Design

Preliminary Sketches

The design phase began with the team creating a set of sketches, for brainstorming purposes. Sketches were created by hand or in AutoCAD. There was no limit to the number or style of the sketches, the only guidance was to keep in mind the number of connections, in attempt to keep construction time at a minimum. A total of 10 sketches were created. The sketches created by the team are shown to the right.

Modeling

Sketches were used as a start for modeling. Three models were created within RISA-3D, based on a visual inspection that determined these sketches would have the easiest construction and lowest weight. Models were made with two footers as pins and two as rollers, any welded connections modeled as fixed, and any bolted connections modeled as pinned. Once the models we functional the loading was applied. For the lateral load a distributed load was modeled for the weight of the decking and the 75 lb counteractive weight, while a point load was modeled for the 50 lb applied load. Loads for the vertical test were all modeled as a distributed load divided in half and placed on each stringer. The image to the left shows an example of how the loading was applied. There were 11 possible loading positions and the loading process was done for each to determine the worst case scenario deflections. All models were successful and within the required lateral and vertical deflections, 0.75 in and 3 in respectively.

Design Selection

With three functional models, a decision needed to be made to move forward to reach a final design. A decision matrix was used to score each model. Bridges were scored based on lateral and vertical deflection, weight, constructability, and aesthetics. Weight and Constructability were weighted the highest at 30% as these categories impact the competition results the most. Lateral deflection was the next highest at 20%, followed by vertical deflection at 15%. Lateral was weighted higher than vertical because the bridge needed to pass lateral to get the vertical load tested. Aesthetics were weighted the least because functionality was considered more important than looks.

Within the decision matrix actual values were listed to be used in scoring. Aesthetics were ranked 1, 2, or 3 with 3 being the best. To determine the raw score, the highest value was given a score of 1, then the remaining bridges actual score was divided by the highest for that category. Raw scores were then weighted and all categories were totaled to determine the overall bridge score. A lower score is favorable.

Option 1 and Option 2 scored closely, which lead to both moving forward to be refined. Both models were modified with the goals in mind of decreasing weight, number of pieces, and deflections. Once the models had been improved another decision matrix was used to select one model. Scores were determined the same way in the second decision matrix, and ultimately led to Option 2 being selected, as shown below. This selection was not the final model, just the only model that would be further improved.

Connection Design

To design bolted connections, first the number of unique connections needed was identified. It was determined that there would be 4 unique connections; one for the top of each truss member: one for the bottom of each truss member, one for straight cross bracing, and one for diagonal cross bracing. The SSBC rules were referenced for the design of each connection as they had specific constraints, such as the number of faying surfaces (times the face of a member touches another). The unique connections are shown below.
Using the AISC Steel Construction Manual, the number of bolts required for each connection was determined to ensure each connection would be adequate for all failure modes. A similar process was followed to design welded connections. Unique weld styles were determined at appropriate bridge locations. Calculations were completed to determine the strength of the weld and base material.

Connection 2: Top of Truss

Connection 3: Straight Cross Brace

Connection 1: Bottom of Truss

Connection 4: Diagonal Cross Brace

Shop Drawings

With design completed, the team went on to create a set of shop drawings. This plan set was crucial as Flagstaff High School (FHS) would be welding the bridge for the team. The drawings included a material and parts schedule, multiple views, and connection details.

See Shop Drawings

Materials and Fabrication

Page Steel

Page Steel donated any steel that was requested by the team, for construction of the bridge. Thank you Page Steel!

Copper State Nut and Bolt

Copper State donated any nuts and bolts needed for construction of the bridge.

Flagstaff High School

Mr. Rust at Flagstaff High School and his welding class welded any required members, as well as drilled holes for some of the connections. Thank you Flag High!

Team Fabrication

With all other fabrication complete the team finalized the bridge. This involved some cutting, welding, and drilling holes.

Cost of Construction

Cost of construction included any materials, subcontracting, or travel needed for completion of the project (all categories of the original cost estimate, except for staffing). Materials were donated, but a dollar amount was included for an actual estimate of cost. The total cost came out to be $13,539.45.

Competition

Summary of Results

With fabrication complete, the team took their bridge to the Intermountain Southwest Student Symposium, where the bridge competed in the SSBC. At competition timed construction was completed, followed by lateral then vertical loading. The results of the competition were not ideal, but the team anticipated a larger deflection than the model predicted due to error in fabrication. Overall the team learned a lot about design, fabrication, and construction from this project and considers it a success.