lateral and vertical pressure in grain bins

7/27/2019 Lateral and Vertical Pressure in Grain Bins

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Lateral and vertical pressures in two different

full-scale grain bins during loading

Presiones laterales y verticales durante el llenado

de diferentes silos para granos

S.A. Thompson*1, N. Galili2 and R.A. Williams1

1Biological andAgricultural Engineering Department, University of Georgia,Athens, GA 30602-4435, USA2Power and Machinery Division,Agricultural Engineering Department, Israel Institute of Technology, Haifa, Israel

Lateral and vertical floor pressures were measured in two different corrugated-walled steel grainbins using load cells mounted on the floor and walls of the bin. Bin one was 12.8 m in diameterand 17.1 m tall and bin two was 11.0 m in diameter and 14.0 m tall. Tests were conducted withcorn. In the 12.8 m diameter bin the largest average lateral wall pressure was 28.2 kPa at a graindepth of 15.2 m, while in the 11.0 m diameter bin the largest average lateral pressure was 26.9kPa at a grain depth of 11.9 m. Design standard EP433 produced only slightly more conservativelateral wall pressure values at larger grain heights than design standard DIN1055. In both the12.8 m and 11.0 m diameter bins a significant decrease in vertical floor pressures was measurednear the wall of the bins (> 0.85r), while at the other load cell locations the vertical floor pressurevalues were very similar in magnitude. In design, both EP433 and DIN1055 underestimated thevertical floor pressures which were measured in these bins. Based on these results some thoughtshould be given to designing metal bins for both initial conditions in which grain slides alongvirgin materials and have a high coefficient of friction, and for worn conditions in which oilsand waxes have been deposited on the bin walls and thus produce a low coefficient of friction.

Keywords: pressure, grain bin, loading

Se han determinado las presiones laterales y verticales de dos silos para almaccnar grano conparedes corrugadas mediante celdas de carga montadas en la base y en las paredes de los silos.El primer silo tenia 12.8m de diimetro y 17.7 m de altura, mientras qm el segundo era de 11.0m de d13nietro y 14.0 m de altura. Se Litiliz6 niaiz para llevar a cabo las pruebas. Las presioneslaterales miximas en el silo de 12.8 m de di6metro fueron de 28.2 kPa a una profundidad delgrano de 15.2 m, mientras que en el silo de 11.0 m de diametro las presiones laterales maximafueron de 26.9 kPa a una profundidad del grano de 11.9 m. En terminos de diseno, el calculomediante la norma EI433 condujo a una presion lateral ligeramente mas alta a mayores alturasdel grano que mediante la norma DIN1055. En ambos silos se obser%6 una disminucion signi-ficativa de las presiones verticales en la base medidas cerca de la pared de estos (> 0.85r). En las

otras celdas de medici6n los valores de presiones verticales en la base fueron muy similares paraambos silos. En t6rminos de diseno, tanto el estindar EP433 como el DIN1055, subestimaron las

presiones verticales en las bases medidas para los respectivos silos. Segllll estos resultados, serecomienda que en el diseno de silos metalicos se tome en cuenta las condiciones iniciales cuando

se emplean niateriales virgenes con los que se obtienen un alto coeficiente de friccion, y las condi-ciones gastadas en las que los residuos depositados de ceras y aceites pueden provocar unadisminucion del coeficiente de fricci6n.

_

PulU~rcm clrn~u: presion, silos, carga

*To mlroru corrc~syorrc~muco slrould LIt smnt.


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INTRODUCTION

While much can be learned from s111111-scdle

experiments, the sizing factor and modelling effectswhich occur during model bin studies are not totallyunderstood. Therefore, full-scale bin studies are also

needed. To address this deficiencv we measured thelateral and vertical floor pressures during filling intwo different full-size, corrugated-waned steel grainbins for various grain heights and bin diameters.The results of these tests were then compared todesign loads suggested bv different design cudes andstanda rds.

MATERIALAND METHODS

Procedure ..

