In an area accustomed to top entertainment and “headliners,” the headline currently getting Las Vegas residents’ attention is that relief has finally come to the I-15/U.S. 95 Spaghetti Bowl Interchange. This interchange lies at the geographic center of the Las Vegas Valley and used to be the central bottleneck in the area’s regional transportation network. As the local population swelled, the upgrade of I-15/US-95 Interchange became Nevada’s number one statewide transportation priority.

Table 1: Growth Trends

Figure 1: Model of the Spaghetti Bowl Interchange

Table 2: Spaghetti Bowl Major Ramp Superstructure Statistics

Table 1 summarizes the growth that led to the need for this major improvement, which was to facilitate smooth traffic flow to downtown Las Vegas from just north of the famous Las Vegas strip, a major tourist destination offering gambling, entertainment and family recreation. Another important need was that existing traffic flows on the intersecting roadways not be interrupted.

The Project Setting: Major Structures and Construction Methods Used

The reconstruction of the I-15/U.S. 95 (Spaghetti Bowl) core interchange was the first major project in the western U.S. to use precast concrete segmental construction with external tendons. The four major ramp structures that comprise approximately 65 percent of the overall interchange superstructures (Figure 1) were constructed of precast concrete segments. Table 2 summarizes the structural arrangement of these ramps. The design of each ramp used a combination of frame structures. Each frame is made up of four to five continuous spans that range in length from 36 m to 65 m (120 feet to 215 feet).

The spans were erected by the span-by-span method except for the four 65-m (215-foot) -long spans of ramp N-W crossing over US 95, which were constructed by the balanced cantilever method. The maximum length for the span-by-span arrangement was 45 m (150 feet).

Making it Possible: Priming the Construction Market

The difficulties of using segmental construction included the need for specialized construction equipment and personnel with the skills to operate and maintain that equipment. In addition, tight geometric control was required during casting and erection of the segments. Recognizing that this level of complexity was not typical to the local construction market, the owner and our design team raised the question:

Can we expect reasonable bids in a market that never had experience with segmental construction technology?

An affirmative answer was made possible through an aggressive contractor awareness campaign aided by the American Segmental Bridge Institute (ASBI). The campaign included extensive advertisement and a series of pre-bid conferences for contractors.

Selecting the Precast Segmental Alternative

The selection of a precast segmental alternative was a result of a comprehensive bridge type study and evaluation of the following alternatives:

• Steel plate girder
• Steel box girder
• Cast-in-place concrete box girder
• Precast I-girder
• “Bath-tub” sections
• The standard ASBI/PCI/AASHTO box girders.

The candidates were measured against a series of evaluation criteria that considered a combination of factors, including:

• Construction in an urban environment
• Minimum required clearances
• Aesthetics
• Maintenance of traffic
• Seismic performance
• Cost.

Figure 2: Overhead Construction Over Uninterrupted Traffic

Challenges Posed by Urban Construction. Construction in an urban environment such as busy downtown Las Vegas requires that projects minimize lane closures, detours and traffic interruptions. Key to minimizing these impacts is minimizing the duration of construction. Fortunately, precast segmental construction using an overhead gantry offered unquestionable advantages in this regard over the other construction techniques considered (Figure 2).

Overhead erection minimized impact on the structure’s footprint and the work envelope required for construction. Segments for the subsequent spans in the erection cycle were supplied from the previously completed span, so the work envelope below the deck level was minimized to a small quantity of “below-deck” false work that was used to erect the pier segments and, in some cases, to provide lateral stability during erection.

Minimum Required Clearances. The resulting segmental alternative provided the structural solution with the minimum horizontal and vertical clearance requirements for the overall interchange layout. Further reductions in encroachment were realized when the pier caps were eliminated at all the interior spans. Pier caps were required at the expansion piers, however. This type of hinge support was selected over an “in-span” hinge because we believed it has inherently greater reliability during a seismic event.

Figure 3: Positive Aesthetic Appearance of Ramp in Service

Aesthetics. The positive aesthetic appearance of the trapezoidal segmental box solution needed minimal architectural enhancement (Figure 3). The treatments used were in the form of fractured fin texture, which was applied to the surfaces of the piers and abutment back-walls; therefore, the aesthetic impacts did not result in the cost penalty that was attached to some of the other alternatives that were evaluated.

Seismic Performance: The Integral Superstructure-Column Connection. Our client, the Nevada Department of Transportation (NDOT), expressed its preference for an integral superstructure-to-column connection right from the start because it provides additional redundancy in the framing system, so greatly improves structural response during a seismic event. Such a connection provided a greater lateral capacity overall when compared to the other conventional framing alternatives that were considered. In the case of the Spaghetti Bowl, this larger seismic capacity gives the ramp structures in the interchange a higher level of serviceability in the event of an earthquake.

