Investigation of the Pier Response during the Lateral Slide of Multi-Span Bridges

Abstract

Lateral slide-in bridge construction—also referred to as slide-in bridge construction (SIBC)—has gained increasing attention as a viable accelerated bridge construction (ABC) approach. Although SIBC has been successfully employed on multiple single-span bridge projects, the use of SIBC on multi-span bridges is rare. Adding more spans creates a more complex system that requires connections (and other details) that were previously not needed in a single-span slide. Also, it is a concern that the multi-span bridge would need to slide on piers, which creates possible uplift and overturning scenarios. As such, it was vitally important to understand the structural response of the substructure during the bridge sliding. Although much general information on lateral slides as an ABC method was available, very little literature had been published related explicitly to such construction for multi-span bridges. The objective of this research was to develop durable design details to be used with the lateral slide concept with a focus on pier connection details and evaluate the performance of the bridge pier during the sliding. To achieve the goal, relevant information was collected and evaluated by conducting a literature review on various details and construction approaches for previously completed lateral slide projects. Further, a three-span, 300 ft long steel girder bridge on IA 1 southwest of Iowa City, Iowa, U.S., was monitored using gauges during the slide-in. The results indicated that the current slide-in practice works well with the multi-span steel girder superstructure and the wall pier. No significant response from the substructure was visually observed during the slide-in, and no cracking occurred on the concrete deck or piers. This indicated that the superstructure with steel girders and concrete diaphragms could be built with the lateral slide-in method. The significant strain was measured from the pile strain gauges. However, the piles were functionally adequate to carry the vertical load, and the moment carried by each pile was minimal. An uplifting action was captured on Pier 1. However, this effect was minimal on Pier 2.

Keywords

multi-span bridges lateral slide-in bridge construction field monitoring substructure response accelerated bridge construction bridge pier

Lateral slide-in bridge construction—also referred to as slide-in bridge construction (SIBC)—has gained increasing attention as a viable accelerated bridge construction (ABC) approach. With lateral slide construction, the bridge superstructure is constructed off the alignment and usually on a system of temporary works while the original bridge is still open to traffic. Once the construction of the superstructure is essentially complete, the original bridge is demolished, and new substructure construction is completed. Then, usually over a relatively short period of time, the new bridge superstructure is slid laterally from the temporary works onto the in-place substructure.

Although SIBC has been successfully employed on multiple single-span bridge projects, the use of SIBC on multi-span bridges is rare. The addition of more spans creates a more complex system that requires connections (and other details) that were previously not needed in a single-span slide. Furthermore, because the multi-span bridge would need to slide on abutments plus piers (as opposed to just abutments in a single-span case), this created possible uplift and overturning scenarios. As such, it was vitally important to understand the structural response of the substructure during the bridge sliding. Although a good amount of general information related to the use of lateral slides as an ABC method was available, very little literature has been published specifically related to such construction for multi-span bridges.

The objective of this research was to develop durable design details to be used with the lateral slide concept with a focus on pier connection details and evaluate the performance of the bridge pier during the sliding. To achieve the goal, relevant information was collected and evaluated by conducting a literature review on various details and construction approaches for previously completed lateral slide projects. Further, a three-span, 300 ft long, steel girder bridge on IA 1 southwest of Iowa City, Iowa, U.S., was monitored during the slide-in process with a focus on the structure behavior near the bridge pier. The successfully implemented design details were evaluated and summarized based on the findings from the research work.

Literature Review

The objective of the literature review was to collect and summarize information relevant to SIBC with a focus on implementing SIBC on multi-span bridges. Sliding a constructed bridge is not a new concept and has been successfully implemented in many projects nationwide. Utah Department of Transportation (UDOT) and the Michael Baker Corporation developed the Slide-In Bridge Construction Implementation Guide for the U.S. Federal Highway Administration (FHWA) to demonstrate the advantages of SIBC and document how state and local agencies can implement SIBC in typical bridge replacements as a part of their standard business practices ( 1 ). The authors pointed out that, most often, these projects have been large bridges with high traffic volumes that limited other construction options. However, state agencies and the FHWA have successfully employed SIBC with small bridge replacements as an innovative option to minimize impacts on the traveling public. The implementation of SIBC involves: (1) constructing a temporary substructure next to the existing bridge as the support for the superstructure of the new bridge; (2) constructing the superstructure on top of the temporary substructure while maintaining traffic on the existing bridge; (3) constructing the substructure under the existing bridge without disturbing traffic; (4) detouring traffic to the new bridge superstructure built on the temporary support and demolishing the existing bridge (the construction of the new substructure sometimes continues during this step); and (5) sliding the new bridge superstructure onto the new substructure. The road closure for the sliding usually takes a few hours to several days.

In this section, the past bridge construction cases that utilized the SIBC method since the 1990s were found in online resources, research project reports, and technical articles. These resources were reviewed with the results presented in Section 2.1. During this process, the cases related to SIBC of multi-span bridges and the study of pier/foundation behavior during the slide were identified. Further, a literature search was conducted to find detailed information on SIBC cases. Different types of equipment and design details used during the slide-in procedure are discussed with respect to their effect on the substructure of the multi-span bridge in Sections 2.2 to 2.10.

SIBC Applications

More than 40 projects in the past 30 years that have used the slide-in method for single- or multi-span bridges were identified from online webpages, research project reports, and technical articles ( 1 – 6 ). During the surveying of these bridge projects, basic bridge information was summarized, including the number of spans, bridge length and width, beam type, pier type, foundation type, diaphragm type, and sliding system. Since the objective of the research focus was on multi-span bridges, with an emphasis on the pier region, only those cases that used SIBC on multi-span bridges are presented (Table 1) and discussed.

Table 1.

Multi-Span Bridges Constructed Using the Slide-in Bridge Construction (SIBC) Approach ( 1 – 6 ).

