Suspension Bridge Construction in Rural Area

CABLE-SUSPENDED PEDESTRIAN BRIDGE DESIGN FOR RURAL CONSTRUCTION by AVERY LOUISE BANG B.S. University of Iowa, 2007 B.A. University of Iowa, 2007

A project submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Civil and Environmental Engineering 2009

Abstract Lack of access to health care facilities, schools and markets is a great impediment. For many communities in the developing world, alleviating rural isolation would help break the cycle of poverty by providing access to educational opportunities, markets, medical clinics and other basic services.

The development of cable suspended

pedestrian bridges are one of the most economical and sustainable solutions to rural isolation. This also presents a challenge for performing engineering analysis with experimental material properties. A review of simple techniques for soil testing, geotechnical models and designs for equivalent structures are reviewed.

Soil

parameters are proven to have a minimal impact on the ultimate uplift capacity required for anchor pull-out design. Recommendations are presented for design and construction in the developing world. A case-study in Ethiopia provides a baseline example as simplifying design assumptions are justified, and design process outlined. Finally, lessons learned from simplifying design for development purposes and general ethical considerations are discussed.

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This report entitled: Pedestrian Bridge Design Best Practices for Rural Construction written by Avery Louise Bang has been approved by the Department of Civil and Environmental Engineering

____________________________________________________ Professor Bernard Amadei (committee chair)

____________________________________________________ Asst Professor John McCartney

____________________________________________________ David Jubenville, P.E., Instructor

Date

The final copy of this paper has been examined by the signatories, and we find that both the content and the form meet the acceptable presentation standards of scholarly work in the above mentioned discipline.

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Table of Contents Chapter 1: Introduction and Background .................................................................... 10  1.1  Cable-Suspended Pedestrian Bridges ............................................................... 10  1.1.1 Rural Transportation .................................................................................. 10  1.1.2 Pedestrian Bridges ..................................................................................... 10  1.1.3 Bridges to Prosperity.................................................................................. 13  1.1.4 Case Study: Sebara Dildi, Ethiopia ............................................................ 15  1.2 Research Objectives .......................................................................................... 19  1.3 Expected Research Contributions ..................................................................... 20  1.4 Organization of Report ..................................................................................... 20  Chapter 2: Typical Bridge Design Scenario ............................................................... 22  2.1 Model of Typical Bridge ................................................................................... 22  2.2 Structural Failure Parameters ............................................................................ 24  2.3 Geotechnical Failure Parameters ...................................................................... 26  Chapter 3: Structural Considerations .......................................................................... 28  3.1 Structural Analysis ............................................................................................ 28  3.1.1 Horizontal Tension..................................................................................... 29  3.2 Pedestrian Bridge Loading ................................................................................ 31  3.2.1 Liveloads .................................................................................................... 31  4

3.2.2 Dead Loads ................................................................................................ 33  3.2.3 Wind Loads (Overturning) ......................................................................... 33  3.2.4 Load Combinations .................................................................................... 34  3.3 Structural Design .............................................................................................. 34  3.3.1 Suspenders ................................................................................................. 34  3.3.2 Main Cables ............................................................................................... 35  3.3.3 Decking ...................................................................................................... 38  Chapter 4: Geotechnical Considerations ..................................................................... 42  4.1 Fine-Grained Soils: Clays and Silts .................................................................. 42  4.1.1 Geotechnical Analysis & Anchor Capacity ............................................... 42  4.1.2 Recommended Parameters ......................................................................... 49  4.2 Coarse-Grained Soils: Sands, Gravels and Non-Plastic Silts ........................... 49  4.2.1 Geotechnical Analysis & Anchor Capacity ............................................... 49  4.2.2 Recommended Parameters ......................................................................... 53  4.3 Geotechnical Design ......................................................................................... 53  4.3.1 Design Process ........................................................................................... 53  4.3.2 Soil Classification & Testing ..................................................................... 55  4.4.3 Design Example: Sebara Dildi Case-Study ............................................... 65  Chapter 5: Quality Control Considerations................................................................. 67  5

5.1 Material Specifications ..................................................................................... 67  5.1.1 Concrete Mixture ....................................................................................... 67  5.1.2 Steel Cable ................................................................................................. 69  5.1.3 Cable Clamps ............................................................................................. 71  5.2 Construction Quality Control ............................................................................ 72  5.2.1 Cable Clamps ............................................................................................. 72  5.2.2 Backfill and Compaction ........................................................................... 74  Chapter 6: Conclusion and Discussion ....................................................................... 76  6.1 Summary ........................................................................................................... 76  6.2 Design for the Developing World ..................................................................... 77  6.2.1 Design Simplification ................................................................................ 77  6.2.2 Ethics of Accountability ............................................................................ 78  6.2.3 Transferring Best Practices to Developing Nations ................................... 80  6.3 Opportunities for Future Research .................................................................... 82  References ................................................................................................................... 84  Appendices .................................................................................................................. 88  Appendix 1: Soil Identification Table (Helvetas, 2001) ......................................... 88  Appendix 2: Computation of Simple Active & Passive Pressures ......................... 89  (DM-7 Section 7.2, Naval, 2009)............................................................................ 89  6

Appendix 3: Breaking Strength Properties of Cable............................................... 90  Appendix 4: Specific Weight of Wood Specimen (CSG) ...................................... 91  Appendix 5: Explanation of Logan’s Pull-out tests for Footings in Sands............. 92  Appendix 6: Abbreviated Unified Soil Classification System (Coduto, 2001) ...... 93  Appendix 7: Bjerrum Correction Factor for Vane Shear Test ................................ 94 

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Index of Figures Figure 1 Suspension and Suspended Bridge Comparison .......................................... 12  Figure 2 Typical Suspended Footbridge, Las Vegas, Honduras ................................. 14  Figure 3 Map of Ethiopia & Sebara Dildi Bridge Site................................................ 16  Figure 4 Sebara Dildi Bridge Rope Crossing ............................................................. 17  Figure 5 Typical Bridge Profile .................................................................................. 22  Figure 6 Typical Abutment Profile for Cable-Suspended Footbridges ...................... 23  Figure 7 Free Body Diagram of Anchor and Tower ................................................... 23  Figure 8 Typical Decking Cross-Section .................................................................... 25  Figure 9 Schematic to Derive Moment at Mid-Span .................................................. 29  Figure 10Typical Decking Detail Plan View .............................................................. 38  Figure 11 Typical Decking Cross-Section with Dimensions ...................................... 40  Figure 12 Variation of Fc' with H'/h Ratio (Adapted from Das, 1990) ...................... 44  Figure 13 Variation of β with Embedment Ratio for ψ=0 .......................................... 45  Figure 14 Net Ultimate Holding Capacity with Variation in Cohesion using Das (1990) .......................................................................................................................... 47  Figure 15 Net Ultimate Holding Capacity with Soil Unit Weights using Das (1990) 48  Figure 16 Minimum Embedment with Friction Angles using Meyerhof and Adam (1968) .......................................................................................................................... 51  Figure 17 Variation of Minimum Embedment with Soil Unit Weight using Meyerhof and Adam (1968) ........................................................................................................ 52  Figure 18 Free Body Diagram Anchor (Adapted from DM-7, 2009)......................... 54  8

Figure 19 Sieve Test (Adapted from Concrete, 2009) ................................................ 56  Figure 20 Typical Triaxial Testing Apparatus ............................................................ 58  Figure 21 Expected UU Triaxial Test Results for Cohesive Soil ............................... 59  Figure 22 Pocket Vane Shear Test .............................................................................. 62  Figure 23 Pocket Penetrometer ................................................................................... 62  Figure 24 Soil Classification and Testing Flow Chart ................................................ 64  Figure 25 Cable Uncoiling Procedure (Helvetas, 2001) ............................................. 70  Figure 26 Proper Cable Transport Technique ............................................................. 70  Figure 27 Failed Nepali bridge: Clamp Slippage ....................................................... 71  Figure 28 Proper Cable Clamp Installation ................................................................ 73  Figure 29 Reduction in Cable Cross-Section with Proper Torque ............................. 73  Figure 30 Proper Cable Clamp Installation and Torque Wrench ............................... 74  Figure 31 Hand Rammer............................................................................................. 75  Index of Tables Table 1 Liveload Schedule for 1.0-meter Deck Width ............................................... 32  Table 2 Assumed Dead Loading ................................................................................. 33  Table 3 LRFD Load Combination Alternatives.......................................................... 39  Table 4 Wood LRFD Resistance Factor Values ......................................................... 39  Table 5 Soil Property Assumptions Summary Table .................................................. 53  Table 6 Correlations for Coarse Grained Soils (Terzaghi, Peck & Mesri, 1996) ....... 63  Table 7 Concrete Ratios by Volume (Adapted Engineers, 2006)............................... 69 

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Chapter 1: Introduction and Background 1.1 Cable-Suspended Pedestrian Bridges 1.1.1 Rural Transportation It is estimated that about 900 million rural people in developing countries do not have reliable year-round access to road networks, and 300 million are without motorized access (Lebo, 2001). Aid dollars being invested into infrastructure improvements for paved highways and major vehicular bridges are only serving those with a standard of living appropriating vehicle use. The remaining 300 million rural citizens have unreliable access to even the most basic services or opportunities. Many governments lack the basic infrastructure capacity to link feeder roads and rural footpaths, and the dilapidated state of the paved roads often is prioritized. Investment in rural transportation improvements would help to reduce poverty through improving access to markets, medical clinics and educational opportunities not currently accessed. Accordingly, a country’s ability to maximize its economic potential is closely linked to the efficiency of its transport system (Haynes, 2003).

1.1.2 Pedestrian Bridges For nearly 50 percent of the world’s population living in rural isolation, the lack of access reinforces the cycle of poverty (United Nations, 2005).

Rural community

members spend a great deal of time and effort on transport activities to fulfill their basic needs. Whether walking miles downriver to reach a river crossing en route to 10

school, or spending a full day to reach the weekend market, the worlds’ poorest people are faced with the disadvantages of lack of direct access to the basic amenities and adequate transport infrastructure necessary to reach them. Rivers and streams isolate villagers of many communities, stranded from the feeder roadways and pedestrian paths during annual floods.

A development strategy that

gives priority to providing reliable, year-round access, to as much population as possible has been proposed in several forms (Lebo, 2001: Blaikie, 1979).

A main

proponent of these strategies is the need for pedestrian bridge crossings. Affordability of an infrastructure project, pedestrian bridge or otherwise, is primarily determined by a population's capacity to maintain its infrastructure over the long term. In rural communities where motorized access is neither existent nor affordable, improvements to the existing trail networks and the provision footbridges over river crossing locations is one of the most cost-efficient investments to create the largest impact. Many countries do not have a single pedestrian bridge in county and those that do are most often over-sized, difficult to maintain and prohibitively expensive structures. A simple footbridge design would provide a cost effective solution to be built without foreign design assistance. Pedestrian bridge technologies vary vastly in design, cost and function. Crossings can be as simple as a fallen tree or as complex as a multi-million dollar work of art. From a structural standpoint, pedestrian bridges have taken a number of forms, each with the function of providing safe transport over an otherwise impassable crossing. 11

Arched bridges, simple beam bridges, truss bridges and cable-stayed bridges constitute four main types of pedestrian bridges: a review of suspended cable-stayed bridges follows. The difference between a cable-suspension bridge and a cablesuspended bridge type is shown in Figure 1, where the blue cable indicates loadbearing in both.

