Agricultural grains are handled as bulk materials

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1 se 1795 ms 7/9/01 11:39 AM Page 1211 MOISTURE-INDUCED PRESSURES AND LOADS IN GRAIN BINS H. V. Kebeli, R. A. Bucklin, D. S. Ellifritt, K. V. Chau ABSTRACT. A 1.00-m-high and 0.60-m-diameter model grain bin was fabricated from 0.4-mm-thick smooth galvanized steel sheet. A humidifying system was used to increase the moisture content of soft red spring wheat in the bin. Lateral pressures and hoop stresses were measured at three levels on the bin wall using strain gages. Two series of experiments were conducted, one with model bin walls supported independently from the floor by three load cells and the other series with the bin wall connected to the bin floor. Vertical forces on the bin floor and wall were measured separately with load cells. The average grain moisture content increased from 15.6% d.b. to 26.9% d.b. and from 14.2% d.b. to 28.2% d.b. during two series of wetting experiments, respectively. In the first series of experiments, an average decrease of 350 N in the vertical wall force was recorded during the wetting process as hygroscopic expansion of grain caused the loads to shift from the bin wall to the floor. Vertical wall load to total load ratio decreased from 0.2 to 0.02 as the grain wetted. The highest measured lateral pressures at the bottom level were five times the initial lateral pressure. Vertical pressures on the floor increased from 5.7 to 8.0 kpa with wetting. The highest overpressure ratio for vertical pressure was two. The hoop stresses on the wall increased as high as five times while the axial stresses switched from compressive to tensile stress. In the second series of experiments, the overpressure ratio was nine for the lateral pressures at the bottom level. This corresponded to an increase of nine times in the hoop stresses on the bin wall. The axial stresses were in compression, and slightly decreased as the grain moisture content increased. Connecting the bin wall and floor produced an increase in lateral overpressure ratio from five times to nine times initial pressure. This shows the importance of considering the type of wall support when evaluating the results of model tests or applying test results to bin design. Keywords. Grain, Grain bin, Moisture, Load. Agricultural grains are handled as bulk materials and typically are stored in circular bins. Bulk materials impose both static and dynamic loads on bins. The moisture content of the stored material affects these loads, and changes in moisture content may produce additional loads on structures. When grain is wetted, grain kernels swell, and the grain bulk tends to expand. This expansion is restricted by bin walls, and consequently additional pressures, termed hygroscopic pressures, are imposed on the bin walls (Whitaker, 1979). Early researchers placed little or no emphasis on the moisture content of stored grain. Kramer (1944) found that the angle of repose of rice was greatly affected by changes in moisture content, especially when the moisture content exceeded 16% w.b. Saul (1953) reported that grain with a moisture increase of from 11% to 16%, at 16 C, produced a lateral wall load seven times larger than that before Article was submitted for publication in October 1999; reviewed and approved for publication by the Structures & Environment Division of ASAE in March Approved for publication as Florida Agricultural Experiment Station Journal Series No. R The authors are H. Volkan Kebeli, ASAE Student Member, Graduate Student, Ray A. Bucklin, ASAE Member Engineer, Professor, Agricultural & Biological Engineering Dept., University of Florida; Duane S. Ellifritt, Professor, Civil Engineering Dept., University of Florida; Khe V. Chau, ASAE Member Engineer, Professor, Agricultural & Biological Engineering Dept., University of Florida, Gainesville, Florida. Corresponding author: Dr. Ray A. Bucklin, University of Florida, Agricultural & Biological Engineering Dept., PO Box , Gainesville, FL 32611, phone: , fax: , <bucklin@agen.ufl.edu>. moisture increase. Risch and Herum (1982) found that aeration air velocity, temperature, and relative humidity were highly significant factors in predicting the magnitude of the hoop stresses induced in grain bins. Hoop stresses at three different levels of the model bin increased for the initial 30 to 100 h of aeration with rewetting of the confined corn. Blight (1986) examined the behavior of grain subjected to wetting and showed that the lateral pressure in grain swelling under constant vertical stress with zero