A Precasting Call

Jan. 1, 2006

Prestressed concrete evolved into a practical solution for the design and construction of highway bridges in the late 1950s and early 1960s. The development of prestressed technology coincided with the rapid escalation of efforts toward the development of the nation’s transportation infrastructure. Prestressed sections also facilitated off-site fabrication leading to rapid construction techniques, thereby reducing on-site construction time and limiting impacts to facility users.

Prestressed concrete evolved into a practical solution for the design and construction of highway bridges in the late 1950s and early 1960s. The development of prestressed technology coincided with the rapid escalation of efforts toward the development of the nation’s transportation infrastructure. Prestressed sections also facilitated off-site fabrication leading to rapid construction techniques, thereby reducing on-site construction time and limiting impacts to facility users. Precast prestressed concrete girder bridges rapidly gained favor as the preferred structure type, serving many departments of transportation (DOTs) well for nearly 50 years.

During this same time, California chose a different path than many states, adopting the cast-in-place (CIP) post-tensioned box girder as the preferred structure type. However, precast pretensioned girder bridges have always had a place in California. In fact, recent statistics reveal an increasing percentage of bridges in California are now being designed with precast girders. This fact is largely due to demands for accelerated project delivery as a top priority in the department and of its regional partners. It is expected that this trend will continue well into the future, particularly as new concrete materials such as self consolidating concrete (SCC) and ultra-high performance concrete (UHPC) become mainstream, enhancing the versatility of precast prestressed concrete.

The I has it

Unlike the rest of the country, the CIP post-tensioned box girder is generally the structure type of choice in California. The first prestressed concrete bridge constructed in California was a precast prestressed I girder pedestrian overcrossing at Arroyo Seco Parkway in Los Angeles in 1951. The California I girder has been in use in California for nearly 60 years and is here to stay.

The California I girder competes with the CIP box girder for span lengths ranging from 60 to 120 ft. The I girder has a depth-to-span ratio of approximately 0.055 for simple spans and reduces to 0.050 for multispan structures made continuous for live loads. This structure type has proven to be an excellent choice for widening existing structures; a commonplace occurrence to enhance system capacity in California. Once the deck is poured and the structural section becomes composite, there are no significant upward or downward deflections to transfer unwanted forces to the existing structure. Also, with no need for ground-supported falsework, precast girder construction usually takes far less time than cast-in-place construction, and the impact to the traveling public is thereby minimized.

The precast industry in California has worked very closely with the California Department of Transportation for many years, resulting in the development of extremely high-quality, cost-effective products. Precast girders are economically competitive with CIP structures, particularly for widths necessitating multiple girders with similar lengths and force requirements. Several California precast plants are capable of casting up to four girders in a single casting bed and achieving initial concrete strengths in excess of 6,000 psi within a 14-hour period. With production rates of up to four girders per day, the California I girder will become even more attractive as prolonged traffic congestion because of construction becomes more of an issue.

With the Cascade Mountains to the north, the Sierra Nevada Range to the east and the San Gabriel Mountains in the south, California has a large number of bridges located in extreme snow environments. Chain wear, deicing salts and freeze-thaw effects all contribute to accelerated deterioration of bridge decks. Precast girder systems are favored because of anticipated future deck replacements in these environments. A main drawback of CIP design and construction in this regard is that the entire box section is stressed as a single unit, making deck replacements virtually impossible due to unacceptable stress redistributions when the deck is poured in a zero stress state.

Bulb-tees in growing number

The California bulb-tee girder was first introduced in the mid-1990s. Although relatively new to California, the bulb-tee has been the structure of choice for dozens of bridges in California. The bulb-tee shape was introduced to compete with the CIP box girder in bridges with span lengths in excess of 90 to 100 ft. When used as a fully pretensioned unit, girders up to 140 ft in length have been transported by truck to various locations across California. When post-tensioning is used to splice several girder segments together, span lengths in excess of 180 ft are possible. The depth-to-span ratio for fully pretensioned simple spans is approximately 0.050 and can be reduced to 0.045 when multiple spans are made continuous through the addition of mild reinforcement across the bent cap for live load. When spliced together with post-tensioning, depth-to-span ratios as low as the CIP post-tensioned box girder (0.040) can be achieved.

Several benefits are attributed to the bulb-tee in comparison to the California I girder. The characteristics of the bulb-tee shape provide a larger section modulus, often eliminating the need for harped prestressing strand. Harping strand is expensive, and often cumbersome, as the hardware used to harp strand results in an abrupt angle change in the strand pattern. Using straight strands and allowing up to 33% of the strands as debonded to control tensile stresses at the top fiber of the girder ends provides a much more desirable alternative to harping. The bulb-tee cross-section, due to significantly wider top and bottom flanges, has a larger lateral moment of inertia than the I shape. The increased stiffness in the weak direction requires minimal, if any, lateral bracing to prevent buckling failure during transportation.