No.	Bridge location	Year	Original bridge			New bridge
No.	Bridge location	Year	Total span #	Total length (ft)	Total width (ft)	Total span #	Total length (ft)	Total width (ft)	Max. # of spans (each slide)	Beam type	Pier type	Foundation type	Diaphragm type	Sliding system
1	I-405 over Northeast 8th Street Bridge ( 1 )	2003	6	293	103	2	328	121.5	2	SG	BCF	Spread footings	Steel	Roller
2	Hood Canal Bridge ( 2 )	2005	6	643	30	5	605	40	5	PSCG	BCF	Drilled shafts	Concrete	Rollers
3	Elk Creek Bridge ( 2 )	2008	6	340	30	3	320.5	38.2	3	SG	BCF	Drilled shafts	na	Bearing pad
4	Ben Sawyer Swing Bridge ( 2 )	2010	>3	NA	NA	>3	1,154	36.5	6	SG	BCF (reused)	na	Steel	na
5	I-44 over Gasconade River ( 2 )	2011	6	670	34	6	670	36.67	4	SG	BCF	na	Steel	Teflon pad
6	Sellwood Bridge ( 6 )	2013	4	1,100	NA	NA	NA	NA	4	Steel Truss	NA	NA	NA	NA
7	I-84 over Dingle Ridge Road ( 3 )	2013	na	140	33.3	3	140	na	3	Double Tee	NA	NA	NA	NA
8	Larpenteur Avenue Bridge ( 2 )	2014	na	185.5	61	2	187	75.8	2	PSCG	na	na	Concrete	na
9	M-50 over I-96 ( 4 )	2014	4	227	37.5	2	198	71.25	2	na	na	na	na	na
10	Poplar Street Bridge ( 5 )	2018	5	2,165	na	na	na	na	na	na	BCF (reused)	na	na	na

Note: BCF = beam column frame; PSCG = prestressed concrete girder; SG = steel girder; Max. = maximum; NA = not available.

The results in Table 1 indicated that most of the bridges have two to six spans and that the whole bridge was built continuously over the piers and slid simultaneously onto the permanent structure. For the bridges with tens of spans and usually constructed over a river, the superstructure was usually divided into units of up to three spans. Each unit was slid into the final position using the SIBC approach (sometimes in conjunction with the float-in method). By comparing the lengths of the new bridges with that of their original bridges, the researchers found that the total length of the new bridges is usually shorter than that of the original bridge. This observation agrees well with the findings from UDOT and the Michael Baker Corporation ( 1 ).

The information in Table 1 also indicates that the beam-column frame pier is the most frequently used pier type for the construction of the multi-span bridge utilizing the SIBC approach. During the review of the literature, it was found that no special consideration seemed to be given to the pier design. Also, no issues have been reported for the use of the beam-column frame associated with the SIBC method. With respect to the foundation type, the limited information indicated that both spread footings and drilled shafts were used. The selection of the material for the diaphragm is mostly based on the type of girder. Both steel and concrete diaphragms were used with the SIBC approach without reports of an issue. For the selection of the sliding system, it appears that when the superstructure in each slide exceeds approximately 300 ft in length or 50 ft in width, the roller support was commonly used, since a large heavier superstructure requires a low coefficient of friction on the sliding track to reduce the lateral sliding force demand. The researchers found that both steel plate girders and prestressed concrete beams were used for multi-span SIBC.

Compared with conventional construction methods, SIBC requires additional equipment to move the new superstructure from the temporary supports to the permanent ones. The special equipment used for SIBC usually includes a sliding system with rollers or bearing pads as the contact between the substructure and the superstructure, an actuating system (sometimes used with a movement control mechanism) to provide the power for the movement, and one or two temporary structures used to support the new or old superstructure.

Sliding Systems

Sliding systems provide and maintain a path for the superstructure during the lateral slide. Polytetrafluoroethylene (commonly known as Teflon) pads and rollers are most commonly used for the slide systems in SIBC, as shown in Figure 1 ( 1 , 17 , 18 ).

Figure 1.

Sliding systems (Utah Department of Transportation and Michael Baker Corporation 2013, U.S. Federal Highway Administration): (a) guided with industrial rollers, and (b) guided with Teflon pads.

Industrial Rollers

Industrial rollers are usually installed under the girders or end diaphragm of the new bridge and used with a sliding track. The sliding track maintains the movement in the slide direction. One of the advantages of using rollers is that, compared with the Teflon pad, sliding friction is very low in roller systems. The coefficient of friction of the roller system is usually less than 5%, and the breakaway friction is close to the kinetic friction since the sliding velocity is very low. The low friction of coefficient means less external force is needed to initiate and keep the movement of the superstructure; as a result, the reaction force on the temporary and permanent substructure is low. UDOT and the Michael Baker Corporation list a few major drawbacks of roller systems, including that the large point load occurs under each roller, binding or jamming of rollers may occur if not aligned properly, and start and stop ability should be provided during the slide since the dynamic coefficient of friction is low ( 1 ). The large point load requires more attention on the design of permanent and temporary substructures. In addition, vertical jacking is required to remove the rollers after the superstructure is moved to its final position.

Teflon Pads

Teflon pads are the most commonly used sliding system in SIBC approaches. This method uses elastomeric or cotton duck bearing pads topped with Teflon to slide the bridge into place. The pads are usually lined along the temporary supports and permanent substructures, and the bottom of the bridge diaphragm becomes the sliding surface. Slide shoes or sliding blocks can be cast into the diaphragm and wrapped with a sliding surface such as stainless steel. With this method, the final sliding pads on which the bridge stops can be left in place to act as the final bearings. Aktan and Attanayake indicated that the coefficient of friction associated with Teflon pads could be as much as 20% ( 5 ). Phares et al. tested the behavior of the bearing pads to determine if excessive shear deformation occurs such that the bearing pads may “roll” during construction ( 7 ). The results indicated that the coefficient of friction was calculated to be approximately 0.11 for the non-lubricated tests and 0.07 for the lubricated tests. For the multi-span bridge, a greater friction coefficient may result in a larger reaction force to the pier column and the foundation structures.

There is no best system for any specific application ( 1 ). Geometry, weight, tolerances, and experience are the parameters considered in the selection of slide systems. The sliding resistance of Teflon pads is relatively greater than that of rollers, resulting in a greater reaction force on the substructure. Parameters that affect Teflon-steel interface friction include sliding velocity, normal pressure, Teflon composition, steel sliding surface roughness, surface treatment (lubricant applied at the interface), temperature, and the angle between the surface polishing of steel and the slide direction ( 8 ).

Actuating Devices

Actuating systems are used to provide force to initiate and maintain the slide. Sliding can be completed by pushing, pulling, or a combination of both ( 9 ). The most commonly used actuating systems include hydraulic rams, mechanical pulling devices, and prestressing jacks (Figure 2). To provide enough force for the slide, multiple actuating devices placed at different locations are usually required. Ridvanoglu indicated that the difference between the applied force and resistance is not constant throughout the slide-in ( 9 ). This may result in binding on one side, with uncontrollable drifting of the superstructure.

Figure 2.

Actuating devices (Utah Department of Transportation and Michael Baker Corporation 2013, U.S. Federal Highway Administration): (a) hydraulic jacks, (b) mechanical pulling devices, and (c) post-tensioned jacks.

Hydraulic Jacks

Hydraulic jacks are usually installed along with Teflon pads and a sliding track system to provide an anchor to push against and guide the bridge to its final alignment. To execute the slide, the jacks extend to full stroke to push the bridge forward while anchoring against the slide tracks or temporary supports. On a multi-span bridge, the hydraulic cylinders are usually connected to superstructure diaphragms over the abutments and piers, and cylinders are capable of pulling and pushing. Ridvanoglu indicated that the capacity and the stroke length of the hydraulic cylinders are important for the slide, especially to prevent binding ( 9 , 10 ). Binding may result in damage to the superstructure in the use of longer-stroke-length cylinders if the binding occurs at the beginning of the push cycle.

