English

One of the most important conceptual risk that we run by reconstructing the bridge, is to confuse the ancient original elements of the abutments and the first stones at the springer level, (if they will remain on site despite all the limits explained in the previous paragraphs), with the new stones of the arch. This should not happen for a conceptual coherence, for respect to the ancient left portions, and because history may be not changed: with war, in fact, something has been lost forever and may be not completely restored. The memory of what has happened should not be cancelled and should leave at least a trace also on the Bridge.

It is suggested therefore to examine the aesthetic matter of the final appearance of the bridge faces which is at the same time a matter of restoration principles.

Even if the new stones will be quarried and worked as the original ones, and even if special attention will be given to the bump and texture of the surface, the final result should be planned to be as a compromise between the complete similarity to the ancient stones, and the requirement for declaration of the new interventions. This way, the bridge at a first glance, may appear all alike to the former one, but to a close view should be possible to gather the differences between the recent portions and the original ones.

This in order to satisfy the demand of documenting, not only with reports, but on the monuments itself, what has been done and what has been added to the bridge. The same purpose may also be achieved with a system of marking the new stones with special small carved tags.

Another device that may be worked out in the aim of declaring the difference among the old structure and the new intervention is to emphasise the connection joints in that spot by making them thicker and with a clearly visible different texture. This would give its best result in the case that it would be possible to leave some stones at the springer level (see §4.4), and may be performed all around the bridge: on the elevations, on the paving, and on the vault intrados.

Final design of the analysed matter has been detailed represented in drawing RE-10, and has been summarised as follow: "In order to declare which are the new interventions a plan has been worked out. The plan may be subdivided in two different sections: declaration of the joint in the pavement trough the use of metal marks and marking of some stones (trough small and light carvings) in the elevations". For notes about final intervention specifications refer to chapter 3 of this report.

This marking of the joints and of the stones is required for two different reasons: one is mainly scientific, and related to restoration requirements for which it is important to identify in future times each different intervention on the monument; the other reason is instead historical, and is aimed at preserving the trace and the course of time and events.

In the Inception Report about this issue it was explained the following:

"One of the most important value to be preserved is the refined construction technique of the former bridge, and the way each stone was linked with cramps and dowels to the adjacent ones. This will be taken into great consideration and should be planned and repeated in the new arch stones, to make the structure work in the same way it has been conceived.

If any strengthening remedial work has to be foreseen, it will be analysed whether it is possible to proceed to a not too invasive technique, so that to preserve the structure as much similar to the previous one also in the interior portions. The use of current construction material will be limited to the cases of static needs required by the calculations results. Anyhow for what concern the matter of using any recent strengthening technique or special reinforcing stirrups or ties we may refer to the principles listed in the methodology description of the current report:

• for the strengthening aim of the structure modern techniques may be used only for static reasons and only when the ancient constructive method would not be adequate.
• interventions should be as less invasive as possible.
• interventions should be as much reversible as possible."

All the above is here, in this report, confirmed, but what it has to be stressed, at this final stage of the design work, is that the mentioned issue is one of the most important of the whole project for the following reasons:

• materials and strengthening works are strictly linked to conceptual design objectives and may not be worked out for practical convenience;
• construction material issue is one of the most urgent matter to be defined, (at the moment not yet finalised for reasons not depending on General Engineering responsibility), because it involves and changes either structural design either technical and architectural design;
• mechanical characteristic of materials should be verified and inquired by special investigations of the laboratory test company (LGA).

For the above reasons it should be noted that this matter is really delaying and stopping the foreseen time schedule of General Engineering work which will remain uncompleted and not finalised even for what concern some architectural issues and specifications.

In the aim of giving an useful contribute, this issue will be here better analysed, so that to define all the requirements and requisite for a coherent development of the design.

Following the methodological approach proposed in the first paragraphs of this report, the portions of the Bridge that will be new should be conceived as a "documented" reconstruction of the former structure. This means that the bridge components should be assembled in the same way as they were previously, including even all the interior portions of the Bridge. Following the same methodology and the same theoretical approach, construction materials may be exclusively traditional and local ones: this means that no different material and no other location different from the ancient used ones should be accepted.

If these principles are neglected we will be building something that will not have any value, not even the documentation value that we would like to guarantee. Even a partial use of different construction materials would easily bring to a different esthetical result that may, time passing by, get even much more different from the former Bridge. Moreover the use in some cases of new or different materials will lead to huge incoherences in the methodological approach: for this reason it is not difficult to understand that the research and the use of an historical type of mortar would be in hard contrast with the use of lime stones coming from abroad, (as it has been hypothesised).

