Structural loads on bunker silo walls: Experimental study

J. agric. Engng Res. (1991) 50, 2 7 3 - 2 9 0

Structural Loads on Bunker Silo Walls: Experimental Study Q I o l u Z H A O , t J. C . JOFRIET* t Gore and Storrie Limited, Consulting Engineers, Toronto, Ontario MZ! 5B6, Canada * School of Engineering, University of Guelph, Guelph, Ontario N1G 2Wl, Canada

(Received 4 April 1990; accepted in revised form 27 April 1991)

An experiment to obtain wall pressures in a large bunker silo was carried out for 2 consecutive years. Normal wall pressures were measured with 18 pressure sensors mounted on a 3.7 m wide by 4.9 m high precast concrete wall panel of a 51 by 90 m bunker silo in Walkerton, Ontario, Canada. The silo was filled with whole-plant corn silage both seasons. Wall pressures were measured during filling, during compaction with a 21 t bulldozer, and for about 3 months thereafter. Results from the 2-year experiment showed that the silage loading increases almost linearly with depth measured from the silage surface. Thus, wall loading exerted by the silage can be expressed as a linear function of a pressure ratio and the silage density; the pressure ratio in this function would be the ratio of the normal wall pressure to the vertical stress in the silage. In the 1987 experiments, silage pressure was found to increase with depth at the rate of about 5-3 kPa/m and in the 1988 experiments at the rate of 6-8 kPa/m. The maximum pressure in 1988 was 28 kPa at the lowest sensor. In 1987, the top of the silage was flush with the top of the wall. In 1988, the silage was piled up well above the top of the wall with a surcharge angle of about 15° . The maximum values exceeded the 1983 Canadian Farm Building Code design load by a factor of 3-8 and 4-5 in 1987 and 1988, respectively. The maximum pressure in 1988 exceeded the new 1990 Canadian Farm Building Code by a factor of about 1.3. The maximum wall pressure due to the weight of the 21 t bulldozer used to compact the corn was 10 kPa near the silage surface, 15% of the pressure under the tracks of the bulldozer.

1. Introduction In C a n a d a , b u n k e r silos are used to store silage for livestock on large beef farms or ranches. M a n y farms feed several t h o u s a n d h e a d of cattle. T h e large a m o u n t of feed required over a year for such a large o p e r a t i o n has led to the construction o f b u n k e r silos with walls up to 6 m high. T h e design o f these m a j o r structures was g o v e r n e d by the 1983 C a n a d i a n F a r m Building C o d e 1 ( C F B C ) which was based on experimental research on very small b u n k e r silos with walls of less than 2.5 m height. Experimental investigations o f pressures on walls of b u n k e r silos can be traced back to the early 1950s. E x p e r i m e n t s were c o n d u c t e d with grass silage on w o o d e n as well as concrete silos. These early investigations were limited to small silos with walls less than 2-5 m high. 2-s C u r r e n t design loads for b u n k e r silo walls in N o r t h A m e r i c a are based on the results o f experiments c o n d u c t e d on these small silos. T h e 1983 C a n a d i a n F a r m Building C o d e 1 ( C F B C ) specified, for b u n k e r silos, a constant design load of 6-7 kPa f r o m a depth o f 0-6 m below the silage surface d o w n to the floor, regardless o f height of the wall. Messer and H a w k i n s 7 investigated several o n - f a r m b u n k e r silos with walls o v e r 4 m high containing grass silage c o m p a c t e d with tractors of up to 7 t. T h e results o f their studies s h o w e d a different load pattern f r o m those r e p o r t e d earlier. T h e lateral wall 273 0021-8634/91/120273 + 18 $03.00/0