Mechanical Pulling Devices

Mechanical pulling devices, such as a winch or crane, can pull the superstructure along rollers or Teflon pads to its final position. Separate pulling devices can be used at each pulling location, or a system of pulleys can be used to allow one mechanical pulling device to pull simultaneously on multiple points. If using one pulling device with a pulley system, the bridge is uniformly moved on all pull points. One of the major drawbacks of mechanical pulling devices pointed out by UDOT and the Michael Baker Corporation is that there is no ability to “back up” the pull without a separate pull system set up on the opposite side of the structure ( 1 ). Consequently, the system is usually used along with hydraulic jacks.

Post-Tensioned Jacks

Post-tensioned jacks are small jacks used to pull an anchored post-tensioned strand or threaded high-strength bar and push the bridge into place on rollers or Teflon pads. UDOT and the Michael Baker Corporation pointed out that it requires abutment or diaphragm designs that allow anchoring of the post-tensioned strands and transfer from a pulling force on the strand to a pushing force on the superstructure, and there is no ability to “back up” the pull without a separate pull system set up on the opposite side of the structure ( 1 ). Ridvanoglu indicated that these systems are generally used with a pulling operation, since jacks can only apply tensile forces ( 9 , 10 ). In addition, cable systems do not require settling for each pulling cycle. UDOT and the Michael Baker Corporation indicated that the cable flexibility and prestressing losses could generate jerks in movement ( 1 ).

Movement Control Mechanisms

Two approaches are used to control movement during the slide: pressure-regulated systems and servo-controlled systems. Pressure-regulated systems are used more commonly than servo-controlled ones.

Pressure-Regulated Systems

Pressure-regulated systems are capable of controlling only the hydraulic pressure applied to a jack. Combined pulling and pushing methods are utilized multiple times. Pressure-regulated systems should only be used with guided slide systems, along with attentive visual monitoring of movement and a contingency plan. While most cases require force applications that result in equal displacements of supports, it is possible to slide the structure to a skewed position along a curved path of travel. Ridvanoglu indicated that pressure-regulated actuating faces differential friction resulting in the drifting of the superstructure ( 9 , 10 ). This event may be prevented by monitoring and/or using a short-stroke-length cylinder.

Servo-Controlled Systems

Servo-controlled systems monitor displacements and calibrate applied pressure automatically to balance the movement. They can be utilized to monitor real-time displacement in different rails to control an equal sliding rate. Servo-controlled systems maintain an aligned slide-in, given the difference in friction resistance is balanced with controlling the applied pressure. Servo-controlled systems should be utilized with unguided slide systems to eliminate the effects of differential friction resistance. Aktan and Attanayake indicated that the control of forces using the pressure control valves at the manifold is often quite slow ( 4 ). To allow accurate and rapid force control during the move operation, a servo-controller is required. The inclusion of the servo-controller requires the use of electronics and, most likely, a field computer.

On a multi-span bridge where more than two actuating devices are used for each slide, multiple controllers can be synchronized to achieve equal force or displacement and reduce the possibility that binding occurs. As a result, it reduces the chance of damage to the superstructure, substructure, and actuating devices.

Locations of Force Application

Most of the bridges constructed in the U.S. utilizing the SIBC approach have been single-span bridges. However, the SIBC approach has been successfully used to move superstructures with up to six spans. The superstructure of bridges with more than six spans is usually divided and prefabricated as six-span units and slid in individually. For single-span bridges, it is a common practice to place an actuating device at each abutment. For bridges with more than one span, the actuating devices have usually been placed at both the abutment and the pier diaphragms. However, coordination of separate mechanical systems is required at each push/pull location to perform a smooth slide-in.

Temporary Structures

To slide a multi-span superstructure using the SIBC method, temporary support structures are required at the pier location before and during the lateral slide. Temporary structures include a foundation, a frame system, and a sliding track. Loads transferred to temporary supports by friction forces need to be considered, as well as gravity loads—such as weight, traffic, and equipment—in the design. Another consideration of the temporary structures is their use is not only limited to the bent for the new superstructure but can serve as a bent for the old superstructure. UDOT and the Michael Baker Corporation indicated that defining the load path for the sliding forces is an important step when designing temporary supports ( 1 ). Force development in the transverse direction of the slide is generally disregarded in the design, yet field observations and slide monitoring studies show that forces develop in the transverse direction. Ridvanoglu classified temporary structures into two categories: inline and in-front temporary structures (as shown in Figure 3) ( 9 , 10 ).

Figure 3.

Temporary structures: (a) inline temporary structure, and (b) in-front temporary structure (Ridvanoglu 2016, Western Michigan University) ( 9 ).

Inline Temporary Structures

Inline temporary structures resist superstructure loads during construction and the initial stage of the slide. Inline support is connected to the permanent structure, and sliding is maintained from temporary supports to the permanent substructure. The design and construction of the connection between the temporary and permanent substructure have significant importance in ensuring a smooth transition during the slide. Axial forces, shear forces, and moments can be transferred through the connection when the temporary support is continuous. Bolts are the most common devices used to provide continuity of the connection. Continuous connections are most favorable since they provide a smoother path for sliding and minimize temporary support-related binding problems. Cold joints, hinges, and solid grout are used and classified as semi-continuous connections. Semi-continuous connections limit load transfer in some directions. A discontinuous connection is not recommended, since it may result in differential deflections at the connection, which prevents a smooth transition resulting in an increase in slide resistance.

An important lesson learned from the I-44 over the Gasconade River Bridge was to cast or erect the temporary bent such that it has a constant elevation across the top. This facilitates an easier process of sliding the structure, but typically requires a minor modification to the original bridge design. However, if constant temporary or permanent bent elevations are not possible, SIBC can still be completed even on a slightly sloped structure, as seen in the 2003 I-405 NE 8th Street Bridge slide. The structure was slid in two halves, split down the length of the structure, and formed a crown in the roadway cross-section of 2%, where the slopes of the temporary or permanent cap beams were sloped to match. Even with each half coming in at 4,400 kip, through the use of an innovative slide system at the time, the structure was successfully rolled up to the 2% grade ( 1 ).

In-Front Temporary Structures

In-front temporary structures include the construction of a temporary support system for the full slide-in operation. A lateral slide is operated on a temporary structure, and transfer to the permanent substructure is performed after the slide for permanent alignment. This system requires vertical lifting after the slide to place the superstructure in its permanent location. In addition to general considerations, eccentric loads can develop on the permanent substructure when an in-front temporary support system is utilized. Using the foundation of the permanent abutment to support the temporary support system and the connecting rail girder to the abutment cap in the permanent location has been documented for past projects. However, no record indicates that this type of temporary system has been used at a pier location.

