A new concept of overhead movable scaffolding system for bridge construction

Transcription

1 A new concept of overhead movable scaffoldng system for brdge constructon Pedro PACHECO CEO BERD Matosnhos, Portugal A. ADÂO da FONSECA Project Drector BERD Matosnhos, Portugal Hugo COELHO Structural Engneer BERD Matosnhos, Portugal Mchael JENTSCH Manager BCE Center Österrechsche DOKA Amstetten, Austra Summary In the last decade, a research and development process, ntated n the Faculty of Engneerng of the Unversty of Porto has brought out an nnovatve technology for brdge constructon: Organc Prestressng. Ths new technology s now beng appled to a brand new generaton of movable scaffoldng systems. The present paper presents a new concept of overhead movable scaffoldng systems for n stu brdge constructon, n whch the scaffoldng structure s smlar to a bowstrng, wth the partcularty of havng an arched upper chord and an actvely controlled lower chord. Keywords: Organc Prestressng; Brdge Constructon; Movable Scaffoldng Systems; Deflecton Control. 1. Introducton Advantages of organc prestressng applcaton n structures wth hgh lve load / dead load ratos and wth relatvely slow loadngs, such as movable scaffoldng systems used for brdge constructon [1], has promoted an ncreasng development of ths technology n the past few years. Inspred on the behavour of nature structures (bommetcs), more specfcally n the muscle behavour, Organc Prestressng System (OPS) s an automatcally adaptve prestressng system whch has the ablty to ncrease or decrease prestressng forces accordng to lve load varaton. It s no more than a prestressng system n whch the tenson appled s automatcally adjusted to the actuatng loads, through a control system, n order to reduce the structural deformatons and mnmze tensons. The frst movable scaffoldng system wth OPS was desgned for the constructon of the Ro Sousa hghway brdge n northern Portugal, a double deck comprsng 15x30 m long spans [2]. The scaffoldng steel structure comprses four ndependent man grders (see Fg. 1), brackets, frcton collars and boges sets. It s strengthened wth an OPS equpment, whch essentally conssts of unbonded prestressng cables, anchorages, devaton shores and saddles, hydraulc actuators (see Fg. 2), sensors and automaton components.

2 Fg. 1 Frst OPS movable scaffoldng system applcaton Fg. 2 Actuator and Organc Anchorage The applcaton of OPS allows for lghter and more functonal structures. The maxmzaton of ts potental, whose lmts are yet to be establshed, justfes the desgn of new types of steel scaffoldng structures, congregatng structural advantages and responses to functonal needs, especally of knematc nature. A new concept of movable scaffoldng system for n stu brdge constructon s presented, namely, an overhead equpment n whch the scaffoldng structure s smlar to a bowstrng, wth the partcularty of havng an arched upper chord and an actvely controlled lower chord. Ths paper performs an overall presentaton of the man concept, ncludng a descrpton of the superor grder and of the transverse structures whch sustan the formwork and an analyss of ther knematcs, comprsng smultaneously structural features and formwork engneerng related ssues. Specal attenton s gven to the OPS system and nherent control strategy. 2. Superor Grder The equpment presented n Fg. 3 s an overhead movable scaffoldng system conceved to buld cast n stu concrete brdge decks wth a maxmum span of 50 m. The superor grder s a steel structure smlar to a bowstrng, wth an arched upper chord and a lower chord actvely controlled by an Organc Prestressng System (OPS) durng the concrete pourng and deck prestressng stages.

