- Email: [email protected]
An alternative technique to the demolition of a prestressed concrete box-girder bridge: A case study
Accepted Manuscript Title: An alternative technique to the demolition of a prestressed concrete box-girder bridge: a case study Author: S.S.R. Pereira M.D.C. Magalhaes H. Gazzinelli PII: DOI: Reference:
S2214-5095(16)30037-7 http://dx.doi.org/doi:10.1016/j.cscm.2017.03.003 CSCM 86
To appear in: Received date: Revised date: Accepted date:
3-5-2016 10-3-2017 17-3-2017
Please cite this article as: Pereira SSR, Magalhaes MDC, Gazzinelli H, An alternative technique to the demolition of a prestressed concrete boxgirder bridge: a case study, Case Studies in Construction Materials (2017), http://dx.doi.org/10.1016/j.cscm.2017.03.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An alternative technique to the demolition of a prestressed concrete boxgirder bridge: a case study
a
Departamento de Engenharia de Estruturas, Universidade Federal de Minas Gerais (UFMG)
cr
Av. Antônio Carlos 6627, Belo Horizonte, CEP: 31270-901, Brazil. Ceprol Consultoria e Engenharia de Projetos Ltda.
us
b
ip t
S.S.R. Pereiraa, M.D.C. Magalhaesa* and H. Gazzinellib
an
Av. Alvares Cabral 593, Belo Horizonte, CEP: 30170-912, Brazil.
ABSTRACT: This is a case study in which the partial collapse of a prestressed concrete box-
M
girder bridge in Brazil happened only nine days after removing the supporting scaffolding. It is believed that the actual reinforcement longitudinal steel bars in the pile caps were
d
underestimated. Although only part of the structure had collapsed, it was decided that the
te
whole structure should be demolished. It was claimed that there was not available alternatives
Ac ce p
for 'in situ' structural recovery that would not compromise local traffic and safety precaution procedures. This paper presents an alternative technique for the bridge structural recovery. The application of this technique was possible because the prestressing process used unbonded pos-tensioned concrete, i.e. the sheaths were not filled with grout. The technique was based on the use of a weld torch to cut the tensioned strands in the box-girders methodically, unloading the pillars and foundations. Experimental tests were performed 'in loco' and proved to be effective and safe. The application of this suggested technique 'in situ' is believed to be an original contribution to the knowledge. Keywords: Prestressed concrete box-girder bridge, structure failure, demolition, recovery.
*Corresponding author: Max Magalhaes ([email protected]) Page 1 of 12
1. Introduction Prestressed concrete is usually adequate for the construction of medium and long span bridges. This type of material has found extensive application in the construction of long-span
ip t
bridges. It has gradually been used in place of steel which needs expensive maintenance due to its inherent process of corrosion under aggressive environment conditions. One of the most
cr
commonly used forms of superstructure in concrete bridges is precast girders with cast-in-situ slab. This type of superstructure is generally used for spans between 20 to 40 m. Box-girder
us
bridges are very popular because of their simple geometry, low-cost fabrication, easy erection
an
or casting and relatively low dead loads. [1-3]
The demolition of prestressed concrete structures is hazardous and the experience of
M
most industries is still limited. Professional advice must be obtained from a suitably experienced registered engineer. A demolition plan or method statement is required. The rapid
d
release of the stored energy in the tendons, by removing the surrounding concrete, and/or
te
burning through the tendons, could cause sudden failure. There is also the possibility of the tendon and its anchorage becoming a missile, especially where the tendons were not grouted
Ac ce p
during the original construction. A sandbag screen should always be put around anchors when the post-tensioned prestressed members are demolished. In general, the only safe way to demolish a structure containing prestressed concrete is
to dismantle the structure in the reverse order in which it was originally erected. Some buildings will be straightforward, but special care will be needed in the following circumstances: (a) Continuous structures over more than one support or cantilevered structures; (b) Suspended structures; (c) Structures that had been progressively stressed during construction; (d) Structures made of precast members stressed together once erected; (e) Shells, ring beams, tension ties, stressed tanks. Care must be taken in handling prestressed components. For example, long “slender” beams may become unstable if allowed to tip onto
Page 2 of 12
their sides. In general, prestressed beams should only be supported near their ends. Demolition using “conventional” methods such as balling or concrete breakers may be unsatisfactory due to the possibility of an uncontrolled collapse, or the sudden release of the
ip t
stressing steel. Ducts for post-tensioned pre-stressing tendons have been known to 'float up' during concreting, causing additional hazards for demolition contractors. It may be necessary
cr
to confirm the location of stressing cables or ducts prior to commencement.