~-) 1991 Silsoe Research In~tit,,te

274

Q. Z H A O ; J. C. J O F R I E T

Notation g K m q p

acceleration of gravity, m/s 2 pressure ratio rate of increase of the pressure with depth, k P a / m vertical silage stress, kPa normal wall pressure, kPa

coefficient of regression depth below silage surface at the silo wall, m 19 silage bulk density, kg/m 3 o one standard deviation r z

pressure was reported in the form of an equivalent liquid pressure. The most recent measurements of silage pressures on bunker silo walls were conducted in Sweden by Kangro. a Based on his experimental results, Kangro also recommended a linearly increasing wall pressure with depth. In 1986, a failure was reported in the support buttress of a precast concrete 6.1 m high bunker silo in Alberta. Severe cracks were found in the support buttresses and in the concrete wall panels of three silos with equally high walls. More recently, cracks have been observed in a number of precast concrete buttresses of a 4.83 m high bunker silo in Walkerton, Ontario, Canada. This 51 × 90 m silo is used to store whole-plant corn silage for a 3000 head beef finishing operation. Two independent engineering reports s'~° concluded that the design loads specified in the 1983 CFBC ~ were not adequate for bunker silos with walls over 3 m high. A research project on wall pressures in large bunker silos was started in 1986 as a result of the recommendations in the engineering reports. A testing program including field measurements of wall pressures exerted by silage and compaction equipment was part of the project. The objective of the experiments were (a) to quantify, by experiment, the normal wall pressures exerted by whole-plant corn silage on a relatively high silo wall, and (b) to determine, by experiment, the effect of a compaction vehicle on the wall pressures. This paper reports the results from 2 years of field experiments on a 5 m high bunker silo wall.

2. Experimental methods The full-scale experiments were carried out at the Jemstar Farms Ltd, near Walkerton, Ontario, Canada, which has two large bunker silos with 4-83 m high walls. The walls slope outwards at 80° to the horizontal. One of the two silos was used in the test program. It consists of precast concrete panels (3.7 × 4-9 m) supported by precast concrete A-frames placed at 3-7 m centres. The overall size of the silo is 90 m long, 51 m wide and 4.84 m high. Wall pressure measurements were made by means of strain gauge-based force transducers designed to provide a contact surface with the stored material flush with the face of the silo wall (Fig. 1). The contact surface had a texture similar to that of the silo wall. A 200 mm diameter concrete-filled disk, supported firmly by an L-shaped stainless steel tubular frame, formed the force transducer. Four pairs of strain gauges, each in a half-bridge arrangement, were attached to the stainless steel support frame to measure the bending moments at four locations of the L-shaped frame. This arrangement permits, with one redundancy, the calculation of the normal and the friction force acting on the face of the concrete filled disk, as well as the eccentricity of the normal force. The geometry of the force sensor frame was selected such that the lateral and rotational

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displacement of the face of the sensor is minimal for the expected ratio of normal to downward friction force of about two. To minimize interference with the farm operation and property, a test panel was built equal in size to one of the wall panels of the experimental silo. The test panel had 18 circular openings and all necessary inserts to allow the force transducers to be rigidly supported by the panel (Fig. 2). Three vertical rows of six transducers provided force measurements over an area of about 0.03 m 2 at spacings of approximately 0.66m vertically and 0.92m horizontally (Fig. 3). The transducers were calibrated in the laboratory before mounting and again in the field before the measurements started in early September for the 1987 and 1988 test seasons. In the laboratory weights were applied to the transducers in 8 to 10 increments. Loads were applied normal and parallel to the loading disks in separate calibration tests. The field calibration was restricted to normal loads applied with a specially designed loading rig. The voltage signals from the force transducers were recorded automatically by a HP3497A Data Logger controlled by a HP85F computer. Raw data were stored on computer tapes for later processing on a personal computer.

