English

7.7 Stone to be used for the reconstruction of the Old Bridge

Stone to be used and cut is Tenelija of the same type, colour and resistance of the ancient one originally used for the bridge construction, quarried in the same quarry where the ancient stones where presumably originally taken. Detail specification to guarantee the above requirements will be given by LGA company (which is in charge of all the laboratory tests).

For what concern the lowest required resistance to compression stress and for the mechanical characteristic of the material it should be referred to the structural design report.

Other specifications concerning:

• stone type
• stone requirements
• criteria of acceptance depending on pathologies
• stone quarry and cut (rough and final cut)
• stone storage
• stone transportation

are included in the bidding document and have been determined by stone expert on behalf of the PCU and with the co-ordination of PCU TA and of General Engineering.

7.8 Bedding of all the bridge elements

Bedding of the stones is the natural stratification of the lime stone Tenelija. Bedding of all the elements should be carefully considered and have been strictly determined in design for the stone cut: the stratification plan should be always orthogonal to the compressive stresses acting in the structure (thrust line for the arch). This because it has been proved and observed (by stone expert) that the durability and efficiency of the construction material is, this way, maximised.

In order to respect the above mentioned requirement it is absolutely necessary to use with extreme care all the final drawings and charts provided for the stone cut in all the different stages. With the aim of giving clear and useful instructions on how to use the charts for the stone cut here next some additional schemes have been included. These schemes shows, for each element of the bridge, the final appearance after final stone cut (SC-final) and related parallelepiped block of the rough cut (SC-rough). In both cases orientation of the bedding has been clearly represented by parallel dotted lines.

At the same time in the schemes are inserted all the codes for the dimensions that have been used in the charts. This codes have to be carefully considered in order to cut the stones with bedding oriented in the proper direction.

If these warnings are not correctly and strictly followed, (during all the stone cut stage and the assembling phase), it will not be possible to respect the requirements, this because either stone dimensions or stone bedding will not match the design and will not be suitable for the reconstruction.

fig.9 – arch stone bedding – representation after final cut (left) and after rough cut (right)

The criteria adopted for the coding has been to consider the "base" of each element the side which is in the lowest position in the assembled configuration of the bridge (for the arch it is always the intrados side, even for voussoirs that are next to the arch springers). The height is always related to the base and lies on the same plan of the bridge front. Thickness is always the side which is oriented in the bridge thickness.

fig.10 – lower cornice stone bedding – representation after final cut (left) and after rough cut (right)

fig.11 – spandrel stone bedding – representation after final cut (left) and after rough cut (right)

fig.12 – upper cornice stone bedding – representation after final cut (left) and after rough cut (right)

fig.13 – parapet stone bedding – representation after final cut (left) and after rough cut (right)

7.9 Numbering and classification for stone cut

Stone cut design has been conceived in a way that it is mostly aimed at respecting the former geometry and shapes of the ancient bridge trough a management "stone by stone" which may be correctly performed only if numbering and classification is strictly followed.

Stone numbering is included in the design charts and may be repeated and followed easily during the on site works by applying metal tags on to the stone blocks either during the SC-rough, either during the SC-final. Tags may be removed only after the stone is well assembled in the foreseen position; tags should moreover contain information about the stone bedding.

Numbering, classification and nomenclature is detailed exposed in this report in a purposely written paragraph which is also about a global classification of all the bridge stones (including the ones of the former structure). The classification is quite versatile and allows different level of accuracy and simplification depending on the different use of it; for what concern the new quarried stones the classification may be easily resumed with the following:

fig.14 – new quarried stones classification system - compact code

Numbering of stones is here next explained:

For the voussoirs (SMS-A) two numbers have been used: the first is the row number which starts from the east springer level and ends in the west springer level; the second is the voussoir number which goes from north to south. Therefore voussoir SMS-A 88-3 (e.g.) will be located in the 88^th row from east and will be the 3^rd one starting from north elevation.

For the bridge elements (SMS-B) one number only has been used with different subdivisions, (of type and location), and together with design drawings (STONE CUT - SC category) is possible to locate any stone. Numbering of different groups is not progressive but starts again for each group. Therefore stone SMS-Bsp_ne 49 (e.g.) will be located in the 49^th position (following drawing) of the north-east spandrel wall.

7.10 Final stone cut - (SC-final)

Final stone cut of the voussoir (SMS-A) is undoubtedly one of the most delicate stage of the bridge reconstruction, and for this reason architectural design has required special care and different evaluations.

