Attachment 0003D USDA_SustainBridgeDesign.pdf

United States Department of Agriculture U.S. Department of Transportation

Federal Highway Administration

Sustainable Trail Bridge Design

Forest Service

National Technology & Development Program

2023–2805P–NTDP

2300–Recreation

March 2020

Sustainable Trail Bridge Design

Cover photo—A field crew and trail maintenance equipment on a trail bridge.

Notice

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Sustainable Trail Bridge Design

James Scott Groenier

Project Leader

U.S. Department of Agriculture, Forest Service

National Technology and Development Program

2E22A30—Sustainable Trail Bridge Design

Project

March 2020

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To file a program discrimination complaint, complete the USDA Program Discrimina-tion Complaint Form, AD-3027, found online at http://www.ascr.usda.gov/complaint_fil-ing_cust.html and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit your completed form or letter to USDA by: (1) mail:

U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov.

USDA is an equal opportunity provider, employer, and lender.

The Forest Service, an agency of the U.S. Department of Agriculture (USDA), has devel-oped this information for the guidance of its employees, its contractors, and its cooper-ating Federal and State agencies. The Forest Service assumes no responsibility for the interpretation or use of this information by anyone except its own employees. The use of trade, firm, or corporation names is for the information and convenience of the reader.

Such use does not constitute an official endorsement or approval of any product or ser-vice to the exclusion of others that may be suitable.

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Acknowledgments The project leader would like to thank David Strahl, Forest Service, Pacific Northwest Region; Eleanor

Oman, Forest Service, Tongass National Forest; Merv

Eriksson, Forest Service (retired); Jaime Schmidt, Forest Service, Washington Office; Chris Murphy, Forest Service, National Technology and Devel-opment Program; and Jerry Barrow, Forest Ser-vice (retired), for assistance with and review of this publication.

iii

Contents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Introduction

Trail Management Objectives

Siting

Bridge Types

Deck Girder and Deck Truss Bridges

Single-Unit Bridges

Side Girder and Side Truss Bridges

Arch Bridges

Cable Bridges

Covered Bridges

Materials

Timber

Steel

Concrete

Fiber-Reinforced Polymer (FRP)

Plastic Wood

Design

Bridge Widths, Grades, and Approaches

Users and Loads

Dead Loads

Live Loads

Snow Loads

Wind Loads

Snow Groomer Loads

Fatigue Load

Seismic Loads

Maintenance Vehicles, Light Vehicles, and Emergency Vehicles

Design Methodology and Load Combinations iv

Forest Service Standard Trail Bridge Plans and Specifications

Timber Bridges

Prefabricated Steel Bridges

Trail Bridge Structure

Substructures

Abutments

Piers

Superstructures

Decking

Decking Types

User Barriers

Signage

Construction Details and Constructability

Shedding Water and Debris

Earth and Wood Contact

Screws Versus Nails

Through Bolts Versus Lag Screws

Weathering Steel Corrosion Problems

Corrosiveness of Wood Treatments

Decking

Approaches

Wearing Surface (Running Planks)

Rub Rails

Inspection and Maintenance

References

Additional References

Websites

Appendixes v

Appendix A—Trail Bridge Matrix

Appendix B—Trail Design Parameters

Appendix C—Example of Trail Bridge Design Folder

Appendix D—Trail Bridge Specification Excerpts From Standard Specifications for Construction of Trails and Trail Bridges on Forest Service Projects vi

Executive Summary The U.S. Department of Agriculture, Forest Service, is one of the most prominent Federal agencies when it comes to trail and trail bridge management. The

Forest Service manages more than 158,000 miles of trails and more than 6,700 trail bridges. Designing trail bridges based on trail-specific Trail Manage-ment Objectives (TMOs) is essential for providing the desired trail experience, for ensuring user safety, and for maximizing bridge longevity. Managing a trail bridge for sustainability requires proper siting, good design details, routine inspections, and maintenance.

Additionally, timely maintenance and repair are less costly than replacing bridges because of neglect or failure.

This report focuses on designing new, short, single-span, wooden trail bridges that the Forest Ser-vice classifies as minor and major trail bridges. This report also briefly addresses other bridge types and materials outside the minor and major trail bridge classifications. The Forest Service “Trail Bridge Cata-log” website <http://www.fs.fed.us/eng/bridges/> provides further pictures and information on trail bridge types, decks, rail systems, materials, and abutments.

http://www.fs.fed.us/eng/bridges/ http://www.fs.fed.us/eng/bridges/

Introduction The U.S. Department of Agriculture, Forest Service, manages about 6,700 trail bridges on National For-est System lands. The Forest Service is continually improving trail bridge design and construction tech-niques as new materials and new uses for existing materials become available. Bridges are an impor-tant feature of many trails, so it is essential to design them based on the desired trail experience (as indi-cated in the Trail Management Objectives [TMOs]) and for user safety and bridge longevity. Bridge de-sign engineers must consider siting, design details and requirements, routine inspections, and mainte-nance when designing a structure for sustainability.

This report focuses on the design of new bridges that the Forest Service classifies as minor and major trail bridges (figure 1) (FSM 7737.05). These bridges are short, single-span trail bridges constructed of wood.

National Environmental Policy Act (NEPA) require-ments, landownership issues, and Federal and State permitting requirements are beyond the scope of this report.