In the next section a brief description of the viaduct design is presented. Next, the
us
collapse of the southern 'handle' of the viaduct is described. After that, an experimental test
an
was used to unload the pillars by cutting-off the box-girder strands. Finally, a brief analyses and discussion is presented on the design and construction techniques which might have led to
M
the viaduct structural failure. Some conclusions are also presented.
d
2. The viaduct design scheme: a brief description
te
The viaduct was composed of two 'handles', namely southern and northern handles which have similar geometry and two spans. Each one has a length equal to 77.5 meters. The
Ac ce p
northern part comprised pillars P1, P2, and P5 and the southern part pillars P2, P3 and P4. The central pillar P3 in the southern part was the one that collapsed. This P3 pillar supported two spans of 77.5 m on each side. The box-girder cross-sections were composed of three-cells and variable heights (see Fig. 1). Each cross-section includes two traffic lanes. The cross-section of P3 had an area equal to 200 x 200 cm at the joint with the
foundation block. Pillar P3 was linked to a group of ten cast ‘in situ’ piles (diameter equal to 80 cm) by a concrete cap with dimensions equal to 930 x 430 x 200 cm.
Page 3 of 12
ip t cr
us
Fig. 1. Cross section of the box girder bridge after demolition.
an
3. The collapse of P3 in the 'southern handle' of the viaduct
The collapse occurred in the central pillar of the south part (pillar P3), nine days after
M
removing the entire scaffolding system. The actual weight of the viaduct was not supported by the foundation. Its collapse occurred during the rush hour.
d
The pile cap was abruptly ruptured at the time that the first crack was formed, i.e. nine
te
days after the scaffolding was removed.
Before the formation of the first crack the pile cap was stiff enough to transmit the
Ac ce p
total loading to all 10 piles uniformly (Fig. 2a). On the formation of the first crack in the cap, the longitudinal reinforcement steel bars in the block had a total area much lower than the minimum necessary (Fig. 2b). As a result these two piles were stuck in the soil by the boxbeam slab which worked as a huge pile-driver (Fig.s 3-4).
Page 4 of 12
ip t cr b)
us
a)
Fig. 2.Sketch showing foundation piles, caps (blocks) and pillars; a) block before
Ac ce p
te
d
M
an
forming the first crack; b) block after forming the first cracks
Fig. 3.Sketch showing the foundation block rupture and sinking of pillar P3
Page 5 of 12
ip t cr b)
us
a)
an
Fig. 4. These photos show the foundation block ruptured and pillar P3 stuck in the
4. Experimental tests 'in situ'
M
ground. a) side view; b) front view
d
As mentioned before, the longitudinal steel at the bottom of the concrete cap might
te
have been underestimated. On the other hand, the concrete pile cap which supported pillar P5
Ac ce p
(located on the north part of the viaduct) did not collapse even though it might have had the same structural project. It was because the slab support system at this part of the viaduct had not been removed yet. As a result, all pillars on the structure ‘northern handle’ were demolished for precaution seventy days after the collapse of the ‘southern handle’. After the pillars demolition by blasting the experimental tests were performed 'in situ',
in 'the girder-box'. The sheath ends were exposed by removing the concrete using pneumatic hammers. The tests were performed by melting the steel wire strands using a welding torch (Fig. 5). In this way the strands were broken and ejected in the opposite end (Fig. 6). This operation is simply, quick, cheap and safe for applying the reverse prestressing process.
