Movable bridges

R. Saul , K. Humpf , in Innovative Bridge Design Handbook (Second Edition), 2022

Bridge deck

The bridge deck has a span of 55.8   m and hoists a four-lane roadway 16.00   m, the walkways 2   ×   1.15   m totaling 2.30   m, for a combined span of 18.30   m.

The distance of the main girders is 13   m and the two cantilevers are 2.65   m long (Figure 18.10 ). The orthotropic deck consists of the deck plate, with a thickness of 12  mm; the bulb-shaped longitudinal ribs, with a distance of 310   mm and a depth of 160   mm; the narrowly spaced (d   =   1.65   m) cross-girders, with a depth of 640   mm corresponding to 1/20 of their span; the 60-mm-thick asphalt layer.

Figure 18.10

Figure 18.10. Section of the bridge deck.

The main girders have a depth—as the approach viaducts—of 2.64   m corresponding to 1/21 of their span. They are stiffened by vertical stiffeners on the outside only, and, therefore, they are an early application of the tension field theory. The weight of the steel structure is 381 tons, corresponding to 360 kg/m2, and the total weight of the bridge deck is 540 tons.

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Bridge structural theory and modeling

Alessio Pipinato , in Innovative Bridge Design Handbook (Second Edition), 2022

Isotropic and orthotropic plates

Isotropic or orthotropic superstructure differences are defined in Figure 5.19 . In this subset, special attention should be given to the orthotropic deck, which can be solved with the following nonhomogeneous differential equation ( Huber, 1923):

Figure 5.19

Figure 5.19. Comparison of deflections and bending moment in a square isotropic and a square orthotropic plate.

(39) D x ∂ 4 w ∂ x 4 + 2 H ∂ 4 w ∂ x 2 ∂ y 2 + D y ∂ 4 w ∂ y 4 = p x y ,

where w is the deflection of the middle surface of the plate at any point (x, y) (Figure 5.20); D x , D y , and H are the rigidity coefficients defined by

Figure 5.20

Figure 5.20. Basic designations of an orthotropic superstructure.

(40) D x = E x t 3 12 1 − v x v y D y = E y t 3 12 1 − v x v y

(41) 2 H = 4 C + v y D x + v x D y ,

and p (x,y) is the loading intensity at any point as a function of the coordinates x and y. Consequently, the solutions to the equations have been inferred (Girkman, 1959). When modeling an orthotropic deck, a rough model could be built up with a plate element, considering different bending stiffnesses in the two principal directions. An advanced model—such as a model considering local effects, the choice of the specific type of rib, a refined analysis of transverse-to-rib connection, and so on—should be made of plate elements representing the local portion of the substructure.

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Steel and composite bridges

A. Pipinato , M. De Miranda , in Innovative Bridge Design Handbook, 2016

2.2.1 General

Composite bridges are more and more popular around the world since they combine some advantages of steel bridges with some key factors of concrete bridges. In fact, a composite bridge presents the following:

•

A steel main structure that is much easier to erect if compared to the construction of a concrete girder

•

A light structure, which imposes smaller loads on piers and foundations, allowing for economy

•

A concrete slab, which is cheaper and easier to build than a steel orthotropic deck, and also presents two other advantages:

•

A higher mass, which induces fewer vibrations, noise, and dynamic loads on the supporting structure

•

A top surface that allows for easy paving with traditional methods, while weak points of the orthotropic deck consists of the difficulty of realizing a strong binding, the delicate execution, and some concern about durability of its paving

Aside from these advantages, a composite deck has the following drawbacks:

•

Longitudinal tension forces can cause cracks in the slab, and the link with steel causes tensile stresses due to restrained concrete shrinkage.

•

With respect to a steel deck, the weight increases, which is a disadvantage for the longest spans

•

With respect to a concrete girder, the steel structure is usually more expensive in terms of material costs

In any case, composite bridges, if well designed, have shown to be competitive with concrete bridges in all small–medium spans and competitive with steel bridges in spans up to 120   m.

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Steel and composite bridges

Alessio Pipinato , M. De Miranda , in Innovative Bridge Design Handbook (Second Edition), 2022

2.2.1 General

Composite bridges are becoming more and more popular around the world because they combine some advantages of steel bridges with some key qualities of concrete bridges. A composite bridge has the following advantages:

•

A steel main structure that is much easier to erect when compared to the construction of a concrete girder

•

A light structure, which imposes smaller loads on piers and foundations, allowing for economy

•

A concrete slab, which is cheaper and easier to build than a steel orthotropic deck and has these two additional advantages:

–

A higher mass, which induces fewer vibrations, noise, and dynamic loads on the supporting structure

–

A top surface that allows for easy paving with traditional methods, whereas, in orthotropic decks, it is difficult to create strong bindings, the paving requires delicate execution, and there are some concerns about the durability of paving

While the composite deck has these advantages, a composite deck it also has the following disadvantages:

•

Longitudinal tension forces can cause cracks in the slab, and the link with steel causes tensile stresses due to restrained concrete shrinkage.

•

A steel deck weighs more than a composite deck, which is a disadvantage for the longest spans.

•

A steel structure is usually more expensive than a concrete girder with respect to material costs.

Well-designed composite bridges have proven to be competitive with concrete bridges in all small and medium spans and competitive with steel bridges in spans up to 120   m.

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Cable-Stayed Bridges

Weiwei Lin , Teruhiko Yoda , in Bridge Engineering, 2017

10.3.3.3 Composite Deck

The deck in cable-stayed bridges can also be built as steel-concrete composite section. Composite construction of steel and concrete is a popular structural method due to its numerous advantages against conventional solutions. The optimal combination of the properties of the two most popular construction materials, i.e., steel and concrete, results in structures that are both safe and economic (Vasdravellis et al., 2012 ). In cable-stayed bridges, the composite concrete slab over the steel orthotropic deck provides a new option. In composite bridges the anchors can be aligned with the stiffening girder or placed in an exterior position (under or in the slab plan).

