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NEWSLETTER 04 - 31.10.02

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Hypothesis about the ancient constructive and assembling techniques


Building a stone bridge, more then 400 years ago, was really a feat and most probably the highest difficulties of the time were the following:

structural and architectural design was not determined trough a knowledge of the theoretical principles related to stability and resistance but trough the knowledge of the causes of collapses and effects of loads: therefore anything new would have meant something unforeseeable;

the building of a wooden false work, (centering), that could bear the huge load of the stone blocks with limited settlings has always been one of the most remarkable issues of large ancient stone structures;

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transporting and lifting heavy stone blocks was a technological issue that was strictly linked to the times of performing the works which had to be controlled to avoid collapsing risks related to the season variations of the river water level.

Unlikely, no historical document is yet available about the tale of the bridge yard, with organisation notes, constructive methods, difficulties found: therefore, what above mentioned, is mainly an hypothesis based on similar cases.

But what follows is more scientifically proved by the architectural analysis held on the ancient bridge surveys.

The centering was most probably a wooden one; nothing is known about the exact configuration of it, apart from the fact that some recurrent discontinuities have been found in the arch intrados, which may have been caused by different sectors of the arch false work, (centering - scaffolding); these sectors may have been defined by the length of the master beams used for the structure.

It is most likely that the ancient centering was not strong enough to bear the loads of the stone blocks without settlings: this may be proved by the fact that, by comparing north and south elevation, it is possible to note that next to the springers the rows were much more regular and well built, with little variations of levels. While, proceeding to the key stones, irregularities increase: rows are proceeding not parallel, levels are changing from north to south in a range of about 10 centimetres. Presumably, the more the centering was loaded, the more it underwent to unforeseen settlings that were more remarkable by the south side, as it can be observed with the numerical analysis of the co-ordinates. To recover the settlings, it is possible that wooden wedges were used, of which there are traces in many spots of the intrados surface of the vault, (some steps between adjacent voussoirs may be surveyed even in the assembled blocks recovered from the river). The use of wooden wedges was probably performed with very small ones next to the springers in order to adjust the voussoirs and to match their joints and was performed with bigger ones towards the top, to correct the geometry following the planned design shape.

Even if arch stones were of remarkably different dimensions, the preparing and assembling procedures were not randomly performed,and what is even more important to stress, the work over the centering was most likely to be quite limited to the assembling: in other words most of the work like stone cut and carvings of slots were performed off-site. The above statement may be proved by the following observations:

positioning of voussoir joints was, as in ordinary masonry works, accurately shifted to guarantee an efficient interconnection of the stone blocks;

positioning of cramps and dowels, and related slots and channels, required the exact knowledge of the stone dimensions and of their joints positions of adjacent rows of voussoirs to avoid interference between metal strengthening devices and joints;

stone voussoirs were quite variable in the vault, but the ones belonging to the same row were of very close transversal base dimensions: average variation range cm 0.5-2 in a length of almost 4 metres, which is an accuracy quite higher than the average followed for all the other parameters.

From the above observations it is possible to deduce that, at the time, they were using different stone voussoirs for the bridge vault of different size and shapes, (due to the natural availability of the quarry, where it seems that some natural weakness veins compelled to limit the dimensions), but each row of the vault was accurately pre-selected and picked from a temporary deposit of rough blocks. Each row was composed of voussoirs of very close intrados dimensions, and was prepared next to the preceding one in regard of the joint positioning and of the metal elements positioning. Dowels were previously assembled off-site and related slots were prepared. This procedure was most likely to be performed on groups of rows and not on couples to avoid that the assembling could be stopped by the lack of prepared arch rows. Over the centering it wouldn't have been possible to manage all the vault requirements unless an efficient communication of dimensions were performed between the working teams over the centering and the working team off-site.

Despite the accuracy concerning the transversal thickness of arch rows was accurately performed, the raising of the rowswas quite irregular towards the top, most probably due to the mentioned centering anomalous settlings. This increasing inaccuracy has been recovered wholly in the three top rows at the key stone level, where assembling has been performed regardless of the previously surveyed criteria, and even a variation of cm 11 of the intrados size has been checked in only one row.

Most probably the arch top rows of voussoirs are the prove that the two teams of workers were not in contact and not co-ordinated in the carrying-on of their works. It may be possible, also, that something was going wrong with the centering and that final rows have been quickly assembled to stop the gradual settling of the vault. We shouldn't forget that the vault was about 145 m3 of stone which weighted almost 300 tons over a wooden temporary structure.