Figure 1 Suspension and Suspended Bridge Comparison

The development of cable-suspended pedestrian bridge construction has played an interesting role in the history of human civilization (Gade, 1972). The first recorded bridge with suspenders connecting handrail and walkway cables was built as early as 285 BC in the Province of Sichuan in China (Peters, 1987). Other known suspension structures during a similar time period were documented in the Eastern Himalayas and consisted of single woven cable, transversed by holding onto either two handrail cables or in a movable basket. Perhaps in a parallel line of invention or speculatively through early Chinese travelers, similar technical knowledge emerged in South America (Peters, 1987). Ancient Incan civilization used rope bridges to span deep gorges, connecting footpaths between villages. These bridges consisted of a pair of 12

stone anchors and massive woven grass cables and two additional woven cables for guardrails. Consistent maintenance and annual replacement of the woven cables made these bridges strong enough to carry the Spaniards while riding horses after they arrived (Gade, 1972). Such primitive rope bridges led to the basic idea of modern cable bridges. The modern cable-suspended bridges constructed by Bridges to Prosperity do not vary greatly from many of the historical bridges. The simple design, constructed using manually-powered tools and only locally available materials are all the same challenges faced by designers for rural developing world bridges today.

1.1.3 Bridges to Prosperity Bridges to Prosperity (B2P) is a United States based non-profit organization that has recognized the need for rural pedestrian bridges. Their work building and training a specific cable suspended footbridge technology has connected rural communities with access and opportunities in over a dozen countries around the world. The suspended cable footbridge design used by B2P was first developed by the Swiss organization Helvetas (2001).

Helvetas took footbridge building practices from improvised

construction to a standardized bridge design manual while creating the world’s largest trail bridge program in Nepal (Nepal, 2008). The suspended design relies on each cable for load distribution and lacks the tall towers equated with suspension bridges. An example of the Helvetas-type suspended bridge is shown in Figure 2.

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Figure 2 Typical Suspended Footbridge, Las Vegas, Honduras

Helvetas successfully accomplished their goal of standardizing the design such that a visual geotechnical evaluation and rudimentary topographic Abney level survey could be used to produce entire construction drawings: only very basic geometry calculations are required. Although the modulated design was appropriate for deep gorge applications as found in Nepal, there is a desire to break-down the design process to allow for easier design modifications more suitable for non-gorge crossings. The author concluded that an example design process would allow a designer to optimize the design to fit local capacity and material availability. 14

Many of the existing bridge design resources are specific to developed nations where constructability and material availability may be considered a lower priority than cost or time of construction (Bridges, 2009).

Rural construction, particularly in the

developing world, creates a number of additional constraints and often present challenges to engineers only experienced with developed-world design practices. With the increase in humanitarian-aid engineering projects through organizations such as Engineers Without Borders (EWB), there is an increased need for both final modular designs as well as design process resources.

B2P identified an

organizational goal to create a best-practices approach to rural footbridge design and construction for general dissemination and internal reference (Bridges, 2009). Bridges to Prosperity started in 2001 by using the Helvetas design manual as it was the most comprehensive design reference available. Several design alterations and modifications have taken B2P away from the original designs as many B2P crossings have topographic situations not addressed in the Helvetas manual. All of these design addendums and calculation assumptions have been posted on their internet site. This document seeks to provide a more complete best-practice document to serve as a resource for potential bridge-builders around the world through B2P’s online database.

1.1.4 Case Study: Sebara Dildi, Ethiopia On behalf of Bridges to Prosperity, the author will be constructing a 100 meter suspended bridge in the Ethiopian state of Amhara in the summer of 2009. A site 15

visit and engineering survey were conducted in June of 2008. During the trip, the need for a more complete design guide for soil testing and design was realized. 1.1.4.1 Background & Location Approximately 40 kilometers from Lake Tana, a broken multi-arched bridge spans the Blue Nile River gorge. The bridge links a major caravan route between two trading regions: the Gonder region and the city of Debra Tabor to the north, and the Gojjam region and the city of Debre Markos to the south. The bridge site is marked in yellow in Figure 3 and the two aforementioned towns marked in red.

Figure 3 Map of Ethiopia & Sebara Dildi Bridge Site

"Sebara Dildi,” or broken bridge in the local Amharic dialect, was built in the mid1600’s of stone, sand, lime, and egg: an early version of an elastomeric adhesive (Bridges, 2009). During World War II, the middle arch of the bridge was destroyed by Ethiopian Patriots to impede Mussolini's Italian invasion force. During the effort to cut away the arch, it collapsed and killed 40 men (Snailham, 1968) but succeeded 16

in slowing the Italian forces. After the Italian retreat, the bridge was never repaired. The current method of crossing is both expensive and dangerous and requires one to pay to be manually pulled across while holding to a knotted rope, as seen in Figure 4.

Figure 4 Sebara Dildi Bridge Rope Crossing

Approximately 450,000 people live directly on either side of the bridge and although dangerous, traffic remains heavy at the crossing to avoid the additional 75 kilometer trip required to use the next closest bridge (Bridges, 2009). Those who operate the rope charge 3 Ethiopian Birr ($0.38) or approximately 20 percent of a person’s average daily salary. In 2002, Bridges to Prosperity attempted to fix the crossing by building a steel truss bridge, set atop bridge remains. The bridge was swept away during the first rainy 17

season that followed, as water levels at the crossing point currently reach a higher elevation than when the bridge was originally designed. This could be attributed to high levels of deforestation in Ethiopia and in turn, higher levels of runoff (Nile, 2008).

The failure of this bridge project led to the creation of the Bridges to

Prosperity organization, and although the first attempt to repair the bridge was unsuccessful, Bridges to Prosperity plans to build a bridge with adequate freeboard and clearance. The crossing point is extremely remote. Flying from the capital city of Addis Ababa, one must arrive by air in Bahir Dar: the closest city to the site. Approximately 3 hour drive south is the township of Mota from which one must walk approximately 8 hours through Ethiopian highlands into the Blue Nile Gorge at Sebara Dildi. The remote nature of this site limits survey and testing equipment to what can be carried. Construction materials will need to be brought in on mule thus designers must limit the size and weight of any particular material or tool required for bridge construction. 1.1.4.2 Site Visit The author visited the site in June 2008 with the intention to choose the best location for a suspended bridge crossing. A site 200 meters downstream and up-trail from the current crossing was selected based on a narrowing of the river and an avoidance of several residuals landslides. A rudimentary surveying approach using an Abney level and string was used to create a topographic cross-section of the site. This process has been well documented: 18

reference the Helvetas Volume 1 Suspended Manual for further detail (Helvetas, 2001). The final span was found to be 100 meters, with a negligible height difference between the abutments. The suggested process for soil identification required only a visual identification by which the surveyor classified the soil based on ability to ‘see’ more than fifty percent of the grains (Appendix 1). Both abutments were excavated to one meter depth and a soil sample was attained for visual classification. The soil visually classified as a sand at both abutments.

The author found it difficult to conclude on design

parameters from such a basic approach.

A greater understanding of design

assumptions was required to conclude whether a more in-depth testing and classification process was feasible or necessary.

1.2 Research Objectives Extensive literature exists for equivalent structure behavior in the developed world, but very limited documentation has been created that adequately addresses design for development applications. Through a review of existing testing and modeling approaches for comparable structures and parametric studies with pertinent geotechnical models, a simplified design approach is desired.

This includes

identifying viable geotechnical classification testing approaches, and offering recommended soil parameter assumptions as needed. A design case-study, inclusive of both structural and geotechnical design methods, will be included to improve general understanding. The end result will allow future bridge designers to identify 19

the underlying assumptions in order to modify the design for sites where the Helvetas standard does not apply.

1.3 Expected Research Contributions Minimal research has been completed specifically on the geotechnical proprieties of anchorages intended for rural pedestrian cable stay bridges. behavior and assumed failure mechanisms will be discussed.

A review of cable Structural design

process will be outlined, including a case-study example for a 100 meter span. Furthermore, a literature review of models intended for similar structures commonly used as foundation systems that require uplift or lateral resistance will be included. By reviewing design and modeling of these well-understood structures, a design approach for soil anchorages for small cable-stayed bridges will be proposed. Furthermore, as testing equipment available in rural developing world applications are often not available, conservative soil parameters will be proposed with respective justification based on impact on geotechnical models. Existing documentation has a greater emphasis on modulated design rather than design process. This document will produce a design guide for soil classification and testing, structural and geotechnical design and engineering quality control for pedestrian bridge design for the developing world.

1.4 Organization of Report The introduction chapter has given context for the cable-suspended bridge. The following chapter will more precisely identify the technical challenge and parameters 20

through discussion of a typical cable-suspended bridge. Chapter 3 will detail the structural design considerations including assumed cable behavior, cited codes and standards, example loading calculations and a complete design process for the casestudy bridge. The main objective of the chapter is to demonstrate how to calculate the anticipated loads being transferred from the structure into the anchorage system. Chapter 4 discusses geotechnical design. Discussion into how to classify as soil as either coarse or fine-grained followed by pertinent models for each. One specific model is detailed for each soil type, from which assumptions are justified through parametric studies. A simplified geotechnical model is proposed for design use for cable-suspended bridges. Testing programs for both soil types are also discussed. Chapter 5 introduces many of the quality control issues faced in footbridge construction including both material property and construction quality assurance. The final chapter concludes with a discussion of engineering design for the developing world. Lessons learned from this report in design simplification are presented and more general concepts on the topics of ethics of accountability are discussed.

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Chapter 2: Typical Bridge Design Scenario 2.1 Model of Typical Bridge The primary objective of this report is to suggest an anchor design approach to resist pull-out failure for footbridge deadman anchors, shown in Figure 5.

Figure 5 Typical Bridge Profile

Where: L = span in meters Lb = Backstay length hsag = cable sag in meters ho.b. = height overburden Typical spans for consideration range from 40 to 120 meters, backstay lengths range from 5 to 10 meters, cable sag range from 2 to 10 percent of the span and overburden heights range from 1.5 to 3 meters.

To limit the scope of this report, the

aforementioned characteristic dimensions will be considered the limits conditions for each respective parameter. The modulated Helvetas cable-suspended bridge design was developed with few components and minimal connection points. The primary components of a cable22

suspended pedestrian bridge are: anchorage (1), ramped approaches (2), foundation tiers and towers (3), handrail and walkway cables (4 & 5 respectively) and deck walkway, as shown in Figure 6.

4 3

1

5

2

Figure 6 Typical Abutment Profile for Cable-Suspended Footbridges

A simplified free body diagram detailed in Figure 7 depicts the typical forces inflicted upon the anchorage and tower. Chapter 3 will discuss the structural analysis process needed to solve for forces in the tower and Chapter 4 will discuss geotechnical analysis for the anchorage design based on those forces.

Figure 7 Free Body Diagram of Anchor and Tower

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Where: Pt = Cable tension =Force imposed on anchor PV and PH = Respective components of the force. Wt = Weight of Block + Weight Soil above Block = WB + WS WB = X * Y * γB Ws = X * h * γs γs = unit weight of soil γB = unit weight of reinforced concrete c = cohesion intercept of the soil ϕ = Angle of Friction of the soil ψ = Angle of anchor cable θ = Cable deflection angle Pp = Passive force (Appendix 2). Therefore, the geotechnical parameters of interest are the angle of friction (ϕ), the soil unit weight (γs) and the cohesion (c). The only structural variable that influences the final anchor design is the loading, (Pt).