lateral strain could double in value. Britton et al. (1993) measured vertical forces on the bin wall during the wetting processes of grain. They found that after about 900 min of wetting, the total vertical force decreased to zero and the grain mass was completely balanced by the upward force of grain swelling. Hao et al. (1994) tested the effects of grain moisture content on dynamic loads during discharge of the bin. They reported that with the increasing moisture content, static lateral pressure on the bin wall decreased while static vertical force on the wall increased. Zhang et al. (1998) conducted an experiment using a model test bin system which consisted of a corrugated steel bin, force and pressure transducers, freshly harvested hard red spring wheat, a data acquisition unit, an air humidifier, and a grain handling device. They made the following conclusions: Vertical pressure on the bin wall decreased as grain was wetted, and an up-lift force resulted from grain expansion. Vertical force on the bin floor increased as grain was wetted, and the force on the floor exceeded the total weight of the grain. Transactions of the ASAE VOL. 43(5): American Society of Agricultural Engineers / 00 /

2 se 1795 ms 7/9/01 11:39 AM Page 1212 Lateral pressure increased as grain was wetted. The increase in lateral pressure was greater in the lower portion of the bin. The highest measured lateral pressure was almost nine times the static lateral pressure which was the result of an increase of 6.9% d.b. in the average grain moisture content. The lateral to vertical pressure ratio increased 1.6 times during grain wetting. Although the problem of pressures induced by grains in grain storage bins has been studied for a period in excess of 150 years, there is not complete agreement among researchers regarding an accurate method of design. The major reason is the extreme complexity of the problem. Considering the moisture content changes in grain makes the design progress even more complex. Grain moisture content affects both particle and bulk properties of grain, as well as grain-wall interactions, and thus it affects bin loads. This effect of grain moisture on loads has not been well described in any design standards and codes. Only EP 433.1, Loads Exerted by Free-flowing Grain on Bins, (ASAE, 1998a) has a caution for design purposes. Janssen s equation (Janssen, 1895) is the most widely used method to estimate grain pressures; however, because of changing material properties and the effects of the expansion of individual grains, Janssen s equation greatly underestimates grain pressures as moisture is added to the grain bulk. Therefore, this study was conducted to determine the effects of grain moisture content on both lateral and vertical wall loads in grain storage bins. MATERIALS AND METHOD Two series of tests were conducted using a model bin in two different configurations to study the moisture-induced pressures in grain bins. In the first series of experiments, the model bin wall was supported by three load cells and the bin wall and bin bottom were independent from each other. For the second series of experiments, the bin wall was attached to the bottom of bin and the three load cells that were carrying bin wall were removed. The equipment used in this study consisted of a model grain bin, wheat as the stored material, an air humidifier system, strain gages, load cells, and a data acquisition unit (fig. 1). The model bin was made from 0.4-mm-thick smooth galvanized steel sheet, and scaled to represent typical grain bins. The model bin was 1.00 m high and 0.60 m in diameter (h/d = 1.67). The model bin had a flat Figure 1 Test bin. bottom and the floor was supported independently from the bin wall in the first series of tests. The bin wall was fastened to the floor with bolts at three points 120 apart from each other. The floor was made from 3.0-mm-thick perforated steel with 40% total open area so that the air could be circulated through the bin. The bin wall was supported by a 6.4-mm-thick ring-shaped steel base which had an outside diameter of 0.68 m and a 0.60 m inside diameter. The base was suspended by three 12-mm steel rods located 120 apart, each of which was attached to a load cell supported by the base table. These three load cells were used to measure the resultant vertical force on the bin wall. Six strain gages were placed in three pairs on the bin wall at locations 180 apart. A 22-cm-high air plenum was located between the wall mounting ring and base table to supply air to the grain. The plenum was constructed from 0.4-mm-thick galvanized