DEMOLITION, CONTROLLED BLASTING
Nitrex® srl
“SICIGNANO – POTENZA” HIGHWAY, Italy Refurbishment of the Pietrastretta bridge in Vietri di Potenza
CONTROLLED DEMOLITION OF THE SOUTHERN CARRIAGEWAY OF THE PIETRASTRETTA BRIDGE WITH THE NORTHERN CARRIAGEWAY IN SERVICE
Via Mantova 61 21017 Lonato - ITALY Tel.+39 03 09 90 40 39 Fax.+39 03 09 90 61 89 info@nitrex.it www.nitrex.it
Customer: COLLINI LAVORI spa, Trento - Italy, year 2016
The job Controlled demolition of one of the two carriageways of the Pietrastretta bridge, in the “Sicignano – Potenza” highway. The bridge was built in the 70's, in reinforced concrete. Single slim-pylons supporting, on transverse cantilevers, two independent carriageways on 47 m long spans each with 3 beams with a cross-section from 1,7 to 2,7 m, for a total of 21 spans and total length of 1 km. The demolition become necessary to replace the old and ruined carriageways with a new one in COR-TEN steel (about half weight, better seismic resistance, longer shelf life).
profile of a section of the the PIETRASTRETTA bridge
The demolition was designed to prevent damage to the Northern carriageway and pylons to be left in service and to the close standing housing.
Background The Southern carriageway was taken out of service due to relevant structural faults. Traffic of both directions was moved to the Northern carriageway with interdiction to vehicles whose mass was exceeding 7,5 tons.
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An “under traffic” refurbishment and reconstruction project was started. This consisting in the replacement of both carriageways, starting from the Southern one, and in the treatment of pylons concrete surface and re-bars.
Profile of the PIETRASTRETTA bridge, spans from 1st to 16th
Layout of the bridge, 21 spans 47 m, 1 km long
Concept A first attempt to remove the spans by means of a launching girder was stopped due to occurrence of an accident. One of the 3 beams of the span, which was cut from the other 2 to be lifted, broke during lifting. In its fall almost drag the launching girder and personnel.
Spans of both carriageways showed structural damages, with cracks, collapse of concrete volumes, re-bars exposition, corrosion and rupture. Similar but less relevant damages, were found in pylons.
This failure was caused by a crack which created a hinge triggering collapse and strong tear on the fixing spots for lifting. The beam, with its 185 tons and 45 m lenght, was originally tied to the other 2 by means of transverse post-tensioned concrete beams and by the reinforced concrete slab. When slab and transverse beams were cut to allow its lifting, all its dead weight become self-sustained, being lost the cooperation previously given by the other two beams. To prevent a next accidental collapse it was decided to remove the spans by demolishing them with explosives, keeping personnel at safe distance. All activities to prepare the the blast were foreseen by means of radio controlled equipment with personnel at safe distance and secured to a safety line fixed directly at the pylons.
Design of this bridge, as many other of that time, was targeted to minimization of concrete and iron, taking advantage to relatively low cost of high qualified manpower. This is to be seen by the number of beams (3 instead of 4) and from their geometry, with variable height of the cross section (increasing from the supports to the centerline, instead as being equal for the whole length). It is also to be seen from the slim and elegant geometry of the pylons with their many angled surfaces which all required extra work for scaffolding and formwork. The two independent carriageways are 3 m staggered to match the slope profile. Both carriageways spans are supported on a transverse box-cantilevers from a single slim box-pylon (height from 12 to 45 m). Number 6 post-tensioned transverse and a slab 9,5 m wide and some 20 cm high, bind together the 3 main beams. Center of mass of each carriageway lays outside the trunk of the pylon. This determined concerns due to the hazard of transverse overturning of the bridge once that carriageway would have been demolished entirely before the new one would have been put in place to counterbalance the weight.