2.2 Structural Failure Parameters For cable to fail, the strands must elongate past the elastic range into the elasticplastic portion of the material’s stress-strain curve. As the deck live load increases, the load to the walkway cables is increased proportionally until the added length due to stretch forces the suspenders to transfer the load onto the handrail cables. Only when both the handrail and walkway cables were fully loaded would cable have the

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potential to go beyond the elastic state required for cable failure. The aforementioned cable connection is shown in Figure 8.

Figure 8 Typical Decking Cross-Section

Steel cable is the primary load-bearing structural component, diverting the decking loads between the towers and anchorage systems. The cable carrying the transverse load results in a geometrical configuration where horizontal force at mid span is inversely proportional to the sag. It follows that cable pulled infinitely horizontal is unable to carry any transverse load as zero sag implies an infinitely large cable force (Pugsley, 1957). Likewise, significant increase in cable sag would result in a greater vertical reaction at the towers. Structural design optimization requires the designer to designate a sag ratio that properly balances these two considerations. The minimum safe working load of steel cable can be found by dividing the manufacturers supplied breaking strength by the safety factor. The recommended factor of safety for load-bearing steel cable is 3.5 (Bureau Reclamation, 2009). Cable clamps are required to reach 80 percent efficiency rating, thus an accumulative factor 25

of safety of 2.8. As the material specifications are highly regulated and guaranteed for a high level of precision, the result is a highly unlikely case for structural cable failure. Discussions into the behavior of the cable and construction elements identify other potential failure mechanisms. Prior to the wide-spread use of computer modeling and finite element techniques, cable typically was assumed to behave as a parabolic curve (Pugsley, 1957). This simplified parabolic model allows a basic understanding of the fundamental interdependence between stability, stiffness, and strength. The cable analysis described in Chapter 3 is overly-simplified but included to provide a basic understanding of cable behavior and to provide the framework for design understanding.

2.3 Geotechnical Failure Parameters The geotechnical failure mode of interest is anchor pull-out. As such, the ultimate uplift capacity (Qu) of the soil must be found. Chapter 4 will outline an empirical approach for calculating Qu for both fine and coarse-grained soils.

The soil

parameters of interest in both models are soil unit weight (γ), friction angle (ϕ) and cohesion (c). The primary geotechnical failure mode of consideration is anchor pull-out.

To

prevent pull-out failure, the anchor must be placed at an appropriate depth and distance from the tower with consideration for soil strength parameters. The soil parameters of interest are the soil unit weight (γ), cohesion (c), and the friction angle 26

(ϕ) of a soil.

Separate design approaches for fine and coarse-grained soils is

recommended for calculating ultimate uplift capacity of anchors (Das, 1990). Smith and Stalcup (1966) suggested that fine-grained cohesive soils attained up to 30% increase in holding capacity as compared to coarse-grained, but 2 to 3 times the horizontal displacement was required to activate the passive earth pressure.

This

initial research indicated that further investigation was needed before assuming a similar design model was appropriate for both fine and coarse grained soils. Rock masses will not be considered herewith in as the following models are not applicable to jointed rock masses where strength is controlled by joint orientation. Furthermore, intermediate rock masses will not be considered as the anchor design for excavatable rock mass is not based on the soil properties but rather the rock and anchor’s ability to attach to surrounding material as a single mass. The designation between intermediate rock mass and a soil will be defined as the later is able to be excavated with man-powered shovels.

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Chapter 3: Structural Considerations Structural design requires few site-specific parameters and thus can be implemented off-site. The best-practice approach to design requires identifying applicable codes and regulations. Under most conditions, structural design codes applicable in the United States or equivalent will be at least as comprehensive and well-proven as those in the country of consideration. A designer must address the differences in codes during his or her work, but may use a design methodology documented herein. The international nature of this design further encourages a designer to consult local codes and learn from the experience of comparison. Redundancy in the modulated Helvetas design has resulted in only one documented failure in over 2000 bridge constructions (Helvetas, 2001).

The failure of this

particular bridge was due to insufficient torque on the clamps used to tie the cable around the anchor. As this is a relevant material and construction quality control item, quality control will be discussed in Chapter 5.

3.1 Structural Analysis Assuming the cable is frictionless and a perfectly flexible material, the cable hangs in a parabolic arc (Pugsley, 1957). The primary assumption is that the intensity of the vertical distributed load

is constant. The perfectly flexible cable is considered to

give no resistance to bending at any point and thus the resultant tensile force is tangent to the curve at any point in the cable. Thus, to find the maximum tension in

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the cable, it is necessary to know the relations involving tension, span, sag and the length of cables (Meriam, 2007).

3.1.1 Horizontal Tension Taking the moments about point A taken at mid-span and assuming that the supports are at equivalent heights, one can solve for the horizontal tension in the cable per the Figure 9.

Figure 9 Schematic to Derive Moment at Mid-Span

8

 

Where: Wc = distributed load Th = horizontal tension L = span in meters h = cable sag, in meters Given the horizontal tension in the cable, solve for the slope of the cable at the towers to acquire the maximum cable tension at the height of the towers.

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The slope of the cable, the corresponding total tension in all cables and the tension in each cable may be calculated from the following relationships: 4

 

 

 

  Where: θ = cable deflection angle T’ = cable tension, in kN N = number of cables Tc = allowable tension per cable

One must chose the number of cables based on the availability of cable and its respective breaking strength. Although each cable supplier must verify the breaking strengths of the cable, Appendix 3 may be used for academic purposes. The sum of the walkway and handrail cable design strengths must exceed the tension in the cable after accounting for allowable stress design factors. Each cable takes a load proportional to its’ cross sectional area and thus if cables of differing sizes are used, each cable will take a proportional load to its cross-sectional area ratio. In accordance with AASHTO (2003) standards, the following design approach and assumptions were used throughout this report. To illustrate the design process, a 30

design example has been included through the text.

By outlining pertinent

assumptions and processes, modifications to the standard design may be used. One such scenario is a community’s request to widen the decking from 1.0 meter width to 1.5 meters to allow for animal-pulled carts or a decrease in deck-width for low traffic crossings.

3.2 Pedestrian Bridge Loading To find the tension in the cable, the load on the cable must be computed. The following details the recommended design approach per American Association of State Highway and Transportation Officials (AASHTO) article 3.16 and the supplemental “Guide Specifications for Design of Pedestrian Bridges” document (AASHTO, 1997). It is recommended that the reader reference applicable codes and specifications in area of intended construction prior to design.

3.2.1 Liveloads A liveload of 85 pounds per square foot is designated unless the walkway area is greater than 400 square feet. Then the live load figure is slowly reduced between 400 square feet and 850 square feet, at which time the minimum standard of 65 pounds per square foot is used. The 65 pounds per square foot minimum load limit is used to provide a measure of strength consistency with the LRFD specifications, which specify 85 pounds per square foot less a load factor indicated in the LRFD Design specifications (AASHTO, 1997). The formula is as follows: 85 0.25

15/√

  31

Where A is the total square feet of walkway surface area. Therefore, using a 1.0 meter walkway cross sectional area, the following live load schedule would apply: Table 1 Liveload Schedule for 1.0-meter Deck Width Span

English Unit Loading

Metric unit Loading

(m)

(lbs/ft²)

(kN/m²)

1-37 m 38-78 m

85 lbs/ft² proportional reduction, from 85 to 65 lbs/ft² 65 lbs/ft²

0.415 kN/m² proportional reduction, from 0.415 - 0.317 kN/m² 0.317 kN/m²

79 + m

As noted in AASHTO (1997), the live load reduction for decking areas exceeding 400 square feet is consistent with ASCE 7-95, “Minimum Design Loads for Buildings and Other Structures.” The reduction accounts for the reduced probability of the large loading area of the structure being fully loaded at any given time. The likelihood of the rural footbridge being fully loaded is somewhat unrealistic, but failure cases have been reported during heavy traffic (Nepal, 2009).

Furthermore, the likely case of

small motor-vehicles and animal-driven carts would require both distributed and point-load analysis to find maximum loading case. As such, the conservative assumption loading of 0.415 kN/m² (85 lb/ft²) is recommended. Total LL = 0.415 kN/m, assuming 1.0 meter width

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3.2.2 Dead Loads Table 2 Assumed Dead Loading

Assumptions/Conversions 8 mm diameter x 1.7 m steel rebar = 8.5e-5 m³ per rod Unit weight steel = 490 lb/ft³ = 7847.3 kg/m³ 0.67 kg per suspender 0.0098 kN/kg Suspender spacing 1 meter on center per side 10 cm x 10 cm x 1.4 m Cross Assume 600 kg/ m³ (Appendix 4) beams Cross beams 1 meter on center 5 cm x 20 cm x 2 m = 0.02 m³ per 2 meter member Decking Assume 400 kg/ m³ (Appendix 4) 8 kg per decking panel 5 decking panels across Assume 6x19 IWRC galvanized steel cable Cable Assume 32 mm cable (1 ¼”) : 2.89 lb/ft Assume 6 cables 1 lb/ft = 1.288 kg/m Total DL = 0.334 kN/m, assuming 1.0 meter width

Suspenders

Loading 0.0134 kN/ m

0.082 kN/m 0.020 kN/m

0.219 kN/m

3.2.3 Wind Loads (Overturning) A wind load applied horizontally at right angles to the longitudinal axis of the bridge shall be applied at 35 pounds per square foot (0.171 kN/m²), assuming that the wind can readily pass through the bridge profile, per AASHTO specifications. The specified wind pressures are for a base wind velocity of 100 miles per hour which in such case a site has higher wind-velocity requirements: AASHTO Article 3.15 (1997) may be referenced. Given the projected profile of the bridge is 1.1 meters in height: the resulting wind overturn force is 0.183 kN/m. Total WL = 0.183 kN/m, assuming 1.1 meter railing height 33

3.2.4 Load Combinations The following load combinations will be used, extracted from Table 3.22.1A in AASHTO (1997): • • •

Group I - (Dead + Live) at 100% of Allowable Stress (i.e., Load Combination Reduction Factor = 1.0). Group II - (Dead + Wind) /125% of Allowable Stress (i.e., Load Combination Reduction Factor = 1.25). Group III - (Dead + Live + 0.3 Wind) /125% of Allowable Stress (i.e., Load Combination Reduction Factor = 1.25).

Group I: Group II: Group III:

.

0.651 

/

0.413 

/

. .

0.565 

Æ directs

/

Wc = 0.651 kN/m²

3.3 Structural Design 3.3.1 Suspenders Suspenders transfer the loads from the deck to main cables, and are attached to crossbeams at 1 meter intervals. Thus, with a 1 meter deck width, each suspender has a tributary area of 0.5 m², and thus must be able to carry 0.325 kN of loading from the 0.651 kN/m loading. The minimum diameter of suspender can be calculated per the following equation:

34

 

Where: Fs = applied force in kN Ps =allowable yield strength in kN/m²

Assuming ASTM A36 Grade 300 (ASTM) with a minimum yield strength of 24.5 kN/cm² (250 MPa) and a factor of safety of 1.5, and assuming a 1.0 meter deck width, the minimum diameter of a suspender would be 1.6 mm. Due to the high surface area to volume ratio of the suspender and thus increased likelihood of corrosion, a minimum suspender diameter of 8 mm is recommended.

