Panel-Lam Timber Vehicle Bridges
TIMBER SLAB SPAN BRIDGES
Currently there are about 45,000 Treated Timber Bridges in the National Bridge Inventory. The largest proportion of these are longitudinal timber slab span structures. These structures do not have separate beam and deck elements, but rather a solid deck slab as the complete superstructure.
In the early 1970’s Wheeler developed the Dowel-Laminated Panel-Lam Deck System as an advanced way to build longitudinal slab span bridges. For over 30 years the treated timber Panel-Lam has remained an effective way to build short and medium span bridges. Wheeler has gained experience through thousands of installations to evolve the Panel-Lam into a bridge system rivaling any contemporary design regardless of material (i.e. reinforced concrete, steel, plastic, etc.).
Built as single or multiple span configurations the Panel-Lam is well suited for most site conditions. Individual span lengths from 10 feet up to 40 feet are typical. Longer spans have been used in special applications.
The longitudinal panel design of the Panel-Lam is successfully used for staged construction, eliminating lengthy detours or temporary structures.
Unaffected by temperature, the Panel-Lam is ideal for winter construction projects in cold weather climates.
Timber treated with an oil-borne preservative is unaffected by corrosive action of deicing chemicals and withstands repeated freeze thaw cycles.
The Dowel-Laminated Deck System is designed in accordance with AASHTO standards. All typical highway loadings can be applied including HS20, HS25 and HL-93.
The superior strength-to-weight ratio of a timber slab design always provides the thinnest depth of structure. This allows for more head room clearance and can reduce approach work by limiting raises in grade.
The entire system is very redundant. Numerous members coupled with the use of ductile fasteners create thousands of load paths. This massive load sharing ensures the weakest member does not define the system’s strength.
Panel-Lam superstructures are compatible with all substructure and foundations designs. Details are available for attaching to all pile types and concrete.
The Panel-Lam is a proven concept for providing prefabricated components to build bridges in minimal time. Quality is controlled with assembly of components at the manufacturing plant. All pieces arrive on site with material certification ready for installation. Fast installation reduces road closure, detour delays and construction inspection expense.
Individual planks are shop assembled with dowels into panels that comprise the slab span. The pattern of the dowels and the deck thickness are a function of the span length and design load. Dowels are usually positioned in two rows near the top and bottom of the panels and spaced at one foot. Longer spans may utilize three rows. Laminates are added two at a time until the desired panel width is achieved. Penetrating four laminates, the dowel pattern is staggered and repeats to avoid driving dowels into each other.
The panel width, approximately seven feet, is a function of the overall width of the structure. Panels are custom built in the shop for each project and the width can be adjusted for easier handling.
The panels are banded for increased stiffness during transport. Banding is removed after the panels have been placed.
A splice block is added as an important part of the Longitudinal Ship-Lap Joint that joins adjacent panels. Field installed dome-head drive spikes fasten the upper and lower splice blocks of adjacent panels. Coupled with a field installed transverse spreader beam, the ship-lap joint increases lateral distribution of wheel loads to a wider portion of the deck. Longer spans may incorporate multiple spreader beams to produce a stiffer deck. This is important for asphalt wear surface performance.
The spreader beam attaches with 3/4 inch diameter dome-head bolts through the deck. Pre-drilled holes for the spreader beam are shipped with eye-bolts used to lift the panels.
For bridges requiring an asphalt surface the individual laminates are allowed to vary in width. When assembled vertically, the bottom surface of the panel is smooth and the top is irregular. This allows the bituminous mixture to grip the panels with no reflection of the irregularities showing through to the surface. This has been demonstrated as the best method of applying asphalt to timber deck systems.
When a timber wear surface is desired the laminates are planed to a consistent width before the panels are assembled. Except for limitations with the deepest panels this provides a near smooth surface.