The purpose of this experiment is to introduce students to the concepts of axial force, loading and modeling. Weights are moved sequentially across a 3 bay truss to model the moving load of a tractor-trailer. Load cell data is collected and analyzed to examine how a truss responds to a moving load.
This laboratory is intended for middle school or high school students. The estimated time to complete the experimentation and analysis is two supervised hours. It is assumed that students have a basic understanding of Microsoft Excel before they embark on this laboratory.
Before you begin the laboratory ask the students to brainstorm and name different types of structures and landmarks.
Structures:
Suspension (Golden Gate) Bridges
Truss
Arch
Buildings
Commercial: stores, skyscrapers, office buildings, stadium
Residential: houses
Industrial: warehouses, plants
Radio Towers
Landmarks:
Eiffel Tower, St. Louis Arch
You may also want to use the King Post Truss poster and PowerPoint presentation to discuss Civil Engineering, Bridges, Trusses, load path and node analysis.
If time reduction were necessary, it would be advisable to set up the three bay truss for the students and allow them to perform the experiment. This will still give the students time to analyze the results and answer questions. If you are planning on utilizing this lab with high school students it is possible to increase the difficulty level by spending additional time on theory such as axial loading and node analysis. For example, you can

provide students with the members shown in Figure C and have each of them perform a node analysis.
Instructors should become familiar with the three bay truss, the load cells and
LabView before the experiment is perform. Additionally, you should supervise your students during the set up of the three bay truss and especially when attaching the load cells to the Military connector.
Civil Engineers
Civil engineers are known to “build the community”. Concentrated areas of study in Civil Engineering include structural design, waste treatment design, transportation, geotechnical, water management and construction management. Civil Engineers may also develop mathematical models to conduct risk assessments or investigate the quality of water and air. Typically, Civil Engineering projects are found to be large and one of a kind.
Bridges
The purpose of a bridge is to cross an obstacle, such as a span river, chasm or highway. When designing a bridge the following design considerations need to be taken into account: span, clear height, traffic, materials, cost and location. The function of bridge is to transfer load from middle of structure out to the abutments or piers.
Trusses
Trusses are an extension of the simple beam bridge – a plank across a gap. In theory, the wider beam you have the stronger bridge you can create. Trusses provide depth while maintaining an efficient use of material.
Models
Engineers often use a computer or scale model to test their design before spending time and money on the real thing. An important concept in modeling is scaling, keeping ratios the same.

In this experiment we are modeling a bridge’s reaction to a certain type of loading: a truck driving across the bridge. For a typical tractor-trailer truck, the front axle load is ¼ of the rear axle loads (Figure A) – we will maintain this loading ratio in our experiment.
Load cells have been placed within certain members of the three bay truss, and at the supports. As we place load on the truss, the load cells will measure the force at each location.
10,000 lbs 40,000 lbs 40,000 lbs
Figure A: Example of a Typical Tractor-Trailer Truck Load Distribution
Axial Load
An axial load is carried only in the direction of a member.
Tension – pulling on a member
Compression – pushing on a member
Figure B: Axial Load Applied to a Single Member

To emphasize the concept of axial loads, have students hold a weight above their head with one arm to feel compression. Then, let the weight hang from their arm to feel tension.
If a member is in tension, a positive sign (+) is used to denote this load.
However, if a member is in compression a negative sign (-) is used to identify this axial load.
Sign Convention : Tension (+) and Compression (-)
Load path
The engineer must provide a way for the load to travel from the point of application to the point of support or reaction. The passageway that this load travels is known as the load path .
Node analysis
To find which members are in tension and which are in compression, we place each node of the truss in equilibrium. For every force to the right, there must be an equal and opposite force to the left. For every upward force there must be an equal and opposite downward force to prevent the truss from moving. This theory is known as
Newton’s 3 rd
Law and is demonstrated in Figure C.
Diagonal members carry both vertical and horizontal loads – the axial load in a diagonal member depends on the angle of the member and the vertical and horizontal loads it must carry.
DiagonalFo rce

VerticalFo rce sin( angle )

Horizontal Force cos( angle )
A member inclined at a 45

angle carries a load 2 times the vertical load (vertical and horizontal loads are equal for such a member.)

Figure C: Truss Nodes in Equilibrium
Lecture
Truss PowerPoint Presentation
King Post Truss poster
Red, green, and black wet-erase markers. 1 each.
Experiment
Military connector with 8 channels
3 Bay Truss
Multi-meter
1 long red banana cable
1 long black banana cable
1 short black banana cable
(4) 10 lb weights
(2) 2 ½ lb weights
ITLL LabStation
Truss Analysis spreadsheet

In the case that younger students are performing this experiment (7 th or 8 th grade), it may be advisable that they only perform the following:
A.
Setup the Truss

Connect military connector to load cells (step 4)

Verify Voltage and connect to LabStation (steps 5-7)

Adjust Offsets (steps 13-14)
B.
Testing of the Truss

Apply Load (steps 1-2)

Write data to file (step 3)
C.
Spreadsheet for data analysis

Importing the data to excel (steps 1-3)
The instructor should carry out the remainder of the setup.
If a four-person high school group is going to run the experiment, you may have two of the students execute steps 1-8 of the setup. The other two students can then complete the truss setup by performing steps 9-15. During testing two students should attend to applying the load as the other two students run LabView and write the data to the file. The group as a whole should evaluate the data in Excel and answer the Results and Analysis questions.