Page 6 of 12
ip t cr us
Fig. 5. Photo showing one end of the box-girder where the steel strands were melted
Ac ce p
te
d
M
an
'in situ' using a weld torch.
Fig. 6. Photo showing the opposite end of the box-girder where the steel strands were ejected.
5. Analyses and discussion
After preliminary analyses it was concluded that the reason for the collapse may have been the use of insufficient longitudinal steel bars in the pile cap of pillar P3. The longitudinal reinforcement bars in the concrete cap comprised a total of 16 bars. Each bar had a diameter ø equal to 16mm and the reinforcement ratio in the block was equal to 0.037%. This ratio is
Page 7 of 12
much lower than the minimum value necessary in the ultimate strength and also lower than that considered as a minimum value for bending of reinforced concrete beams. Considering the lower and upper limits of the concrete tensile strength and assuming
ip t
the load P acting on the top of the pillar is centered, the range of the load variation which could lead to the first crack (positioned at a distance of 1.6m from the gravity center of the
cr
block) and consequently to failure was 14,370kN
te
d
M
an
us
of concrete (fck) used was 30 MPa.
Fig. 7. Equal reaction forces on each pile emerge as a load P is acting in the gravity centre
Ac ce p
of the pillar. No crack is present in the concrete cap (block).
The load acting on the top of the pillar after removing the scaffolding of the southern
‘handle’ of the viaduct was estimated on 17,750 kN. At first glance, this load was incapable of provoking the appearance of the first crack (Fig. 7). Due to the ‘long term’ effect of maintained load, known of ‘Rüsch’ effect, nine days after the application of load ‘P’ on the pillar top the foundation block cracked and failure abruptly. At this time load ‘P’ was entirely transmitted to the two central piles that were stuck in the soil by the concrete gear-box (Figs. 3 and 4).
Page 8 of 12
As mentioned previously, the northern part of the viaduct had the same structural project. However, this part of the structure did not collapsed and remained stable for seventy days after the collapse of the southern part. It is believed that the decision for recovering the
ip t
northern part could have been possible by removing partially or totally the transmitted loading to the central pillar.
d
M
a)
an
us
cr
Fig. 8 below illustrates the types of loading actuating on the structure.
Ac ce p
te
b)
c)
Fig. 8.Sketch showing scaffolding system and loads being transferred to the pillars
during the prestressing process; a) Before applying pre-tension on the cables (the pillars only support their own weight); b) After prestressing some of the cables (the pillars are also loaded with part of the box-girder weight); c) all cables on the box-girder are tensioned (loading: weight of the box-girder plus their own weight).
Page 9 of 12
It is believed that the concrete block of pillar P5 could have been collapsed without warning. This fact was crucial on making the decision for not allowing workers on the viaduct. Among various ways of reducing the load on the central pillar the release of the
ip t
prestressing forces on the cables might have been one of them. Doing that, the greatest part of the box-girder slab weight would have been supported by the scaffolding, that was set-up
cr
again after the collapse of the northern handle. In other words, this would be the reverse
te
d
M
an
us
process of prestressing.
Ac ce p
Fig. 9. Picture showing several holes on a particular pillar for explosives allocation
Several holes were made on the bridge pillars for allocation of explosives (see details
on Fig. 9). As the pillars were demolished the box-girder bridge went collapsed. Before the demolition of the bridge pillars, the supporting scaffolding that was repositioned after the collapse of the southern part was removed. After that, a hydraulic hammer and a diamond wire cutting machine were used for the demolition completion.