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The structure
Survey In a preliminary survey it was stated that all beams were interested by widespread degradation with lost of concrete, exposition of reinforcement and corrosion both of re-bars and tensioning wires. Locally reinforcement appeared to be deeply rusted and also broken. Degradation showed a lower grade in the middle beam in comparison with the external ones. An higher degradation was noticed in the lower flange in comparison with the upper one. Similar deep degradation also involved the slab but not the cross beams. Three of the spans appeared to be critical, close to ultimate resistance. At first instance it was not possible to understand the reason of such a different intensity of degradation among one span and the other. After the testing it was proven that this was depending from different quality of execution and consequent relevant differences in the mechanical resistance of the concrete and, in part, due to the different exposition to weathering (freeze and lack of proper drainage for percolation of water with the salt used in large quantity in winter for defrosting). Geometry of the components was found matching with the construction design. At the time of the construction of the bridge it was an habit to adjust the design in execution, also with relevant changes. This to simplify the construction and/or to compensate lack on site of specific components. Not always those changing were recorded in the official documents so that when demolishing old bridges, a double check for conformity is always needed. No control were possible for the tensioning wires due to a concrete cover the tensioning bulbs on the beams heads. Also no data were found on wires post-tensioning sequence, if executed before or after the connection of the slab to the beans. Therefore for that, assumptions had to be made for the model of the spans, to predict its behavior while weakening the Uniaxial compression structure to prepare it for blasting. Mechanical characterization Actual values of the geometrical and mechanical parameters of the the bridge were defined to permit 3D modeling of the two acceptors “bridge-to-beweakened-for-blasting” and “bridge-to-be-left-in-place”. Prospecting campaign and tests took a month. One hundred ten samples of concrete were cored, at least two in each beam. This to permit execution of 340 tests and define values of elastic and ultimate mechanical resistance parameters of each single beam.
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Tests were including unit-weight, uniaxial and triaxial compression, static elastic module (according to the norm EN 12390-13), ultrasound (for the computation of the tangent dynamic elastic modulus according to the norm EN12504-4), tensile strength. A minimum value of uniaxial compressive strength (UCS) of 30 MPa was found. This value is low, when compared with minimum foreseen by law which is of 45MPa. The higher UCS was 46MPa. For all the spans, the minimum UCS was equal to the mean value less the standard deviation value. The standard deviation value reached 30% of the average value and never falls below 11%. This proves a high variation of the resistance in the beams of the same span. Average tensile strength values from "Brazilian" test were found generally below 10% of UCS. Triaxial compression strength (confining pressure equal to 4 MPS) were some 40% higher than UCS (from 57 to 77 MPa).
SPAN 1 SPAN 2 SPAN 3 SPAN 4 SPAN 5 SPAN 6 SPAN 7 SPAN 8 SPAN 9 SPAN 10 SPAN 12 SPAN 15 SPAN 16 SPAN 17 SPAN 18 SPAN 19 SPAN 20
Unit weight [Kg/m³]
strength [MPa]
2.378 2.411 2.412 2.402 2.409 2.379 2.404 2.397 2.423 2.404 2.414 2.331 2.381 2.412 2.387 2.394 2.394
32,1 33,0 40,7 45,9 31,9 36,6 38,4 51,8 37,0 38,3 30,9 44,8
35,9 33,0 36,0 41,8 35,0
MINIMUM VALUE AMONG THE 17 SPANS min AVERAGE = 2.322 max STD DEV = 63 min AVERAGE - STD DEV = 2.266 min MIN = min AVG max STD DEV = max AVG min STD DEV =
2.232
38,9 14,1 31,8
30,9
24,8
80%
2.387
MAXIMUM VALUE AMONG THE SPANS max AVERAGE = 2.401 15 min STD DEV = max AVERAGE - STD DEV =2.377 max MIN = 2.374 max AVG min STD DEV = min AVG max STD DEV =
58,5 3,4 53,6 51,8
150% 24% 168% 168%
55,0
222%
2.259
AVERAGE VALUE AMONG THE SPANS AVERAGE = STD DEV = AVERAGE - STD DEV = MIN = average AVERAGE average STD DEV =
2.369 30 2.342 2.334
47,9 9,0 39,1 37,9
2.339
39,0
157%
“Applied elements model� of the structure to be demolished and of that to be left in place The bridge was modeled and analyzed in 2 demolition scenarios involving the drilling of holes for placement of explosives and cutting of slab and transverse beams for weakening and separation of each single beam before the to blast, to permit their removal in sequence. These scenarios were defined in the search for the best compromise among practicability and safe execution for personnel, and no risk of transverse overturning of the bridge for the sudden release of weight due to the demolition of the one carriageway. The Applied Element Method was used, through its Extreme Loading for Structures (ELS) program. Output included reports and movies of the stresses and oscillation to define the behavior of the super-structure during the preliminary drilling and mechanical cutting and the behavior of the supporting pylons demolition process, based on the data found in the surveys and testing. Analysis results showed how the bridge would have sustained the weakening foreseen to prepare it to the blast, without failure, for both scenarios.
