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|ARCHITECTURAL DESIGN - PART 04||
3.6 Dismantling of the bridge remnants
Everyone would have liked that this work stage weren't necessary, proceeding to rebuild over the bridge remaining portions. But for many reasons, (explained in chapter 4, §4.4 of this report), this seems to be technically not possible and a portion of the ruins will have to be dismantled. Of course for the stone elements of the bridge, which are still not fractured and in good condition, it is required a dismantling with survey, numbering, storage and remounting in the original locations.
Exact quantification of dismantling is not possible without checks on-site during the excavation works, therefore, different hypothetical excavation profiles have been worked out, while, evaluation for the bill of quantities has been the highest, in order not to need any review of the contractual terms.
Of course any technical device, any on-site check, any possibility of reducing the dismantling works, (due to the finding of good masonry that can bear loads), is desirable and should be carefully investigated. At the same time it is necessary to be well aware of the risk that is taken by loading released masonry: if the new portions of the bridge lay on unstable layers, structural cracks would develop and a collapse may happen as well. For the above reason the work Supervisor will have the responsibility of deciding whether it is possible to reduce dismantling or not.
3.6.1 Stabilisation of the cantilevering portions
As it has been already underlined, in order to avoid any collapse and in order to dismantle the foreseen portions of the remaining parts of the arch, it is necessary to design and fix provisional structures that may ensure the stability of the cantilevering stones and may allow their dismantling in complete safety. Moreover scaffoldings will be necessary either to perform excavations, either to perform strengthening works: temporary structures should guarantee a good accessibility of workers to the spot and the positioning of small working machinery.
3.6.2 Repair works of structural cracks of the abutment walls
Before any excavation or dismantling may take place, it is absolutely necessary to make sure that abutment walls, which are bordering the bridge springers and which contained the bridge thrusts, should be repaired. It is necessary that at least structural cracks, detached masonry portions and leaning outwards walls are repaired with tie rods.
Design for structural repairs of the abutment walls has been worked out by CONEX Company on their responsibility, as foreseen by assignments. General Engineering, (not responsible for that type of works), simply would like to suggest to use different strengthening devices from the one proposed: instead of using anchor bars it would be better to use tie rods for their better efficiency with released masonry. Anyhow it is absolutely important that CONEX may have the possibility of reviewing the design for the abutment walls being informed of the dismantling works which have been worked out by current design, since CONEX was not aware of the excavations that would have been necessary. A co-ordination of this peculiar matter is required by PCU-TA.
The above mentioned repair works are absolutely necessary to avoid collapse of abutment walls during the dismantling; of course the works on the walls will not be finalised and completed with all the non-structural repairs, because the dismantling intervention is most likely to cause some settlings and some surface damages. Finalisation of the repair works of the abutment walls may take place after excavation works and bridge have been completed: when all the other abutment walls will be repaired.
3.6.3 Dismantling by numbering of the bridge stone pavement
Stone pavement should be accurately dismantled only in the portions where excavation will have to be performed. Dismantling should avoid any further damage to the bridge stone flooring and it is required that stones are surveyed and numbered in order to be remounted, (if not cracked), in their original locations. This procedure should be guaranteed by storing stone flooring, tiles and rows, in wooden boxes lying in the correct position one next to the others. Storing of the mentioned pavement elements should be in a safe and dry place.
Single damaged stones of pavement, not repairable, should be dismantled with accuracy and care in order not to ruin the adjacent stones and the underneath mortar layer (which has a waterproofing function). These damaged stones are sometimes in isolated locations (quantification is by estimation) and some others are in drawn areas (see drawing DS-01). Remedial works should preserve or repair mortar layer underneath.
Portions of the pavement which are covered by rubbles should be cleaned before flooring remedial works, and anyhow all the area should be cleaned to allow a detailed evaluation of the damaged portions. Moreover all the joints of the remaining portions of the pavement should be cleaned from any recent sealing intervention (joints have been sealed with plastic materials).
