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Experiment: Truss - Member Force Objective: 1. To study the member force distribution of a loaded pin-jointed truss by using strain gauges. 2. To verify that equilibrium of joints by using method of joint". 3. To verify the equilibrium of a free-body by using "method of section". 4. To identify the zero force members by using "method of inspection". Apparatus TecQuipment Pin Jointed Frameworks STR8 Theory Let &; be the measured axial strain of member no. i. By the stress-strain relationship, 0(N/mm2) = EE (N/mm2) where o and E are the axial stress of the number i and Young's Modulus of the material respectively. Let Fi (N) be the axial force in member no. i, and it can be determined from the axial stress o. Fi(N) =0 (N/mm2)* A (mm) Where A is the cross sectional area of the truss member i. By measuring the strain €; in the member i, it is possible to obtain the axial stress of and hence the axial force Fi. Page. 1/9 Hong Kong Institute of Vocational Education (Tuen Mun) Department of Construction Truss-Member Force Construction Materials and Science Laboratory Experimental Procedures 1. Measure and draw the configuration of the truss before testing in fig.1. 2. With the load cell (located at the center of the truss ) reading zero, select the channel no. and use the bridge balance control knob (see fig.2) to set the zero reading of strain gauge. 3. Repeat 2 for other channels. Record the initial reading in Table 1. (position of strain gauges refer to fig. 1) 4. Apply a load of 50N by turning the load adjustment of the load cell. 5. Take the 7 strain gauges readings in turn by switching the display input of the Digital Strain unit. 6. Increase the load in 50N intervals to 500N and record the strain gauge readings for each load in Table 1. 7. Decrease the load in 50N intervals to O N and record the strain gauge readings for each load in Table 1.
Results 1. Applied Load and their Corresponding Strain Gauge Readings Table 1 Applied Strain Gauge Strain Gauge Strain Gauge Strain Gauge Strain Gauge Load (N) Strain Gauge Strain Gauge No. 1 Reading (*10%) No. 2 Reading (*10) No. 3 Reading (*10%) No. 4 Reading (*10%) No. 5 Reading (*10) No. 6 Reading (*106) No. 7 Reading (*10%) 0 0 -001 -124 -088 -052 -071 -220 -100 50 -010 -124 -088 -061 -063 -214 -091 100 -020 -124 -088 -070 -056 -207 -082 150 -028 -124 -088 -078 -049 -202 -074 200 -038 -124 -088 -088 -041 -195 -065 250 -048 -124 -088 -098 -032 -187 -055 300 -059 -124 -088 -109 -023 -177 -042 350 -064 -124 -088 -115 -018 -172 -037 400 -073 - 124 -088 -123 -010 - 166 -029 450 -084 -124 -088 -133 -001 -157 -018 500 -092 -124 -088 -140 +008 -150 -004 450 400 350 300 250 200 150 100 -084 -073 -064 -059 -048 -038 -027 -019 - 124 -124 -124 -124 -124 - 124 - 124 -124 088 -088 -088 -088 -088 -088 -088 -088 - 133 -123 - 115 -109 -099 -088 -078 -070 -001 -010 -017 -023 -031 -040 -063 -157 - 166 - 171 -177 -187 -194 -201 -207 -018 -029 -037 -042 -054 -064 -074 -082 -055 Page. 3/9 -088 50 0 -010 -001 - 124 -124 -061 -051 -062 -070 -213 -219 -088 -091 -100 2. Graphical Presentation for the Test Results a) Use the data in Table 1, plot the best-fit straight line for applied loads against strain gauge no. 1 readings in the graph paper provided. b) From the graph in (a), determine the strain at load W = 500 N c) Repeat (a) and (b) for strain gauge no. 2 to 7.
(6) Identified zero force members and the comparison of the theoretical results with that of the measured values. (h) Calculations of the equilibrium equations for joints A, B and C by using the measured values 0 Calculation of the equilibrium equations for the free-body shown in fig 3 by using the measured values 0) Discussion on the errors involved in this experiment and suggestions for further improvement. 2. Hand in the report on/before G 2 B D W Fig. 1 - Position of Strain Gauges Page. 5/9 Hong Kong Institute of Vocational Education (Tuen Mun) Department of Construction Truss Member Force Construction Materials and Science Laboratory Fig. 2 TecQuipment STR8 B с Fig. 3 - Free Body Diagram