2003 Lucko DelaGarza Constructability Considerations for Balanced Cantilever

8/11/2019 2003 Lucko DelaGarza Constructability Considerations for Balanced Cantilever

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Constructability Considerations for Balanced Cantilever Construction

By Gunnar Lucko1, Student Member, ASCE, and Jess M. de la Garza2, A.M. ASCE

Abstract

Constructability considerations are of importance in segmental bridge construction as the

occurrence of failures of bridge superstructures under construction have highlighted. For a safe

and economical construction process the interactions between construction loads and the

permanent structural system, depending on the chosen erection method, need to be evaluated.

Segmental bridges can be constructed with methods like Balanced Cantilever Construction,

where individual spans are counterweighted about their substructure support. Time-dependent

material properties like strength of the newly cast concrete, as well as shrinkage, creep, and

relaxation influence the structural system resistance. Resulting stresses in the unfinished bridge

structure during construction can even exceed the final stresses during service.

This paper makes an educational contribution by illustrating these concepts with the case study

of the Wilson Creek Bridge in Virginia. This five-span cast-in-place bridge was constructed

using Balanced Cantilever Construction. Two form travelers were used to construct cantilever

arms about the pier tables until the full span was finally connected at midspan; casting cycle

duration for a single segment was 7 days. The contractor implemented major constructability

1

Graduate Research Assistant, The Charles E. Via, Jr. Department of Civil and Environmental Engineering,Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0105, [email protected].

2 Professor, The Charles E. Via, Jr. Department of Civil and Environmental Engineering, Virginia Polytechnic

Institute and State University, Blacksburg, VA 24061-0105, [email protected].

Keywords: Segmental bridges, cast-in-place concrete, erection methods, balanced cantilever construction,

construction loads, constructability.

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changes in both the design and the construction of the bridge to facilitate a more economical

construction process.

Introduction

Construction of segmental bridge superstructures is a complex process due to the multiple effects

that arise from the erection method that is employed. Depending on the erection method, which

relates to the physical means of constructing the foundations, bridge substructure, and especially

its superstructure, the structural resistance of the bridge changes with each construction stage,

with the load conditions, the time-dependent materials behavior, and with environmental

influences. These complex interactions need to be considered in the construction engineering

structural analysis and for developing the flow of work tasks in the construction schedule.

Furthermore, each erection method requires its specific equipment and site installations to carry

out the necessary work tasks, which impacts the project budget.

Analyzing these constructability concepts on a particular project the case study of the Wilson

Creek Bridge in Southwest Virginia helps to develop better understanding of the construction

engineering practice and contributes to the safety and economy of future bridge structures. The

objective of this paper thus is to give an educational contribution that can be used for a senior

capstone course in bridge engineering. It draws heavily on the work described in detail by Lucko

(1999) that in turn initially arose from reports on failures of bridge superstructures during the

ongoing construction process (Anon. 1988, Royal Commission 1971).

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Part I. Construction of Segmental Bridges

Concrete segmental bridges were built as early as 1925 (Plougastel Bridge) by French engineer

Freyssinet (1879-1962), who was among the first to implement prestressed concrete in bridge

construction, as Menn (1990) reports. Since then, prestressed concrete bridges, both as pre-

tensioned and post-tensioned have seen a rapid development. In most cases post-tensioning is

employed, i.e. the prestressing tendons are stressed with hydraulic jacks after the concrete has

been placed and gained a minimum strength. Usually the post-tensioning tendons are located in

steel ducts within the concrete segment and are terminated in special anchorages.

Concrete segmental bridges utilize box girder superstructures, which according to Troitsky

(1994) have been used in the U.S. since 1973. These superstructures consist of bottom slabs,

webs (that can be inclined), and a cantilevering top slab to provide maximum deck width.

Concrete box girders have multiple advantages, e.g. their versatility in alignment, width, and

depth, high torsional and bending stiffness of the closed cross-section, and an aesthetically

pleasing geometric appearance.

Segmental Bridge Construction

Segmental construction is a method of construction in which primary load-carrying members

are composed of individual members called segments post-tensioned together (Podolny and

Muller 1982, p10). Segmental construction limitations logically follow from the technical

limitations of erection methods and the construction equipment. Cranes, concrete pumps, form

travelers, and other pieces of equipment have certain physical limitations as to the volume and

weight of material that can be erected at one time.

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Subdivision of the superstructure into segments can be made both in the transverse and the

longitudinal direction. Separation of segments in the vertical axis is found less frequently.

Vertical segmentation is used e.g. in composite bridge superstructures that consist of steel beams,

trusses, or steel box girders with a concrete deck slab. Longitudinally divided segments are load-

carrying members that span the complete length of one bridge span, e.g. the use of multiple

prestressed concrete AASHTO (American Association of State Highway and Transportation

Officials) girders, which are laid parallel and then covered with a deck. This paper focuses on

segmentation in the transverse plane only. Segments in such a bridge usually are 3 to 5 m long

(Fletcher 1984). They can weigh up to 250 metric tons in precast cantilevering construction

(Podolny and Muller 1982) or up to 300 metric tons for cast-in-place (CIP) cantilevering

construction (Fletcher 1984). Podolny and Muller (1982) caution to keep these segments as

regular in their geometry and as straight in alignment as possible and to keep obstructions

through e.g. diaphragms in the box girder at a minimum.

The erection of segments divides the overall construction process into repetitive steps that

facilitate a learning process (Fletcher 1984) and improving productivity in the subsequently

erected spans. Segmental construction thus leads to economic and rapid erection of the bridge

superstructure. A major advantage of segmental construction also is the ease with which it can be

adapted to the specific requirements of the project (e.g. geometry, span lengths, etc.) and to the

capacity of the equipment available to the contractor.

