A3-Durkin

1.            The test bridge used when calculating the Methods of Joints analysis had a span of 24", a height of 6", and a load of 10lbs on the middle joint.

Figure 1: Page One of Sample Bridge Methods of Joints Analysis

Figure 2: Page Two of Sample Bridge Methods of Joints Analysis

2.

Figure 3: Test Bridge Forces Diagram

Member	Force
TBA	-26.0N
TBC	26.0N
TBD	-27.4N
TCD	26.0N
TAC	13.7N
TCE	13.7N
TDE	-26.0N

3.

Figure 4: Test Bridge Truss Bridge Designer Analysis

4.         Bridge Designer, a computer program, can calculate nearly the same analysis as shown above in Figures 1 and 2.  This program can scale the length, width, and load of any trust bridge design that is entered into it.  This allows for easier adjustments when doing the analysis.  The problem with scaling the bridge in Bridge Designer is that the outputs Bridge Designer gives then need to be adjust to fit the scaling.  The outputs can be adjusted by multiplying or dividing the numbers by a factor such as two.

5.

Figure 5: Knex Truss Bridge Designer Analysis

           As shown in figure 5 our group scaled down our bridge and built it on the Bridge Designer website.  We found that the greatest force on our bridge according to the website was 90 when 20lbs of weight was applied to the midpoint.  The forces on each member on the outer edge increased as you approach the midpoint.  The cross pieces each seemed to have the same force on them which would mean that the weight was exactly evenly distributed throughout the cross sections which is not very likely. Since none of our members had a force greater than 100 the bridge would hold up if this weight was applied.  According to the Bridge Designer website the bridge would stay standing with a 20lb weight applied but it would not be able to carry much more since the max force of 90 was so close to 100.

6.         The “Testing Information About Knex Joints” gives the average, median, and minimum forces (lb.) that it would take to cause a member to pull out of a joint.  The website does tests with anywhere from one to three member and the necessary forces differ depending on the number of members connected to each joint. The tests show that the greater the number of member connected to each joint the greater the carrying capacity of that joint before the members pull out.  This is a logical result as the greater the number of member the more the weight can be distributed putting less of a strain on one individual piece. This knowledge can be used to improve our bridge because we now know to make sure that there are at least three pieces attached to each joint.