3.3.2 Main Cables Assuming a 100 meter crossing, the design process is as follows for selection of the primary cable. For further details on the mathematical derivation of the following equations, see reference (Meriam, 2007). 0.361 

/ ² 

Therefore: 0.651 8

8

100  5 

²

162.75 

 

Where: 35

Th = horizontal tension, in kN. L = Span in meters hsag = cable sag, in meters 4

11.3° 

Where: θ = cable deflection angle  

166 

 

Where: T’ = cable tension, in kN   Where: N = number of cables Tc = Allowable tension per cable The sum of the walkway and handrail cable design strengths must exceed the tension in the cable. In this example, 166 kN may be distributed either 4 cables (2 walkway and 2 handrail) or 6 cables (4 walkway and 2 handrail) in order to support the load. Using example design breaking strengths in Appendix 3 (including the factor of safety of 3.5), the load can be split between 4 cables. Assuming each cable is to be the same, each must have a minimum design strength equivalent to Tc. Thus, the result is the minimum cable size of 16 mm, per Appendix 3. Each cable takes a load proportional to its’ cross sectional area. The total design load imposed on the anchor 36

is equivalent to T’ or 166 kN in the case of a 100 meter bridge with a 1.0 meter decking. To calculate the amount of cable to order, one must first know the length of each cable between towers: 8 3

1

For a 100 meter span and 5 percent sag,

 

is equal to 100.67 meters per cable

between towers. To calculate the total length of cable required to purchase, one must first decipher the length of the backstay which requires geotechnical design. For practical purposes, a simplified equation would allow field supervisors to order cable without an intimate understanding of the design process. As the length of the cable is only increased by less than one percent when accounting for sag, neglecting this added length and alternatively including a four percent contingency is practical. The following is proposed: 1.04

14

 

 

 

 

 

 

 

The 14 meter addition allows for cable wrap-back, seven meters at either anchor, approximate but standard on all crossings. There are several design alterations that may be considered to reduce the length and thus cost of cable, but while the length of the span may reduce to lessen this cost, the tower height likely would increase thus increasing masonry costs. 37

3.3.3 Decking There are two primary code sources used for bending stress problems in The United States: Load and Resistance Factored Design (LRFD) and Allowable Stress Design (ASD). LRFD will be discussed herein. A full design process is not detailed herein as it is beyond the scope of this report, but the following provides context and background for how the modular decking design alternatives currently in use by Bridges to Prosperity were developed. A typical decking plan view is shown in Figure 10. Note that the crossbeam spacing is 1.0 meters. The deck width will be assumed 1.0 meters as well.

Figure 10Typical Decking Detail Plan View

3.3.3.1 LRFD Loadings LRFD use slightly different nomenclature from the loading section at the beginning of the chapter but for consistency, the following will continue to use similar nomenclature. 38

Table 3 LRFD Load Combination Alternatives

LRFD Load combination alternatives: 1. 1.4DL 2. 1.2DL+1.6LL+.5(Lr or S or R) 3. 1.2DL+1.6(Lr or S or R)+(.5LL or .8WL) 4. 1.2DL+1.6WL+.5LL+.5(Lr or S or R) 5. 1.2DL+/- 1.0E+.5LL+.2S 6. 0.9DL +/- (1.6WL or 1.0E)

Table 3 lists the six fundamental factored load combinations from Minimum Design Loads for Buildings and Other Structures (ANSI/ASCE 7-88) used for safety analysis in LRFD. There will be assumed no roof (Lr), no snow (S) no rain (R), and no earth (E) loadings.

Therefore the LRFD design strength is 1.065 kN from load

combination 2. As the member is being design for compression, the required nominal strength, assuming a resistance factor of 0.9 from Table 4, would be 1.183 kN. Table 4 Wood LRFD Resistance Factor Values

Mode Compression Flexure Tension Shear

ϕ 0.90 0.85 0.80 0.75

Structural design includes resistance to shear failure, flexure failure and for serviceability, a maximum displacement must not occur.

The crossbeams and

decking panels must be considered independently and for two load case scenarios: fully loaded and point loaded in cross-section. 39

3.3.3.2 Cross beams Crossbeams are the members that are spaced perpendicular to the length of the bridge. There are three initial design choices: crossbeam spacing, and the width of the decking.

Figure 11 Typical Decking Cross-Section with Dimensions

Figure 11 depicts a cross-section for a typical decking cross-section with a small spacer board that is for constructability, but will not be considered in the following calculations. The crossbeam bending calculations will be based off a cross beam dimensioned (X+36 cm) by Y cm by Z cm (into the page). The additional 36 cm is included for connection spacing on either side, as recommended from practical experience. 3.3.3.3 Decking Planks The length of the decking planks is recommended to be 3.0 meters, although 2.0 meter decking planks are also acceptable with a slight reduction in longitudinal rigidity, as shown in the plan view of a typical decking in Figure 10. Design for longitudinal beams should assume a multi-support, simple beam analysis. To identify 40

the maximum shear and applied moment, both point load and distributed load scenarios should be considered. For both cross-beam and decking plank design, one must state the material properties and assume an initial member size. Material properties of interest are the material yield strength, Fy, the ultimate flexural strength, Fu and the modulus of elasticity, E. An initial member size is selected for the following parameters: cross-sectional area, Ag, moment of inertia, I, the radius of gyration, r, and the corresponding slenderness ratio, kL/r, the section modulus, S, and the maximum deflection (∆ = L/360). As detailing every potential consideration and design alternative for decking design is beyond the scope of this project, a modulated design is recommended for use. Bridges to Prosperity has provided modular designs for both wood and steel decking solutions: both provided with several size alternatives on their website (Bridges, 2009).

41

Chapter 4: Geotechnical Considerations The main geotechnical design component of the bridge is the anchor block. The primary purpose of these anchors is to transmit a tensile load from the cables to the anchor and soil to prevent pull-out failure. Accordingly, it must have adequate weight and placed at an appropriate depth and distance from the abutment to provide adequate resistance (Das, 1990). Design models used to find the ultimate uplift capacity of the anchor are separated for fine and coarse-grained soils due to their difference in behavior. They behave differently due to the rate of pore water pressure dissipation during loading.

The following will discuss fine-grained-specific soil

analysis and recommendations, followed by coarse-grained-specific soil models and recommendations. A recommended design process will summarize the findings from these two proceeding sections.

4.1 Fine-Grained Soils: Clays and Silts 4.1.1 Geotechnical Analysis & Anchor Capacity There are relatively few studies relating the holding capacity of inclined anchors embedded in fine-grained materials under a tensile load (Das, 1990).

One of the

most comprehensive studies of inclined plate anchors was completed by Das (1983). The results showed that the net ultimate holding capacity of an inclined rectangular anchor is related to an empirical breakout factor Fc’ as follows: 

42

Where: Fc’ = average breakout factor

Qu = net ultimate holding capacity A = area of anchor plate = Bh B = width of anchor plate Cu = undrained cohsion of the clay soil (ϕ = 0 condition) Ws = Weight of soil above anchor ψ   anchor inclination with respect to horizontal  H’   average depth of embedment  Therefore, for rectangular anchors, the breakout factor can be recalculated as follows:

 

The breakout factor Fc’ increases with the average embedment ratio H’/h to a maximum value, at which point it asymptotically approaches a maximum value, as depicted in Figure 12.

43

Figure 12 Variation of Fc' with H'/h Ratio (Adapted from Das, 1990)

The first step in solving for the ultimate holding capacity of the anchor is to calculate the critical average embedment ratio (H’/h)cr for a rectangular anchor as follows:

 

0.73

 

0.27

1.55

 

Where:

 

0.107

2.5

7.0

Cu = undrained cohesion in kN/m² If the design ratio H/h is greater than

 

, then it is considered a deep anchor and

the breakout factor assuming a zero degree anchor angle is as follows:

7.56

1.44

44

If the design ratio H/h is less than

, then it is considered a shallow anchor and

 

the breakout factor assuming a zero degree anchor angle is as follows:

7.56

1.44

Where β is found using Figure 13.

1 0.9 0.8 0.7 β

0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.2

0.4

0.6

0.8

1

α=(H'/h)/(H'/h)cr

Figure 13 Variation of β with Embedment Ratio for ψ=0

The next step is to estimate the breakout factor for an anchor with a cable angle of 90 degrees. Although unrealistic, the process eventually correlates the actual backstay angle to the ratio of the breakout factor at 0 degrees and 90 degrees.

   

0.5 0.5

0.9

0.1

1.31

45

Where:

0.0606

 

If the design ratio H/h is greater than

4.2

6.5

, it is considered to be a deep anchor and

 

the breakout factor assuming a zero degree anchor angle is as follows:

9

If the design ratio H/h is less than

0.825

0.175

, then it is considered a shallow anchor and

 

the breakout factor assuming a zero degree anchor angle is as follows:

0.41

0.59

Where :

   

0.5 0.5

Das details the process where Fc’ is determined through the following equation, relating the variation of the average breakout factor as follows:

90

²

46

With Das’ (1990) empirical procedure outlined above, a parametric study was conducted with several assumed backstay cable inclinations, as shown in Figure 14.

Minimum Cohesion to resist pull‐out failure (kN/m²)

70 60 50 40 30 20 10 0 0

200

400

600

800

1000

1200

Net Ultimate Holding Capactiy (kN) 30 degrees 40 degrees 60 degrees Linear (35 degrees) Linear (45 degrees)

35 degrees 45 degrees Linear (30 degrees) Linear (40 degrees) Linear (60 degrees)

Figure 14 Net Ultimate Holding Capacity with Variation in Cohesion using Das (1990)

Using Das’ approach for shallow-anchor design, Figure 14 summarizes the dependency on cohesion for a load of 166 kN (which is representative for a 100 meter span), assuming an anchor with a 1.2 m by 3.0 meter surface. A soil unit weight of 19 kN/m³ was assumed because the dependence on the unit weight is insignificant for all ranges of loading types, as shown in Figure 15.

47

Minimum Cohesion to resist pull‐out failure (kN/m²)

70 60 50 40 30 20 10 0 0

100

200

300

400

500

600

700

Net Ultimate Holding Capactiy (kN) 17 kN/m3

19 kN/m3

21 kN/m3

25 kN/m3

Figure 15 Net Ultimate Holding Capacity with Soil Unit Weights using Das (1990)

As the dimensions of interest are considered ‘shallow’ by Das’ (1990) definition, the deep anchor scenario was not modeled. The assumed parameters for this model included a surface length perpendicular to cable (h) of 1.2 m, an average embedment depth (H’) of 1.2 meters, and an anchor width (B) of 3.0 meters. For typical loading, detailed in the structural analysis in Chapter 3, the minimum cohesion values required to resist pull-out failure for a 166 kN load are below realistic in-situ cohesion values for fine-grained soils.  