To minimize the displacement in the middle span, a combination of deck types such as steel deck, concrete deck, and composite deck can be used for the mid-span and side spans. In such a case, heavier section (i.e., concrete section or composite section) should be used in side span, while lighter section (i.e., steel section or composite section) should be used in midspan to reduce the down-ward deflection in midspans and avoid the upward defection in side spans. The proposed sections for a cable-stayed bridge are shown in Fig. 10.24, in which the steel section was proposed for the midspan to reduce the self-weight, and the composite section was designed for the side span.

Fig. 10.24. Steel deck and composite deck. (A) Steel section for middle span. (B) Composite section for side span.

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Research and Training in ABC Structural Systems

Mohiuddin Ali Khan Ph.D., M.Phil., DIC, P.E. , in Accelerated Bridge Construction, 2015

3.9 Choosing the accelerated construction route in New Jersey

Since the author's work for ABC has mainly involved the New Jersey DOT, NJ Turnpike, and Port Authority of New York and New Jersey, progress made in ABC implementation is discussed here. Other states have similar administrative, design, and rapid construction issues. Northeast states such as New Jersey have been at the forefront of promoting and implementing innovative technologies to achieve improved work-zone safety, as well as motorist safety and comfort, by using jointless decks and integral abutments with minimal environmental disruption.

Audits are in practice at the New Jersey Department of Transportation (NJDOT) to ensure that designers and project managers are studying alternatives, new manufacture processes, connection details for prefabricated elements, management programs, and quality assurance. (Refer to the report on ABC presented by the author and New Jersey State Bridge Engineer at the FHWA Conference, Baltimore, MD, 2007.)

3.9.1 Superstructure work

Crews can cut the old bridge spans into segments and remove them, prepare the gaps for the new composite unit, and then set the new fabricated unit in place in an overnight operation. The quicker installation minimizes huge, daily, delay-related costs and daily traffic-control costs.

Construction is usually scheduled for the fall months, when the weather is more predictable. A single-course deck will save a minimum of 6   weeks in construction time compared to a two-course deck.

•

On the Route 46 Bridge spanning Overpeck Creek in Bergen County, NJDOT decided to use prestressed, precast beams to prevent painting cost.

•

Utilizing a precast superstructure (Inverset), NJDOT replaced a structure in South Jersey, Creek Road over Route I-295 SB.

•

Prefabricated deck panels (Inverset, which is no longer proprietary) for three single-span Route 1 bridges over Olden Avenue and Mulberry Avenue in Trenton, NJ were constructed in 2005, over weekends.

•

Besides exodermic and orthotropic decks, new materials used include High Performance Concrete (HPC) and corrosion inhibitor aggregate. Precast or steel diaphragms for prestressed beams have been allowed. Precasting has quality control and avoids reinforcement placement, concrete pouring, and weeks of curing of HPS: The author recently designed bridges with HPS 70W hybrid girders in New Jersey. It allowed for a longer span and lighter girders. Shallower girders improve vertical underclearance, reduce the number of girders to be constructed, and eliminate painting. Weathering steel provides enhanced resistance to fracture.

3.9.2 Parapets

A variety of parapets are used in New Jersey. NJDOT permits its contractors to use slip forms to increase the speed of construction, as done successfully with the Route I-195/I-295 Interchange.

3.9.3 Substructure work

Integral abutments with fewer piles have been successfully used in New Jersey. They can be constructed more quickly than conventional bridges. An example of an integral abutment bridge using prestressed concrete box beams on Route 46, over Packman's River, was designed by the author (Figure 3.4).

FIGURE 3.4. Great attendance and rapt attention by senior engineers at the Philadelphia SEI meeting.

Currently, a design is in progress for lesser-used, semi-integral abutments for bridges on Nottingham Way over Assunpink Creek in Mercer County, and Garden State Parkway Bridges over Jakes Branch.

Lighter piers or precast concrete pile bents save costs and duration, as exemplified by the Albany Street Bascule Bridge carrying Routes 40 and 322 into Atlantic City. Precast, posttensioned pier caps were recently used on the Route 9 bridge over the Raritan River and by the author on at the interchange of US Route 322 and NJ Route 50.

Drilled shaft foundations and concrete cylinder piles of 36–66   in in diameter are in use. Precast sheeting has been used for retaining walls and abutment. MSE abutments have performed extremely well.

NJDOT has used RFP material for fender systems for two bridges along the Jersey coast, Route 9 over Nacote Creek, and Route 9 over Bass River. It is environmentally friendly and eliminates marine borers.

3.9.4 Installing scour countermeasures

Minimal marine life disruption and quick construction are achieved by using gabion baskets, articulated concrete, or cable-tied blocks in lieu of traditional sheet piles. The author has prepared a "Handbook for Scour Countermeasures" for NJDOT jointly with the City University of New York (CUNY), which was approved by the FHWA, and in addition helped developed relevant sections of the NJDOT Bridge Design Manual.

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Long-span bridges

M. De Miranda , in Innovative Bridge Design Handbook (Second Edition), 2022

2.4 Structural elements

The main structural elements of CSBs are as follows:

•

Decks

•

Pylons

•

Stay cables

These will be discussed next.

2.4.1 Decks

Decks can be realized entirely in steel, entirely in concrete, or as a steel-concrete composite structure. Generally, the concrete solution is the most convenient for spans up to approximately 250   m. The composite structure can be used successfully in all spans up to about 600   m, but it adapts well to spans from 200 to 500   m (i.e., crossings of large rivers). The steel solution is the most expensive, but it is also the most suitable for bridges with spans exceeding approximately 500   m (that is, for long-span bridges).

