Details of Final Design

Final Project Deliverable

Thursday, April 25th, 2013

Introduction

Pons Pontis Inc. has collaborated with the City of Flagstaff for the replacement of a FUTS trail pedestrian bridge in Picture Canyon. Currently, a small wooden bridge on the Arizona Trail crosses the Rio de Flag within Picture Canyon. The City of Flagstaff is planning to add two new trails, the Flagstaff Urban Trail and the Educational Loop Trail, which will intersect with the Arizona Trail at this crossing. Due to recent restorations of the Rio de Flag and expected increase use of the trail system, the current bridge is inadequate. Therefore, a larger bridge will be necessary to accommodate floodplain regulations as well as trail personnel.

Background

Picture Canyon

Picture Canyon is located on the east side of Flagstaff, Arizona near Wildcat Hill Wastewater Treatment Plant, see Figure 1 below.
Figure 1 above displays the location of Picture Canyon within Flagstaff, Arizona. Previously, Picture Canyon had wastewater running through it, was filled with garbage, and was not well maintained (Appendix H). In recent years, the City of Flagstaff has cleaned up the canyon, restored the Rio de Flag, and purchased the canyon for recreational purposes. The stream restoration project has added the meanderings back into the Rio de Flag within the canyon. The City of Flagstaff plans to add two additional trails, the Flagstaff Urban Trail and the Educational Loop Trail, through Picture Canyon that will serve hikers, bicyclists, and equestrian personnel.

Current Bridge

The intersection of the Arizona Trail and the two proposed trails will cross the Rio de Flag. This location currently has a small wooden bridge, as displayed in Figure 2 below.
As Figure 2 shows, a small wooden bridge is used to cross the Rio de Flag within Picture Canyon. This bridge is about 15 feet long and 4 feet wide, and has rock foundations. Due to the small size of the bridge and the foundation conditions, it is difficult for bicyclists and equestrian personnel to cross the Rio de Flag within Picture Canyon. The bridge is also within the 100-year floodplain and requires a high degree of maintenance since it is made of wood. Due to these conditions, a replacement bridge is necessary.

Replacement Option

The City of Flagstaff has been offered an out-of-service steel truss bridge. The Flagstaff trail planning committee is interested in using this bridge as a replacement for the wooden bridge within Picture Canyon. The truss bridge was originally built in 1917 and was along the route of Obed Road near Joseph City, Arizona. An in-service picture of the bridge can be seen in Figure 3 below.
As can be seen in the above photograph of the Obed Bridge, the entire bridge contained six trusses with each truss spanning 83 feet and had a wooden deck. Originally, the bridge was used to accommodate cattle and pedestrians but later accommodated vehicles. The trusses were removed from their original location in 2011. After the bridge was removed, each truss was sand blasted and repainted; the current condition of the trusses can be seen in Figure 4 below.
The City of Flagstaff is considering buying one truss to use as a replacement bridge within Picture Canyon. In order to use this bridge as a pedestrian bridge on the Flagstaff Urban Trail System (FUTS), the structural stability of the truss needed to be checked, a deck design was required, and the foundations as well as an approach had to be designed.  Pons Pontis Inc. has investigated these three aspects of the bridge replacement design and the results are outlined below.

Design

Pons Pontis Inc. has collaborated with the City of Flagstaff to analyze the use of a steel truss bridge as a pedestrian bridge within Picture Canyon. Pons Pontis Inc. has looked at the structural stability of the truss, a deck design, and an abutment and approach system.

Design Criteria

The City of Flagstaff has expressed the following criteria for the replacement structure:

Correspondences related to these criteria are attached in Appendix E.
Several design references were used for this project including:

Documents that are not readily available are attached in Appendix H. Details about the design of the truss, deck, and foundations are discussed in the following sections of this report.

Truss Evaluation

Due to the age of the truss, a structural evaluation of the bridge needed to be completed. Since the original plans for this truss were not available, the actual yield point of the truss could not be used in calculations. This also meant, dimensions and bridge weight needed to be detemined. A measurement of all bridge components was necessary so a bridge weight estimate could be made. A detailed list of bridge measurements and the total weight of the bridge can be found in Appendix A. Since the actual actual strength of the truss steel is unknown, an estimate of the yield point of the steel needed to be determined. Therefore, using AISC Design Guide 15, a range of yield points were found and used to evaluate this truss. The evaluation was done using SAP 2000 finite element analysis. The results of this analysis indicated the truss would be unable to support the loads as defined by The City of Flagstaff and therefore will need to be retrofitted before it can be used for the Flagstaff Urban Trail System.