Figure 1—A 15-foot span timber bridge classified as a minor trail bridge, which was constructed using Forest Service standard trail bridge plans.

This report is not intended as a comprehensive guide about trail bridge design, but should serve as a guide for basic knowledge about designing national forest single-span, wooden trail bridges. Having a back-ground in construction and in-service inspection of trail bridges is valuable when designing trail bridges.

An important aspect of effectively managing trail bridges is providing timely and quality trail bridge in-spection and maintenance. It costs far less to main-tain and repair a bridge than it does to replace a bridge that has failed because of neglect. Appendix

A provides a copy of the “Trail Bridge Matrix,” which the Forest Service uses for inspection requirements.

The Forest Service “Trail Bridge Catalog” website

<http://www.fs.fed.us/eng/bridges/> provides further information and images of trail bridge types, decks, rail systems, materials, and abutments. This report also briefly touches on bridge types and materials that are outside the Forest Service definition for ma-jor and minor trail bridges.

Trail Management Objectives One important criterion for designing trail bridges is meeting the TMOs for each trail. Trail bridge de-signs for National Forest System trails must reflect trail-specific TMOs (Forest Service Handbook [FSH]

7709.56b).

TMOs influence trail bridge design criteria by identi-fying the intended trail uses, trail experience, and ac-cessibility requirements of each trail, the associated trail class and recreation opportunity spectrum, the trail width and other associated design parameters, and operation and maintenance requirements. Refer to Forest Service Manual (FSM) 2353 for further guid-ance on TMOs.

Siting Site selection is critical in trail bridge design. Select-ing a suitable site usually involves input from an inter-disciplinary team (IDT) that typically consists of sub-ject matter experts in engineering, trail management, hydrology, geoscience, and biology. It requires careful attention to hydrology, hydraulics, trail align-ment, environmental, and geomorphic concerns. The

IDT must address all of these concerns to ensure that the structure is appropriate for the site. The Forest

Service National Technology and Development Pro-gram (NTDP) publication “Locating Your Trail Bridge for Longevity” <https://www.fs.fed.us/t-d/pubs/pdf-pubs/pdf10232808/pdf10232808dpi300.pdf> (figure

2) covers siting in more detail.

Figure 2—The Forest Service’s “Locating Your Trail Bridge for Longevity” publication.

Additionally, FSH 7709.56b, Section 22, recommends that the IDT conduct site investigations at the most promising sites. The IDT must select an appropriate site where the trail and the trail bridge will achieve the Trail Management Objectives (TMOs). The IDT should also consider:

• Placing the crossing on a straight stretch of stream. However, in valleys where stream channels meander, the team should not rule out sharp bends as possible structure sites because these bends often indicate the presence of erosion-resistant material and a stable point in the stream reach.

• Avoiding crossing at a skew whenever possible.

• Avoiding curves and steep grades in trail alignment.

• Crossing at a narrow stream-channel section.

• Selecting sites with stable soils and slopes.

• Avoiding stream crossings where channels are subject to channel degradation, shifting, aggrada-tions, or excessive scour. For example:

◦ A poor site has easily erodible streambed and streambank material. Determining the depth of erodible material can help the team assess the site’s suitability.

◦ The stream reach at a good site location has been stable in both the horizontal and vertical direction in recent history.

◦ Any upstream or downstream man-made structures might affect stream stability by changing waterflow patterns.

◦ Crossings in alluvial fans may be unstable because streams through alluvial fans are subject to aggradation and sudden shifts in alignment. Preferably, cross near the apex or head of the fan.

• Using straight approaches to bridges of at least 50 to 100 feet in length if possible.

• Placing bridges on a slight grade where the trail geometry permits. A 2-percent grade works well for shedding water from a structure. Avoid bridge deck profile grades greater than 5 percent.

https://www.fs.fed.us/t-d/pubs/pdfpubs/pdf10232808/pdf10232808dpi300.pdf https://www.fs.fed.us/t-d/pubs/pdfpubs/pdf10232808/pdf10232808dpi300.pdf https://www.fs.fed.us/t-d/pubs/pdfpubs/pdf10232808/pdf10232808dpi300.pdf https://www.fs.fed.us/t-d/pubs/pdfpubs/pdf10232808/pdf10232808dpi300.pdf

• Ensuring adequate clearance under a bridge to allow floodwaters, floating ice, or debris to pass during high runoff.

• Locating bridge substructures out of stream chan-nels. Avoid constricting waterways with approach fills, abutments, or piers.

• When sizing a trail bridge and selecting the type of material to use, bridge design engineers should consult the trail-specific TMOs for guidance on the intended type of trail experience, the specific managed uses, and trail design parameters, in-cluding appropriate material types. Additionally, bridge design engineers should consider using long-term materials, such as steel, concrete, alu-minum, or appropriate preservative-treated wood.

Bridge design engineers may use untreated logs in some situations (e.g., for log foot bridges and trail bridges in designated wilderness areas) or for short-term uses, such as temporary bridges.

