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3.4 Design technical report: introduction

Here next it has been reported about the foreseen design works giving a general description and some technical specifications, warnings and requirements. A strict connection with the bill of quantities (BoQ) and with the final design drawings (Drw), has been carried on for a better understanding of the different interventions and for a better explanation of the design contents. It is suggested to read the following paragraphs having as reference either the drawings either the bill of quantities.

As preliminary introduction, the site has been described with notes concerning the current situation of the involved architectural elements.

3.4.1 Current condition analysis: site description

The site where former bridge was located is characterised by a scarce accessibility trough ancient and narrow streets: (ulica Kujundziluk and ulica Oneskukova). The bridge remnants are reduced to small portions and only few voussoirs are still on site with some spandrel elements and some cornices. The bridge look as if it were cut at the arch reins (east) and next to the abutment (west): fill has been subjected to rain water infiltration which may have ruined inner portions. Masonry due to the lacking of the bridge thrust has probably released. Bridge ruins are in a precarious situation, (east side more than west) and cantilevering over the Neretva; for what concern walls of adjacent buildings, collapse is most likely to happen if remedial works are performed over the abutments, (see building located south-east and facing the river). (A co-ordination with the yard for the rehabilitation of the towers is required in order to strengthen building walls and adjacent structures before beginning of works).

In the abutment located north-east, an huge void has been found and surveyed; this void is so wide that partially interferes with the bridge thrust bearing masonry, and may be reached during the dismantling works next to the springers. That void may also be an impediment during the settling of flooring adjacent to the bridge paving (which is over mentioned room).

The Neretva river may be considered an impediment to the performing of the works, since the stream is vehement and the flow changes quite frequently and quickly during daytime. It is quite unlikely that any structure may be anchored to the river bed, also because this would easily lead to dangerous consequences in case of river floods.

All the area nearby the bridge is in a state of complete abandonment since the war period (for almost ten years): buildings, floorings, walls and finishing are ruining and many concrete plinths for provisional structures have been placed all over the area damaging the left portions of the bridge pavement which prosecutes, (few metres), outwards on the two opposite banks.

Some more plinths and tie cables have been placed by the north side, working as foundation structures of a gangway which actually is located eight metres north and parallel to the former bridge axe. All the above may constitute also a limit to the installation and settling of the bridge yard, and this is why no portion of that area has been proposed in the site organisation drawings, where settling is foreseen to be fixed by the south side.

Some small pavement areas are covered by rubble and ruined masonry, vegetation has grown but not in a remarkable way except for some special cases on the abutment walls where branches are coming out and damaging the masonry layout.

Stone elements of the bridge, which fell into the river water, due to the shootings, have been recovered after about five years of permanence on the river bed. These stones of former bridge have been stored on a platform located approximately 50-60 metres south-west to the bridge original location, their quantity is 36% of the load bearing arch and 16% of the bridge elements, their condition is quite variable. Traces of bevelling and of fractures are present either due to the shootings, either due to the destruction event and consequent falling from high level. Metal cramps and dowels conservation is again quite variable but, considering the period of permanence in the water, are not so much rusted. Two huge blocks of assembled voussoirs have been left by the shore being too heavy to be moved away for the currently available crane, and they still are subjected to the river waters and to high deterioration.

3.4.2 Current condition analysis: abutment materials and pathologies

Abutment walls are currently heavily damaged by shootings and they have structural and superficial cracks, some of which developed before war events. Most injured walls are located on the north east side, where shootings have caused wide masonry lacks, (one of which is currently used as access to the inner void); surface is highly deteriorated in many spots. Most remarkable structural crack is located on the north west abutment and wall is partially leaning outwards, while other walls, located on the south side, are apparently in better condition and with isolated damages.

Ordinary traces of deterioration due to the ageing are present all over the walls, as well as stains due to the rich vegetation growing. Now that most of the green has removed, (during investigation drilling works), traces of different ancient repair works are visible and recognisable because of masonry layout discontinuities.

Slight leaning outwards of the abutment walls may have been recently caused also by the scarce protection to rain water infiltration which can get inside the ground from the top and from the ruining portions of the bridge.