Bridge deck (Figure 16.9a) with two planes of stays are realized effectively with two lateral girders, a series of cross-members with pitches varying from 3.50 to 7.00 m, and one slab in a concrete or orthotropic deck. Aerodynamic fairings, improving aerodynamical stability, and reducing drag, can be required for larger spans or higher winds ( Figure 16.19b and c , and see de Miranda and Bartoli, 2001). The decks of bridges with a unique central plane of stays are always realized with a central box girder; two lateral overhangs are usually envisioned with variable length from 2 to 8   m. Bridge decks, mainly for longer spans, must be streamlined, with minimum depth and good aerodynamic properties (Figure 16.19b). This not only reduces the wind drag and increases the deck flutter stability, but also reduces vibration amplitudes in the deck so it can withstand vibrations of the stay cables.

Figure 16.19

Figure 16.19

Figure 16.19. Typical cross sections for side suspension of deck: (a) Kniebrucke; (b) Rande Bridge; and (c) Higuamo Bridge.

2.4.2 Towers

Like decks, towers can be realized in steel, concrete, or in a steel-concrete composite. Typical configurations are illustrated in Figure 16.20.

Figure 16.20

Figure 16.20. Types of towers for cable stayed bridges.

The main technical problems are related to the upper part of the pylons, where very high vertical loads must be transferred in a limited space to the tower shaft and the horizontal components of the stay cables have to be equilibrated. Towers are not only a fundamental structural element, but they become the main aesthetical element in a CSB. For this reason, their design is a difficult, challenging task of integrations of structural/engineering statements and aesthetical/architectural aspects.

2.4.3 Stay cables

Stay cables are the main and more special elements in this type of bridges. Their behavior, and mainly their axial stiffness due to the sag effect, are nonlinear. The nonlinear axial stiffness can be taken into account in an effective engineering form by the equivalent Ernst modulus (Figure 16.21).

Figure 16.21

Figure 16.21. Equivalent Elastic Modulus for stay cables.

The following types of cable are mainly used (Figure 16.22):

Figure 16.22

Figure 16.22. Typical cross sections of stay cables.
•

Locked coil rope

•

Parallel wire cables

•

Parallel strand cables

Solid bars and twisted ropes are used less frequently today.

The locked coil rope system was used in the first German CSBs, and it is still used today, especially for bridges with small and medium spans. The advantages of prefabrication and the consequent high executive quality are balanced by the difficulties regarding transport and installation of these very long elements (which, for larger bridges, may weigh a great deal).

Cables with parallel wires are very stiff and have high resistance to fatigue and low aerodynamic resistance and therefore, except for the difficulty of installation of the large prefabricated elements, they are suitable for bridges with large spans. Cables with parallel strands, in which the strands are installed on site one after the other, are currently the most popular system just by virtue of their easy installation, which requires light and easy tensioning in the cantilevered construction.

The design of stay cables is influenced by four main aspects: strength, fatigue, durability, and aerodynamic stability, listed in order of increasing severity. In fact, strength aspects are well addressed, knowledge is sufficient, and the codes seem to cover all aspects. Fatigue, although there is more uncertainty, is a clear issue from a design point of view, even if the uncertainties of aerodynamic aspects must be taken into account. The durability of stay cables, related to frequent lack of proper inspection and maintenance, is an important issue that requires more research.

Although much knowledge has been acquired over the last decades, the aerodynamic stability of stay cables still presents some degree of uncertainty. This instability is basically due to direct aerodynamic sources, such as:

•

Von Karman vortices

•

Wake galloping of closely spaced cables

•

Buffeting, induced by wind turbulence

•

Galloping of inclined cable, or ice accumulation

•

Rain/wind induced vibration

In addition, there can be dynamic sources, like cable excitation due to deck/towers vibration from wind or traffic.

The aerodynamic causes depend on the wind actions on the cables. The parameters involved are:

•

Wind speed V

•

Cable diameter D

•

Cable damping c, ρ

•

Cable unit mass m

Their relative influence can be studied by looking at the dynamic equilibrium equation:

y ¨ ⋅ m + y ̇ ⋅ c + y ⋅ k = F V t D ,

where

y = transverse displacement of cable

k = stiffness of cable, inversely proportional to its length: k α L −   1

c = damping.

Also, it can be seen that the response y to wind action F are directly proportional to V, D, and L, and inversely proportional to c and m.

The nondimensional Scruton number takes most of these factors into account:

Sc = m ⋅ c c CR ρ ⋅ D 2 .

In order to avoid cable instability, the following empirical/experimental criteria were proposed:

•

Von Karman vortices usually induce small oscillations and the inherent damping of stay cables results sufficient.

•

Wind/rain oscillations can be kept small enough if

Sc ≥ 10 for smooth cable surfaces

Sc ≥ 5 for cable surfaces with helical ribs or protuberances that can prevent the stabilization of rain rivulets on the cable

According to this criteria, for Sc lower than 5, an additional damping system should be provided in most stay cables, independently of their length. Since the vibration of cables that are shorter than approximately 100   m is rare, the Sc criteria should be used only above this length threshold.

•

Wake galloping of closed-spaced cables and dry galloping of inclined cables occurs (PTI, 2007) only above a critical speed of

V CR ′ = 25 ÷ 80 ⋅ f ⋅ D ⋅ Sc .

Further investigations (FHA, 2007) showed that this statement is too conservative for real stay cables, and that, if Sc is greater than 3 and if the criteria for wind/rain vibration are fulfilled, no risk of dry galloping occurs.

The last cause of vibration is the forced oscillation of the cable ends. This effect does not depend on aerodynamic effects on cables and is often more difficult to control.

