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Architectural
description
The Old Bridge of Mostar, (Stari Most), was a stone bridge of very slender and
elegant shapes: its profile and its skyline were so thin and so high over
the river waters that it was hardly to believe that such a structure could
be worked out of huge stone blocks. Built in tenelija
stone, it was of a light tone colour, bright and changing during daytime
depending on sun colours.
The bridge was mainly conceived as a functional structure, aimed
mostly at connecting the two banks of the river: it may hadn't,
originally, any additional ceremonial and monumental meaning, and its
design has been quite influenced by the morphology of the site which is
totally matched by its abutment walls.
The whole monumental complex, including the adjacent fortification
towers, is totally part of the site, castled on the rocks and winding the
banks, it is not the result of a single design work, but of a development
during time, that has followed the historical happenings and the need for
protecting and keeping the bridge, that has marked the town even in the
name of "Mostar".
The bridge had few esthetical devices and no ornamental element; its
architectural beauty and value were to be found in the simplicity and in
the essentiality of the structure: the shapes of the bridge were not linked
to any time, to any style and to any fashion, in a way that the bridge of Mostar has always been admired as symbol.
Two cornices, only, with angled section profile, ran on each
elevation and met at the keystone with a refine balance among tangency and
jutting out: both of them had a protective function to rain waters, and
both of them marked different structural elements of the monument, being
part of it and not pasted. One more bigger
cornice, per side, determined the bridge arch springer
level, prosecuting along the abutment walls upstream and downstream.
The stone surfaces were nor
polished neither regular: they were perfectly cut and hand worked, but
characterised by frequent ordinary constructive inaccuracies from which it
was possible to perceive their materiality and that they were
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made of stone. Out of more
than one thousand stone blocks, there were not two elements of identical
dimensions, and even the arch voussoirs, were all
different and with remarkable variations, as a consequence of the random,
natural-determined, availability of the stones in the quarry. Undoubtedly
most of the architectural beauty of this monument was in the refined tuning
among a globally compact appearance of the structure, caused by the thin
joints of the masonry, and an unforeseeable and fragmentary close-view
appearance due to the small assembling imperfections of stone blocks of different
shapes and sizes.
The one-span bridge had an intrados curvature which was close to a
circle having the center lowered of approximately
cm 296 compared to the east springer level; but
the circle shape may be not enough to describe the curvature wholly, that
slightly changed at the imposts level to better match with the straight
profile of the abutment wall, and was slightly raising at the key stone
level. The curvature exact original shape was anyhow perturbed by
irregularities that may have been caused by settlements and by ordinary
construction imperfections, (for a detailed analysis of
the curvature refer to following paragraphs).
The bridge main dimensions were the following: span was of cm 2871
by the north side and of cm 2862 by the south side and the arch raise was
approximately of cm 1206. Generally speaking measures were obviously often
variable and moreover the bridge impost level by the west bank was
approximately 12-13 cm higher than the east level.
The load bearing arch had a depth of about cm 395 and an height of
cm 80, and was composed of 111 rows (a number which was probably planned
and not randomly obtained), and rows counted from 2 to 5 voussoirs, (more frequently 3-4). Voussoirs,
(arch stones), were of variable shapes and sizes but the average stone
block had the following dimensions: cm 40 × 80 × 100.
The bridge arch and elevations were made of tenelija
stone, (local lime stone), and connections were performed with the use of
mortar and of metal reinforcing cramps and dowels. The use of forged iron
devices to strengthen the structure was one of the peculiarities of the
monument and was applied almost to every element
of the bridge following different assembling methodologies.
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Being the bridge arch, at the top, considerably higher than the
adjacent street levels (about cm 270), the footpath over the bridge was
steep and tilt in a way that all the architectural elements, like
spandrels, parapets and upper cornices, were following these directions
until the top. Spandrel walls of the bridge were divided from the arch by a
stone cornice, (lower cornice), that followed the arch curvature, and were
limited on top by another cornice, (upper cornice), of straight but tilt
profile.
