Chapter 5-- Designing Trail Crossings and Structures--Continued
The design of above-grade crossings--bridges and overpasses--is complex and beyond the scope of this guide. Designing an appropriate above-grade crossing that meets the special needs of riders requires qualified and knowledgeable engineers, as well as other key resource specialists, who may include hydrologists, soil scientists, bridge and geotechnical engineers, and landscape architects. Design must comply with regulations established by the authorizing agency and Federal and State laws. Bridges require regular certified inspection according to governing regulations. Bridges on Forest Service lands, for example, must undergo inspection every 5 years.
Bridges and overpasses on horse trails require careful design to accommodate animal behavior. Horses and mules may hesitate if a bridge or overpass is narrow, sways, swings, vibrates, or is constructed of unfamiliar materials. Stock also are uncomfortable if the structure creates or amplifies noise. Even well-trained stock may balk at ramp approaches to bridges, especially where there are no approach rails. If a structure or tread appears dangerous, horses and mules usually refuse to go any farther. Incorporate skid-resistant surfaces and avoid designing steps on equestrian overpasses and bridges.
In general, there are six types of trail bridges:
- Cable bridges
- Deck girder/truss bridges
- Side girder/truss bridges--pony-truss bridges
- Arch bridges--deck or suspended bridges
- Miscellaneous single-unit bridges
- Covered bridges
Each bridge type and construction material has different span limitations that must be matched to site conditions. Longer crossings may have a very limited selection of suitable bridge types or materials. Prefabricated bridges, shipped in sections for reassembly on the site, may be appropriate for some situations. For example, the remote bridge shown in figure 5-17 consists of sections that were packed in and bolted into place. Engineering analysis is required for these products, along with strict adherence to the manufacturer's installation and maintenance instructions.
A simple bridge (figure 5-18) is adequate for many stream crossings. Horse and pedestrian trails frequently cross suspension bridges (figure 5-19). Long or swaying suspension bridges (figure 5-20) can be daunting to stock and riders that are not accustomed to crossing them.
Bridges with the horizontal alignment perpendicular to the stream are the shortest and usually the least costly to build. Avoid sharp and blind curves on the immediate approaches to bridges, because curves adversely affect sight distance. The vertical alignment--or grade--of bridges also affects sight distance, drainage, and footing. Adjusting the trail alignment to address these issues usually costs less than modifying the bridge.
Bridges with a slight grade or camber shed water better than flat bridges. However, grades that are too steep can cause footing problems. Bridge grades on trails should not be greater than any part of the trail itself and when possible, should not exceed 5 percent. Camber on arch bridges also should not exceed 5 percent. Figure 5-21 shows a trail bridge with camber.
The minimum suggested bridge width on horse trails in areas with low levels of development is 5 feet (1.5 meters). In areas with high levels of development, 12 feet (3.6 meters) is preferred. Bridges in areas with moderate levels of development often range between 5 and 8 feet (1.5 and 2.4 meters) wide. Bridges that are wider than 6 feet (1.8 meters) and narrower than 10 feet (3 meters) are only suitable for riding single file, but riders may be tempted to pass or ride two abreast, a potential source of conflict. For facilities subject to the AASHTO guidelines, match the clear bridge width to the width of the shared-use trails that lead up to them. Then add an additional 2 feet (0.6 meter) on each side (AASHTO 1999). This extra width gives all trail users the minimum horizontal shy distance from the railing or barrier. It also provides maneuvering space when trail users encounter others who have stopped.
Bridges, causeways, and boardwalks on horse trails must meet engineering specifications to support the weight of a large group of stock. Structures designed primarily for pedestrians and bicycles are not strong enough for horses and mules, because the decking cannot withstand the force of horseshoes or the point load per hoof. In addition, bridges must be engineered to withstand the vibration caused by single or multiple animals. Stock, including their riders or loads, usually weigh from 1,000 to 1,700 pounds (454 to 771 kilograms).
Design bridge beams and stringers for the anticipated maximum loading or load combinations, including live loads, snow, wind, snow groomers, earthquakes, and light vehicles. Covered bridges in snow country have additional load considerations.