Experiments were conducted to determine thevariation in lateral and vertical floor pressures in twodifferent corrugated-waned grain bins. Bin one was12.~ lm in diameter and had a side mall height of17.1 m. Bin two was 11.0 m in diameter and had a

side cvall height of 14.0 m. Both bins had horizontalcorrugations which were 66.7 171171 apart and 13.3 m111

deep. Vertical internal wall stiffeners were locatedaround the circumference of the bins which had a tlat

hopper bottom (0) with elevated tloors. These binswere located at a commercial feed mill used to

manufacture feed products for the poultry industry.Lateral and vertical floor pressures were measured

using diaphragm-type load cells located at discretepositions on the floors and watts of the bins.The diaphragm-type load cclls were each equippedwith a pressure compensation device (PCD)(Gailli and Thompson, 1989) which consisted of aplywood plate and a flexible rubber bag filled withan incompressible liquid. The pressure cell was

installed in the middle of the ptywood plate, Hushwith its surface, and the flexible bag was then placedin contact with the sensing element of the pressurecell and bonded to the plywood plate around itscircumference. The PCD was intended to overcome

problems normllly associated with diaphragm-typetoad cells by: (i) reducing the load concentrationeffects close to the sensor; (ii) reducing the deflectionat the grain-bag interface; and (iii) maintaining auniform pressure distribution across the sensing area,independent of the cell-dielphrlgm deflection. The bagmembrane connection was designed to transferfrictional shear stresses, such as those normllly foundin bins, to the ptywood plate through the bondbetween the bag membrane and platc boundary.Therefore, it was assumed that only pressures pJerpJeii-dicular to the sensing face should be measured duringtesting.

Load cells were tocated on the floors and waits ofthe bins as shown in Table 1. During any given testtwo different sets of toad cells were locelted ellong twodifferent lines in a bin. Both lateral and vertical floor

pressure readings were taken during filling attapproximately 30 min intervals, which represents thetime required to empty approximately one half of arailroeld car (capacity of H)1 tonnes) into either binbased on the grain hand ling system.A single testconsisted of filling the bin and then untoading itshortty after filling ceased. During any given test, the

grain was not attowed to stay in the bin longer than2 weeks. Bin one (12.8m diameter) took approx-imately 1.5-2.0d to fill (capacity of 19 train cars) andbin two (11.0 m diameter) approximately 1 d (capacityof 11 train cars). The time required for filling wasdictated by worker schedules, rail car schdulesand the amount of grain currently in storage.

For all tests corn 41~d5 used which had an average

uncompacted bulk density of 718 kg/m and anaverage moisture content of 15% (by weight) (as

Table 1. Load cell locations in the two grain bins.

Tabla 1. Posiciones de las celdas de carga en los dos silos.

The load cell locations for determining lateral pressures were measured with respect to the floor of the bin.

The load cell locations for determining vertical floor pressures were measured with respect to the centre of the bin.


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reported to the authors by personnul at the feed mill).Because this is a commercial feed mill, the same grainwas never used for any two tests.

Bin one (12.8 m diameter) was equipped so that itcould be filled by one auger system, while bin two(11.0 m diameter) was equipped such that it could befilled by two different auger

systemsat the same time.

The inlet spouts to both bins were centrally locateci.However, the grain How out of the spout into bin onewas not straightened prior to flowing into the bin, soa slightly off-centre pile (grain peak < 1 m from thecentre of the bin) was formed at small grain heights.As the height of the grain in the bin increased, theeccentricitv of the pile decreased until a symmetric,1llyshaped grain mass was formed when the bin was full.Bin two was centricallv filled.

Grain heights were measured in the bin bytowering a weighted tape down to the grain mass

througha man-hole locited in the roof of the bin nearr

the side-w


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Figure 1. Lateral pressures in a 11.0 m diameter grainbin measured during filling.

Figura 1. Presiones laterales del silo de grano de 11,0m de di6metro medidas durante el llenado.