Abutments may be mainly divided in two different types: one lower portion which is characterised by being built in Tenelija ashlars, and an higher portion which is built in natural conglomerate masonry. Connection between the different types is marked by a Tenelija cornice which is the direct prosecution of the bridge cornice at the springer level. Tenelija ashlars are present also in the upper masonry portions as strengthening devices at the corners. Top edges of masonry abutment walls were covered by grey Tenelija sills which seem to be a recent protection. Of the above mentioned sills only small portions are left on-site. A small lower portion of the abutment walls is built with Marl stone (south west).

3.5 General: preliminary workstage

Some preliminary operations should be performed before any rehabilitation work could start, here next are commented and briefly described. These works have been divided in two categories:

• works that should have been already performed
• works that should be preliminary performed

The first group of works includes also interventions in which General Engineering is absolutely not involved, and for which no specification and advice will be given. In this report, only for completeness sake, procedures will be mentioned: no constraint and no impediments should be attributed to this simple list of work stages, the aim of which is only to point out that these works have been foreseen by PCU and PCU TA and that General Engineering has been assured of that.

3.5.1 Works that should have been already performed

Refer to chapter six of this report for more details about the work construction phases, even for what concern stages not included in current design work (only generally mentioned).

BoQ: XXXX – foundation strengthening

Of course one of the work, that should have been completed long before, (enough to have it matured), is the strengthening of the foundations and their protection from erosion. As far as it has been communicated to General Engineering by PCU TA, these works should be performed until the bridge arch impost (springer) level, and they are supposed to guarantee a foreseen maximum settling under load action, given by General Engineering structural design (as defined by Prof. Vignoli).

Also foundation for the load bearing centering should be fixed on site in strict co-ordination with the Company that will be in charge of the centering design. General Engineering is not anyhow involved in any foundation work and for that no design and no specifications are provided; nevertheless following notes are given as a reference (but should not considered as work constraint):

Works on foundations seem to be necessary to face the critical conditions of the ground underneath the abutments as it has been investigated by CONEX Company. For what it concerns the structural design requirements it may be said the following:

• Abutments shouldn't be loaded without making the necessary remedial works on the foundations, (and on the abutments as well).
• As proposed by PCU TA, the settling at the springer level shouldn’t be more than one centimetre.
• It is required that the abutments will be loaded about 6 months after the completion of the foundations strengthening, (to be confirmed by foundations design). This way, the possible settlings will happen gradually and will only be due to the loads and not to the maturing of the repair works.
• The analysis on the results coming from drillings will determine which is the best intervention for the stability and resistance of the foundations, anyhow it seems likely that it will be necessary to operate trough special concrete injection to fill some caves. In any case it is required the control and the limiting of the total amount of injected concrete and of any other additional load to avoid settlings of the deep layers of the ground that would be of future difficult management.
• From on site surveys and from CONEX Company investigations it seems that the erosion phenomena is intense, therefore the strengthening of the current situation may be not enough: the erosion phenomena should be limited to avoid any future problem to the foundations.
• The erosion phenomena, because of the wide excursion of water level of the river, should be better achieved not with concrete protections but with naturalist engineering systems.

BoQ: 2.0.3.0 – load bearing centering

Arch false work (centering), presumably steel made, should be fixed and transported in a safe place close to the work site. See chapter eight of this report for more notes about this matter.

General Engineering is not anyhow involved in any centering design.

BoQ: 2.0.2.0 – wooden deck

Wooden decks, either for stone final cut, either to be put over the load bearing centering should be completed. See chapter seven and eight of this report for more notes about this matter.

General Engineering is not anyhow involved in any wooden deck design.

BoQ: XXXX – stone rough cut

Stone rough cut (SC-rough) should be completed in order that stone blocks with quarrying tolerance are all available and transported in a safe place close to the site. Ultrasound check of their integrity should have been performed in order that non inner fracture is present. See chapter seven of this report for more notes about this matter.