In this case, there are two control criteria:

•

To increase damping of the cable

•

To tune the cable frequencies in order to avoid the range of forcing frequencies

The increase of damping, for this and other instabilities, can be achieved in various ways:

•

By installing internal dampers between the cable and the protection pipe, which can be made by high-damping elastomers and viscous or friction dampers

•

By installing tuned mass dampers on the stay cable

•

By installing external hydraulic/oil/viscous fluid dampers

The change (typically an increase) in the cable frequencies can be achieved by means of cross-cables, or cross-ties, or "aiguilles," interconnecting the stay cables in various ways.

The idea of introducing cross-cables connecting the main stay cables was first proposed by Fabrizio de Miranda, who patented the system (de Miranda, 1969) in the previously mentioned design of a CSB for the Messina Strait Crossing. The purpose of the cross-cables was mainly to reduce the sag effect of the longest cables in order to increase their stiffness. These cables can accomplish this very well, but also intuitively, to reduce their tendency to move and vibrate.

Later, the first experiences of cross-ties with the purpose of stabilizing vibrating cables occurred with the Stormsund Bridge in Denmark in 1971 and later in Japan. And more recently, cross-ties have been used in many large bridges which, after their opening, presented excessive cable vibrations, like the Dames Point Bridge and the Pont de Normandy.

Cross-ties segment the free length of the stay cables, in the cable planes, increasing their first vibration frequencies and adding damping to the system due to interference due to the connected cables vibrating at different frequencies. Research on optimal cross-tie configurations is in progress, but cross-ties have already proved to be effective, and they are the most efficient way of counteracting the vibration of cables in very long-span bridges.

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Modular Bridge Construction Issues

Mohiuddin Ali Khan Ph.D., M.Phil., DIC, P.E. , in Accelerated Bridge Construction, 2015

5.4 Advancements in prefabrication technology by AASHTO and the prestressed concrete institute

5.4.1 Introduction

There are several types of rapid construction technologies currently used in the United States. One technology uses precast concrete bridge components that are fabricated off-site, allowed to cure, and then transported to the construction site for installation (shown in Figure 5.1). This technology allows bridges to be constructed faster than traditional construction methods, reducing the amount of time the bridge and/or associated roads are closed to the public, and reducing the total construction time. For bridges above waterways, the construction time is also reduced; thus the amount of debris that falls from the construction site is reduced, which in turn reduces the environmental impact.

FIGURE 5.1. View of a semitrailer traveling to the site for erection by crane.

The widely used PCI Bridge Design Manual provides concrete girder shapes with standard dimensions and properties of typical sections. Examples of standard sections are as follows:

AASHTO solid and voided slab beam—For small spans

AASHTO box beams—For small and medium spans

ASHTO I-beam—For small and medium spans

AASHTO-PCI bulb-tee—For small and medium spans

Deck bulb-tees—For small spans

Double tee beam—For small and medium spans

AASHTO-PCI-ASBI standard segment for span-by-span construction—For long spans

AASHTO-PCI-ASBI standard segment for balanced cantilever construction segments—For long spans.

Their selection is based on the following design considerations:

•

Live load intensity such as HS 20, HS 25 and H 20

•

Other AASHTO-specified loads such as braking forces and earthquake, etc.

•

Girder spacing of 5–12   ft (or using adjacent box beams)

•

Boundary conditions such as partial continuity at supports provided by the deck slab

•

Other special requirements such as transportation and erection

There are six different types of AASHTO I beams : Type I (28″ deep) to Type VI (72″ deep).

The precast prestressed units are available off the shelf from the manufacturing companies, and only a limited notice of delivery to the construction sites is required. However, it is not easy to connect I shapes with the precast deck slabs. Composite sections are possible with the cast-in-place deck slabs. To make composite sections with the deck slab, vertical rebar shear connectors are required. Transverse diaphragms provide stability for the longitudinal girders and help transfer the loads.

5.4.2 Ready-made technology for full-depth decks

Quick assembly of bridges has advanced in the recent years, in part through the use of superstructure proprietary systems for new bridges and bridge rehabilitation, such as:

CONSPAN: A complete assembled small-span reinforced concrete bridge.

Inverset: The method uses composite rolled steel joists and concrete deck panels. Prefabricated deck panels for three single-span Route 1 bridges over Olden Avenue and Mulberry Avenue in Trenton, New Jersey were constructed in 2005, over weekends.

Effideck precast systems: Prefabricated deck panels using "Effideck" were used for the replacement of a Route 1 Bridge in Trenton, New Jersey, paving the way for future rapid construction and minimal traffic impacts. Unique details are provided in the NJDOT Bridge Design Manual for reference.

SpaanSpan: A low-profile, precast concrete, through-girder bridge system that uses post-tensioned edge girders and precast drop-in deck panels in which after installation, the deck is post-tensioned in the longitudinal and transverse directions.

Exodermic bridge deck: This combines a reinforced concrete slab on top of, and composite with, a steel grid. Exodermic decks are made composite with the steel superstructure with headed studs welded to stringers, floor beams, and main girders through blackouts in the precast concrete.

Orthotropic decks : Rebars allow two-way bending and load transfer.

Open steel grid bridge flooring: A steel grid–reinforced concrete deck system, which can be precast prior to installation, for both temporary and permanent bridge decking applications.

Use of prefabricated trusses.

Low-cost design alternates, which include reducing the number of steel girders with HPS.

Figure 5.2 shows a full-depth slab-beam precast deck section. The voids allow selected utility pipes to pass through by concealing them against exposure to weather.

FIGURE 5.2. Precast prestressed slab beams used in New Jersey.

5.4.3 Partial-depth prefabricated deck panels

These act as stay-in-place forms that help accelerate and control construction for decks that are more durable than fully cast-in-place decks. Full-depth prefabricated bridge decks also facilitate construction; bridge designers are finding innovative ways to connect full-depth panels.