Figure 2. Lateral pressures in a 12.8 m diameter grainbin measured during filling.

Figura 2. Presiones laterales en un silo de grano de12,8 m de di6metro medidas durante el Ilenado

grain bins. Figure 3 illustrates the lateral pressures inthe 11.0 m bin measured by the load cells located1.0 m above the floor of the bin, and Figure 4 showsthe lateral pressures in the 12.8 m bin measured bythe load cells located 5.1 m above the floor of the bin.Like Figures 1 and 2, less variation occurred in the

11.0 m diameter grain bin than in the 12.8 m diam-eter grain bin. As expected, the lateral pressuresincreased exponentially as predicted by Janssensequation (Janssen, 1895).

Best-fit equatioll for the lateral pressllre drrtn

Table 2 lists the best-fit values of p and k determined

for the 11.0 m and 12.8 m diameter grain bins for theoverall data set. Ross et rtl. (1987) proposed that J.. andk are interrelated in that the greatest effect of ~.involves displacing Janssens equation up and down

Figure 3. Lateral pressures, measured during filling,in a 11.0 m diameter grain bin for load cells located 1.0m from the floor of the bin

Figura 3. Presiones laterales durante la carga del silo

de grano de 11,0 m de di6metro medidas con celdas de

carga situadas a 1,0 m de la base.

Figure 4. Lateral pressures measured during filling ina 12,8 m diameter grain bin for load cells located 5.1 mfrom the floor of the bin.

Figura 4. Presiones laterales medidas durante lacarga del silo de 12,8 m de di6metro medidas con celdasde carga situadas a 5,1 m de la base.

while greatest effect of k is on the slope of the Janssenequation. In the 11.0 m diameter grain bin much

higher valueswere

found using the previouslydescribed form of Janssens equation, and twodifferent values of p were determined which could

have been used to obtain almost identical results.

Table 3 catalogues the results of the NLIN pro-cedure using Janssens equation for each bin atdifferent grain heights. Values of ju were tested overa range of 0.4-0.6. In the 12.8 m diameter grain bin,k varied from 0.43 to 0.61 for all grain heights and allvalues of .t. In this bin, a A value of 0.6 was associ-ated with the best-fit condition for all grain heights.In the 11.0 m diameter grain bin, k varied between


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Table 2. Best-fit values ofA and k for the lateral pressures measured during filling.

Tabla 2. Valores del mejor ajuste de tt y k para las presiones laterales medidas durante la carga de los silos.

Table 3. Best-fit values of u and k at different grain heights (lateral wall pressures).

Tabla 3. Valores del mejor ajuste de J.1 y k a diferentes alturas de grano (presiones laterales en la pared).

Grain height refers to the adjusted grain height above a load cell at any load cell location.

0.48 and 0.58 for all grain heights and all values of u.In this bin, no one value ofA could be used to obtain

the best fit at each grain height. However, in manycases only slight improvement in the R2 value wasobserved between two or three different A values.

Conrpcmisou of mc~c~strr~co~ Ul1l/1es to design standards

Table 4 lists the design values for p (the side wallfriction coefficient), k (the lateral to vertical pressurecoefficient), and 7t (the bulk density of the storedmaterial) as suggested by DIN1055 and EP433 forthese storage conditions. .

Table 4. Design values ofA, k and w as specified byEP433 and DIN 1055 for Janssenss equation..

Tabla 4. Valores de diseno de p, k y w especificadospor los est6ndares EP433 y DIN1055 para la ecuaci6n

de Janssen.

For the 11.0 m diameter grain bin (Figure 5), bothDIN1055 and EP433 estimated pressures that

exceeded themean

measured lateral pressure for allconditions except one. For the 11.0 m diameter grainbin, the lateral pressures estimated bv DIN1055 were

considerably more conservative at lower grainheights, while EP433 was much more conservative atlarger grain heights. For the 12.8 m diameter grain bin(Figure 6), DIN1055 exceeded the mean lateralpressures for all grain heights, while EP433 exceededthe mean lateral pressure for grain heights greaterthan 3.0 m. Both DIN1055 and EP433 exceeded the

upper boundaries (mean plus one standard deviation)for grain heights of 13.7 and 15.2 m.

