COLLAPSE AND R E P A I R OF T A L L C O N C R E T E SILOS

WITH SUSPENDED STEEL H O P P E R

By German Gurflnkel, 1 Fellow, A S C E

ABSTRACT: A tall reinforced-concrete grain silo collapsed suddenly on a sunny,

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windless, day in the Midwest. Field observations and analysis showed that failure
of the suspended steel hopper triggered the collapse. Attachment of the suspended
hopper to the concrete wall of the silo had been designed awkwardly and was
subject to substantial overstressing in service. The quality of fabrication and weld-
ing of the various components for the hopper-supporting system was poor. The
question of why it took four years of service before the silo collapsed but an ad-
jacent, twin, silo remained standing, is answered. Design of the strengthening re-
quired for the remaining twin silo is also described. Advice to designers of sus-
pended hoppers is also given at length.

INTRODUCTION

A reinforced-concrete silo used for the storage of grain collapsed sud-

denly, and without warning, on a sunny, windless day, on August 28, 1986
at Roachdale, which is a small town situated in west-central Indiana. A twin
silo remained standing (see Fig. 1). Both silos had been built adjacent to
each other in 1982 and had been in operation since then.
Events preceding the collapse of the silo took place as follows. The pre-
vious day the silo was full of corn; four truckloads, for a total amount es-
timated at 4,000 bu and weighing 118 tons (107.3 Mg), were removed using
two existing side-withdrawal outlets in the wall. Later that day 3,000 bu,
weighing 88.5 tons (80.5 Mg), were placed back in the silo using the ele-
vator leg and the roof inlet. The same operation took place in the morning
of the day of the collapse, except that the last 800 bu, weighing 23.6 tons
(21.5 Mg), were placed back in the silo by 1:20 pm. The silo collapsed only
ten minutes later.
The history of usage of the twin silos was obtained from the plant oper-
ators. The collapsed silo had been used almost exclusively for the storage
of soybeans, although it was loaded with corn when it fell down. The "turn"
of the tank, defined as the time elapsed between complete volume changes,
was one week. Compare this to the twin silo which was mostly used for
corn, and for which a "turn" would take at least three weeks. In four years
the collapsed silo had been subjected to a much larger number of filling-
and-emptying cycles than its surviving twin.
When the silo collapsed it was fully loaded with approximately 66,000 bu
of corn weighing an average of 59 lb/bu (47.2 lb/cu ft) (756 kg/m 3 ) at
13% moisture content. The owner determined that this was the heaviest corn
on record for the last two years. To place this in the proper perspective it
should be noted that, even at that record value, the unit weight was lower
than 50 lb/cu ft (801 kg/m 3 ) (which is commonly used for the design of
grain-storage structures), say nothing of an expected 1.7 load factor that
'Prof., Dept. of Civ. Engrg., Univ. of Illinois, Urbana-Champaign, IL, 61801.
Note. Discussion open until April 1, 1990. To extend the closing date one month,
a written request must be filed with the ASCE Manager of Journals. The manuscript
for this paper was submitted for review and possible publication on November 10,
1988. This paper is part of the Journal of Performance of Constructed Facilities,
Vol. 3, No. 4, November, 1989. ©ASCE, ISSN 0887-3828/89/0004-0243/$1.00 +
$.15 per page. Paper No. 24014.
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FIG. 1. Tall Concrete Silo Collapsed and Damaged Surrounding Structures while
Twin, Adjacent, Silo Remained Standing

would guarantee reasonable stresses in the reinforcing steel under service

conditions, and adequate structural strength at ultimate.
In addition to the fundamental question of the cause for the silo collapse,
various other questions remained unanswered at the time. To wit, the fact
that one silo collapsed and its twin remained standing was a puzzle. Why
did it take four years before the silo collapsed? Finally, what should be done
with the remaining silo?
Out of concern for the safety of the remaining silo, which was also full
of corn at the time, the writer recommended that it be slowly emptied of
half of the contents and not be filled beyond that volume until further advice.
The writer was asked by the owner to determine the cause for the collapse
of the silo and recommend a strengthening procedure, if necessary, that would
restore the twin silo to full service. It was not difficult to make the owner
realize that the latter task, although of utmost and immediate importance to
him, could not be carried out without first understanding the collapse. The
investigation that followed was conducted using field observations, a review
of the original design drawings, and independent structural analyses.