Select materials for bridges and overpasses based on durability as well as for strength, esthetics, cost, and appropriate level of development. Common bridge materials include timber, steel, concrete, and fiberglass. Many companies have engineered plans for standard bridge lengths of wood, steel, and fiberglass. Table 5-1 shows suggested structural materials suitable for different levels of trail development. Esthetics and the setting--wildland, rural, urban--also affect choices.
|Material||Low development||Moderate development||High development|
|Sawn timber or engineered wood||X||X||X|
Select surface treatments for bridges and wetland structures carefully. Most stock will hesitate to step from the tread to the bridge unless the transition between tread and bridge is as smooth and uninterrupted as possible. The surface of the tread and bridge should be flush and have similar colors. A step up or down to the bridge draws the trail animal's attention to the change in material.
Common bridge decking materials include wood, concrete, steel grates, fiberglass, and composites made from plastic and wood. Wood decking can be planks or glue-laminated panels. Because wood surfaces may be slippery when wet, they work best in areas that don't get a lot of rain. Concrete bridges surfaced with appropriate natural soils, sand, crushed rock, or a rough surface generally are horse friendly. Avoid steel grates because stock may be frightened when they look through the grate or hear a horseshoe striking it. Fiberglass decks must have a wearing surface that can withstand the impact of horseshoes. The surface of plastic laminates can be slick, requiring that they be manufactured with a roughened surface. Avoid decks that sound hollow when stock travel across them.
Bridge wearing surface refers to a temporary layer of decking that is easily replaced when worn. Often less expensive, untreated wood is used for this purpose. The wearing surface frequently is the same width as the trail on each end and tapers to a narrower width toward the center (figure 5-22). This gradual reduction in width serves to funnel trail traffic to the center of the bridge tread. This pattern is less costly than providing a wearing surface that extends the full bridge width for the entire span length.
In areas with low levels of development, the Forest Service often constructs decking from wood planks that are 3 inches (76 millimeters) thick if no wearing surface is included. When used along with wearing surfaces, the decking consists of transverse wood planks 2 inches thick by at least 8 inches (51 by 203 millimeters) wide, placed on the bridge stringers. The wearing surface consists of longitudinal planks 2 inches thick by 12 inches (51 by 305 millimeters) wide. Horse loads normally are concentrated loads. Horse loads determine the thickness of bridge decking and wearing surfaces. Pedestrian live loads are uniform loads over the entire deck. Pedestrian live loads determine the size of bridge stringers.
Select tread surface materials that don't become slick from use, particularly if the bridge has any slope. Timber cleats, rubber matting, or other wearing surfaces can be installed to improve traction.
Trail bridges require railings, except in certain circumstances. Trail bridges that don't have railings must have longitudinal edging, commonly called curbing or curbs. Before constructing bridge curbs instead of railings, agencies may require documentation that substantiates the decision. For example, if an analysis shows that the potential hazards along the trail are the same or greater than the hazards of a bridge without a railing, curbs may be used in place of railings. The Forest Service requires an engineering analysis to determine whether the hazards along the trail are the same or greater than those on a bridge without a railing. In general, trail users in rural and urban settings are more likely to be small children or less experienced adults who will need a railing. In wildland settings, trail users normally are more experienced and railings may be unnecessary.
The first consideration in selecting railings must be safety. According to the Trail Bridge Catalog (Eriksson 2000), guidelines for rail systems fall under the following:
Building Code--Railings on trail bridges in urban settings must meet building code requirements, such as the International Building Code (IBC). These railings are designed for pedestrians, not riders, and must have vertical balusters that are not easy to climb. The code requires a handrail at least 42 inches (1.067 meters) high that does not allow a 4-inch (101.6-millimeter) sphere to pass through.
AASHTO Code--Horizontal railings on trail bridges frequently used by children must meet AASHTO Standard Specifications for Highway Bridges. A 6-inch (152.4-millimeter) sphere must not pass through the railing in the bottom 27 inches (685.8 millimeters), and an 8-inch (203.2- millimeter) sphere must not pass through the area higher than 27 inches (685.8 millimeters). The code also requires a handrail at least 54 inches (1,372 millimeters) high for equestrian traffic.
