English

The issue concerning the adaptation between ancient and new arch stones, explained in next sub-paragraphs, has been developed following the indications coming from the structural design team and therefore in co-ordination with Prof. Eng. Andrea Vignoli and Eng. Maurizio Orlando – Department of Civil Engineer – University of Florence.

Even if the bridge has been almost totally destroyed, some portions of it, next to the springers, are still on site: in the west side these remnants are quite small and not so much ruined, in the east side there are wider portions but with more war damages (especially in the north elevation).

Being these ruined portions part of the original ancient bridge of Mostar, they are very precious and they still represent with the abutments and the recovered stones the historical ruins of the monument. Any intervention on these elements should be carefully analysed to avoid any other damage to the structure.

Willing to build a "new old bridge", the most desirable option would be of keeping these portions intact on site and with no changing and no dismantling, starting therefore the new structure from the ancient stones. But for technical and structural reasons this doesn’t seem to be completely possible and a compromise has been necessary.

What it has to be stressed is that, even in the hypothesis that we would not build any bridge on the spot, (following the most conservative position of preserving the current condition of the abutments), an intervention of partial dismantling and of repair would be necessary because right now the east portion cantilevers in an unstable condition, and both sides are not protected by water infiltration.

The construction of the "new old bridge" in the spot raises delicate issues concerning these remaining portions because restoration requirements do not match the demand for structural security. The analysed technical impediments and limits are the following:

• ruined and cracked masonry should not bear the loads of a new structure and may not be left on site;
• unforeseeable behaviour of the two structures (old portion and new arch) that may crack at the connection point and that may be not easily modelled in a finite element investigation;
• unknown condition of the ruined portions: stone resistance, internal fractures and metal stuff (like cramps and dowels) conservation.

For a better understanding of the issue, we may first refer to the classification of §4.3.5 that is here next repeated for what concern the category under evaluation. The classification allows a better definition of the different interventions that should be performed on the structure and that are worked out depending on the subcategory of each stone or bridge element.

The stones are mainly split in load bearing arch stones (that have special strength requirements), other bridge worked-stones, fill and flooring. These have been then subdivided in two parts evaluating only if the stones are clearly broken or not: this of course doesn’t mean that the stones that look intact are not cracked in the interior portions; for the above reason this has to be considered a preliminary step towards detailed laboratory tests. The classification is here next reported:

• BS: remaining portions of the bridge, still on site, next to the abutments (built stones)
• BS-A: load bearing arch voussoirs
• BS-A1: intact stones
• BS-A2: broken stones
• BS-B: other bridge elements
• BS-B1: intact stones
• BS-B2: broken stones
• BS-C: bridge fill and stiffening rib
• BS-D: pedestrian flooring

fig.02 – Built Stone (BS) classification - schematic drawing

As it has just mentioned, it is not possible to start the construction of a "new Old Bridge" directly from the former bridge remnants for structural and technical reasons. The scarce quality of the ruined portions and the damaged connections wouldn’t guarantee the necessary safety and security to the new structure. In other words it is strongly recommended not to load the current cantilevering portions of the bridge, (as they are now), in order to avoid dangerous cracks, fractures, and even a collapse. Masonry in fact, after the destruction of the arch, has released and may not work again has before without performing any intervention; moreover no repair seems to be possible on the load bearing arch leaving on site all the elements that are insisting over it.

What it seems to a first analysis, (that has to be confirmed anyhow by on-site tests while works are ongoing), is that a part of these elements will be dismantled to allow all the necessary repair works, but the conclusions on this subject may not be worked out at this final design stage because many investigation results are needed and it may be also necessary to begin the works to understand exactly the amount and the limits of this peculiar step.