Final cut main objective is to cut all the stones blocks (SC-rough) following final shape and size foreseen by design and making sure that each one matches with the adjacent ones. The easiest and most practical way of performing the above task, with little use of measures, is to work the stones on an intrados wooden model of the arch.

Final stone cut is therefore supposed to be performed off site, on a wooden deck, that should have the intrados shaped as the load bearing centering (most likely a regular curvature) and the extrados equal to the former bridge intrados (following design geometrical specifications). This wooden deck may be subdivided in to different sectors and settled over the ground: by performing the final cut over the deck the intrados curvature will be respected with no difficulties and no use of measures. For details about centering and related wooden deck refer to next chapter of this report.

• Once the first sector of the wooden deck is ready and stable on the ground, the rough blocks (SC-rough), following the code numbers, may be moved next to it and subsequently placed on the proper position which is marked on the deck.
• Metal sheets should be prepared and numbered shaped with the front face of the stone voussoir as defined and dimensioned in the stone cut charts.
• First stones to be put on the wooden deck should be, for each row, the ones that are at the outer edges (north and south side).
• Edge voussoir will be settled by the use of wedges and its intrados face will be hand worked and cut coherently with the front face shape (metal sheet).
• With the use of ropes, of the metal front sheets and working over the deck, it will be possible to define the final shape of the whole row and to determine the longitudinal cut of the voussoirs.
• Edge voussoirs will be placed with their axe parallel to the direction of the row (marked on the deck) and, front faces, due to the quarrying tolerance, will be projecting slightly out of the deck outer profile.

fig.15 – arch stones (voussoirs) during final stone cut off site, placed on a sector of the wooden deck. In the sketch we have following notes: D: wooden deck structure - sup: sand support for the deck - sc: voussoirs that have already been cut and prepared - qt: edge portions of the block that should be cut due to quarrying tolerance - mf: metal front face reference sheet - m: metal protection for wooden edges of the deck following the arch intrados profile - ref: reference signs of connection joints among rows as planned in design specifications.

• Once that the two edge voussoirs are correctly positioned and cut, it will be quite simple to do the same job for the voussoir that are inside the row: these will be put, as well, on site by the use of wedges in order to face gradually and with small steps the differences of height among north and south elevation.
• No problem will arise for the stone cut of the extrados, being quite rough even in the former bridge, and moreover a sharp reference dimension will be again given by front face metal sheets.
• By the time that adjacent rows are completed small cut corrections may be performed in order that all the stone sides matches. Among rows thin wedges may be placed in order to simulate the mortar thick.
• Front faces will be hand cut following the direction and the elevation plan.

All the channels, the carvings, the slots for cramps and dowels foreseen by design, should be performed at this stage; with special care for their mutual position in order that they do not interfere with joints position of different rows. Range dimensions given in final design drawings should be verified and respected.

7.10.1 Warnings about final stone cut

Stone final cut measures are reference dimensions, (given for all the rows and all the voussoirs), that define a theoretical volume which does not consider the thickness of joints, (measures are referred to the joint center line), and which has been worked out by linking with straight lines the extreme faces of each row. This means that this volume with all its dimensions will have to be followed as a reference limit and all the geometrical variations between north and south elevation will be performed by small steps of adjacent voussoirs and not by straight ideal profile which would make the stone cut much more complex and the result quite different from the former bridge.

Even slightly differences (1-2 centimetres), that are reported in stone chart final cut, among opposite faces of a single voussoirs may be managed either with the joint thickness among rows, either by making each voussoir profile constant and with a small step with the adjacent arch stone (which can be performed trough the use of wooden wedges during the assembling procedures, and which was a peculiarity of the former bridge as well).

fig.16 – arch stones (voussoirs) during final stone cut off site, placed on a sector of the wooden deck (transversal section). In the sketch a very important warning is underlined: stones should not progressively change their dimensions in their length, but there will be small steps between two adjacent stones of different heights (see also enlarged detail on the right). In the sketch we have the following notes: s1-s4: arch adjacent voussoirs in current row - w1-w2: two adjacent wedges of different heights to be used to settle correctly the arch stones - nd-sd: north and south dimensions of the deck depending on levels of north and south intrados curves of the bridge arch - iP: inner stiffening wooden profile - eP: external wooden profile of the deck - m: metal protection for the edge of the wooden profile - d-c: connection joint among deck and steel centering - C: future position of the steel centering (regular profile) - dd: difference of height among two adjacent arch stone = step dimension.