48

4.1.2 Recommended Parameters The conclusion is that the soil unit weight and the minimum cohesion are fairly insignificant soil properties within the pertinent parameters of interest. As detailed in Chapter 2, the longest typical bridge of consideration is 120 meters. Accordingly, even though the cohesion is important for high loads, these will not be observed in these bridges. Figure 14 proves the relative insignificance of soil parameters at given conditions of interest. If no testing is available, a conservative cohesion of 20 kN/m² may be assumed. Assuming that the structure is quickly loaded and the undrained strength parameters direct, assuming a zero friction angle is also appropriate. Figure 15 shows that the depth of embedment is relatively insensitive to the soil unit weight (19 kN/m³). Non-plastic silts will exhibit little or no cohesion and friction therefore they are included with coarse grained soils.

4.2 Coarse-Grained Soils: Sands, Gravels and Non-Plastic Silts 4.2.1 Geotechnical Analysis & Anchor Capacity Meyerhof and Adams (1968) proposed a semi-empirical relationship for estimating the ultimate uplift capacity of strip, rectangular and circular anchors in coarse-grained materials.

It is one of few methods available for estimating the capacity of

rectangular anchors. Many alternative models are presented in literature for circular or square anchor plates, but those will not be discussed as the shape of the anchor requires a shape factor not included in other design models (Das, 1990).

The 49

introduction of further empirically derived correction factors ideally would be accompanied by experimental validation for use with anchorages similar in size and use as footbridges.

The Meyerhof and Adam’s model includes fewer assumed

assumptions than other models and thus will be discussed. To find the ultimate uplift capacity per unit width of anchor, the following equation may be used: 1 2

 

Where: Qu’ = ultimate bearing capacity per unit width Kb = Passive Pressure Coefficient h = height of embedment H = depth of bottom of anchor H’ = average embedment of anchor ψ = Angle of cable from anchor, from horizontal γs = unit weight of soil

Several parametric studies were completed analyzing the impact of material assumptions on the Meyerhof procedure for inclined anchors in a cohesionless soil (Meyerhof, 1973). A study comparing the impact of backstay angles and minimum embedment depth for a 166 kN load (from Chapter 3, structural loading for a 100 meter bridge) with various friction angles is summarized in Figure 16. 50

Backstay Angle (degrees from horizontal) 20

30

40

50

60

70

80

0.00

Minimum embedment (m)

0.50

1.00

1.50

2.00

2.50 Phi = 25 degree

Phi = 30 degrees

Phi = 35 degrees

Phi = 40 degrees

Figure 16 Minimum Embedment with Friction Angles using Meyerhof and Adam (1968)

As the backstay angle had a minimal impact on the embedment depth, a simplifying conclusion specifying a minimum embedment of 2.0 meters may be suggested. As this analysis is specific to a 100 meter bridge loading, these results may not be extrapolated to all spans. The outlined analysis may be followed to produce similar simplifying conclusions for any span of interest. This particular study assumed a soil unit weight of 19 kN/m³, because the minimal impact on the model for this parameter as shown in Figure 17.

51

Assumed Unit weight of soil (kN/m³) 15

17

19

21

23

25

27

0.00

Minimum Embedment depth (m)

0.50

1.00

1.50

2.00 Phi assumed 30 degrees, backstay assumed 45 degrees

Figure 17 Variation of Minimum Embedment with Soil Unit Weight using Meyerhof and Adam (1968)

Varying the backstay angle of the anchor was found to have relatively minimal impact on required friction angles within the range of feasible anchor angles from 20 to 60 degrees from the horizontal. The variations of Kb for shallow strip anchors can be obtained from the earth pressure coefficients of an inclined wall, and were summarized in a chart (Das, 1990). Each anchor angle and assumed soil friction angle will have a unique empirical value. Given the opportunity, pull-out tests on various sandy soils may provide further insight and negate the need for conservative friction-angle assumptions.

Logan

(Logan, 1976) completed an experimental series of pull-out tests for footings in sand. Footings were loaded to failure and the failure mechanism was documented. Future 52

work in this area could find his testing procedures and findings applicable. For further details of Logan’s study and findings, reference Appendix 5.

4.2.2 Recommended Parameters Figure 16 shows an increase in load when the friction angle is increased from 25 and 30 degrees. If no testing is available, it is recommended that a value of 26 degrees be assumed for the value of friction angle. This is relatively low for quartz sands, as it is the angle of repose. Figure 17 depicts a relative insensitivity for the assumed soil unit weight thus 19 kN/m³ may be assumed. The most conservative strength for a coarsegrained soil is when it is fully-drained in which case it will have a zero cohesion intercept. Table 5 is a summary of recommended soil assumptions. Table 5 Soil Property Assumptions Summary Table

γsoil ϕ

c γsoil ϕ

c

Fine-grained soil 19 kN/m³ 0 Degrees

20

kN/m²

Coarse-grained soil 19 kN/m³ 26 Degrees

0

4.3 Geotechnical Design 4.3.1 Design Process Sections 4.1 and 4.2 detailed two distinct analysis approaches for anchors in fine and course grained soils respectively to identify design simplifications. The following is 53

modified DM-7 design approach (2009) that may be used for design of anchors in fine or coarse-grained soils.

Soil parameter assumptions justified in the

aforementioned sections may be used, or further testing approaches detailed later in the chapter may be used to reduce material uncertainty.

Figure 18 Free Body Diagram Anchor (Adapted from DM-7, 2009)

Where: Pt = Force (can be found from the Structural Considerations section) PV and PH = Respective components of the force. Wt = Weight of Block + Weight Soil above Block = WB + WS WB = X * Y * 2300 kg/m³ (unit weight concrete) WB = X * h * γ γ = unit weight of Soil c = cohesion ϕ = Angle of Friction Pp = Passive pressure (Appendix 2).

54

The design process is very straight forward and only requires verification that the anchor of interest is able to resist the vertical force and the horizontal force with independent calculations. 1) Step 1: Check resistance to vertical force: 1.5 2) Step 2: Check resistance to horizontal force : 1.5

4.3.2 Soil Classification & Testing 4.3.2.1 Soil Classification The geotechnical component of the design for rural bridges involves an estimate of the resistance to pull-out of an anchor. The parameters governing the mechanical response of the soil to such loadings as well as the recommended testing approaches are dependent on the rate of loading and the drainage characteristics of the soil. The main parameters needed are the shear strength, usually represented by the MohrCoulomb failure envelope where the strength is sensitive to the water content and density:  tan  For the case of short-span pedestrian footbridge design, the anchorage systems have been proven in Sections 4.1 and 4.2 to be relatively insensitive to input soil parameters. As such, it is recommended by the author that the soil at a minimum be classified with the objective to choose between the two modeling alternatives. If no 55

further testing is possible, use of the conservative soil parameters are suggested for these groups. The Unified Soil Classification System (USCS) groups soils using their grain-size distribution and plasticity characteristics, in order to separate them by their expected engineering behavior (Appendix 6). The USCS assigns a group symbol to the soil, along with standardized descriptions appropriate for that group name which is useful for selection of design strategies. The USCS begins by separating the soil into either coarse-grained or fine-grained, depending if greater that 50 percent of the material is larger or smaller than a 200 sieve, with the exception of highly organic soil. Highly organic soils often will smell have fibers and are typically dark in color. If found on site, organic soil should be excavated and discarded due to their poor properties and thus will not be discussed herein.

Figure 19 Sieve Test (Adapted from Concrete, 2009)

56

USCS further differentiates between the coarse-grain into ‘gravels and sands’ and fine-grain into ‘silts and clays’.

This second classification step requires further

sieving for coarse-grained soils and laboratory work including the Atterberg limit tests for fine-grained soils. For on-site feasibility, the use of a 0.074 mm screen, equivalent to sieve size #200 should be used. If the in-situ soil is clumped, the soil must be washed prior to using the sieve. To collect the soil sample, the site investigator shall dig a small trench and sieve one 5-gallon bucket of material onto a standard 75 micrometer mesh (Wovenwire, 2009).

The action of digging a test-pit also gives one a better

understanding of soil variability and an increased awareness of drainage issues to better identify where the soil may present excavation difficulties. A second required classification step is to administer the dilatancy test detailed in Section 4.4.2.2. Given the results of the sieve and dilatancy test, respective field testing approaches should be completed for soils classified with greater than 50 percent passing the 0.074 mm sieve. The test requires a sample with a soft putty consistency. Observe the reaction during shaking, followed by squeezing the soil in ones hand with vigorous tapping. During the test, if the soil behaves as a fine-grained soil, the vibration would densify the soil and water would appear on the surface. In a clay sample, no change occurs and thus may be classified as fine-grained (Field, 2009). Silt has a tendency for dilatancy so excess water would disappear from the

57

surface. In such case, this soil behaves b simiilarly to a cooarse-grainedd soil and shhould be tested and modeledd as such. hear Strengtth of Fine-G Grained or Cohesive C Soiils 4.3.2.2 Sh For a dessigner intereested in opttimizing thee size of thee anchor, sooil testing would w reduce co onservative assumptions. a . For fine-grained soils, it is rellatively straaight forwarrd to obtainn an undistuurbed a test it inn the laborattory. In a trriaxial test a cylindrical soil specim men is sample, and confined within a fllexible mem mbrane whicch permits the applicattion of isottropic w permittting the speccimen to defform under axial a loads. The stress-sstrain stresses while curve can n then be obbtained for different coonfining pressures (Saadda & Townnsend, 1981).

Figure 20 Typ pical Triaxial Testing T Apparattus

v can be b defined onn the stress-strain curve plotted on a Mohr-Couulomb Strength values diagram to t get c and ϕ. As drainnage does noot occur quicckly in the field, f excess pore 58

water pressure does not n dissipatee quickly. Therefore, T thhe shear strenngth correspponds to short-teerm or undraained condittions. With ideal i testing and laborattory accessibbility, an uncon nsolidated, undrained u (U UU) triaxiall test wouldd simulate a similar loaading (Coduto, 2001). The UU test is performed in i the triaxial cell with the drain valves v closed thrroughout thee test. For the bridge desccribed in Chapter C 2, the t structuraal loading condition would w correspon nd to a suddden, large voolume of bridge traffic. Sudden briidge loadinggs are common in the case of festivals, post-schooll departures. During thhe rainy seasson it would nott be expected to have neearly saturateed soil alongg the banks of o a river. Figure F 21 shows representatiive data expeected from UU U triaxial tests in a labooratory.

Figu ure 21 Expected d UU Triaxial Test T Results for Cohesive Soil

The UU tests t on saturrated fine-grrained soils may be carrried out eitheer on undistuurbed or remold ded samples. With the σ3 acting onn the entire sample, the axial pressuure is increased until failurre occurs at the deviatoor stress (σ1- σ3), from which the m major 59

principle stress is determined. Several tests should be completed to create a similar plot to that detailed in Figure 21. In this case, (σ1- σ3) is not sensitive to σ3 as the increase in total stress is carried completely by the pore water. The input parameter from the test to use in the design models is su which is related to the maximum principal stress difference (σ1- σ3), by the following relationship.

2 The VST is often used in-situ to obtain approximations of shear strength of saturated cohesive soils, specifically where undisturbed samples of acceptable quality are difficult to obtain (Terzaghi, Peck & Mesri, 1996).

