| Hudson-Bergen Light Rail Transit System |
| Structural Design for a DBOM Project |
| By Peter W. Fedun, Philadelphia, Pennsylvania
1-215-790-2325, fedun@pbworld.com
and Alex Krasilovsky, Newark, New Jersey 1-973-353-7650
krasilov@pbworld.com |
| Good engineering information provided in the early
stages of the DBOM process helped to lay the foundation for successful
implementation of the design and minimize changes and control costs. |
|
PB was responsible for advancing the HBLRTS project from the planning
stage through preliminary design as NJ TRANSIT’s general design
consultant. The design calculations are a valuable part of the initial
preliminary design because the resultant drawings are used to solicit
bids from prospective design/build contractors. If the preliminary
design drawings reflect the construction requirements accurately,
then the owner will receive more realistic bids with less potential
for changed conditions.
Preliminary design was defined on a discipline specific basis. For
the structural aspects of the project, the engineering and associated
contract plan preparation were to be advanced to a 30 percent level
of completion. The following sequence of activities was performed
to achieve this design objective:
- Field surveys
- Design calculations
- Hazard surveys
- Contract drawings
- In-depth inspections
- Design report.
Field Surveys
The
Alignment. At the commencement of conceptual design, two
members of our structural design division conducted a general survey
of almost the entire alignment. This 32-km (20-mile) survey excluded
only those areas that were inaccessible by foot, principally a 0.8-km
(0.5-mile) segment of the alignment that traversed wetlands near the
north terminus of the system. The survey was used to identify significant
constraints and establish all of the structural requirements for the
project.
Existing Structures. We made an
effort to collect construction drawings and as-built plans for any
existing structures that were in the LRT right-of-way or within the
construction zone of influence. Many of the structures were old. Most
were built before World War II with some going back more than 80 years,
and most of them had had multiple owners. Some of the bridges were
without owners, so New Jersey DOT had classified them as orphan structures.
Consequently, it was difficult to obtain drawings and those that were
available were limited for the most part to plans of recent rehabilitation
or modification work.
We also performed an initial survey of all the existing bridges to
assess their structural integrity and usefulness for the light rail
transit operations. The number of bridges surveyed was greater than
the project total because we included structures that could be used
for alternate alignments that were still under consideration. The
bridges were classified into three principal categories, as needing
either minimal repair or modification, major structural rehabilitation,
or replacement. The survey results, photographs and a general description
of the findings were included in the HBLRTS Report of Existing
Bridge Inspections, which proved to be a valuable reference document
throughout the design life of the project.
Hazard
Surveys
The LRT alignment crosses beneath many existing bridges that are owned
or operated by different jurisdictions. During preliminary design,
all of these structures were surveyed to identify any potential problems
that could impact light rail operations adversely. At this stage,
it was important to establish the ownership of each structure so that
any needed repairs or modifications could be performed or coordinated
by the proper authority.
The recommended repairs were restricted to those items that could
impair future transit operations. In most cases, NJ TRANSIT opted
to perform the required repairs as part of the HBLRTS construction
project, and contract documents were prepared to define these work
items. It should be noted that the hazard surveys did not supersede
the mandated in-depth bridge inspections that are performed on a regularly
scheduled cycle.
In-Depth Inspections
We performed in-depth inspections of the existing bridges that would
carry LRT tracks because LRT revenue service depends on the capacity
and future integrity of these structures. In general, the inspections
were conducted according to the guidelines specified by the American
Association of State Highway and Transportation Officials (AASHTO)
Manual for Condition Evaluation of Bridges.
Each bridge was checked to determine if it had the geometric adequacy
to carry the LRT vehicle and associated guideway attributes. The load
carrying capacity was calculated based on the condition of the structural
elements that were documented during the inspection. All of the railroad
bridges were rated using the Cooper E type load.
The bridges identified as viable candidates for the system were checked
for conformance with the requirements specified by the NJ TRANSIT
Manual of Design Criteria, using actual LRT vehicle live
loads in combination with any other applicable loads.
Contract documents were prepared based on this design and any repair
recommendations that were documented in the in-depth inspection report.
Design Calculations
We advanced the structural calculations to a sufficient level of detail
to support the information in the corresponding structural drawings.
Most of the drawings were advanced to a preliminary level of completion
with only two exceptions:
- Designs were brought to the conceptual level
when uncertainties regarding property acquisition were identified.
- Designs of the standard passenger station
canopy and the station beacon were advanced to a 100 percent level
of completion.
Calculations for the preliminary design drawings considered the following:
- Stress and deflection analysis of the primary
framing members
- Global stability
- Seismic performance
- Foundation design
- Earth pressures
- Buoyancy.
In addition, a series of boundary problems were evaluated to verify
that the aerial structures would adequately resist the interactive
forces that are generated by the continuous welded rail.
Contract Drawings
The objective of the structural drawings was to provide a sufficient
level of information to permit competing DBOM contractors to develop
a realistic basis for their construction bids. The preliminary structural
drawings:
- Showed the location and overall geometry
of each bridge or associated transit facility
- Established the arrangement and overall
dimensions of each primary framing member
- Detailed principal structural components, with their dimensions.
Based on the results of the geotechnical investigation, a foundation
system was selected for each structure. Spread footings were dimensioned
and, where deep foundations were required, the size and depth of piles
were defined. Although the bridge bearings were not included as part
of the preliminary design, the as-designed bearing arrangement was
identified on the plans. The preliminary structural drawings specifically
excluded secondary members, reinforcing bars, joint details, bearings
and connection details. The corresponding design calculations were
advanced to a level of completion that demonstrated that these features
could be accommodated by the depicted design solutions.
Design
Report
Design reports were prepared for each of the preliminary design packages.
The structural chapter of each design report included:
- A written description of each structure
- A list that identified the exact codes used
for design, by year or edition
- A summary of design assumptions
- A discussion of critical constraints and
unresolved design issues.
These reports are particularly important for design/build projects
because they are the only reliable way to transfer design logic from
the engineers who prepared the preliminary design to the contractor’s
final designers. The two parties typically do not have a contractual
relationship.
Summary
One aspect of the DBOM project is to accelerate construction. As the
HBLRTS project demonstrates, good engineering information provided
in the early stages of a DBOM lays the foundation for successful implementation
of the design by the contractor. It also helps to controls costs and
ensures a final design consistent with the owner’s criteria. |
|
Peter Fedun has spent the majority of his
19-year career designing transit systems. In addition to the HBLRTS
project, he has worked on the Dallas Area Rapid Transit, Taipei
Rapid Transit System, Honolulu Rapid Transit Program, Taiwan High
Speed Rail Project and Central Florida Light Rail Transit System.
Peter currently serves as director of facilities engineering for
the D/B procurement phase of the proposed Hiawatha LRT project in
Minneapolis, Minnesota.
Alex Krasilovsky is a structural engineer with 27 years experience
in transit/railroad facilities, marine structures, tunnels, bridges,
sewer systems and buildings. He was involved in the EWR Monorail-Northeast
Corridor Connection project and the Kearny Connection project, both
in New Jersey. Among his contributions to this project, Alex developed
structural design criteria for a high-speed turnout rail connection
and recommendations for strengthening the bridge foundations and
underpinning.
|
|