Remote Areas--Railings on remote trail bridges must be at least 54 inches (1,372 millimeters) high for equestrian traffic. The handrail system also must have one or more intermediate rails so that the vertical distance between rails does not exceed 15 inches (381 millimeters). The Forest Service requires handrail systems on bridges to have at least two horizontal rails above the tread level.
Table 5-2 gives selected design criteria for Forest Service bridges on horse trails. Live load pressures for hikers, ATVs, motorcycles, bicycles, snowmobiles, stock, or packstrings are grouped together under pedestrian live loads.
Other considerations may justify railings or barriers. For example, horses and mules may become frightened if they can see high-speed vehicles or other distractions passing beneath or near the bridge. Provide a solid barrier or panel topped with an open-view railing (see figures 3-18 and 5-23). Use a similar design on the bridge approach to ease the transition from the trail onto the bridge deck. Such panels on approaches guide a reluctant trail animal onto the bridge. Construct the panels on one or both sides to extend a distance appropriate to site conditions. Angle the extensions outward from the bridge structure to form approach rails (figure 5-24).
Figure 5-23--A solid barrier topped with an
open view railing is often more acceptable to
horses and mules than an open view fence
alone. This open view railing is on the Marjorie
Harris Carr Cross Florida Greenway Land Bridge.
Figures 5-27 and 5-28 show additional views
of the land bridge.
Railings should be free of protrusions that can catch on legs, feet, stirrups, or tack. Install all connecting hardware with the smooth side toward the trail user.
Safety is compromised when riders are forced into areas with narrow or low clearance. Construct bridges with a minimum overhead clearance of 10 feet (3 meters) in the equestrian trail corridor. The preferred overhead clearance is 12 feet (3.6 meters). Pedestrian and bicycle bridges over freeways frequently have vertical curved fences or roofs to prevent anything being thrown from the bridge. Tread location and inadequate trail clearance (horizontal or overhead) should not force riders to the center of the corridor or make it difficult for riders to pass stopped users safely. Loud traffic noises on these bridges may make them questionable for equestrian use.
Figure 5-26 illustrates a shared-use bridge for nonmotorized travel over a freeway. It has a separate, 12-foot- (3.6-meter-) wide equestrian tread in the center of the bridge where the vertical clearance is greatest. Pedestrians, bicyclists and other nonmotorized users use the separate treads on either side of the horse tread.
Figure 5-26--Separation barriers on this
shared-use bridge are short enough
for horses and mules to see over, so stock are
more comfortable. The roof over the equestrian tread
(center) has a high overhead clearance to
Sight distance can be restricted by a bridge's arc or because approaches are placed at a poor angle. A long sight distance on bridges allows riders to see problems in advance, preferably the entire length of the bridge, plus approaches. When sight distance or visibility on bridges is limited, work with bridge and traffic engineers to determine proper remedial action. In urban and rural areas, this may include installing signs and signals.
Many stock are unfamiliar with bridges that also have vehicle traffic. The speed of the traffic on the bridge, noise level, and vibrations can make some stock nervous. Occasionally, managers designate a bridge for equestrians only. For bridges where motorized use is very low, if budgets and bridge conditions permit, separate riders from vehicles and other trail users. Where feasible, bridge design can incorporate barriers between two or more treads to separate riders and slow motorized traffic. The barrier would be subject to careful analysis and regulatory approval.
It is best if bridges over high-speed roads separate stock and traffic. Some shared bridges route traffic on one level and trail users on a different--usually lower--level. The traffic is not visible to the animal, and the sound of traffic is contained in the separate corridor.
Several specialty land bridges over major roads in the United States have grass and shrubs planted in a soil-covered deck. Many user groups appreciate this design, which is costly. Figures 5-27 and 5-28 show the Marjorie Harris Carr Cross Florida Greenway Land Bridge over Interstate 75 just south of Ocala, FL.
Figure 5-27--The Marjorie Harris Carr Cross
Florida Greenway Land Bridge across
Interstate Highway 75 south of Ocala allows
riders, hikers, and bicyclists to cross six
lanes of traffic. The bridge is 52.5 feet wide
and 200 feet long.
