bridge development.Theorem 1 When they are set,include external loads,structure systems,structural boundary conditions,lengths and curvatures of elements under unstressed states,internal forces and displacements of final structure is determined.Cor

37、ollary 1 Internal force and displacement of structure changes with changing of external loads,structure systems and tension of cables.Length of an element under unstressed state will change only having been adjusted artificially. When external loads and structure systems are settled,change of elemen

38、t length under unstressed state corresponds to change of the element axial force;change of element curvature under unstressed state is respond to change of the element bending moment.Theorem 2 Having erected a non-closed part under unstressed state on the last stage structure, the new structures int

39、ernal forces and displacements is determined, no matter whether the part is one-step forming or not, no matter the part is built on falsework or suspended scaffolding.2.1 Principle of plan of bridge closureDuring design of prestressed concrete continuous beam (girder) bridge,it is assumed that beam

40、is built in segment,and the internal force distribution under the weight of bridge self is still same as continuous beam, not cantilevers(bad state).Based on the theory of unstressed state, the best construction technique for a given structure depends mainly on local conditions as below: (1)At any s

41、tage under construction,structure or any portion is safe.(2)When element or portion of structure is being erected,it must be under unstressed state.(3)It is necessary to use special technique to make the ends of adjacent cantilevers smoothly joint,thus,the internal fore distribution under completion

42、 stage of bridge is near to target of design. (4)In order to obtain a bridge with the right elevation,the required camber of the bridge at each construction stage must be calculated. Note: set up of camber cannot change distribution of internal force,so it cannot take the place of reasonable techniq

43、ue for changing of system. (5)Construction plan is economical and reasonable as well as safe. 2.2 Special technique of closure planBased on the theory of unstressed state,it is necessary to use special technique to make the ends of adjacent cantilevers smoothly joint.The special technique are as bel

44、ow:Plan 1#:having loaded a bending moment at each end of adjacent cantilevers,then joint ends,finally,cancel the bending moments.Plan 2#:having loaded a force at each end of adjacent cantilevers,then joint ends,finally,cancel forces.Plan 3#:rotate cantilevers at adjacent mid-supports,then joint ends

45、 of adjacent cantilevers,finally,cancel the torsion.Plan 4#:lift-up cantilevers at adjacent mid-supports,then joint ends of adjacent cantilevers,finally,cancel the force.2.3 Digital exampleFig.2-1 Model of continuous beam and target configuration under complete stage Fig.2-2 Target distribution of b

46、ending moment under complete stage Fig.2-3 Cantilever state before mid span closureLets regard a continuous beam (see Fig.2-1)with (0.7l+l+0.7l) spans as an simple example,comparing with internal forces and displacements under complete stage,applied various construction plans.The result is in table2

47、-1.Plan 1# :structure system dont change during construction.Its complete state is really continuous beam(see Fig.2-2).Plan 2#:it is similar to present plan.Its complete state is cantilevers.it is a bad plan.Plan 3#:under cantilever state(see Fig.2-3),use special technique-lift-up/drop-down supports

48、.The plan is suggested for continuous beam bridge.Plan 4#:its special technique is lift up the ends of adjacent cantilevers.The plan is suggested for continuous girder bridge.Table2-1 Compare of construction plans for (0.7l+l+0.7l)continuous beam bridgePlan numberItemPlansConstruction PlanCross Sect

49、ion at the Center of Mid-spanBending momentRotating angleDeflectionPlan 1#cast-in-place beam over framingover framing,cast-in-place,one-step forming 0.0487ql200.00521ql4/EIPlan 2#current cantilever constructionFirst,cast-in-place cantilever beam over framing.second,remove support.third,joint the end

50、s of adjacent cantilevers.00.0357ql3/EI0.01525ql4/EIPlan 3#lift-up/drop-down support(1)cast-in-place cantilever beam over framing;(2)remove support;(3)lift-up middle support,vertical displacement-0.025ql4/EI;(4)joint the ends of adjacent cantilevers;(5)drop-down middle support,put it back in place.0

51、.0487ql200.00521ql4/EIPlan 4#Lift up the ends of adjacent cantilevers(1)cast-in-place cantilever beam over framing;(2)remove support;(3)lift-up the end of cantilever ,force P=0.1477ql;(4)joint the ends of adjacent cantilevers;(5)cancel the force.0.0487ql200.00521ql4/EINote 1:EI-flexural rigidity of

52、continuous beam;Note 2:q-intensity of load on the pull span;Note 3:l-middle span;Note 4:values in table are from structural mechanics;Note 5:P from the equation 0.2417Pl2/EI=0.0357ql3/EI (2-1) 3 Shortcoming and result of present plan of bridge closure3.1 Shortcoming of the internal fore distribution

53、 under completion stage of bridge During design of bridge,for designer,the target of the internal fore distribution under completion stage of bridge is not clear and scientific.After having enforced the present plan of system change, the completion state is often non-continuous.For example,the state

54、 which should be continuous beam ,is really numerous cantilevers ; The state which should be continuous girder ,is really numerous T-shaped cantilevers .3.2 Deficiency of design of system changeCompare section 1 to section 2,it is founded that the present closure plan is lack of section 2.2,i.e spec

55、ial technique for closure.Further more,the self-weight of formwork ,falsework and additional weight is harmful forever for structure.The result is the state which should be continuous beam ,is normally cantilevers indeed.The larger is span,larger is difference of internal forces between them.The bigger is deflection at the center of each span.It is a pity that,at present, almost prestressed concrete beam/girder bridge ,with changing system during construction,is far from section 2.4 ConclusionsIn the authors opinion, the shortcoming