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BRIDGE DESCRIPTION - PART III

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2.3.5 Technological devices


As already mentioned, the bridge stone elements were reinforced trough the use of peculiar technological devices made in hand forged iron, and placed across the connection joints following different typological assembling methods. Basically these iron connectors were of two different types: cramps and dowels, and they were applied to the stone elements into slots that were purposely carved with their bottom slightly widen in order to avoid untying. Metal connectors had, as well, their edges widen, and, once assembled, it was poured melted lead in the slots to finalise the assemblage.

The above described technique allowed, together with the use of mortar, a quite efficient connection system which was extensively performed for the voussoirs, in the vault, where three different groups of connectors were adopted: dowels among row joints, cramps for side joints, and extrados cramps mounted in a continuous layout to guarantee a tying action. All the other stone elements of the bridge were linked by metal connectors, including the parapets which were doweled to the upper cornices and linked one to each other trough visible cramps placed on top edge.

Most of these iron strengthening were hidden and protected in the inner parts of the joints, also to avoid rust; nevertheless, being those elements hand forged and partially winded by the lead, they could not get easily rusted: as it has been noted for the recovered stones, which after being 4 years in the river waters, were quite damaged but considerably in good condition.

For what concern detailed explanation of the assembling techniques, the methodology used, and the dimensioning criteria, refer to chapter 9 of this report.

 


2.3.6 Construction materials


A detailed and scientific analysis of the construction materials used for the bridge construction, has been provided trough laboratory tests by LGA Company; here next only a brief list of the construction materials used is provided:

  • most of the stone elements of the bridge like vault, cornices, spandrels and parapets were made in tenelija stone (Category I), which is a local oolithic limestone rock of light and warm colour and high porosity of a resistance to compressive strengths of about 20 MPa; (availability Mukoša Quarry);

Tenelija stone

Porosity

23%

Specific density

27KN/m3

Wet density

21KN/m3

Dry density

19KN/m3

Moisture content

8%

Uniaxial compressive strength

20Mpa

E modulus

13.000 MPa

  • the pavement and the stone slabs over the lightening voids were in krecnjak stone, a limestone hard and resistive marble-like of light colour;
  • metal connectors, cramps and dowels, were in forged iron as well as the fence;
  • lead was used for the connectors and for the assembling of the fence;
  • mortars of different types and compositions have been found all over the structure: refer to LGA reports for detailed analysis;
  • "terra rossa": red coloured aggregates with a remarkable content of bauxite were settled in a layer underneath the pavement.

A special document, as summary of the construction materials and their correct denomination, has been asked to LGA by GE. For this special issue credits are given to LGA and to eng. Gregor Stolarski, the document is here fully reported:

  • Tenelija (local name) for the oolithic limestone, uniformly granulated (visible grains), ivory - whitish coloured stone; denomination used in the LGA reports is either Tenelija or oolithic limestone; also more grey coloured type is encountered;
  • Breca (local name) or natural breccia or conglomerate; greyish, very coarse (gravel size well rounded grains) and very porous stone, which is encountered directly at the location of the bridge in Neretva river bank formations; it presents a secondary cementation of a large size river sediment; in the LGA report mostly named Breca, also as conglomerate by others (e.g. CONEX);
  • Krecnjak - dark grey, the dark-grey and black-grey section of limestone blocks at the right bank downstream wing wall foundation (few rows of stones only, possibly of origin older than Stari Most) covered partially with modern restoration - plaster; this section was wrongly named "marl" in early LGA paper;
  • Krecnjak - massive, a massive, white - grey limestone used in the rough irregular masonry works e.g. of the towers, in the fill masonry of the Stari Most and in the stiffening rib; sharp-edged stones of small size, very hard stone;
  • Krecnjak - marble, massive, marble - alike limestone, which is used in the pavement slabs; also encountered locally in aggregates of fill mortar;
  • "Sandstone" (as direct translation of the local name), geologically a conglomerate of white smaller grains (when compared to Breca) - not a classical "sandstone"; Its individual grains are very sharp edged and are embedded into a sandy, iron-oxide coloured cementation matrix; the stone is hard and porous; it is represented by only few stones within the monument, found e.g. as window frame in Helebija;
  • Miljevina, a massive limestone of oolithic origin but with no visible grains, harder than Tenelija (found in the same quarry Mukoša), a "mudstone" in a sense; used obviously for recent or younger repair works only on very few spots;