Vertical floor pressures ,

Figure 7 displays the vertical floor pressures for theload cells located 2.74m from the centre of the 11.0

m bin. The relevant data comprise five different tests,but like the lateral pressures, those data points on theboundaries (either very high or very low) werenormally measured by a single load cell during onediscrete test. The percent variation in vertical floor

pressure at any given grain height was much less thanthat of the lateral wall pressures measured during


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Figure 5. Comparison of the mean lateral pressuresmeasured in the 11.0 m diameter grain bin to thosepredicted by EP433 (-) and DIN1055 (- - -) standards.

Figura5. Comparac16n de las

presioneslaterales

medias del silo de grano de 11,0 m de di6metro,

con las predichas por los est6ndares EP433 (-) yDIN1055(- - -).

Figure 6. Comparison of the mean lateral pressuresmeasured in the 12.8 m diameter grain bin to thosepredicted by EP433 (-) and DIN1055 (- - -) standards.

Figura 6. Comparaci6n de las presiones lateralesmedias del silo de grano de 12,8 m de di6metro, con las

predichas por los est6ndares EP433 (-) y DIN 1055 (- - -).

these same tests. Schwab et rrl. (1989) observed similarresults in a bin 4.1 m in diameter: the variation in

vertical floor pressures was much less than that of the

lateral wall pressures during filling of the test bin.Figure 8 shows the data for two different sets of

load cells for the 11.0 m diameter grain bin, includingthe vertical floor pressures measured by the load cellslocated 1.83 m and 3.66 m from the centre of the bin.

In the derivation of Janssens equation (1895), constantt

Figure 7. Vertical floor pressures measured in the11.0 m diameter grain bin for load cells located 2.74 mfrom the centre of the bin.

Figura 7. Presiones verticales medidas en la base del

silo de 11,0 m de di6metro con celdas situadas a 2,74

del centro.

Figure 8. Vertical floor pressures measured in the

11.0 m diameter grain bin for load cells located 1.83 m(0) and 3.66 m (j} ) from the centre of the bin.

Figura 8. Presiones verticales medidas en la base del

silo de 11,0 m de di6metro con celdas colocadas a 1,83m (0) y 3,66 m (j}) del centro.

Figure 9. Vertical floor pressures measured in the

12.8 m diameter grain bin for load cells located 3.66 m(8 and 4.72 m (*) from the centre.

Figura 9. Presiones verticales medidas en la base delsilo de 12,8 m de di6metro con celdas situadas a 3,66m (0) y 4,72 m (*) del centro.


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pressures were assumed to occur over a vertical plancin the bins.At these two locations very similar verticalfloor pressures were measured. In the 11.0 111 diameter

grain bin similar vertical pressures were measured attoad cell locations 1.83, 2.74 and 3.66 m from thecentre of the bin. Figure 9 illustrates the vertical floorpressures measured

bythe load cells located 3.66 and

4.72 m from the centre of the 12.8 m diameter bin.Atthese locations very little overlapping of the dataoccurs; in this case the measurements from the loadcells located close to the wall were much less thanthose from the load cells near the centre of the binwhose vertical tloor pressures were measured ant threediscrete locations. The vertical floor pressuresmeasured by the load cells located at 2.13 and 4?7 mfrom the centre of the bin were found to be very

similar, while those measured by the toad cells located5.48 111 from the centre of the bin were significantlysmaller. This decrease in

pressureat this location is

believed to be caused by the interaction of the floorwith the wall.

