®
M o d e l N o . M E - 6 9 9 1
P r o p e r t i e s o f I - b e a m s
7
Example: Bridge with Load Cells
The bridge shown in Figure 11 incorporates six load
cells to measure the tension or compression in various
members. A hanging mass is used to apply load. The
mass is adjusted so that the compression in one of the
legs is 1.0 N. Compression is registered as a positive
value and tension as a negative value.
If the screws are loose, the theoretical analysis of the
bridge can be carried out by assuming that the net force
at each node is zero. Thus, the vertical component of compression in the left-most diagonal member must be 1 N
(to oppose the force applied by the leg). The horizontal component must also be 1 N since the member is at a 45°
angle. The predicted resultant force is:
The actual measured force confirms the theory.
Calibration of Load Cells
.
Load cells are factory calibrated; however, you can
recalibrate them in software or on the datalogger. See
the documentation for your software or datalogger for
instructionsWhen calibrating a load cell, it is necessary
to apply a known load. Assemble the fixture shown in
Figure 12 to support the load cell. Hold or clamp the
fixture at the edge of a table and hang a mass from it as
shown. Note that the hanging mass applies tension to
the load cell; therefore the known force that you enter
into the software or datalogger should be a negative
value. For example, if the mass is 1.0 kg, the applied
force is -9.8 N.
Properties of I-beams
This demonstration shows the difference between
the X and Y bending moments of an I-beam.
Simple Triangles
Most structures are made of isosceles right trian-
gles as shown in Figure 14
.
Figure 11: Bridge with load cells
1.0 N
2
1.0 N
2
+
1.4 N
=
Figure 12: Calibration fixture
Figure 13: Bending an I-Beam
#4
#4
#5
#4
#3
#3
#3
#2
#2
#2
#1
#1
Figure 14: (Left) A triangle made from a #5 beam and two #4 beams.
(Right) Combinations of beams to make triangles.