steel sheet, and was slightly smaller in diameter than the bin. A 6.4-mm-thick and m-diameter steel disk provided the base for both the bin and the air plenum. Three load cells located 120 apart under the base were used to measure the total mass of grain in the bin. These three load cells carried the bin, air plenum, wall mounting ring, and base table. The model bin had a full capacity volume of m 3. The bin was filled from 22.5 kg bags by pouring grain freely into the top and was unloaded from the top pneumatically. Wheat was selected as the stored material in the bin for this study because of its high bulk density and because it has been used as the test material in many studies involving grain in bins. The wheat used in all tests was soft red spring wheat. Bulk density was calculated as the mass of the grain in the bin divided by the total grain volume. The initial moisture content of the wheat before each wetting process was determined by the oven method given by ASAE Standard S352.2 DEC92 (ASAE, 1998b). During the wetting process, it was difficult to measure the actual local grain moisture content which varied within the bin. The average moisture content of grain, defined by equation 1, was used as an indicator of the moisture content of the wheat: AMC = m c 1 + MC i m i 1 (1) where AMC = average moisture content of grain (% d.b.) MC i = initial moisture content of grain (% d.b.) m c = current mass of grain (kg) m i = initial mass of grain (kg) The air humidifying system consisted of a centrifugal fan, a humidifying chamber, and an air plenum located under the bin floor. The fan was a high pressure fan with a capacity of 0.24 m 3 /s airflow rate at 1.25 kpa. The humidifying chamber was a m wooden box, with two spray nozzles that injected mist into the chamber to humidify the flowing air. Moist air with a relative humidity up to 98% was blown through a 100-mm flexible duct into the plenum. All tests were performed inside an environmental chamber (8.75 m 3 ) to control the temperature of the air surrounding the model bin. An electric heater and a refrigeration unit were used to 1212 TRANSACTIONS OF THE ASAE

3 se 1795 ms 7/9/01 11:39 AM Page 1213 increase or decrease the ambient temperature. The ambient temperature was measured by thermocouples, and temperature data were recorded by a CR-10 data logger. The average recorded temperature in the chamber was 28 ± 1.2 C during the wetting experiments. Six two-element 90 (tee) rosette strain gages were used to measure strains used to calculate lateral pressures, hoop and axial wall stresses. Two columns of gages were placed 180 apart from each other at three levels on the model bin wall. All three pairs were placed 30 cm apart vertically starting 15 cm from the bottom of the bin as shown in figure 1. Lateral grain pressures and hoop stresses were calculated from the measurements made with horizontal strain gages, and vertical stresses in the model bin wall were calculated from the measurements made with the vertical strain gages. It was assumed that the principle stresses on the bin wall were acting in the x and y directions. Six tension/compression load cells were used in this study. Three of the load cells had a 1,000 Newton maximum load capacity with an accuracy of 99.98%. These load cells were used to measure vertical loads on the model bin walls in the first series of experiments, and were placed 120 apart under the wall support ring. The other three had a 5,000 Newton maximum load capacity with an accuracy of 99.95%. These load cells were used to measure both grain weight and floor loads, and were placed 120 apart under the bottom support base. All six load cells were factory calibrated for both compression and tension. The factory calibration data were compared to the calibration tests performed with an Instron (Universal Testing Machine). It was found that both calibration data sets matched each other exactly. A signal conditioning card for strain gages, an excitation and conditioning accessory for load cells, a data acquisition board, a personal computer, and the software package (LabVIEW 4.0; National Instruments, 1998 ) were used for data acquisition in this study. All data were collected at 30- min intervals during the wetting tests. The output signals from strain gages and load cells were stored in units of volts, and then the raw data were converted to strains and forces. RESULTS AND DISCUSSION WALL SUPPORTED INDEPENDENTLY FROM FLOOR Six wetting tests were performed with the wall supported by three load cells and separated from the bin floor. The average total grain