Experiment 1.5 – Pin-Jointed Truss
Learning outcomes
By the end of this experiment you should hopefully:
• be able to calculate the internal forces in a pin-pointed truss subject to a set of external loading conditions
Figure 1: Pin-jointed frame. including loading frame. (units: mm)
EXPERIMENT
Examine the relevant equipment and observe the effect that varying the load has on the readings obtained from the strain gauges positioned on the truss’s members. Make sure the equipment is set up properly (this can be checked with the Teaching Assistant), then follow these steps:
❖ Reduce the load until there is little to no resistance when turning the load dial.
❖ Check the pin in the joint between members 1 and 2, it should be easy to move with little friction.
❖ If the pin binds adjust the load dial in a small increment either clockwise or anticlockwise and check the pin again.
❖ If there is less resistance to the pin moving keep moving the dial in increments until the pin nearly moves freely.
❖ If the pin becomes stiffer move the dial in the opposite direction and repeat.
Once the above steps have been completed, the Truss is ready to be loaded and strain on its members can be measured:
o Apply loads in increments of 100 N up to a maximum of 500 N, recording the true member strain in Table 1.
o Record the deflection of the girder under the dial gauge.
o Calculate the theoretical member forces for the framework with a load of 500 N.
o Compare the experimental and theoretical results.
o Plot a graph of Load (N) against Joint Deflection (mm) and comment on the resulting graph.
o Evaluate potential errors in the measurements and calculations performed.
Figure 2: Labelled members on pin-jointed truss.
Table 1: True Member Strain (μϵ)
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Load (N)
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Strain 1 μϵ
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Strain 2 μϵ
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Strain 3 μϵ
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Strain 4 μϵ
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Strain 5 μϵ
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Strain 6 μϵ
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Strain 7 μϵ
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Joint
deflection mm
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0
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100
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200
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300
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400
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500
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Subsequently, calculate the equivalent member forces for an applied load of 500 N and complete the “Experimental Forces” column in Table 2. This requires using the measured strain, ε, to calculate the induced stress, σ, noting their relationship as per the material’s Young’s modulus, E, and that members are made from steel:
Where, the Young’s Modulus for steel is denoted by Es, F is the force acting through a given member and A is the cross-sectional area. Measure the diameter of the rods to calculate their cross-sectional area:
Rod diameter = _______mm
Table 2: Comparison of Experimental and Theoretical Force when the Pin Jointed Truss is loaded at 500 N.
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Member
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Experimental Force (N)
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Theoretical Force (N)
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1
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2
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3
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4
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5
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6
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7
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Finally:
❖ For members 4 and 6, plot a graph of load applied to the truss (N), on the horizontal axis, against Strain (με) measured (vertical axis).
❖ For members 4 and 6, plot a graph of the load applied to the truss (horizontal axis), against the force calculated to be acting within these members (vertical axis); this can be plotted on the same graph as load vs strain, employing a secondary vertical axis.
❖ Comment on the relationships observed and any anomalies in your graph.