Problem Statement

For the K'NEX truss structure shown in Figure 1, predict the strength-to-weight ratio (SWR) and estimate the cost in K'NEX dollars. Assume the structure is co-planar and linear-elastic and that the displacements are small such that the principle of superposition may applied. Apply a force of ½ pound at the top red joint of the truss. Assume that the left lower red connector is a pin support and that the lower right red connector is a roller support. Apply a unit load at the top of the structure (the red connector). Let the distance between joints for a yellow rod is 4.25 inches. Also, let E = 377,000 psi and I = 8.8 x 10^-5 in⁴.

Figure 1. K'NEX Truss Structure.

Use the Euler's buckling model with pinned connections for predicting failure in compression members. Use the pull-out forces, listed in Table 1, for prediction failure in tension members. The values of the pull-out force in tension listed above are preliminary estimates and should be used only in this assignment and not in your final design calculations for Project #1.

Table 1. K'NEX Pull-Out Force for Various Modes

Approach

1. Analyze the K'NEX structure shown in Figure 1 using SAP2000 and report the forces in each member.

2. Using the results of SAP2000 and the failure models listed above, predict the ultimate strength of the K'NEX truss.

3. Estimate the weight of the K'NEX truss (use the values listed in the table K'NEX Weights).

4. Estimate the cost of the K'NEX truss (use the values listed in the table K'NEX Costs).

5. Predict the SWR of the K'NEX truss.

It is suggested that you develop a Excel spreadsheet to compute estimates for the ultimate strength, weight, and cost of the K'NEX truss. This spreadsheet will be very valuable in the design of the K'NEX structure for Project #1.

Hand in the following items for this assignment:

1. A printout of this assignment page

2. A printout from SAP2000 of the geometry of the truss with the frame elements labeled

3. A printout of your spreadsheet used to predict the SWR

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.