However, treated wood or naturally decay-resistant wood will last significantly longer. Bridge design engineers should consider the following when selecting materials:

◦ When practical, bridge design engineers should use wood species that are either naturally resistant to deterioration (refer to

American Association of State Highway and

Transportation Officials [AASHTO] M 168) or are treatable using appropriate pressure treatments. Treatment plants should treat, clean, and handle wood in conformance with the requirements of the American

Wood Protection Association (AWPA) publi-cation “AWPA Book of Standards” available at <http://awpa.com/standards> and the

Western Wood Preservers Institute (WWPI) publication "Best management prac tices for the use of preserved wood in aquatic and sensitive environments" <http://pre-servedwood.org/portals/0/documents/BMP.

pdf>.

◦ Bridge design engineers should specify galvanized, painted, or weathering (corro-sion-resistant) steel to reduce damage from oxidation on steel bridges.

◦ Bridge design engineers should specify air-entrained concrete in regions subject to freeze and thaw cycles.

• Designing permanent bridges to last at least 50 years. Design short-term bridges for a lifespan appropriate to their intended use and in confor-mance with the TMOs for the trail.

• Designing all bridges to, at a minimum, withstand a 100-year flood. Provide for additional vertical clearance for the passage of woody debris and ice, as necessary. Refer to applicable regional guidance, channel configuration at the bridge site, and the requirements in Forest Service Handbook

7709.56b, Section 62.2, to determine the amount of additional vertical clearance.

Because a trail bridge is more susceptible to impact damage from debris, bridge design engineers should consider designing critical or high-value trail bridges with 1 foot more vertical clearance than the clearance required for a road bridge. Refer to the AASHTO pub-lication “LRFD Guide Specifications for the Design of

Pedestrian Bridges” for additional guidance on verti-cal clearances.

Bridge Types There are six basic types of structures that bridge design engineers typically use for trail bridges:

• Deck girder and deck truss bridges

• Single-unit bridges

• Side girder and side truss (pony truss) bridges

• Arch bridges (deck or suspended)

• Cable bridges

• Covered bridges http://awpa.com/standards http://awpa.com/standards http://awpa.com/standards http://awpa.com/standards http://preservedwood.org/portals/0/documents/BMP.pdf http://preservedwood.org/portals/0/documents/BMP.pdf http://preservedwood.org/portals/0/documents/BMP.pdf http://preservedwood.org/portals/0/documents/BMP.pdf

The bridge design engineer should determine the appropriate bridge types and materials based on careful consideration of TMOs and site-specific factors, including crossing and span lengths. Dif-ferent bridge types combined with various material types result in differing span capabilities and limita-tions. Not all bridge types and materials will work at all site locations. Longer crossings, in particular, may have a very limited selection of suitable bridge types and materials.

This guide focuses on the design of single-span, wooden deck girder and single-unit bridges in-cluded in the “USDA Forest Service Standard Trail

Plans and Specifications” <https://www.fs.fed.

us/managing-land/trails/trail-management-tools/ trailplans>.

Bridge design engineers primarily use trail-specific

TMOs to determine trail bridge type and material selection. The Forest Service “Trail Bridge Catalog” website <http://www.fs.fed.us/eng/bridges/index.

htm> provides some general guidance on where dif-ferent types of bridges will work and also has infor-mation about other types of structures. The website provides pictures, drawings, span guidelines, and comparisons.

Deck Girder and Deck Truss Bridges

Deck girder and deck truss bridges generally consist of two or more longitudinal stringers or trusses that support the top-mounted deck (figure 3). Deck girder and deck truss bridges span from 10 to 120 feet

(table 1). Their decks are usually made of timber (log, sawn, or glued-laminated [glulam]), but may be fiber-reinforced polymer (FRP), concrete, or steel. Bridge design engineers recommend a minimum of three stringers or trusses for structural redundancy of the bridge.

Figure 3—A glued-laminated (glulam) deck girder trail bridge.

Table 1—Types of material and typical span lengths for deck girder and deck truss bridges

Bridge type Material Typical span length (feet)

Deck girder Timber (log) 10 to 50

Deck girder Timber (sawn) 10 to 30

Deck girder Timber (glued-laminated)

20 to 100

Deck girder Steel 30 to 120

Deck girder Fiber-reinforced polymer

10 to 20

Deck truss Timber 50 to 80

Deck truss Steel 50 to 100

Deck truss Fiber-reinforced polymer

20 to 80

Single-Unit Bridges

A single-unit bridge is a single, self-supporting unit that spans from 10 to 120 feet (table 2). People some-times refer to timber-laminated and concrete bridges as “slab” bridges (figure 4).

http://www.fs.fed.us/recreation/programs/trail-management/trailplans/ http://www.fs.fed.us/recreation/programs/trail-management/trailplans/ https://www.fs.fed.us/managing-land/trails/trail-management-tools/trailplans https://www.fs.fed.us/managing-land/trails/trail-management-tools/trailplans https://www.fs.fed.us/managing-land/trails/trail-management-tools/trailplans http://www.fs.fed.us/eng/bridges/index.htm http://www.fs.fed.us/eng/bridges/index.htm http://www.fs.fed.us/eng/bridges/index.htm

Table 2—Types of material and typical span lengths for single-unit bridges

Material Typical span length (feet)

Timber (log) 10 to 40

Timber (nailed or glued-laminated) 10 to 40

Prestressed concrete 30 to 120

Figure 4—A single-unit bridge with two timber slabs.