BoQ: 3.2.0.0 – stone final cut of the voussoir

Arch voussoirs should be already cut, (final cut - SC-final), on the wooden deck in order that they are ready for a quick and accurate assembling. Ultrasound check of their integrity should have been performed in order that non inner fracture is present. See chapter seven of this report for more notes about this matter.

3.5.2 Works that should be preliminary performed

BoQ: 1.0.2.0 – work plan

A program of the works should be proposed by the Company that will be in charge of the rehabilitation works, in order to respect season times (see chapter six of this report).

BoQ: 1.0.2.0 – construction drawings

Drawings of the yard settlings and of any foreseen provisional scaffolding will have to be prepared, mostly referring to the ones which have a structural function or have to bear the load of machinery.

BoQ: 1.0.3.0 – topographic survey

A topographic survey should be held, (by 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 coherence of all the design, (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.

BoQ: 1.0.4.0 – removal of blocks from the platform

Blocks of the platform should be moved and stored to a temporary location that the City of Mostar will provide in the meanwhile that a museum is settled. Temporary location should be indoor or at least protected from rain water. Stones should be moved with care avoiding any further damage or cracking due to transportation.

BoQ: 1.0.5.0 – protection of assembled blocks on the river bank

Two huge blocks are currently located on the river shore since they couldn’t be moved over the platform due to their weight. General Engineering has suggested to save those assembled blocks from quick deterioration and to move them in a museum with all the other ancient bridge elements, (see starting paragraphs of this chapter); while ICE has decided to leave them on the shore as monuments of war events. If the ICE decision will not change, those assembled blocks will have to be protected to avoid any damages due to the yard ongoing works.

BoQ: 1.0.6.0 – monitoring

Monitoring of micro-shifts of the abutment walls is required to allow structural controls of the monumental complex before, during and after rehabilitation works. Monitoring will allow also checks during time with season changing. Monitoring sensors are required also during the bridge construction to be inserted in the inner structural sections with remote control. A design of monitoring is required with location and type of sensors for the abutment walls, for any structural crack and for the bridge inner portions. In the bridge inner portions sensors to monitor water infiltration should be provided as well. After sensors have been installed it is required a periodical and frequent check of the output results with related structural interpretations given on written reports. When works will be over, a control cabin should be fixed in one of the adjacent buildings so that it will be possible to locate the instruments for saving the outputs coming by remote sensors.

In detail, as guideline, following control devices may be installed to have a complete monitoring of the structure:

• rock meters (extension-meter bars) to check any displacement due to the bridge thrusts;
• inclinometers to check leaning outwards of the abutment walls;
• optical instruments to check any displacement of key level, arch reins and arch springers;
• thermal sensors to check inner structural variation;
• dynamic control sensors: one on the bridge at the key stone level, and another one on a stable reference spot.

BoQ: 1.0.7.0 – quality compliance control

Before the beginning of the works a test for the Company in charge of the rehabilitation works is required: some works should be performed in 1:1 scale with final finishing and layout on some samples or models. Required tests are the following:

• assembling of a small portion of the load bearing arch with front face finishing work;
• assembling of a cornice with parapet with related finishing work;
• assembling of a spandrel small portion with related finishing work;
• injection of masonry;
• repair of masonry and ashlar walls;

Results of the above tests should be approved by the ICE and by UNESCO, and after approval they will be taken as model for the on-site works. If tests are not approved, the Company should be rejected from the performing of the works.

Once the parapet and related cornice have been correctly assembled finished and approved, an additional test is required to have some data for the structural design which could not be gathered from scaled laboratory tests. The parapets should be loaded with an horizontal thrust at an height of about cm 120, (see structural design requirements), until failure of the 1:1 sample is reached. Failure thrust has to be determined and communicated to General Engineering in order to work out if any additional strengthening devices is required to match current standards of safety for the people.

BoQ: 1.0.8.0 – public information

PCU and PCU TA will give specifications and requirements for this peculiar item. General Engineering suggests about this matter the following:

• to inform public long before works begins in order to receive and use any observation;
• to be clear about design objectives and contents.