Structural Analysis

The dimensions of the truss were put into SAP 2000 along with the concrete deck load and the service vehicle load of 10,000 lbs. The self weight of the individual components of the truss were added into SAP 2000 as well. A table listing all weights added into SAP 2000 can be found in Appendix A.
Using Table 1.1a from AISC Design Guide 15, the yield point was determined to be somewhere between 27.5 ksi and 32.5 ksi. Table 1.1a from AISC Design Guide 15 can be found in Appendix A. Both 27.5 ksi and 32.5 ksi, the worst and best case scenarios respectively, of these yield points were used to analyze the truss. After running the steel frame design analysis, the demand to capacity (D/C) ratios on the members of the truss indicated that the truss would not be able to support the loads in the truss’ current state. The D/C ratios are shown in Figure 5 below.
Figure 5 displays the D/C ratios for each truss member and the colors further illustrate how stressed the member is, with red being the most stressed and blue being the least. Truss members with D/C ratios above 1.0 are failing and will need to be retrofitted in order to be used on the Flagstaff Urban Trail System. All calculations and SAP 2000 print outs, including tables showing loading and demands, can be found in Appendix A.

Retrofit Design

Due to the high D/C ratios of the top and bottom chords, the truss will need to be retrofitted in these locations so the truss can be used as a pedestrian bridge. The top chord will need a 11” x 1/4” plate added to the bottom of the element. The bottom chord will need a 12” x 3/8” plate also added to the bottom of the element. Drawings of these details can be found in Appendix D. The addition of these plates decreases the D/C ratios to below 1.0 as shown in Figure 6.
Figure 6 indicates the D/C ratios for each truss member have decreased due to the addition of the plates. This is evident by the lack of red coloring on any member. The lower D/C ratios indicate the City of Flagstaff can use the truss as a pedestrian bridge. The SAP 2000 print outs for the analysis of the retrofitted truss can be found in Appendix A.

Alternative Design

In order to have the most efficient and cost effective design, an alternative where the truck load of 10,000 lbs was removed, was considered. All other loads remained the same.  This allowed the design to be conservative so the pedestrian bridge could still hold bicyclists, equestrian, and pedestrian personnel. In this case, the service vehicle would have to be re-routed to Rain Valley Road.  A map of the service vehicle detour can be seen in Figure 7 below.
Figure 7 illustrates the detour the service vehicle would take to get to Rain Valley Road.
The location of the bridge is indicated by a red triangle, the detour is shown with a red line and the location of where Rain Valley Road crosses the Rio de Flag is highlighted by a red rectangle. This detour would allow the retrofit of the bridge to be less expensive due to the decreased loading which will decrease the amount of reinforcement necessary. This will therefore reduce the overall cost of using this truss as the bridge replacement at this location in Picture Canyon.
The truss without any retrofit was analyzed to determine if the truss could withstand all other loads, excluding the truck load, as this would be the least expensive option. Figure 8 shows the results of this analysis.
Figure 8 indicates the truss will not be able to be used as a pedestrian bridge, even without a truck load. The truss would not be able to withstand the weight of the concrete deck and pedestrian, equestrian and bicycle loads due to the high D/C ratios. Therefore, it will be necessary to retrofit the truss even without a service truck load. 
An analysis of a retrofitted truss without the truck load was run. This significantly reduced the D/C ratios, which indicates smaller retrofitting members can be used. The results of this analysis can be seen in Figure 9. Detailed printouts from SAP 2000 can be found in Appendix A.
As seen above in Figure 9, an analysis of the truss without a truck load was run. Due to the considerably reduced D/C ratios, smaller retrofitting elements can be used to strengthen the truss members. The analysis done above, was done using a 9” x 3/8” plate added to the bottom of the bottom chord and a 9” x 1/4” plate added to the bottom of the top chord. Drawings of these details can be found in Appendix D.

Deck Design

Concrete Deck

The City of Flagstaff has recommended that the re-evaluation of the Obed Road truss include a brand new decking system. This deck will be made of Portland Cement Type I based concrete to minimize future maintenance of the truss and to prevent corrosion in the case of water over-topping the bridge (Appendix E). The concrete deck was designed around a general purpose use for animals, service vehicles, pedestrians, bicyclists, and equestrian personnel. According to AASHTO LRFD (5th Edition, 2010), the concrete deck slab thickness shall not be less than 1/20th of the overall design span. The truss is approximately 83.667 feet in length, and 1/20th would be about 4.18 inches. With this in mind, Pons Pontis Inc. designed the concrete deck to be 5.00 inches in thickness.
The Picture Canyon area is very rural and is not highly traversed by many users at this time. With the ultimate goal of designing a deck to hold a curb weight 10,000 pound service vehicle, the current truss can be cut down from 16 feet in width to 10 feet. This will lighten up the overall weight of the truss plus the decking and reinforcement, and still provide the service vehicle enough clearance room to safely cross the Rio de Flag. According to the AASHTO’s Guide for the Development of Bicycle Facilities (2012), Section 5.2.1 states:
“The minimum paved width for a two-directional shared use path is 10-ft… In very rare circumstances, a reduced width of 8-ft. may be used where bicycle traffic is expected to be low even on peak days or during peak hours…”
Expecting a higher volume of users in the near future of Picture Canyon, the City of Flagstaff has recommended that the minimum width be set to 10 feet. The concrete decking was designed for both a narrowed width of 10 feet, an overall length of 83.667 feet, and a standard thickness of 5.00 inches. With Portland Cement Type I concrete weighing in at 150 pounds per cubic foot, the overall deck will add approximately 52.3 kips of weight to the entire truss. Additional details and drawing designs can be seen attached as Appendix B and Appendix D.