In both the 11.0 (Figure 10) and 12.8 m (Figure 11)diameter grain bins the vertical floor pressuresmeasured by the outermost load cells vvere slightlysmaller than the vertical tloor pressures measured at

the other load cell locations. Vertical floor pressuresmeasured by the innermost load cells were not

significantly different from each other, while the loadcells nearest the wall in both bins were significantlysmaller.At grain height-to-diameter ratios less th,in

1.6,Schwab ~ al.

(1989)determined similar

verticalpressure distributions in which the vertical floor

pressures nearest the centre of the bin were

slightly larger than those near the will uf the bin. Inthe study by Schwab et nl. (1989), a second pressurepeak at approximately 83% of the bin radius wasdetermined which became more prevalent at grainheight-to-diameter ratios greater than 1.6. In both binsused in this work the grain height-to-diameter ratiowas 1.5 or less. Schwab et rtl. (1989) reported a signif-icant decrease in vertical floor pressures at 95% of the

bin radius, while a significant decrease was observed

to occur in both bins at 85% of the bin radius. Thisdifference in location could have been caused byfactors such as: (i) difference in bin size; (ii) differencein measuring technique; (iii) test materials; and (ia)depth of material at which the vertical floor pressureswere measured.

C(1111y(7!ls0ll Of tire Z~(IflC(1J_f~(~l~J yJ(ss111(s ill tire two ~~Ills

Vertical tloor pressures were measured in both binsat 0.33, 0.67 and 0.85 of the bin radius. For all

locations and grain heights the vertical floor pressures

Figure 10. Mean vertical floor pressures at discretegrain heights and load cell locations within the 11.0 mdiameter grain bin.

Figura 10. Presiones verticales medias medidas adeterminadas alturas y posiciones de las celdas dentrodel diametro del silo de 11,0 m.

Figure 11. Mean vertical floor pressures at discretegrain heights and load cell locations within the 12.8 mdiameter grain bin.

Figura 11. Presiones verticales medias a alturas yposiciones de celdas discretas dentro del di6metro delsilo de 12,8 m.

were (as expected) larger in the 12.8 m diameter grainbin than in the 11.0 m diameter grain bin. For thesethree positions the vertical floor pressures in the12.8 m bin were 1.05 to 1.75 times those in the 11.0 m

diameter bin, with an average pressure ratio of 1.23.

Ignoring the 3.1 m grain height, the ratio of verticaltloor pressures in the two bins was reduced to 1.19.

Using Janssens equation and the material propertiessuggested by EP433 (ASAE, 1995b) as shown in Table4, the pressures in the 12.8 m diameter grain bin wereestimated to be 1.01 to 1.06 times those in the 11.0 m

diameter grain bin over these same grain heights.

Bc~~t-fit equation for the ucrtical f700r hrc~~~urr~o data

Values of pk (the product of the coefficient of wallfriction and the lateral-to-vertical pressure ratio)


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Table 5. Best-fit values of pk during filling (verticalfloor pressures).

Tabla 5. Valores del mejor ajuste de pk durante lacarga de los silos (presiones verticales en la base).

were determined statistically while holding the othervariables in Janssens equation constant (Table 5). Forboth bins no single value of J1.k could be used as abest fit for both bins or all load cell locations.

In order to use Janssens equation, a value for zv(the bulk density of the stored materials) was

required.An

uncompactedbulk

densityof 718

kg/m-was used for all calculations with a pack factor of5.5%, which is similar to the analysis used withthe lateral pressure data in both the 11.0 and 12.8 m

diameter bins.

The walls of the bins were observed to be slick andwell coated with oil and wax deposits from the storedgrain. Therefore, the coefficient of friction between the

grain and the bin walls was thought to be very small.Thompson et al. (1988) determined that during exper-iments in which wheat was repeatedly passed overa galvanized steel surface, a 39% decrease in wallfriction occurred

duringa

wearingin

process.This

decrease was attributed to the deposition of oils andwaxes from the seed coat of the grain onto thegalvanized steel surface. In EP433 the product J.Lk is0.185. If a 39% decrease in the coefficient of friction

is assumed to be caused by the repeated process ofgrain sliding on the galvanized steel walls of thesebins, a new value of J1.k of 0.113 would be obtained.In these two bins the overall best-fit values of J1.k were

0.113 for the 12.2 m diameter bin and 0.135 for the

11.0 m diameter bin. This supports the hypothesis thatdifferent values of J.L may need to be assumed during

the design life of a grain bin. Fornew

bins inwhich grain slides along virgin materials, a highercoefficient of friction may need to be assumed whichcauses higher lateral wall pressures and vertical wallloads, while on worn materials a lower coefficientof friction may need to be assumed which causes

higher vertical floor pressures.