BoQ: 2.0.5.0 – temporary stabilisation of the remaining part of the arch

In order to avoid any collapse and in order to dismantle the foreseen portions of the remaining parts of the arch, it is necessary to design and fix provisional structures that may ensure the stability of the cantilevering stones and may allow their dismantling in complete safety.

3.5.3 Site organisation

Site organisation drawing and report notes are mainly a guideline for a possible settling of the yard: it should not be taken as a constraint, and any different settling of the yard, that could optimise the performing of the works, may be proposed and performed by the Company that is in charge for the rehabilitation works of the bridge. Anything that concern the site organisation has been defined by PCU TA trough sketches that have been drawn in final layout by General Engineering.

Anyhow, limit of the yard should be protected by fencing in order that access is reserved to workers and supervisors only; moreover an access to the site by visitors, (e.g. local authorities and press), should be guaranteed trough separated and protected paths, (as it has been required by PCU TA). Gates should be provided as well, to allow a secure closing of the yard during the non working time.

Main access to the site for workers, construction materials and for working machinery, may be the one located south west by the main city road; this because works should not interfere with another adjacent yard that will be ongoing for the rehabilitation of the towers and annexed buildings. Only during final stages, for the bridge rehabilitation works, it will be possible to get trough the narrow streets leading to the bridge (Onescukova and Kujundziluk streets), as required by PCU TA.

Position of the crane, and related working range, may be considered only as a proposal, and should be evaluated mostly by the Company in charge of the works depending also on available facilities and on other technical evaluations.

General Engineering about this matter underlines the fact that a crane of 60 metres operative range may be not suitable for all the foreseen works, also because the assembling phase of the voussoirs requires a more accurate lifting system that do not have wide swinging. Concerning this matter, in design drawing (GE-03), also another proposal has been showed in a sketch: a travelling crane over lattice girders may run all over the area of the bridge length, and a direct access to its base may be fixed (similar approaches have been followed also in other bridge yards).

The yard might have its headquarters and facilities on the west side of the main town road, while all the other yard operative sectors may be located towards the river shore on platforms high enough to avoid river waters.

Nevertheless all the above mentioned hypothetical proposals should be technically verified for what concern their feasibility and their efficiency, evaluating also that no risk is taken due to the wild regime of river waters. General Engineering is absolutely not responsible of any configuration of the settling of the yard, of any provisional work and of any consequences due to river floods over the site.

Another matter that should be carefully evaluated, by the Company that is in charge of the rehabilitation works, is that north to the bridge location there is a footbridge that may be of impediment or constraint for the rehabilitation works either of the bridge, either of the abutment walls: plinths and tie cables have been placed by the north side, working as foundation structures of the gangway which actually is located about 8 metres north and parallel to the former bridge axe. For the above mentioned reason no portion of that area has been proposed in the site organisation drawings, where settling is foreseen to be only by the south side.

3.5.4 Site limits

With the aim of giving a clear limit that includes all the areas that should be rehabilitated, and for bidding purposes, PCU TA has asked General Engineering to work out a plan in which the limits of the interventions would have been marked. In drawing GE-04 it has been represented the limit of the works with a bold line. This line includes the areas that should be rehabilitated, and all the abutment walls that should be repaired. Small areas next to doorsteps of the entrances of other buildings should be planned and co-ordinated with the design for the Towers (and adjacent buildings) since in some spots stairs and steps may be restored. Limit line has not been drawn for what concern the elevations since they are fully included in rehabilitation works with no limit: from the rocks until the top.

It is included in the works (even if not included by limit line):

• water drainage, waterproofing and piping fixing as described in drawing AD-01
• stabilisation of adjacent bordering walls and structures to avoid collapse

It is included in the works (even if not included by limit line and to be performed after completion):

• existing gangway, (footbridge), should be dismantled with related reinforced concrete foundation plinths and tie rods;

• existing platform for ancient stones storing should be dismantled with related reinforced concrete foundations plinths;
• all the provisional structures, scaffoldings, centering plinths and any other element used for the yard purpose should be removed;

• working area should be cleaned and left in a decent appearance with no trace of the yard and of the stored construction materials.

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