Side Girder and Side Truss Bridges

Side girder and side truss bridges generally consist of two longitudinal girders or trusses with floor beams or ledger beams attached to the inside of the main girders or trusses. Ledgers support transverse deck planks, and floor beams support longitudinal deck planks. On larger bridges, floor beams may support the stringers. The girders or trusses usually function as all, or part of, the handrail system (figure

5). Decks are usually timber (sawn plank or glued-laminated), but may be concrete, steel, or FRP. Side girder and side truss bridges typically span from 40 to 240 feet (table 3).

Figure 5—A fiber-reinforced polymer bridge where the truss is also part of the handrail system.

Table 3—Types of material and typical span lengths for side girder and side truss bridges

Bridge type Material Typical span length (feet)

Side girder Glued-laminated 60 to 120

Side girder Steel 60 to 120

Side truss Timber (log) 40 to 80

Side truss Timber (sawn) 50 to 100

Side truss Steel 50 to 240

Side truss Fiber-reinforced polymer

20 to 100

Arch Bridges

Deck arch bridges generally consist of two arches below the bridge deck supporting longitudinal beams or walls, which in turn support the deck (figure 6).

Suspended arch bridges consist of two arches, which extend above the bridge deck; longitudinal beams hung from the arches above with steel rods or cables support the deck. Decks can be timber (sawn or glued-laminated), concrete, or steel. The longitudinal beams often function as tension members, holding the ends of the arch together. Often these arches are referred to as “tied arches.” Arch bridges span from

20 to 200 feet (table 4).

Figure 6—This masonry arch trail bridge was originally constructed for vehicle traffic.

Table 4—Types of material and typical span lengths for arch bridges

Component Material Typical span length (feet)

Deck arch Timber 60 to 120

Deck arch Masonry 20 to 60

Deck arch Concrete (filled) 20 to 80

Deck arch Concrete (open spandrel)

40 to 100

Suspended arch Timber 60 to 120

Suspended arch Steel 60 to 200

Cable Bridges Two main steel cables support cable suspension bridges. The deck hangs from suspender cables or steel rods, which hang from the main cables. The main cables drape over towers at each end of the bridge. The embankment anchors the main cables, and the towers support them. Decks are usually sawn timber planks. Bridge design engineers usually design longer span cable suspension bridges with a stiffening truss under the deck.

Two main steel cables support simple suspension bridges (figure 7). The two main cables directly sup-port the deck, so the deck follows an arc down and up from each abutment along the sag of the cables.

The abutments anchor the cables, and intermediate towers may or may not support the cables. Decks are usually sawn timber planks. Two cables above the deck serve as handrails.

Figure 7—A deck cable bridge supported by two main cables on timber towers.

Multiple steel cables connected to the tops of one or more towers support cable-stayed bridges. Decks may be timber, concrete, or steel grid. The differ-ence between suspension bridges and cable-stayed bridges is that the main support cables drape over the towers in suspension bridges; they connect di-rectly to the towers in cable-stayed bridges. Cable bridges usually span from 40 to 400 feet (table 5).

Table 5—Types of material and typical span lengths for cable bridges

Type of cable bridge

Type of tower Typical span length (feet)

Suspension Timber 80 to 200

Suspension Concrete and steel 80 to 400

Suspension Tower without stiffening truss

80 to 120

Simple suspension None 40 to 80

Simple suspension Timber 40 to 120

Simple suspension Concrete 80 to 300

Cable-stayed Steel 100 to 400

Covered Bridges Traditional covered bridges were essentially side truss bridges (figure 8). Most modern covered bridg-es use conventional side girders or deck girders. The covering is simply added to the top of the bridge to protect the structure from weather and deterioration.

Covered bridges span from 10 to 300 feet (table 6).

Figure 8—Many old covered bridges are being converted into pedestrian bridges.

Table 6—Types of material and typical span lengths for covered bridges

Component Typical span length (feet)

Timber side girder 50 to 80

Timber deck girder 40 to 80

Longitudinal glued-laminated deck

10 to 25

Traditional covered bridge 40 to 300

Materials The Forest Service constructs most trail bridges us-ing either timber or steel. However, in recent years the Forest Service has used aluminum, FRP, and con-crete more often. Bridge design engineers should de-sign and specify fabrication of bridges using materi-als that meet aesthetic, economic, and environmental constraints, while also taking into account installation and maintenance requirements.

Timber Viable timber bridge designs use native logs, sawn timber, or glued-laminated (glulam) products and can use different types of surfacing (planed or rough-sawn) to alter the appearance of wood. Rough-sawn wood is not planed and is more appropriate for primitive locations or where bridges require antiskid characteristics. All wood products, including glulam, are available as rough-sawn. Rough-sawn glulam is generally referred to as “resawn” glulam.

Preservative treatments can significantly change the appearance, smell, and suitability of wood. A timber bridge design can specify preservative-treated wood, untreated wood, or a combination of both.

For example, a design can specify untreated, Alaska yellow cedar railing with treated, glulam girders. The

Forest Service follows requirements in the publica-tion “AASHTO LRFD Bridge Design Specifications,” which indicates that timber must be treated with a preservative (AASHTO M 133) unless it is considered high in heartwood decay resistance (AASHTO M 168)

(table 7). Untreated wood that is not high in heart-wood decay resistance typically has a design life of one-twentieth to one-fifth the life of treated wood and is usually an unacceptable alternative. Field crews can cut trees adjacent to the bridge site for native log stringers, handrails, and sills. These native logs are not required to be treated or high in decay resistance, although it is advantageous to locate trees such as those listed in table 7.

