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.
Photogrametricly rectified picture of the
bridge
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 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.
Photogrametricly rectified picture of the
remnants of the bridge after it's destruction (1996)
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
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).
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
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.
Visit of Mr Hannah from
the World Bank
On September the 18. and 19. 2002, Mr.Lawrence
Hannah visited Mostar.
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
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.
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.
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| crane foundations, view to the columns |
the crane is lifted to its position |
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| concrete ballast weights production |
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For two foots closest to the bridge, ordinary foundations were made, but remaining two had
to have a special construction. 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.
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
Stone blocks
removal
On 21.09.2001, before the erection of the crane, one big stone block
that was laying for the long time in the waters of Neretva, north east from the existing
platform, was finally taken out. The operation had to be executed wit maximum care because
the block shouldn’t be damaged on harmed in any way.
This part of the bridge weighted approximately 20 tons, and it is a beautiful example of
the bridge constructional elements. It contains the parts of the arch, spandrel walls and
the cornice.
The Libherr crane is taking the
block out of the water
It was pulled out with the same mobile Leibherr crane as the Potain site crane. Operation
lasted approximately one hour.
The block is now placed on the access road under the platform, but as all the other
remainig bridge blocks it will be dislocated to the Harem site and stored in covered shed
on the wooden tannin-free pallets.
Dismantling of
the pavement
One of the main tasks for the Contractor in these days is to start with
the dismantling of the existing pavement.
Due to its irregular surface and its particularly visible position, the pavement is one of
the most delicate parts as regards dismantling and rebuilding. It is therefore essential
to prepare very precise stone-by-stone records and to mark each of the stones in all three
dimensions. In addition to that, the position in the row and in the band of the pavement
needs to be individually identified for each stone.
Marked pavment on the right river bank
The remnants of the pavement on both sides of the bridge have now been numbered, marked
with different colours, and preliminarily measured.
It is very important that all of these works are done with maximum care, because, almost
all of these stones will be reassembled to its original place, when the bridge is
finished.
Remaining pavment on right and left side |
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