mass changed from 204 to 224 kg as the result of the wetting process. The average moisture content of grain changed from 15.6% d.b. to 27.1% d.b., or an average of 20 kg of water was added to the grain over a period of approximately two days (fig. 2). The moisture content of wheat at three levels was measured by the oven method according to ASAE Standard S352.2 DEC92 (ASAE, 1998b) to evaluate moisture distribution inside the model bin after each test, except for the first test. Grain samples were collected with a probe at three measuring levels after the end of wetting tests by probing the grain. These values are presented in table 1. It Table 1. Moisture content of wheat found by oven test method after wetting tests at three measuring levels in the first series of tests, and final average moisture contents Measur- Measured Moisture Content (%d.b.) ing Test Test Test Test Test Test Levels Average Bottom ± 0.5 Middle ± 0.5 Top ± 0.4 Final ± 1.0 Average Moisture Content (AMC) Figure 2 Changes in average moisture content of wheat during first series of wetting experiments (bin wall and floor independent). VOL. 43(5):

4 se 1795 ms 7/9/01 11:39 AM Page 1214 was found that the real moisture content values obtained by the conventional oven method were in acceptable agreement with the average moisture content values obtained by load cell readings and equation 1. Although the grain mass increased and grain expanded, the vertical force on the bin wall decreased as the grain wetted. When the wetting process started, the vertical wall forces showed a sudden decreasing trend. Vertical wall forces reached a minimum after an average moisture content increase of 4.2% d.b. Statistical analysis (ANOVA, analysis of variance and F-test) of vertical wall force changes in six wetting tests did not detect any significant difference among replications(p > 0.05). The average initial vertical wall force for six tests was 401 N (without the weight of the bin wall, 106 N), but the force dropped to 45 N after 600 min of wetting. From that point, vertical wall forces were stable until the end of wetting process (fig. 3). There are two possible contributions that could cause this behavior in the vertical wall force. Load distribution changes resulted in shifting of the vertical forces from the bin wall to the floor. After the average moisture content of the grain increased by 4% d.b., the floor supported a load almost equal to the total grain weight. Hence, no more decrease was possible in the vertical wall force. In addition, the internal friction between grain kernels and the friction between wall material and grain have an opposite effect on vertical wall force. Under initial conditions, grain tended to slide downward along the bin wall. This resulted in a downward (positive) vertical wall force on the bin wall. Grain kernels expanded as the grain wetted, and the internal friction between grain kernels increased (Kramer, 1944; Lorenzen, 1960; Moysey, 1984; Zhang and Britton, 1994; Giner and Denisienia, 1996). This might cause grain movement upward along the bin wall, and an equilibrium between wall friction forces and upward forces because the floor prevented the downward movement of grain. This upward movement and the increase of internal friction could contribute to an upward (negative) vertical wall force. Similar results for vertical wall forces were reported by Britton et al. (1993). They observed a rapid decrease in vertical wall forces up to 700 min of wetting time, then the vertical wall forces became stable and until the end of wetting test (1,470 min). Their model bin wall was also independent from the floor. Table 2.Comparison of initial average measured lateral pressures observed in the first series of experiments and lateral pressures predicted by Janssen s equation (k = 0.5, α = 0.3) Measuring Predicted Pressures Measured Pressures Levels (kpa) (kpa) Bottom Middle Top Initial lateral pressures measured on the bin wall at three levels were compared with the values predicted by Janssen s equation (k = 0.5 and µ = 0.3 as recommended by ASAE Standard EP433.1, 1998a). The measured values were in reasonable agreement with the predicted values indicating that the measuring system was operating properly (table 2). The average lateral pressures at the three measuring levels increased as the grain wetted for all tests (fig. 4). Analysis of variance results indicated that the replications were not significantly different (p > 0.05). Lateral pressures at all three measuring levels started to increase slowly at the beginning of the wetting process. However, the