Table 7—Wood considered high in heartwood decay resistance, according to American Association of State Highway and Transportation Officials (AASHTO) M 168

Softwoods Hardwoods

Bald cypress, old growth Chestnut

Cedar Black locust

Juniper White oak

Redwood, old growth Osage orange

Pacific yew Black walnut

As mentioned in the “Siting” section of this guide, untreated, native log stringers may be appropriate for bridges in wilderness areas and other remote locations to meet visual requirements and in areas where transport by helicopters or mules is either too expensive or is not allowed. A bridge design engi-neer can evaluate trees near the bridge site for size and strength. The field crew can cut and move the suitable trees into place at the bridge site with ca-bles, pulleys, and winches. Construction details that minimize water ponding on the bridge will maximize the life of the bridge.

The Forest Service typically uses Douglas fir or hem-fir on the west coast and southern pine on the east coast as preservative-treated timbers.

The AWPA introduced the Use Category System

(UCS) in 1999 to provide a simple way to use the

AWPA Standards (table 8).

The UCS assists the bridge design engineer with se-lecting and specifying the treatment for the end use of the product. The Forest Service uses the following treatments and UCS for their trail bridges:

• All treated stringers, decking, running planks, and handrails must be treated after fabrication in accordance with AWPA U1, UCS, using pentachlo-rophenol or copper naphthenate (CuN) in light oil, (type C solvent) for use category UC3B.

• All treated substructures (sills, backing planks, cribs, timber walls, etc.) shall be treated after fab-rication in accordance with AWPA U1, UCS, us-ing pentachlorophenol or CuN in heavy oil (type

A solvent) for use category UC4B.

The Forest Service also requires that all treated tim-ber members comply with the requirements of the current edition of WWPI’s "Best management prac-tices for the use of preserved wood in aquatic and sensitive environments" <http://awpa.com/stan-dards>.

If railing components are not treated or cut from naturally decay-resistant wood, the bridge design engineer should specify waterborne treatments or light solvent, oil-borne treatments for wood that trail users will frequently touch. Table 9 lists some of the most common preservative treatments and handling restrictions.

http://awpa.com/standards http://awpa.com/standards http://awpa.com/standards http://awpa.com/standards http://awpa.com/standards

Table 8—A table summarizing the American Wood-Preservers’ Association use category designations.

Use category

Service conditions Use environment Common agents of deterioration

Typical applications

UC1 Interior construction, aboveground, dry

Continuously protected from weather or other sources of moisture

Insects Interior construction and furnishings

UC2 Interior construction, aboveground, damp

Protected from weather, but may be subject to sources of moisture

Decay fungi and insects Interior construction

UC3A Exterior construction, aboveground, coated, rapid water runoff

Exposed to all weather cycles, but not exposed to prolonged wetting

Decay fungi and insects Coated millwork, siding, and trim

UC3B Exterior construction, aboveground, uncoated, or poor water runoff

Exposed to all weather cycles, including prolonged wetting

Decay fungi and insects Decking, deck joists, railings, fence pickets, and uncoated millwork

UC4A Ground contact or freshwater, noncritical components

Exposed to all weather cycles, normal exposure

Decay fungi and insects Fence, deck, and guardrail posts, crossties, and utility posts (low-decay areas)

UC4B Ground contact or freshwater, components that are critical or difficult to replace

Exposed to all weather cycles, high decay potential, includes saltwater splash

Decay fungi and insects, increased potential for biodeterioration

Permanent wood foundations, building poles, horticultural posts, crossties, and utility poles (high-decay areas)

UC4C Ground contact, freshwater, critical structural components

Exposed to all weather cycles, severe environments, extreme decay potential

Decay fungi and insects, extreme potential for biodeterioration

Land or freshwater pilings, foundation pilings, crossties, and utility poles (severe decay areas)

UC5A Salt or brackish water and adjacent mud zone, northern waters

Continuous marine exposure (saltwater)

Saltwater organisms Pilings, bulkheads, and bracing

UC5B Salt or brackish water and adjacent mud zone, New Jersey to Georgia, and south of San Francisco

Continuous marine exposure (saltwater)

Saltwater organisms, including creosote-tolerant Limnoria tripunctata

Pilings, bulkheads, and bracing

UC5C Salt or brackish water and adjacent mud zone, south of Georgia, Gulf Coast, Hawaii, and Puerto Rico

Continuous marine exposure (saltwater)

Saltwater organisms, including Martesia and Sphaeroma

Pilings, bulkheads, and bracing

UCFA Fire protection as required by codes, aboveground, interior construction

Continuously protected from weather or other sources of moisture

Fire Roof sheathing, roof trusses, studs, joists, and paneling

UCFB Fire protection as required by codes, aboveground, exterior construction

Subject to wetting Fire Vertical exterior walls, in-roof surfaces, or other types of construction that allow water to drain quickly

Table 9—A table summarizing the properties of preservatives

Standardized use

Preservative Solvent characteristics

Surface/ handling restrictions

Color Odor Fastener corrosion

All uses Creosote Oil-type Oily, not for frequent human contact

Dark brown Strong lasting

No worse than untreated

Ammoniacal copper zinc arsenate

Water Dry, but contains arsenic

Brown possible blue areas

Mild short term

Worse than untreated wood

Chromated copper arsenate

Water Dry, but uses are restricted by the U.S.