Partial ABC retains conventional design-bid-build construction management but uses precast and partly assembled superstructure and substructure components. For example, NJDOT uses precast, prestressed hollow adjacent girders for small spans, without an 8-in. thick structural slab. This may be regarded as achieving partial ABC. It reduces the dead loads on the substructure.

Full ABC requires design-build management of precast and assembled components. Conventional, partial ABC, and ABC-managed bridges will all be designed for the same live load, wind and snow loads, etc. In each case, lighter density materials can be used; therefore member cross-sections will be smaller and dead load, thermal, and seismic forces will be lower on the substructure and the foundations.

5.4.4 Deck replacement applications with prefabricated full-depth panels

Table 5.1 provides information from selected states (Kentucky, New York, and Virginia) on completed full-depth panel projects. For further details, see the FHWA ABC Website. Since there is no design code available, the listed design details can be used for guidance.

Table 5.1. Examples of Successful Projects Completed in the United States for Prefabrication of Full-Depth Panels

Name Location Built Description Prefabrication
U.S. 27 over Pitman Creek Somerset, KY 1993 700-foot bridge Full-depth deck panels
Troy–Menands Bridge Between City of Troy and Village of Menands, Rensselaer/Albany Counties, New York 1995 Six panels: 900 square feet of deck area per night Exodermic precast concrete full-depth deck panels using lightweight concrete
Route 7 over Route 50 Route 7 over Route 50 bridges, Fairfax County, Virginia 1999 Replace approx. 14,000 square feet of bridge deck Precast deck panels (lightweight); placement of rapid-setting concrete overlay supporting full traffic after only 3   h of curing

5.4.5 Partial prefabricated bridge elements and systems

Prefabrication technology in the United States is not new. When the deck slab and girders are prefabricated separately, partial advantages of composite behavior will result. Welded or rolled steel girders and precast, prestressed girders have been used in many of the older bridges, since AASHTO standardized the I-shaped girders a long time ago. Hawaii has successfully used partial-depth panels for their superstructures (as shown in Table 5.2).

Table 5.2. Example of Successful Project Completed in the United States for Prefabrication of Partial-Depth Panels Only

Name Location Built Description Prefabrication
Keaiwa Stream Bridge Route 11 near Pahala, Hawaii 2000 Seven-span, 230-ft long concrete bridge Precast pretensioned partial-depth deck panels

5.4.6 Prefabrication of full precast concrete superstructure components

Table 5.3 shows many U.S. states, such as New York, Pennsylvania, Texas, Virginia, Washington, and West Virginia, using a fully prefabricated superstructure. Design details may vary for each state, but Inverset applications are more popular. The units are brought to the site by SPMTs, lifted by high-capacity cranes, and placed into position on top of the bearings, to which they are anchored.

Table 5.3. Examples of Successful Projects Completed in the United States for Full Prefabrication of Superstructure

Name Location Built Description Prefabrication
I-10 over Lake Pontchartrain 2002 Span 65   ft long and 46   ft wide 7.5-in. concrete slab cast on precast prestressed concrete girders
Tappan Zee Bridge Hudson River, 13 miles north of New York City 1998 16,000-ft Tappan Zee Bridge carries 130,000 vehicles per day Exodermic precast concrete, full-depth deck panels
Main Street over Metro North Railroad Tuckahoe, New York 2000 Through-girder bridge Precast prestressed concrete and steel composite superstructure
Norfolk Southern Railroad Bridge over I-76 I-76 east of U.S. Rte. 202 Interchange, Upper Merion Township, Montgomery County, Pennsylvania 2002 240-ft long, 42-ft high, 740-ton steel truss railroad bridge Truss raised onto four 330-ton Hillman rollers and hydraulic winches used to pull it to its final position over the expressway
Lavaca Bay Causeway Between Port Lavaca and Point Comfort, over Lavaca Bay, Texas 1961 Existing causeway Precast monolithic beams, precast prestressed deck composite units
U.S. 59 under Dunlavy, Hazard, Mandel, and Woodhead Streets Arch bridge on U.S. 59 (TxDOT) 1995 Attractive tied-arch bridges; structures suspend a thin slab from two tied arches 45 ft apart Existing bridges used as work platforms for erecting arches. Prestressed deck panels precast in segments and bolted to erection beams to eliminate the need for falsework under the bridge.
Wesley Street Bridge Ragsdale Creek in Jacksonville 2002 Precast prestressed slab beams
Dead Run and Turkey Run Bridges George Washington Memorial Parkway, Virginia 1998 Dead Run Bridge has three spans, with two structures 305 ft long. Turkey Run has four spans, two structures 402 ft long Full-depth noncomposite deck panels used for both the bridges
Richville Road Bridge Manchester, Vermont 2001 Single-span bridge 69 ft long and 32 ft 8 in. wide Three Inverset units consisted of two rolled beams with a precast reinforced concrete deck
Northeast 8th Street Bridge NE 8th over IH 405 in Bellevue, Washington 2004 328 ft long and 121.5 ft wide Totally prefabricated bridges
Lewis and Clark Bridge SR 433 across Columbia River between Oregon and Washington state 2004 Steel truss bridge 5478 ft long and 34 ft wide, with 34 spans Full-depth deck panels and precast approach slabs
I-5/South 38th Street Interchange Tacoma, Washington 2001 Two-span, 325-ft replacement bridge Precast post-tensioned box girder, tub girder segments, full-depth deck panels
Howell's Mill Bridge County Road 1 over Mud River in Cabell County, West Virginia 2003 245-ft long bridge and 32.5 ft wide, with two spans. Fiber-reinforced polymer (FRP) deck panels (8 by 32.5-ft) on weathering steel beams
Market Street Bridge Wheeling, West Virginia 2001 180   ft 6   in. long with a single span 177   ft long FRP deck and sidewalks replacement with half-inch wearing surface of polyurethane concrete and granite chips

5.4.7 Lightweight prefabricated trusses using timber and aluminum

For pedestrian, small-span bridges such as those required in public parks and for private gardens, prefabricated open parapets serve as longitudinal girders. Complete prefabricated bridges in lightweight timber and aluminum are being manufactured not on-site but under controlled conditions in a factory and are brought to the construction location, ready for installation.