Foundation selection for temporary support generally depends on the soil conditions. Driven piles, drilled shafts, micro-piles, or spread footings can be used. The foundation of permanent piers can also be used as a foundation for temporary support in specific applications. Ridvanoglu pointed out that the settlement and deflection of the system subject to the full bridge load should be calculated to determine the elevation to initially set the temporary support ( 9 , 10 ). It is also recommended that a moving load analysis be performed for the temporary support system considering forces developed in the direction of gravity, the slide-in, and the transverse of the slide-in. Furthermore, if traffic is shifted to the new superstructure while on the temporary structures, a traffic live load analysis should be performed.

Foundation Solutions

Many studies have been conducted to study the available foundation types for construction under existing bridges. The use of shallow foundations, drilled shafts, and micro-piles were recommended. Furthermore, supported excavation is recommended to assure the structural stability of the in-service bridge ( 7 ).

Spread Footings

Spread footings are the simplest and most cost-effective foundation alternative when soil conditions permit. Spread footings do not require excessive headroom during construction, and performance is the same as that with a traditional construction project.

Drilled Shafts

Drilled shafts are another alternative for the new bridge foundation. Note that the construction quality of drilled shafts with unsupported excavations could be a concern. Supported excavation for drilled shafts can assure the stability of the in-service bridge as well as foundation construction quality. Crosshole sonic logging can identify concrete consolidation problems with drilled shafts. Technologies such as compaction grouting and jet grouting have been successfully used to remedy drilled shaft construction flaws.

Micro-Piles

Micro-piles can be used when deep foundations are required, and traditional piles cannot be driven under the existing bridge because of limited vertical clearance. A micro-pile is a small-diameter pile (typically less than 12 in.) that is drilled and grouted. Micro-piles can be used in areas with low headroom because of their smaller size and segmental installation, which allow the use of smaller equipment. A new bent with micro-piles constructed near an existing foundation must avoid conflicts with any existing battered piles. Since micro-pile cross-sectional areas are smaller compared with other deep foundation systems, the buckling and lateral load capacities could be a concern.

Sometimes, the existing foundation can be reused for the new structure. However, only one project report was found where the existing foundation was reused. This is because the new bridge footprints are usually different from that of existing bridges. If the new bridge is on the same or partially on the same footprint, foundation reuse potential or replacement can be evaluated. The foundation reuse decision heavily depends on the availability of good quality design and construction records as well as the current condition of the foundation. Assessment of an unknown foundation requires a detailed investigation to collect the necessary data. Illinois DOT developed a comprehensive procedure and guidelines for foundation reuse ( 11 ). According to that, the existing substructure and foundation elements are assumed to have adequate load capacity for reuse without a detailed structural analysis when the following conditions are satisfied:

The substructure elements are in good condition (National Bridge Inventory [NBI] condition rating of 6 or higher) and show no significant structural distress under existing live loads.

The proposed service dead load is not greater than 115% of the original design service dead load.

There is no significant reconfiguration of loads (i.e., no changes to bearing locations or substructure fixities).

Diaphragm Types

A detailed literature search on the use of different diaphragm types over the pier for SIBC projects was conducted; however, little relevant information was found. More information related to the end diaphragm was found, which might give some hints on the design of the diaphragm over the pier. For example, UDOT and the Michael Baker Corporation indicated that the solid end diaphragm on semi-integral abutments provides a large, rigid member to jack up the bridge and mount the various sliding systems ( 1 ). The continuous diaphragm allows rollers or sliding shoes anywhere along the abutment (not just underneath the girders). Avoiding bearing points in the center of the abutment beam can minimize permanent moment loads and deflections. In addition, excessive deflections of the seat can cause sliding supports on the end diaphragm to lose contact with the abutment seat and require the end diaphragm to span between two adjacent slidings supports that still have contact. One solution to this is to design the end diaphragm to span over one slide support that loses contact. Another solution is to design the end diaphragm stiffness to allow flexibility and redistribution of the load as the seat deflects.

A review of bridge plans indicated that both steel bracing diaphragms and concrete diaphragms had been successfully used for steel plate girder bridges. For example, the I-405 bridge over N.E. 8th Street in Washington utilized steel bracing diaphragms over the pier, while the Hood Canal Bridge, also in Washington, used concrete diaphragms. On the I-44 over the Gasconade Bridge slide, steel W-shape diaphragms were used for the end and pier diaphragms. Originally designed for conventional construction, these items required design modifications to meet the needs of the SIBC system. The diaphragms had to transfer the pushing loads into the superstructure more effectively and were redesigned accordingly. Bearing stiffeners and connection plates outside the diaphragm provided the connection to the jacks for lateral movement. Additionally, because of clearance limitations, the diaphragms were designed to handle the vertical jacking loads necessary for the transition of the bearings. These two design modifications show the need for special considerations of the bridge diaphragms when using SIBC. Lastly, it was also noted that the flexibility of the steel superstructure was prevalent in ensuring no damages or cracking occurred from moving the structure into place. Another design detail to consider from New York DOT’s I-84 Bridge over Dingle Ridge Road is the use of diaphragms as the sliding surface rather than the beam bearings. This bridge was comprised of precast NEXT beams that sat on a prefabricated rigid diaphragm outfitted with four slide shoes. The design was done this way to meet the slide elevation and avoid conflict with other structures onsite.

Special Considerations

The slide-in process in the SIBC approach has special requirements for the design and construction of the substructure near the pier. The most commonly experienced challenges for the selection and construction of substructures include the large horizontal loading induced by the slide-in process, the influence of the new foundation on the existing substructure, and limited headroom.

The first challenge to overcome is the large horizontal loading during the slide-in process. The magnitude of the force required to initiate and maintain the movement of the superstructure depends on the weight of the superstructure and the coefficient of friction between the superstructure and the substructure. Aktan and Attanayake indicated that the weight of the superstructure to be moved is generally in excess of one million pounds, so the force required to initiate the motion will be about a half million pounds ( 4 ). Usually, the designs of bridge foundations do not consider the large horizontal forces induced by ABC implementations such as SIBC caused by pull or push mechanisms. Therefore, it is essential to evaluate the capacity of the substructure and foundation before the slide-in. If the foundation lacks the required lateral load capacity, temporary bracings can be designed to support the substructure and foundation. For the pier structure, the challenge can be overcome by using the in-front temporary structure, or through the reinforced design of the substructure.