changes in the pressures increased more rapidly after 2,500 min of wetting which corresponds an increase of approximately 8% d.b. in average moisture content. The greatest increases in lateral pressures were observed at the bottom level, thus the main concern should be on bottom level pressures. At the bottom level, a significant regression (p < 0.01) with R 2 = 0.97 was found for average change in lateral pressures of six tests with increase in average moisture content of grain. The prediction equation is: P L = 0.06 MC MC (2) where P L = amount of change in lateral pressure (kpa) MC = amount of increase in average moisture content (% d.b.) The prediction equation for changes in the lateral pressures at the bottom level with increasing moisture content was compared to that given by Zhang et al. (1998) for a m-high and 0.96-m-diameter model bin with the bin wall supported independently from the floor. Zhang et al. measured an initial lateral pressure of 3.0 kpa at 14 cm above the bin floor, and reported a prediction equation for changes in lateral pressures with increasing moisture content as: P L = 0.39 MC MC (3) Figure 3 Average change in vertical wall force with the increase in average grain moisture content during the first series of experiments. The results are presented in figure 5. As seen in the figure, equation 3 from Zhang et al. predicts lateral pressures much higher than equation 2. In addition, equation 3 is inconsistent with moisture content increases above 7% d.b. because the equation was derived for moisture content increases up to 6.9% d.b. For an increase of 7% d.b. in average grain moisture content, equation 3 predicts a lateral pressure of 24.2 kpa while the equation 2 predicts 7.6 kpa TRANSACTIONS OF THE ASAE

5 se 1795 ms 7/9/01 11:39 AM Page 1215 Figure 4 Average changes in lateral pressures at three measuring levels (first series). MC = change in grain moisture content (% d.b.) Figure 5 Comparison of the prediction equations for lateral pressures with increasing grain moisture content. The average lateral pressure observed at the end of wetting was 14.2 kpa at the bottom level. This pressure increase represented an overpressure factor (ratio of final to initial pressure due to wetting) of 4.8 for an average moisture increase of 11% d.b. (fig. 6). The final pressure values for middle and top levels were 8.6 and 7.5 kpa, respectively. The overpressure factors were 3.7 and 6.4, respectively. Although the highest overpressure ratios were observed at the top level, the final lateral pressures at the bottom level were higher than those at the top and middle levels. The regression equation (p < 0.01) for the 11% d.b. moisture content increase for the bottom level overpressure ratio with R 2 = 0.98 is: where R ovp R ovp =e MC (4) = dimensionless overpressure ratio for the bottom level The behavior of changes in lateral pressures showed different results than those of previous experiments reported by Hao et al. (1994), and Zhang et al. (1998). Hao et al. observed a decrease in lateral pressures while Zhang et al. reported a rapid increase in lateral pressures at the commencement of wetting, then a slower increase with increasing average moisture content in the similar experimental setup. They also reported an overpressure ratio of eight for lateral pressures. This might be attributed to the differences in instrumentation (they used diaphragm pressure sensors), tests conditions, and initial moisture content of grain. The lateral expansion of grain bulk was restricted by the bin wall, and because of this restriction, the lateral pressures increased as the grain kernels swelled. Therefore it is possible to assume that the grain expansion caused a radial displacement in the model bin as the grain expansion caused an increase in the grain height. Based on this approach, final lateral pressures could be estimated by using radial displacement formula for interference fits (Deutschman et al., 1975; Timoshenko and Goodier, 1951): P L = r E g r (5) 1 ν g where r = radial displacement (m) r = inner radius (m) E g = modulus of elasticity of grain bulk (854 kpa at 26% d.b., based on Thompson, 1980) ν g = Poisson s ratio of grain bulk (0.3, given by Arnold and Roberts, 1969) VOL. 43(5):