Comparison of measured val lies to design Sfl7ltC~171l~S

Values of the mean vertical floor pressures measured

in each bin appear as: (a) which is the average of all

measured values within the bin at a discrete grainheight, and (b) which does not take into account thepressures measured by the load cell nearest the wall.

In the 11.0 m diameter grain bin (Figure 12), theaverage floor pressures were (a) 1.04-1.08 and (b)1.13-1.17 times the predicted values using EP433. Inthe 12.8 m diameter grain bin (Figure 13) the averagefloor pressures were (a) 1.08-1.18 and (b) 1.18-1.29times the predicted values using EP433. UsingDIN1055, the average floor pressures were 1.19-1.55

times the predicted pressures in both bins. Suggestedmaterial properties for corn used by DIN1055 andEP433 in Janssens equation are shown in Table 4.

Figure 12. Mean vertical floor pressures compared tothe vertical floor pressures predicted by EP433 (0) andDIN1055 (*) for the 11.0 m diameter grain bin.

Figura 12 Presiones verticales medias medidas en labase del silo de 11,0 m de di6metro, predichas por losest6ndares EP433 (S) y DIN1055 (*).

Figure 13. Mean vertical floor pressures compared tothe vertical floor pressures predicted by EP433 (8) andDIN1055 (*) for the 12.8 m diameter grain bin.

Figura 13. Presiones verticales medias medidas en la

base del silo de 12,8 m de di6metro, predichas por losest6ndares EP433 (8) y DIN1055 (*).


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REFERENCES 4C

ASAE (1995a). Procedure for establishing volumetric capacities ofcylindrical grain bins.ASAE Standard S413. St. Joseph, MI:American Society ofAgricultural Engineers.

ASAE (1995b). Loads exerted by free flowing grains on bins.ASAE Standard EP433. St. Joseph, MI:American SocietyofAgricultural Engineers.

DIN (1987). Design Loads for Structures, Loads in Silo

Compartments. DIN1055, Part 6, Berlin: DeutscheNormen.

Galili, N. and Thompson, S.A. (1989). Force and membrane

pressure cells for stress analysis in granular media. In:2nd European Symposium on the Stress and Strain Behaviourof Particulate Solids - Silo Stresses. CHISA, Praha,Czechoslovakia,August 26-31.

Janssen H. A. (1895). Versuche ber Getreidedruck inSilozellen. Zeitschrift, Verein Deutscher Ingenieure 39:1045-1049.

Moore D.W., White G.W. and Ross I. J. (1984). Friction ofwheat on corrugated metal surfaces. Transactions of the

ASAE 27: 1842-1847.

Ross I.J., Bridges T.C. and Schwab C.V. (1987). Vertical wallloads on conical grain bins. Transactions of theASAE 30:753-760.

SAS (1985). SAS Users Guide. Cary, NC: SAS Institute.

Schwab C.V., Ross I.J., White G.M and Colliver D.G. (1989).Investigation of the grain pressure phenomenon in a full-scalebin. Part I: Grain loads and flow characteristics.ASAE PaperNo. 89- 4007A. St. Joseph, MI:ASAE.

Thompson S.A., Bucklin R.A., Batich C.D. and Ross. I.J.(1988). Variation in the apparent coefficient of friction ofwheat on galvanized steel. Transactions of theASAE 31:1518-1524.

lateral and vertical pressure in grain bins

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