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IBC 19-77: Dynamic Analysis Evaluating Human Induced Vibrations in a Lightweight Suspension Bridge
Martin Hudecek, Ph.D., E.I.T., Stantec Consulting Ltd., Victoria, BC Canada; Eduardo Arellano, P.Eng., Stantec Consulting Ltd., Kamloops, BC Canada
In lightweight slender bridges, human induced vibrations can result in significant discomfort and yet compromise structural integrity if frequency of passing load, represented by walking or running pedestrians, synchronizes with the natural frequency of the bridge. This paper discusses a method of advanced dynamic analysis considering human induced vibrations in a suspension bridge. The method is demonstrated on the Bear River Siphon Suspension Bridge situated south of Grass Valley, CA. Predicted structural response is compared with the actual response obtained from onsite testing. Although the vital function of the bridge in question is to carry a water line (54-inch in diameter), the deck comprises also two walkways for maintenance crew. Therefore, with the main span of 200 ft and cable sag-to-span ratio of 0.12, this structure qualifies under category of slender bridges and may be susceptible to human induced vibrations. The presented analysis method, utilizing finite element analysis software, is developed to obtain dynamic accelerations of the deck. Accelerations of the deck represent the main concern affecting comfort of walking at serviceability limit state. The developed method considers frequency and load magnitude of walking or jumping pedestrians crossing the structure in various groups. Natural frequencies, serving as main input, were calculated and verified against onsite dynamic testing. Ritz and Eigen vectors and structural damping factors are employed in the developed method to simulate load imposed on a real structure. The developed method is described in detail and accompanied with flowcharts to provide a practical guideline for industrial applications.
IBC 19-80: Design and Construction of the 2nd Street Bridge – Austin, Texas
Robert Anderson, P.E., S.E. and Trevor Kirkpatrick, P.E., AECOM, Tampa, FL
As part of the revitalization of a decommissioned water treatment plant site in down town Austin, Texas, the new 2nd Street Bridge provides a vital link for vehicles and pedestrians over Shoal Creek between the new city library to the west and residential/retail areas to the east. The new bridge is designed, proportioned and detailed to offer an elegant solution to connect the two sides of the 2nd Street over Shoal Creek with an iconic structure that is friendly to vehicles and pedestrians, and integrated with the future vision for this location.
Through a series of design charrettes, a tiered process was used to elicit input and obtain decisions from key stakeholders. During those workshops the team analyzed and evaluated the following full spectrum of options against a comprehensive list of project goals.
The preferred bridge type was a canted network arch concept spanning approximately 160 ft. The over deck supporting elements of the arch bridge are a pair of trapezoidal shaped steel ribs each with a network arrangement of galvanized wire rope hangers connected above deck to the girder framing. A central utility corridor between the box girders will accommodate the multiple utility lines which cross the bridge. The bottom soffit of the utility corridor will be screened by a metal deck bar grating. Outrigger beams carry a curved pedestrian sidewalk, varying from 12 to 14 ft wide. The thrust of the arch ribs is resisted by a foundation system with 6 foot diameter drilled shafts anchored to bedrock.
IBC 19-69: Novel Approach towards Improving the Seismic Response of Irregular Bridges: Tuned Structures
Samantha Chaudhari and Max Stephens, University of Pittsburgh, Pittsburgh, PA
Irregular bridge structures (e.g. bridges with large skew or bent-to-bent stiffness irregularities) are susceptible to greater damage from seismic loading due to increased displacement demands resulting from torsional effects. Current codes governing the design of bridges recognize the vulnerability of these structures, and impose stricter standards to improve performance, however these provisions have not always been effective in controlling deformations and reducing damage. To improve the seismic performance of irregular bridge structures, this research is focused on the concept of structural tuning, where the dynamic characteristics of irregular bridges are strategically modified to produce an aggregate behavior free of damaging irregular modes of response. First, practical methods to modify the dynamic characteristics of irregular bridges were identified. Two methods are introduced here including (1) strategic mass redistribution in the superstructure and (2) bent stiffness modification using post-tensioning and/or plastic hinge relocation. These methods were selected because they can be implemented without significantly altering the global geometry or column strengths. Next, two irregular case study bridges were re-designed using the aforementioned tuning methodologies; one with large skew and one with large variations in bent stiffness. Finally, validated modeling procedures were implemented in the opensource structural analysis software OpenSees to evaluate the seismic performance of the original and tuned case-study structures. Results from the numerical evaluation indicate that the structural tuning methodologies introduced here eliminate lateral responses dominated by irregular modes and result in decreases in overall bent drifts.