Page 10 of 12
6. Conclusions Investigations into the collapse indicated that the steel reinforcement bars used on the foundation of pillar P3 was underestimated.
ip t
The performance of the experimental tests showed that the operation of releasing the steel strands, which can be seen as the application of the reverse method of prestressing, by
cr
melting the steel strands inside the sheath did work very well. It is believed that the application of this technique 'in situ' is an original contribution and an alternative to
us
demolition. It is a cheap, safe and rapid way. In addition, safety conditions could have been
an
reached for the northern part of the viaduct within a short period of time (less than 70 days needed to implode and demolish the viaduct) and the traffic lanes released during the process
M
of the structure recovery. The experimental tests were a breakthrough in non-destructive tests for 'in situ' recovery of prestressed structures.
d
Usually, the set-up preparation of demolition using blasting requires a few hundreds of
te
workers under the bridge structure. The procedure is certainly much more risky in terms of the safety of workers than that related to the structural recovery. Besides, the cost involved in
Ac ce p
the demolition process is much higher than that needed for the structure recovery. Whereas the cost of reconstruction must be added on the demolition process, the overall cost is usually much higher.
Acknowledgments
The authors gratefully acknowledge the support provided by the Federal University of Minas Gerais (UFMG) and the National Counsel of Technological and Scientific Development (Cnpq).
Page 11 of 12
References [1]
Yang Y, Sneed LH, Morgan A, Saiidi MS and Belardi A. Behaviour of instrumented prestressed high performance concrete bridge girders. Construction and Building
ip t
Materials 2010; 24(11):2294–2311. [2]
Choi YC and Oh BH A. Transverse modeling of concrete box-girder for prediction of
cr
deck slab ultimate load capacity. Journal of Bridge Engineering 2013; 10:1373–1382. [3]
S.S.R., Pereira. Desprotensão de um cabo de 27 cordoalhas de 15,2mm com as
Ac ce p
te
·
The physical insight into the behavior of the prestressed systems used in the bridge; Experimental tests presented herein considers cutting-off the strands 'in situ' for the release of pillar loadings before commencing the recovery procedure; These experimental tests are a breakthrough in non-destructive tests for 'in situ' recovery of prestressed structures.
d
· ·
M
an
us
extremidades já cortadas. YOUTUBE 2014; 50:540–548. (in Portuguese)
Page 12 of 12
S2214-5095(16)30037-7 http://dx.doi.org/doi:10.1016/j.cscm.2017.03.003 CSCM 86
To appear in: Received date: Revised date: Accepted date:
3-5-2016 10-3-2017 17-3-2017
Please cite this article as: Pereira SSR, Magalhaes MDC, Gazzinelli H, An alternative technique to the demolition of a prestressed concrete boxgirder bridge: a case study, Case Studies in Construction Materials (2017), http://dx.doi.org/10.1016/j.cscm.2017.03.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An alternative technique to the demolition of a prestressed concrete boxgirder bridge: a case study
a
Departamento de Engenharia de Estruturas, Universidade Federal de Minas Gerais (UFMG)
cr
Av. Antônio Carlos 6627, Belo Horizonte, CEP: 31270-901, Brazil. Ceprol Consultoria e Engenharia de Projetos Ltda.
us
b
ip t
S.S.R. Pereiraa, M.D.C. Magalhaesa* and H. Gazzinellib
an
Av. Alvares Cabral 593, Belo Horizonte, CEP: 30170-912, Brazil.
ABSTRACT: This is a case study in which the partial collapse of a prestressed concrete box-
M
girder bridge in Brazil happened only nine days after removing the supporting scaffolding. It is believed that the actual reinforcement longitudinal steel bars in the pile caps were
d
underestimated. Although only part of the structure had collapsed, it was decided that the
te
whole structure should be demolished. It was claimed that there was not available alternatives
Ac ce p
for 'in situ' structural recovery that would not compromise local traffic and safety precaution procedures. This paper presents an alternative technique for the bridge structural recovery. The application of this technique was possible because the prestressing process used unbonded pos-tensioned concrete, i.e. the sheaths were not filled with grout. The technique was based on the use of a weld torch to cut the tensioned strands in the box-girders methodically, unloading the pillars and foundations. Experimental tests were performed 'in loco' and proved to be effective and safe. The application of this suggested technique 'in situ' is believed to be an original contribution to the knowledge. Keywords: Prestressed concrete box-girder bridge, structure failure, demolition, recovery.
