Table of Contents
Task 2- BNSF Railroad
Bridge Assessment
Task 2.2 – Structural
Evaluation
Task 5- Prefabricated
Bridge Research
Introduction
Pons
Pontis Inc. will collaborate with the City of
Flagstaff in the design of a new pedestrian bridge located along the trail
system east of Flagstaff in Picture Canyon Preserve and Cultural Park. Our
proposal for the replacement bridge is outlined in the following sections:
-
Project
Understanding
-
Scope
of Services
-
Schedule
Work
hours and fee estimates are attached in a separate envelope.
Project Understanding
Purpose of the Project
The
goal of the project is to determine the best solution for the replacement of a
wooden walkway bridge located in Picture Canyon, crossing the Rio De Flag northwest
of the Wildcat Hill Waste Water Treatment Plant, at the junction of three
separate trails. The Arizona Trail, the Flagstaff Urban Trail System, and the
Educational Loop Trail all connect at one location where a small (15-feet wide)
pedestrian bridge is currently located, see Figure 1.
As
the figure above displays, the project site is located on the east side of
Flagstaff. Figure 2 below is a trail systems map for the Picture Canyon
Preserve and Cultural Park. The intersections of the three trails, where the
bridge is located, is outlined in red.
As
Figure 2 displays, the bridge is located at the intersection of three trails,
the Arizona Trail, the Flagstaff Urban Trail System, and the Educational Loop
Trail, within Picture Canyon Preserve and Cultural Park.
Recently
the embankments of the Rio De Flag in this area were widened due to changes
made to the stream conditions, causing the current bridge to be too small for
the system. The current condition is not ideal but it is also not a critical
concern, therefore the project will be implemented in about three to four
years. Figure 3 below displays the current condition of the wooden walkway
bridge.
The
purpose of this project is to evaluate the current state of a larger (16-feet
by 80-feet), donated Burlington Northern Santé Fe (BNSF) railway bridge to be placed
in lieu of the existing wooden walkway bridge. Figure 4 displays the BNSF
railway bridge under evaluation.
Displayed
above is the BNSF railway bridge that is under consideration as a pedestrian
bridge as a replacement for the wooden walkway bridge.
Project Specifications
With
the ever-changing flood plains of the local area, the required larger bridge
will support pedestrians, equestrian personnel, and bicyclists as well as
service vehicles. Since the City of Flagstaff does not have a bridge design
code, the AASHTO Guide for the Development of Bicycle Facilities (2012) and the
AISC Steel Code will be the technical standards designed to. The abutments for
the new walkway will be built higher to accommodate the 100-year flood plain of
the Picture Canyon Preserve and Cultural Park, as well. All site details,
including the 100-year flood plain elevation and topographic maps, will be
provided to us by the City of Flagstaff. A condition evaluation of the BNSF
bridge will take place, checking for any signs of rust, serious damage, and bent
bars or beams. Existing conditions of the site will later be determined after a
site visit is scheduled with the main stakeholders which include: Bret Petersen
(Senior Project Manager), Karl Eberhard (Community
Design and Redevelopment Manager), and Martin Ince
(Multi-Modal Planner), all of whom work for the City of Flagstaff. Additionally,
the users of the trail systems, including hikers, bicyclists, and equestrian
personnel, are important investors in this pathway.
Challenges
Some
of the many potential challenges that face this project include possibly
changing the location of the bridge crossing, realigning the trail, and the
transportation of the donated BNSF railway bridge from its current location in
Joseph City to the project site. Also determining the exact point at which the
bridge BSNF is deemed unusable will have to be under significant evaluation.
Succeeding
with this project will entail many project site visits to determine existing
conditions of the channel and conditions of the BNSF bridge. This project’s
main scope is to replace the current walkway with one with a much better fit to
the channel, raised to accommodate to the 100-year flood plain, while keeping
the costs low.
Scope of Services
In
order to consult in the replacement of the bridge located in Picture Canyon, at
the intersection of the Arizona Trail, Flagstaff Urban Trail System and the
Educational Loop Trail, our team will complete several aspects of this project.