Technical description of procedures for amendment of the wall from the level of abutment bottom to the level of cornice


INTRODUCTION

The assumptions of work performance on amending the wall from the level of abutment bottom to the level of cornice, which are the subject of this project, are:

quality performed works on amending the wall beneath the level of abutment bottom

excavation and archaeological works inside the abutments of the bridge until the level of injected mass appearance (assumption is that injected mass appears on the cornice level on both sides of the bridge)

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stone wall reparation

Works on amending the wall in mentioned segment are composed of amendments of abutments (injecting the wall through joints, replacement of damaged stones on the wall surface and re-pointing), and injecting the wall approximately 1.0 m into zone of injection works already done in the section of foundation.


Amendment of wall

Amendment of wall is done by injection and placing the constructive reinforcement into drills. The main purpose of injection procedure is filling the possible smaller cavities and cracks on joints between the layers.

Previously done injections in the mentioned wall in section of foundation have shown small consumption of mixture (respectively small penetration into the wall) in between the weak and strong connected layers of conglomerate. Only in unconnected layers of conglomerate the consumption of mixture was somewhat larger.

THE WALL

The wall in the mentioned zone of this project, according to geotechnical research works, is composed of layers of firmly connected conglomerate alternately with layers of weakly connected conglomerate, on both banks. Layers are approximately horizontal with irregular changes of layer thickness.

That kind of structure is also seen on upstream and downstream parts of the bank.


AMENDMENT OF THE ABUTMENTS ON THE MENTIONED LEVEL

Amendment of the abutments is done by injecting the wall through joints, replacing damaged stones on the wall surface, re-pointing, as well as re-building on the surface where the stones are missing. The purpose of injecting is better solidity of the wall, decreasing the amount of voids in the wall, and accomplishing better touch with the wall. Minimizing the voids in the wall is necessary for making the wall less permeable in order to stop the leaking of grouting mixture when injecting behind the wall.

The works will be done according to following schedule:

Cleaning the joints.

Replacement of the damaged parts of the stone

Re-building on the places where the stone is missing

Closing the joints with lime mortar that has the colour and content according to demands of the conservator.

After consolidation of mortar in joints, drilling of injecting drills
f 25 mm needs to be done. 5 drills on 1 m2 of the wall surface are foreseen. Injecting the wall will be done only from one side so the length of the drill is foreseen in the wall thickness. According to researches, the wall thickness of the mentioned segment is 60 – 80 cm.

Injecting is done with factory-made mortar “Calx Romana” under the pressure of 1 bar.

Control of performed work is done after completed consolidation of grouting mixture. Two procedures are suggested: either ulterior injection of a specific zone where additional filling of grouting mixture can not be more then 10% of previously determined quantity that was injected, either by disclosure of few square meters of the wall surface in order to visually determine penetration of grouting mixture.

Reinforcement is placed constructively for strengthening cracks between approximately horizontal layers (drills are vertical on layers).

Injecting will be done with downward method. Approximately one drill on each 1.5 x 1.5 m of surface is foreseen, but the real schedule of drills will be adjusted according to the existing situation of the walls within the surface of abutments. The amendment includes the zone of 6.0 m from the bridge foot, which is characterized as a zone of possible influence of the bridge load. Schedule of drills on the right bank shown in supplements is conditional because the archaeological researches and excavations to the level of cornice are not done yet. Some drills are done slantingly with the purpose of penetration into the wall beneath the walls of larger thickness.

On the right bank vertical drill 6.5 m length is foreseen, and on the left bank 8.5 m drill. Length is determined as a distance from cornice level to the depth of penetration

conglomerate (breca) layers is not expected in the mentioned zone, according to geotechnical researches.



Control of performed work

Control of performed work on amending the abutments will be done with method of ulterior injecting or with disclosure of injected part of the wall.

Conducting control is foreseen on three places on each abutment wall.

The control of performed work on improving the wall will be done by taking out cylindrical testing samples by ulterior drillings. Vertical drilling on three places on each bank is foreseen. The locations of performing control works will be determined by supervising engineer.


Technical description of preliminary scaffold design


1.1 GENERAL

In order to enable undisturbed work on construction of new abutments and the remaining parts of the arch, the main scaffold girders in base shall be mounted nearby bridge, as well as concrete foundation and piers. Fully protection of stone walls will be achieved in a way that connection between concrete and stone will be coated with appropriate folie. Fully structural stability of piers, i.e. stone part of abutment shall be carried out with pre-stressed anchors, which shall have function of abutment stability even after removal of scaffold, because they'll be completelly injected.