• mortar thickness should be simulated by the use of thin wooden elements during the stone-cut, and dimensions of stone charts final design are referred to center line of joints (as if joints were zero);
• stone final cut should be performed by hand cut only;
• slots for dowels should be cleaned carefully after carvings;
• slots for dowels should have a slightly trapezoidal section (larger at the bottom) and should be performed with sharp tools and accuracy. (Detail of trapezoidal section may be not gathered from the final design drawings);
• slots for dowels will be of different sizes depending on the stone side (refer to design drawings);
• channels for lead pouring will be carved only on one side of the voussoir coherently to design drawings;
• All the channels, the carvings, the slots for cramps and dowels foreseen by design, should be performed with special care for their mutual position in order that they do not interfere with joints position of voussoirs of adjacent rows. Range dimensions given in final design drawings should be verified and respected. Position of dowels and cramps is given in design drawings;
• key stone voussoirs will not have channels nor dowels (refer to design drawings);
• each stone voussoir should maintain its tag (sign) with numbering and bedding direction;
• refer for more notes also to assembling specifications given in this report;
• design drawings should be carefully examined before any final cut begins: any doubt and question should be forwarded to General Engineering, any unforeseen problem should be evaluated and examined by works Supervisor and by General Engineering Workgroup.

7.10.2 Different approaches to final stone cut

The above described procedure for stone final cut (SC-final) is mainly aimed at avoiding any incoherence between design dimensions and their future use: in other words a different procedure for stone cut may be not suitable with design dimensions given in stone charts and may lead to unforeseen problems and to a bridge with a different geometry from the planned one.

At the same time, it should be pointed out that the wooden deck for the stone cut is here reported and represented as a sketch: it shouldn't become a constraint as it has been proposed, and a purpose design of it is required. During structural and architectural design of the deck, any variation to its shape and functionality is possible and probable but should be approved by work Supervisor and by General Engineering Workgroup.

Any completely different approach to stone cut is not desirable specially if it is not aimed at an accurate control of the geometry. General Engineering will not be responsible of any different procedure and of any variation that has not been agreed; nevertheless General Engineering, since work is quite peculiar and atypical, will be pleased to receive any additional suggestion and observation about the matter and will be available for any changing before works or while works are ongoing.

The philosophical approach to final stone cut has been worked out by General Engineering Workgroup knowing that it was not exactly the same way probably followed in the ancient times. By following the ancient time procedure we would have had as result a quite similar bridge but not equal in geometry to the bridge of Mostar. Respect of geometry is one of the main requisite and objective required for current design by the ICE and by the City of Mostar represented by the PCU.

Of course respect of geometry requires more expertise and accuracy by workers, and this expertise shouldn't be asked to stone workers but to carpenters that are used to make accurate wooden provisional structures (nowadays usually formworks for cast concrete in r.c. structures).

The wooden deck will allow therefore:

• to devolve all measuring work to carpenters
• to mount a regular steel centering with a circular profile
• to cut stones properly off-site with plenty of time and in a comfortable yard
• to follow geometrical design requirements easily
• to check all the joints and all the metal anchoring system before assembling stones

7.10.3 Final stone cut (SC-final) of the bridge elements - SMS-B

Stone final cut of all the bridge elements (SMS-B) is quite more simple than final cut of the load bearing arch stones (SMS-A) and doesn't need many comments and warnings.

Once all the stones have been prepared, cut (SC-rough), and numbered, the stone final cut may be started following drawings and stone cut charts.

Best moment and place, in which stone final cut may be performed, depends mostly on the decision of the works Supervisor and on the organisation of the yard. It is suggested in this report that, by the time the load bearing arch is under construction (assembling), trough a constant verify of all the measures on site, a final cut may be started next to the yard at least for the lower levels of the spandrels. Small tolerances may be maintained in order that, on the centering, during the assembling, there is still the possibility of managing any ordinary construction inaccuracy trough the joint thickness and trough a quick adjusting and finishing work (SF-adjusting).