Below the pavement slabs two different mortar layers are defined:

  • pink lay mortar for the stones, few centimetres thick only below the stones and present also still in the joints between the slabs of the pavement; laid as real mortar
  • dark red and brick red mortar layers below the pink one; defined as foundation or levelling layer for the final pavement cover; this "mortar" has a composition of an ancient mortar, have been placed under low workability (low moisture content) and compacted manually; the thickness is probably less than 10 cm but cannot be clearly defined without precise archaeological studies, since the red colour has spread down to the mineral layers, originally not belonging to the pavement formation (levelling) layer.
  • Additionally a mineral fill layer has been placed historically under the dark red mortar as equalising foundation layer. This layer contained sand and clay of intense red colour and has been defined by comparison to local materials as Crvenica - the red earth, geologically: very old soil, typical for the region and for the limestone formations. From the point of view of LGA it is not recommended to put the local red earth into the reconstructed bridge due to the high clay content, which is above the frost resistance tolerance level. Instead the fill mortar below should be laid to the original red-earth level.

 


2.3.7 The destruction event


The Old Bridge of Mostar was destroyed in November 1993 by shelling during recent war events; the moments in which shelling were ongoing have been filmed and from that documentation it has been possible to gather some technical data and observations.

 

fig15 – moments before the destruction event: shelling are ongoing by the south side

From analysis of the remaining portions of the bridge located by the east abutment, it is possible to observe an higher surface deterioration on the north side than in the south one, but, as far as it was possible to see from the movie, shelling were coming from south side and hit mostly the south east portion of the bridge over the arch reins: nowadays the widest remaining portions of the bridge are located on that side. This seems quite strange but it is most likely that shootings were directed also to the north elevation during other war attacks, and this is the reason why the bridge had been protected by tyres, (temporary structures over the footpath were instead aimed at the protection of the people from shootings). Moreover shelling was performed with accuracy almost on the same spot in order to cause the collapse using the minimum numbers of shells and the structure was divided in two main parts: a small one below the arch reins by the east bank, (still built-in and on-site), and a big one which ruined wholly in the river.

fig16 – the sequence of images of the collapsing of the bridge

From observations of the movie it is possible to note how shells have gradually brought to the bridge collapse:

  • the load bearing arch was the main target element of the shelling: a wide portion of it was destroyed before final collapse, (from the reins to the key stone for a thickness of about a meter);
  • other shelling which were higher than the target perforated the spandrel wall with escaping of fill;
  • the attack has been performed by people that knew the basic functioning of a bridge structure;
  • shelling was performed with the aim of causing the collapse of the bridge and not of ruining the bridge; the bridge collapsed when a shell broke definitely the continuity of the load bearing arch.

The above mentioned movie has helped the work for the repositioning of the recovered stones, (see 4.3.7), since it has been possible to determine the voussoirs that were most likely definitely lost and reduced to powder due to direct hit.

fig17 – current condition of the bridge (north and south elevation)

 


2.3.8 Reference dimensioning: the numbers of the bridge


A schedule with most significant data of the bridge is here next provided to allow preliminary evaluations during the on-site works:

data concerning geometry

unit

value

north span

cm

2871

south span

cm

2862

north maximum raise – measured from east impost level

cm

1206

south maximum raise – measured from east impost level

cm

1205

average height above river level (depending on seasons and on river waters)

cm

1600-1800

lowering of the east springer in respect of the west one (by north side)

cm

13

lowering of the east springer in respect of the west one (by south side)

cm

12

intrados curvature length north side

cm

4058

intrados curvature length south side

cm

4036

extrados curvature length north side

cm

3607

extrados curvature length south side

cm

3638

thickness of load bearing arch

cm

395

thickness of spandrel walls

cm

60-85

thickness of lower cornices

cm

60-70

thickness of upper cornices

cm

80-90

thickness of parapets

cm

20-26

average height of parapets (measured by the outer side)