Cantilevering Method

Cantilevering is an erection method in which individual bridge segments (the primary load-

carrying members) are sequentially erected at the tip of the self-supporting superstructure. Post-

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tensioning with longitudinal prestressing tendons is employed to hold the segments in the

cantilever arms (i.e. span halves) together and to provide the needed moment resistance to

withstand dead loads and live loads. The Cantilevering Method can be applied to both precast

and cast-in-place construction. Particularly for cast-in-place construction the influence of the

early loading of newly cast segments as well as the different segment ages in the superstructure

need to be considered in the structural analysis of the erection process. German engineers first

used cast-in-place cantilevering in 1950 for a 62-m long span, as Fletcher (1984) reports.

However, the cantilevering method is much older and had already been used in Asia for wooden

structures of the earliest times, according to Podolny and Muller (1982).

Special construction equipment is employed in cast-in-place cantilevering. Form travelers are

attached to the tip of the cantilever arms and provide external and internal formwork for cast-in-

place construction of box girder superstructures and incorporate several work access platforms.

The form travelers consist of rhomboid-shaped steel framework with transverse front and back

main trusses and bracing and allow adjustment to the required bridge superstructure geometry for

each segment.

Cantilevering commences at one of the supports of the superstructure, i.e. a pier table or an

abutment. As shown in Figure 1, a distinction is made between Balanced Cantilever

Construction, where an erected span half is counterweighted about the pier support by a

concurrently erected span half in a scales-like manner, and the one-directional Progressive

Placement Method, where the erected span can additionally be supported by overhead stay

cables. For both Balanced Cantilever Construction and for the Progressive Placement Method,

temporary towers (with vertical prestressing against uplift, if necessary) or even counterweights

may be employed for temporary support of the span.

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Other erection methods for concrete bridge superstructures are e.g. use of falsework, Incremental

Launching, and Span-By-Span Erection. Various authors, e.g. Podolny and Muller (1982),

Mathivat (1983), and Liebenberg (1992) have provided comprehensive introductions into these

erection methods. This paper focuses on Balanced Cantilever Construction, which is also known

as Free Cantilever Construction (Podolny and Muller 1982).

Part II. Constructability Considerations

The design and construction of bridge superstructures requires thorough planning that both

calculates and evaluates the effects of the erection method. Figure 2 provides a comprehensive

schematic for cast-in-place Balanced Cantilever Construction and the complex interactions of the

multiple variables: Time-dependent materials behavior, construction loads, and the permanent

structural system that is sequentially erected. This graphic is based on information provided by

Barker and Puckett (1997, pp455-466), Bishara and Papakonstantinou (1990), and Shiu and

Russell (1987).

Figure 2 shows the complete construction process of a bridge superstructure with Balanced

Cantilever Construction, beginning with the design of the individual segments and the pier table,

i.e. geometry, the properties of the concrete mixture design, and the reinforcement and

prestressing tendon layout. Also, cast-in-place segments may be partially composed of precast

segments, e.g. precast webs. After the pier table has been constructed, individual segments will

be cast in the formwork of the form travelers with the concrete being placed in several lifts and

cured until sufficient compressive strength has been achieved. Laboratory tests are performed to

determine the actual strength of the concrete. In some cases, to keep the segment casting cycle

duration short, segments will be stressed only two days after casting when the concrete has not

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yet developed the full specified 28-day strength. Then, the longitudinal prestressing tendons

within the newly cast segment will be post-tensioned to rigidly connect it with the existing

cantilever arm. Next, the formwork will be stripped. Upon advancing the form traveler into its

new position, the whole casting cycle can begin again to construct the subsequent segment, as

shown in the vertical center axis of the diagram in Figure 2. Different segment ages within the

bridge superstructure from this successive casting naturally also lead to different concrete

strength. Each concrete segment must possess the structural capacity to support the subsequently

cast segments. The early loading of newly cast segments usually creates span deflections that are

larger than for precast cantilevers. The camber of the superstructure compensates for these

deflections to reach the planned alignment after construction has been completed and further

time-dependent material effects have occurred. A detailed scheme of a single segment casting

cycle including the individual work tasks is provided in Part III of this paper.

The alternating stepwise growth of the cantilever arm lengths influences the post-tensioning

tendons by causing both initial losses and long-term losses of prestressing force in the

longitudinal tendons (Barker and Puckett 1997). Each segment that is added to the structure

changes the structural system itself as well as the construction loads that are applied to the

system, e.g. from advancing the form travelers. Initial loss of prestressing force results from

incremental slippage of the tendon strands in their anchorages before the wedges or nuts that

hold the strands grip seat firmly. Also, applying a prestressing force on a newly cast segment and

its predecessors will partially relieve tendons that have been stressed earlier because of the

elastic shortening of the previously cast segments. Finally, friction between the tendons and the

ducts in which they are encased reduces the overall prestressing force, as tendons in curved ducts

will contact the interior surface of the ducts (wobble effect) (Barker and Puckett 1997).

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Apart from the aforementioned immediate losses, long-term materials behavior will further

reduce the prestressing force. Especially the young concrete with its developing compressive

strength while already being loaded through the prestressing force and segment weight is

susceptible to time-dependent effects. Creep of the concrete segments, i.e. a plastic shortening

under permanent compressive stresses, concrete shrinkage from evaporation, and finally

relaxation of the steel strands of the tendons, i.e. a stress loss under constant strain, all contribute

to reducing the prestressing force. On some bridge projects in response to the segment concrete

strength, a smaller prestressing force sufficient to support the current segments may be applied

initially with the full prestressing force applied later onto the segments when higher concrete

strength is achieved. Time-dependent materials behavior of the concrete is also dependent on the

environmental influences, particularly relative humidity and ambient temperature. Construction

loads also contribute to time-dependent effects, e.g. when the weight of the form travelers affects

the behavior of the structure throughout construction.