In some cases, underpasses--or below-grade crossings--are more suitable than at-grade crossings or bridges. Large-diameter structures--culverts and underpasses--generally serve riders well. Prefabricated underpasses are available in aluminum, steel, and concrete. They can be round, elliptical, arched, or box-shaped. Examples of underpasses are shown in figures 5-29 and 5-30. Trails with belowgrade crossings must meet design regulations or guidance such as AASHTO specifications, and they require the expertise of engineers. The advantage to recreationists and wildlife can sometimes justify the higher cost of below-grade crossings rather than at-grade crossings. When designing belowgrade crossings, carefully consider the safety of approaches, drainage structures, the tread surface, clearance, sight distance, and lighting. Figure 5-31 shows separate, adjacent underpasses for motorized traffic and trail users.
It is often difficult to provide the necessary overhead clearance required by riders when approaches slope down into below-grade passages. Design new structures so approaches are level with the trail tread. If drainage or site conditions require a slight slope, make it constant from one end of the passage to the other. Retrofitted below-grade trail approaches sometimes slope downward at both ends, reducing clearance and making drainage difficult. Avoid this situation wherever possible. When sloped approaches to retrofitted culverts or underpasses are unavoidable, design them with no more than a 5-percent grade. Avoid hard, smooth tread treatments for approaches.
Relatively level, natural tread surfaces leading into underpasses generally require no additional treatment. The exception is a tread surface that is frequently wet or muddy. Sloping trails that are frequently wet may benefit from geosynthetic materials. If culverts don't drain adequately, they are unsuitable for horse trails. Design the approach and surfaces of the underpass to prevent water, snow, sand, soil, or other materials from collecting where they will hamper traction or interfere with clearance. Use horse-friendly surface materials. Make sure that the below-grade crossings are large enough for the equipment needed to maintain them. See Chapter 6--Choosing Horse-Friendly Surface Materials for more information.
If a trail animal startles while in an underpass or below-grade culvert, the animal, rider, and other trail users may be injured. This is especially true in narrow underpasses or those with low, curved ceilings. For safety, design culverts and underpasses on horse trails so the vertical clearance is no lower than 10 feet (3 meters) across the entire width of the tread. The preferred height is 12 feet (3.6 meters). Horizontal clearance often extends 2 to 3 feet (0.6 to 0.9 meter) beyond the tread edge on both sides of the trail. Horizontal and vertical clearance in passages should be no less than the clearing limits on the rest of the trail.
When figuring horizontal and vertical clearance in underpasses, allow space for maneuvering and passing. Box-shaped structures should meet the standard height guidelines and be no less than 8 feet (2.4 meters) wide. A preferred width of 12 feet (3.6 meters) allows space for trail users to pass. The culvert in figure 5-32 appears wide enough for riders, but the vertical clearance is suspect. Culverts that curve near the top must provide 10 to 12 feet (3 to 3.6 meters) of overhead clearance without forcing riders to the center of the trail. Riders can suffer severe injuries if they hit their heads. The horizontal clearance at head height should be at least as wide as the trail itself and no narrower than 5 feet (1.5 meters) wide. This may be difficult to achieve with tapered culverts (figure 5-33).
Figure 5-33--Riders need more lateral
clearance near the top of underpasses than other
users. Provide 12 feet of overhead clearance
that is the entire width of the trail. Avoid
sloping roofs that force riders to the center
of the tread.
Adequate lighting and sight distance are important inside, outside, and at approaches to enclosed trail corridors. The eyes of stock don't adjust quickly to lighting changes, and many animals stop or hesitate when they can't see well.
In highly developed areas, artificial lights may be helpful, especially if the corridor approach is sloped. If possible, install fixtures flush with the approach walls. In trail corridors, locate fixtures at least 10 feet (3 meters) above the trail surface where they will not encroach on clearance. Keep the scale appropriate to trail users, and vary the light intensity for trail conditions or location. Consult a professional lighting designer or engineer for a site-specific plan.
Some divided highways provide a light well--or opening--in the median to allow sunlight into the passage below and enhance visibility during the day.
With careful design, some culverts that carry water can include a separate trail tread
Successful designs prevent trail tread material from being eroded at either end of the culvert.
Figure 5-35 illustrates a culvert that carries water and also includes a trail. Inside the culvert, a channel along the outer edge of the trail carries water out of the culvert. Abutments direct the water to a catchment pond below the trail tread.