The second challenge to overcome is the influence of the new foundation on the existing foundation since, most of the time, the SIBC approach requires the construction of the new foundation next to the original foundation, and the fill must be excavated and retained against the existing foundation. Aktan and Attanayake indicated that parameters such as the amount of displaced soil within the vicinity of the constructed foundation and the equipment used have a significant impact when the foundation is built in proximity to a structure ( 4 ). The dynamic effect of installing a new foundation adjacent to an in-service bridge is also a consideration in SIBC projects. To overcome this challenge, spread footing foundations, drilled shafts, auger piles, and micro-piles with proper installation methods are recommended. The effect of vibrations on the old foundation caused by pile installation should also be considered. Zekkos et al. developed a tool to estimate ground vibration caused by pile driving ( 12 ). This tool has been verified for a limited number of soil types. Even with limitations, such tools need to be utilized to predetermine the potential dynamic effects for planning purposes. Finally, during foundation installation, the existing bridge response needs to be monitored to ensure its stability.

Usually, the SIBC approach requires the construction of the substructure underneath the existing structure when traffic remains open on the existing structure. The headroom limits the construction of the foundation and the use of various equipment. This challenge can be overcome by both design and construction methods. UDOT and the Michael Baker Corporation indicated that it is a common practice to design the new bridge with a span length shorter than the existing bridge, which enables the new abutments to be constructed underneath the existing bridge before its demolition ( 1 ). For the pier, a straddle bent can be used to install foundations outside the existing bridge footprint. The bent is designed to span between the two foundations. When using a straddle system, deflection of the spanning element (seat) during the slide and in the final configuration should be considered. In addition to using typical columns and bent caps, hammerhead piers and piers with two outriggers, precast post-tensioned segmental piers, and prestressed or post-tensioned bent caps are also options for SIBC projects.

Field Monitoring of the Sliding Process

Although SIBC has been used for decades, few research activities have been conducted to study the structural performance during the slide-in. It was found that most of the SIBC projects were monitored with conventional monitoring tools to ensure the successful completion of the project. UDOT and the Michael Baker Corporation recommended the use of a conventional monitoring plan in each SIBC project to control the horizontal and vertical alignment of the bridge superstructure during the bridge slide-in ( 1 ). They suggested monitoring superstructure rotation around the longitudinal and transverse axes by measuring elevation or by using other methods approved by the project engineer. The authors also suggested observation and reporting of excessive deflections, twists, and changes in longitudinal and transverse gradients.

Ridvanoglu et al. suggested including a monitoring plan regardless of the selected structural system for construction ( 9 ). The report suggests using an actuating system under displacement control that utilizes synchronized self-monitoring systems to control superstructure movement and maintain the move at a steady rate. Typically, a conventional monitoring plan includes monitoring the hydraulic manifold pressure and displacements in the direction of the slide and transverse to the slide.

Shutt indicated that, to prevent drift, displacement in both actuating systems should be monitored during the slide ( 13 – 15 ). Uneven movements are frequent, and monitoring the displacement is essential for early corrections, which may prevent misalignments. Displacements are usually monitored using measuring tapes, total stations, or servo-controlled monitoring systems. Hydraulic manifold pressure is measured using pressure gauges, load cells attached to actuators, or computerized servo-controlled monitoring systems.

In addition to conventional monitoring of the slide-in process, SIBC projects have been conducted monitoring for specific interests. For example, the pier displacements were monitored during the slide of a four-span bridge on M-50 over I-96 in Michigan. During the bridge slide, the pier was instrumented with targets, and the movement was measured with non-contact laser equipment. The targets were mounted on the bent cap and the columns. The laser tracker was located with a view of all targets but about 150 ft away from the targets. The displacement data measured during monitoring were used to calculate forces applied to the pier during the slide in all three directions.

Field Monitoring of IA 1 Bridge Over Old Man’s Creek

The objective of conducting field monitoring on the IA 1 bridge over Old Man’s Creek was to monitor the construction effects on a bridge and its associated structural elements during the lateral sliding. To achieve this objective, specific monitoring goals were developed as follows: (1) Monitor the overall lateral slide force effects on the piers; (2) Monitor the uplift and overturning effects on the piers caused by the lateral slide forces; (3) Monitor the temporary work during slide-in operations; and (4) Assess the efficiency, drawbacks, and advantages of the slide-in procedures used.

Bridge Construction and Sliding Process

The IA 1 Bridge over Old Man’s Creek southwest of Iowa City was instrumented for field monitoring. The bridge has a length of 300 ft and a width of 47 ft 2 in., with three spans (90 ft, 120 ft, and 90 ft). Figure 4 shows the bridge orientation. The traffic flow is in the north-south direction, and the lateral slide was conducted from east to west. For better identification of the orientation, a Cartesian coordinate system was established with y in the vertical direction, x in the bridge transverse direction, and z in the traffic direction. As shown in Figure 4, the permanent pier on the south was labeled Pier 1, and the other was labeled Pier 2. The bridge superstructure consisted of seven rolled steel girders (W40 x 249 at the middle span and W40 x 199 at end spans) and an 8 in. reinforced concrete deck, as shown in Figure 5a. The permanent bridge substructure consisted of two wall piers founded on 14 H.P. 16 x 101 driven piles each, as shown in Figure 5b. The temporary pier consisted of six 1 ft diameter steel pipe piles capped with a steel beam, as shown in Figure 5, c and d . The permanent piers were nearly identical in size and construction. Further information about the monitored bridge could be found in the reports ( 16 , 17 )

Figure 4.

IA 1 bridge orientation.

Figure 5.

IA 1 bridge construction: (a) bridge superstructure, (b) Pier 1 permanent structure, (c) Pier 1 temporary structure, and (d) Pier 2.

Figure 6 shows the equipment that was essential to the slide-in process. As shown in Figure 6a, a fixed connection between the temporary and permanent piers was achieved by fastening the steel pile cap of the temporary pier to the face of the concrete permanent pier cap. A continuous steel channel was used across the temporary and permanent piers to guide the slide-in, as shown in Figure 6b. The Hillman rollers were used, equipped with horizontal guide rollers that allowed them to use the guide channel and to be guided and maintained on the path. Figure 6c shows the sliding shoes that carried the bridge superstructure. Eight sliding shoes were used at each pier, with each placed between the two adjacent girders under the concrete diaphragm (Figure 5a). Four hydraulic jacks were used, with one at each pier and abutment location (Figure 6d). The bridge slide took about 3.5 h on the afternoon of September 9, 2020. The superstructure was moved 2 to 3 ft during each push, and the hydraulic jacks were repositioned. At each push location, two to three team members worked to observe the slide-in process, apply the lateral forces, measure the movement, and communicate via walkie-talkies to ensure equivalent pushing at each jack.

Figure 6.

Slide-in equipment: (a) track over the connection between the permanent and temporary piers, (b) sliding track on the permanent pier, (c) sliding shoes, and (d) hydraulic jack.