Structural analysis of the stresses and deformations during the construction stages and for the

finished structure must account for the aforementioned initial and long-term losses of prestress.

During construction stages, the formwork is aligned according to the camber data that have been

determined during structural analysis. Geometry control data from the structural analysis and

from daily field surveys are used to control the alignment of the bridge during its erection. This

includes accurately meeting the two span halves for midspan closure, and after completion of the

bridge superstructure by performing on-site surveying of the constructed bridge geometry and

comparing this to the calculated profile.

In Balanced Cantilever Construction of a bridge with several spans, the individual cantilever

arms grow until they meet at midspan. Before midspan closure, the cantilevers will have reached

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their most unfavorable condition with their length and weight being at a maximum while

structural redundancy through a continuous span structure has not been achieved yet. Stresses in

such an unfinished structural system can considerably exceed the stresses in the final structure

under its anticipated live load. Commonly, a closure segment will be placed to connect both

halves and form a continuous superstructure span, which will be a statically indeterminate

system. In some cases, hydraulic jacks and dead weights are used to carefully correct minor

misalignments between the two cantilever tips and also to compensate some of the long-term

effects described above by jacking the cantilever arms apart before placing the closure segment

(Matt et al. 1988). However, introducing additional stresses into the structure upon the

completion of each span should be avoided, if possible. The form travelers will be dismantled

and removed, resulting in an upward deflection of the cantilever tips. So-called continuity or

integration tendons at the bottom of the segments (as opposed to the cantilever tendons in the top

part of the segments) will be stressed after the concrete of the closure segment has reached

sufficient strength (Mathivat 1983). Upon closure at midspan, the statical system changes from a

cantilever (with tension at the top over the support) to a beam on several supports (with tension

at the bottom in the spans). Internal stress redistribution takes place in the superstructure, shifting

the moments from the supports more towards the free span and changing the stress distribution

within the segments. It is caused by the boundary conditions of the newly continuous

indeterminate statical system which restrain the former cantilevers in rotation and deflections

that otherwise would have taken place (Bishara and Papakonstantinou 1990). Time-dependent

effects with respect to materials behavior, however, still continue to take place, as indicated in

the diagram in Figure 2. Settlements of the substructure (i.e. horizontal or vertical movements or

rotation of the foundations) now affect the overall structure.

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Finally, the completed structure will undergo additional loading from superimposed dead loads,

e.g. pavement, barriers, and accessories, as well as from the in-service dynamic live loads from

vehicle traffic and wind. Moreover, environmental influences will affect the structure both

during its construction and its service life.

The structural analysis of the bridge and, consequently, construction engineering must consider

the complex integration of the following factors: Construction method and erection sequence,

bridge geometry, statical system, materials behavior, and loading conditions. The Construction

Industry Institute (CII), an organization composed of many key players in the U.S. construction

industry, which was founded to foster safety and economy of construction projects through

research, defines constructability as follows (CII 1986, p2):

Constructability is the optimum use of construction knowledge and experience in planning,

design, procurement, and field operations to achieve overall project objectives. Maximum

benefits occur when people with construction knowledge and experience become involved at the

very beginning of a project.

Planning for constructability of a structure as outlined in the above paragraphs thus integrates

both design and construction and contributes to the safety and quality of bridges and other

engineering structures.

Part III. The Wilson Creek Bridge Case Study

The Virginia Department of Transportation (VDOT) is the owner of the bridge structure. VDOT

was represented through its Structure and Bridge Division, which in turn assigned the structural

design work for bidding purposes to Figg Engineers, Inc. of Tallahassee, FL. The low bidder,

PCL Civil Constructors, Inc. of Coral Springs, FL was awarded the contract for the $

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14,647,120.00 project in June of 1998. PCL selected Janssen & Spaans Engineering, Inc. of

Indianapolis, IN as their construction engineering consultant to prepare the detailed erection

calculations and shop drawings for actual execution of the work. Janssen & Spaans Engineering,

Inc. also did calculations and shop drawings for the changes in the design and construction,

which included pier table and segment geometry, and pier erection sequence. Construction of the

Wilson Creek Bridge and the surrounding works commenced in Summer of 1998 and was

completed in Spring of 2001.

Location and Background

Wilson Creek Bridge crosses Wilson Creek and Ellett Road (Route 723) near Blacksburg, VA

and will serve to improve the regional traffic infrastructure in Southwest Virginia. It is part of

Virginias Smart Road Project, a state-of-the-art research site with a fully instrumented roadway

to conduct research on safer highways and automated vehicles. The Smart Road Project ties into

Route 460 and shall eventually connect it to Interstate I-81. It is carried out by VDOT, the Center

for Transportation Research (CTR) at Virginia Tech, and by the Federal Highway

Administration (FHWA).

Dimensions and Geometry

The bridge superstructure has a total length of 605.00 m between the abutments and is divided

into three center spans of 144.00 m length and two side spans that are 86.50 m long, measured

center-to-center. The cross-section of the bridge is a single-cell box girder with cantilevering

flanges and inclined webs, a total width of 12.00 m and a variable depth that reduces from 9.50

m at the pier table to 3.70 m at midspan in a quasi-parabolic curve of the box girder soffit. Due to

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superstructure rest. Stiffening 1.875-m thick diaphragms are located in the box girders above the

bearings. Expansion joints are located at both ends of the bridge superstructure.