Design and Construction of the Integral Superstructure-Column Connection

Although this integral connection could be constructed without adversely affecting the cost competitiveness or time efficiency of segmental construction, special details had to be incorporated to accomplish frame integrity. The integral pier segment was subject to the following unique conditions compared with the traditional pier segments supported on bearings:

• Less space was available for reinforcement and post tensioning anchorages because the precast segment had a large center block-out reserved for the cast-in-place integral connection with the column.
• Transverse post tensioning and additional reinforcement was required to resist vertical loads because of the single column support.
• The plastic hinge moment from the column had to be resisted by extra reinforcement projecting into the pier segment and the cast-in-place beam-column joint.
• The support condition and forces would vary during the construction phases before the final integral condition could be accomplished.

Figure 4: Integrated 3D CADD Drawing of Face (Front/Rear) Reinforcement.

Acknowledging the pier segments’ importance, we addressed these technical challenges by using unique techniques for designing and detailing. Computer aided design and drafting (CADD) was used to expedite the design and improve the accuracy of the reinforcement design. Perspective renderings, such as the one shown in Figure 4 of the integral pier segments, were created to help potential contractors understand the design in order to prepare their construction bids.

The bridge erection sequence required special considerations to accommodate the integral superstructure-column connection. Ducts of longitudinal post tensioning bars were inserted through the segment diaphragms and the column reinforcement cage. The concrete was then placed through the deck opening into the segment to achieve the integral connection between the precast segment and the cast-in-place column. Upon completion, the launching gantry was then advanced forward and fastened on top of the integrated segments. To complete the cycle of span assembly, the typical span segments were erected individually while supported by the overhead launching-gantry. The gantry spanned between the previously assembled integral pier segments.

Special Design Characteristics

The special characteristics of this project reflect how the different issues presented above were addressed.

Selecting the Cross Section of the Segments. The cross sections had to specifically satisfy the single-lane ramp and double-lane ramp structural requirements. ASBI/PCI AASHTO standard boxes were considered; however, they did not meet the project-specific seismic design requirements and they required a greater footprint for the piers. As a result, custom designed cross sections were developed.

Frame and Column Selection. Continuous frames were proportioned to balance the conflicting effects of creep, shrinkage, temperature and seismic demands. The designers by and large found that creep and shrinkage produced the maximum moment demands on the outer piers of the continuous frame. In some cases, erection loads were found to control. Because seismic demands did not govern the column designs, an added challenge was introduced to the seismic resistant design.

Figure 5: Ramp N-W Showing Cross-Section of the Deep Segments and Longitudinal Elevation of Transition Between Span-by-Span and Cantilever Construction

Figure 6: Ramp N-W

Remember that seismic design criteria were based on the capacity design principle of the “weak column-strong beam”; therefore, we had to enable column plastic hinging while also ensuring that the superstructure and the foundation remained elastic. The design objective was to keep the substructure stiffness as consistent as possible. In some specific cases, column casings were required to increase the flexibility of the shorter piers. The overall goal was to keep the plastic capacity of the columns within manageable limits so that a practical superstructure and foundation design could be achieved (Figures 5 and 6).

Checking the Columns for Estimated Erection Loads. We had anticipated that the curved alignment of the ramps was going to put additional demands on the piers. An estimate of the eccentricities resulting from the horizontal offsets of the gantry’s tangent alignment and the ramps’ curved alignment was developed. These offsets, coupled with the assumed gantry reaction magnitude and location, formed the basis of the column demand estimate.

Keeping in mind that the designer has no way of predicting exactly what equipment the successful bidder will use to erect the bridge, erection schematics were offered to the contractor in the bidding documents. Analyses of this erection scheme resulted in some cases where construction controlled the design of the piers and once the real erection loads were known, some modifications were necessary. Incorporating these changes into an efficient design required the team (owner/designer/ constructor) to maintain a delicate balance in order to develop a solution that was suitable for construction and yet maintained the design intent of the single shaft pier. This balancing act continued to be sorted out as the project matured.

Many Contributed to this Successful Project

In closing, it is only proper to acknowledge the support we received from NDOT in its encouragement of a segmental solution for this complicated urban interchange project. Acknowledgement is also due to Meadow Valley, the general contractor, along with its segmental contractor, Walter-SCI, and construction engineer, Finley McNary Engineers, for the execution of the project; and to Figg Engineers for their field supervision support to NDOT. This project is a benchmark for further applications of segmental construction in urban areas, as well as in areas of high seismicity.