Deck Support

The recently mentioned concrete deck slab to be used on the truss requires the addition of extra steel support beams to ensure longevity of the bridge. A series of hand calculations and Microsoft Excel 2010 files can be seen later on, attached as Appendix B, that depict the series of steps undertaken to design such a support system. The support beams were required to be able to support the concrete decking while leaving the minimalist effect of stress on the original truss. According to the American Concrete Institute 318-08 Building Code Requirements (2008 Edition), the maximum allowable stress of steel is 58% of the maximum 50,000 pound requirement. Resulting in a design stress of 29,000 pounds per square inch, a design vehicle weight of 10,000 pounds, and a vehicle dimension base of 8 feet in length, the steel beam supports can be detailed for a final design (Appendix D).
The attached MS Excel file for the I-Beam Design of Obed Steel Truss displays that once the dead load moment and live load moments are computed and added together, various beam designations can be depicted according to their low weight per foot and allowable stress. Page 28 of the Bethlehem Structural Shapes Catalog 3277 (1989 Edition) designates the W12x22 wide flange I-beam size to be suitable for the concrete deck design. With an allowable stress of 19.83 kips per square inch, 5 of these longitudinal beams will be used. The beams will be spaced evenly at 2.5 foot intervals perpendicular to the existing support beams of the truss. This will evenly spread the concrete deck load across additional support beams without adding a significant weight to the existing truss or to the foundations and approaches tying into the bridge. Final design drawings can be seen in Appendix D.

Foundation Design

The foundation for the bridge contains two aspects: concrete abutments and soil embankments. The concrete abutments consist of a wall-wing wall system and deep pile foundations. The soil embankment will span from the wall-wing wall system to ground level. In order to prevent the bridge from having a significant effect on the floodplain, the bridge has to be at a sufficient height above ground level. The bridge height has been determined to be at an overall five feet from ground level, two feet to accommodate for the 100-year floodplain level and three feet for freeboard. The details of these two systems are outlined below.  

Abutments

The abutment system consists of a wall-wing wall system and pile foundations. One wall spans 16 feet between the two piles is 18-inches thick and has a height of six and a half feet. Two wing wall configurations were considered, one in which the walls are at 90 degrees from the 18-inch wall and one in which the walls angle away from the main wall at 45 degrees. The angled configuration was selected to minimize concrete, safety rail configurations, and aesthetics. All of these walls extend 30 inches into the ground in order to maintain a depth below the frost level. For calculations and drawing details see Appendix C and Appendix D, respectively.
The pile system consists of four piles, two at each end of the bridge. The length and diameter of the pile is dependent on the soil conditions, and was determined based on the standards presented by ADOT in ADOT Poly Memorandum: ADOT DS-3. As specified by ADOT, an appropriate analysis program, Ensoft, Inc.- LPILE 6.0, was used to determine the length of the pile subjected to an axially and laterally loads. The loading was determined using the Resistance Factor (LRFD) Methodology as specified in AASHTO LRFD Bridge Design Specifications. A web-soil survey (see Appendix G) indicated that the ground is classified as clay-loam. Since a geotechnical report is not available for this area, clay was used in the model in order to determine the worst case situation. During this process, two diameters were considered, 12 inches and 18 inches. In order to minimize deflections and for constructability purposes, an 18 in diameter pile was selected. With an 18 inch diameter, the length of the pile was determined to either extend down to bedrock with a rock socket, or down to ten feet. For calculations and drawing details see Appendix C and Appendix D, respectively.