The VST consists of a metal

vane which is inserted into the ground and torque is applied until the soil fails in shear, when the test is completed according to ASTM D2573. It is pertinent to note that the rate of vane rotation is intended to ensure undrained conditions at failure. As such, it is very beneficial to sample the soil either before or after testing, to understand the drainage conditions of the soil tested because the presence of a silt or coarse-grained soil will not produce usable results (ASTM D2573, 2008). Furthermore, as the soil must be saturated prior to testing, it is advised to take a sample near the stream bed rather than in the intended area of excavation, assuming homogeneity between the two sites. The undrained shear strength of a fine grained soil is correlated to the torque required for failure, the vane dimensions and the plasticity index per the following equation:

60

6 7

 

Where: su = undrained shear strength Tf = torque at failure d = diameter of vane = Bjerrum correction factor

To properly identify the Bjerrum correction factor (Appendix 7), the plasticity index, Ip, must be found. The Plasticity Index of a soil is the numerical difference between the liquid limit and the plastic limit, (LL-PL) (Coduto, 2001). The water content is one of the parameters which is very difficult to ascertain in the field without access to an oven. The pocket vane shear tester is a more portable and inexpensive version of the VST. The pocket VST test should be completed according to ASTM

D 4648 which

designates the rotation of a 12.7-mm high x 12.7-mm diameter vane at approximately 90 degrees per minute (Geotest, 2009).

The vane may be advanced to depths of

interest by first excavating a small pit, to 1.5 to 2.0 meters in depth, or to a depth more closely correspond to the soil properties at the depth of the anchor.

61

Figure 22 Pocket Vane Shear Test

The pocket penetrometer is another method to obtain the undrained shear strength of a saturated soil. By pushing the small probe into a fine-grained soil, the measured unconfined compressive strength measured can be converted to shear strength by diving by 2 (Coduto, 2006).

Figure 24 shows a picture of a typical pocket

penetrometer.

Figure 23 Pocket Penetrometer

The spring operated pocket penetrometer is a small and transportable device that measures the undrained compressive strength by pushing a 0.25” (6.4 mm) diameter loading piston into the material of interest, to the depth of a calibration groove machined on the piston 0.25 cm from the end. The strength in kN per square cm is obtained by noting the position of the indicating ring on the scale, which is retained 62

until reset (Professional, 2009). Both of these testing devices are highly mobile and inexpensive thus providing a viable testing solution for rural applications. 4.3.2.3 Strength of Coarse-Grained or Cohesionless Soils If the soil is classified as coarse-grained, obtaining undisturbed samples is nearly impossible, especially in rural areas. Accordingly, it is difficult to quantify strength without field tests like the Standard Penetration Test (SPT) or the Cone Penetration Test (CPT). However, these tests require specialized equipment unavailable in the field. Accordingly, it is recommended to use correlations. Correlations involve an estimate of the soil density. Efforts should be made to estimate the density in the field and use correlations such as those presented in Table 6. Table 6 Correlations for Coarse Grained Soils (Terzaghi, Peck & Mesri, 1996)

If advanced testing is not available, conservative soil strength parameters are given in Figure 24. These were developed based on the findings of the analyses in Sections 4.1 and 4.2. For every soil type, the first step is to sieve with a 0.074 screen. The second step is the dilatancy test, outlined in Section 4.3.2. If the soil shows properties of dilatant silts, it will be modeled as a coarse-grained soil. 63

Based on the classification of the soil, either tests or correlations should be used to identify soil strength properties. If adequate testing is devices are not available, the analyses suggest that conservative values can be used.

Figure 24 Soil Classification and Testing Flow Chart

64

4.3.3 Design Example: Sebara Dildi Case-Study 100 meter span results in 166 kN load onto the anchor, as detailed in the Structural Design section.

Soil classification resulted in a coarse-grained soil on either

abutment. Using the Meyerhof method detailed in Section 4.2.1, and Figure 16, an initial embedment depth of 1.7 meters was chosen, and similar anchor geometries were chosen: block X, Y, L = 1 meter x 1 meter x 3 meter wide at a depth of 1.7 meters. Assumptions γ_soil

19 kN/m³

ϕ

26 Degrees

c ψ h B Qu(total) Qu (g)

0 30 1.7 3 166 55.3

Degrees m m kN kN/m

Pt = 166 kN PV = Pt * sin(ψ) = 83 kN/m = 83/3 = 27.67 kN PH = Pt * cos(ψ) = 143.76 kN/m = 47.92 kN WB = X * Y * 25 kN/m³ (unit weight reinforced concrete) = 1 x 1 x 25 = 25 kN Ws = X * h * γ = 1 x 1.7 x 19 kN/m³ = 32.3 kN Wt = 25 kN + 28.5 kN = 57.3 kN Pp = Passive pressure (Appendix 2 for Granular Soil) tan 45

 

tan 45

 

19

.

= 74.04 kN

65

1) Check 1.5 57.3  27.6 

2.07

1.5

2) Check 1.5 74.04  47.9 

1.55

1.5—

In conclusion, the 1 meter by 1 meter by 3 meter anchor at a depth of 1.7 meters is acceptable for a 100 meter span with coarse-grained soil conditions. As detailed in Chapter 4, further design iterations could increase the depth of embedment with the objective to reduce the size of the anchor depending on priorities for optimization.

66

Chapter 5: Quality Control Considerations Although engineers and project designers intend for designs to be constructed exactly per specification, as-built drawings even in the developed world often vary greatly from the original designs. Design factor of safety and the in-situ factor of safety are rarely the same.

Due to inadequacies in workmanship, material quality, quality

control, etc., the capacity of the completed bridge is not actually known, thus the design factor of safety must be liberal to account for those conditions. Designers must include added factors of safety in design to account for the probable occurrence of inadequate craftsmanship and material specifications. The following will detail a few of the critical quality control issues that a field supervisor must account for, but further research is needed to adequately address quality control measures.

5.1 Material Specifications 5.1.1 Concrete Mixture Concrete is one of civilization’s oldest building materials and most often is a material already widely used in most rural communities. Teaching the local laborers the importance of proper mixing techniques and mixture types will improve the quality of all concrete construction and thus may be one of a project’s primary successes (Ruskulis, 1996).

67

Concrete is produced by mixing water, Portland cement and sand and gravel. To produce a good concrete block, care needs to be taken in the quality of the sands and gravels used in the mixture. Construction quality control of the sand and gravel materials often requires preparatory work as natural conditions rarely leave wellgraded deposits. The fine aggregate with a diameter less than 5 mm, more commonly referred to as sand, is often available on rural construction sites. No silt or clay passing a #200 sieve or about 0.074 mm may be used. Similar to the process for soil classification, if sand is sourced locally out of a riverbed, a mesh screen must be used to ensure proper grain-size. Sands need to be washed and sifted through a screen with 5 mm openings. Coarse aggregate or gravel is a mixture of rock with a range of 6-20 mm diameter which may be found in-situ or created by crushing larger locally available rock. The gravels and sands should have regular grain-size grading without one specific size dominating the size distribution: with sand, particularly too many fines (Ruskulis, 1996). Water content controls the workability of the mixture and chemically reacts with the cement to bond the resulting concrete. One of the critical components of quality control is to ensure that the proper ratio is maintained during construction for increased portions of water will improve workability but decrease material strength (Engineer, 2002). For hand-mixing, a water to cement ratio of about 0.55 produces a workable and durable concrete (Davis, 2002) but for more specific cement ratios, Table 7 may be referenced.

68

Table 7 Concrete Ratios by Volume (Adapted Engineers, 2006)

Mix Ratio by Volume (Cement:Sand:Gravel:Water) 1 : 3 : 6 : 1.6 1 : 2.5 : 5 : 1.6 1 : 2 : 4 : 1.6 1 : 2.5 : 3.5 : 1.6

Typical Use on Bridges Tower Foundations, Block Anchors Tower Foundations on poor soil Non-structural Approach walls Structural Column in Tower

Approximate Yield (m³) 0.24 0.21 0.17 0.17

Mixing technique is another aspect of quality concern. Many rural laborers are familiar with mixing concrete but local methods of mixing are often inferior as there is a lack of quality control standards. Common is the volcano approach, in which aggregates and cement are mixed by hand, forming a pile. A hole dug out of the top provides a bowl-form for the water to be poured and mixed. Although common, this approach is not appropriate as it is difficult to attain an even mixture. Alternatively, to hand-mix concrete, one must specify that the water is to be splashed into the mixture in lifts while being manually mixed using a shovel. Once set, the fresh concrete must be kept wet during the curing period. Concrete will set in three days but reaches workable strength after seven days (Hazeltine,2003). For greater detail on appropriate methods for concrete mixtures, reference Engineers Without Borders, Concrete Mixes Guidelines (Engineers, 2006).

5.1.2 Steel Cable Steel cable has two types of elongation: elastic stretch that fluctuates with the applied load and the permanent stretch that corresponds with the cable strands rearranging 69

and tightening in cross-section. The type of cable purchased dictates the amount of hoisting sag. Cable may be purchased as either non-prestretched or prestretched, the latter which will be considered herein. It is pertinent to not that if non- prestretched cable is used, the design engineer must increase the anticipated sag onset from loading which would have a greater impact on the hoisting sag set. Cable handling is of paramount importance. It is critical not to unwind the cable incorrectly, as this may cause kinks in the cable which result in weak points in the set cable and thus potential failure points. Figure 26 shows the proper way to unwind the cable.

Figure 25 Cable Uncoiling Procedure (Helvetas, 2001)

Cable transport from the drop point to the bridge site is also critical. Figure 27 shows the proper way to transport cable.

Figure 26 Proper Cable Transport Technique

70

5.1.3 Cable Clamps U-bolt clamps, often referred to as bull-dog clamps, are used to tie the cable around the anchors. The singularity of the clamping method is one of the few design aspects that does not include redundancy. As such, the material properties of the steel used to create the clamps and the process used to attach the clamps is critical for the quality assurance a bridge project. The structural integrity of the clamps used to connect the steel cable is an area of concern, as clamp failure is the source of the only known bridge failure to date (Nepal, 2008) as shown in Figure 28.

Figure 27 Failed Nepali bridge: Clamp Slippage

Malleable steel clamps are most common but are inadequate for continuous loadbearing design (Crosby, 2009) such as in the case of cable-suspended bridges. Drop71

forged are of superior quality for bridge-type loadings but are often difficult to locate in developing countries. The difference between the two is the process used to create the clamp. As with all steel, the principal mechanical properties of interest to designers are strength, ductility and hardness all of which are dependent on the process used to create the clamp. In the casting process to create malleable clamps, the mold has the shape of the desired component and the liquid metal flows into the desired shape. Malleable clamps are able to attain the same efficiency ratings based on breaking strength of wire rope, but are apt to continuously loosen with continued load and thus reduce their ‘grip’ on the cable. With forged steel, the original shape is an ingot that is forged into shapes by presses. The resulting product has a greater material strength and lower ductility. As such, the torque specified to reach maximum efficiency rating is greater than a malleable clamp of comparable diameter but once torque, the clamp is far less likely to slip. It is the engineering field supervisor’s responsibility to ensure that the clamps used on-site are per specification.

5.2 Construction Quality Control 5.2.1 Cable Clamps Proper installation of cable clamps is one of the most critical components of construction quality control. Correct installation is shown in Figure 29 and shows both ropes are arrayed parallel and in contact with the bow clamp screws twisted on

72

from the side of the carrying c ropee. It is essenntial that thee clamp sadddle surroundds the ‘live end’ of the cablee, as shown.