The four bridge piers were constructed vertically cast-in-place with placement heights of about

6.00 m (variable to facilitate an about equal volume of concrete in each placement). Two

different sets of formwork were used, first a starter form for the base segments and then a

climbing form that consisted of exterior and interior modular panels on a steel framework for

rapid assembly and dismantling. Formwork panels were adjusted to the varying dimensions of

the tapering pier for each placement. On top of the piers the pier tables were constructed cast-in-

place in separate formwork that rested on steel falsework, as shown in Figure 4. These pier

tables, due to their massiveness were constructed in four major placements, each consisting of

several individual lifts (Sean Bush, personal communication, September 17, 1999). They have a

depth of 9.50 m, incorporate additional reinforcement in the box girder webs, and feature two

1.00-m thick, heavily reinforced vertical diaphragms within the box girder cross-section to

facilitate the flow of forces into the rigidly connected substructure. The pier table, whose cross-

section is shown in Figure 5, provides the necessary resistance to bending moments from some

segment imbalance that remains due to a time lag between identical construction operations on

both cantilever arms, although the special asymmetry of the pier table already reduces bending

moments.

Material Properties and Volumes

The superstructure of Wilson Creek Bridge (6,451 m3concrete) and its piers (1,615 m3) are built

from concrete with a 28-day compressive strength of 55 MPa that has high early strength and

low permeability (VDOT 1997). Minimum required strength of 30 MPa for prestressing in some

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cases has been reached after only 24 hours, according to the Project Engineer (Sean Bush,

personal communication, September 17, 1999). Abutments (162 m3), abutment footings (126 m3)

and pier footings (2,236 m3) are constructed from concrete with a 28-day compressive strength

of 30 MPa (VDOT 1997).

The reinforcement of type ASTM A615M has a yield strength of 420 MPa and is by

specifications required to be epoxy-coated for both superstructure and abutments. Prestressing

tendons are composed of 19 individual seven-wire strands that consist of ASTM A416 grade 270

steel with low relaxation and have an ultimate strength of 1,862 MPa (VDOT 1998).

Arrangements of prestressing tendons and their sequence of post-tensioning and stressing forces

are provided in the plans (VDOT 1997) and in the detailed shop drawings for Wilson Creek

Bridge. Longitudinal prestressing tendons are post-tensioned from one of their ends only,

alternating between both ends of the cantilever arms, which is expected to give a more even

distribution of prestressing force in the superstructure (Sean Bush, personal communication,

September 17, 1999). The tendons are anchored in special anchorage blocks called blisters,

which are located on the inside bottom edges of the box girder; empty blisters are provided in

case additional post-tensioning is needed in the future due to increasing live loads. All

longitudinal tendons are installed in galvanized corrugated steel ducts. Transverse tendons,

which are found in the cantilevering top slab of the box girder use polyethylene or steel flat ducts

for transverse tendons as per the specifications (VDOT 1997).

Construction Engineering Considerations

Based on the description of Balanced Cantilever Construction in Part II of this paper that was

illustrated by Figure 2 and the previous information on Wilson Creek Bridge itself, it now

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becomes obvious which special considerations the construction engineers had to make in

planning its construction. Since Wilson Creek Bridge was erected cast-in-place, the

specifications called for concrete with high early strength, so that newly cast segments could

withstand the loads from advancing the form traveler and from placing subsequent segments,

through which the cantilever arms grow step-by-step. The different ages and compressive

strengths of these segments cause the modulus of elasticity E of the concrete to be quite

heterogeneous along the spans, in addition to the moment of inertia Ichanging with the variable

box girder height. Structural analysis had to be carried out for each of these construction stages,

so that the necessary amount of prestressing tendons and their layout could be determined, as

well as calculation of the necessary camber of the superstructure. Two major types of

longitudinal prestressing tendons are located in the superstructure, so-called cantilever tendons in

the top slab and so-called continuity tendons in the bottom slab (VDOT 1997.

Casting of the individual segments for the Wilson Creek Bridge followed a closely prescribed

procedure to ensure the required accuracy. Concrete samples were tested to confirm that at least

the minimum specified compressive strength had been reached in a segment by the beginning of

the subsequent casting cycle. At each construction stage, surveying was performed prior to

sunrise (to avoid temperature changes creating deformations of the bridge) on a series of

prominent points around the circumference of the newest segment, as well as along the cantilever

arms. Elevations were compared with the computer-generated profile for the respective step to

ensure that deviations from the ideal alignment would be kept within the permissible range for

the accumulated error of 1/1000 of the span length (VDOT 1998).

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Segment Casting Cycle

Based on Mathivat (1983, p201) an overview of casting cycle steps and typical values for their

duration is provided in Table 1. It should be noted that the given steps are generic and would be

broken down further for planning of the actual execution. Construction of the superstructure of

Wilson Creek Bridge closely followed the duration for the individual steps given by Mathivat

(1983), with the difference of allowing three days instead of two for installation of reinforcement

and prestressing tendons and reducing curing duration to two days. With respect to the overall

duration of one casting cycle, British engineer Fletcher (1984, p17) gives information that it

generally settles down to one pour [i.e. segment placement] per shutter [form traveler] per

week. He also names important factors that influence casting cycle duration:

A learning effect takes place due to the repetitive nature of the segmental work tasks;

Speed of reinforcement and tendon installation and speed of concrete placement depend on

the size of the segment;

Form traveler adjustment takes longer when changes in segment geometry, e.g. variation in

the box girder depth or slab width occur;

Form traveler adjustment takes longer for incorporation of special details, e.g. diaphragms

and tendon anchorages, due to additional formwork and reinforcement installation;

Adverse conditions, such as inclement weather (e.g. low temperatures) can delay sufficient

gain of compressive strength of the concrete, which provides the lower limit for the casting

cycle duration. The quality of each individual segment in the cantilevering chain of cast-in-

place segments is critical to the structural integrity of the entire superstructure.