Embankment

The embankment will be made of soil as specified by the City of Flagstaff Engineering Detail, see Appendix H. The embankment will extend away from the 6.5 foot wall down to ground level at a grade of 6%. The cross section of the trail is determined by the typical cross section specified by the City of Flagstaff, see Appendix H. The embankment will require a rail fence that will be located one foot away from the shoulder with a slope of 3:1. After the rail fence, the soil will have a 2:1 slope down to the wing wall or the ground level. Since a survey of Picture Canyon has not been conducted since the restoration of the Rio de Flag, the original ground level at this site cannot be accurately determined. In order to account for this, the total embankment volume considering level ground was calculated. The volume was then multiplied by 1.5 to determine the total embankment needed in order to account for no-level ground conditions. For calculation details see Appendix C.

Impacts

Global/Regional

This project will not have a global effect but will have a small regional effect. The region affected by this project will be users of the Flagstaff Urban Trail Systems. This project will allow many trail users to experience the beauty of Picture Canyon. The bridge will link the two sides of the Rio de Flag within Picture Canyon allowing for greater accessibility within the canyon.

Economic

This project is a long-term investment, well worth the upfront cost. Although there is a high upfront cost of revitalizing the area, the costs will pay off. The replacement structure is designed for low maintenance. The structure is outside of the 100-year floodplain, which will allow the structure to withstand large storm events without washing away or replacement. The concrete deck will require little maintenance, and have a long lifespan. The retrofitted truss will be able to withstand the loads and increase the truss’ usable life.

Environmental

In recent years, Picture Canyon has undergone many conservation efforts. The canyon has undergone trash removal, clearing and grubbing, and Rio de Flag channel restorations. Due to all these recent preservations, the environmental impacts of the replacement bridge are a huge consideration.  The main environmental impact that was considered was the effect of the replacement bridge on the floodplain. The replacement system should have minimal impacts on the floodplain. In order to ensure this, the new system should be placed outside the width and height of the floodplain. A FEMA map of the floodplain can be seen in Appendix H.

Societal

The City of Flagstaff has purchased Picture Canyon with the intent not only to preserve the environment, but also to allow for recreational activities. The trails that are planned for this area will accommodate pedestrians, bicyclists, and equestrian personnel; therefore, the recreational uses of the bridge have a high impact on its design. To take this into account, many trail specifications, for example a 6% slope leading up to the bridge, were considered. Also, since so many people will be using this bridge, the safety and structural stability of the bridge is a high priority.

Cost Estimate

The cost estimate, also known as the Engineer’s Quantity Take-off, can be seen attached as Appendix F. This rough estimate of quantities and corresponding prices will be used as the base model for any future purchases made towards this project. The cost estimate includes item delineations such as the structural concrete necessary for the concrete deck system and the abutment design. Other items include the reinforcing steel for the embankments and the structural steel for the truss retrofit model. The unit costs were derived from the “Historical Bid Unit Price Lookup” in accordance with the Arizona Department of Transportation website. The unit prices were averaged from multiple past construction bids of the same materials required for the project site.
The attached cost estimate proposal does not include various estimations of some materials that have yet to be specified. These item descriptions include:

A contingency determinant of approximately 1.3 was factored into the subtotal of the cost estimate to produce a value of $17,907.98 extraneous costs. These contingencies are considered to be future emergency costs and any other possible quantities that were forgotten and/or not yet considered for the project. The final project estimate comes out to $78,232.97, not including any of the aforementioned listed quantities.

Recommendations

Yield Point of Truss Steel

Since the original plans for the trusses are not available, it will be necessary to determine the actual strength of the steel on the trusses before using them as a pedestrian bridge. A tensile test will need to be performed on the steel to determine the actual yield point of the members. Then, the SAP 2000 analysis will need to be rerun to ensure the truss does not need to be retrofitted differently.

Design Vehicle

In order to reduce the cost of the bridge replacement project, an alternative to consider would be to re-route the design vehicle so it does not cross the bridge. This will allow for less retrofitting of the bridge and therefore, will reduce the cost of the structural steel necessary for this project by $1,486.92. A detailed cost estimate of this alternative can be found in Appendix F.

HEC-RAS Model

In order to determine that new bridge does not have a significant impact on the floodplain within Picture Canyon, a HEC-RAS model should be completed. The model should include the entire embankment, abutments, and superstructure of the bridge. Once the model is completed, the results should be analyzed to determine that the replacement system does not have significant impacts on the floodplain.

Topographic Data

The recent restorations of the Rio de Flag stream channel have caused the landscape within Picture Canyon to change. The most recent topographic survey of Picture Canyon was in 2007, prior to the stream restorations. A new survey in which the stream channel is accounted for will give more accurate data about the topography within Picture Canyon, which has the potential to decrease the amount of embankment required.

Geotechnical Investigation

A geotechnical investigation should be conducted for the site in order to determine the subsurface characteristics of the site. The investigation should determine the depth to bedrock, and the type of subsurface soils. The data retrieved from the investigation could allow the piles to be shorter.


 

 

 

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