Environmental Protection Agency*

Greenish brown, weathers to grey

None Similar to untreated wood

All uses (except in seawater)

Pentachlorophenol in heavy oil

No. 2 fuel oil Oily, not for frequent human contact

Dark brown Strong, lasting

No worse than untreated wood

Copper naphthenate

No. 2 fuel oil Oily, not for frequent human contact

Green, weathers to brownish gray

Strong, lasting

No worse than untreated wood

Alkaline copper quat

Water Dry, okay for human contact

Greenish brown, weathers to gray

Mild, short term

Worse than untreated wood

Copper azole Water Dry, okay for human contact

Greenish brown, weathers to gray

Mild, short term

Worse than untreated wood

Aboveground, fully exposed

Pentachlor-ophenol in light oil

Mineral spirits Dry, okayfor human contact if coated

Light brown, weathers to gray

Mild, short term

No worse than untreated wood

Oxine copper Mineral spirits Dry, okay for human contact

Greenish brown, weathers to gray

Mild, short term

No worse than untreated wood

Aboveground, partially protected (such as millwork)

Iodopropynyl butylcarbamate

(IPBC) +

permethrin

Mineral spirits Dry, okay for human contact

Colorless Mild, short term

No worse than untreated wood

Indoors (usually for insect protection)

Borates Water Dry, okay for human contact

Colorless, blue dye often added

None No worse than untreated wood

* A few uses of chromated copper arsenate are still allowed for treatment of sawn products less than 5 inches thick (12.7 centimeters, such as dimension lumber). Pilings, poles, large timbers, and plywood are still allowed for highway construction.—Courtesy of USDA Forest Service, Forest Products Laboratory.

Steel The bridge design engineer should specify steel that is painted or galvanized, unless it is a corrosion-re-sistant, weathering steel. Each of these types of steel treatments has its place in the field. The decision to paint a structure or use weathering steel depends on the local environmental conditions, the TMOs

(including recreation opportunity spectrum classifica-tion), and consideration of the Forest Service “Built

Environment Image Guide” <https://www.fs.fed.

us/recreation/programs/beig/>. The Forest Service usually specifies using a Forest Service brown paint color for painted steel bridges.

Weathering steel is the steel of choice for most For-est Service steel bridges. However, bridge design engineers should consider important factors, such as environment and location, before selecting weath-ering steel for trail bridge construction. In general, bridge design engineers should not specify using uncoated, weathering steel in wet environments, including coastal areas or areas with high rainfall, high humidity, or persistent fog. Refer to the U.S. De-partment of Transportation, Federal Highway Admin-istration, Bridges and Structures, Technical Advisory website <https://www.fhwa.dot.gov/bridge/t514022.

cfm> for additional guidance on using weathering steel in specific locations.

Bridge design engineers and landscape architects generally do not prefer using galvanized steel for trail bridges on National Forest System trails; its bright, shiny, silvery color does not blend in well with the natural environment.

Concrete Concrete is typically identified by its industrial grey appearance. The bridge design engineer can specify concrete to be texturized, colored, stained, or paint-ed to better match aesthetic constraints. Texturizing tends to have long-lasting results and can be done with surface rollers, forms, form liners, sandblast-ing, or chemicals. Coloring can be done by adding coloring agents to the concrete or by staining and painting. Without periodic maintenance, concrete coloring tends to fade. The seal coat placed on col-ored and textured concrete may cause the surface to become slippery. One way to remediate this is to pro-vide better surface traction by applying silica sand to the surface when the seal coat is tacky.

Concrete should have an air entrainment of 4 to 6 percent if it will experience exposure to freezing temperatures and should have a minimum design compressive strength of 3,000 pounds per square inch.

Fiber-Reinforced Polymer (FRP) Fiber-reinforced polymer (FRP), commonly referred to as fiberglass, is a composite material made of a polymer matrix reinforced with fibers. The fibers can be glass, carbon, aramid, or basalt. The polymer is usually an epoxy. FRP materials are lightweight and durable. Common shapes match those of the rolled-steel materials used for trail bridge components, such as tubes and channels. Fiberglass should have a waterproof, colored surface treatment (surface veil) to protect the fiberglass from ultraviolet radiation.

Also, the structural components should be pultruded and not extruded. Fiberglass is available in a limited number of colors: green, brown, and gray. If appli-cable, the bridge design engineer can specify using higher cost components that manufacturers fabricate with a fire-retardant resin for bridge sites that are prone to fire damage.