The casting cycle for Wilson Creek Bridge can be broken down into eight major steps as shown

in Figure 6. In Step 1 the form traveler that is shown in Figure 7 is detached from its rails, which

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are then moved forward, anchored firmly with hydraulic tie-downs, and the form traveler is set

back onto the rails. In Step 2, the form traveler with its exterior forms is advanced with hydraulic

jacks, brought into alignment according to the calculated camber data, and is anchored in the

new position over the end of the cantilever, as shown in Figure 8. Once the form traveler has

been set up, a work crew can manually install reinforcement in the bottom slab and webs in Step

3, as the form traveler framework and bracing does not provide space to lift preassembled

reinforcement cages into place. Also, a carpenter builds a custom-made plywood bulkhead that

will close the formwork at its front face during casting and curing. Step 4 comprises of

advancing the interior formwork as well, which would previously have obstructed the

reinforcement installation. The bulkhead, interior formwork, and exterior top slab formwork can

be seen in Figure 9. To keep both forms together, wall ties are installed. Finally, the

reinforcement for the top slab is put in place. In Step 5 the reinforcement crew installs and

couples ducts for the longitudinal and transverse prestressing tendons and inserts the tendons into

the ducts. In Step 6 the concreting crew places concrete into the bottom slab, the webs, and the

top slab of the new segment. Concrete is placed through tremie pipes that run through openings

in the formwork panels. The individual concrete lifts are consolidated using internal vibrators.

During Step 7 the concrete is curing to achieve sufficient compressive strength and durability, so

that it can withstand the construction loads and the in-service loads as well as environmental

influences. Finally, transverse tendons in the top slab and the longitudinal cantilever tendons are

stressed and their ducts are filled with grout in Step 8, the forms will subsequently be stripped

from the segment, and a new casting cycle can begin.

The aforementioned casting cycle was repeated a dozen times on both cantilever arms, as shown

in Figure 10, until midspan had been reached and the closure segment was placed, as shown in

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Figure 11. Only one set of two form travelers was used in this project for economical reasons.

Thus, cantilevering was performed subsequently about all pier tables in the order of Pier 2 Pier

3 Pier 4 Pier 1 to construct the bridge superstructure. However, work tasks were not

performed concurrently on both cantilever arms, as mentioned above. Figure 12 provides a linear

schedule for the beginning cantilevering operation about Pier 2. For both cantilever arms, which

were referred to as the surveying Downstation (in the direction of Abutment A) and

Upstation (in the direction of Abutment B), it clearly shows the curing time for newly placed

segments before the casting cycle begins anew. Construction of each segment is shown as a

continuous operation to keep this schematic linear schedule easy to read; in reality each segment

involves many individual tasks like formwork setup, reinforcement installation, and concrete

placement, as described above. Note also the planned lag of two days between repetitions of

identical operations for pairs of equivalent segments on both sides, like Segments 1 and 2,

Segments 3 and 4, etc. This time lag allowed optimized allocation of work forces by reducing

lead-time for each crew, as the crews for formwork setup, reinforcement installation, and

concrete placement alternated between the cantilever arms. Some reduction of the duration of

work activities could be observed because of (a) the reduction of segment dimensions, i.e. less

volume of concrete per segment towards midspan and (b) due to a learning effect from the

repetitive nature of the casting cycle (Sean Bush, personal communication, September 17, 1999).

A schematic of the cantilevering operations that combines the information from Table 1 and

Figure 12 is provided in Figure 13.

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Constructability Changes for Superstructure

Segment and Pier Table Lengths

Initially, plans foresaw a pier table length of 6.00 m with an asymmetry of to the pier centerline,

and several types of segments, being 4.50 m long in the spans and 3.00 m at the closure segments

but only 2.25 m near the pier table (VDOT 1997). Keeping deeper segments shorter was

intended to level volumes of concrete placement during construction. As permissible under the

contract, the contractor modified the segment lengths and the length of the pier table as follows

to facilitate a smoother construction process with the Balanced Cantilever Method. The segment

length was changed to be 5.00 m each with a 4.00-m long closure segment, as shown in Figure

14 in comparison with the original arrangement. It should be noted that a segment length of 5.00

m is at the upper end of what is considered technically common (Fletcher 1984).

Moreover, the pier table design was changed to 15.00 m length and an asymmetry of 2.50 m to

the pier centerline. Several initial segments were now already incorporated in the pier tables

themselves. The number of casting cycles for the individual cantilever arms was thus reduced

and at the same time less form traveler adjustments would become necessary, as all segments

would have equal length throughout the spans. Cantilevering was to begin with a segment at the

Downstation end of the pier table. This major design change required redesigning the form

traveler specifications, changing the shop drawings containing segment geometry data as well as

performing structural calculations completely anew, but reduced the overall duration of

cantilever construction.

Due to crew availability and labor cost, segments were to be placed about the pier tables in an

alternating manner. The aforementioned initial asymmetry of 2.50 m, i.e. half a segment length,

of the pier tables effectively reduces the overturning moments due to segment imbalance in the

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bridge superstructure. At any point in time the imbalance between both cantilever arms will

equal only half a segment weight because the asymmetric pier table compensates for the

remaining imbalance. In other words, the side with the newly added segment will be half a

segment longer and heavier until another segment is cast on the other cantilever arm, which

in turn makes that side longer and heavier. Had the alternating segment placement been

undertaken without this asymmetry, the imbalance would always have amounted to a full

segment weight. A schematic of this concept is provided in Figure 15.

