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Design of a Wooden Truss
Group 6
Engr 220
Sec 002
Statics and Mechanics of Materials
Dr. Hadala
10-18-00
Executive Summary
Our Engineering team was requested to design
a wooden truss made solely of 20 3/8”x36” dowels and 20 1.5”x3.5”x3” blocks.
The truss must be connected together with our choice of glue. For
the glue we chose elmer’s wood glue, mainly because we have tested the
glue before and know it’s strength. Realizing our inexperience in
truss building we took our capabilities into consideration when choosing
a truss. After we reviewed many designs we narrowed it down to four
designs, which we then analyzed and came to a conclusion. In testing
our truss will be supported at the ends (50 inches apart), and loaded in
the center. The design we chose was possible for us to build and
the member with the most stress has only 73.53% of the total load.
Our joints are glued and inserted 1.5” into the blocks. In an analysis
done earlier it was found that a 3/8” dowel glued 1.5” into a block of
wood could hold an average of 705 pounds. Using this information
it was calculated that our truss could hold up to 1916 pounds, almost one
ton. We concluded that through our design’s simplicity we used less
material and constructed the truss in less time, while still supporting
a very large load.
Design Objectives
Our problem was to design the strongest possible
truss with the available materials. This must be built using no more
than 20 3/8”x36” dowels, and no more than 20 1.5”x3.5’x3” blocks.
These truss components are to be joined with the glue of our choice.
We decided to use Elmer’s wood glue as was mentioned earlier, we know it’s
strength, which will be helpful in calculating the failure load.
While considering many truss designs we were restricted to a truss that
could be no more than 20 inches tall and 10 inches in width. The
truss would have two supports, 50 inches apart and a load applied at a
point, centered between these two supports. The supports are 1 inch
steel bars. See figure 1.
Figure 1.
When analyzing different truss designs we found that a taller
truss design optimized angles between members and allowed the truss to
withstand a greater load. We also found several truss designs that
could hold similar loads, but choose the simplest design, due to our inexperience
in truss construction, to insure quality truss.
Design Assumptions
When designing the truss, we assumed that the materials would
perform, in the same manner, as in the Strength of Glued Joints experiment.
We also assumed that the supports would be exactly 50” apart and the load
applied at the center. Another assumption was that buckling would
not be a factor in failure since the members in compression were under
a much smaller load than the member under the highest load, which was in
tension.
Material Properties
In the strength of glued joints experiment, we discovered the
“3/8” inch dowels glued into 1.5 inch holes could withstand a mean force
of 526 pounds. We also discovered that the majority of the time the
glued joint failed, and not the dowel.
Design Alternatives Considered
The truss we selected (figure 2.) was chosen
because of it’s ability to handle a heavy load and it’s simplicity of design.
Here we have each of the four trusses we analyzed (figure 2.-5.).
Figure2.
Figure 3.
Figure 4.
Figure 5
Design Calculations
Calculating the forces in our truss, we used a software package by Timothy A. Philpot, called MD Solids. The Software calculated every force, stress, and angle, in each truss we were analyzing. We of course checked the calculations of the software using the method of joints and the method of sections. In Figure 6 you will find all the calculations for truss #1, the truss we decided to build.
Figure 6.
Predicted Failure Load
We predict that the center bottom horizontal member (Figure 2.
CE) will fail when a load of 1916 lbs is applied at the top center of the
truss. We found the load in the member using the method of joints
and feel that the member will fail at one of the joints because of information
gathered during a previous experiment.
Fabrication
To begin fabrication of the truss, we first laid out the position
of the wooden blocks to be used as the joints, and then measured the lengths
needed for the top and bottom members compensating for the depth of the
hole. A bandsaw was used to cut the members to the desired lengths,
and a drill press was used to drill the holes one and a half inches into
the blocks. We then measured the distances for the interior members
of the truss, and also cut these to length with a bandsaw. Elmer’s
wood glue was used to glue the members into each block. To complete
the fabrication, we cut a notch with a bandsaw in each support block.
Estimated Cost of Design
After completing the truss, our final cost of design came to
eight-hundred and sixty dollars. We arrived at this figure using
a billing rate of sixty dollars an hour, for twelve hours of engineering
services, and thirty-five dollars an hour, for four hours of fabrication
work.
References
Hall, David E., “Course Notes for Engineering 220,” Louisiana Tech
University,
College of Engineering and Science, Fall 2000.
Hadala, Paul, “Course Notes for Engineering 220,” Louisiana Tech University, College of Engineering and Science, Fall 2000.
Riley, Sturges, & Morris, Statics and Mechanics of Materials: An
Integrated Approach, pp. 68-70, John Wiley & Sons Inc. 1995.
Philpot, Timothy A., MD Solids Version 1.7.5,
Educational Software for Mechanics of
Materials, www.mdsolids.com
Appendix 1.
Excerpt From “Strength of Glued Joints Lab”
Load at Breakage for .375” dowels, glued in wood at a depth of 1.5”. All measured in pounds.
787
557
1093
707
744
674
768
677
339
The Average load at Failure is 705 lbs
The Standard Deviation is
200lbs