cm

90-92

raising of the bridge footpath

%

18-19%

raising of the bridge footpath

cm

270

 

data concerning the quantification

unit

value

number of rows in the load bearing arch

n

111

number of voussoir per row (the average is anyhow 3-4)

n

2-5

average dimensions of a voussoir (volume = 0.32 m3; weight about 640 kg)

cm

4080100

highest length of a voussoir

cm

258

highest volume of stone block before final cut (weight about = 2500 kg)

m3

1.25

     

number of main stone elements of the whole bridge (pavement not included)

n

1006

number of stone elements of the whole bridge (pavement not included)

n

1088

number of voussoirs in the load bearing arch

n

456

number of spandrel stones

n

425

number of cornice stones

n

157

number of parapet stones (including small element by south-east side)

n

50

     

number of metal cramps for the side joints of the vault

n

666

number of metal cramps for the extrados of the vault

n

550

number of metal cramps for the lower cornices

n

91

number of metal cramps for the spandrel stones

n

197

number of metal cramps for the upper cornices

n

124

number of metal cramps for the parapet stones

n

46

total number of metal cramps in the bridge

n

1674

     

number of metal dowels in the vault

n

717

number of metal dowels in the parapets

n

93

total number of metal dowels in the bridge

n

810

     

volume of the load bearing arch

m3

145

volume of the load bearing arch plus quarrying tolerance

m3

191

required volume of stone to be cut for the load bearing arch

m3

202

required volume for spandrel stone

m3

102

required volume for cornices stone

m3

33

required volume for parapets stone

m3

19

required volume of stone for the whole bridge (pavement not included)

m3

356

 

data concerning co-ordinates

unit

X value

Y value

Z value

north east springer (a.s.l. level - referred to local origin)

m

23.02

41.02

46.71

north west springer (a.s.l. level - referred to local origin)

m

50.69

33.35

46.84

south east springer (a.s.l. level - referred to local origin)

m

24.12

44.81

46.72

south west springer (a.s.l. level - referred to local origin)

m

51.70

37.17

46.84

         

north east springer (referred to relative origin: north east springer)

m

0.00

0.00

0.00

north west springer (referred to relative origin: north east springer)

m

28.71

0.00

0.13

south east springer (referred to relative origin: north east springer)

m

0.04

3.95

0.01

south west springer (referred to relative origin: north east springer)

m

28.66

3.95

0.13

 


2.3.9 Classification and nomenclature


In order to avoid any misunderstanding due to naming of different bridge stone structures and elements, here next it has been given a reference scheme with which it is possible to identify clearly the terms that have been used in the report.

 

fig18 – bridge elevation with identification of the terms used

fig19 – bridge axonometric cut view; on the left classification codes according to flow chart of next page; on the right other terms used in the report and in the design

In addition to what showed in the above pictures, some other terms may be underlined:

  • bridge span = arch span = distance among springers
  • bridge raise = arch raise = distance among springer and key stone levels
  • elevation = front view
  • joint = connection line between two adjacent stone elements
  • abutments = masonry structure that bears the thrust of the bridge
  • masonry work = stones assembled with mortar
  • ashlar work = accurately cut stone blocks assembled with mortar
  • metal cramp = forged iron U shaped element used to assemble bridge stone blocks
  • metal dowel = forged iron I shaped element used to assemble bridge stone blocks
  • stone carve = surface stone cut – incision
  • carved slot = hole in the stones for dowels
  • carved channel = surface long incision for pouring the lead - feedhead
  • arch centering = temporary structure used for assembling the bridge arch
  • arch falsework = arch centering
  • archivolt = bridge arch, vault

To allow an easier explanation of all the matters and issues concerning the stone elements it has been worked out a simple classification an code system that is here next showed in the flow chart. This classification system has been used and enlarged in the report for the purposes of the design.

fig20 – stone classification and code system

The main subdivision among arch stone and other bridge elements has been performed since the arch voussoirs are quite peculiar and are supposed to match higher structural requisites.

This classification has been adopted in the report, in design drawings and in design charts for stone final cut.


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

- General Engineering Workgroup -

SOURCE:

Final Design Report

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