Lower cornice stones were jutting out from the load bearing arch and
determined the base from which started the
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spandrel walls; at the same time upper cornice was jutting out from
the spandrel walls and finally parapets were almost
aligned with the below spandrel walls, but slightly leaning outwards
while getting to the bridge top, as to give an optical effect of a wider
footpath.
The pavement was made in krecnjak stone,
(limestone hard and resistive marble-like), and was characterised by
transversal rib-steps to avoid slipping; flooring was assembled on a mortar
layer which had, most probably, also a waterproofing function together with
the below layer made of “terra rossa”, (heated
aggregates of red colours given by the presence of bauxite).
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Structural description
On the
structural side, the bridge was quite interesting, and from the analysis of
the inner elements, it is possible to appreciate fully the engineering
level of the time and the wise devices adopted for the long lasting of the
structure.
Main structural element of the bridge was the load bearing arch,
which was undoubtedly the portion of the monument that required the maximum
care and resources. The arch, thank to its shape, and to the configuration
of the above dead loads, was subjected to compressive strengths and the
stone blocks would have been enough and perfectly suitable for the purpose,
but additional devices were provided to strengthen the vault: forged iron
dowels were inserted between adjacent voussoirs,
and forged iron cramps were put over the extrados and across the side
joints. This way each connection joint was guaranteed either by the mortar,
either by the metal reinforcements.
Over the load bearing arch there was a masonry rib with an important
structural function that allowed, together with the spandrel walls, a
stiffening action of the whole bridge. Among spandrel walls and stiffening
rib there were two lightening voids that contributed in reducing the loads
over the arch, while fill was wisely provided only next to the arch springers to stabilise the structure and the vault.
The lightening voids were covered by stone krecnjak
slabs and above them only thin layers of aggregates were settled until the
pavement.
Undoubtedly the structure of the bridge shows an high and impressive
level of knowledge of all the requisites that were necessary for the stone
bridge and shows, as well, a refine
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VISIT OF Mr. HANNAH
FROM THE WORLD BANK
On September the 18. and 19.
2002, Mr.Lawrence Hannah visited Mostar.
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He made several
meetings with the PCU ( Project Coordinating
Unit), Supervisor (Omega Engineering), and the Contractor (ER – BU).
Mr. Hannah was
informed about necessity to change the type of the centering
proposed in the Technical Specifications.
THE CRANE
Crane
foundations, view to the columns
As it was stated
in the last Newsletter, some investigation drillings were made to determine
if the position chosen for the crane is stable and strong enough to carry
the weight of the crane and it’s loads.
After the examination works,
which lasted for several days, every single of the four chosen points for
placing of the crane footings showed up to be satisfying and completely
acceptable.
According to the
technical specifications, the crane stands on four concrete footings, on
approximately 5x5m wide foundations.
The Crane is lifted to it’s position
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bridge
transversal section with technological details
well, a refine constructive
technique; moreover, it has to be noted, that all the structural choices
were purposely performed and the elements were carefully optimised and
dimensioned to guarantee the resistance and long lasting of the monument.
The bridge
architectural appearance and its structure were strictly related, in a way
that it was possible to gather partially the inner structure even trough
the observation of the outer elevations, where cornices marked the most
important structural sections of the bridge.
One of them was on the cantilever
cave and it was necessary to support it from the firm ground. The fourth had to be raised up by the
reinforced concrete column
, to achieve the same
elevation as the other three footings.
For two foots closest to
the bridge, ordinary foundations were made, but
remaining two had to have a special
construction.
View to a crane from the foot
bridge
The
foots were connected with concrete beams.
When the bedding was released, very elegant but safe structure
appeared.
On 21.09.2201 the Leibherr mobile crane was brought to site, and the
crane erection started.
In two hours, the crane was
removed from the temporary location, and lifted to it’s planned location.
Crane needed some
ballast weights, so 10 of them 4.3 tons each, were made on harem site.
Concrete ballast
weights production
The main idea of placing the
crane on that particular spot was it’s safety
Recent height water levels proved that the chosen
position of the crane was the best possible solution for this particular
task and site.
This positioning also
secures the uninterrupted
progress of the works
even in the rainy winter period when the access road is flooded, because
the connection between the crane and the platform is secured.
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