Using the simple statical systems without (A) and with (B) pier table asymmetry on the right

cantilever arm as shown in Figure 16, the effect of the segment imbalance on the overturning

moment Mat the base of the pier table can be determined as per Equation 1.

M = w (l12 l2

2) Equation 1

It shall be assumed that all superstructure segments in this model have a constant geometry, i.e.

equal length liand weight wi(with the subscript ibeing the segment number) in the cantilever

arms of total length l1and l2. For simplification the load per unit length wshall be assumed to be

equal to 1 and the individual segment length lishall also be 1. Subsequently adding segments to

the systems increases the overturning moment M, however in a different manner for Systems A

and B. Use of System B results in a reduction in M that ranges from initially 25 % to a

maximum reduction of 50 % as compared to System A without the pier table asymmetry. Since

the lengths l1 and l2 in Equation 1 are squared, the percent moment reduction exhibits an

asymptotic behavior towards 50 % reduction with increasing cantilever length. Table 2

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summarizes the calculated overturning moments for the first 12 segments in the two systems and

gives the percentage of moment reduction achieved by System B (with asymmetry).

Pier Erection Sequence

In order to optimize the erection sequence of construction for the Wilson Creek Bridge the

contractor chose to alter the order in which its piers would be erected and cantilevering from

their pier tables would be started. The actual order used was Pier 2 Pier 3 Pier 4 Pier 1, so

that construction of the highest one, Pier 2, with its shaft height of 41.314 m could be carried out

first. While the cantilever arms of one pier were constructed the next pier was already

constructed so that cantilevering could continue on that following pier without delay as soon as

the available set of form travelers had been dismounted and set up again. This staged erection

sequence also generated more time on the schedule to deal with Pier 1 at the sloping edge of the

valley, where accessibility with heavy-lift cranes for the form traveler setup and materials supply

was limited and required additional earthwork to create sufficient workspace.

Discussion of Construction of the Wilson Creek Bridge

The Wilson Creek Bridge is an excellent example to show the complexities that arise during

Balanced Cantilever Construction with cast-in-place segments. Using the diagram shown in

Figure 2 enables understanding short-term effects during construction as well as long-term

effects that take place long after the bridge superstructure has been completed. In particular, the

importance of the specific casting cycle within the overall superstructure erection is emphasized

in the diagram. Casting cycles of one week including the weekend for curing of the newly placed

concrete are most common in Balanced Cantilever Construction. In the case of the Wilson Creek

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Bridge, the casting cycle as introduced in Figure 6 used only two days for curing of any newly

cast segment (as shown in Table 1) to achieve sufficient strength for being post-tensioned to the

previous segments but alleviated this by using concrete with high early strength. Furthermore the

contractor chose to use only one set of form travelers and have the crews alternate between the

two cantilever arms where identical operations would be performed with a two day time lag, as

was depicted in Figures 12 and 13. A special asymmetry of the pier table was implemented to

reduce the imbalance arising from these alternating operations. Designing the pier tables to be

asymmetric by exactly half a segment length reduced the potentially overturning moments from

segment imbalance by as much as 50 % as compared with an alternating segment placement

about a pier table without asymmetry. A learning effect on some tasks related to segment casting

could be observed during project execution, according to the Project Engineer (Sean Bush,

personal communication, September 17, 1999). Altogether, the large scale of the Wilson Creek

Bridge and its carefully planned operations show impressively how Balanced Cantilever

Construction has developed since its introduction in the Construction Industry.

Conclusion

This paper provided an introduction to constructability concepts in segmental bridge

superstructure erection. The various influences that arise from the chosen erection method, the

bridge geometry, and the materials behavior and their complex interactions were outlined

focusing on Balanced Cantilever Construction. Short-term effects and long-term effects were

introduced and put in relation with loading conditions and environmental influences in a

comprehensive diagram.

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Construction loads depend on the actual erection method and may create higher stresses in the

structure than any loads anticipated for the bridge under service conditions could cause.

Furthermore, construction loads affect the structure in its weak stages prior to completion the

structural system has not yet reached continuity and additional redundancy from being statically

indeterminate in its final configuration. During construction the segments may have reached only

a minimum level of compressive strength that may be less than the specified 28-day compressive

strength when successor segments are added. Consideration of all construction stages with their

respective geometry and boundary conditions, structural details, time-dependent material

properties, and construction loads therefore is essential for adequately analyzing the structure

and designing against failures.

Providing the case study of the Wilson Creek Bridge in Virginia, which was erected using

Balanced Cantilever Construction, enhanced the concepts outlined in this paper and gave insight

into how these concepts are applied in real engineering practice. The segment casting cycle with

its individual steps was explained and illustrated in detail. Finally, two constructability changes

that the contractor implemented on the Wilson Creek Bridge to optimize operations and reduce

schedule duration were presented. In particular, a redesign of the segment length in conjunction

with a planned asymmetry of the pier tables considerably reduced overturning moments from

cantilevering operations of the superstructure.

Future research could include examination of other erection methods, such as e.g. Incremental

Launching and analyze these with respect to constructability issues. Providing a collection of

several different construction methods, each illustrated with case studies of real-life examples

would be a valuable source of information in teaching future construction engineers. This can

ultimately lead to yet safer and economical construction of bridges to serve the public.