The above issue is one of the most important in this project, involving at the same time either structural matters, either conceptual and restoration requirements, and for this reason a detailed analysis of all the different stones has been carried out to perform the best intervention on each case.

classification code: BS-A2 and BS_B2

For what concern all the elements that are not anymore intact, that are ruined or cracked due to the shootings, and that are in an unstable position (not anymore linked to the masonry structure or cantilevering), it seems most likely that they should be removed. The reuse of these elements is for structural reason improbable. The final destination of these fractured portions may be the settling for exposition in a purpose built museum.

classification code: BS-B1 and BS-D

Bridge elements that are still intact and that are not supposed to bear the loads of the arch thrust, (like spandrel portions, cornices, parapets and flooring), being still on site in the original location may be first carefully surveyed and numbered. As it has been underlined, the position of these elements (which are over the load bearing arch) do not allow any intervention on the underneath portions, and for this reason we are compelled to dismantle them with an accurate classification in order to perform a reassembling work during the construction phases. Reassembling should be performed trough the anastilosis technique: following the original positioning of each element after a quality control of the level of conservation; some of them may be restored with accuracy in the case that only the surface is ruined (restoring methodology is explained in chapter 3 of this report). Most of the examined stones of this type are located next to the east abutment, and the ones that are in a better condition are on the south elevation, while on the north elevation it seems that many of them have been harshly stricken. Before the proposed intervention may take place it should be analysed how and if it is possible to easily dismantle this type of stones with no damages, and it has to be investigated a technique to take off any eventual metal cramps without cutting or bending them.

classification code: BS-C

Bridge fill, being one of the element which is over the arch load bearing vault, has to be removed to allow the repair intervention on the lower structural portions. The fill of the bridge is not as precious as the other stone bridge elements, since it is not an hand-worked portion; nevertheless it is suggested to perform inquiries on it, to know its composition and the percentage of the single components and the average weight. The reuse of the fill may be not performed being its quantity very low, its composition heterogeneous and limited to small areas next to the springers (the bridge section is, in fact, purposely characterised by wide voids). For what concern the stiffening ribs, (masonry structures that are over the axis of the vault and that determine the lightening voids), it can be repeated the same of the above, even if, this stiffening rib needs a detail observation because it has to be inquired why a structural element like that has such an incoherent composition. What it can be anticipated, (from calculations - by Prof. Eng. Vignoli), is that this stiffening rib will have, in the rehabilitated bridge, an important structural function and therefore it will be composed of good quality and well assembled masonry.

classification code: BS-A1

The intervention on load bearing voussoirs that look intact may be not easily determined and this leads to many unsolved matters concerning the issue of the adaptation although here two different approaches will be proposed.

BS-A1: proposal n°1: The first proposal is to leave these stones on site and to start the new arch directly over them, in detail we should point out what follows:

• Not all the stones of the group would be left on-site, since if any intact stone is located next or over a ruined one, it will not be possible to take off the damaged one leaving on site the good one.
• The springer level arch stones are load bearing stones and they should be checked with special investigations and diagnostic techniques to guarantee their structural resistance and their integrity.
• Metal stuff like cramps and dowels should be verified making sure they are not ruined or rusted.
• It should be verified that leaving these stones on site there will not be any impediment to the abutments repairs that CONEX company has designed.
• All the necessary data for the F.E. structural model of this group of stones have to be investigated.
• This group of stones even if in the final structural configuration are not very much loaded, they are loaded during the bridge construction (as represented in the following picture) when the arch bridge has been erected and when no other load is over.

fig.03 – arch structure with its thrust line compared to the bridge structure

The main advantages of this proposal is that, leaving these stones on site, it will be possible to limit as much as possible the interventions on the remnants of the Old Bridge respecting the ancient structure; moreover keeping on site that stones would lead to limit the excavation works at the springer level and of all the ground that is over (see next paragraph). On the other side this proposal leads to more delicate evaluations about the structural matter that should be carefully considered to avoid any future risk or damage.

For a better understanding of this proposal it has been worked out a sketch (see following figure) in which are drawn directly on to pictures the preliminary limit lines of the intervention: the stones below the lines that are marked as "I" may be the ones that are most likely to be left on site if all the mentioned conditions are verified; while stones which are located between the limit lines marked as "I" and "II" are the ones that even if they look almost sane may be not left because are interposed among other damaged ones and because they are located in a position that also in the final bridge configuration is heavily loaded.

fig.04 – First proposal for the BS-A1 stones: limit lines of intervention

As it is clearly visible, in the picture of the east abutment - north side, all the stones are very much ruined on their surface and harshly stricken by shootings, even next to the springer. For this reason the limit line marked as "I" may have to be lowered in that area until the arch springer. In final design drawings another integrity limit line has been hypothesised based on the results of the ultrasound checks performed by CONEX company (see drawings DS-02 and DS-03).