Existing stone krecnjak socles (east side) between bridge pavement and adjacent flooring should be dismantled with accurate survey and numbering of all the elements. This work is required in order to perform an efficient waterproofing layer and connection among different types of paving and in order to manage the adjacent flooring slope. Storage and transportation in a safe and dry place will be performed. During the repair works, elements that are not damaged will be remounted in the original locations. Any stone element that may be not reused, due to the damages and cracks, will be transported to the temporary storing place where all the recovered stones have been collected.
3.6.4 Excavation of the abutment and ground stabilisation
Dismantling works of the abutments is a very delicate intervention that should be held with high care and expertise: more than dismantling we should define it as high accuracy excavation works. As already described, in previous paragraph, bridge pavement should be partially dismantled, first, following the minimum configuration as possible. Dismantling should be performed proceeding from the top of the abutment going downwards, by manual excavation. Machinery may be used only with the approval of the work Supervisor and on his responsibility being aware of the following:
Dismantling works of the abutment should be performed while strengthening the adjacent ground and while doing provisional works to avoid collapse of the abutment walls. Sheet piles may be placed to brace the ground, as well as tie rods may be used to avoid collapse, as represented in schemes of drawing PH-01.
Dismantling works of the abutment, even if over-quantified, should be performed following the smallest configuration as possible. This should be evaluated on site depending on the stability of the abutment and on the safety of the workers. Works Supervisor will be responsible of the above choice.
Dismantling works of the abutment and of the bridge fill have been represented and quantified. Three different configurations have been hypothesised: the smallest one is the most desirable and the widest is the one that has been used for quantification. Dismantling should be performed until a good and coherent masonry layout is reached. All rubble should be removed as well as unstable elements.
Works Supervisor will be responsible either of the dismantling level to be reached, either for dismantling line configuration. Dismantling limit should be, anyhow, stair-shaped in order to guarantee a more efficient connection surface.
During dismantling works of the abutment special care should be put to the room which is located inside the east abutment in order that no inner structural damage takes place.
3.6.5 Dismantling by numbering of the bridge stone elements
Remaining portions of the bridge, (spandrel, cornices and parapets), will be dismantled with accurate survey and numbering of all the elements. Special care should be put during dismantling works of stones and of metal stuff, in order not to provoke any damage. Storage and transportation in a safe and dry place will be performed. During the reconstruction works, elements that are not damaged, (to be decided on supervisor responsibility), will be remounted in the original locations. Quantification of dismantling includes all the remaining portions of the former bridge; remounting is equal to dismantling and it is over-quantified since it will not be possible to re-use all the elements.
Before proceeding to the works some tests should be performed in order to find the best way of disassembling metal cramps from the stones with no damage. Integrity of stones that will be reused should be checked by ultrasound tests. Some of the spandrel stones, (mostly the ones located on the north east side), have high surface damages due to the shootings: repair of some of those stones should be evaluated following similar techniques to the ones proposed for the tenelija ashlars of the abutment walls.
Even krecnjak stone slabs, which are over the lightening voids, should undergo to the same dismantling procedures as the other elevation stone elements.
3.6.6 Dismantling of voussoirs: integrity limit
Structurally speaking, the issue about the arch voussoirs to be left on-site is even more undertaking than the previous one. During the first stages of the work many studies and hypothesis have been worked out about the matter: for more notes refer to chapter 4, §4.4 of current report.
Load bearing arch should be perfectly integer and should guarantee a full performance with no mechanical difference with all the other arch voussoirs: if the above may be not fully demonstrated, those stone will have to be dismantled until the arch springers, (following the highest level of proposed dismantling profiles).
Final design has preliminary proposed, (see drawings DS-02 and DS-03), an integrity limit which has been worked out from investigation results carried by CONEX Company. Only voussoirs, that had acceptable ultrasound test values, have been included below the integrity limit, and those are the ones that may be left on-site. But this may be worked out only after ultrasound test is performed with ultrasonic instrumentation from intrados to extrados side, (this way results will be much more reliable), and depending on the work Supervisor responsibility.
Arch voussoirs should be numbered and surveyed before dismantling; dismantling works should be performed with special care to avoid damages.