5.4.8 Prefabricated glue-laminated wood sections or planks

An older method was to use sawn lumber. Smaller spans and small live loads are required. Wooden bridges are popular for pedestrian bridges. Timber planks are used as deck panels. Processed special-quality sawn wood is used. Wooden bridges are lighter in weight and are economical, especially in regions where tall timber trees grow abundantly.

5.4.9 Full prefabrication of bridge components off-site

Figure 5.3 shows typical prefabricated components commonly in use. Examples are precast box piers, pier caps, box beams, composite parapets, and deck wearing surfaces on the box beams. In most cases, the footing is cast in place; but for small spans and firm soils, precast footings are increasingly being used, subject to the geotechnical investigation and report. However, box beams are only one example. There are several types of full-depth or partial-depth precast girders that are also being used in lieu of the box beams, the details of which are provided in this chapter. For small and medium span bridges, prestressed concrete is more economical than steel.

FIGURE 5.3. Typical prefabricated components.

 (Photo courtesy of FHWA.)

AASHTO requirements are for a bridge life of 75   years, which can be accomplished with prestressed concrete.

5.4.10 Types of precast components for superstructures

The following components can be used:

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Precast prestressed deck panels

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Precast prestressed I-beams

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Precast diaphragm forms

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Precast pier cap forms

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Precast traffic barriers

The other components are precast parapets, cylinder piles, and precast approach slabs.

5.4.11 Use of precast concrete girders

Unlike building structures, where reinforced concrete beams of less than 20   ft are required, prestressed concrete girders are widely used for longer spans. Prestressing techniques have revolutionized the construction of bridge girders. Reducing tensile stress due to bending by inducing compressive stress has resulted in small-depth girders. Due to prestressing, girders can be of medium span lengths of 100   ft or even longer, but the longest lengths are unlikely to exceed 140   ft. The standard precast girders shapes are:

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Rectangular with depth exceeding the width

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I-shaped with bottom flange wider than top flange

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T-shaped

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Hollow or box girder.

Holes can be rectangular or round. Segmental construction is widely used for longer spans, and precast or steel diaphragms for connections in the transverse direction of prestressed beams have been allowed. Diaphragms help to distribute dead and live loads in both directions.

5.4.12 Use of prefabricated trusses

Increasingly, innovative bridge designers and builders are finding ways to prefabricate entire segments of the superstructure. This may involve prefabricated truss spans and pre-constructed composite units that are fabricated or assembled at or away from the project site and then lifted into place in one operation. Low-cost design alternates include reducing the number of girders with HPS.

5.4.13 Demolition first

In an overnight operation, crews can cut the old bridge spans into segments and remove them, prepare the gaps for the new composite unit, and then set the new unit in place.

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Fatigue cracking of welded railway bridges: A review

Guilherme Alencar , ... Rui Calçada , in Engineering Failure Analysis, 2019

3.4 Vibration-induced fatigue

Vibration-induced fatigue cracking in steel bridges can result from either primary or secondary stresses, and can originate from different type of actions, e.g. wind loading, railway traffic, earthquakes and others. Due to low local stiffnesses in correspondence of critical details, vibration phenomena can occur simultaneously or not with distortion-induced fatigue, and can in fact strongly affect steel bridges safety [55,127]. Vibration caused by wind was already recognized as being very critical since the well-known failure of Tacoma Narrows Bridge, which took place in 1940. In this case, the wind caused an excessive number of stress reversals of the bridge deck (made of steel riveted girders), which resulted in resonance with a 0.2 Hz torsional vibration mode, leading to failure due to low-cycle bending fatigue [49]. The sources of railway traffic-induced vibration in welded components can be separated as follows: i) due to the complex dynamic interaction phenomena between the train and the bridge; ii) due to dynamic effects caused by the contact forces between the wheel and the train considering the rail irregularities and the wheel defects and iii) due to the global and local resonance that can be created by the crossing of repeated and equally spaced axle loads. In the following subsection some examples of vibration-induced fatigue regarding the type of phenomenon are cited and discussed.

3.4.1 Examples of vibration-induced fatigue cracking

More recently, fatigue cracks on tensioned welded hanger connections of railway bowstring bridges, due to wind and traffic vibrations, were reported by Haghani et al. [56] and Klinger et al. [128], with an illustration example shown in Fig. 17. Hangers usually have very low bending stiffness, which makes them very sensitive to resonance with many stress cycles in the free vibration phase [129,130]. Since the goal of current work is to review fatigue due to traffic loads, herein a focus will be given for cracks caused by traffic loads.

Fig. 17

Fig. 17. (a), (b) Cracks induced by vibration, e.g. by wind or traffic in welded hangars of railway bridges [131].