3 Fg. 3 Elevaton (top) and plan vew (bottom) of an overhead movable scaffoldng system strengthened wth OPS 2.1 Superor Grder steel structure The steel structure of the superor grder comprses the followng man components: arch, man grder, upper te, front nose, front crane and rear nose. The steel arch, wth a maxmum heght of 9 m and a maxmum dstance between supports of 40 m, s an HEB 400 profle that dverges nto 2 HEB 300 n the abutments. The man grder a modular truss wth a transversal secton of 4.00m x 3.00m has a total length of 60 m, of whch 40 m are suspended from the arch, 15 m form the front cantlever and the remanng 5 m consttute an extraordnarly short rear cantlever. Its man purpose s to suspend the transversal structures that support the formwork. The man grder nferor chords are actvely controlled by an OPS system between the arch abutments, retanng abutments dsplacements and subsequent arch openng. Ths partcularty allows the structure to behave lke a bowstrng durng the concrete pourng stage. The arch and actve te structural effcency allows a drastc reducton of the man grder deformatons between supports durng the concrete pourng stage, wth a maxmum md-span vertcal deflecton nferor to L/2000 (approxmately 2.5 cm). The cantlever concrete pourng extenson (1/5 of the followng span) s not controlled by OPS. The cantlever deflectons are reasonably restrcted through ncluson of two superor passve tes wth a maxmum eccentrcty of 6 m n the per support secton relatve to the grder nferor chord, resultng n a consderable stffness ncrease. The feasblty of the equpment launchng s guaranteed by the front nose, the front crane and the rear nose. The front nose has a total length of 27.5 m and a trangular transversal secton wth a heght of 3 m and a wdth of 4 m. The rear nose conssts on a prolongaton of the man grder, formed by two vertcal plane trusses wth capacty to open (through ndependent rotaton around a vertcal axs), enablng the rear per frame dsassemblng wth the movable scaffoldng structure n the concrete pourng poston. On the other hand, the front crane conssts of a rotatory nose prolongaton equpped wth a temporary frame and an elevaton wnch. Wth the structure stll n the concrete pourng poston, the temporary frame lands on the front per (by means of hydraulc jacks) allowng the front crane to elevate, from the ground, and to assemble the prevously dsmounted per frame.

4 2.2 OPS system OPS s nothng more than an actve control prestressng system [3] whose objectve s to reduce deformatons and/or tensons due to lve loads. The man elements (see Fg. 4) are the actuator n the organc anchorage, the unbonded cables, the sensors and the electronc controller n the grder control unt [4], [5], [6], [7]. Fg. 4 3D scheme of an Overhead MSS grder equpped wth OPS The control strategy s smlar to the frst OPS applcatons, adoptng the md-span vertcal deflecton as prmordal control varable. To mplement ths technque, a reservor s ftted n a fxed locaton near a per, and pressure sensors are spread along the structure, connected by a flud crcut (Fg.5a). Man grder deflecton varaton can be determned through changes n hydrostatc pressure [6], [7]. The sensors transmt the nformaton to an automaton whch processes t accordng to a control algorthm, and then decdes between mantanng or changng the prestressng force [1]. Typcally, n a concrete pourng stuaton, the concrete pourng mode s turned on and f the md-span deflecton exceeds a pre-defned lmt, the automaton decdes to ncrease the hydraulc jack (actuator) stroke, movng the organc anchorage beam (see Fg. 2) and smultaneously tensonng four rectlnear prestressng cables. In smplfed mathematcal terms, durng the concrete pourng stage, the control algorthm s manly stated by expressons n Eq. (1): ( t ) > c ( t ) c where, nc( t nc( t + t) = nc( t ) + 1 ξ ( t ) + t) = nc( t ) ( t ) s the fltered md-span deflecton at nstant t ; c s the predefned md-span deflecton control lmt; t s the tme step adopted n the control algorthm ( t = t t 1 ) ; nc ( t ) s the number of stroke unt-step changes performed by the actuator at nstant t ; ξ ( t ) s the overall valdaton functon at nstant t (assumes values 0 or 1) [8]. The symmetrc algorthm controls the brdge deck prestressng stage (reverse process). In both stages, software flters are used to overlook vbratons. Indeed, ths control algorthm s vald for statc control, thus, to avod control nstablty, unt-step changes performed by the actuator (output) must not depend on vbratons. Software safety features provde contnuous evaluaton of the hardware components ntegrty state and of the whole system operatonal state. If ths contnuous evaluaton suggests any abnormal stuaton, OPS reaches a breakng level (actuator blockage) and an alarm s trggered [9]. To acheve ths fundamental prncple, software codes are developed accordng to Eq. (1), where any unt-step (1)

