Design Project

Design of a Wooden Truss

(Adapted from David Hallís Web Page)


Design a wooden truss or a set of parallel trusses to achieve the highest load capacity to truss weight over a span of 40 inches, as shown below. 

The truss will be simply supported at both ends, and a point load will be applied at the center.

Each group will be supplied with:

9 dowels 1/4 inch in diameter and 36 inches long;

9 dowels 3/8 inch in diameter and 36 inches long;

14 pieces of 3 inch x 1.5 inch x 3.5 inch pine boards for strengthening the joints.

 

The following design criteria must be maintained:

 

1)     The height of the truss is less than or equal to 24 inches;

2)     The width of the truss or trusses will be less than or equal to 10 inches (the distance into the paper).††

3)     Because all external loads must be applied at the joints of the truss, a joint must be in the center of the top cord.

4)     The truss must be one that can be analyzed using the methods given in class (no beams are allowed).

5)     Only the wooden blocks and dowels provided by the college may be used.

6)     The blocks cannot be cut to bring the total number of joints to above 14.

7)     No shaving, drilling, sawing, or sanding is allowed so that the weight of a block or dowel can be decreased.

8)     Students must use Elmer's wood glue, since joint strength experiments (lab 1) used this glue.

9)     No other types of fasteners such as nails, screws, bolts, tape, bondo, or wire is allowed.

All contact between the truss and the loading fixture must occur at a joint, and only the wooden blocks at the joints should touch the loading fixture, as shown below.  Contact between a member and the supports will likely lead to early failure. 

 

To prevent the truss from sliding off of the supports, cut a small notch in the center joints to receive the loading bar, as shown below.

A drill press will be provided for drilling holes into the wood blocks to receive the dowels. A small handsaw will also be provided for sawing the dowels. The use of power tools other than a drill is discouraged, as injury could occur. You must supply your own safety glasses and measuring devices when working in Bogard Hall, and you must be given permission and safety instructions before operating any machinery in the lab. We recommend that you assemble your truss prior to gluing to be sure that all of the parts fit properly and that the overall dimensions are correct.  All gluing done in Bogard Hall must be completed on the cardboard covered table in BH 118.

The grading will be based on the design report (60%), the design presentation (30%), and the performance of the truss (10%).  Each group should supply the following in their report:

1)     A dimensioned sketch of their truss, with members in compression in blue, tension in red, and zero force members in yellow;

2)     Calculations showing the stress in each member for an applied load of 1,000 lbs;

3)     A prediction of the mode (pullout of a dowel at a joint, buckling of a dowel, breaking of a dowel outside a joint) and location of failure; and

4)     An estimate of the peak load that their truss will carry.

The required format for the report is given HERE! The required joint labeling scheme is given HERE, and the required format for summarizing your truss calculations is given HERE.

GRADING BASED ON PERFORMANCE:  The performance of the truss will be evaluated based on the performance of the best truss in the class.  The performance (which will count as 10% of the project score) will be computed as follows:   

1)     the truss from all sections combined which achieves the highest load to weight ratio will receive the full 10 points;

2)     the trusses which achieve the mean load to weight ratio of all sections combined will receive 6 points;

3)     the truss which achieves the lowest load to weight ratio of all sections will receive 2 points; and

4)     any truss which carries less than 150 lbs will receive 2 points.

The grades for teams whose truss achieves a load to weight ratio between the minimum and mean will receive between 2 and 6 points based linear scaling, while those teams whose truss achieves a load to weight ratio between the mean and the maximum will receive between 6 and 10 points.

Grading guidelines for the presentation are given HERE!