6 se 1795 ms 7/9/01 11:40 AM Page 1216 Figure 6 Overpressure ratio of lateral pressures at three levels with increase in average grain moisture content (first series). Table 3. Comparison of estimated and measured final lateral pressures Wetting Tests Test 4 Test 5 Test 6 Average h/h ± Estimated lateral pressure (kpa) ± 3.04 Measured lateral pressure (kpa) ± 0.65 By assuming the ratio of vertical displacement ( h/h) equal to the unconfined radial displacement ratio ( r/r), final lateral pressures were estimated for three wetting tests for which the change in grain height was recorded (table 3). Based on this approach, the estimated final lateral pressures were found to be in reasonable agreement (p > 0.05) with the measured final lateral pressures at bottom level. These results showed that if the unconfined radial displacement of grain bulk can be measured and the proper values of grain properties are known at a given condition, useful estimates of moisture induced pressures may be possible. Vertical floor pressures were determined by dividing net vertical floor loads (caused by grain) by floor area. The measured initial vertical grain pressure values at the floor were found to be in close agreement with those predicted by Janssen s equation when the recommended k (0.5) and µ (0.3) values by ASAE Standard EP433 (ASAE, 1998a) were used in pressure calculations. The measured mean vertical pressure was 5.7 kpa while the predicted pressure was 5.9 kpa. As wetting progressed, vertical pressures at the bin floor started to increase. However, the vertical pressures changed less than lateral pressures. Vertical grain pressures showed a sudden increasing trend up to 600 min of wetting, but then the magnitude of changes became smaller until the end of wetting tests (fig. 7). This was believed to be caused by shifting of forces acting vertically from wall to floor of the bin with the commencement of wetting. The maximum final vertical pressure was 8.0 kpa Figure 7 Average change in vertical pressure at floor level with increasing grain moisture content (first series). which corresponded to a pressure increase of 2.1 kpa. This was attributed to the lack of restraint in vertical direction while the grain expansion was restricted by the bin wall in the horizontal direction. According to the results, it is possible that the hygroscopic expansion might have less effect on vertical pressures than lateral pressures. Statistical analysis of the vertical pressure changes in six wetting tests, using analysis of variance, did not detect any significant difference in replications (p > 0.05). The best fit curve for the increases in vertical grain pressure at the bin floor was a third order polynomial with R 2 = The regression equation (p < 0.01) which predicts the amount of increase in vertical pressure at the floor level with an increase in average moisture content is: P ν = MC MC MC (6) 1216 TRANSACTIONS OF THE ASAE

7 se 1795 ms 7/9/01 11:40 AM Page 1217 where P ν = amount of change in vertical pressure (kpa) MC = amount of increase in average moisture content (% d.b.) Zhang et al. (1998) measured an initial vertical pressure of 6.6 kpa at 14 cm above the bin floor for a 1.57-m-high and 0.96-m-diameter model bin with the bin wall supported independently from the floor. They reported a prediction equation for vertical pressures with increasing average moisture content as: P ν = MC MC MC MC (7) Figure 8 shows the comparison between equation 7 and equation 6. Equation 7 predicts a decrease of 1.5 kpa for an average moisture content increase of 1% d.b. However, Figure 8 Comparison of the prediction equations for vertical pressures with increasing grain moisture content. from that point, it shows a rapid increasing trend, and predicts larger vertical pressures than equation 6 for a 7% d.b. moisture content increase. At that point, equation 6 predicts 7.5 kpa while Zhang et al. equation predicts 11.0 kpa. During the experiments, circumferential (hoop) and vertical (axial) stresses changed as the grain kernels swelled as water was added. It was observed that the greatest increase in stress on the bin wall was in the circumferential stress at the bottom level. The most critical stress changes were observed in circumferential stresses at the bottom level for six wetting tests (fig. 9). The initial hoop stress at the bottom level was kpa, and this value reached kpa after an increase of 11% d.b. in average grain moisture content which corresponded to a ratio of 4.3. However, the axial compressive stress at three measuring levels decreased with increasing moisture content of wheat (fig. 10). This indicated that the axial wall stress switched from compressive to tensile stress as the grain kernels swelled. In addition, the changes in axial stresses corresponded to the moisture movement (migration) inside the model bin. First, the bottom level reached zero stress, then the middle level and finally the top level. This