Refer to the AASHTO publication “Guide Speci-fications for Design of FRP Pedestrian Bridges” and the NTDP publication “A Guide to Fiber-Rein-forced Polymer Trail Bridges” (0623–2824P–MTDC)

<https://www.fs.fed.us/t-d/php/library_card.php?p_ num=0623%202824P> for further information about

FRP trail bridges.

https://www.fs.fed.us/recreation/programs/beig/ https://www.fs.fed.us/recreation/programs/beig/ https://www.fs.fed.us/recreation/programs/beig/ https://www.fs.fed.us/recreation/programs/beig/ https://www.fhwa.dot.gov/bridge/t514022.cfm https://www.fhwa.dot.gov/bridge/t514022.cfm https://www.fhwa.dot.gov/bridge/t514022.cfm https://www.fhwa.dot.gov/bridge/t514022.cfm https://www.fhwa.dot.gov/bridge/t514022.cfm https://www.fhwa.dot.gov/bridge/t514022.cfm https://www.fs.fed.us/t-d/php/library_card.php?p_num=0623%202824P https://www.fs.fed.us/t-d/php/library_card.php?p_num=0623%202824P https://www.fs.fed.us/t-d/php/library_card.php?p_num=0623%202824P https://www.fs.fed.us/t-d/php/library_card.php?p_num=0623%202824P

Plastic Wood Plastic wood, or “wood-plastic composite” (WPC), is an alternative building material that manufactur-ers usually produce using recycled plastics and wood flour. WPC has a lower tensile strength and stiffness than wood and a much greater coefficient of thermal expansion than steel or wood, meaning that it expands and shrinks more. This contributes to significant long-term creep and deformations. For these reasons, when using plastic wood as decking and other nonstructural members, the bridge design engineer should develop special details to account for the creep and greater expansion.

Refer to the NTDP publication “Plastic Wood and Al-ternative Materials for Trail Structures” (1123–1804P–

SDTDC) <https://www.fs.fed.us/t-d/php/library_card.

php?p_num=1123%201804P> for further information about plastic wood.

Design The Forest Service requires that all trail bridge design engineers follow “AASHTO LRFD Bridge

Design Specifications” and AASHTO’s “LRFD Guide

Specifications for the Design of Pedestrian Bridges.”

These specifications, along with the additional ref-erences listed in FSH 7709.56b, Section 80.6, are the nationally accepted guidance for national forest trail bridge designs. In addition, trail bridge design engineers must follow AASHTO’s “Guide Specifica-tions for Design of FRP Pedestrian Bridges” when designing FRP bridges. Engineers must meet specific qualifications to design national forest trail bridges.

A Forest Service employee can become a certified bridge design engineer by meeting education and ex-perience requirements (FSM 7723.2). If a consulting engineer provides the design, a professional engineer must stamp the design (FSH 7709.56b, Chapter 80).

Bridge design engineers must design trail bridges to reflect the TMOs established for the trail. The TMOs indicate the appropriate trail fundamentals, recre-ational opportunity spectrum, and national qual-ity standards for trails and applicable accessibility requirements. TMOs specify the trail class, intention-ally managed uses, and trail-specific design param-eters, including the trail width and other factors.

Bridge design engineers should design trail bridg-es for the maximum loading or load combinations, taking into account such factors as pedestrian live load, snow, wind, snow groomers, seismic events, and light vehicle loads. Design loads must reflect the trail fundamentals, including the design parameters that the TMOs identify for the trail (appendix B).

Bridge Widths, Grades, and Approaches The required bridge width, bridge grade, approaches, and any accessibility requirements that the TMOs identify are important considerations that bridge de-sign engineers must consider when designing a trail bridge.

Bridge Widths The required trail bridge width depends on the in-tended uses of the trail as the TMOs specify, such as pedestrian, off-highway vehicles (OHVs), snow groomers, etc. Trail bridge widths can range from 18 inches wide (single-log foot bridge) to 14 feet wide or more (groomed, snowmobile trail bridge). A bridge design engineer who designs a bridge primarily for hiker and pedestrian use during the summer may also have to design the bridge to accommodate wider and heavier loads, such as trail maintenance equipment or emergency vehicles. Bridge design engineers should design trail bridges managed for both standard/terra trail uses during the summer and snow trail uses during the winter to accommodate the most demand-ing designed use that the TMOs identify.

https://www.fs.fed.us/t-d/php/library_card.php?p_num=1123%201804P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1123%201804P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1123%201804P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1123%201804P

Refer to the TMOs design parameters and “Forest

Service Trails Accessibility Guidelines (FSTAG) Pock-et Version” (1523–2812–MTDC) <https://www.fs.fed.

us/t-d/php/library_card.php?p_num=1523%202812> for specific widths and other accessible trail require-ments.

Grades and Accessibility Grades for trail bridges should be consistent with the TMOs for the trail and should meet accessibility requirements, as applicable. Where trail geom-etry permits, bridge design engineers should place bridges on a slight grade (2 percent) to shed wa-ter. When grades exceed 4 percent, bridge design engineers should consider skid-resistant surfaces on high-use trail bridges or equestrian trail bridges.

Bridge design engineers should avoid bridge deck grades of more than 5 percent on trails if they plan to meet accessibility requirements.

Approaches Trail bridges should have straight approaches of 25 feet or more leading up to the bridge. Motorized trails should have straight approaches of at least 50 to 100 feet, if possible. Adequate line of sight is important for motorized trails; it enables users to see vehicles crossing the bridge or stopped on the bridge, al-lowing them time to stop. Another consideration for approaches is surface drainage and erosion con-trol to keep water and debris from collecting on the bridge deck. The “Siting” section of this manual and the NTDP publication “Locating your Trail Bridge for Longevity” (1023–2808P–MTDC) <https://www.

fs.fed.us/t-d/php/library_card.php?p_num=1023%20

2808P> provide further information about fitting a trail bridge to a site.