One of the early reported cases of cracks due to vibration induced by traffic loads in railway bridges occurred in the Tokaido Shinkansen high-speed line [55]. Although high-speed passenger trains spacing are not considered very relevant in terms of ultimate limit state condition (ULS) due to their relatively low weight compared to freight trains [132], they are able to originate high dynamic effects on bridges. The advent of high-speed traffic led to an increase in reported fatigue cracks of welded bridges due to secondary stresses created by excessive vibrations observed at certain circulating high-speeds. Since the start of operation in 1964, speeds have been increased progressively and from about the time that 200 km/h was exceeded, vibrations of bottom flanges of stringers in plate girder bridges and truss girder bridges in directions perpendicular to bridge axes began to appear prominently when crossed by trains, after only 8 years of service [133]. Vibrations in out-of-plane directions have also been reported in diaphragms of box section girder bridges in the Tokaido Shinkansen, Fig. 18 [134]. The presence of local vibration modes of the web characterized by high-frequencies in the range 20–45 Hz, typically excited during train transit, see Fig. 18b could cause high local "modal" stresses leading to fatigue damage and reducing the remaining fatigue life [134]. The natural frequencies of these local modes were identified by means of the FFT from both numerical simulations and experimental measurements in the actual bridge due to the crossing of high-speed trains with speeds of about 270 km/h, considering a coupled dynamic analysis of the train-bridge system for the former. Accordingly, peak frequencies at about 30 Hz were distinctive with relatively large magnitude, which indicated the possibility of resonance of this high-frequency mode with the train moving loads [135].

Fig. 18

Fig. 18. (a) fatigue cracking in a steel box girder of the Tokaido Shinkansen and (b) out-of-plane vibration modes of the web leading to increase "modal" stresses in the web gap [134–136] (Bridge case study o), Table 3).

Another structure typology often used in long-span high-speed railway bridges which is very sensible to vibration-induced damages according to Lippi et al. [8] are orthotropic steel decks (OSDs). It is well-known that fatigue phenomenon is a worldwide problem to orthotropic bridge decks [137 ]. Recently, several cases of fatigue cracks in railway bridges with orthotropic decks were reported and investigated by the China Academy Railways [ 83,138]. In China, OSDs were adopted in the Chengdu-Kunming Railway for the first time during 1960s–1970s and have been developing at an unprecedented speed since 1990s. Cracks in OSDs are caused by the secondary bending stress concentration generated by the out-of-plane distortion at the welding restraint provided by the so-called rib-to-deck connection. OSDs have been widely adopted in high-speed railway bridges in China, and other numerous bridges are planned to be built in near future [139]. The study of the vibration phenomena caused by high-speed trains in orthotropic decks and hence the impact on the fatigue life has increasingly attracted research interest in recent years [140,141].

On the other hand, fatigue cracks due to vibration were also reported for train speeds lower than 200 km/h, specifically for HAL traffic for the welded bridge at FAST (bridge s), see Table 4). In particular, it was observed a strong correlation between rail irregularities and increased rate of fatigue crack initiation and growth. In railway bridges, the irregularities of the rail have a strong influence in the dynamic behaviour of the structure, and it can be separated between two parts: (a) static track irregularity, due to the inherent defects of the rails and the train wheels and (b) dynamic track irregularity, due to rail elastic deformation under train loads [142]. In the welded bridge at FAST, the static irregularities, i.e. without the effect of the live load were measured to have an amplitude of 2.30 mm on the north rail and 2.80 mm on the south rail, which were considered of moderate level. However, they were enough to cause strains about 10% higher than with smoother rails. The higher strain ranges observed for non-smoothed rails resulted in about a 50% increase in fatigue accumulation per train crossing [81].

Other example which occurred for speeds under 200 km/h was at the steel structure of one of the elevated parts of the Washington D.C. Metro, specifically in the crossing over the Potomac River (bridge u), Table 4). The Washington D.C. Metrorail system was built in 1970's and inaugurated in 1976. Each train is 22.86 m long with a unique track gauge of 1429 mm and being able to operate at a maximum speed of 121 km/h. The crossing within this portion is composed of multiple simple spans across concrete piers, varying between 21 and 36 m approximately, with two steel box girders section, which support a RC slab. Within each box girder, K-type diaphragms are weld connected to transverse connection plates which are fillet weld connected to the box girder webs. The referred fatigue cracks occurred in the transverse connection plate weld ends [98]. Herein, vibration with frequencies with magnitude in the order of 20 Hz were observed (Fig. 19), in a very similar manner with the steel box girders fatigue cracking found in the Tokaido Shinkansen bridges.

Fig. 19

Fig. 19. View of the Metrorail double-box steel girder corner of the bridge over the Potomac River with out-of-plane vibration of the girder web [adapted from [98]] (Bridge case study u), Table 4).

3.4.2 Designing to vibration-induced fatigue

Due to their lightness and low damping capacities, it is likely that steel bridges could experience resonance-induced vibrations produced by trains running slower than the maximum design speed usually set to 350 km/h for high-speed lines. If a frequent operating speed of a HS train passing through high-speed railway bridge corresponds to the resonant speed, excessive span deflections and vibration may be caused and, therefore, the dynamic behaviour of railway bridges subjected to high-speed train loading should be taken into consideration in the fatigue assessment [143]. Most of the first bridges built for the Tokaido Shinkansen in the 1960s, where most fatigue cracks due to vibration-induced fatigue were early reported, adopted pure steel solutions (ballastless track half-through girder, through truss, box girder or girder bridges), which are well-known to have very low mass and damping ratios. In the case of modern high-speed railway bridges, very strict design criteria in terms of deformability (vertical, horizontal and torsional) and dynamic behaviour are now recommended by standards and technical-scientific literature [143–145]. These criteria can be fulfilled by designing a main steel or composite section, providing the required stiffness, incorporating a concrete slab underneath the track and adopting ballasted tracks. The ballast and the concrete introduce additional mass and damping under the track, thus reducing the dynamic vibrations induced by the live load. This design philosophy has been employed throughout the French HS lines since the TGV Nord, opened to traffic in 1990. The efficient use of materials, alongside with the design of simple details, lead to a good fatigue behaviour of these structures and, subsequently, to a longer lifecycle and lower maintenance [146].