Figure 288 Proper Cable Clamp Installaation

To attain maximum efficiency rating r of thee clamp, thee manufactuurer designaates a minimum m torque requuired. It is the t author’s experience that t for the clamp c to be fully torqued, the t cross-seectional areaa of the deaad-end of thhe cable willl be reduceed by approxim mately 25% as shown in Figure F 30.

Figurre 29 Reduction n in Cable Cross-Section with Proper P Torque

73

To attain the required torque, one must reference manufactures standards. As the installation of cable clamps occurs within a short span (a 26 mm cable requires 7 clamps, each spaced at 15 cm on center), it is very difficult to exert excess force. Based on the 26 mm diameter of the cable, the required torque is approximately 300 ft-lbs. Assuming a typical laborer may be able to exert 80 to 100 pounds of force, a 3-foot wrench barely achieves full torque. It is unreasonable to require a torquewrench to measure actual torque applied at rural construction sites, thus one clamp in addition to manufacturer’s specifications is recommended for each cable. Figure 31 shows a completed clamp installation in with the wrench used for cable installation.

Figure 30 Proper Cable Clamp Installation and Torque Wrench

5.2.2 Backfill and Compaction Care must be taken when backfilling the approaches. Soil should be placed in layers no greater than 15 cm thick. In the case of clays or silt backfill, a hand-rammer should be used to compact the soil, shown in Figure 32. 74

Figure 31 Hand Rammer

Alternatively, community members or livestock walking thoroughly atop each layer will ensure proper compaction. With soil placed in lifts the weight of the soil above the anchor relied on in the design can be achieved. Furthermore, compaction of the backfill ensures excessive settlement will not damage the approach ramp concrete. Several other quality control items are critical to ensure the safety of the pedestrian footbridges. The inclusion of these few is intended to encourage bridge designers and field supervisors to consult the Helvetas manual (2001) for a more complete coverage. Further research and publication in this area would also be extremely beneficial for those working with rural footbridge technologies.

75

Chapter 6: Conclusion and Discussion

6.1 Summary Pedestrian bridges ensure access to education and health, commerce and opportunity. Rural pedestrian bridges contribute towards the improvement of living conditions for some of the world’s most economically and socially disadvantaged. The simplicity of the technology and the availability of a design example will ensure many more bridges are built. A review of pertinent structural design codes and geotechnical models were reviewed. Parameter assumptions were justified through parametric models, and a simple design approach adapted from DM-7 (Naval, 2009) was proposed for design use for both fine and coarse grained soil types. The loading assumptions and structural design approach was presented in Chapter 3, including a case-study based on a 100 meter span and 1.0 meter decking. Chapter 4 detailed pertinent academic approaches to anchor design for fine and coarse grained soils.

Separate consideration for either soil type was given and soil parameter

assumptions concluded upon. A recommended soil testing and classification flow chart was provided to acquire soil parameters for use in the DM-7 anchor design process. Structural loading assumptions and codes are provided in the context of a case-study example. The final product found that the 100 meter Ethiopia bridge case study with a sandy-soil at either abutment must resist 166 kN of loading. One anchor design solution is detailed. Several of the key quality control measures were outlined 76

in Chapter 5, with the intention to introduce the reader to the importance of material and construction quality control for footbridge projects.

6.2 Design for the Developing World The spectrum of potential benefits for infrastructure projects in developing countries ranges from improved beneficiary access, for example improved educational standard allowed from year-round access to schools, to the introduction of construction technology transfer. Despite the benefits of any type of international aid work, ethical dilemmas of accountability and safe practice are pertinent. The process of design simplification presents a number of technical and logistical challenges. Furthermore, professional ethics must be considered when detailing the operations, maintenance and project lifespan accountability. A general discussion of transferring a technology from the developed to developing world is also considered from a lessons-learned context.

6.2.1 Design Simplification Great care must be taken not to reduce the quality of a design when simplifying. Many technologies needed in the developing world have well-documented design approaches for use in developed countries. To make these technologies appropriate for rural applications, modifications for material availability, low cost and limited tools and equipment must be accounted for. The process of simplification requires the engineer to make many of the same design decisions as in an industrialized context, but with a varying hierarchy of priorities. For example, in the case of 77

infrastructure projects, a designer in The United States may be willing to sacrifice an increase in budget to reduce the construction time. In the developing world, most often time and labor are least expensive and thus lowering the cost of a project would be prioritized.

Finding the balance between cost, and construction time is of

paramount importance. Standardizing designs and design processes specifically for development work provides a greater level of comfort in a design, reducing this likelihood of project design failure. But, modulated designs require a number of assumptions: a design code to be followed, material availability and project objectives by the beneficiaries. Local engineering design codes and community usage requests must be taken when simplifying a design from the original context in the developed world to that of the developing.

As such, even a simplified and modulated design must have the

flexibility to be modified. Design manuals are often created for use in the developing world as was the case of the Helvetas manual. It is the suggestion of the author that development projects may have the greatest impact when a modulated design also is supplied with a detailed explanation of the design process and assumptions used. This allows for a more general use of the work as secondary contributors are able to modify to better suit their local community and national standards.

6.2.2 Ethics of Accountability Accountability for humanitarian-aid projects requires one to consider the professional ethical codes for practice in a country other than where one is licensed.

In typical 78

industry work, the engineering profession has a very high level of professional accountability but design codes and regulations allow an engineer a level of confidence in his or her work.

Abroad, the same codes and regulations are

applicable, but the designs often are impractical and thus additional individual consideration must be given to each project. Furthermore, as developing world construction techniques and quality control are often inferior to those considered standard in the developed world, making standardized quality assurance and control documents for each type of project further ensures project reliability and safety. The inadequacy of the legal framework in many developing countries measures reduces the liability of contractors to ensure quality control by their own measures (Leisninger, 2009). If a developing country has no regulation or has one but does not enforce it, it is likely that additional margin of safety should be included in the design as well. A complete best-practice design guide includes the assumed factors of safety, but an additional document improving the quality control would allow a designer to fully understand the areas of concern and more assign appropriate factors of safety considering local capacity for local accountability. Additionally, project engineers and implementing organizations need to take the initiative to be personally accountable for each project. A plan of how to avoid failure as well as what happens in the case of failure is essential.

Insurance

companies in the developed world play an essential role in the guarantee of an engineered project: the developing world projects deserve a similar level of project 79

assurance. Attention in all humanitarian projects should address the issue of ethical responsibility and how to address a failure situation. Bridges to Prosperity takes great lengths to ensure quality control throughout each project. A document is currently being created that would be inclusive of all critical quality control and maintenance issues. Ultimately, it is the engineer’s responsibility to take personal accountability for a project’s enduring success and thus operation and maintenance instructions and training should be included as a required component of every project. Returning to assist with maintenance also helps to reduce the risk of a failure.

6.2.3 Transferring Best Practices to Developing Nations Many lessons were learned in the attempt to transfer a technology fairly wellunderstood under typical engineering conditions into a setting for development work. Perhaps the greatest lesson learned was not attempt to reinvent the wheel. Many military and emergency engineering documents exist. An academic understanding of the design issue is necessary but the most pertinent and useful reference materials are those which consider the lack complexity in simple design. In the case of footbridge design, the first step was to identify the intended audience for the report. Ideally a document would be produced that could be used as a field manual in the developing world. This particular document targeted a more academic audience. With the vocabulary of choice more technical, further steps were taken to identify the specific engineering problem and pertinent parameters.

When the 80

number of input parameters exceeded the feasible ability of in-situ testing, the model was used for parametric studies to compare possible material assumptions. As the intended audience was identified as having a working knowledge of geotechnical engineering, a greater focus was placed on justifying assumptions. Documents with a more general intended audience may chose to include technical assumptions and models in an appendix. Constructability is another critical issue. Many of the design references for soil anchors for power-lines assumed that changing the depth of embankment would be the easiest control variable. In the case of rural construction, each meter of added excavation could add weeks to a project as only man-powered excavation is possible. This additional construction time may be preferred over additional cost, but the balance between design cost and time is vastly different from the original design intent outlined in academic sources. A considerable amount of effort should be taken to consider both the theoretical and practical sides of a testing program or design. Designers interested in creating a best-practices guide to design for developing applications are suggested to limit the amount of theoretical information gathered and to focus on what is already being done. This report is primarily concerned with existing academic literature applicable to a somewhat specific product. In future research and publication, a lesser focus would be placed on academia and a greater emphasis would be placed on constructability and cost issues, as these are of paramount importance to application in developing world applications. 81

6.3 Opportunities for Future Research Future experimental research is needed to verify the correlation between assumed soil parameters and the ultimate uplift capacity of rectangular dead-man anchors. Although research pertinent to equivalent structures were reviewed, the lack of studies with similar loading and geometrical scenarios was disappointing. Further research could address one of the following: comparison between increases in anchor width versus burying the anchor to a greater depth, correlation between rudimentary field tests and laboratory tested friction angles and changes in ultimate pull-out capacity for various coarse-grained and fine-grained soils. From experimental data, a more complete database and design assumption matrix may be created. Furthermore, the need for an improved testing approach would need to be developed or a testing device, such as those detailed in Chapter 4, would need to be correlated to the empirical findings to calibrate the devices. A Best Practice Guide for Construction Quality Assurance and Quality Control, including Safety precautions also would be an excellent contribution to the field of pedestrian bridge design. Chapter 5 briefly addressed a few of the key quality control components, but a document properly addressing this topic was beyond the scope of this report. Included in any effective construction document should be the design process used and the assumed quality of each component. For example, a structural engineer in The United States must specify A50 steel if he or she assumed 50 ksi yield stress in their design. Without this declaration of material standard and without the proper system of quality control, an unknowing contractor may choose to use a 82

less expensive and more readily available A36 steel with inferior yield strength. In the developing world, this component of construction and material quality control must be documented very clearly.

