DESIGN OF A STEEL BRIDGE TRUSS SYSTEM

Figure 1 shows a 15 m span steel bridge truss in Townsville, which was designed and built many years ago. This bridge truss system is based on two steel trusses, which include two parallel chords (top and bottom) and a series of web (diagonal and vertical) members. The trusses span between two concrete piers and are simply supported at the ends. At present, this bridge is not in use, and the Main Roads Department is considering replacing it with a new bridge that has the required capacity to support the design traffic loads. For the new bridge, a new truss system is proposed as shown in Figure 2. As part of the design of the new bridge, there is a need to undertake detailed load evaluations, structural analyses and design checks. Figure 2 shows the schematic diagram of the new truss system, its dimensions and member and connection details. The two trusses located 7.2 m apart support a lightweight concrete deck system. A bracing system consisting of steel members provides lateral stability by connecting the two trusses at the top and bottom via their joints as shown in Figure

2. Preliminary design calculations have shown that the following steel member types and sizes are suitable for top and bottom chords, diagonal and vertical members with welded connections.

Top chord: 150UB14 Bottom chord: 250UC72.9 Diagonal and Vertical Members: 125PFC

Verify the structural adequacy of these steel sections and determine whether the new bridge truss system can be used safely. Your group (four members) has been asked to investigate the structural adequacy of the new bridge truss system and to recommend suitable modifications. If the design is found to be unsafe or over-conservative, revise the design to have the effective use of UB, UC and PFC steel members for top/bottom chords, diagonal and vertical members, respectively. In your analyses and calculations, assume that the truss joints and supports are pinned.

The design actions for the strength limit state of this bridge truss were calculated using the Australian action codes. They are given next.

• The permanent actions due to the various bridge components are as follows: Self-weight of each steel truss including its bracing members = 2.0 kN/m; Lightweight concrete floor system and its bracing members = 1.8 kPa.

• The imposed action due to vehicle traffic can be taken as 2.5 kPa. This is a simplified static load case, considered equivalent to the loading given in AS 5100.

• The wind actions on the truss are 1.5 kPa and (-) 4.0 kPa in the downward and upward directions, respectively.

• Serviceability deflection of the truss should be limited to span/250. However, the deflection check for trusses can be assumed to be satisfactory.

Your tasks are as follows:

Task 1: Investigate the bridge truss structure and the supporting system, develop a good understanding of the load path (permanent, imposed and wind actions), identify the main load carrying members in the bridge truss system including their connections and group them appropriately.

Nominal loads x Load factors

• 1.35 G • 1.2 G + 1.5 Q * – common combinations

• 1.2 G + 1.5 l Q l – long term factor

• 1.2 G + Wu + c Q c – combination factor

• 0.9 G + Wu *

• G + Eu + E Q (earthquake action): E – Earthquake combination factor

*l, c and E are given in Table 4.1 of AS1170.0 & notes

Task 2: Undertake the load evaluation for the bridge truss structure and calculate the loads to be applied at the truss bottom chord joints, determine the design action effects for the truss members using the method of joints and/or method of sections, and investigate the strength limit state adequacy of the new steel bridge truss system using AS 4100 (ie. top and bottom chord, diagonal and vertical members). If the chosen member sizes are found to be inadequate (either unsafe or over-designed), recommend suitable improvements based on revised section sizes. Assume welded connections between continuous top and bottom chords to web members.

All in Australian standards

Figure 1: Existing steel bridge truss in Townsville

Figure 2: New bridge truss system – Schematic Diagram

Figure 3: Typical Bolted Connection System