Both scenarios showed no effect on the pylons stability with reference to transverse overturning Maximum transverse dynamic displacement was found in the range of some centimeters. Risks of overturning by sudden release of the weight of the whole one carriageway, with the one remaining in place not being counterbalanced by the new one, could be excluded.
No tensile stresses developed below the foundation confirmed that no uplift due to the sudden removal of the bridge span was to be expected.
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The Maximum compressive stresses in the vertical direction below the foundations were computed.
Execution Due to reduced safety margins resulting from the A.E.M. model, maximum care was given to accuracy in execution of cuts and holes to prepare the structure to the blast. This to maintain execution as much as possible matching the model and minimize risks of occurrence of a sudden collapse. To eliminate any risk, the personnel was secured against the hazard of an accidental collapse of the span, by means of a safety line fixed at the pylons and procedure based activities with remote controlled equipment.
pylon
160
160 160 160
160 160 160 160
100100 100100
160 160
160
32 holes and cuts alignments
160 160 160 160
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160
160 160 160
Spans falling down from the lower pylons, due to reduced impact speed, remained in pieces tied together by reinforcement steel, juxtaposed to the slope contour.
160
The spans from the higher pylons impacted the ground with a speed in the range of 20 km/h and higher, and disintegrated totally.
160 160 160
For this purpose drilling and preliminary slab demolitions were extended to the whole span, for a widespread fragmentation with the explosive.
100100100
The spans, after having been drawn down, had to remain stable on the steep slope. This to prevent sliding away down, with consequent difficult access for the excavators for the completion of the demolition with breaker and jaws crunch, and also for mucking.
demolition of transverse beams and slab with a radio controlled excavator personnel secured to a safety line fixed to the puylons
pylon
drilling with a radio controlled rig personnel secured to a safety line fixed to the puylons
firing sequnce
spans being demolished, laying juxtaposed to the steep slope
Displacement and seismic monitoring Achievement - costs and time minimization By controlled blasting with remote controlled equipment, it was possible to complete in 2 months the demolition of the bridge Pietrastretta in alternative to its deconstruction by means of a launching girder (this would have taken 8 months instead). This last technique, due to the reduced resistance of the structural components of the bridge, proved to be unsafe and had to be abandoned Controlled blasting allowed not just best safety conditions for personnel due to hazards and risks minimization but, at the same time costs and execution time minimization. This successful result was achieved thanks to a stringent approach with accurate and extensive surveys, efficient modeling, competence and experience in designing as well as precise execution. Impact induced in the nearby receptors such as below standing pylons and the by side standing spans, as well as the housing in the near, were within thresholds such to guarantee at 100% nooccurrence-of-damage. Seismic and strain monitoring, also executed extensively, confirmed the previsions at the design stage.
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Continuous displacement and seismic monitoring helped to confirm previsions of the model proving that safety limits according to the norms and state of the art, were not exceeded.