showed the upward movement of moisture through the grain bulk, and it was assumed that the lower portions of the grain bulk reached the actual average moisture content earlier than the higher portions of the bin. BIN WALL CONNECTED TO FLOOR The second series of three wetting tests was conducted with the bin wall and floor connected to determine if the moisture induced pressures differed from the results obtained from the first series of experiments with the bin wall and floor supported independently. The initial average moisture content of grain was 14.2% d.b., and increased to 29.2%, 24.6%, and 30.9% d.b. for three wetting tests, respectively. The change in moisture content of wheat with time during experiments is given in figure 11. Figure 9 Changes in hoop stress at three levels with increasing grain moisture content (first series). VOL. 43(5):

8 se 1795 ms 7/9/01 11:40 AM Page 1218 Figure 10 Changes in axial stress at three levels with increasing grain moisture content (first series). Figure 11 Changes in average moisture content of wheat during the second series of wetting experiments (bin wall and floor connected). Lateral grain pressures at three measuring levels increased with increasing average moisture content (fig. 12). Analysis of variance results indicated that there were no significant difference among the replications at the corresponding measurement levels at three wetting tests (p > 0.05). At the bottom level, a statistically significant regression equation (p < 0.01) with R 2 = 0.96 was found for average change in lateral pressures of three tests with increase in average moisture content of grain. P L = MC MC (8) The lateral pressure at the bottom level increased from 3.2 kpa to 29.7 kpa with an increase of 12% d.b. in average moisture content. This corresponded an overpressure ratio of 9.3. Middle and top-level lateral overpressure ratios were 5.4 and 4.8, respectively. The overpressure value at the bottom was much higher than the one obtained from first series of tests while the middle and top level values were close to ones obtained from the first series. The 1218 TRANSACTIONS OF THE ASAE

9 se 1795 ms 7/9/01 11:40 AM Page 1219 Figure 12 Average changes in lateral pressures at three measuring levels for the second series of tests. regression equation (p < 0.01) for the bottom level overpressure ratio with R 2 = 0.98 is: R oνp =e MC (9) The hoop and axial stresses changed as the average moisture content of grain increased during the wetting experiments. The most significant increase in stress on the bin wall was in the hoop stress at the bottom level. The initial hoop stress at the bottom level was kpa. This value reached kpa after an increase of 12.5% d.b. in average grain moisture content which corresponded to a ratio of 9.1. However, the axial compressive stresses at the three measuring levels did not change much with increasing moisture content of grain (fig. 13). Only compressive axial stresses were observed at all three measuring levels during wetting of grain. Figure 13 Changes in axial stress at three levels with increasing grain moisture content (second series). VOL. 43(5):

10 se 1795 ms 7/9/01 11:40 AM Page 1220 COMPARISON OF THE RESULTS OBTAINED FROM TWO SERIES OF EXPERIMENTS In the first series of experiments with the model bin wall supported independently from the floor, the vertical force on the bin wall decreased as the grain wetted. This corresponded to a decrease in compressive axial stresses. The axial stresses at all three measuring levels switched from compressive to tensile stresses as the average moisture content increased. This was attributed to the upward movement of grain along the bin wall caused by increasing internal friction. On the other hand, the axial stresses in the second series of experiments did not show the same behavior. The axial stresses at all three measuring levels were in compression through out the entire wetting process. Only slight decreases were observed. This might be attributed to a slip-stick behavior between the grain bulk and the stable bin wall. As the grain wetted, the combined expansion of individual kernels caused grain bulk to move upwards but the grain bulk slipped because of the stiff and stable bin wall. The other noteworthy difference between two experiments was the difference observed in the increase in lateral pressures. The maximum lateral overpressure ratio at the bottom measuring level observed in the first series of experiments was 4.8 for an 11% d.b. increase in average moisture content while it was 9.3 for a 12% d.b. increase in the second