Users and Loads The three major types of loads that bridge design engineers use in designing National Forest bridges are dead loads, live loads, and snow loads. Other types of loads that bridge design engineers should consider are wind, seismic, and fatigue loads.

Bridge design engineers should design trail bridges for maximum loading or load combinations, taking into account such factors as pedestrian live load, snow, wind, snow groomers, seismic events, and light vehicle loads. Design loads must reflect the trail fundamentals, including the design parameters that the TMOs identify for the trail (appendix B). At a minimum, bridge design engineers must apply the provisions from AASHTO’s “LRFD Guide Specifica-tions for the Design of Pedestrian Bridges” covered in the following sections.

Dead Loads Basic dead loads (stationary loads) consist of girders, stringers, beams, bracing, decking, and railing sys-tems. Bridge design engineers may add additional dead loads, such as running planks, utility lines, a gravel surface, covers, etc.

Wearing surfaces (running planks), are a dead load and the Forest Service highly recommends applying them to protect the main decking for OHV, snowmo-bile, and equestrian trail bridges.

https://www.fs.fed.us/t-d/php/library_card.php?p_num=1523%202812 https://www.fs.fed.us/t-d/php/library_card.php?p_num=1523%202812 https://www.fs.fed.us/t-d/php/library_card.php?p_num=1523%202812 https://www.fs.fed.us/t-d/php/library_card.php?p_num=1523%202812 https://www.fs.fed.us/t-d/php/library_card.php?p_num=1523%202812 https://www.fs.fed.us/t-d/php/library_card.php?p_num=1023%202808P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1023%202808P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1023%202808P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1023%202808P https://www.fs.fed.us/t-d/php/library_card.php?p_num=1023%202808P

Live Loads Live loads (moving loads) consist of pedestrians, bicycles, OHVs, equestrians, snowmobiles, and other temporary loads. The bridge design engineer should refer to the trail-specific TMOs to determine live loads to use in the design, taking into consider-ation all allowed uses, including managed, accepted, and administrative uses. The bridge design engineer should design trail bridges that are managed for both standard/terra trail uses during the summer and snow trail uses during the winter to accommodate the most demanding designed use that the TMOs identify. The bridge design engineer should also consider vehicle loads for emergency access and trail maintenance when determining live load requirements.

Per the AASHTO publication “LRFD Guide Specifica-tions for the Design of Pedestrian Bridges” loads include pedestrian loading (people), vehicle loading

(all-terrain vehicles [figure 9], motorcycles, bicycles, over-snow vehicles) and equestrian loading

(packstock). The minimum pedestrian live load is 90 pounds per square foot (lb/ft²). AASHTO prepared the

“LRFD Guide Specifications for Design of Pedestrian

Bridges” for urban settings with large numbers of po-tential users. Many national forest trail bridge sites are remote and only small groups of hikers access them.

When the TMOs indicate such limited use, the bridge design engineer can reduce the pedestrian load to 65 lb/ft². The bridge design engineer should document the justification for a reduced design live load and in-clude it in the permanent bridge file.

Equestrian loads require bridge design engineers to design bridge decks for a patch load of 1,000 pounds over a square area measuring 4 inches on each side.

Equestrian loads usually require thicker decking than pedestrian loads. Typical decking material for eques-trian loads is 3 by 8 (or larger) planks. Bridge design engineers must design the entire trail bridge, includ-ing the deck, for the required uniform pedestrian or equestrian load in combination with other loads.

Figure 9—A single-unit timber bridge designed to accommodate all-terrain vehicles, bicycles, and hikers.

Bridge design engineers must design elevated view-ing platforms for a minimum pedestrian live load of

100 lb/ft², according to Forest Service requirements

(FSH 7709.56b, Chapter 80).

Snow Loads Snow loads vary with geographic location and are often the heaviest load on national forest bridges, es-pecially at high elevations and in northern latitudes

(figure 10). Snow loads can vary from 0 lb/ft² in the

South to 200 lb/ft² in the Northeast, and from 300 lb/ ft² in California to more than 1,200 lb/ft² in the Cas-cade Range of Washington State. To determine snow loads, bridge design engineers should use the maxi-mum ground snow load conditions, accumulation, and water content for the 50-year recurrence interval without reduction or use load values that local build-ing or public road authorities have developed. Bridge design engineers should also consider snow loads as live loads and combine snow loads with other loads when applicable.

If designing for seismic events, a reasonable design methodology is to add 20 percent of the snow load to the dead load for determining total seismic weight.

The “Natural Resources Conservation Services Snow

Telemetry (SNOWTEL) and Snow Course Data and

Products” website <https://www.wcc.nrcs.usda.gov/ snow> is one good resource for determining snow https://www.wcc.nrcs.usda.gov/snow https://www.wcc.nrcs.usda.gov/snow https://www.wcc.nrcs.usda.gov/snow https://www.wcc.nrcs.usda.gov/snow https://www.wcc.nrcs.usda.gov/snow loads in the West. Site-specific snow load informa-tion is also available on the Applied Technology

Council (ATC) website <https://hazards.atcouncil.

org/>. The NTDP “National Snow Load Information” website <http://www.fs.fed.us/eng/snow_load/> provides further information about snow loads.

Figure 10—Trail bridges should be designed for snow loads.

Wind Loads Wind loads for trail bridges are another live load.

Bridge…