Utilizing bulb-tees as spliced girders, providing not only superstructure continuity but also an integral cap-to-column connection, couldn’t become a reality in California until physical testing of developed seismic details was accomplished. A 50% scale model was designed and constructed in 1996 at the University of California San Diego Structural Testing Lab, where a large-diameter column was subjected to incremental displacement cycles, with the last three cycles at a ductility of eight. Although there was severe plastic hinging at both the top and bottom of the column, with moderate joint shear cracking in the bent cap, minimal cracking in the girders was noted. This was due, in part, to the torsional rigidity of the post-tensioned bent cap providing a mechanism to distribute forces fairly equally to all four girders in the model. Keeping the superstructure “essentially elastic” during an extreme seismic event was the goal in developing the design methodology and details, which were validated by the test results. NCHRP Research Project 12-57, resulting in NCHRP Report 517 titled Extending Span Ranges of Precast Prestressed Concrete Girders, provides much needed spliced girder design guidelines and examples on a national level. AASHTO LRFD Bridge Design Specifications language was developed, and spliced girders have now been recognized as a unique structure type in this code. The design examples in the NCHRP report include:

  • Design Example 1: 200-ft single span spliced PCI BT96 girder, made up of three segments;
  • Design Example 2: Two-span spliced U-girder; 318 ft total length with unequal spans; and
  • Design Example 3: Continuous three-span haunched girder, with a 280-ft main and 210-ft end spans.

Tub of alternatives

The California bathtub girder, developed at the same time as the bulb-tee, hasn’t been quite as successful as a competitive structure type. Detailed with sloped webs to mimic the appearance of the CIP box girder, the bathtub section weighs about 50% more than an equivalently deep bulb-tee section, making it difficult and costly to transport. The bathtub section is much more difficult to fabricate, adding significant cost compared to the I and bulb-tee sections. Because of these cost and transportation issues, the construction industry is reluctant to invest in standard steel forms, further escalating the girder cost.

The California Voided Slab is used routinely for spans up to the 60-ft range. The depth-to-span ratio of these girders is 0.030, making it the shallowest concrete section available as a function of depth. Slab widths of both 3 and 4 ft are pre-engineered, allowing for both quick design as well as rapid construction. The voided slab is commonly used to span small creek crossings, where ground-supported falsework is prohibited. It also is used in remote areas of California where ready-mix concrete is difficult to obtain.

Other shapes, including box beams, rectangular girder and delta girders, are used in certain situations, but on a much smaller scale. From an engineering standpoint, it is clearly advantageous to possess an assortment of precast alternatives, each providing a unique solution for site-specific issues.

It’s happening everywhere

California is nearing completion of a massive effort to seismically retrofit the nearly 12,000 state-owned bridges in its inventory. Structural deficiencies were identified in a systematic process based on criteria established by the department and validated by an independent panel of academicians and professionals engaged in structure design and analysis. The evaluation criteria were developed from knowledge gained through post-earthquake evaluations, as well as a vigorous, continuing research program established to improve seismic design details.

The most widely publicized projects stemming from this effort relate to the major toll structures primarily located in the San Francisco Bay area. This region also is widely recognized for its large-scale seismic events, including the 1989 Loma Prieta and the 1906 San Francisco earthquakes. Precast elements played a major role in the retrofitting and replacement of three of these structures: the east spans of the San Francisco-Oakland Bay Bridge (SFOBB) Skyway project, the San Mateo-Hayward Bridge widening and retrofit and the Richmond-San Rafael Bridge lower trestle replacement.

The SFOBB carries I-80 traffic across the Bay and includes three distinct crossings: the 1.95-mile-long west bay spans connecting the east waterfront of San Francisco with Yerba Buena Island, the island crossing and the east bay spans into Oakland. The east bay spans consist of a new crossing comprising a new architecturally inspired self-anchored suspension span bridging the main shipping channel near the island and a nearly 1.3-mile-l precast segmental bridge referred to as the “Skyway,” which touches down in Oakland. The superstructure comprises 452 precast segments, cast in a temporary yard in Stockton, Calif., and shipped to the site on barges. Nearly 75% of the segments had been cast as of October 2005, with an average production rate of three to four per week. Fifty percent of the units were erected and post-tensioned in place during the same time frame.

Work on the 4.9-mile-long San Mateo-Hayward Bridge involved retrofitting 20 piers to enhance lateral capacity, as well as widening the structure. The existing piers lacked the lateral capacity deemed necessary to resist collapse during a major seismic event. Precast “dog bones” fabricated in halves on barges off site were post-tensioned together around the piers and pin connected to four large-diameter piles designed to limit seismic displacement. Each dog bone weighed 550 tons, with the reinforcing steel cage weighing 55 tons. The design of the widening incorporated precast elements in 90% of the structure. A CIP deck was placed on thin precast stay-in-place deck panels supported by precast bulb-tee girders resting on precast bent cap shells integral with precast piles. A total of 19,000 precast deck panels, 2,168 precast prestressed 42-in.-deep by 86-ft-long bulb-tee girders and 826 precast concrete piles, each 42 in. by 118 ft long, were included in the structure.

Finally, the Richmond-San Rafael Bridge project addressed replacement of a 1.25-mile-long deteriorated trestle section. The design incorporated a 43.75-ft-wide single precast double-tee girder. Each 100-ft span comprised three match-cast precast segments; a 40-ft pretensioned center segment and end segments with linear haunches. The three segments were post-tensioned on a barge prior to erection, then four simple spans post-tensioned on the piers for continuity to create 400-ft-long frames.

Precast power

As seismic connection detailing improves and material technology continues to develop new high-strength concrete materials, precast elements will continue to compete for applications in the California transportation infrastructure. The advantages of rapid construction coupled with tighter fabrication controls allows implementation of new materials in precast that might otherwise be difficult to utilize in on-site applications. Precast elements will continue to enable California designers to bridge otherwise difficult areas with speed, an arguable measure of safety for the traveling public not otherwise attained with CIP applications and more secure assurances of the resulting product quality.

About The Author: Wolfe manages one of two bridge design offices at Caltrans responsible for project delivery. Pope is chair of Caltrans’ Prestressed Concrete Technical Committee.

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