Instrumentation Plan

The primary objectives for the field monitoring system were to monitor lateral force effects on the piers and temporary works and flexural stresses in the continuous girders. The lateral force effects were monitored to determine if the bridge slide-in was inhibited or if uneven forces were applied during the slide. The pier behavior was monitored for factors such as uplift and overturning effects. In addition, the transition from the temporary structure to the permanent structure was monitored for settlement or movement that has the potential to inhibit a smooth slide onto the abutments and piers. In addition to the pier monitoring systems, superstructure monitoring systems were developed to capture any flexural stresses in the horizontal plane of the bridge deck and continuous girders.

The pier instrumentation was designed to capture data to estimate the loads applied to the pier, calculate the movement of the pier cap, calculate the forces in the pier piling, and estimate the effects of binding or racking events. To accomplish this, 20 strain gauges, four accelerometers, 12 tilt meters, and four displacement transducers were installed on the bridge substructure. Piers 1 and 2 on the structure were nearly identical, and there was no significant difference in the superstructure above them. Accordingly, Pier 1 was outfitted heavily with sensing equipment to capture more data. Equipment was similarly installed on Pier 2; however, the number of sensors was fewer, and the focus of Pier 2 instrumentation was to provide validation of the results on Pier 1.

The strain gauges (H1 to H12) were installed on the piles 6 in. below the pier concrete, as shown in Figure 7. The gauges were installed on three piles (Piles 1, 8, and 14) on Pier 1 and two piles (Piles 1 and 14) on Pier 2. Figure 7b shows the gauge labels for each instrumented section. These strain gauges were arranged in four-gauge groups to monitor strains from bending effects, uplift, and gravity loads. This instrumentation arrangement allows abnormalities from imposed loads to be identified via strain values in the pile flanges and for the reactions in a pile to be calculated.

Figure 7.

Strain gauges installed on the pile: (a) instrumented piles, and (b) strain gauge on the pile.

Figure 8 shows the locations of the accelerometers, tilt meters, and displacement transducers instrumented on Pier 1. Four accelerometers (A1 to A4) were used, and all were installed on Pier 1. A1 and A3 were installed on the south face of the permanent pier cap to capture the acceleration in the z (traffic) direction, and A2 and A4 were installed on the east and west sides of the pier cap, respectively, to measure the acceleration in the x (bridge transverse) direction. Twelve tilt meters (T1 to T12) were utilized during the monitoring, with eight on Pier 1 and four on Pier 2. As shown in Figure 8, for the instrumentation on Pier 1, T1 to T6 were installed on the south face of the pier cap, and T7 and T8 were installed on the side. Pier 2 was instrumented only at the middle of the pier front face and the side of the pier cap, with T9 and T10 replacing T3 and T4, and T11 and T12 replacing T7 and T8. Among these tilt meters, T1, T3, T5, T8, T9, and T12 measured the rotation in the x-direction, and T2, T4, T6, T7, T10, and T11 measured the rotation in the z-direction. These tilt meters were used to monitor the pier caps for rotation. In addition to the accelerometers and tilt meters, four displacement transducers (C1 to C4) were used to measure the movement in the vertical direction, with C1 and C2 installed on the permanent pier, and C3 and C4 installed on the temporary structure.

Figure 8.

Pier 1 instrumentation: (a) pier front face, and (b) pier side view.

The superstructure was instrumented with a focus on capturing any flexural stress in the deck and girders induced by the lateral slide-in forces. To achieve this objective, the 14 extended BDI strain gauges (E1 to E14) and 16 regular BDI strain gauges (S1 to S16) were installed on the edge of the deck and the girders, respectively. Figure 9 shows the strain gauge locations on the deck elevation view and the bridge plan view. At each instrumentation location, the deck strain gauges were placed at the middle depth of the deck and used to measure the strain along the traffic direction. Figure 10 plots the BDI strain gauge locations on the girder cross-section view and bridge plan view. All of the girder strain gauges were installed on the exterior girder at the top surface of the bottom flange. They were placed to measure the strain in the traffic direction.

Figure 9.

Extended strain gauges installed on the deck: (a) strain gauge on the edge of the deck, and (b) deck strain gauge instrumentation plan view.

Figure 10.

Strain gauges installed on the girders: (a) strain gauge on the girder, and (b) girder strain gauge instrumentation plan view.

Monitoring Results

Over the course of the 3.5 h bridge slide, data were captured at all locations at a frequency of 20 Hz. During the slide, a signal loss occurred at approximately 75 min into the slide. The data collection was stopped, saved, and restarted. About 15 min after this stoppage, data collection was resumed, and the remainder of the slide was captured. It should be noted that the second set of data was offset to the final minute of collected data for all sensors. During the day of the slide and installation of most equipment, there was a steady rainfall event throughout the day, wetting most of the concrete surfaces. In most cases, the bonding agent was adequate even with this limitation; however, it was noted that, on removal, some strain gauges were removed more easily than what is typical and, thus, were potentially de-bonded.

In general, the slide-in procedure worked well for the multi-span bridge with respect to the steel girder superstructure and the piers. No noteworthy response from the substructure was observed during the slide-in, and no visible signs of distress (e.g., cracking) were observed on the concrete deck or piers. No indications of significant binding or restrictions were observed during the slide-in process. The field data collected from the gauges were processed and presented based on gauge type. Each type of data was interpreted with an emphasis on investigating the effect of the lateral slide on the bridge super- and sub-structure.

Acceleration Data

Although four accelerometers (A1 to A4) were used during the bridge slide-in, only one (A2) indicated significant bridge acceleration that matched the timing of each impulse force/push effort. Figure 11 shows the acceleration data from the A2 accelerometer, which shows the acceleration in the x (transverse) direction. As can be seen from Figure 11, there are many distinct spikes in the data corresponding to each jacking event. It is common in long periods of data acquisition for the data to drift. The data were corrected for this drift effect. The results indicated that the maximum acceleration in the x direction was 0.002 g. This level of acceleration is similar to the findings by Ridvanoglu, where the pier acceleration during the slide-in of a 198 ft long, two-span, precast concrete box girder bridge had a maximum measured acceleration because of impulse forces of about 0.0018 g ( 7 ). Acceleration levels in the other accelerometers (A1, A3, and A4) were minimal, and individual peaks could not be identified, further reifying the lack of impact. The A2 accelerometer was installed on the east end of the pier cap, connected to the temporary piers. It is reasonable to expect this side of the pier to be more sensitive to the slide forces than the other parts of the pier.

Figure 11.

Acceleration data from A2.

