Attributes linked to this design project (CEAB's Graduate Attributes):

> Knowledge base for engineering

> Problem analysis

> Investigation

> Design

> Use of engineering tools

> (Individual and) Teamwork

> Communication skills

> Life-long learning

OBJECTIVES:

·  Design and build a bridge which can support 400N trainload over a 1060 mm span.

·  Design and build the strongest yet the lightest weighing bridge.

·  Apply concepts and theories from the lecture to predict the maximum load the bridge can take before failure occurs.

·  Use the engineering drawings to explain the design of the bridge.

·  Imply project planning and prototyping to build the bridge based on the design.

·  Demonstrate design decisions and documentation of the process.

BRIDGE CONSTRUCTION CONSTRAINTS:

• The full length of the bridge must be between 1280 mm and 1325 mm. 

• The height of the bridge at the two support points must be less than 200 mm. 

• The deck of the bridge must be horizontal, be at least 100 mm wide and permit unhindered passage of the train. There can be no steps on the top surface for the train wheels to climb over. 

• The support plates and loading plates used to apply point loads in Load Case 2 are 30 mm x 100 mm.

INTRODUCTION

In a group of three, we were given a sheet of matboard with dimensions 32’’ x 40’’ x 0.05’’ (813 mm x 1016 mm x 1.27 mm) using which we had to design a (thin-walled) box-like bridge that could pass “Load Case 1” of 400N and withstand a maximum combined load of 603.5N in “Load Case 2”. 

DESIGN MEASUREMENTS

Some changes were made from Design0 (the sample design provided):

HEIGHT

The total beam height from the Design0 value of 75 mm was increased to 83.81 mm (80 mm web height + 3*1.27 mm layers). This increased the bridge’s second moment of area, which would make it more resistant to the bending moment that the beam experiences. This in turn would make it less likely to fail from both tensile and compressive failure of the walls.

FLANGE WIDTH

The bottom flange width from the Design0 value of 80 mm was decreased to 60.70 mm, and the top flange width from the Design0 value of 100 mm was slightly increased to 100.28 mm.

GLUE-TAB WIDTH

The glue tab width from the Design0 value of 11.27 mm (10 mm + one 1.27 mm layer, assuming a 90-degree fold which is a safe overestimate) was increased to 16 mm. All four of these design decisions were made to increase the second-moment area (I) of the beam.

LENGTH OF THE BRIDGE

We increased the total length of the bridge from the Design0 value of 1280 mm to 1300 mm. 1280 mm was the bare minimum length allowed but proceeding with this was risky due to the high likelihood of construction errors and imperfections. So, we decided to add an additional tolerance of 10 mm on either side to account for this. 

CONSTRUCTION PROCESS AND DECISIONS MADE

Based on the Metric/Criteria/Constraint given, we evaluated each design separately and tried to find any solutions on how each design could be improved. For a zigzag design, increasing the self-weight of the matboard bridge was required and the number of steps to finish the building of the bridge but endured the failure load. But the PI beam design, compared to the zig-zag design, could endure the failure load, merely increased the self-weight of the bridge and the design was not complicated to build as well. The predicted failure load for the train load-case was 1540 N, which was enormously great. For loadcase-2 the failure load was 1400 N. After taking estimates, we realized that the zigzag design was feasible and went along with it.

ENGINEERING DRAWING

Before starting the construction of the matboard bridge, I made an accurate engineering drawing with all dimensions required for construction, as shown below:

CONSTRUCTION

As can be seen in the figure below, we took a zig-zag design approach for the construction of the bridge.

TESTING

The bridge eventually failed the first Load Case test: the 400N train test. We were expecting a similar outcome (Figure 3.3.6) as the contact cement used to attach the piece of matboard together required 72 hours to strongly bond, but we only allowed less than 8 hours for it to settle.