*Corresponding author: Max Magalhaes ([email protected]) Page 1 of 12
1. Introduction Prestressed concrete is usually adequate for the construction of medium and long span bridges. This type of material has found extensive application in the construction of long-span
ip t
bridges. It has gradually been used in place of steel which needs expensive maintenance due to its inherent process of corrosion under aggressive environment conditions. One of the most
cr
commonly used forms of superstructure in concrete bridges is precast girders with cast-in-situ slab. This type of superstructure is generally used for spans between 20 to 40 m. Box-girder
us
bridges are very popular because of their simple geometry, low-cost fabrication, easy erection
an
or casting and relatively low dead loads. [1-3]
The demolition of prestressed concrete structures is hazardous and the experience of
M
most industries is still limited. Professional advice must be obtained from a suitably experienced registered engineer. A demolition plan or method statement is required. The rapid
d
release of the stored energy in the tendons, by removing the surrounding concrete, and/or
te
burning through the tendons, could cause sudden failure. There is also the possibility of the tendon and its anchorage becoming a missile, especially where the tendons were not grouted
Ac ce p
during the original construction. A sandbag screen should always be put around anchors when the post-tensioned prestressed members are demolished. In general, the only safe way to demolish a structure containing prestressed concrete is
to dismantle the structure in the reverse order in which it was originally erected. Some buildings will be straightforward, but special care will be needed in the following circumstances: (a) Continuous structures over more than one support or cantilevered structures; (b) Suspended structures; (c) Structures that had been progressively stressed during construction; (d) Structures made of precast members stressed together once erected; (e) Shells, ring beams, tension ties, stressed tanks. Care must be taken in handling prestressed components. For example, long “slender” beams may become unstable if allowed to tip onto
Page 2 of 12
their sides. In general, prestressed beams should only be supported near their ends. Demolition using “conventional” methods such as balling or concrete breakers may be unsatisfactory due to the possibility of an uncontrolled collapse, or the sudden release of the
ip t
stressing steel. Ducts for post-tensioned pre-stressing tendons have been known to 'float up' during concreting, causing additional hazards for demolition contractors. It may be necessary
cr
to confirm the location of stressing cables or ducts prior to commencement.
In the next section a brief description of the viaduct design is presented. Next, the
us
collapse of the southern 'handle' of the viaduct is described. After that, an experimental test
an
was used to unload the pillars by cutting-off the box-girder strands. Finally, a brief analyses and discussion is presented on the design and construction techniques which might have led to
M
the viaduct structural failure. Some conclusions are also presented.
d
2. The viaduct design scheme: a brief description
te
The viaduct was composed of two 'handles', namely southern and northern handles which have similar geometry and two spans. Each one has a length equal to 77.5 meters. The
Ac ce p
northern part comprised pillars P1, P2, and P5 and the southern part pillars P2, P3 and P4. The central pillar P3 in the southern part was the one that collapsed. This P3 pillar supported two spans of 77.5 m on each side. The box-girder cross-sections were composed of three-cells and variable heights (see Fig. 1). Each cross-section includes two traffic lanes. The cross-section of P3 had an area equal to 200 x 200 cm at the joint with the
foundation block. Pillar P3 was linked to a group of ten cast ‘in situ’ piles (diameter equal to 80 cm) by a concrete cap with dimensions equal to 930 x 430 x 200 cm.
Page 3 of 12
ip t cr
us
Fig. 1. Cross section of the box girder bridge after demolition.
an
3. The collapse of P3 in the 'southern handle' of the viaduct
The collapse occurred in the central pillar of the south part (pillar P3), nine days after
M
removing the entire scaffolding system. The actual weight of the viaduct was not supported by the foundation. Its collapse occurred during the rush hour.
d
The pile cap was abruptly ruptured at the time that the first crack was formed, i.e. nine
te
days after the scaffolding was removed.