Tilt Data

The tilt data are plotted in Figures 12 to Figure 15. Figure 12 and Figure 13 plot the tilt about the z direction for Pier 1 and Pier 2 (See Figure 8 for gauge locations), respectively. Figure 14 and Figure 15 plot the tilt about the x direction for Pier 1 and Pier 2, respectively. The results indicate that the maximum tilt caused by the jacking force occurred about the x direction and was approximately 0.2 degrees (from T1). The maximum tilt about the z direction was 0.05 degrees. It was found that all the gauges measuring the tilt in the x direction on Pier 1 showed residual rotation (0.04 degrees) on completion of the slide (Figure 14). This indicates that a tilt in the z direction remained at the end of the slide because of the slide forces. Such a phenomenon was not observed on the gauges measuring tilt about the z direction or on Pier 2. This corresponds to the pier moment of inertia about the z direction being greater than that about the x direction, and Pier 1 has a larger portion above ground than Pier 2, which results in lower lateral stiffness.

Figure 12.

Tilt data from T2, T4, T6, and T7.

Figure 13.

Tilt data from T10 and T11.

Figure 14.

Tilt data from T1, T3, T5, and T8.

Figure 15.

Tilt data from T9 and T12.

To estimate the horizontal displacement in the z direction at the top of the pier cap, the tilt data about the x-direction were used. The pier wall was considered a cantilever, and a fixed condition was assumed at the bottom of the pier concrete encasement. This assumption aligned with the current Iowa DOT Bridge Design Manual assumption of a fixed condition occurring 6 ft below the ground for concrete-encased piles, and the bridge drawing indicated that Pier 1 had about 6 ft embedment into the soil. When Pier 1 was subjected to the lateral slide impulse forces, the maximum tilt was approximately 0.2 degrees about the x direction, which resulted in a maximum displacement of 0.6 in. and a force in the z direction of 400 kip at the top of Pier 1. This result shows similar magnitudes as those in a previously completed study by Aktan and Attanayake ( 4 ). In this case, Aktan and Attanayake found that the maximum pier cap displacement in the bridge traffic direction induced by the lateral slide impulse load was about 0.6 in., which corresponds to a force of 501 kip. The residual horizontal displacement in the z direction was calculated utilizing the tilt of 0.04 degrees. This resulted in a displacement of 0.14 in. and the force in the z direction of about 80 kip.

Displacement Data

Figure 16 plots the displacement data for four displacement transducers (see Figure 8 for gauge locations). Positive data indicate an upward movement and vice versa. The results indicate that the permanent pier has less vertical displacement than the temporary structure, which one would expect based on the materials and construction methods associated with each. The data from the C2 displacement transducer (at the side near the temporary structure) initially showed negative values after the superstructure moved onto the permanent piers, and, as the sliding continued, an uplifting action was observed. Comparing the data from the C1 and C2 displacement transducers, an opposite trend was observed during the second half of the slide-in. This is because, as more weight from the superstructure was transferred to the substructure, a bending effect about the z direction occurred on the pier. Both the C3 and C4 displacement transducers showed positive data with a maximum of about 0.1 in. The positive data are reasonable as, when the superstructure moved to the permanent piers, an upward movement occurred on the temporary structure. This movement was quite small and did not create issues during the slide-in process.

Figure 16.

Displacement data from C1, C2, C3, and C4.

Pile Strain Data

Figures 17 to 21 show the strain data collected from the piles (see Figure 7 for gauge and pile locations). Note that, during construction, strain gauge cables for H5, H7, and H14 were damaged, and, thus, no data were collected from these gauges. The maximum strain was measured at Pile 1 on Pier 1 at about 285 microstrain, which corresponds to stress of about 8.3 kips per square inch (ksi). The H.P. piles were Grade 36, with a yield strength of 36 ksi.

Figure 17.

Pile strain data from H1, H2, H3, and H4.

Figure 18.

Pile strain data from H5 and H7.

Figure 19.

Pile strain data from H9, H10, H11, and H12.

Figure 20.

Pile strain data from H13, H14, H15, and H16.

Figure 21.

Pile strain data from H17, H18, H19, and H20.

Figures 17 to 19 plot the strain for Pier 1. Generally, positive strain values were captured from Pile 1, and negative strain values occurred on Pile 14. This indicates that a moment about the z direction occurred on Pier 1 and an uplifting action occurred on the east side of Pier 1, which shows an agreement with the displacement results. Comparing these data with those collected from Pier 2, the results indicate that the pile strain measured from Pier 2 was small (less than 70 microstrain). The strain data collected from each pile were further processed to provide axial force, bending, and warping torsion forces at the instrumentation section of the pile. Equation 1, developed by Hibbeler and Kiang, was utilized to calculate these loads ( 18 ).

ε = \frac{P_{axial}}{EA} + \frac{M_{X} y}{E_{X} I_{X}} + \frac{M_{Z} y}{E_{Z} I_{Z}} + ε_{torsion}

(1)

where

$ε$ = the strain at the instrumentation location,

$P_{axial}$ = the axial force on the pile,

$y$ = the distance from the neutral axis to the strain measurement location, and

$X$ and $Z$ = the global coordinates (as shown in Figure 4).

Table 2 presents the results from the calculation. Given the four unknowns in Equation 1, $P_{axial}$ , $M_{X}$ , $M_{Z}$ , and $ε_{torsion}$ , four strain measurements are needed at each instrumented section. Because H5, H7, and H14 were damaged, only the instrumented section that had data for all four strain gauges was calculated for the moments and axial forces. The results indicated that an uplifting force of about 95 kip occurred on Pier 1 Pile 1. The same behavior was not observed on Pier 2, and both exterior piles (Pile 1 and Pile 14) on Pier 2 were subjected to downward axial forces. The calculated results indicate that the moment about the x direction ( $M_{x}$ ) at the base of Pier 1 ranged from 94 to 101 kip-in. Comparing this with the moment generated by the forces at the top of the pier cap in the z-direction (80 kip) with a lever arm of 24.5 ft (pier height), these $M_{X}$ values are quite small. The moment about the z direction ( $M_{z}$ ) of 27 to 86 kip-in is also minimal. In general, Pier 1 had larger responses than Pier 2 in both the longitudinal (z) and transverse (x) directions of the bridge. This could be explained by multiple reasons, such as the different embedment heights, different coefficients of friction, uneven distribution of the weight, and so forth.

Table 2.

Moment and Axial Forces Acting on the Selected Piles.

Gauge location	Gauge ID	Residual strain after sliding (microstrain)	Resulted force/moment component	Resulted force/moment
Pile 1, Pier 1	H1	−98	$P_{axial}$ (kip)	95
	H2	200	$M_{X}$ (kip-in.)	−101
	H3	285	$M_{z}$ (kip-in.)	86
	H4	50	na	na
Pile 1, Pier 2	H9	−120	$P_{axial}$ (kip)	−99
	H10	0	$M_{X}$ (kip-in.)	94
	H11	−120	$M_{z}$ (kip-in.)	33
	H12	−216	na	na
Pile 14, Pier 2	H17	−20	$P_{axial}$ (kip)	−33
	H18	−55	$M_{X}$ (kip-in.)	0
	H19	−65	$M_{z}$ (kip-in.)	27
	H20	−10	na	na

Note: na = not applicable.

