LAUNCHING OF THE NEW SAN CRISTOBAL BRIDGE.pdf

7/25/2019 LAUNCHING OF THE NEW SAN CRISTOBAL BRIDGE.pdf

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LAUNCHING

OF THE

NEW SAN

CRISTOBAL

BRIDGE

MARWAN NADER

RAFAEL MANZANAREZ

JACK LOPEZ-JARA

CARLOS DE LA MORA

BIOGRAPHY

Marwan Nader is an Associate

Vice President at T.Y. Lin

International in San Francisco,

CA. Born in 1962, he receivedhis Ph.D. from the University

of California at Berkeley. He

has worked on the design and

construction of bridges for

over 18 years.

Rafael Manzanarez is a Vice

President and Project Manager

at T.Y. Lin International inSan Francisco, CA. Mr.

Manzanarez has more than 25

years of experience in bridge

design and retrofit.

Jack Lopez-Jara is a Senior

Project Engineer at T.Y. Lin

International in San Francisco,

CA. Mr. Lopez-Jara has been

involved in the design and

construction of several

suspension and cable-stayed

bridges, with a special

emphasis on seismic analysis

and design.

Carlos de la Mora is Director

of Construction at ICA,

Mexicos largest construction

company. Mr. De la Mora has

more than 15 years of

experience managing the

construction of complex

infrastructure projects.

SUMMARY

The San Cristobal Bridge in

Chiapas, Mexico is a 3-span

(235'591'235') curved steel

composite and orthotropic boxgirder erected by incremental

launching. The following

paper discusses the lessons

learned from the collapse

during launching of this

bridge, as well as the re-design

and re-launching of the new

bridge. T.Y. Lin International

was hired to investigate the

cause of the collapse of the

original bridge, check the

redesign of the bridge and

perform the erection

engineering of the bridge.


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LAUNCHING OF THE NEW SAN CRISTOBAL BRIDGE

Marwan NaderRafael Manzanarez

Jack Lopez-Jara

Carlos de la Mora

Description of The San Cristobal Bridge

The San Cristobal Bridge is part of a new highway connecting the cities of Tuxtla-Gutierrez and San

Cristobal in the State of Chiapas, Mexico. The bridge crosses a deep canyon with a 3-span, continuous curved

box girder supported on two intermediate piers and two end abutments. The total length of the bridge is

1,060 ft (591 ft main span and two 235 ft side spans).

Figures 1 and 2 show elevation and plan

views of the San Cristobal Bridge.

Due to the steep topography of the site, the

designer considered the incrementallaunching of segments from both abutments

to reach first the piers, and then to connect

both ends at midspan.

The superstructure is an unconventional mix

of post-tensioned composite (steel/concrete)

and orthotropic-steel deck sections. The

central portion of the main span (Segments

0105) is formed by orthotropic segments,

whereas the rest of the main span and the

side spans (Segments 0614) are comprised

of composite sections. Figures 3 and 4 show

a typical composite segment, and a typicalorthotropic-steel segment, respectively.

The Designers selection of light steel o

Fi ure 1 San Cristobal Brid e - Elevation

rthotropic segments for the central segments of the main span, and

heavier steel-concrete composite segments for the back segments was intended to reduce the weight of the

launched cantilevers and to provide heavier back segments that would prevent the overturning of the bridge

during launching.

Figure 2 San Cristobal Bridge Plan View

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Figure 3 Typical Composite Section

This solution induces large negative moments at composite segments 07, 08 and 09 in the zones adjacent to

the piers. Deck post-tensioning was provided to resist those negative moments and to prevent tensile stresses

in the concrete slab.

Collapse of the Tuxtla Gutierrez Structure

The construction of the San Cristobal Bridge started in early 2003. On October 26, 2004, soon after all

segments had been launched, but before the closure of the bridge, the original structure on the Tuxtla-

Gutierrez side collapsed, (See Figure 5).

The Secretaria de Comunicaciones y Transportes de Mexico (SCT) hired a new contractor (Ingenieros Civiles

Asociados ICA) to rebuild the structure. Their work involved the fabrication and launching of the entire

bridge and rebuilding of Pier-2 on the Tuxtla Side, and the retracting, retrofitting, and relaunching of the

bridge on the San Cristobal Side.

