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 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.

A main span consists of two knee-braced trusses, two transverse beams, two longitudinal beams, and transverse decking. The knee-braced trusses and the longitudinal beams are connected by the transverse beams (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.

Figure 5 shows a knee-braced truss sector viewed from above.

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

The upper and bottom chords consist of two side-by-side placed boards transversely connected by nails but not longitudinally connected (gaps). The verticals and diagonals consist of a single board; they are connected to the cords on the external vertical side by bolts and nuts. Figure 6 shows a connection between a knee-braced truss and a trestle.

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

The chords are connected to the trestle's columns on the internal vertical sides by a single bolt and nut per chord. The figures below show asymmetrically arranged truss members, round holes, cracks, missing connections, and buckling deformation.

Asymmetrically arranged truss members.
Irregular diagonal layout of a knee-braced timber truss.

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

Missing connection and two round holes.
Diagonal without connection.

The bottom nut and bolt of diagonal 1 are placed near the edge, and two round holes are near them. Diagonal 2 does not connect the bottom chord (missing bolt and nut).

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

Longitudinal cracks are noticeable (connections regions).

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 connections.
Knee braces do not connect the trestle column.

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

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.

    A shore span consists of two longitudinal beams and transverse decking.

  • What are other possible main span variants made of timber that probably use fewer resources?

    • © Copyright 2022 - 2025, all right reserved, used with permission.