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6.1 Construction phases main warnings

Construction phases drawings are a sequence of schemes which are aimed at focusing important details concerning the proceeding of the works. Being the drawings represented as schemes they can not be taken as detailed drawings but as sketches of the most relevant aspect that is underlined.

It is extremely important therefore to point out the following:

• Construction phases drawings are only sketches and not detailed design (as foreseen in ToR).

• Configuration and shape of the centering, of scaffoldings and of any provisional structure is purely indicative and symbolic: it shouldn’t be taken as a constraint for the performing of the works. Company that is in charge of the rehabilitation works is responsible for the provisional structures.

• The proposed construction phases drawings do not include any reference to the season time, to the works maturing time and to periods of time that have absolutely to be respected.

• Specifications about methodology for dismantling strengthening and calculations of the structure during the different construction phases is included in structural report.

• For the above mentioned reason, and for a good readability of the schemes, most of the scaffoldings have not been represented.

Flow chart of global construction phases should be considered as a proposal for work organisation which will be co-ordinated by the PCU-TA and responsibility of it is not up to General Engineering.

6.2 Construction phases technical warnings

Most of the difficulties related to the current work may be found in the methods and in the procedure to be followed, therefore construction phases and assembling techniques should be carefully evaluated. Nowadays not many companies in the World have the expertise in stone bridge constructions and moreover this peculiar work has got requisites that are somehow unique.

Technical warnings are given in the following paragraphs.

6.2.1 Dismantling works

• Dismantling works of the abutment, even if over-quantified, should be performed following the smallest configuration as possible. This should be evaluated on site depending on the stability of the abutment and on the safety of the workers. Works supervisor will be responsible of the above choice. In the drawings dashed lines represent smaller dismantling configurations.
• Dismantling works of the abutment should be performed after remedial structural works of the abutments are over and are matured.
• Dismantling works of the abutment should be performed while strengthening the adjacent fill and while doing provisional works to avoid collapse of the abutment walls.

6.2.2 Season times 

• The centering should be built when the good season begins and should be dismantled when it ends; (presumably from May until half of August).
• The best season for the works should be evaluated by the work Supervisor trough the use of rain charts and trough a monitoring of the river level of all the past years available.
• Special agreement and emergency plans should be organised with the upstream dams in order that it would be possible to face any unforeseen river flood.
• If the times can not be respected, the construction of the bridge is absolutely to be postponed of one year.
• It has clearly to be underlined that, in case of river flood, if the arch false work. (centering), is still on-site, the collapse of the structure and of the load bearing arch is almost sure.
• Responsibility of timechart is up to the work Supervisor and to the Company that will be in charge of the reconstruction works.

6.2.3 Construction materials maturing times

During the construction works, special care should be taken about the different times of all the works, in order that no structure is loaded before the proper maturing time. To gather the right prescriptions for this peculiar aspect it has to be referred either to structural design final report, either to LGA’s report concerning the characteristics of all the types of mortars.

Also the right time of maturing of the works for the foundations has to be carefully known trough the specifications, warnings and recommendations given for that group of strengthening works in order that no unforeseen settlements take place. A schematic reference list may be here next reported, and reconstruction works should be planned to suit all the time requirements:

• maturing time of the foundations strengthening works (6 months according to PCU TA);
• maturing time of the abutment walls strengthening works;
• maturing time of the bridge load bearing arch;
• dismantling of the load bearing centering after right time, (to be carefully evaluated in order that no cracks take place in the load bearing arch).

proper time for loading of the arch with the bridge elements, (to be evaluated depending also on calculations of the structure during the working phases).

6.2.4 Load bearing arch assembling times

The load bearing arch should be assembled following a strict timetable in order that no delays take place and the execution of the works doesn’t exceed the foreseen season time. Assembling times of the voussoirs should be preliminary evaluated, with on site tests, by the Company that is in charge of the reconstruction works with the approval of the Supervisor and with a verify of the correct organisation of the yard. The yard organisation should be improved and optimised whether times are not matching the requirements.

Anyhow, during the assembling phase of the arch, until the centering has been dismantled, works may be performed also in the night-hours with 2 or 3 group of workers, (of course the yard in this case needs more facilities for the mentioned purpose). The full-time performing of the works is a procedure that has been followed also during other stone bridge construction works and shouldn’t be considered as an anomalous proposal.

Being the times so much important, while the centering is on site, it is desirable that all the construction materials for the works were already prepared off-site, ready for final assembling, including: stones, mortar, cramps, dowels and lead with a secure quantity margin to avoid delays in supplying of additional construction materials.

6.2.5 Symmetrical proceeding of the construction works

The construction of the load bearing arch, as well as all the other portions of the bridge, should be mounted at the same time, proceeding from both sides of the arch springers and from lower to upper portions.

If the works are not symmetrically performed, from both the springers (east and west), the load bearing centering may be subjected to unforeseen movements that may lead to very dangerous loading configurations, to a different geometrical shape of the arch and to collapse of the structure.

Same symmetrical approach should be followed for what concern the transversal section loading of the bridge: spandrels and other bridge elements should be mounted at the same time on both south and north elevations in order that no torsion stress is caused.

CREDITS:

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

© - General Engineering Workgroup -

SOURCE:

Final Design Report