General Information
Figure 1 shows a pedestrian truss bridge.

Main span | ≅ 24 m |
Type | Deck truss bridge |
Truss material | Steel |
Truss height | ≅ 1 m |
Deck width | ≅ 1 m |
Truss Configuration
Figure 2 shows a schematic three-dimensional view of the bridge.
The girder consists of a bow barrel truss with upward-sloping diagonals toward the mid-span; the rise r ≅ 1.5 m. The deck is made of timber planks, and the mid-span region has a width enlargement of about 0.5 m. Figure 3 shows a schematic three-dimensional view of a girder sector.

The bracing consists of inverted V-braces and plan cross braces; the latter are placed on the bottom chord level. Mid-width verticals (red shown) are installed along the entire span, and the floor system is framed. Figure 4 shows a truss girder sector viewed from below.

The stringers are connected to the upper cross members, and the mid-width verticals and the inverted V-braces join under the upper cross-members and are connected to them. The stringers are made of rectangular hollow sections, the plan cross-braces are made of rebars, and the remaining members are made of circular hollow sections.
Deck Truss Bridge Vibration
Figure 5 shows a lateral view of the bridge in the abutment region.

The bottom chords are connected to the abutments by base plates and anchor bolts; vertical and horizontal displacements are assumed constrained. The upper chords are connected to the timber structure of the entrance stairs. Figure 6 shows the bridge in the mid-span region.

The two arrows represent the foot-induced vibration forces by a single person in a stationary position. The camera was placed on the deck at mid-width axis; video 1 shows the vibration.
What are some possible reasons for the above shown vibration?
A schematic lateral view of the above shown bridge and a variant with a rise of 2r is shown in figure 7.

Except for the rise, the two bridges are equal .