Deck Strain Data

Rainfall occurred during the installation of the deck strain gauges. These conditions are not conducive to adhering the strain gauges to the deck, and, while removing the strain gauges after completion of the slide, it was observed that several strain gauges had become de-bonded. Because of this, strain gauges E3, E4, E5, E7, E11, and E12 did not measure significant data. Figure 22 and Figure 23 plot the strain data collected for the remainder of the gauges near Pier 1 and Pier 2, respectively (see Figure 9 for the gauge locations). For the gauges near Pier 1, the strain data collected from E2 and E10 showed an opposite trend. This indicates that a flexural moment occurred on the deck near Pier 1. Although the data from E8 and E14 (near Pier 2) also indicated a similar flexural bending, the strain magnitude was comparatively small.

Figure 22.

Deck strain data from E1, E2, and E10.

Figure 23.

Deck strain data from E6, E8, E9, E13, and E14.

Girder Strain

Figures 24 to 26 plot the strain data collected from the girder strain gauges. These gauges were installed on the top surface of the bottom flange of the girders (see Figure 10 for the gauge locations). Some strain gauges (S4, S5, S6, S8, S13, and S14) de-bonded because of the wet surface, and, thus, no strain data were obtained. After a careful investigation of the girder strain data, no conclusions with respect to the superstructure flexural behavior from these girder strain data could be made.

Figure 24.

Girder strain data from S1, S2, and S3.

Figure 25.

Girder strain data from S7, S9, and S10.

Figure 26.

Girder strain data from S11, S12, S15, and S16.

Conclusions

Based on the results from the literature review and field monitoring, the following conclusions can be made with regard to the practical and usable design guidance:

For most of the bridges with two to five spans, the whole superstructure was usually built continuously over the piers and slid simultaneously onto the permanent structure. For bridges with more than six spans, the superstructure was usually divided into units of up to a few spans and then slid into the final position using the SIBC approach. The investigation indicates that the maximum number of spans in each slide that has been performed is six.

It was found that the total length of the new bridge was usually shorter than the original bridge, since the SIBC method required the construction of the substructure for the new bridge under the original bridge without disturbing traffic on the original bridge. The common practice to achieve that is to build a new abutment in front of the original one. The new bridge is usually wider than the original bridge because of the increase in traffic volume.

Both spread footings and drilled shafts were commonly used for the pier foundation. The most frequently used substructure type is the beam-column frame pier with a spread footing foundation, although drilled shafts and driven piles were also used. The most commonly experienced challenges for the selection and construction of substructures include limited headroom, influence on the existing substructure, and the large horizontal loading induced by the slide-in process. During foundation installation, the existing bridge response needs to be monitored to ensure its stability.

With respect to the bridge girders, both prestressed concrete beam and steel plate girders have been used with SIBC. However, no special consideration for the lateral flexural stress level in continuous girders has been given to the design of the girders in the past. Both steel and concrete diaphragms were used with the SIBC approach without report of an issue. In general, the lateral forces were applied at all of the diaphragms over the abutment and pier. The diaphragms are expected to be designed as a large, rigid member to jack up the bridge, transfer the lateral load to the deck and girders, and place the rollers and sliding shoes in multiple locations to prevent load concentration.

Both Teflon-pad and roller systems have been used with multi-span bridges. For the selection of the sliding system, it appears that, when the superstructure for each slide exceeds about 300 ft in length or 50 ft in width, the roller support was commonly used, since a large, heavier superstructure requires a low coefficient of friction on the sliding track to reduce the lateral slide-in force demand. The past literature indicated that the coefficient of friction for the Teflon pads was usually assumed to be from 7% to 20%, while, for the roller system, the friction was usually assumed to be less than 5%.

Both steel and concrete temporary structures have been used with an inline setup. No outline setup had been used for a multi-span bridge. The inline setup slides the superstructure from the temporary structure directly to the permanent structure. Therefore, the connection between the temporary and permanent structures is critical. The different settlement between the permanent and temporary structure during the slide-in of the superstructure is usually a concern. A common practice to capture it is to perform a trial slide before the full slide-in to measure the different settlements. It was suggested that the settlement and deflection of the system subject to the full bridge load should be calculated to determine the initial elevation for the temporary support setup.

Usually, the design of bridge foundations does not consider the large horizontal forces induced by SIBC caused by pull or push mechanisms. Therefore, it is essential to evaluate the capacity of the substructure and foundation before the slide-in. The substructure should be evaluated for the effect of the uplifting force in the pier column and the overturning of the pier structure, the effect of the transverse forces (transverse to the sliding direction), especially for the unguided sliding system, and so forth.

It was found that the difference between the applied force and resistance is not constant throughout the slide-in, which may result in binding and uncontrollable drifting. To allow accurate and rapid force control during the move operation, a servo-controller is required.

The field monitoring results indicate the slide-in procedure worked well for the multi-span bridge with respect to the steel girder superstructure and the piers. No noteworthy response from the substructure was observed during the slide-in, and no visible signs of distress (e.g., cracking) were observed on the concrete deck or piers. No indications of significant binding or restrictions were observed during the slide-in process.

The flexural bending (about the z direction) of the superstructure on the horizontal plane occurred during the slide-in. However, the deck strain data were minimal, and the resulting forces were inconsequential to the structure. The superstructure consisting of steel girders and concrete diaphragms performed well during the slide-in.

An uplifting action was captured on Pier 1 with a strain of 285 microstrain measured at the pile strain gauge locations. However, the corresponding stress (8.3 ksi) is low compared with the steel yield strength (36 ksi).

A rough calculation indicates that the greatest forces in the z direction induced by the impulse pushing load experienced at the top of the pier was about 400 kip, with approximately 80 kip of residual force after the slide.

In general, Pier 1 had larger responses than Pier 2 in both the longitudinal (z) and transverse (x) directions of the bridge. This could be explained by multiple reasons, such as the different embedment heights, different coefficients of friction, or uneven distribution of the weight, and so forth.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: B. Phares, K. Freeseman; data collection: J. Dahlberg, Z. Liu; analysis and interpretation of results: J. Dahlberg, Z. Liu, K. Freeseman; draft manuscript preparation: J. Dahlberg, Z. Liu, B. Phares. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research team would like to thank the Iowa Department of Transportation (DOT) for sponsoring this work using state planning and research funding.

ORCID iDs

Justin Dahlberg

Zhengyu Liu

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