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
§ 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
§ 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;
§ 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Unlikely, no historical document is yet available about the tale of the bridge yard, with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
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
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
Technical description of
procedures for amendment of the wall from the level of abutment bottom to the level of
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)
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
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 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
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
description of preliminary scaffold design
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.
longitudinal view to the bridge
1.2 DISPOSITION OF THE SCAFFOLD
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 Æ 159/150 (5 pipes in each esction) shall be
supported. Those steel pipes shall be mutual stiffened with steel pipes Æ 48,3 mm and standard rigid coupling in
horisontal way (two pipes in both, longitudinal and cross direction).
cross section of the bridge
Besides, steel pipes joints shall be diagonally stiffened in accordance with enclosed
At the top of the pipes Æ 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
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.
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.
lying of the foundation tubes for
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 its first task was the
removal of stone blocks from the platform.
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
view to Neretva on 15th of October