ICA hired T.Y. Lin International (TYLI) to

perform the following tasks:

Investigate the Collapse of the

Original Structure

Check the Re-Design of the Bridge

Figure 4 Typical Orthotropic Section

Figure 5 Collapsed Structure on the Tuxtla-Gutierrez

Perform the Erection Engineering of

the New Structure

The principal cause of the collapse was later

identified as failure of the shear connectors in

the concrete slab. This failure induced the loss

of composite action and the collapse of the

structure due to the lack of redundancy of thecantilever system. The shear failure of the

shear studs was caused by their insufficient

number and due to poor workmanship of the

welds connecting them to the top flanges of

the steel box girder, (See Figure 6).

A site evaluation of the collapsed structure

found several additional problems:

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Significant delamination of concrete slab in Tuxtla-

Gutierrez side.

Figure 6 TOP: Failed shear studs onTuxtla side of bridge, BOTTOM:

Existing shear studs on San Cristobal

Significant cracking of the concrete slabs on both the

Tuxtla-Gutierrez and San Cristobal sides.

Signs of local buckling during launching in the lower

web panels, especially on Segments 6 and 11.

Significant damage on Pier 2 due to impact of the

collapsed superstructure onto the pier.

Further evaluation of the design found the following:

Overstress in the bottom flanges of the box girders

due to inadequate spacing and sizing of longitudinal

stiffeners.

Insufficient post-tensioning in the concrete slab to

prevent cracking and tension during launching.

Insufficient concrete slab strength.

Changes To The Original Design And

Construction Sequence

For the construction of the Tuxtla-Gutierrez side of the bridge and the retrofit of the superstructure on the San

Cristobal Side, the Designer implemented several changes in the details and in the construction sequence.

Changes to the Top Concrete Slab: Addition of Shear Studs and PT Tendons, Increase

in Concrete Slab Strength

The new details provided additional shear studs (almost double the original number) and required a higher

level of quality control for the welds connecting the studs to the top flange.

The original design had the deficiency ofplacing many of the shear studs required

to provide composite action of the main

girders on top of a small I-Beam located in

the middle of the concrete slab instead of

placing them directly on top of the top

flange of the girders where they would

have be more effective.

Figure 7 Existing and Additional Shear Studs

Additional shear studs (See Figure 7) were

welded over the girder top flanges to

guarantee full composite action during

launching. Also, additional PT Tendonswere added to reduce the tensile stresses

in the concrete slab and prevent cracking

during launching.

Cracking of the concrete slab during launching of a composite cantilever has negative consequences since it

causes a change in the section properties, moves down the centroid of the section and causes redistribution in

the state of stresses and deformations in the structure.

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Changes to the Bottom Flange: Additional Plate Stiffeners

An evaluation of the details of the original design found deficiencies in the dimensioning of the longitudinal

and transverse stiffeners on the steel bottom flange.

Additional plates and angles were added to

prevent local buckling and to ensure adequate

capacity of the bottom flange in compressionzones. Figure 8 shows a typical stiffened

compression panel with the various forms of local

and global buckling.

The AASHTO Specifications (3) provide

guidelines for the dimensioning of compression

flanges in Composite Box Girders with both

longitudinal and transverse stiffeners (AASHTO

10.39.4.4). The intention of the Specifications is to

prevent local buckling of the flange plates or local

stiffener buckling, and to allow the panels to

develop their capacity without buckling inbetween stiffeners.

Several changes had to be implemented in the

design of the compression flanges to satisfy the

AASHTO Specifications and to limit the

compressive stresses to the allowable limits

permitted by the code.

In many situations, it was not possible (or

practical) to fully implement the provisions of the

AASHTO specifications, especially on segments that had already been fabricated and in zones in the deck

where accessibility was an issue. For those situations, it was necessary to perform detailed finite element

analyses to more accurately calculate the capacity of the panels and evaluate the possibility of local or global

buckling. Figure 9 shows a FEM model of a bottom panel with its corresponding buckling mode shape.

Figure 8 Buckling of Box Girder Compression

Changes to the Web Plates:

Reinforcement and Stiffening of Web

Panels

Figure 9 Buckling Analysis of Bottom Flange

Several Failure modes control the design of the

webs on steel- plate girder bridges that are

constructed using incremental launching.

Figure 10 show some of the predominant

failure modes.

An evaluation of the web panels of the San

Cristobal Bridge found that the stiffeners

provided in the original design were not

adequate to resist the bearing forces during the

launching of the segments. Signs of local

yielding were already observed during the

launching of the original structures. This

prompted the designer to add triangular web

stiffeners on the lower web panels. A potential

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stability problem due to web panel

buckling was also identified. This problem

becomes more evident on Segments 10

and 11 where the web panels used

relatively thin 13mm plates.

