Design of Floating Structures

Site conditions may force you to modify a prefabricated dock or even to design your own floating structure. This report is limited to small, simple structures and is not intended to cover the details of planning and designing floating bridges. Floating trail bridges should be designed by a qualified engineer. The Forest Service Manual delegates authority for trail bridge design to the forest engineer.

For more information, consult books on dock building, military manuals, and experienced dock builders and engineers. The following sections provide general information about the design of floating structures.

Decks

Four elements need to be considered when designing the deck: width, structural adequacy, traction, and appearance. The deck needs to be wide enough to carry its intended users. In most cases decks should be handicapped accessible. A wider deck provides a feeling of security and safety and is more stable. A bridge should be at least as wide as the trail leading to it. The absolute minimum width is 3 feet; 4 feet is the minimum recommended width. If users might congregate on the structure, the deck may need to be wider or an additional section may be attached to the side of the deck to provide room where users can stop while others move by (figure 15).

Figure 15—Side extensions to this floating trail bridge provide
places where users can stop without obstructing others (photo by
Paul Schrooten, USDI Fish and Wildlife Service).

Decking material must be strong enough to prevent failure and stiff enough to give a feeling of stability. Additional framing support may be needed for plastic or thin wooden decking. Strength is even more important if the decking also serves as the frame or is used to connect to anchorage supports.

Deck material should not be slippery. A floating structure is often muddy and wet. Plastic or metal gratings allow mud and water to flow through, leaving the grating's surface dry. Timber planks should be spaced at least ½ inch apart to allow for drainage. Planks that are 4 to 8 inches wide generally work best for decking. When additional traction is needed, try painting the planks with an anti–skid paint, gluing sand to the deck, securing rubber matting to the deck, or mounting timber cleats on the deck. When designing the deck surface, consider the deck's intended users, the Recreation Opportunity Spectrum setting, and any accessibility requirements. Make sure that the surfacing will not catch the tires of bicycles or wheelchairs.

Appearance is important. Deck overhangs, which hide the floats, are encouraged, but should not reduce the strength of the decking or the stability of the structure. Decking is the most visible part of a floating bridge or dock. A rustic deck can often hide or disguise the appearance of a modern frame or floats. The Missoula Technology and Development Center's Trail Bridge Catalog (available to Forest Service and USDI Bureau of Land Management employees at http://fsweb.mtdc.wo.fs.fed.us/bridges and to the public at http://www.fhwa.dot.gov/environment/rectrails/trailpub.htm) contains styles of bridge decks for different Recreation Opportunity Spectrum classes.

Frames

In most designs the frame bears the brunt of the forces placed on the structure. These forces include vertical forces from users and wave action as well as lateral forces from wind, currents, boats, and debris. Connections securing the frame to the floats, and the deck to the frame, are critical.

Dock frames are similar to the superstructure of a traditional trail bridge. At least two longitudinal stringers provide the main structural integrity. The stringers' spacing determines the thickness of the decking (figure 16). Cross braces or floor beams are placed between, or under, the stringers. Cables sometimes are used to brace the frame. Header planks are placed at each end of the section to be flush with the top of the decking. Skirts are sometimes used along the perimeter to protect the frame and to hide the floats and frame. Cross braces and diagonal bracing resist horizontal loads. Stringers resist vertical loads. Longer spans may need stronger or additional stringers. Additional cross braces and diagonal bracing may be needed for sections that are subject to strong currents, high winds, or other lateral stresses. Consult a qualified engineer for design recommendations.

Figure 16—Typical components of a frame. Plastic
or think decking, or a wide deck, will require
additional stringers for intermediate support.

Design recommendations into the frame—not the deck. If possible use backing plates to reinforce the section connections and accessory mounting points.

Floats

The flotation system must be designed to carry all dead loads and anticipated vertical live loads. Calculate dead loads before beginning the design of the float system. The dead load refers to the weight of the materials making up the structure. The following table gives the weights of common deck and frame construction materials.