A.
Setup of the Truss
1.
Turn OFF National Instruments SCXI-1000 chassis power, by the PC tower.
P1A
Plug
P1B
Plug
SCXI-1000 Chassis
Power Switch
2.
Make sure that a load cell and platform is located under each leg (FR, RR, FL, RL) of the three bay truss. A space should remain between the load cell and the platform. If you do not see a space, turn the load cell clockwise until it is flat against the platform.
Next, turn the load cell counter-clockwise 360

so a small space is present.
Load Cell
Space
Platform
3.
Attach the military connector to plug P1A or P1B on the Military Connector Panel on the right side of the Lab Station. Make sure the P1A or P1B plug corresponds to the
LabStation side that you are sitting at.

LabStation
Side
Military
Connector
4.
Attach the military connector to the load cells. The following table shows the correct channel for each load cell. Figure 2 displays a schematic that will assist in finding the location of the load cells.
CH CH 0 CH 1 CH 2 CH 3 CH 4 CH 5 CH 6 CH7
Cell FL RL FR RR 3 4 9 11
5.
Connect a short black banana plug to the HP 6205 power supply: one end to ground and the other to negative output.
6.
Turn ON the power supply and adjust the voltage output to +10V BEFORE connecting to the panel.
7.
Verify the voltage reading using a multi-meter set to read DC voltage (V=). Plug the black lead from the multi-meter plugs into the negative (-) output on the power supply
Voltage Readout
DC Voltage
Lead
Terminals

and the red lead into the positive (+) output. Make sure the lead terminals are connected correctly on the multi-meter.
8.
Turn OFF the power supply, after confirming +10V on the multi-meter.
9.
Use black and red banana cables to connect the power supply to the lab station. The following table shows the connections for each cable.
Cable From HP
6205
Red (+) output
Black (-) output
Voltage
Value
10V
Ground
To Lab Station DIRECT
CHANNELS
XTRA 0+
XTRA 0-
XTRA 0-
XTRA 0+
10.
Turn ON the SCXI Chassis.
11.
Turn ON the HP 6205 power supply.
12.
Open H:\Courses on ‘itll-users’\ITLL Documentation\ITLL Modules\Static Loading of a 3 Bay Truss\ 3BayTruss.vi
13.
Double click on the 3BayTruss.vi file
14.
Choose LabStation side (either A or B). Click Run

. Name the LabView file and save it to the computer desktop.
15.
Adjust offsets using the

and

arrows. Enter the values in the force components box into the offset box so that the force components read close to zero (see figures on the next page).

16.
Click Write to File
Offset
Boxes
Force Component
Boxes
B.
Testing of the Truss
1.
For each load stage place the required load on A, C and B, D. If the required load for
A, C is 5 lbs, place 2 ½ lbs on plate A and 2 ½ lbs on plate C.
2.
Enter the corresponding load for A, C and B, D on the screen.

3.
Click Write to File.
4.
Repeat for all 6 load stages. Refer to Table 1 for load stages.
C.
Spreadsheet for Data Analysis
1.
Press STOP on the LabView window.
2.
Look for your LabView file on the desktop. Then, change the extension of your file name to .xls
. To do this, click on your filename so it is highlighted. Click two more times on the filename until a cursor appears at the end of the name you gave the
LabView file. Next, type .xls
and press Enter on the computer.
3.
To open the file, double click on the file icon and Microsoft Excel will open with your data in a spreadsheet.
4.
Once your data has opened in Microsoft Excel, delete the first row of your data set.

4
5
6
Stage
1
2
3
Load on A,C (lbs)
0
5
20
20
0
0
Table 1: Three Bay Truss Load Stages
Load on B,D (lbs)
0
0
5
20
20
0
Figure 1: 3 Bay Truss and Load Cells

FL
3
4
C
A
A
D
B
B
9
Figure 2: Three Bay Truss Schematic
11
FR

1.
What law of physics tells us that total reaction = total load?
2.
What is it called when the front and back reactions are the same?
3.
Members 3 and 9 are in compression. Members 4 and 11 are in tension. Why?
4.
Discuss how the location of the load affects the member forces.
5.
Discuss Member Load Relationships.
6.
What is the greatest ratio of total load any one member carries?
7.
What are other tests we could do on this truss?

1.
What law of physics tells us that total reaction = total load?
Newton’s 3 rd
Law
2.
What is it called when the front and back reactions are the same?
Symmetry of Loading
3.
Members 3 and 9 are in compression. Members 4 and 11 are in tension. Why?
For positive bending, bottom horizontal members will always be in tension – top horizontal members will be in compression.
4.
Discuss how the location of the load affects the member forces.
Loading curve is not linear, so member force depends on location as well as magnitude of load. Member 9 is a zero force member when there is no load on B,D.
5.
Discuss Member Load Relationships.
Diagonals are sqrt 2 of vertical and horizontal.
6.
What is the greatest ratio of total load any one member carries?
50% - Member 3 when the front axle of the truck is on A,C.
7.
What are other tests we could do on this truss?
Uniform loading to model lane of traffic. Wind loading. Impact loading.