83

References AASHTO's Standard Specifications for Highway Bridges: Guide for Design of Pedestrian Bridges, 1997. ASCE Bearing Capacity of Soils, Technical Engineering and Design Guides as Adapted from the U.S. Army Corps of Engineers, No. 7. ASCE Design of Sheet Pile Walls, Technical Engineering and Design Guides as Adapted from the U.S. Army Corps of Engineers, 1996, pp 36-37. ASCE Soil Sampling, Technical Engineering and Design Guides as Adapted from the U.S. Army Corps of Engineers, No. 30. ASTM D 2488, Standard Practice for Description and Identification of Soils (VisualManual Procedure) ASTM International, West Conshohocken, PA. Accessed online October 2008: (http://www.dem.ri.gov/pubs/sops/wmsf5.pdf). ASTM D 5878 -08 Standard Guides for Using Rock-Mass Classification Systems for Engineering Purposes. ASTM International, West Conshohocken, PA. ASTM D 6032-08. Standard Test Method for Determining Rock Quality Designation (RQD) of Rock Core. ASTM International, West Conshohocken, PA. ASTM D1556, “Standard Test Method for Density and Unit Weight of Soil in Place by the Sand-Cone Method.” ASTM International, West Conshohocken, PA, www.astm.org. Bjerrum, L. (1972). “Embankments on soft ground,” ASCE Conference on Performance of Earth and Earth-Supported Structures, Purdue University. 2, pp. 1-54. Blaikie, M. P., Cameron, J. and Seedon, J. D. 1979. The Struggle for Basic Needs in Nepal. Development Center of the Organization for Economic Cooperation and Development. Bowles, Joseph E., 1996. Foundation Analysis and Design, 5th Edition. McGrawHill. Bridges to Prosperity. Accessed online January 2009: www.footbridges.org. Field Determination of Texture for Sand, Silt & Clay. British Columbia Ministry of Environment, Lands & Parks. Accessed Online: February 2009: http://ilmbwww.gov.bc.ca/risc/pubs/teecolo/terclass/appii.htm 84

CSG Wood Specifications. Accessed online October 2008: http://www.csgnetwork.com/specificgravwdtable.html Das, B.M., 1990. Earth Anchors: Developments in Geotechnical Engineering, Vol. 50. Elsevier, New York. Das, B.M., 1983. A Procedure for Estimation of Uplift Capacity of Rough Piles. Soils and Foundations, Japan, 23(2):122-126. Davis, J., Lambert, R. 2002. Engineering in Emergencies: 2nd Edition. Intermediate Technology Publications, London. Dayaratram, P. International Conference on Suspension, Cable Supported and Cable Stayed Bridges. Nov. 19-21 1999, Hyderabad. Indian Institute of Bridge Engineers. FM 3-34.343 Military Nonstandard Fixed Bridge. Chapter 8: Suspension-Bridge Design. Accessed online October 23, 2008. www.sachs.us/nsfb.pdf Foster + Partners. London Millennium Bridge Project. Accessed online March 2009. http://www.fosterandpartners.com/Projects/0953/Default.aspx Gade, D. W. 1972. Bridge types in the central Andes. Annals of the Association of American Geographers, v. 62 (1), p. 94-109. Geotest Instrument Incorporated. E-285 Pocket Vane Shear Tester. Accessed online February 2009: http://www.geotestinst.com/Catalog/ItemInfo.phtml?id=E-285 Haynes, R., Lovett, A., Sunnengerg, G. 2003. Potential Accessibility, Travel Time and Consumer Choice: Geographical Variations in General Medical Practice Registrations in Eastern England. Environment and Planning A 35: 1733-1750. Hazeltine, B., Bull, C. 2003. Field Guide to Appropriate Technology. Academic Press, London. Helvetas International. 2001. Short Span Trail Guide Survey Guide, First Edition. Volume 1. His Majesty’s Government, Ministry of Local Development. 2005. Integrated Rural Accessibility Planning. Kathmandu, Nepal. Second Edition. Kulhawy, Trautman and Nicolaides. 1987. Spread Foundations in Uplift: Experimental Study: Foundations for Transmission Towers. Geotech. Spec. Pub. 8 ASCE, 110. Kulhawy, 1983. Transmission Line Structure Foundation for Uplift-Compression Loading: Final Report. Research Project EL-2870. 85

Lebo, J. and Schelling, D. 2001. World Bank Technical Paper No. 496. Design and Appraisal of Rural Transport Infrastructure: Ensuring Basic Access for Rural Communities. The World Bank. Washington, D.C. Leisinger, K. Ethical Challenges of Agricultural Biotechnology for Developing Countries. Accessed online March 2009: http://www.doylefoundation.org/icsu/CG%20Leisinger.pdf Logan, C.E. , 1976. Footing Tests for Transmission Line Towers: A Collection of Data. Report No. SA-9. United States Department of the Interior, Bureau of Reclamation. Martinette,C.V., 2007. Trench Rescue: Awareness, Operations, Technician: 2nd Edition. Jones and Bartlett Publishers, Sudbury, MA. Meriam, J. L. & Kraige, L. G. 2007. Engineering Mechanics: Statics, Sixth Edition, John Wiley and Sons, Inc. Meyerhof, G.G. , 1976. Bearing Capacity and Settlement of Pile Foundations, Journal of Geotechnical Engineering Division, ASCE, Vol. 102, No. GT3. Meyerhof, G.G. , 1973. The Uplift Capacity of Foundation Under Oblique Loads. Journal of Geotechnical Engineering Division, ASCE, Vol. 10, pp. 64-70. Meyerhof, G.G. and Adams, J.I. (1968). The Ultimate Uplift Capacity of Foundations. Canadian Geotechnical Journal. 225-244. Murray, E.J., & Geddes, J.D. 1987. Uplift of anchor plates in Sand. J. Geotech Engineering, Div. ASCE 113, No. 3, 202-215. Naval Facilities Engineering Command, Soil Mechanics Design Manual 7.01. Accessed online October 2008: http://www.geotechnicaldirectory.com/publications/Dm701.pdf Naval Facilities Engineering Command, Foundations and Earth Structures Design Manual 7.02. Accessed online October 2008: http://www.ce.washington.edu/~geotech/courses/cee523/manuals/NAVFAC72.pdf Nepal Trail Bridge Section. Accessed online December 2008: http://www.nepaltrailbridges.org. Nile Basin Initiative (NBI). Accessed online February 2008: http://www.nilebasin.org/_borders?theNileRiver.htm Peters, Tom. 1987. Transitions in Engineering: Guillaume Henri Dufour and the Early 19th Century Cable Suspension Bridges. Birkhauser, Geneva. 86

Professional Equipment: Pocket Penetrometer Standard. Accessed online January 2009: http://www.professionalequipment.com/pocket-penetrometer-geotest-e280/soilsampling/ Pugsley, A. 1957. The Theory of Suspension Bridges. London: Edward Arnold Publishers. Ruskulis, O. 1996. Micro-Concrete Roofing Tile Production. IT Technical Enquiry Service in Appropriate Technology, Vol. 23, No.1. Ryall, M. J., Parke, G. A. R. and Harding, J. E. (Editors). 2000. The Manual of Bridge Engineering. First Edition. London: Thomas Telford. Saada, A.S. and Townsend, F.C. 1981. Laboratory Strength Testing of Soils, ASTM STP 740, American Society for Testing and Materials, Philadelphia. Snailham, Richard. 1968. The Blue Nile Revealed: The Story of the Great Abbai Expedition. Chatto & Windus, London, Smith, J. E., Stalcup, J.V., 1966. Deadman Anchorages in Various Soil Mediums. Naval Civil Engineering Lab Port Hueneme California. Terzaghi, K., Peck, R., Mesri, B. 1996. Soil Mechanics in Engineering Practice, 3rd ed., John Wiley and Sons, New York. Unified Facilities Criteria (UFC) Geotechnical Engineering Procedures for Foundation Design of Buildings and Structures. 2005. Accessed online January 2009: http://www.wbdg.org/ccb/DOD/UFC/ufc_3_220_01n.pdf. United Nations. 2005. Department of Economic and Social Affairs, Population Division, World Population. U.S. Department of Energy, Richland Operations Office. Hanford Site Hoisting and Rigging Manual. Accessed Online February 2009: http://offroadrecovery.zoovy.com/category/riggingoffroad#P164_6009 U.S. Department of Interior, Bureau of Reclamation. Safety Manual: Appendix D: Wire Rope. Accessed Online April 2009: http://www.usbr.gov/ssle/safety/RSHS/AppD.pdf  

Vesic, A.S. , 1977. Design of Pile Foundations, National Cooperative Highway Research Program Synthesis 42, Transportation Research Board. Woven Wire Resources. Accessed online December 2008: http://www.wovenwire.com/reference/sievescreen.htm 87

Appendices Appendix 1: Soil Identification Table (Helvetas, 2001)

The above table is provided for reference only but is not implicitly recommended through inclusion. The table is used in Helvetas’ design manual (2001).

88

Appendix x 2: Compu utation of Siimple Activee & Passive Pressures

(DM-7 Seection 7.2, Naval, N 2009)

89

Appendix 3: Breaking Strength Properties of Cable Typical IWRC 6x19 Cable Properties: Assumed tensile strength of 1770 MPa) Verify with Supplier Nominal Diameter

Weight

Breaking Breaking strength strength

Design Breaking strength (FS = 3.5)

Inches

mm

lbs/ft

kg/m

tons

kg

kN

kN

1/4

6.4

0.116

0.17

2.94

2667.12

26.16

7.47

5/16

7.9

0.18

0.27

4.58

4154.91

40.75

11.64

3/8

9.5

0.26

0.39

6.56

5951.13

58.36

16.67

7/16

11.1

0.35

0.52

8.89

8064.87

79.09

22.20

1/2

12.7

0.46

0.68

11.5

10432.62

102.31

29.23

9/16

14.3

0.59

0.88

14.5

13154.18

129.00

36.86

5/8

15.9

0.72

1.07

17.9

16238.61

159.25

45.50

3/4

19.1

1.04

1.55

25.6

23223.93

227.75

65.07

7/8

22.2

1.42

2.11

34.6

31388.59

307.82

87.95

1

25.4

1.85

2.75

44.9

40732.59

399.45

114.13

1 1/8

28.6

2.34

3.48

56.5

51255.94

502.65

143.61

1 1/4

31.8

2.89

4.30

69.4

62958.62

617.41

176.40

1 3/8

34.9

3.5

5.21

83.5

75749.93

742.85

212.24

1 1/2

38.1

4.16

6.19

98.9

89720.57

879.86

251.39

.

90

Appendix 4: Specific Weight of Wood Specimen (CSG) Wood - dried Afromosia Apple Ash, black Ash, white Aspen Balsa Bamboo Birch (British) Cedar, red Cypress Douglas Fir Ebony Elm ( English ) Elm ( Wych ) Elm ( Rock ) Iroko Larch Lignum Vitae Mahogany ( Honduras) Mahogany ( African ) Maple Oak Pine ( Oregon ) Pine ( Parana ) Pine ( Canadian ) Pine ( Red ) Redwood ( American ) Redwood ( European ) Spruce ( Canadian ) Spruce ( Sitka ) Sycamore Teak Willow

kg/m³ 705 660 - 830 540 670 420 170 300 - 400 670 380 510 530 960 - 1120 600 690 815 655 1280 - 590 1370 545 495 - 850 755 590 - 930 530 560 350 - 560 370 - 660 450 510 450 450 590 630 - 720 420

91

Appendix 5: Explanation of Logan’s Pull-out tests for Footings in Sands Logan (1976) completed a series of pull-out tests for pad and stem footings in sand which appear appropriate to the footbridge anchors due to similarities in geometry and loadings. The test method described a series of instruments installed around each tests footing to evaluate the uplift movement of the surrounding ground due to pullout, taken to failure. During the tests, movement was negligible up to 30 kips (133 kN) of load with geometries within reasonable footbridge anchor sizes. As detailed in Chapter 3, this loading is within 15% of expected tensile loads incurred on an anchorage for a 100 meter bridge. The tests found that upward movement of the ground was confined to the effective volume of soil included within a slope of 30 degrees from the top corner of the anchor pad (Logan, 1976, pg 72). It should be noted that Logan’s tests were conducted by pulling the footing at a slope 78.9 degrees which is significantly greater than found in footbridge applications. Thus, although the experimental objective was to model failure patterns, the study provided insight into the true behavior of anchors in coarse-material under tensile loadings.

92

Appendix 6: Abbreviated Unified Soil Classification System (Coduto, 2001)

93

Appendix 7: Bjerrum Correction Factor for Vane Shear Test

Bjerrum’s Correction Factor for use in the Vane Shear Test is for use with saturated, normally consolidated clays. Ip is the plasticity Index of a soil is the numerical difference between the liquid limit and the plastic limit, LL-PL, presented in a percent form (Coduto, 2001).

94