Figure 10 Local Failure Modes above Launching Bearings

(2)

Finite Element Models of entire webpanels were developed to study these

problems. Figures 11 and 12 show FEM

models of Segment 11 with and without

the addition of longer triangular web

stiffeners. The study shows that the

inclusion of those stiffeners limits the

extent of local yielding and reduces the

potential of web instability by increasing

the bearing reaction required for panel

buckling and shifting the stability problem

to the upper web area at much higher

loads.

Figure 11 Web Panel Buckling Mode Segment

11 Original Design

Figure 12 Web Panel Buckling Mode Segment

11 Re-Design

Changes to the Original Construction Sequence

Tuxtla-Gutierrez Side

A completely new structure was fabricated and launched incorporating all the recommended changes to the

shear studs, slab post-tensioning and additional stiffeners to the bottom flange and web plates.

The construction sequence resembled the original design with some changes made in the sequence of

application of post-tensioning forces.

The new structure was fabricated and assembled behind Abutment-01 and then launched towards the pier and

mid-span. Given the space limitations in the launching platform, the assembly and launching operations had

to be performed simultaneously.

As the launching progressed and more space was available in the launching platform, more segments were

assembled behind the abutments and prepared for launching.

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The PT force and sequence were modified to provide for a larger compression force in the concrete slab

before the full cantilever is launched and the deck reaches the position of maximum negative moment over the

pier.

San Cristobal Side

After the collapse of the original structure on the Tuxtla-Gutierrez side, the structure on the San Cristobal side

was pulled back with Segment 01 resting on top of Pier-03.

Due to restrictions and space limitations at the job site, it was impractical to completely pull back all 14

segments to perform the retrofit/strengthening work. The original design considered a maximum of 6

segments behind the abutment during launching, with additional segments to be assembled as the launching

pushed forward.

With the superstructure supported on the pier, abutment, and temporary supports behind the abutment, the

concrete slab (with all PT tendons de-stressed) was completely removed and replaced including additional

shear studs. The steel box was also reinforced with additional stiffeners in the bottom flange and lower web

panels.

The change in the slab casting sequence on the San Cristobal side caused a different stress distribution

compared to the structure on the Tuxtla side. The new construction sequence caused the weight of the

concrete slab on Segments 04, 05 and 06 to be carried by the steel girder only instead of the entire composite

section as in the case of the Tuxtla-Gutierrez Structure.

A reduction in the post-tensioning was necessary to prevent generating large compressive stresses in the

concrete slab in the back span where positive bending moments were expected during the service load of the

structure.

Figures 13 and 14 show the new construction sequences for the Tuxtla-Gutierrez and San Cristobal structures.

Fi ure 13 New Construction Se uence Phases I - VIII

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Stage Construction Analysis

Figure 14 New Construction Sequence Phases IX - XVI

A SAP2000 computer model was created to calculate the forces and deflections in the structure during the

launching of the bridge. Three-dimensional frame elements were used to model the composite and orthotropic

deck segments as a spine consistent with the actual geometry and alignment of the bridge. Tendon elements

were used to model the post-tensioning effects in the deck.

The actual modeling of the launching of the segments was accomplished with the use of non-linear springs to

simulate the support of the structure on temporary bearings behind the abutments and on top of the piers.

Since no vertical restraints were provided on those bearings, the non-linear springs had to allow uplift of the

structure under certain construction stages.

To simulate the movement of the superstructure on the launching platform, the non-linear springs were added

or removed accordingly to reflect the boundary conditions at each launching stage.

The force demands from the construction stage analysis were used to do a stress check of the structure at eachconstruction stage according to the Project Design Criteria.

Analytical Studies and Field Measurements

The AASHTO Specifications (3) indicate the use of the moment of inertia of the gross cross-sectional area to

compute the deflections of composite beams and girders (AASHTO 10.6.5), and the use of effective slab

widths for the calculation forces and stresses in the section (AASHTO 10.38.3).

Recent studies (4) also suggest the use of the full effective width for the calculation of both deflections and

stresses in the cross section.

Nevertheless some divergence was observed between the analytical results and the field measurements.

Therefore there was a need to calibrate the effective inertia and effective slab width of the section in order to

match the observed deflections with the analytical results.

The difference can be attributed in the particular case of the San Cristobal Bridge to the more predominant

effect of shear lag and differences in the assumed full-effective-width and actual-effective-slab-width on the

composite segments with a pre-stressed concrete slab.