IBC 19-71: Seismic Design of Integral Abutment Bridge
Mohamed Zawam, Trevor Small, and Xiaocen Jia, WSP Canada Inc., Oakville, ON Canada
This paper discusses challenges associated with the design of Highway 401 underpass at 3rd line road, Bainsville, Ontario. The superstructure was an integral abutment type structure with cast-in-place reinforced concrete deck composite with five (5) precast prestressed concrete NU 1600 girders. The integral abutments were founded on steel H-piles driven to bedrock. The piers comprise a reinforced concrete bent supported by two circular columns. The seismic design was according to the new provisions in CHBDC 2014.
The abutments were subjected to high deformations due to thermal effects, concrete shrinkage, and creep. Such deformations created a challenge for optimizing the piles design to ensure sufficient flexibility at the abutments, while achieving adequate strength to resist the seismic forces. The high mass of the concrete superstructure in addition to the seismic class of the site (class E) have led to significant design seismic forces. The seismic design was carried out using a multi-mode response spectrum analysis on a three-dimensional, linear elastic finite element model. Non-linear soil springs were utilized for the model using P-Y iteration method in order to account for the large deformations at the abutments. Seven HP 310×179 piles were used at each abutment. The pile was a class 1 section which allowed the piles to undergo plastic deformations under non-seismic ultimate loading conditions. Corrugated steel pipes around the piles, which was filled with loose sand, was increased to 900mm diameter compared with conventional 600mm diameter to accommodate the larger movements. The construction is scheduled to finish in spring 2019.
Topic: Special Interest
IBC 19-63: Implementation for Design of Bridges to Resist Dynamic Barge Impact Loads
Michael Davidson, Ph.D., P.E., Henry Bollmann, and Gary Consolazio, University of Florida, Gainesville, FL
Bridges spanning navigable waterways are designed to resist vessel collision loading, including loads generated during barge impact events. While design provisions have promoted improved assessments of bridge structures (e.g., risk assessment, computation of nonlinear response), the existing design approach involves static characterizations of vessel collision load and bridge response. However, 15 years of Florida Department of Transportation research on barge-bridge impacts have shown that: (1) impact load and bridge response are dynamic; and, (2) incorporating dynamic effects can lead to advantageous bridge designs. Implementation of automated analysis features for quantifying dynamic risk for barge impact loads is needed to bring about improved uniformity of safety. The objective of this research is to establish an implementation of bridge design for resisting dynamic barge impact loads. Presented herein is the dynamic design implementation itself and resources for practicing engineers to refer to when making use of the implementation. As demonstration of the methodology, two unique bridges are considered, where the dynamic barge collision risk assessment procedure is illustrated for each case. Additionally, major modeling steps for conducting dynamic vessel collision analysis (using currently available bridge finite element analysis software) are identified to exemplify selection of bridge structural configurations that help to minimize design loads.