main warnings

• each stone should be checked and measures should be verified on site;
• stone final cut are given in the final design stone charts for what concern the parallelepiped shape of each block, any additional cut that makes the block trapezoidal or differently shaped is documented in stone cut (SC) drawings; also for what concern all the other elements like cornices and parapets, drawings and charts should be used together;
• Assembling and final position of all the stones related to the numbering system is represented in stone cut (SC) drawings;
• Thickness of spandrel, cornice and parapet elements have been given by range values being this dimensions unknown and quite variable from the study of the bridge transversal section; detailed notes about the adopted criteria have been provided with description of stone final design charts;
• stones of the spandrels which are close to the load bearing arch (next to lower cornice) will have to be cut with care in order that the angled weak edges do not break;
• not all of the spandrel rows are horizontally assembled: in order to partially reduce angled weak edges, (refer to archaeological study of Mr Bessac), this has to be respected following design drawings;
• parapets are to be considered as sculptures: their final cut should be carefully evaluated and may be performed partially off-site and partially on-site, in order to globally manage the final effect of all the parapets with the direct control of the works Supervisor and with the approval of the ICE;
• the more we proceed to the top of the bridge, the more parapets are leaning slightly outwards: this has to be respected as well during the assembling phase;
• after final cut, any element which is not directly assembled, should have its numbering tag (sign) applied again coherently with its bedding clearly declared;
• refer for more details and specifications to the chapter concerning the assembling techniques in this report.

7.11 Stone Finisching and adjusting (SF-adjusting)

Stone finishing and adjusting is actually the final cut refining work which is aimed, during the assembling and immediately afterwards, at matching all the adjacent stones and at working the surfaces in the proper way.

One of the things which, in this report, is not intentionally defined and described is about all the specifications concerning how stone cut should be performed: this mainly for two reasons. The first one is that wider specifications for stone cut will be included by stone experts in bidding documents; the second is that it is not possible to define with the use of words the final result that is to be obtained, not even by the use of pictures or drawings. For the above reasons here next will be given guidelines and suggestions that should be followed in order not to have any final unforeseeable result of the whole stone structure; anything that is here next reported may be valid also for most of the aspects which concern the stone final cut.

Main objective of the SC-final and of the SF-adjusting is to gathered the same surface, texture and layout of the former bridge in its different portions and areas, trough a careful observation of all the remaining portions of the bridge and of any worked inner joint (which may still have the traces of the tools, used at the time, intact and not eroded).

Specifications by stone experts an by LGA should be followed in order that the right direction is taken and warnings and requirements are respected. But the above is not enough and, following guidelines are given:

• from the beginning of the works, next to the works Supervisor, and to the ICE, two professional expertise should be present while works are ongoing: one must be a stone and sculpture expert (SE) and another one should be an artistic consultant (architect) (AC), chosen with a purpose competition based on previous experiences on ancient façades restoration works;

• company that will win the competition for the bridge reconstruction will have to perform a sort of test by building a small portion of the bridge off-site in a 1:1 scale: stone final cut of the surfaces and finishing will be performed; the above test will be evaluated and analysed by the ICE and will need approval before the company could start works; if the test is not approved company won't be accepted for the works, as already agreed with PCU TA;
• The new professional figures should control works in order to check that finishing works and final stone cut is correctly performed for what concern the final aspect and the following:

• no machinery should be used for stone cut;
• the level of polishing of the surfaces should be monitored;
• the level of intensity of the stone work should be monitored;
• the level of density of stone work should be monitored;
• the distribution of the stone work should be monitored;

• The artistic consultant and the stone expert will also evaluate and judge the proceeding of works during other stages like:

• evaluation of joint thickness
• spandrel assembling
• adjusting of cornices and spandrels in crucial turning spots
• parapets stone cut and assembling (either bridge parapets, either nearby ones);
• parapets gradient quantification
• load bearing arch intrados finishing
• stone angles smoothing (specially on parapets)
• settling of organic admixture for light patina
• carvings over the pavement

Other specifications for the above mentioned works will be defined later in this report. Among what it has been mentioned two finishing works should be better underlined and defined:

• The load bearing arch intrados, due to the method of construction, trough the use of wooden wedges (as probably happened for the ancient one), will be characterised by steps and irregularities, some of which may result to much prominent and should be therefore slightly smoothed with a refine finishing works to be supervised by the artistic consultant and by the stone expert.
• Bridge flooring was probably originally built with no carvings on the stone tiles, and (as Mr Bessac has noted), these carvings may have been performed later on: like from 50 to 200 years later, when stone got slippery. In the rehabilitation works of the bridge, these carvings will be performed again as they were, without waiting for the flooring to get slippery: this because the objective of the current work is to rehabilitate the monument as it was right before destruction, (a declared scientific copy of the former bridge), and is not to live again the experience of building a similar bridge with same techniques and same construction materials.

General Engineering will be available at any time for definition and explanation of any single details which has not been fully defined in current paragraph, (due to the mentioned reasons), and which is concerning the stone final cut and the finishing works.

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