3.6.7 Demolition of existing concrete blocks
Existing concrete blocks should be demolished; those which are over the pavement should be demolished before pavement remedial works; the others may be demolished at the end of the works, once they are not necessary for any provisional work. Demolishing of reinforced concrete blocks should be performed with no vibration and taking care for the adjacent unstable structures. Underground thickness of the blocks has been estimated and quantified, (about 70 centimetres, as communicated by PCU TA). Demolition of underground portions may be not performed if there is any structural risk: to be decided on the Supervisor responsibility.
During demolition of concrete blocks, special care should be reserved to the ones which are located over the abutments void (room): it is most likely that the ceiling of the void has been overloaded by the blocks and a collapse may take place during the works.
Moreover, there is only one case of demolition of a small masonry wall that is almost completely ruined: it is located at the borders of the works limit by the north-east side (drawing DS-01 position E-10). It should be partially demolished until a good layer is found.
3.7 Centering erection
As already mentioned, centering erection should be performed during the right season time, (refer to chapter 6), and once centering foundations are matured. General Engineering is not in charge of centering design, and therefore no specifications will be given, but some warnings about the matter have been given in chapter 8 of this report.
General Engineering suggests that centering foundations, centering itself and wooden deck should be designed by one single Company or at least with a tight co-ordination by PCU-TA, being all those structures quite connected. Company that is in charge of the rehabilitation works will have to foresee a lifting system powerful enough to bring and place the centering on site.
Centering may be composed, (as it has been proposed in Construction Phases schemes), by five arches, (with three hinges), at a distance of about 80-100 centimetres; load bearing beam arches may be identically shaped with a regular profile smaller than the bridge intrados arch.
3.7.1 Hydraulic jacks
Centerings may have, at the arch impost level, hydraulic jacks to allow adjustment and releasing when it is required. These devices could allow the following curvature adjustments:
Company that is in charge of the rehabilitation works should provide assistance to jacks maintenance and regulation to gain the required adjusting.
3.7.2 Wooden deck placement
Following specifications given for the stone final cut, (SC-final) in chapter 7 of this report, and following additional specifications given in chapter 8 of this report, a wooden deck is foreseen in order to obtain the irregular arch shape of the former bridge. This wooden deck should be designed in order that it is resistant enough to bear the load of the arch voussoirs, and should be placed over the load bearing centering. Wooden deck will be, most probably, composed by different sectors that could be re-assembled and well connected once fixed over the arch false-work.
After wooden deck is placed, small adjustments will be performed either with the load bearing centering hydraulic jacks, either by settling the wooden deck itself: adjustments will be defined by a permanent survey system of the reference points located on the wooden deck.
Special care should be put when the deck is positioned next to arch springers, due to the scarce workability caused by the narrow spot. Deck should be placed over the centering making sure that its intrados curvature matches perfectly the load bearing centering profile: if this is not respected, unforeseen discontinuities will be caused on the bridge arch intrados.
3.7.3 Control of the geometry
Geometry control is required to allow the performing of the bridge shape and mostly to check the bridge intrados curvature. First geometry control is foreseen to adjust load bearing centering following the specifications given in the design of the arch false-work, and subsequently, to adjust the wooden deck over the centering. The above task may be performed trough the survey of reference points located on the temporary structures: those points may be surveyed determining all the 3d co-ordinates with a professional survey instrumentation, (electronic total station), that can have some base marks unmoveable and linked to the ground. Survey instrumentation may be, this way, replaced at any time in the same spot and periodical checks will allow the correct proceeding of the works.
Geometry control should be repeated while works are ongoing and at least every five rows of voussoirs are assembled per each side. Co-ordinates survey should be constantly compared with design drawings and data: reconstruction works should be supervised with the help of these verifies in order to manage their progression and their developing.
If geometry control is not constantly and frequently performed it will be extremely difficult to correct or recover any mismatching. Even if the bridge curvature is supposed to be the most delicate shape to be repeated, also other bridge elements like spandrels, cornices and parapets should undergo to these type of controls to avoid remarkable differences among new bridge and former one worked out trough the ancient survey studies and researches. Of course temporary structures for the bridge construction should be designed and built following co-ordinate data (refer to chapter 8 of this report for more notes). The performing of this task is necessary to respect the main objective of the assignment and therefore may be not neglected.
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 -
Final Design Report
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