Design two identical bridge trusses for a bridge that spans across a river, as s

Question

Design two identical bridge trusses for a bridge that spans across a river, as seen in the image below (from https://en.wikipedia.org/wiki/Truss_bridge). Your responsibility is only to design and produce two complete bridge trusses. Because the bridge uses two identical trusses for stability, each truss must therefore support one-half of the load.

Here are the requirements: Span: 30m Load: total load of 3000 kN, equally distributed over all of the horizontal pin joints Structure Requirements: Good starting designs for bridge trusses are the Warren, Howe, or Pratt. Use your textbook and the internet for additional information. Each truss member must be a steel beam as specified from the table below The joints are held together with gusset plates but are modeled as pin joints One support is a pin joint and the other support is a roller

Explanation / Answer

Bridge Basics

Because of the wide range of structural possibilities, this Spotter's Guide shows only the most common fixed (non-movable) bridge types. Other types are listed in the Bridge Terminology page. The drawings are not to scale. Additional related info is found on the other Terminology pages which are linked to the left.

The four main factors are used in describing a bridge. By combining these terms one may give a general description of most bridge types.

The three basic types of spans are shown below. Any of these spans may be constructed using beams, girders or trusses. Arch bridges are either simple or continuous (hinged). A cantilever bridge may also include a suspended span.

Decide on a truss configuration.

Create the structural model.

Check static determinacy and stability.

Calculate reactions.

Calculate internal member forces.

Determine member sizes.

Check member sizes for constructability.

Draw plans.

Create a schedule of truss members and a schedule of gusset plates.

Build the bridge.

Create the Structural Model Having selected a truss configuration, we will now model the structure, by defining

(1) the geometry of the truss,

(2) the loads, and

(3) the supports and reactions—just as we did in Learning Activity #3. We idealize the three-dimensional bridge as a pair of identical two-dimensional trusses. The geometry of one main truss is shown below. The dimensions indicate the locations of the member centerlines. Joints are identified with the letters A through M. Note that the dimensions of our structural model are all consistent with the dimensions shown for Truss 16 in the Gallery of Structural Analysis Results. The Gallery shows that each of the six top chord members has a length L. To achieve a total span length of 60cm, as the design requirements specify, we must use L=10cm. Now the remaining dimensions are calculated using this same value of L. For example, the Gallery shows the overall height of the truss as 1.375L. Since we have defined L as 10cm, the height of our structural model is Once we have determined the geometry of the truss, we can calculate the loads. According to the design requirements, the bridge must be capable of safely carrying a 6-kilogram mass placed on the structure at midspan. The weight of a 6-kilogram mass is Again we will apply this load by placing a stack of book.

Again we will apply this load by placing a stack of books onto the top chord of the truss. The weight of the stack will be supported on six joints—C, D, and E on each of the two main trusses. Assuming that the weight of the books will be distributed equally to these six joints, the downward force applied to each joint is

Geometry of the main truss. 5-8 Note that we could have gotten this same result directly from the Gallery of Structural Analysis Results. The diagram for Truss 16 shows that a downward load of 0.1667W is applied to each of the three center top-chord joints. For a total load W=58.86N, the load at each joint is A complete free body diagram of the truss looks like this: Free body diagram of the main truss. The bridge will be supported only at its ends; thus, the reactions RA and RG are shown at Joints A and G.

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