Before the formation of the first crack the pile cap was stiff enough to transmit the
Ac ce p
total loading to all 10 piles uniformly (Fig. 2a). On the formation of the first crack in the cap, the longitudinal reinforcement steel bars in the block had a total area much lower than the minimum necessary (Fig. 2b). As a result these two piles were stuck in the soil by the boxbeam slab which worked as a huge pile-driver (Fig.s 3-4).
Page 4 of 12
ip t cr b)
us
a)
Fig. 2.Sketch showing foundation piles, caps (blocks) and pillars; a) block before
Ac ce p
te
d
M
an
forming the first crack; b) block after forming the first cracks
Fig. 3.Sketch showing the foundation block rupture and sinking of pillar P3
Page 5 of 12
ip t cr b)
us
a)
an
Fig. 4. These photos show the foundation block ruptured and pillar P3 stuck in the
4. Experimental tests 'in situ'
M
ground. a) side view; b) front view
d
As mentioned before, the longitudinal steel at the bottom of the concrete cap might
te
have been underestimated. On the other hand, the concrete pile cap which supported pillar P5
Ac ce p
(located on the north part of the viaduct) did not collapse even though it might have had the same structural project. It was because the slab support system at this part of the viaduct had not been removed yet. As a result, all pillars on the structure ‘northern handle’ were demolished for precaution seventy days after the collapse of the ‘southern handle’. After the pillars demolition by blasting the experimental tests were performed 'in situ',
in 'the girder-box'. The sheath ends were exposed by removing the concrete using pneumatic hammers. The tests were performed by melting the steel wire strands using a welding torch (Fig. 5). In this way the strands were broken and ejected in the opposite end (Fig. 6). This operation is simply, quick, cheap and safe for applying the reverse prestressing process.
Page 6 of 12
ip t cr us
Fig. 5. Photo showing one end of the box-girder where the steel strands were melted
Ac ce p
te
d
M
an
'in situ' using a weld torch.
Fig. 6. Photo showing the opposite end of the box-girder where the steel strands were ejected.
5. Analyses and discussion
After preliminary analyses it was concluded that the reason for the collapse may have been the use of insufficient longitudinal steel bars in the pile cap of pillar P3. The longitudinal reinforcement bars in the concrete cap comprised a total of 16 bars. Each bar had a diameter ø equal to 16mm and the reinforcement ratio in the block was equal to 0.037%. This ratio is
Page 7 of 12
much lower than the minimum value necessary in the ultimate strength and also lower than that considered as a minimum value for bending of reinforced concrete beams. Considering the lower and upper limits of the concrete tensile strength and assuming
ip t
the load P acting on the top of the pillar is centered, the range of the load variation which could lead to the first crack (positioned at a distance of 1.6m from the gravity center of the
cr
block) and consequently to failure was 14,370kN
te
d
M
an
us
of concrete (fck) used was 30 MPa.
Fig. 7. Equal reaction forces on each pile emerge as a load P is acting in the gravity centre
Ac ce p
of the pillar. No crack is present in the concrete cap (block).
The load acting on the top of the pillar after removing the scaffolding of the southern
‘handle’ of the viaduct was estimated on 17,750 kN. At first glance, this load was incapable of provoking the appearance of the first crack (Fig. 7). Due to the ‘long term’ effect of maintained load, known of ‘Rüsch’ effect, nine days after the application of load ‘P’ on the pillar top the foundation block cracked and failure abruptly. At this time load ‘P’ was entirely transmitted to the two central piles that were stuck in the soil by the concrete gear-box (Figs. 3 and 4).