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The silo was filled with whole-plant corn silage in the 1987 and 1988 test periods. The average length of cut was 10 mm. During the filling process the chopped corn plant crop was transported from the fields to the silo where it was pushed up and compacted by a 21 t bulldozer (Allis-Chalmers 12G Series B). The contact area of the tracks of the 21 t bulldozer is 3-0m long and 0.5 m wide. The compaction was intended to produce uniformly dense silage. One week after the filling process in 1987, the silage was covered with plastic sheets and old tyres and sawdust. Severe flow of silage juice through the transducer holes was encountered in the first few days after filling in 1987 resulted in electronic malfunctions of the datalogger. No reliable data were obtained during the silo filling process and again for about a month between September 23 and October 26. Reliable data became available after the instrumentation problem was solved and weather protection was provided for the electronic instruments. In 1988, pressure data were taken when the bulldozer was passing by the test panel. A

L O A D S ON S I L O W A L L S

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Fig. 3. Test panel installed in position; otew from the rear of the panel before installation of the pressure transducers

remote control device allowed a person sitting on top of the wall to note silage depth and the distance of the machine from the wall and, at the same time, to control the data logging operation. Silage samples were taken of the freshly cut crops. As well, undisturbed silage samples were taken from the vertical unloading face with a core-sampler during unloading of the silo. These samples were used to determine the moisture content and the in situ density of the settled silage. The moisture content was determined according to the standard procedure recommended by the American Society of Agricultural Engineers. 11 3. Results and discussion 3.1. In situ moisture content and bulk density In 1987, the initial moisture contents on a wet basis (w.b.) of the freshly chopped corn varied from 67.7 to 73.5%, with a mean of about 71%. The moisture content of the settled silage in that year varied from 68.1 to 64.6% and increased with depth. The settled silage moisture content was thus on average 5.6% lower than the initial value. Saturation occurred at the floor-silage interface and juice flow was observed. The density test results for the 1987 test period showed that the average bulk density of the silage was slightly higher at the top and the bottom than at mid-height. An analysis of variance, however, showed no significant variation of silage density with depth. This indicates that a fairly uniform compaction was reached at the time of filling. The overall mean bulk density for the 1987 silage was 874 kg/m 3. For the 1988 test period, the observed initial moisture contents of the freshly cut silage (w.b.) ranged from 62-9 to 68.9%, with a mean of about 66%, 7% less than that for the 1987 test period. The moisture contents of the settled silage mass were quite uniform and no significant changes with time were observed. The average moisture content of silage was 66%, which was the same as the average initial moisture content. It should be pointed out that the silage was not covered in 1988. Precipitation therefore may have helped to maintain the silage moisture content.

278

o . Z H A O ; J. C. J O F R I E T

The density of the 1988 settled silage was again fairly uniform. The overall average value of all samples was 822 kg/m 3, 5.7% lower than that in the 1987 test period. Based on the initial moisture content, the average dry matter density in 1988 was 280 kg/m 3 and that in 1987 was 290 kg/m 3. The difference in dry matter density was only about 3.5%. The results from an A N O V A analysis of the 1988 density data showed that the variation of the silage density with height was not significant. 3.2. Silage pressures Force measurements were made every 6 h. The force measurements were divided by the area of the face of the transducers to provide pressures. The normal force and the friction force divided by the contact area will be referred to as normal pressure and friction stress, respectively. Fig. 4 shows a partial cross-section of the silo as it was filled in 1987. The wall slope is 80° with the horizontal and the silage surface for the year was flat (surcharge angle zero deg.). As far as the geometry of the cross-section of the silage mass is concerned, the top surface of the silage is the only parameter that was different in 1987 and 1988. As is shown in Fig. 4, the silage was virtually level with the top of the silo wall in 1987. This was not the case in 1988 as may be seen in Fig. 4. Fig. 5 presents the 1987 normal wall pressures for all six levels versus time. In all cases, the data from the three transducers at one level were averaged. Each data point represents a daily average of 12 measurements. The few available data show that the wall pressure remained quite constant up to the time when the unloading of the silo took place near the test panel in early November. The juice flow problem through the openings in the test wall caused the three transducers at level 6 (0.75 m from floor) to malfunction. As a result, only two points are available for that level in Fig. 5. The friction stresses were about one-half of the corresponding normal wall pressures. The few normal wall pressures measured in 1987 were used to construct the vertical pressure profile shown in Fig. 6. The average of the three measurements at one level

~urfaee (1 Wall panel Flat surface (1987 test)

