Grain Silos Collapse On Farms

Press, Volume CIV, Issue 30686, 27 February 1965, Page 8

Grain Silos Collapse On Farms

(This article was written by

C. J. Crosbie,

iarm advisory

officer (machinery). Department of Agriculture. Christchurch.)

The sudden collapse of two 41-ton bulk grain silos in Canterbury in recent weeks amply illustrates that some fanners do not appreciate the forces involved in restraining a mass of bulk grain.

The 41-ton silo is built from sheets of 12ft galvanised roofing iron and is 15ft in diameter. It is built on a concrete base and has a galvanised conical roof. In Case I, the silo had been built this season, filled one afternoon, the top of the grain had been levelled out and the staff had departed. Shortly after they left there was a roar and the silo collapsed, spewing nearly 1600 bushels of wheat out over the ground —the staff were amazed how much ground such a quantity of grain covered. Urgent telephone calls brought the ready assistance of a local transport firm with their trucks and augers and by midnight the main mass had been augered up and carted away. After midnight it rained, but only a small quantity of grain was dampened. The history in Case II was slightly different The silo had been built last season and filled to a depth of about 9ft and the grain had stored well. This season, however, it was filled to the full depth of 12ft and about 10 days after filling it collapsed. The weather after filling had been average to cool, but on the day in question it had been hot. The silo collapsed at 9.30 p.m. in the cool of the evening. Neighbours rallied with trucks and augers so that little grain got wet when it rained the second night. The wet grain was dried on the farm in question.

The Department of Agriculture hand-out plans of such silos calls for use of two jin bolts in the band joints. In both cases the initial failure was in the jin gutter bolts, which sheared off and allowed the seams to open. When grain is stored in a silo it presses out on the walls with a force of about 201 b per sq. ft. for each foot of depth. Thus, a 12ft deep mass exerts a force of 2401 b per sq. ft. at the base of the walls. This linear multiplication of the forces involved is not the true condition — complicated formulae involv-

In both cases a vertical seam in the walls opened up and the adjoining sheets tore away from the foundations allowing the grain to spew out As it did so, the silo leaned towards the fractured side. This caused a secondary fracture allowing release of more grain and adding to the confusion. It is estimated that only about 10 tons of wheat remained on the concrete base of each silo. In both cases the horizontal galvanised bands encircling the silos had been bolted together with two {in gutter bolts through the lapped ends.

ing several factors are necessary—but it approximates the conditions in shallow silos as used on farms. The tension in a band holding the bottom foot of a silo wall is obtained by multiplying this force by the silo diameter and halving the result.

So for a 12ft diameter silo x 12ft high (27 tons) the tension in the bottom band is 12 x 20 x 12 over 2 14401 b; and for a 15ft diameter silo (41 tons) the tension is 18001 b, or 36001 b when heavier bands are used at 2ft spacing.

Tests carried out last week for the Department of Agriculture by the Engineering School, University of Canterbury, using their Universal Tester to obtain the breaking strain of bands and the sheaf strength of bolts, were most interesting. A IJin x 14 gauge galvanised strap as used at 12in centres on the bottom of 27ton silos yielded at 43001 b and broke at 58501 b. For the purpose of calculations the yield strength only is used because the band may be said to have failed when this point is reached. Allowance must also be made for

the reduced strength resulting from drilling the threeeighths inch holes in the bands. Other tests indicated that Ijin x 10 gauge strap as used on 41ton silos at 2ft centres yielded at 90001 b. On the other hand a jin gutter bolt sheared at 14001 b, and two of them together at 29501 b. Three of them used in the one joint sheared at 46001 b. A five-sixteenths inch bolt sheared at 34001 b lbs (two of them at 67501 b). However, a three-eighths inch bolt sheared at 42501 b. This bolt was jin long and had been threaded right up to the head so that less than the true diameter of the bolt was acting to resist the shear forces. A three-eighths inch bolt by lin long which has a short section of shank of the true diameter was tested, but the band broke before the bolt sheared. It was calculated to shear at about 56001 b. Two pieces of Ijin x 14 gauge bands were lapp joined by electric welding with a light rod using four welds (each 'jin long) on either side over a distance of about four inches. Two other pieces of It inch by 17 gauge were similarly joined with a total of 10 welds each f-inch long. In both cases the bands broke in the test before the welds failed —at 5300 and 48001 b respectively. Thus, the recommended use of two three-eighth inch bolts in the band joints when acting together are equal to or stronger than the bands being used. Important It is most Important when drilling the holes in the bands to clamp the ends together and to drill both holes straight through both bands, for only in this way will the faces line up and transmit the forces equally on both bolts. If the four holes are drilled separately, the forces come first on one bolt, which may shear and overload the second bolt. It can now be appreciated that gutter bolts are inadequate for the forces involved, and they should be replaced if they have been used. Recent inspection revealed three other farmers had used such bolts and their replacement was recommended.