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Acknowledgements

The authors would like to thank Dr. R. M. Barker and Dr. C. L. Roberts-Wollmann of Virginia

Polytechnic Institute and State University for the discussions on the topic of this paper, and Mr.

Sean Bush, Project Engineer at PCL Civil Constructors, Inc., for his friendly support of this

study.

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Appendix I: References

Anon (1988). Zilwaukee Bridge. (Excerpt from The Zilwaukee Bridge: From the Beginning.

published by Michigan Department of Transportation) Concrete International10(5), 68-

75.

Barker, R. M., Puckett, J. A. (1997). Design of Highway Bridges: Based on AASHTO LRFD

Bridge Design Specifications. A Wiley-Interscience Publication, John Wiley & Sons,

Inc., New York, NY.

Bishara, A. G., Papakonstantinou, N. G. (1990). Analysis of Cast-in-Place Concrete Segmental

Cantilever Bridges.Journal of Structural Engineering, ASCE, 116(5), 1247-1268.

CII (The Construction Industry Institute) (1986). Constructability: A Primer. Prepared by The

Construction Industry Institute Constructability Task Force, Publication 3-1, Third

Printing April 1990, Bureau of Engineering Research, The University of Texas at Austin,

Austin, TX.

Fletcher, M. S. (1984). In-Situ Free Cantilever Concrete Bridges. Highways and

Transportation31(11), 6p between 10-18.

Liebenberg, A. C. (1992). Concrete Bridges: Design and Construction.Longman Scientific &

Technical, Longman Group UK Limited, Burnt Mill, Harlow, Great Britain.

Lucko, G. (1999). Means and Methods Analysis of a Cast-In-Place Balanced Cantilever

Segmental Bridge: The Wilson Creek Bridge Case Study.Thesis submitted to the Faculty

of Virginia Polytechnic Institute and State University in partial fulfillment of the

requirements for the degree of Master of Science, Blacksburg, VA.

Mathivat, J. (1983). The Cantilever Construction of Prestressed Concrete Bridges. A Wiley-

Interscience Publication, John Wiley & Sons, Inc., New York, NY.

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Menn, C. (1990). Prestressed Concrete Bridges.(Original title: Stahlbetonbrcken.) Birkhuser

Verlag AG, Basel, Switzerland.

Podolny, W., Muller, J. M. (1982). Construction and Design of Prestressed Concrete Segmental

Bridges.A Wiley-Interscience Publication, John Wiley & Sons, Inc., New York, NY.

Royal Commission (Barber, E. H. E., Bull, F. B., Shirley-Smith, H.) (1971). Report of Royal

Commission into the failure of West Gate Bridge. C. H. Rixon, Government Printer,

Melbourne, Australia.

Shiu, K.-N., Russell, H. G. (1987). Effects of time-dependent concrete properties on prestress

losses. Canadian Journal of Civil Engineering14(5), 649-654.

Troitsky, M. S. (1994). Planning and Design of Bridges.John Wiley & Sons, Inc., New York,

NY.

VDOT (Commonwealth of Virginia Department of Transportation) (1997): Proposed Bridge on

Smart Road over Wilson Creek and Route 723 Montgomery County 0.3 km E. of NS

RWY. Project IVHS-060-101, B603. Plan drawings, Structure and Bridge Division,

Richmond, VA.

VDOT (Commonwealth of Virginia Department of Transportation) (1998): Proposal of the

Commonwealth of Virginia Department of Transportation.Order No. A81. Contract No.

C00016931C02, Richmond, VA.

Appendix II: Notation

The following symbols are used in this paper:

E = modulus of elasticity;

I = moment of inertia;

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l = cantilever arm length;

w = load per unit length of cantilever arms;

M = overturning moment for statical system.

Subscripts

A = statical system;

B = statical system with asymmetry;

i = segment number;

1, 2 = cantilever arm number.

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Figure and Table Captions

Table 1: Typical Duration of Casting Cycle Steps

Based on Mathivat (1983, p201) and Bush (Personal communication, September

17, 1999).

Table 2: Reduction of Overturning Moments through Pier Table Asymmetry

Figure 1: Comparison of Balanced Cantilever Construction and Progressive Placement

Method

Figure 2: Effects of Cast-In-Place Balanced Cantilever Construction

Based on Barker and Puckett (1997, pp455-466), Bishara and Papakonstantinou

(1990), and Shiu and Russell (1987).

Figure 3: Overall Elevation of Wilson Creek Bridge

Figure 4: Pier Table Formwork on Pier 3

Figure 5: Pier Table Cross-Section Looking Upstation

Based on VDOT (1997).

Figure 6: Scheme of the Segment Casting Cycle

Figure 7: Form Traveler Rear View with Rails

Figure 8: Form Traveler Setup on Pier 2

Figure 9: Segment Bulkhead and Top Slab Formwork

Figure 10: Cantilevering about Pier 2 at a Later Stage


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Figure 11: Midspan Closure Placements about Pier 4

Courtesy of Dr. Michael C. Vorster, Virginia Polytechnic Institute and State

University.