1 A-frame SiLage mass

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Fig. 4. Partial cross-section of the test silo filled with silage (1987 and 1988 tests)

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Fig. 5. Normal wall pressures versus time (1987 tests) taken at the same time were averaged and treated as a single value before determining the mean, m a x i m u m , minimum and standard deviation. Included in Fig. 6 are: (1) m a x i m u m and minimum values for the test period; (2) the mean value plus and minus one standard deviation, (connected by a horizontal line), and (3) the mean value. The mean values at the various levels are connected by a solid line to improve clarity. The mean normal wall pressure (Fig. 6) varied from 3-64 kPa at level 1 to 18-8 kPa at level 5. The standard deviation varied from 0.42 kPa at level 1 to 1-3 kPa at level 2. If a linear relationship is assumed with a forced intercept, the rate of increase of normal wall pressure with depth is 5.3 k P a / m (fl = 0.9752). Fig. 6 also shows the friction stresses measured in the 1987 tests. T h e average friction stress varied from 0-69 kPa at level 1 to 11-57 kPa at level 5. T h e m a x i m u m standard deviation of the friction stress ( o -- 1-3 kPa) also occurred at level 2. In 1988 the silage was piled up with a surcharge angle of about 15 ° (Fig. 4). The filling of the silo near the test panel started on S e p t e m b e r 6 and ended on S e p t e m b e r 27. T h e progressively increasing normal wall pressures during filling are indicated in Fig. 7 which presents plots of the normal wall pressures at the six levels versus time. Four data points are shown for each day for about 90 days. The pressures underwent little change after completion of the silo filling process on S e p t e m b e r 27. The decrease in normal pressures starting on N o v e m b e r 15 was due to the

280

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Fig. 6. Normal pressures ( ) and friction stresses ( - - - - - - ) versus depth (1987 tests), n, minimum measured value; ×, maximum measured value; - - , range of one standard deviation above and below the mean unloading of silage in the vicinity of the test panel (Fig. 7). The data after that date were not used in the vertical pressure profiles that follow. Fig. 8 presents the normal pressures versus depth for the period from S e p t e m b e r 27 to N o v e m b e r 15, 1988. The friction stresses versus depth for that same period are also shown in Fig. 8. T h e m a x i m u m normal mean pressure at level 6 remained almost constant during the test period at about 28 kPa. The means of the normal pressures at the other levels lie close to a straight line connecting the m a x i m u m pressure at level 6 with zero at the top of the wall. If a linear increase of pressure with depth below the top of the wall is assumed, the rate of increase is about 6.8 k P a / m (r 2 = 0.978). The range of the standard deviations are from 0.26 kPa (at level 3) to 0-79 kPa (at level 5). Similar comments can be m a d e about the means of the friction stresses (Fig. 8). The magnitude of the stress at level 1 was 0-91 kPa, 18% of the normal pressure at that level, and that at level 6 was 12 kPa, 43% of the normal pressure. The increase of the mean friction stress with depth was close to linear.

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Fig. 7. Normal wall pressures versus time (1988 tests)

The average normal wall pressures from both the 1987 and 1988 test data are plotted versus depth in Fig. 9, together with the results reported by Messer and Hawkins 7 and by Kangro 8. The 1983 CFBC 1 design curve is also shown. Messer and Hawkins' results were averages based on six model silos containing grass silage. Three of the silos had vertical walls and the other three had sloped walls (8, 6 and 3 ° respectively with the vertical). Kangro's curve is based on tests on a silo with 2 m high vertical walls. In both Messer's and Kangro's experiments the top of the silage was flush with the top of the silo wall as is the case in the present 1987 tests. The 1987 results were obtained with a set of conditions that was close to that of Messer and Hawkins. 7 The pressures are very similar despite the difference in silage and in density. In 1988, when the silage was piled up above the silo wall, the rate of increase of the pressure with depth was 28% greater than in 1987. This difference is no doubt related to the difference in the surcharge angle which was 0 ° in 1987 and 15° in 1988. The rate of increase in pressure with depth found in this project is much greater than that suggested by Kangro. This may be due to the different test silos and different compaction methods. A wheeled tractor was used in Kangro's experiments. The present results indicate that the design loading in C F B C ' i s not appropriate. BS 5502 specifies a linearly increasing wall pressure with depth with two different rates of increasefl 2 However, it does not consider such factors as wall slope, surcharge angle, and the friction at the silage-wall interface. Nor does the recently published 1990 CFBC~a; it specifies a lateral wall pressure that increases linearly with depth 3.5 + 3.5 z kPa. A linear increase of normal wall pressure (p) with depth (z) can be expressed as: p = mz = Kpgz