Material	Weight (pounds per cubic foot)
Wood	30 to 50 (depending on species and treatment)
Steel	490
Concrete	150
Plastics	80 to 100
Aluminum	165

The live load refers to the weight applied on the structure by users and their equipment. In most cases, live loads can be assumed to be 85 pounds per square foot of deck area. If the weight of the water displaced by the floats is greater than the dead loads (including the weight of the floats) and the anticipated live loads, the structure will stay afloat during the maximum loading. To reduce bouncing and the possibility that the structure might overturn when live loads unevenly balanced, a safety factor of at least twice the expected live load is recommended. Design the floats to carry the dead load plus twice the anticipated live load.

Floats usually come with built–in connection points, allowing them to be bolted to the frame. If this is not the case, chambers can be built into the frame so the floats fit securely. Floats can also be lashed to the frame with cable or rope. Some floats have specific load–bearing areas that should be used to connect them to the structure. Failure to use these floats properly can cause them to fail.

Placing the floats along the perimeter of the structure provides the most stability. For very wide structures, such as a viewing platform, place additional floats in the interior for extra support. When using pontoons in areas with a current, orient the pontoon with the end pointing into the current.

Most floats are constructed of polyethylene. Polyethylene floats enclose a large volume with little weight, providing excellent buoyancy. The main shortcoming of polyethylene floats is their poor load–bearing strength when they are placed on land or when they contact debris trapped beneath them. At least 3 feet of water is needed for most floating structures. Polyethylene floats will often fail (fracture, chip, puncture) when they are forced to rest on a solid surface. If polyethylene floats might contact the ground, incorporate additional protection into their design.

Several methods can be used to protect floats from grounding. Self–adjusting docks can be used or the cable anchorage system can be adjusted to keep the structure over open water. Other ways to protect floats include anchored support posts that support the frame when water levels have fallen. The posts block the brackets on the frame or by an underlying crossmember between two posts (figure 17). Another common method of protecting the floats is to lash a timber frame to the bottom of the floats, distributing the pressure on the grounded section. Use support frameworks on all floats subjected to grounding to protect them from damage.

Figure 17—A stop attached to a pile or crossmember placed between two
piles (similar to a trail bent) is an excellent way of preventing a float
from grounding. Ensure that the stop or crossmember and the
structure's framing can withstand the stress.

Many polyethylene float and pontoon manufacturers offer stronger, more expensive floats that will withstand repeated groundings. Metal or rubber floats may be another option where grounding is expected.

Hardware and Connectors

Bolts are better than screws and screws are better than nails when you are assembling a floating structure. Wood swells and shrinks as it becomes wet and dries, loosening nails and screws. Set screw and nail heads even with, not below, the surface of the wood. Use aluminum or stainless steel screws and bolts for aluminum hardware and structural members.

Use hardware specified by the designer or dock supplier. Common brackets, hangers, and angles from the local hardware store or builder's supply center may not be strong enough for the rigors of a floating structure. The hardware distributes loads and stress to adjacent members. Inadequate hardware will cause a floating structure to fail quickly.

Anchorage Connections

An important part of dock design and placement is anchorage and the dock's connection to land. Many floating bridge or dock failures are due to inadequate attention to anchorage and the connection of the structure to the anchorage. Anchorage and connection issues are especially important when the structure is placed in an area subjected to debris, wind or wave action, currents, and ice. Before trying to beef up the bridge or dock, the designer should consider whether the structure could be placed in a more protected area, such as an eddy or a cove.

Usually a floating bridge or dock is made by joining many small sections. The individual sections can move independently as waves pass under them, allowing the dock to move with the force of the waves. This behavior reduces the strength required for the structure, but can make for a wobbly ride. A structure made by joining many small sections often has less surface area (the cross–sectional area of the floats below the water's surface) resisting water currents, than single–piece structures. This reduces horizontal forces on the structure. Repairs to the structure can be accomplished more easily, because the damaged section can be taken out and repaired without affecting the rest of the structure.

The floating end of docks should be anchored, especially when multiple small sections are used. Connections between sections and the connections to the shore should be made as strong as possible by using fasteners and backing plates (figure 18) at connection points.

Figure 18—Fasteners, especially designed for docks, are stronger
than the hardware store hinges.