Page 8 of 12
As mentioned previously, the northern part of the viaduct had the same structural project. However, this part of the structure did not collapsed and remained stable for seventy days after the collapse of the southern part. It is believed that the decision for recovering the
ip t
northern part could have been possible by removing partially or totally the transmitted loading to the central pillar.
d
M
a)
an
us
cr
Fig. 8 below illustrates the types of loading actuating on the structure.
Ac ce p
te
b)
c)
Fig. 8.Sketch showing scaffolding system and loads being transferred to the pillars
during the prestressing process; a) Before applying pre-tension on the cables (the pillars only support their own weight); b) After prestressing some of the cables (the pillars are also loaded with part of the box-girder weight); c) all cables on the box-girder are tensioned (loading: weight of the box-girder plus their own weight).
Page 9 of 12
It is believed that the concrete block of pillar P5 could have been collapsed without warning. This fact was crucial on making the decision for not allowing workers on the viaduct. Among various ways of reducing the load on the central pillar the release of the
ip t
prestressing forces on the cables might have been one of them. Doing that, the greatest part of the box-girder slab weight would have been supported by the scaffolding, that was set-up
cr
again after the collapse of the northern handle. In other words, this would be the reverse
te
d
M
an
us
process of prestressing.
Ac ce p
Fig. 9. Picture showing several holes on a particular pillar for explosives allocation
Several holes were made on the bridge pillars for allocation of explosives (see details
on Fig. 9). As the pillars were demolished the box-girder bridge went collapsed. Before the demolition of the bridge pillars, the supporting scaffolding that was repositioned after the collapse of the southern part was removed. After that, a hydraulic hammer and a diamond wire cutting machine were used for the demolition completion.
Page 10 of 12
6. Conclusions Investigations into the collapse indicated that the steel reinforcement bars used on the foundation of pillar P3 was underestimated.
ip t
The performance of the experimental tests showed that the operation of releasing the steel strands, which can be seen as the application of the reverse method of prestressing, by
cr
melting the steel strands inside the sheath did work very well. It is believed that the application of this technique 'in situ' is an original contribution and an alternative to
us
demolition. It is a cheap, safe and rapid way. In addition, safety conditions could have been
an
reached for the northern part of the viaduct within a short period of time (less than 70 days needed to implode and demolish the viaduct) and the traffic lanes released during the process
M
of the structure recovery. The experimental tests were a breakthrough in non-destructive tests for 'in situ' recovery of prestressed structures.
d
Usually, the set-up preparation of demolition using blasting requires a few hundreds of
te
workers under the bridge structure. The procedure is certainly much more risky in terms of the safety of workers than that related to the structural recovery. Besides, the cost involved in
Ac ce p
the demolition process is much higher than that needed for the structure recovery. Whereas the cost of reconstruction must be added on the demolition process, the overall cost is usually much higher.
Acknowledgments
The authors gratefully acknowledge the support provided by the Federal University of Minas Gerais (UFMG) and the National Counsel of Technological and Scientific Development (Cnpq).
Page 11 of 12
References [1]
Yang Y, Sneed LH, Morgan A, Saiidi MS and Belardi A. Behaviour of instrumented prestressed high performance concrete bridge girders. Construction and Building
ip t
Materials 2010; 24(11):2294–2311. [2]
Choi YC and Oh BH A. Transverse modeling of concrete box-girder for prediction of
cr
deck slab ultimate load capacity. Journal of Bridge Engineering 2013; 10:1373–1382. [3]
S.S.R., Pereira. Desprotensão de um cabo de 27 cordoalhas de 15,2mm com as
Ac ce p
te
·
The physical insight into the behavior of the prestressed systems used in the bridge; Experimental tests presented herein considers cutting-off the strands 'in situ' for the release of pillar loadings before commencing the recovery procedure; These experimental tests are a breakthrough in non-destructive tests for 'in situ' recovery of prestressed structures.
d
· ·
M
an
us
extremidades já cortadas. YOUTUBE 2014; 50:540–548. (in Portuguese)
Page 12 of 12