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longitudinal view to the bridge scaffolding



1.2 DISPOSITION OF THE SCAFFOLD CONSTRUCTION

In longitudinal sense, centring consists of two main triangle steel trusses, mounted nearby bridge (from the both sides).

Steel trusses are supported on neoprene bearings with consoles, anchored with pre-stressed anchors in finishing phase of scaffold mounting.
Those console pre-stressing of steel trusst contribute reduction of bending moment in the field, as well as time i smaller deflection skele.

In order to eliminate the wind influence, the steel trusses shall be mutual connected with bracing in three positions, in the middle and two meter from the bearings from the both sides. Wind bracing shall be fitted in upper and lower zone according to principles of propping and tensioning.In accordance with enclosed disposition, on main longitudinal steel trussed girder shall be fitted steel cross girders, on which heavy scaffold 159/150 (5 pipes in each esction) shall be supported. Those steel pipes shall be mutual stiffened with steel pipes 48,3 mm and standard rigid coupling in horisontal way (two pipes in both, longitudinal and cross direction).

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cross section of the bridge centering

Besides, steel pipes joints shall be diagonally stiffened in accordance with enclosed drawing.

At the top of the pipes 159/150 shall be fitted typical height regulators, which provide up and down scaffold lifting up to 170 mm.
On height regulators shall be packed wooden beams – centring girders. Centring shall be constructe by 4 board, 4,8 mm thickness, 150 mm minimal height. On centring shall be fitted small board formwork beams, which follows the level of intrados, i.e. geometry of arch.
The main girders of scaffold shall be fitted above the level 50,70 mnm, i.e., the bottom edge of the steel truss is at the level 51,00 mnm.


1.3 SCAFFOLD CONSTRUCTION CAMBER

In enclosed structural analyses for main steel truss girders were given three phases of analyses.In first phase were given influences of own weight and weight of formwork centring and scaffold. For this influence, deflections are 1,96 mm. This influence could be eliminated at once with above described height regulators. In second phase of analyses, besides own weight, was given influence of mason work on bridge stone arch. The biggest deflections of those influences were in the middle and they are 6,25 mm. This influence could also be eliminated with height regulators.

In third phase of analyses were given deflections of total weight of bridge, arch and own weight, and it is 12,39 mm. Concerning that deflection of own weight is permanent, and that builded arch take over loading up to 3%, so the total deflection of bridge loading could be assumed approx 7,00 mm.

Besides those deflections, in scaffold construction camber shall be counted deflection of bridge construction, which is 2,2 - 25 mm.

In consultation with Supervisor, in each point of centring supporting shall be established precised level of camber, on which the scaffold construction shall be constructed.


1.4 RELEASING OF SCAFFOLD CONSTRUCTION

Releasing of scaffold construction means separation of scaffold from the main construction and taking over of loading to basic construction. Releasing of scaffold in this case shall be done step by step in four phases in a way to release first in top of the arch, i.e. around top of the arch, in each phases 25% of scaffold camber. skele. In Main design of scaffold construction shall be done precised schedule of scaffold releasing, plan of working manpower etc.


Electricity supply for the crane


From 19th to 23rd of October 2002, preparatory works for the crane power supply were performed. The electricity was brought to the site from the nearby electricity transfer case. Five pillars have been erected to carry the electric cable.

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lying of the foundation tubes for the pillars

On the Site, close to the crane, the electric switchboard was installed.

On the 25th of October, after authorised approval, the electricity for the crane was officially switched on.

Fully operating of the crane started on Monday the 28th, and it’s first task was the removal of stone blocks from the platform.

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crane is in function


High water level


From 10th to 24th of October, water levels of rivers Neretva and Radobolja were extremely high. The reason for this level incensement were heavy rains in the area.

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view to Neretva on 15th of October


CREDITS:

This NEWSLETTER was made in accordance with prevailing articles of the Contract No 002/2002 about the Rehabilitation of the Old Bridge, by ER-BU CONSTRUCTION / TURKEY

Paragraph concerning the hypothesis about the ancient constructive techniques is extracted from final architectural report - General Engineering

Intellectual property of final architectural report and of 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
- General Engineering Workgroup -

SOURCE:

Final Design Report  - General Engineering

ER-BU CONSTRUCTION / TURKEY

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