where m is a constant, K is the pressure ratio and p silage bulk density.

282

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If the density of the silage is fairly uniform, the vertical stress, q, in the silage increases linearly with depth as well. The normal pressure, p, can therefore be written as the product of the vertical pressure, q, and a ratio of the normal to vertical pressure, K. This ratio is referred to as the pressure ratio and its magnitude can be calculated from the experimental data for normal pressures and from the observed silage density, p: K = p/q = p/(ogz)

(2)

Since the silo wall panel is very rigid relative to the compressible silage, the stress condition in bunker silos corresponds to an " a t - r e s t " state. Fig. 10 displays the mean of the pressure ratios calculated with Eqn (2) at levels 1-5 inclusive for the 1987 test data. The average pressure ratios ranged from 0.56 to 0.73, with a mean of all data of 0.63. Fig. 11 presents the pressure ratios for O c t o b e r and N o v e m b e r in 1988 and Fig. 12 shows the mean values of the pressure ratios at all six levels. After the middle of N o v e m b e r 1988, the ratios for levels 2 and 3 decreased as a result of the unloading

283

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Comparison of 1987 and 1988 test results of normal wall pressures with CFBC (1983) and with research results by Messer and Hawkins ~ (1977) and Kangro a (1986)

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Fig. 10. Average pressure ratios versus time (1987 tests)

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LOADS ON SILO WALLS

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Table 1 Average pressure ratios for 1987 and 1988 tests

Vertical depth (rn)from top of wall Test

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0-63 0-87

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process mentioned earlier. Therefore, the data after N o v e m b e r 15 were not included in Fig. 12. The average pressure ratios ranged from 0.77 to 0.91. The overall mean of the pressure ratios shown in Fig. 12 covering 1.5 months of measurements is 0.84. Table 1 provides a summary of the average K values calculated at the various levels of measurement from the 1987 and 1988 data. In both years, the K values were high at level 2. The values at the other levels were surprisingly consistent. It may be noted that the pressure measurements at level 2 (Figs 6 and 8) always appeared to be higher than expected. The ratio of friction stress to normal pressure was calculated from the measured normal wall pressures and the measured friction stresses. T h e 1988 calculated ratio was in the Normal pressure, kPa 0

5

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Level 2 E

Level 3 E Level 4 ¢-,

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Fig. 13. Normal wall pressures versus depth during filling in the week September 21-27, 1988. II, September 21; +, September 23; ~ , September 24; A, September 25; x , September 26; V , September 27

286

o . Z H A O ; J. C. J O F R I E T

range from 0.14 to 0-53. The ratio c o m p u t e d from the available data in 1987 varied from 0.2 at level 1 to 0-6 at level 6. The average ratio was 0-43. Since the friction coefficient is the ratio of the friction to normal force when the contacting surfaces are just about to move and since there was no way to record m o v e m e n t it is not clear what conclusions can be drawn. The m a x i m u m measured force ratios did compare favourably with the friction coefficient of 0.41 to 0.45 reported by Nilsson. 14 Bounds have been suggested for the values of the friction coefficient. 15 The lower bound is 0.4 and the upper one is 0.5 for corn silage against concrete. The present results a p p e a r to reinforce this recommendation. 3.3. Wall pressure due to compaction During the week of September 21-27, 1988, the filling operation was in process in the vicinity of the test panel. Fig. 13 shows a plot of m e a s u r e m e n t s taken at various times during that loading week versus depth. They are the mean of three sensors located at the same level. As the level of the silage increased, the pressures increased along parallel lines. The m a x i m u m pressure at level 6 measured on 27 S e p t e m b e r was about 28 kPa. The data in Fig. 13 also shows the effect of the compacting equipment at levels 1 and 2. The high m e a s u r e m e n t s on 21 S e p t e m b e r 1988 (at time zero) were due to the proximity of the compacting equipment. In order to determine the effect on pressures of the 21 t Normal pressure, kPa 0 0