Our team’s scope of services includes the following tasks:
Task
1- Site Assessment
Task
2- BNSF Railroad Bridge Assessment
Task 2.1 - Visual Assessment
Task 2.2 – Structural Evaluation
Task
3- Site Design
Task
4- Abutment Design
Task
5- Prefabricated Bridge Research
Task
6- Cost Analysis
Task 1- Site Assessment
The
site assessment will entail a site visit and an analysis of the channel slopes,
channel widths and current path locations. The Rio de Flag flood plain data will
be gathered from the City of Flagstaff. Once this information is gathered, the
best crossing location will be determined based off of vicinity to current
trails, accessibility for construction of the bridge, grades of the trail and
dimensions of the bridge. If the location of the bridge is changed, the slopes
of channel walls, and grades leading to the bridge will need to be assessed for
usability, and the paths will be realigned. Transportation of the truss into
site will need to be analyzed, determining if any vegetation of the surrounding
area will need to be cut, there are semi-truck assessable roads leading into
the site, and how the truss will be placed exactly in the desired location over
the channel.
Deliverables: Report of site assessment
Task 2- BNSF Railroad Bridge Assessment
An
option for the replacement bridge is to use an out of service BNSF railroad
bridge located in Joseph City. The railroad bridge that is under consideration
as the replacement bridge will undergo a conditions assessment. The conditions
assessment of the bridge will include a structural stability evaluation,
historical background study of the bridge, and a health and safety evaluation,
for example the presence of lead paint. A lab test will be completed to
determine the presence of lead based paint.
Task 2.1 – Visual Assessment
A
visual assessment will be conducted to determine if any rust, section loss or large
truss member deflections. The bridge will also be assessed based on additional
fixtures that would need to be added in order to make the bridge usable. Any
section losses or deflections will be measured.
Deliverables: Bridge assessment
report
Task 2.2 – Structural Evaluation
An
evaluation of the existing bridge will be conducted. Our team will determine the material
properties of the bridge either from the BNSF bridge records or similar time
period bridge records. Our team will be modeling and analyzing the truss based
on the capacity of the bridge verses the demand on the bridge. The capacity of
the bridge will be determined based on the material properties, type and
dimensions of the truss, and the presence of section loss and deflections. The
demand on the bridge will be related to the maximum expected design loading.
The maximum loading will be based on the heaviest weight service vehicle as
specified by the City of Flagstaff. Checks will be performed comparing the
capacity to the load demand of the BNSF truss. The capacity to demand
specifications will be evaluated against the standards set by the AASHTO Guide
for the Development of Bicycle Facilities (2012) and the AISC Steel Code. A finite
element analysis (FEA) computer software program titled SAP2000 will aid in the
analysis.
Deliverables: Capacity and loading report
SAP2000 report
Task 3 – Site Design
Any
topographic data and maps will be provided by the City of Flagstaff to aid our
team in the overall site layout. The abutments required for the truss to fit
properly will have established locations and elevations, once the site is
assessed. A visual analysis of the slopes of each trail will need to be
performed to determine the height of the abutments and how much, if any, fill
will be required to maintain ease of accessibility.
Deliverables:
Plan and profile sheets
Task 4- Abutment Design
Abutments
will be sized by our team and designed respectively to fit the given bridge
design to prevent any “washing away” and/or damage to the bridge during times
of flooding. This portion of the Rio de Flag channel has been recently widened
to accommodate a 100-year flood plain of the local area. The widening has
provided easement for the meandering stream of the channel and any wastewater
runoff from the nearby Wildcat Hill Wastewater Treatment Facility located
upstream of the ideal bridge location. Once an assessment of the 100-year flood
plain for the Picture Canyon area has been compiled, abutments will need to be
designed accordingly for the new bridge. The abutments will provide a more
stable structure, prevent any damage from the channel onto or underneath the
bridge, and to benefit the users with easier access to and from the pathway.
Deliverables: Structural design calculations
Plan and profile sheets
Task 5- Prefabricated Bridge Research
Another
option for the replacement bridge is to order a prefabricated bridge. The main
purpose of this task is to gain a cost determination for a prefabricated bridge
that would span the river channel versus the cost determination for
refurbishing the BNSF truss.
Deliverables: Cost analysis
Task 6- Cost Analysis
The
cost of the BNSF railroad bridge and the cost of the prefabricated bridge will
be compared. The cost of the railroad bridge will be dependent on additional
fixtures, transportation, accessibility, and adjustments made to the bridge for
health and safety reasons. On the other hand, the prefabricated bridge will
have set cost based on the dimensions of the bridge.
Deliverables: Cost comparison
Bridge recommendation