Bridge Problem Statement

A century-old bridge that crosses a river valley in a mountainous region must be replaced. The bridge carries heavy truck traffic to and from mines that are the basis for the economy of this rural region, as well as providing access and emergency services to residences. A quick replacement is necessary because no other crossing is available for miles.

The Tennessee State Department of Transportation (TDOT) has requested design/build proposals for replacing the existing bridge. Any appropriate type of bridge will be considered, but TDOT has specified wood as the material because of its availability and ease of construction. The bridge must carry specified patterns of traffic and wind loads without exceeding deflection limits. In order to expedite environmental approvals, no piers may be used in the river.

The stone abutments of the existing bridge are in good condition and will serve for the new bridge, provided that no lateral thrust or uplift is applied to the abutments. TDOT will not permit modifications of the existing abutments. The new bridge, when complete, must be supported only by the existing abutments; for example, stays and anchorage to the riverbanks are prohibitive.

Your company's design/build proposal is among those that TDOT has deemed responsive. TDOT has asked each competing firm to submit a 1:20 scale model to demonstrate its concepts. TDOT will evaluate the models by multiple criteria including efficiency and economy. The contract will be awarded to the company that submits the best model.

For design purposes, assume Southern Yellow Pine has a modulus of elasticity of 1 x 10⁶ psi, an approximate unit weight of 0.025 lbs./in.³ and an allowable stress in bending of 875 psi (extreme fibers in bending).

Safety

Safety has the highest priority. Judges are directed to disqualify bridges that cannot be safety constructed or load tested using the abutments and other equipment provided. Collapse or deflection in excess of limits specified in these rules will result in disqualification.

Scoring

Categories of competition are efficiency and economy. The efficiency of a bridge is measured by the sum of the normalized weight and deflection (SNWD). The SNWD is computed as:

SNWD = Bridge Weight (lbs.)

+ [50 lbs./in. x End-Span Vertical Deflection (in.) Load Case 1]

+ [50 lbs./in. x End-Span Vertical Deflection (in.) Load Case 2]

The total cost of the model must not exceed $50.00. If the cost of the bridge exceeds the upper limit, an additional weight penalty will be imposed as follows:

Total Weight = Bridge Weight (lbs.) + (Cost - $50) x 10 (lbs./$)

Bridge Construction Rules

1. Bridges must be constructed using Southern Pine lumber and plywood. Standard shapes may be cut to form any section required by the design team.
   

2. Any type of fastener or glue is allowable.
   

3. Members of the bridge cannot be coated or treated in any way.
   

4. The bridge must be designed to fit on the support shown in Figures 1 and 2. Members must span between the tops of the support to simulate a road surface. Members may brace off only the top surfaces of the support. Members may not brace off the sides or the horizontal bottom of the support.

Figure 1. Bridge Supports and Geometrical Constraints

5. The bridge span is 6 ft. with two 3-ft. cantilever sections. The minimum width of the bridge is 24 in. The bridge must accommodate the traffic simulator shown in Figure 3.

Figure 2. Clearance and Deck Supports

Figure 3. Traffic Simulator

6. All bridges must support two load cases. The first load case is a 1,000-pound load, distributed over 2-ft. at the center of the bridge (see Figure 4). The second load case consists of an additional 200-pound load distributed over the end 2-ft. of the cantilever (see Figure 5). Failure is defined as collapse or deflection resulting in contact between the bridge and the horizontal base of the support.

Figure 4. Load Case 1

Figure 5. Load Case 2

7. The traffic deck may be no more higher than 6 in. and must span the entire length of the bridge.
   

8. Each team may submit only one bridge.
   

9. Bridge performance will be measured by a SNWD. The team with the lowest SNWD will be awarded the contact.

Bridge Report

1. A written report is required for each wood bridge submitted for evaluation. The content and quality of the report will account for 70% of the project score. The remaining 30% of the project grade will be determined by the strength of the bridge based on the sum of normalized total weight and deflection (SNWD). There are two strength criteria for bridges: 1) all bridges must have a SNWD < 75 (20% all-or-nothing - bridges with a SNWD < 75 receive 20 points; bridges with a SNWD > 75 receive no points); and 2) the bridge with the lowest SNWD is awarded full points (10%) and the remaining bridges will be awarded scores commiserate with their performance (first place will be awarded 10 points, second place 8 points, third place 6 points, etc.). If a bridge does not meet the construction rules, listed below, the submitting student will receive no bridge points.
   

2. The bridge report should include, but not limited to the following:

   I. Title page - name, date and course information

   II. Introduction - design and report requirements

   III. Bridge Design - a complete set of plans for the bridge

   IV. Bridge Analysis - a complete design and analysis of the bridge including: the location of the point on the bridge where the shear force and the bending moment are maximum for the applied moving loads, the variation of shear force and bending moment for the dead loads, and estimation of the deflection at mid-span for both load cases. Also, include a prediction of the SNWD for the bridge. Show evidence that you refined and optimized your structure during the design process. Assume the modulus of elasticity of Southern Pine is 1 x 10⁶ psi, the unit weight is 0.025 lbs./in.³ and the allowable stress in bending is 875 psi (extreme fibers in bending).

   V. Summary - summarize the strengths and weaknesses of your design and give a prediction for the SNWD.

3. Your report should be printed on a laser quality printer. If possible, all bridge sketches should be produced using graphical software.

Schedule

This web site was originally developed by Charles Camp for CIVL 3121. This site is maintained by the Department of Civil Engineering at the University of Memphis. Your comments and questions are welcomed.