5 change s multpled by an overall valdaton functon ξ ( t ) whch establshes, at any nstant t, f all OPS subsystems and components verfy smultaneously operatonal and ntegrty predefned crtera ( ξ ( ) = 1) or not ( ξ ( ) = 0). t t OPS commands allow the operator to choose the desred operatonal mode, accordng to each constructon stage. The control software s computed by a Programmable Logc Controller (PLC) located n the grder control unt (Fg. 5b, c). Fg. 5 (a) Sensor Cabnet (b) and grder control unt nsde (c) and outsde vews and (d) HMI Through a Human-machne nterface (HMI) (Fg. 5d) the operator s permanently nformed about the state of the system. It dsplays, among other nformaton, the grder deflecton, warnngs and alarms. Fundamental data s contnuously recorded for subsequent analyss. 3. Transversal structure An overhead movable scaffoldng system constructs the brdge s deck beneath the man grder. Therefore, the transversal structure that supports the formwork s suspended from the man grder by means of transversal grps that guarantee the requred wdth for the formwork. The transversal structure s materalzed by two pars of steel trusses, each of them consttuted by a horzontal and a vertcal truss. Durng the concrete pourng stage, the horzontal trusses are nterconnected n order to poston the formwork. Durng ths stage, the hgh level of loadng and deformaton requrements mply the nstallaton of a par of hgh strength steel threadbars n an nner poston, suspendng the transversal structure and reducng ts span (see Fg.6). The threadbars are convenently postoned to facltate the placng of the prefabrcated deck steel renforcement. Superor grder Threadbar "Transversal grp" Rotaton jack Wnch Formwork Transversal structure horzontal truss Transversal structure vertcal truss Fg. 6 Movable scaffoldng system n concrete pourng poston (cross secton)

6 Before the launchng stage, the grders are lowered to free all formwork from the deck, the horzontal trusses are dsconnected, the steel threadbars are dsassembled and the transversal structures are opened. The openng moton s performed actuatng on hydraulc jacks, allowng the automatc rotaton of transversal structures and formwork. a) b) c) d) Fg. 7 Grder descendng, threadbars dsassemblng and transversal structures rotaton The transversal structures were conceved to construct brdge and vaduct decks wth a maxmum longtudnal slope of 5% and varable transversal slope up to a maxmum of 8%. Unlke the tradtonal scaffoldng systems, n whch the deck s constructed n a straght lne between pers, ths transversal structure allows the constructon of a polygonal wth 5 m long segments, obtanng a better approxmaton to the drectrx shape (crcular or clothod). The transversal grps support pars of wnches (see Fg. 6), makng t possble to transport the prefabrcated steel renforcement and prestressng cables ducts drectly from the lorry to the constructon front, wth no need for auxlary elevaton equpment. 4. Formwork The formwork of movable scaffoldng systems s, n general, specfcally conceved for each applcaton. The frst applcaton of the present equpment was developed for the constructon of a box cross secton concrete deck, m wde and wth a constant heght of 2.40 m. It was assumed that the concrete pourng was to be carred out n two dfferent stages: the frst stage comprses the constructon of the bottom slab and vertcal walls and the second stage comprses the constructon of the top slab, cantlevers ncluded (see Fg. 8). In order to avod the appearance of cracks n the frst stage concrete (manly n the vertcal walls) the scaffoldng structure mustn t experment sgnfcant deformatons durng the second concrete pourng stage. The ncluson of an OPS system s synonym of deflecton control and a guarantee of low deformatons.