After the initial launching of the structures a reduction factor to the gross inertia of about 20% was used to

calibrate the results. This reduction factor is consistent with the ratio of the gross inertia to the inertia of the

section considering an effective slab width equal to 12x the slab thickness. With this adjustment it was

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possible to achieve a good match between

the analytical results and the field

measurements deflections during the

remainder of the launching operations.

Figure 15 Comparison of Calculated and Field

Deflections during Launching

A comparison of the deflections calculated

using the SAP2000 model, and the fieldmeasurement was also performed. Figure

15 shows a plot of the tip cantilever

deflections during launching for both the

Tuxtla and San Cristobal ends.

A discrepancy on the deflections of the

of the

the canyon is an economical

San Cristobal side was observed when the

full cantilever was launched (Full

launching of the superstructure but before

jacking of tendons C and D Stage 22 in

Figure 15). This discrepancy may be

explained by the occurrence of some

minor cracking on the concrete slab,which has the potential of causing a

reduction on the effective inertia

introducing larger deflections than the

ones estimated with the computer model.

The difference in the elevation of the

cantilever ends before closure was

corrected by raising the abutment supportsand introducing a rigid body motion of the

Tuxtla-Gutierrez structure. The advantage

of this adjustment procedure is that no

permanent or residual forces were induced

in the structure and the behavior and stressdistribution in the bridges was not

affected. Figure 16 shows a picture

completed structure after closure.

Summary And Conclusions

Due to the restrictions imposed by the

steep topography of the site, erecting the

San Cristobal Bridge using incremental

launching of the superstructure from both

sides of

solution.The selected structural system of the deck

is an unconventional mix of orthotropic

steel segments and composite

(steel/concrete) post-tensioned box girder

segments. The ratio of the main span

length of 591 ft to the back span length of

235 ft is larger than 2.5. Overturning and

uplift at the abutments were expected. The

Figure 16 Completed Structure of the New San Cristobal

Bridge July 2006

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use of lighter steel orthotropic segments for midspan and heavier composite segments on the back span was

intended to prevent the overturning and uplift at the abutments, and to provide stability during launching.

This solution involved the launching of composite segments with the concrete deck already cast in place.

Therefore the composite segments adjacent to the pier were subjected to large negative moments.

Longitudinal slab post-tensioning had to be provided to overcome the negative moment and to prevent tension

and cracking of the concrete slab.The collapse of the original structure was primarily caused by inadequate design of the shear connectors

combined with poor workmanship of the welds connecting the shear studs to the top flange. The failure of the

shear connectors caused the loss of composite action and led to a catastrophic failure due to non-redundancy

of the cantilevered box girder.

For the new design with additional shear studs, additional PT tendons and higher strength of the concrete slab,

a difference was noted between the predicted deflections (from computer models) and the values measured in

the field. These differences are attributed to shear lag effects and a discrepancy with the assumptions made for

the effective slab width of the concrete slab.

Corrections had to be made to calibrate the effective stiffness of the segments to the actual effective slab

width after the initial launching. With this calibration it was possible to achieve a good match of the analytical

and field deflections for the remainder of the launching operations.

Nevertheless additional discrepancies were still observed on the San Cristobal side when the full cantilever

was fully launched. Minor cracking in the concrete slab caused a change in the effective inertia of thesegments and resulted in an increase in the cantilever deflections.

It is concluded that the launching of a composite section with slab post-tensioning is not a practical solution

due to the complexities and uncertainties in the actual stress distribution and effective width of the slab. Even

with a careful analysis and control of the loads some cracking in the slab was experienced, inducing larger

deflections than the ones predicted by the structural analyses.

Due to the configuration of the bridge, it was possible to correct the discrepancy in the deflections on the

Tuxtla-Gutierrez and San Cristobal sides by raising the abutment supports at the Tuxtla side, inducing rigid

body rotations of the deck to match the tip elevations at both ends.

The behavior of longitudinally post-tensioned composite decks needs to be further investigated in order to

better understand the behavior of such deck segments under negative moments and the extents of the effective

slab width.

References

1. Galambos, T.V. Guide to Stability Design Criteria for Metal Structures.John Wiley and Sons, Inc., New

York, 1998.

2. Rosignoli, M.Bridge Launching. Thomas Telford Ltd., London, 2002.

3. AASHTO, Standard Specifications for Highway Bridges, Washington DC, 2002.

4. Chen, S.S. et al. NCHRP Report 543 Effective Slab Width for Composite Steel Bridge Members,

Transportation Research Board, Washington D.C. 2005.

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LAUNCHING OF THE NEW SAN CRISTOBAL BRIDGE.pdf