BS-A1: proposal n°2: The second proposal is to be adopted only in the case that the first one may be not performed because of technical impediments and of structural risk. If these stones have not the resistance requirements to be left on side, they should be dismantled and not replaced. Their final destination may be with the recovered stones: the museum of the Old Bridge. In this case all the arch stones should be removed until the springer level is reached.

This proposal requires to be carefully evaluated because it represents the only step of the design in which an irreversible work on ancient stones is carried out, (it is the only one if other design steps are correctly performed). Partial dismantling works on these remnants should be performed anyhow (even in the case of a simple preservation "as it is") but in this case and in this proposal, because of the "new old bridge", a wider dismantling work is foreseen with an undoubtedly damage for the ancient stones and for the ruins of the monument. For the above reason this proposal should be adopted if it is absolutely necessary for structural demand.

To start the reconstruction works, it is required either to dismantle portions of the ancient bridge (as specified in the previous paragraphs), either to make excavations of the abutments and to fix the ground with provisional works. This is needed to allow the performing of all the strengthening operations without any risk of collapsing of portions of the abutment.

These works may be not finally defined even at this stage because they mostly depend on the type of ground or masonry which is right behind the load bearing arch: in the abutment. And they also depend on the amount of stones that have to be dismantled: the more voussoirs (classification code: BS-A1) have to be removed the more excavations have to be performed.

But two hypothesis have been here next made: one follows proposal n°1 of the previous paragraph (in which most of the voussoirs were left on place) and the second follows proposal n°2 (in which all the stones were removed until the springer level). Therefore it is always a matter of proposal 1 and 2.

In the next figure are clearly represented both the excavations needed for the different proposals: in the first case the ground of the abutment is not so height as in the second, and for this reason may be easier to be performed although in both cases it is required to do excavation following a step by step procedure and going slowly from top to the bottom by fixing the ground with scaffoldings and sheet piles that could bear the load of the ground of the abutment.

fig.05 – The excavation works foreseen for the two proposals

In the figure are also represented with arrows and letters the thrusts of ground and of masonry structures that may collapse due these excavation works. Of course special load bearing scaffoldings should be erected to avoid any structural problem during the works (General Engineering, anyhow, has no responsibility for provisional works and followed methods to gain the designed objectives).

The arch is supposed to be borne and protected by the centering structure, while thrust C should be faced with sheet piles and scaffoldings by the time the excavations reach lower levels; thrust A and B are limited to the edges of the masonry but have to be controlled by the use of provisional protections to avoid wide damages and cracks along the abutments walls.

In the next figure is again represented the amount of the excavation works in the different hypothesis, but this time with the use of elevations. For a better understanding of the adaptation works, have been added the limit lines of interventions (which locate the stones that may left following proposal one).

As it can be easily gathered from these sketches, the works next to the abutments are the most important ones, (apart from foundations), for the future stability of the arch, and at the same time they require care and attention on the architectural side because these operations should be held inside the Old Bridge and its abutments and should respect as much as possible the ruins and their historical value.

fig.06 – The excavation works foreseen for the two proposals and limit line of the intervention

Before any final intervention may be planned it should be underlined the following:

• In any case it must be given priority to the strengthening of the inner portions of the abutments mostly next to the springers;
• In any case it can not be determined, at this stage, which are all the provisional works that may be necessary (for which General Engineering is not responsible);
• In any case some data are necessary to work out conclusions related either to what’s behind the springers either to the condition of the stones that we want to maintain.

What it can be underlined is that too many things are by now not properly defined because of the lack of data, and it may be not possible to know everything proceeding by investigations: some of the design steps will have to be worked out during the reconstruction works, although different hypothesis have been prepared for any event.

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