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ARCH CENTERING AND WOODEN DECK

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8.1 Preliminary remarks 


General Engineering's assignment doesn't foreseen any design of the temporary structures of the bridge including the centering; therefore no specification and no design for the above mentioned works will be given. Nevertheless some opinions and warnings are here next reported to be of help for future developments of the work. But it has to be clearly stressed that these notes shouldn't be of any constraint and of any impediment for the future developing of the work: General Engineering may be not considered, in any way, responsible of any of the following observations and of any of the works which are not foreseen to be designed by GE according to the assignment.

 


8.2 Topographic survey


A topographic survey should be held, (by the Company in charge of the works), in order to verify and confirm the position of the arch springer levels and of the pedestrian flooring levels, being those co-ordinates related to the matching of all the design project, (matching of new interventions with the still existing portions of the abutments). Co-ordinates should be communicated to General Engineering and any eventual incoherence will be checked and analysed. After this check is completed, the Company in charge of the rehabilitation work will accept those measures and co-ordinates as reliable and will not be allowed to complain about any incoherence pertaining matching of ancient and new interventions in the bridge span.

 


8.3 Centering and deck design


Being the former bridge arch intrados of an irregular shape, and being the final stone cut, (SC-final: see chapter 7 of this report), quite undertaking, it is advisable to use for the bridge reconstruction:

  • a load bearing centering regularly shaped and smaller than the final arch;
  • a wooden deck to be superposed to the centering to fit the former curvature.

In order to perform the bridge reconstruction works correctly, in respect of the geometry requisites, it is suggested to provide a detailed design of the load bearing centering and of its foundations, being those temporary structures structurally and architecturally remarkable. It is also suggested that the design of the wooden deck and of its shape and dimensions is provided in co-ordination with the design of the load bearing centering, being these two elements strictly linked one to each other and linked to evaluations of dead loads actions, settlings and geometry requisites.

 


8.4 Arch centering


Arch centering has been hypothesised to be composed of five parallel structures outdistanced of about a meter and placed on to hydraulic jacks for position adjusting.

In the following paragraph (8.4.1), some other observations have been given by the structural design team (Department of Civil Engineer - University of Florence) Prof. Eng. Andrea Vignoli.

Other general observation about geometry are given in paragraph 8.4.2 by the architectural design in order to respect geometry requisites.

 


8.4.1 Arch centering: structural observation


Arch centering will be a special structural scaffolding, and since it will not bear only its own weight but also the voussoirs weight, it will need to be structurally designed with calculations of it. And the scaffolding will be subjected to deformations due to different working phases and related loads: if a very stiff centering is used, the form of the arch centering, (plus superposed deck), can be adopted equal to the one of former bridge vault.

The use of a traditional centering is proposed, as shown in figure 1. Under the dead weight of the voussoirs, the centering exhibits a vertical deflection. Therefore a regular shape of the centering may be designed to follow approximately the ancient shape of the arch, at a predefined distance from the intrados. The deformation is partly due to the elastic deformation of the centering, (which can be predicted easily and limited increasing the rigidity of the centering), and partly to the long and short time deformations of the soil at the springers, (which are difficult to predict). The elastic deformation of the centering should be limited to not more than 5 mm and the deformed shape should be approximately homotetic to the original shape.

At each springer an oleodynamic jack should be placed under the foot of the centering to recover eventual soil settlements and to allow dismantling of the centering.

Fig. 01 Traditional centering scheme.

To assure the stability of the arch during the phases of installation of the spandrels, especially against eventual asymmetrical load conditions, it is suggested to maintain the centering during the construction of the whole bridge. It is suggested to dismantle the centering after the completion of all the works with the exception of the pavements and the parapets. The paving and the parapets will be mounted only after the dismantling of the centering to avoid their damage due to unexpected deflections, especially at the bridge springers.

 


8.4.2 Arch centering: geometrical rquisites


The shape of the centering needs to be carefully evaluated since it should match different requisites at the same time, and the following observations may be underlined:

  • the arch intrados is not close to a round arch but more to a basket-handle arch;
  • usually, regular shaped load bearing structures, are more desirable;
  • load bearing centering, if smaller than the bridge arch, would allow deck superstructure insertion;
  • the less centering obstructs the area over the river the better it is;
  • arch centering settlings should be carefully foreseen.

The arch false work, (centering), should have on its profiles some reference marks that may allow the positioning in foreseen spots that should be planned according to bridge shape as represented in GE final design. This way the adjusting of the final position of south and north centering will be managed trough a monitoring by instrumental survey, while centerings in between may be adjusted accordingly to the outer ones.

 


8.5 Wooden deck superstructure


As before mentioned, it is required, to re-perform the bridge irregular profile and maintain the former geometry, a sort of wooden deck to be superposed to the regular profile of the load bearing centering. This device should be designed to bear the loads of the arch voussoirs, and may be used also in the phase of the stone final cut, (refer to chapter 7 of this report), to allow an easier, more accurate and off-site cut. Advantages of the proposed devices are the following:

  • accurate performing of the geometry;
  • easier dimensioning and joint matching for stone final cut;
  • easier assembling on-site;
  • stone final cut off-site and quick assembling on-site;
  • regular shaped load bearing centering.

fig.02 intrados profile marking for wooden deck: ins: survey instrumentation pln: planar ground surface with reference grid ref: reference marks for intrados curvature wdn: wooden planks to be shaped following the intrados profile

Sketches of the wooden deck have been reported in chapter 7 of this report, and any similar configuration may work efficiently for the purpose. Basically the deck should be built by carpenters and co-ordinated with the use of survey instrumentation as here next explained:

  • The arch intrados curvature should be marked wholly in real scale over a planar surface with the aid of survey instrumentation and of a grid of reference points.
  • Co-ordinates are provided by design charts and drawings.
  • Once the arch intrados is marked point by point, wooden planks may be cut following the profile on the ground. Planks should be enough to cover the whole length of the curvature.
  • The above procedure should be repeated as many times as necessary to have a number of planks enough to make a beam, (as foreseen by design), and by shifting their joints.
  • The above procedure should be repeated for both elevations.
  • Wooden deck may be designed to be subdivided in sectors (presumably 8-12).
  • Other details proposed for the wooden deck may be found in chapter 7 of this report.

It is advisable to carefully evaluate the need of having two identical wooden decks, (twin decks), since during the stone final cut, the decks get easily ruined and it may be necessary to have a different deck to be superposed to the load bearing centering. If special devices are adopted, it may be possible to use only one deck, for saving economical resources, but on-site tests are required.

The wooden deck should have on its profiles some reference marks that may allow the positioning in foreseen spots that should be planned according to bridge shape as represented in GE final design. This way the adjusting of the final position of south and north deck will be managed trough a monitoring by instrumental survey. Special accuracy should be reserved to the joint among adjacent sectors of the wooden deck, in order that no discontinuity arises in the intrados curvature profile.


CREDITS:

Intellectual property of this report and of the design drawings is owned by General Engineering s.r.l.

author of the text: arch. Manfredo Romeo – other contributes have been mentioned in related paragraphs

yard consultant: Bruno Bonuccelli

- General Engineering Workgroup -

SOURCE:

Final Design Report

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