Review of Concrete Segmental Bridges by Dongzhou Huang and Bo Hu

Authors Dongzhou Huang and Bo Hu should be commended for their detailed tome, Concrete Segmental Bridges. The textbook is sure to become a useful reference for both students and practicing engineers due to its substantial breadth of information, from basic engineering principles to detailed bridge design examples. This book covers engineering basics such as Mohr's circle, moment distribution, and basics of the finite-element method. Fundamentals of concrete design concepts are covered in sufficient detail for this textbook to be useful for students or young engineers without significant experience in reinforced concrete or prestressed concrete design. Those fundamentals include LRFD theory, extreme fiber stress calculation, flexural strength capacity, prestress losses, tendon placement limiting zone, partial prestressing, modified compression field theory, and the strut and tie method. For those interested in how concrete segmental bridges came to be, there is also a section on the history of concrete segmental bridges that documents span length milestones. As a recent bridge type having found its start after World War II, design methods for concrete segmental bridges have evolved quickly and now is an appropriate time for a comprehensive text such as this.

Approximately 30% of the book is dedicated to history, design loads, material properties, and general theory of segmental bridge design and analysis. Useful design flowcharts are provided. The remainder of the text is devoted to specific segmental bridge types. Segmental box superstructure types are typically classified by their construction method, namely, span-by-span, cantilever, and incrementally launched segmental bridges. Other types of segmental superstructures such as spliced girder, arch, and cable-stay are included along with segmental substructures. Design methods documented in the text follow primarily the AASHTO LRFD Bridge Design Specifications, although guidance is presented from the American Concrete Institute (ACI) and the CEB-FIP Model Code (CEB-FIP) when appropriate. For each section, information specific to the bridge type is presented, such as component design and geometric details. Typical cross sections and examples are included with step-by-step summaries of the design procedures. The information presented extends beyond basic design to issues such as maintenance and required clearances for post-tensioning.

Span-by-span construction is presented as the most cost effective and simplest of segmental box bridge superstructures. In this erection method, the entire span is joined together and becomes self-supporting before the next span is cast. Closure joints are located between spans. The span-by-span bridge example presented consists of a continuous five-span bridge with equal spans of 45.7 m (150 ft). The bridge has a 12.2 m (40 ft) clear width and carries two lanes of traffic with shoulders. The typical box section was selected from the AASHTO-PCI-ASBI segmental box-girder standards. The design example includes longitudinal, transverse, diaphragm, and deviator design of the box section.

Longer spans of up to 137 m (450 ft) can be achieved with cantilever construction in which segments are placed symmetrically extending from each intermediate pier. Cast-in-place closure joints connect the cantilevers within each span. Unlike bridges constructed via the span-by-span method, cantilever bridges typically do not have equal span lengths. Accordingly, the cantilever design example has a span configuration of 49.7–70.1–49.7 m (163–230–163 ft). Details are provided in the example for the tendon layout, construction sequence, and variable bottom slab depth. Analysis is presented for longitudinal design including both final and construction loads, with construction loading being most critical.

The final construction method for a segmental box superstructure involves incrementally launching the segments. Typical spans constructed using this method are limited to lengths of 30.5–61 m (100–200 ft), but longer spans have been built. The construction method requires the constant depth segments to be cast behind the abutment and then pushed forward on the piers to bridge the span. Careful dimensional control is required, which can be more easily accomplished with straight bridges or bridges with constant curvatures. Launching methods and mitigating negative effects are described in the text because the method must be well understood by the design engineer. The calculations follow the AASHTO LRFD Bridge Design Specification, but the design example is for a bridge constructed in China and so some details do not meet current AASHTO requirements. The bridge consists of five spans, with 30-m (98.4-ft)-long end spans and 40-m (131.2-ft)-long intermediate spans. The example presents longitudinal design and analysis of the bridge.

Spliced I-shaped and newly developed U-shaped girders are covered in detail, with typical section diagrams, span configurations, and tendon layouts. In comparison to box segments, the segments used to construct spliced bridges are typically much longer. The design example presents a three-span spliced I-shaped girder bridge with 68.6-m (225-ft)-long end spans and a 86.9-m (285-ft)-long intermediate span with two traffic lanes and two shoulders. The example covers construction sequences, tendon layout, mild reinforcement, longitudinal analysis, and end zone reinforcing.

Concrete can be effectively used in arch bridges due to comparatively high compression demand compared to bending moment and the bridge type is a suitable choice for crossing long and deep valleys with span lengths between 61 m (200 ft) and 396 m (1,300 ft). Different types of arch bridges and boundary conditions are described with typical sections, details, and construction methods provided. The design example is based on the AASHTO LRFD Bridge Design Specifications, but the bridge used for the example was built in China according to applicable codes, with a maximum clear span of 160 m (525 ft), two travel lanes and two shoulders. The example focuses on the arch ring design.

The last several decades have seen an increase in the use of concrete segmental deck systems in cable-stayed bridges. Compared to steel deck systems, for spans between 152 m (500 ft) and 914 m (3,000 ft), concrete is suitable for resisting compression forces in the deck, has favorable damping behavior, and is less susceptible to aerodynamic vibrations. Cable-stayed bridges are classified as either conventional or special depending on the ratio between the pylon height and the center span length, with conventional cable-stayed bridges having a higher ratio of pylon height to span length. Details are provided for pylons, girders, and cable anchors. Again, the design example follows the AASHTO LRFD Bridge Design Specifications based on a bridge built in Shanghai, China, with a 300-m (984-ft)-long main span with two traffic lanes and shoulders. Load effects are examined, and the capacity is checked for the main girder, pylon, and cables.

The final two chapters cover segmental substructures and construction considerations for segmental bridges. Various pier types are discussed along with bearing types, design requirements for them, and expansion joints. The substructure design example is a rectangular-shaped pier for a bridge built in the balanced cantilever method. The final chapter on construction covers segment fabrication, geometry control, tolerances, handling erection, tendon installation, and filling.