For the same reasons here explained, in this restoration work it is strongly recommended not to use concrete injections except in the cases of demonstrated structural need. Injections should be carefully planned since they are irreversible interventions on the masonry, and repair works should be respectful of the ancient ruined portions of the abutments and of the bridge cantilevering portions.

All the above, concerning materials and strengthening, may be exceptionally neglected only for specific cases related to structural requirements and for the protection of the ancient portions of the Monumental Complex. Specific matters are here next analysed:

Drainage and waterproofing:

It seems from on site observations that the original and ancient drainage system of the Bridge was not working properly, (even if this should be confirmed by investigations on the abutments by CONEX), this because on some assembled blocks, on the extrados side, there are traces of water flowing. Since the drainage is not only a protection for the "new Old Bridge", but also a protection for the remaining ancient portions and for the abutments, it may be acceptable to use a new technology or material in the aim of preserving the historical stones from deterioration. This waterproofing anyhow should be conceived as a layer as much independent as possible from the other elements as an additional protection. Final architectural design has anyhow foreseen the use of the same type of waterproofing used in the former bridge, performing quality and integrity checks and depending on construction material choosing from LGA laboratory tests (see chapter 3 of this report for more notes).

Metal connections (cramps and dowels) and lead:

About metal stuff like cramps and dowels, in the Inception Report was hypothesised the substitution of them with stainless-steel ones, for structural requirements and being worried for future deterioration of iron that gets rusted. But recently, after the study on other similar restoration cases, detailed notes have been found on a bridge reconstruction work in which dowels were used as well as in the Bridge of Mostar. In that occasion iron dowels have been used also in the rehabilitated construction because it was noted that a good hand forging of the iron is already a protection from rusting, and moreover, if lead is poured correctly and winds the dowels, then we have an additional protection from deterioration.

So after more accurate verifies of the above matter trough laboratory tests, considering that drainage may be performed as specified in the previous paragraph, and considering that ancient metal dowels and cramps, even after being six years in the water, do not look so much ruined, we may evaluate the possibility of assembling the "new Old Bridge" with the same type of metal stuff and of lead (which should be investigated trough laboratory tests). Final decisions on this matters depend on laboratory tests which are still ongoing.

In the interior portions of the bridge there are some structural anomalies that may have been partially caused by ordinary construction imperfections, and others that are due to the construction changes in time. It has to been analysed whether to maintain or not such structural peculiarities.

Transversal section observation

From literature it results that the upstream interior lightening void in the bridge structure was larger of about 20 centimetres than the one located downstream. As far as we know trough the analysis of ancient drawings, and trough the direct observation of the bridge remnants, this difference may be partially surveyed and confirmed only on the west bank where the upstream void is 15 centimetres larger than the downstream one. The same may be not said for what concern the east bank where the voids are almost alike.

Evaluating the following matters:

• This structural peculiarity may be not considered as a special device to face the thrust of the river flow during floods. It is not therefore a structural advantage. (As confirmed by Prof. Vignoli).
• This structural peculiarity is not well documented, and we do not know how and where it was located trough a detailed map.

For the above reasons we deem, (if no other different data come to our knowledge), that this structural anomaly should not be repeated in the "new Old Bridge" and the lightening voids may be almost of identical sections, apart from ordinary constructive imperfections.

Longitudinal section observation

Moreover there is also another structural anomaly that may be found in the longitudinal section representation of the Bridge: it always concerns the lightening voids, which by the west bank, get shorter (their height decrease). This is most probably due to a construction changing in time of the Bridge itself (see §2.4.3): it seems, in fact, most likely that the pedestrian level, by that side only, was heightened in a subsequent moment and it seems that the bridge was then first conceived asymmetrically with the voids of different sizes.

From publications it results that there were not two lightening voids but three even if it wasn’t specified the exact location of the third one. But it is most likely that this third void (they say it was horizontal) was created during the heightened of the pedestrian paving level by the west bank, because of structural reasons: in fact it wouldn’t have been wise to load the bridge arch asymmetrically with an heavy fill. But as far as we know, the inner longitudinal section of the bridge is not documented, and until an ancient survey with drawings will be available, anything that is designed is arbitrary.

Therefore we have two possibilities that are here next explained:

• The first one is to rebuild trough an hypothesis the three voids configuration
• The second is to rebuild the voids identically to the east bank configuration

Both of these options are arbitrary and are not proved by any document. But in the case that nothing will be added to the current knowledge of the bridge structure, it is most desirable to follow a structural solution which is simpler to be performed. Therefore the second option of a symmetrical structural configuration is to be preferred being data so scarcely on the matter.

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