IBC 19-65: Poplar Street Bridge Slide
Gregory Kuntz, P.E., HDR, St. Louis, MO; Stacy McMillian, P.E., Missouri DOT – Bridge Division, Jefferson City, MO
The Poplar Street Bridge is a five span (300’-500’-600’-500’-265’) 2165’ long structure which carries I-64, I-55 and I-70 over the Mississippi River in downtown St. Louis and connects Missouri and Illinois. The bridge is actually twin Eastbound and Westbound Structures carrying 4 lanes of traffic each and consisting of two variable depth steel box girders (25’ max. depth) with an orthotropic steel deck on a shared substructure. The Missouri Department of Transportation hired HDR to provide the following improvements to the Poplar Street Bridge: 1) Increase lane capacity of the EB Bridge from 4 to 5 lanes in conjunction with improving interchange ramp structures on the Missouri side, 2) Rehabilitate the superstructure and substructure and 3) Provide a new and improved wearing surface. The project is currently under construction with a completion date of December 2018. The two unique aspects of the project the presentation will focus on are the new wearing surface and the bridge slide. The new wearing surface consists of a fiber reinforced lightweight concrete overlay mechanically connected to the steel deck through shear studs. In lieu of a traditional widening, the EB superstructure was successfully slid 9’ to the south onto widened piers on March 31, 2018 over the course of 2.5 hours and was widely reported as the 2nd longest bridge slide ever in the by length. The WB & EB superstructures were then connected together.
Topic: Design 1
IBC 19-22: Centennial Bridge – A New Connection to the Historic Cabrillo Bridge in Balboa Park
Anthony Sanchez, PhD, P.E., Patrick Chang, and Jason Hong, Moffatt & Nichol, San Diego, CA
The Cabrillo Bridge, and many historic buildings at Balboa Park, were built in 1915 for the California-Panama Exposition to celebrate the opening of the Panama Canal. Over a hundred years later, the City of San Diego and the Plaza de Panama Committee, a philanthropic organization headed by Qualcomm founder Irwin Jacobs, is planning 80 million dollars of improvements for the park. A major feature is the “Centennial Bridge”, a new connection to the historic Cabrillo Bridge, which will allow the Plaza de Panama to again become pedestrian only, as the park founders intended.
The bridge was both architecturally and structurally challenging. Architecturally, the bridge must be visually compatible with the 103-year-old Cabrillo Bridge. It must display good architectural qualities and be visually interesting. But must also be subtle, to not distract from the beauty of the Cabrillo bridge.
Structurally, the bridge must resist large radial and torsional forces from the horizontal curvature of 1/182’, which is 5x standard. The bridge must also resist large bending in the overhangs, which at 13’ long, are 3x standard. Finally, seismic forces must be resisted by the end piers, which are 500x stiffer than the interior piers.
The designers used a “spine” girder, to conform to the curved alignment, provide a simple appearance, and keep costs reasonable, and shaped the columns to pay homage to the Cabrillo Bridge.
In this discussion, we will describe how we designed the bridge to meet the architectural and structural challenges, including design for torsion and seismic forces.
IBC 19-24: NJDOT Sign Structure LRFD Design Standard Upgrades
George Zimmer, Rama Krishnagiri, P.E., and Steve Esposito, P.E., WSP USA, Lawrenceville, NJ; Eddy Germain, P.E., and Xiaohua “Hannah” Cheng, P.E. and Kimberly Sharp, New Jersey DOT, Ewing Township, NJ
NJDOT tasked WSP with updating their Sign Structure Standard Drawings and associated Design Manuals to comply with the 2015 AASHTO LRFD Specifications including the 2017 Interim Revisions. This was the first time that NJDOT sign structures would be designed by LRFD. A key component of this upgrade was the 130 MPH design wind speed for all new sign structures, which created an increase in wind pressure over the previously utilized 80 MPH design wind speed/pressure. This significant increase in wind speed presented many challenges in standardizing structures, sizing and grouping steel members for the Strength Limit State, maintaining Fatigue Category II resistance, satisfying fatigue detailing requirements and the sizing of foundations. Previously, the standards provided the option to select a spread footing or pile foundation for all sign structures, however due to the increase in wind load and the LRFD limiting criteria, these foundation types showed a significant increase in size for shallow foundations. To economize foundation size and avoid right of way and potential utility impacts, drilled shaft foundations were standardized and included for the first time for NJDOT sign structures. WSP performed extensive coordination with leading fabrication experts to implement the recommended AASHTO fatigue details into the standards to satisfy AASHTO requirements, meet fatigue criteria, and consider constructability. A standardized approach to analysis was a key component of this project, requiring extensive repetitive modeling, analysis and design of hundreds of sign structure configurations. We will also discuss the comparative differences of the older standards to the current standards.