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ON SILO

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WALLS

bulldozer, additional tests were carried out with the machine positioned parallel to the wall at clear distances of 0.33 m, 0.6 m, 1.0 m and 2.3 m. The bulldozer was lined up symmetrically about the centre line of the test panel. Readings were taken at each of the four positions and they are plotted versus depth in Fig. 14. Then the machine was removed and measurements were made of the silage pressures which are shown in

Fig. 15. The difference between the total pressures (Fig. 14) and the silage pressures (Fig. 15) are presented in Fig. 16. They can be\considered as "indirect" experimental pressure measurements of the effect of the compacting equipment alone. As might be expected, the pressures were highest nearest the surface of the silage at level 1. The maximum recorded normal pressure was about 10 kPa. It is also obvious that the proximity of the bulldozer should have an effect on pressures. The observed pressures, especially those at level 1, show this clearly. It is also of interest that the normal pressure measurements from the bulldozer at level 3 and below are insignificant. T h e vertical a p p l i e d p r e s s u r e u n d e r o n e track was 70-0 kPa o n an area of 0-5 m x 3 . 0 m .

Therefore, the 0.75 m depth, respectively. Messer and They reported

portion of the applied pressure that was transmitted laterally to the wall at was 5% and 14% for clear distances from the wall of 2.3 m and 0-33 m, Hawkins 7 reported similar findings for wall pressures during compaction. a maximum packing pressure of 14 kPa to a depth 1 m below the silage

Normal pressure, kPa II

0

5 r

10 I N,

N_

15

20

25

30

I

I

I

I

35

Surface Level I

Level 2 E

Level 3 oE -E-

Level 4

Level 5

Level 6

Floor

Fig. 15. Normal wall pressures versus depth from silage only (1988 tests)

288

o.

ZHAO;

J. C. J O F R 1 E T

N o r m a l pressure, kPa 5 I

0

10 L X,

N_

15 20 I I Surface

25 [

30 I

35

Level 1

E 2

E • -~

~. .................................................................

Level 4 •

i

Level 5

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4

Level 6 I ...................................................................

Floor

Fig. 16. Normal wall pressures versus depth .from 21t bulldozer only. Clear distance: • 0.33m;

+0.6Ore; ~ 1.Ore; /~ 2.3m surface when the wheel of a tractor was 6 mm away from a vertical wall. By comparison, the maximum pressure was 10 kPa when the bulldozer was 0-23 m away from the wall (Fig. 16). The effect of the compacting live load at a depth greater than 1.4 m was not recorded by Messer and Hawkins. The effect of compaction was insignificant at a depth greater than 2 m in the present tests (Fig. 16). The 21 t bulldozer, the only vehicle used by the farmer on whose farm the experiments were carried out, is heavier than is typically used in Canada for the compaction of silage in bunker silos. More typical for use are tractors with a mass of 8 to 12 t. 4. Conclusions The following conclusions were drawn from the field tests. 1. Corn silage was found to exert normal wall pressures that increased almost linearly with depth below the silage surface. The rate of increase of the pressure was affected by the profile of the top of the silage. The rate of increase of this pressure with depth calculated from the 1988 data was 6.8 k P a / m when the silage was piled up with a surcharge angle of 15 °. The average silage density was 822 kg/m 3. In 1987 data, the rate of increase was 5.3 k P a / m when the silage was flush with the top of the wall and the average silage density was 874 kg/m 3.