DESCRIPTION OF TWIN SILOS

Some background on the silos and their design is necessary at this point.
Each silo consisted of three significant components, namely: (1) A rein-
forced-concrete cylindrical shell; (2) a hybrid concrete-steel roof structure;
and (3) a suspended steel hopper in the shape of an inverted truncated cone
(see Fig. 2). The silo rested on a reinforced-concrete ring foundation.
The concrete wall of the silo was built by jumpforming [in 4-ft (1.22-m)
high increments] a cylindrical shell having a 30-ft (9.14-m) inside diameter
to a height of 128 ft (39.01 m). The thickness of the shell was 10 in. (0.25
m) for the lower 24 ft (7.32 m) of wall above the foundation and 6 in. (0.15
m) for the rest of the wall to the top. The abrupt change in wall thickness
of the shell took place at the outside surface; this avoided creating an inside
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 -4 in. RC Slab on Corruform
Sheets

228'" P^r7C7ygg^S7^C^7^ l i " 16 in. H Steel Joists at 2ft

'23° iq—Equiv. Free Surface
224' of Grain
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• &

222'

GRAIN
\\ '•'.:.,

30ftI.D.

/ - S i l o Wall,
' Reinforced Concrete

t = 6"

lJ2l.5'(Topof
Compression Hopper)
Ring, RC
•Suspended Hopper,
Steel
t =10-
,102.5'
vlOO' - ^
mmmsg- ••to
7777777/ 777777/
G_ \u-Ring
Ri Foundation

FIG. 2. Vertical Cross Section of Silo

ledge and, thus, left the inside surface of the wall continuous throughout the
full height of the silo. The 10-in. (0.25-m) thick wall was reinforced with
No. 8 hoop and vertical bars, at 24-in. (0.61-m) spacings. The 6-in. (0.15-
m) thick portion of the wall was reinforced with No. 6 hoop bars, at a spac-
ing that increased from 5.5 in. (0.14 m) at the bottom to 12 in. (0.30 m)
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\ ,.»...^J-£5-6"Well Compacts
Compacted
_
Gravel
\ f&sfH-'s"- or Stone as Req'i
'd
<--#8xlO'-0"at9"O.C.
IO'-6" Wide
—*»"|
Ring Foundation

FIG. 3. Suspended Hopper and Lower Silo Wall

at the top, and No. 4 vertical bars at 18-in. (0.46-m) spacing. No special
reinforcement was provided for the portion of the wall directly above and
below the level where the abrupt change of thickness takes place.
The roof structure was made of a 4-in. (0.10-m) thick concrete slab at the
perimeter, sloping to a 6-in. (0.15-m) thickness at the center. The slab was
reinforced with 6 X 6-10/10 steel mesh and No. 3 bars at 18 in. (0.46 m)
on centers; it was cast in place on heavy-duty corruform steel sheets sup-
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FIG. 4. Hopper-Supporting System at Concrete Wall

ported by 16-in. (0.41-m) deep steel joists at 2 ft (0.61 m) on centers. The

roof slab "floated" on the silo wall; none of the vertical reinforcement of
the wall was anchored into the roof slab.
Most grain withdrawal took place using a suspended steel hopper in the
shape of an inverted cone. The latter sloped down 45° from the 30-ft (9.14-
m) diameter silo walls towards a 2-ft (0.61-m) diameter outlet at the bottom
(see Fig. 3). The hopper was attached to the concrete walls through a ring
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J. Perform. Constr. Facil. 1989.3:243-264.

 plate (see Fig. 4). The ring plate was 5/8 in. (0.016-m) thick by 6 in. (0.15
m) deep and was embedded to the inside surface of the 10-in. (0.25-m) thick
concrete wall; the hopper was welded to the ring plate all around the perim-
eter. In addition, 9 x 3 X 1 / 2 in. (0.229 x 0.076 X 0.013 m) steel straps,
spaced 18 in. (0.46 m) on centers, were also welded to both the ring plate
and the suspended hopper.
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The ring plate was attached to the concrete walls of the silo by means of
stirrups made out of 5/8-in. (0.016-m) diameter bars, spaced 8.5 in. (0.22
m) on centers; the stirrups were welded to the ring plate. Although the de-
signer showed closed stirrups in his drawings, the contractor used open stir-
rups instead (see Fig. 4). This departure resulted in only 1 in. (0.025 m) of
each stirrup leg welded to the ring plate, instead of the full 6 in. (0.15 m)
required by the designer.
Where the hopper was attached to the concrete walls the designer created
an ad-hoc composite compression ring [within the 10-in (0.25-m) thickness
of the wall] by placing nine No. 8 hoop bars, 8 in. (0.20 m) apart, sym-
metrically located about the supporting stirrups and opposite to the ring plate
(see Fig. 3).
The concrete was specified at a 4-ksi (282-kg/cm 2 ) 28-day standard-cyl-
inder strength, and the reinforcing steel at 60-ksi (4,227-kg/cm 2 ) yield strength.
Presumably, ASTM A615 Gr. 60 rebars were procured by the contractor,
although this fact is not certain. No specification could be found for the steel
used for the hopper structure.

