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Friday, October 23, 2009

Forces in plane Redundant Truss - EXPERIMENT


INTRODUCTION

Definition; A truss is a structure comprising one or more triangular units which are constructed with straight slender members whose ends are connected at joints. A plane truss is one where all the members and joints lie within a 2-dimensional plane, while a space truss has members and joints extending into 3 dimensions.

In engineering, a structural member usually fabricated from straight pieces of metal or timber to form a series of triangles lying in a single plane. (A triangle cannot be distorted by stress.)
A truss gives a stable form capable of supporting considerable external load over a large span with the component parts stressed primarily in axial tension or compression. The individual pieces intersect at truss joints, or panel points. The connected pieces forming the top and bottom of the truss are referred to respectively as the top and bottom chords. The sloping and vertical pieces connecting the chords are collectively referred to as the web of the truss.
OBJECTIVE OF THE EXPERIMENT

To determine the forces in members of a redundant plane truss.

APPARATUS

1. A Redundant plane cantilever truss with seven members and 2 pinned supports.
2. A Screw jack for applying load to the truss.
3. An eight channels data acquisition system.



METHODOLOGY

1. Ensure that the pinned support is properly secured to the frame.
2. Attached the screw jack to the joint to be loaded.
3. Loosen the screw jack so that the truss is free from applied load.
4. Connect the wire from loaded cell to the data acquisition module, each load cell occupying one channel of the module. Connection of the load cell to the data acquisition module is shown on the data acquisition panel.
5. Run the wincp32 software.
6. Change the sampling rate from 10 seconds per sample to 1 second per sample of any other desired time interval.
7. Select the setting option to set the channels to be acquired and the conversion factor to covert mV output from load cell to Newton.
8. When the setting is complete, return to the main menu and press the start button.
9. Note the reading of the screw hack
10. Preload the truss by turning the screw jack handle tin the counter clockwise direction to apply load downward and observe the readings of the screw jack (for preload apply approximately 10N to 20N).
11. Allow the data to be recorded for approximately 20 Sec. this is the initial reading for each member. Alternatively record the data manually and fill the table below.
12. Increase the apply load by turning the screw jack handle in the counter clockwise direction and wait for 20 sec for the data to be recorded.
13. Increase the load for another 5 readings
14. Unload the truss by turning the jack in the clockwise direction.

RESULT


CONCLUSION

The force in member 1 and member 2 is having a less chance of error, however the rest of the members seems to be having a error when compared with theoretical calculations. In member 3, the percentage of error decreased as the force was increased, as well in member 4. Member 5 error was there at the beginning at slightly increasing as it progressed and than it decreased slightly when the load was increased. Member 6 and member 7 played a similar roll since they are both redundant members.

There can be error in the practical readings, possible instruments error, reading error since the values keeps on changing for all the members.

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