289

L O A D S ON SILO W A L L S

2. The pressure ratio calculated from the experimental results of the 1987 field tests was 0-63; the data collected in 1988 provided a m e a n pressure ratio of 0.84. In 1987 the top of the silage was level with the top of the wall, in 1988 it was piled up at an angle of about 15 ° with the horizontal. 3. Normal wall pressures due to compacting equipment were found to be 15% of the pressures under the track of the compactor. The pressure decreased rapidly with depth below the surface and was insignificant below a depth of 2 m. The pressure was largest when the equipment was positioned closest to the wall. 4. The observed average silage bulk density was 822 k g / m 3 in 1988 and 874 k g / m 3 in 1987. Silage bulk density was related to the moisture content at time of compaction. Statistical analyses showed that for the settled silage no apparent relationships existed between the silage bulk density and the silage depth, nor between the silage moisture content and the depth. 5. Observed m a x i m u m silage pressures near the b o t t o m of the wall exceeded the 1983 CFBC 1 specified design load by a factor of 3-8 in 1987 and by a factor of 4.5 in 1988; the m a x i m u m pressures observed in the 1988 test exceeds the design load in the 1990 CFBC 13 by about 30%.

Acknowledgements The authors are grateful to the Cooke brothers, the owners of the Jemstar Farms Ltd, for their generous and cooperative supports to this project. Also, funds from the Ontario Ministry of Agriculture and Food, Alberta Agriculture and the Natural Sciences and Engineering Research Council of Canada made this project possible.

References 1 Canadian Farm Building Code. NRCC No. 21312. National Research Council of Canada. Ottawa, ON., Canada, 1983 2 Esmay, M. L.; Brooker, D. B. Lateral pressures in horizontal silos. Agricultural Engineering. St Joseph, Michigan 36(10): 651-653, 1955 3 Esmay, M. L.; Brooker, D. B.; McKibben, J. S. Design of above-ground horizontal silos. Agricultural Engineering. St Joseph, MI. 1956 37(5): 325-327,333 4 Young, H. Pressure on walls in bunker silos. South Dakota Farm Home Research; 8(4): 16-18, 1957 Hendrix, A. T.; McCahnont, J. R. Horizontal silos. ASAE paper. Chicago. MI, 1958 a Zoerh, G. C.; Young, H. G.; Delong, H. H.; Moe, D. L. Storage structures for grass silage. Bulletin No. 477. South Dakota Agricultural Experimental Station. SD., 1959 7 Messer, H. J. M.; Hawkins, J. C. The influence of the properties of grass silage on bulk density and horizontal pressure. Journal of Agricultural Engineering Research 1977, 22:55-64 s Kangro, A. Load measurements in bunker silos for silage. Report 48. Department of Agricultural Buildings, Swedish University of Agricultural Sciences, Lund, Sweden, 1986 9 Jofriet, J. C. Comments on the Lawton Bros. bunker silo failure. Private communication with Dennis Darby, Alberta Agriculture, Canada, 1987 lo Turnbull, J. E.; Masse, D. I.; Reid, W. S. Wall pressures for design of horizontal silos. Contribution 1-920, ESRC. Submitted to NRC Standing Committee on Farm Buildings. Ottawa, ON., Canada, 1987 11 ASAE Standards. ASAE $358.1. (3rd Ed.) p. 92. American Society of Agricultural Engineers. St Joseph, MI, USA, 1986 12 BS 5502 Code of Practice for Design of Buildings and Structures for Agriculture. British Standard Institution. London, UK, 1987

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13 Canadian Farm Building Code. National Research Council of Canada. Ottawa, ON, Canada, 1990 1+ Nilsson, L. The Properties of Silage and the Design of Forage Tower Silos Report 19. Department of Farm Buildings, Swedish University of Agricultural Sciences, Lund, Sweden, 1982 is Negi, S. C.; Jofriet, J. C.; Buchanan-Smith, $. Densities, pressures and capacities of corn silage in tower silos. Canadian Agricultural Engineering 1984, 26:43-47