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|STONE CUT - REQUIREMENTS AND SPECIFICATIONS||
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:
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 88th row from east and will be the 3rd 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 49th 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.
stone final cut main working steps:
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.
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
Most important warnings:
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.
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:
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).
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:
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:
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.
author of the text: arch. Manfredo Romeo other contributes have been mentioned in related paragraphs
drawings: arch. Paola Marrone arch. Alessio Talarico - sketches: arch. Manfredo Romeo
database management: arch. Manfredo Romeo - arch. Giovanni Anzani
software development and calculation: ing. Francesco Cenni - ing. Niccolò Baldanzini
yard consultant: Bruno Bonuccelli
© - General Engineering Workgroup -
Final Design Report
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