series of experiments. In the second series of experiments, the bin wall was connected to the bin floor which caused the bin wall to be more stable and stiff. This resulted in a stiffer wall that displaced less and offered greater restriction of the grain bulk movement in horizontal direction. Similar differences were observed for hoop stresses on the bin walls in the same pattern as lateral pressures. CONCLUSIONS Based on the data collected and analyzed during two series of wetting experiments, the following conclusions were drawn from this study. From the first series of experiments with the bin wall supported independently from the floor: Vertical floor loads increased as the grain wetted, and the load on the bin floor reached a slightly higher value than the total weight of grain. The vertical wall forces on the bin wall decreased as the grain was wetted, caused by forces in the upward direction due to grain expansion. Changes in grain moisture content have a large effect on the lateral pressures. With an increase in the average moisture content of approximately 11% d.b., the lateral pressure on the bin wall increased as high as five times the pressure prior to wetting. Janssen s equation was found to be adequate and safe for predicting static pressures (without change in moisture content) with the proper values of the constants µ and k. However, Janssen s formula is inconsistent when the moisture content of grain increases. The hoop stresses on the bin wall increased five times with an increase of 11% d.b. in grain moisture content. The increasing grain moisture content did not have considerable effects on the axial stresses on the bin wall. The axial stresses switched from compressive to tensile stress with increasing grain moisture content. From the second series of experiments with the bin wall connected to the floor: The lateral pressures increased dramatically with increasing grain moisture content. The average overpressure ratio for the bottom level was nine with an increase of 12% d.b. in grain moisture content. The grain moisture content increase corresponded to a large increase in the hoop stresses on the bin wall. The average hoop stress on the bottom level reached a value which was nine times greater than the initial average stress. The increase in grain moisture content caused only slight decreases in the compressive axial stresses on the bin wall. Based on the results and conclusions from both series of experiments, it is shown that the increase in grain moisture content causes large increases in lateral pressures and hoop stresses. Furthermore, larger increases were observed when the bin wall was connected to the floor compared to tests when the bin wall and floor were supported independently. The differences observed between the two series of tests shows the importance of considering the wall support conditions when evaluating the results of model tests or designing grain bins for grain moisture content increase. REFERENCES Arnold, P. C., and A. W. Roberts Fundamental aspects of load-deformation behavior of wheat grains. Transactions of the ASAE 12(1): ASAE Standards, 45th Ed. 1998a. EP Loads exerted by freeflowing grain on bins. St. Joseph, Mich.: ASAE b. S352.2 DEC92. Moisture measurement Underground grain and seeds. St. Joseph, Mich.: ASAE. Blight, G. E Swelling pressure of wetted grain. Bulk Solids Handling 6(6): Britton, M. G., Q. Zhang, and K. McCullagh Moisture induced vertical loads in model grain bin. ASAE Paper No St. Joseph, Mich.: ASAE. Deutschman, A. D., W. J. Michels, and C. E. Wilson Machine Design Theory and Practice. New York, N.Y.: Macmillan Publishing. Giner, S. A., and E. Denisienia Pressure drop through wheat as affected by air velocity, moisture content and fines. J. Agric. Eng. Res. 63: Hao, D., Q. Zhang, and M. G. Britton Effects of grain moisture content on dynamic loads during discharge in a model corrugated steel bin. Canadian Agric. Eng. 36(2): Janssen, A Versuche uber geitreidedruck in silozellun (Research about grain pressure in silos). Zeitschrift des Vereines deutcher Igenieure 39: Kramer, H. A Factors influencing the design of bulk storage for rough rice. Agric. Eng. 25: Lorenzen, R. T Moisture effect on ratio of principal pressures in stored grain. ASAE Paper No St. Joseph, Mich.: ASAE. Moysey, E. B The effect of grain spreaders on grain friction and bin wall pressures. J. Agric. Eng. Res. 30: National Instruments Labview 4.0, Graphical Programming for Instrumentation. Austin, Tex.: National Instruments TRANSACTIONS OF THE ASAE

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