7 Fg. 8 Formwork and concrete pourng stages: 1 st stage (left) and 2 nd stage (rght) The formwork desgn process consdered several pecular features related wth the movable scaffoldng system geometry and knematcs. A formwork table, wth a length of 5 m and a longtudnal dstance between support algnments of 2.80 m, was acheved. The bottom external formwork s dvded n two symmetrc parts, n order to make the transversal structures openng moton vable. The supports are materalzed by 6 ponts of vertcal contact and 2 ponts of horzontal contact (fundamental throughout the openng moton) wth each transversal structure. The formwork was developed amng at functonalty, beng noteworthy the connecton between the bottom and the lateral external panel whch allows vertcal and transversal adjustments as well as the formwork table adaptablty to the deck geometry. In addton, the top slab formwork s launched to the followng span by ts own means. Conclusons The conjugaton of OPS technology wth the structural effcency of an arch resulted n an actve bowstrng steel structure, allowng the achevement of an extremely lght and functonal movable scaffoldng system. The followng advantages standout, n comparson wth a tradtonal equpment: - Reducton of the equpment weght (25 to 30% of steel volume); - Reducton of the acquston costs (about 15%); - Reducton of operatonal costs (10 to 20%); - Md-span deflecton control and ablty to program pre-cambers; - Contnuous montorng of the scaffoldng structure, enablng hgher safety levels; - Easer transportaton and on ste assemblage of the scaffoldng equpment; - Smplcty of steel connectons (maxmum tensons substantally reduced). Moreover, mplementaton of OPS technology n movable scaffoldng systems and, partcularly, n the present equpment, enables the constructon of hgh speed ralways brdge decks, whch are substantally heaver (about 30%) than both common ralway brdge decks and hghway brdge decks. Acknowledgements The authors wsh to thank all members of BERD producton team for all the work and dedcaton throughout the development of the present project: Pedro Borges, Antóno André, André Resende, Inês Ferraz, Frederco Fonseca, Teresa Olvera and Antóno Guerra, who are, ndeed, co-authors of ths project. Fnally, the authors are also grateful to DOKA Portugal, specally to Sousa Lma and Patríca Gomes for ther commtment n the search of the best formwork soluton.

8 References [1] PACHECO P. and ADÃO DA FONSECA A., Organc Prestressng, ASCE Journal of Structural Engneerng, Vol. 128, No. 3, 2002, pp [2] PACHECO P., GUERRA A., BORGES P. and COELHO H., A Scaffoldng System strengthened wth Organc Prestressng the frst of a new generaton of structures, Structural Engneerng Internatonal Journal of IABSE, Vol. 17, No.4, pp , [3] MONTENS S., A Global Concept for 21 st Century Brdges: Parastressng, FIP Symposum on Post-Tensoned Concrete Structures, London, [4] PACHECO P., Organc Prestressng an Effector System example (n Portuguese), PhD Thess, Dep. Cvl. Eng., Faculty of Engneerng of the Unversty of Porto, [5] ANDRÉ A., Expermental study of a movable scaffoldng system reduced scale model strengthened wth organc prestressng (n Portuguese), MSc Thess, Dep. Cvl Eng., Faculty of Engneerng of the Unversty of Porto, [6] PACHECO P. et al, Strengthenng by organc prestressng of exstng launchng gantres n the constructon of hgh speed ralway brdge decks, Workshop Brdges for Hgh Speed Ralways, Porto, pp , [7] ANDRÉ A., PACHECO P. and ADÃO DA FONSECA A., Expermental study of a launchng gantry reduced scale model strengthened wth organc presstressng, Structural Engneerng Internatonal Journal of IABSE, Vol. 16, No.1, pp , [8] COELHO H., BORGES P. and PACHECO P., Specfcatons for OPS Software Development, Techncal report, Faculty of Engneerng of Porto Unversty, [9] CEN, Safety of machnery Safety related parts of control systems Part 1: General prncples for desgn, EN 954-1: 2001 E, European Commttee for Standardzaton, Brussels, 2002.