Figure 12: Linear Schedule for Alternating Cantilevering Operations

Figure 13: Scheme of the Alternating Cantilevering Operations

Based on Information from PCL Civil Constructors, Inc., Coral Springs, FL

Figure 14: Comparison of Original and Changed Segment and Pier Table Lengths

Figure 15: Pier Table Initial Asymmetry for Reduction of Overturning Moments

Figure 16: Simple Statical System for Calculation of Moment Reduction


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Typical

Duration(Mathivat

1983, p201)

(1)

Actual Field

Duration(Wilson Creek

Bridge)

(2)

Activities

Performed

(3)

1 Day 1 Day Post-tensioning tendons(previous segment)

Stripping formwork

Advancing form traveler

2 Days 3 Days Placing reinforcement,

ducts, and tendons

1 Day 1 Day Placing concrete for

bottom slab, webs,and top slab

3 Days 2 Days Curing concrete

(including weekend)

Total 7 Days Total 7 Days Continuous Surveying


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Segment

Numberi

(1)

Added

ToSide

(2)

Moment MA

WithoutAsymmetry

(3)

Moment MB

WithAsymmetry

(4)

Percent Moment

Reduction(MA- MB) / MA

(5)

1 Left 0.500 0.375 25.00 %

2 Right 0.000 -0.625

3 Left 1.500 0.875 41.67 %

4 Right 0.000 -1.125

5 Left 2.500 1.375 45.00 %

6 Right 0.000 -1.625

7 Left 3.500 1.875 46.43 %

8 Right 0.000 -2.125

9 Left 4.500 2.375 47.22 %10 Right 0.000 -2.625

11 Left 5.500 2.875 47.73 %

12 Right 0.000 -3.125

13 Left Etc. Etc. Etc.


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Balanced Cantilever Construction

Form

Traveler

Pier

with

Forms

PierTable

Temporary Support

Tower with Vertical

Prestressing

Progressive Placement Method

TemporaryErection

Support

Erection

DerrickPrecast

Segment

Overhead Stay Cables


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Structural Analysis

Stresses, DeformationsCamber Data

(Form Alignment)

Geometry Control

Data (Surveying)

Relaxation of steel strands

(stress loss under constant strain)

Creep of concrete member

(plastic deformation under stress)

Shrinkage of concrete member

(volume change from evaporation)

Long-term Loss of Prestress

Elastic shortening of member

(relieves tendons stressed earlier)

Friction between tendon and duct

(wobble effect of curved ducts)

Slippage of strands in anchorages

(before wedges or nuts grip firmly)

Initial Loss of Prestress

Segment 1

Segment 2

Segment ...

Midspan

Closure

SegmentJacking for

alignment

Redistribution (Internal Stresses)Moments shift from

pier towards span

Interaction

Dynamic Live Loads

(In-Service)Vehicle traffic, wind

New boundary

conditions ofindeterminate

system as

restraints

Settlements cause

rotation or hor. orvert. movement

of foundations

Continuous Structure

Cantilever System

EnvironmentalInfluences(Rel. humidity, temp.)

Superimposed DeadLoads(Pavement,barriers, accessories)

GeometryConcrete Mixture Design

Reinforcement

Tendon Profile

Segment Design Segment Composition

Single placement (in lifts)or prefabricated parts, e.g.

precast webs for segment

CastingCy

cle

forStepwise

Construction

Developing

SegmentA

ge

andStreng

th

Cast, Cure,Prestress

and Grout,

Strip Form

Construction Engineering

Pier Table

Move, Set, Anchor Form Traveler


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Wilson Creek Bridge

Abutment A

Pier 1 Pier 3 Pier 4Pier 2

Abutment B

Ellett

RoadWilsonCreek

86.50 m 144.00 m 144.00 m 144.00 m 86.50 m


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Acces

sOpeningin

BothDiaphragms

0.40

12.00

0.90 1.65 0.60 1.75 2.20 1.75 0.60 1.65 0.90

0.8

5

1.2

5

6.0

5

0.7

2

0.23

9

.50

Note:

Segment has a2 % transverse

slope and thusis tilted to the

vertical axis ofthe pier.

All dimensions

are in [m].


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2.Advance and

anchor traveler

with exteriorforms on rails

1.Detach formtraveler from

railsAdvance and

anchor railsSet traveler

on rails

3.Install rebar

in bottom slab

and websBuild bulkhead

7.Cure

concrete toachieve strength

and durability

6.Place

concrete intobottom slab,

webs, top slab

8.Stress

and grouttransverse and

cantilever

tendons

Strip forms

4.Advance the

interior form-work

Install wall ties

Install rebar in

top slab

5.Install

longitudinaland transverse

top slab tendon

ducts and

tendons


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Downstation Pier 2 Upstation

Work crews alternate

5. 3. 1. 2. 4. 6.

Time[days]

Location

Time lag

Curing

Curing

Curing


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Day 1:Prestress,

strip form, move

Day 5:Place

segment concrete

Day 5:Place

segment concrete

Day 1:Prestress,

strip form, move

Day 6 - 7:Cure

concrete (cover,

keep newlyplaced segment

moist and warm)

Day 6 - 7:Cureconcrete (cover,

keep newlyplaced segment

moist and warm)Day 2 - 4:Place

reinforcement in

bottom slab and

webs, moveinterior forms,

place top slab

reinforcement,

ducts, tendonsDay 2 - 4:Place

reinforcement in

bottom slab and

webs, move

interior forms,place top slab

reinforcement,

ducts, tendons


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Original Segment and

Pier Table Lengths [m]

Changed Segment and

Pier Table Lengths [m]

3.00 13 x 4.50 4 x 2.25 6 x 2.25 12 x 4.50 3.00

4.00 13 x 5.00 6.25 8.75 12 x 5.00 4.00

6.00

Downstation

Upstation


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Adding New Segment Shifts

Imbalance to Other Side

Segment 1 Pier Table Segment 2Segment 3

Half a Segment Length

Initial Asymmetry


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M M

System A

Without Pier Table

Asymmetry

System B

With Pier Table

Asymmetry

l1 l2 l1 l2

w w

Asymmetry

2003 Lucko DelaGarza Constructability Considerations for Balanced Cantilever

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