sysprob5

sysprob5 - 1 Massachusetts Institute of Technology...

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1 Massachusetts Institute of Technology Department of Aeronautics and Astronautics Cambridge, MA 02139 Systems Problem Out: Wednesday, October 19, 2005 Due: Wednesday, October 26, 2005 Name: Time Spent:
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2 Systems Problems and Labs #7 The Truss Overview In this laboratory exercise, you will: (a) Test a three-dimensional truss under transverse loading; (b) Measure its load-deflection characteristics and the strain in its internal bars/members; (c) Use the strain to determine the loads in the internal bars/members; (d) Compare the loads to values from analysis of a two-dimensional model of the truss; (e) Write a short laboratory report documenting your findings. The concept of strain has not yet been introduced in material and structures lectures. However, a simplified explanation of strain, for a bar that is being pulled on at both ends with a constant load, is that it is the change in the length of the bar (ill) divided by the initial length of the bar (l). As you might expect, stain is an important thing to predict in an aerospace structure so you should keep the deflection results and the strain results for possible use later in the course when these concepts are introduced in lecture. In performing the laboratory, you will be introduced to a number of types of instrumentation and associated concepts. These are strain gages and the measurement of strain; deflection meters; loads cells; load-displacement relations; and the use of experimental/empirical calibration to obtain indirect measurements of one parameter via the direct measurement of another parameter (in this case member loads via member strains). A very important part of this exercise is that you will get to compare the performance of an actual truss to the performance predicted by an analytical model of the truss, and therefore examine the validity of some of the assumptions made in forming the model.
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3 Background As was discussed in lecture, trusses are used for a number of different structures. They are structurally very efficient (i.e. they can carry high loads for a given mass of structure), and so they are widely used for space structures, and were used for early airframes. In the context of Unified Engineering trusses are also useful to us in introducing you to structural analysis, as they are relatively simple structures to analyze. However, it is important to realize, that for trusses, and for any other structure that we design or analyze, the analysis is performed on an idealization of the actual, physical structure. In conducting any structural analysis it is crucial at some point to assess whether the modeling assumptions required to perform the analysis are justified. This laboratory exercise will give you a chance to explore the assumptions we have made in order to analyze trusses. Truss Model:
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sysprob5 - 1 Massachusetts Institute of Technology...

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