Phenomena like vibration-induced fatigue are still partially uncovered by actual design codes and represent a critical aspect for the assessment of existing bridge remaining life and for the design of new bridges. One relevant attempt to change this scenario was performed by the European project FADLESS [8], whose main objective was to clarify the uncertainties concerning vibration/distortion induced fatigue into railway steel bridges. In particular, the most important aspects outlined include the dynamic behaviour, the influence of train-bridge interaction phenomena, the local vibration/distortion effects, the actual stress patterns into critical details, the real composition of traffic spectra and the influence of secondary stresses on the fatigue resistance of components. Several bridge case studies in Europe were selected in order to apply the methodology developed under FADLESS project, which included a variety set of bridges with different construction methods (riveted-only, welded, composite steel-concrete, etc.). Regarding vibration-induced fatigue on welded joints, the FADLESS project highlighted that the structural damping has an important effect on the dynamical behaviour of a railway bridge because it is one of the main factors limiting the vibration amplitudes. Its knowledge is therefore essential for the assessment of existing bridges subjected to high-speed trains. Moreover, the computation of the local stress effects by vibration phenomena in order to perform the fatigue assessment of critical details requires the consideration of the whole dynamic system for the global model. Because the task of numerical simulation is to check the local vibration of the steel bridge, one sufficiently precise numerical model is necessary, both from a global and a local point of view.

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Assessment of train running safety on bridges: A literature review

P.A. Montenegro , ... W.M. Zhai , in Engineering Structures, 2021

4.7 Safety assessment based on normative approaches

The research works based on normative approaches are mostly related with the analysis of the maximum vertical accelerations of the deck due to resonant effects and its comparison with the maximum allowances for ballasted (3.5 m/s2) and non-ballasted tracks (5 m/s2).

Museros et al. [146] proposed closed-form expressions to define resonant acceleration amplitudes and the train speeds for which maximum accelerations or cancelation effects take place. To test these equations, the authors compared the results given by them with those obtained with complete dynamic analysis carried out with the normative HSLM defined in [49]. They conclude that, although the close-form expressions do not give an exact estimation of the values associated with the resonant effects and should not replace the full dynamic analysis specified in the norms, they could be used in a first assessment of simply supported bridges according to the norms. Later, the same research group [147] analyzed the dynamic performance of existing short-span simply-supported bridges considering the normative approach. The authors studied two bridge typologies traditional used in the conventional Spanish railway network, namely the box-girder and slab bridges, and concluded that the 3.5 m/s2 limit value for the vertical accelerations due to the passage of the HSLM was exceeded in the slab bridges only for speeds higher than 270 km/h. The critical speed for the majority of the other type of bridges, however, was much lower, around 200 km/h, which justifies why the box-girder are rarely used for short spans in the recently constructed HS lines. Martínez‑Rodrigo et al. [148] evaluated the resonance effects on orthotropic decks caused by a series of moving loads and analyze their behavior in relation to the maximum vertical acceleration limit proposed by the norms, which, according to the authors, is one of the strictest requirements for HS railway bridges. Galvín et al. [149] performed experimental tests on five Spanish railway bridges with different topologies to identify their modal parameters, measure their dynamic responses under railway traffic under operational and maintenance conditions and analyzing the soil-structure interaction effects. The maximum registered acceleration was 2.96 m/s2 due to the passage of a HS train at 304 km/h in one of the bridges, showing that all of the analyzed bridges comply with the norm.

The normative acceleration criterion was also object of study in skewed bridges, such as in the work carried out by Galvín et al. [150], where the authors developed two distinct numerical models of a 45° skewed bridge, one defined by orthotropic plate elements and another with isotropic plates complemented with beam elements to simulate the longitudinal girders. After comparing the results obtained with both models, they concluded that the model type did not significantly affected the predicted response at midspan. However, and although still below the normative limits, the numerical accelerations obtained with the two models significantly overestimated the experimental data, possibly because the vehicle-structure interaction effects and the variations in the damping with the vibrations amplitudes were neglected. Later, Nguyen et al. [151] performed a dynamic analysis of skew bridges and evaluated the traffic safety based on the maximum acceleration criterion stipulated in [48]. They concluded that, although the dynamic vertical deflections of the bridges were significantly dependent on the bridge skewness, the maximum accelerations (resonance) were not affected by the skew degree, showing that the normative criterion is also valid for this type of bridges.

Schneider and Marx [152] studied the dynamic behavior of several structural systems for HS bridges, including single and continuous span bridges with different cross-section types, such as box girders, double T-beams and slabs with cantilevers. They concluded that, for single-span bridges, only those with spans less or equal to 35 m exhibited strong resonant effects, leading to maximum accelerations above the allowable limit specified in the norms for traffic safety. Moreover, the authors discuss the advantages of continuous bridges over multi-span bridges with simply supported decks, namely the fact that the dynamic responses of the former are smaller in the lower resonance speeds, due to the cancelation effects caused by the loading of the adjacent spans, and the critical first-order resonance speeds are shifted outside the typical range of operating speeds.

Peixer et al. [153] conducted a detailed evaluation of the traffic safety in a steel-concrete composite viaduct belonging to the French HS network based on the deformation and vibration criteria stipulated in the European Norms [48] and [49]. The main innovation of this work consisted of the evaluation of the influence of the viaduct's structural solution in the traffic safety, since the authors evaluated the normative safety criteria not only considering the original viaduct, but also considering an alternative solution based on a double composite action deck. Apart from leading to a more economic deck (12% reduction in the steel weight due to the introduction of a lower concrete slab in the lower compressed region of the deck above the supports), this alternative solution also led to an improvement in the normative safety indicators, namely reductions up to 17%, 36% and 34% of the maximum vertical displacements, accelerations and twist of the deck, respectively.

More recently, Ribeiro et al. [154] performed a numerical study on a metallic centenary bridge under light railway actions and concluded that, for certain train speeds, some of the modes, including torsional models, where particularly excited, leading to resonant effects that could reach accelerations close to the normative limit. This study showed that, even for low speeds, the accelerations can reach undesired values depending on the train configuration.

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