This experiment tests two parameters, deflection and strain, associated with a simple truss subjected to a loading at its bottom end as would be the case if a constructed bridge was subjected to loadings by cars, trucks, trailers, pedestrians, etc. The set up contained an instrumented truss mounted in a test frame with strain gauges attached to the center of each truss member and a linear potentiometer displacement transducer (LPDT) placed underneath the truss assembly to measure displacement. Experimental data collected as loading on the bottom of the truss increased was compared to calculated results obtained by applying the theoretical formulas.
The main aim of the Simple Truss lab was to observe the behaviour of a simple truss that was subjected to a symmetrical loading, by measuring the deflections and strains in the truss and comparing analytical and experimental results of these deflections and strains.
III. Instrumentation and Test Procedure:
Fig 1.1 Truss Test Set-up
The test specimen consisted of two truss networks as can be seen in Fig 1.1 above. The six outer members are hollow whilst the inner eight members have a solid cross section. The hollow members are 9.48 mm outer diameter and a 0.87 mm thickness, whilst the solid members have a cross sectional diameter of 6.35 mm; all members are of length 335 mm. These length measurement were taken from unattached members, but if the measurements were to be taken from the truss set-up, it would have been appropriate to take measurements from the center of the joint rather than the beginning of the member.
Below are the procedures used to conduct this experiment: * Using the appropriate instruments, we measured the dimensions of the truss and distance between the supports. * We measured the length, cross-section properties (diameter, etc.) of all the members. * We then made sure that the loading screw had sufficient ‘travel’ to apply the load. * We checked the data acquisition system, the personal computer and driving software and familiarized ourselves with their operation. * We Set the initial values of load, axial displacement and lateral displacement to zero. * We loaded the truss (gently) in increments as explained by the TA or the Technician. After placing each load increment, we allowed the system to stabilize before logging the data. * We loaded until the displacement became noticeable or until we were instructed to stop. * After reaching the maximum load, we released the load in the same decrements as in the loading stage. Again we logged the deflections for each decrement.
IV. Analysis and Discussion:
Fig 2 - A sketch of the truss identifying all the members and joints. Some of the useful parameters for calculations are listed below: * Length of each truss member = 335 mm
* Inner diameter of hollow members = 9.48 mm
* Thickness of hollow members = 0.78 mm
* Diameter of solid members = 6.35 mm
In this truss set-up, the joints were rigid, which in reality is not perfect as compared to pinned joints. A perfectly jointed truss is important in theory will help for a more accurate calculation of deflections. Another characteristic of this lab set up is that we have a determinate truss in which its forces can be calculated analytically using static equations. This is because we are aware of the loading at P, thereby leaving only two unknowns of the vertical forces at the supports.
Table of Values Force (N)| Deflection (mm)| Strain Gauge 1| Strain Gauge 2| Strain Gauge 3| Strain Gauge 4| Strain Gauge 5| Strain Gauge 6| Strain Gauge 7| 0| 0| 0| 0| 0| 0| 0| 0| 0|
500| 0.43| 0.000051| 0.000053| 0.000047| 0.000074| 0.000038| 0.000057| 0.000048| 1000| 0.69| 0.000171| 0.000173| 0.000122| 0.000204| 0.000103| 0.000177| 0.000148|...