General Information

Figure 1 shows a truss bridge that is used by motorcycles, bicycles and pedestrians.

Truss bridge.
Wooden truss bridge over a river.
Main span ≅ 10 m
Type Underslung truss bridge
Truss material Timber
Truss height ≅ 1 m
Deck width ≅ 1.5 m

Knee-Braced Timber Trusses

Figure 2 shows a schematic lateral view of the bridge.

Lateral view.
Schematic lateral view of a timber truss bridge.

A shore span s ≅ 5 m, and a main span m ≅ 10 m. The former consists of two longitudinal beams (shown in red) and transverse decking, while the latter consists of two knee-braced trusses, two transverse beams, and two longitudinal beams and transverse decking (as the shore span). The shore and main spans are supported by timber trestles, and the foundation consists of RC piles. Figure 3 shows a main span.

Main span.
Main span of a timber truss bridge.

Longitudinal beams and knee-braced trusses are connected by the transverse beams, which are placed in the mid-span region. The mid-span vertical has a larger cross-section and is also used as a barrier post, as shown in figure 4.

Mid-span vertical.
Mid-span region of a timber truss bridge.

Upper and bottom chords consist of two side-by-side placed boards, while verticals and diagonals consist of a single board. The truss members are connected by nails and bolts and nuts. Verticals and diagonals are connected to the upper and bottom chords on the external vertical side. Figure 5 shows a view from above of a knee-braced truss sector.

Knee-braced truss sector (viewed from above).
Knee braced timber truss sector with gaps.

The chords (upper and bottom) are connected in the transverse direction (nails), but not longitudinally (gaps). The connection to the trestle columns is placed on the internal vertical side and consists of a single bolt and nut per chord, as shown in figure 6.

Knee-braced truss to trestle column connection.
Connection between knee-braced timber truss and column.

The following figures show an asymmetrical arrangement of the truss members, empty holes, timber cracks, missing connections, and buckling deformation.

Asymmetrical arrangement of the truss members.
Irregular diagonal layout of a knee-braced timber truss.

The diagonal layout is asymmetric, and there is a field without diagonal.

Missing a bolt and nut and two drilled holes.
Diagonal without connection.

The bottom chord connection region of diagonal 1 has three drilled holes (two of them are empty), and the bottom bolt and nut are placed near the edge. Diagonal 2 does not connect the bottom chord.

Longitudinal cracks on a diagonal.
Longitudinal cracks on a diagonal.

Two longitudinal cracks are noticeable.

Knee brace size and arrangement.
Different knee braces of a timber truss.

Knee braces 1 and 2 are different in size, inclination, and bottom chord connection position (internal and external sides).

Missing bolts and nuts.
Knee braces do not connect the trestle column.

The knee braces are not connected to the trestle column, as bolts and nuts are not installed.

Buckling deformation of knee braces.
Buckling deformation of two timber knee-braces.

Buckling deformation is noticeable, especially on the marked brace.

  • How simply and precisely can the structural behavior of the knee-braced trusses be predicted?
    How efficient are the knee-braced trusses?

    • Main Span Variant

    Figure 13 shows a schematic three-dimensional view of a bridge sector.

    Three-dimensional view.
    Three-dimensional drawing of a timber truss bridge sector.
  • What are other possible main span variants made of timber that probably use fewer resources?

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