ENGN0030_Lab_2 - ENGN 30 Measurement Lab Measurement of...

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1 ENGN 30: Measurement Lab Measurement of Forces in Truss Members Important dates Introduction to the lab: Thursday, October 27, 2011 (in the lecture) Everyone must sign up by: Friday, October 28, 2011, 5pm. Lab sessions will be held in Prince Lab: Monday, October 31 - Wednesday Nov 9, 2011. Each day has 3 slots: 11am 1pm,1-3pm, 3-5pm; Sign-up sheets are in room 096 (5 students per lab slot). Lab reports are due on Friday, Nov 11, 2011 at 4pm at Ms. Gesualdi’s desk. 1. Introduction The objective of this lab is to measure the internal force induced in each member of a truss when external forces are applied on the structure. The measurements are then compared with the corresponding calculated values obtained by applying the principles of statics. Sources of experimental error are then discussed to assess the accuracy of the measurements. Knowing the internal compressive or tensile force generated in a truss member is essential in designing a safe structure that can withstand specified external loads. Excessive tensile force in a member can cause it to plastically deform and/or fracture. Excessive compressive force can lead to plastic yielding or buckling. Knowledge of the internal forces in the truss members allows designers in choosing materials, dimensions, cross-sectional shapes, etc, which can safely withstand the applied loads. Even with all the modern tools available (computational, analytical and experimental), real-life structures do fail for a variety of reasons; see Fig. 2 for a recent example of a failed bridge in Minnesota, in which a bent gusset plate (the plate that connects truss members at the joints) is believed to have caused the collapse. The truss analysis you will B D A C E F G P Figure 1. Schematic illustration of the truss structure used in the measurement lab. All joints are pin joints. The structure is pinned at A and supported by a roller at G.
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2 conduct in this lab is the first step on the path to acquiring more sophisticated skills needed to design and analyze the large scale structures such as that in Fig. 2. 2. Truss Structure Consider the truss shown in Fig. 1, which consists of 11 members joined by pins at points A through G as shown. The truss is pin-supported at A and roller supported at G. It is subjected to a variety of loads at the joints, such as the force P shown in Fig. 1. Note that this force is shown as a representative example; in your lab, the truss may be subjected to loads at multiple joints. 3. Measurement of Internal Force The internal force in each member is obtained by attaching a load cell along its length. Load cells measure by transducing applied force into a measurable quantity such as strain, deflection of a spring, electrical polarization of a crystal, etc. In our case, the load cell is simply an elastic structure that deforms and the corresponding strain is measured by a strain gage ( ).
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