FIELD OBSERVATIONS

Field observations were made that contributed significantly to understand-

ing the collapse of the silo:

1. The upper portion of the silo, i.e., the 6-in, (0.15-m) thick reinforced-
concrete shell and roof, toppled over in one piece and collapsed against adjacent
structures (see Fig. 5).
2. The lower portion of the silo, i..e, the 10-in. (0.25-m) thick reinforced-
concrete shell, split open into two portions which separated from the base and
each other and moved away laterally and outwardly. Most of the wall eventually
toppled over, as the corn rushed out, but a portion did not because it came to
rest against the adjacent twin silo (see Fig. 6).
3. There was a horizontal fracture surface, just above the level of the stirrups,
and a vertical fracture surface, where the ring plate used to be. Both fracture
surfaces developed along the circumferential direction (see Figs. 6 and 7).
4. During the first inspection, made shortly after the collapse, the vertical ring
plate and the suspended hopper were conspicuous by their absence. Actually,
the fracture surfaces indicated that they had sheared off (from their stirrup at-
tachments to the concrete wall) and laid under the corn. Further observations
disclosed a number of fractured stirrup legs.
5. Removal of the corn and the concrete debris exposed the remnants of the
hopper and ring plate completely flattened out (see Fig. 8). Additional obser-
vations disclosed the failure of all (calculated at 133) stirrup attachments of the
ring plate to the concrete wall.
6. All stirrups fractured at either their welded attachments to the ring plate or
at a section just away from it (see Figs. 9 and 10). An incomplete survey showed

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FIG. 5. Concrete Silo Toppled Over

that, of 106 stirrup legs examined, 82 had fractured while only 24 had failed at
the welds. Of the latter, 22 failures had taken place in the upper stirrup legs and
only 2 in the lower legs.
7. The hopper ring plate was fabricated in segments that were welded together

FIG. 6. Concrete Wall Fractured at Hopper Top Level; Note Stirrups

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 Bent Bar

Fractured Stirrup
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Leg

Fractured Concrete
Surface

FIG. 7. Wall Cross Section Indicating Fracture Surfaces

upon installation in the field. The evidence indicated that welding was highly
defective and mostly cosmetic in nature (see Figs. 11 and 12).
8. Instances of insufficient or nonexistent amount of staggering, for lap splices
of hoop bars, were found in the remnants of the concrete wall (see Fig. 13).

FIG. 8. Portion of Vertical Ring Plate and Flattened-Out Hopper; Note Remnants
of Welded Stirrup Legs on Ring Plate Face

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FIG. 9. Ring plate at Stirrup Connection; Stirrup Upper Leg Fractured, Note Welded
Remnant; Stirrup Lower Leg Failed at Welding, Note Absence of Remnant

9. The inside surface of the concrete wall was covered by a smooth coating
that had considerably reduced its initial roughness.

SILO W A L L

The silo wall was designed for hoop tensions generated by the radial pres-
sures exerted by the grain; the current provisions of Standard 313-77 (Amer.
Concrete Inst. 1977) were used to check compliance. For this purpose, the
following values were used: unit weight of grain, y = 50 pcf (801 kg/m 3 );
angle of internal friction, p = 23°; and coefficient of friction between grain
and wall, u,' = 0.29. Note that the values selected for p and p/ are on the
low side of the usual ranges recommended for grain, namely: 23° =£ p =£
37° and 0.29 « p,' =£ 0.47.
For corn and wheat it would not be unusual for designers to take p = 28°
and p/ = 0.38. However, a low value of p,' was in order because the silo
had contained soybeans, which could smoothen the concrete wall surface by
coating it with an oily residue. Combined with a low value of p this produced
the largest hoop tensions on the wall and the largest vertical load on the
suspended hopper. Because of the pronounced height-to-diameter ratio of the
silos, i.e., H/D = 3.5, overpressure factors, Cd, ranging between 1.8 at
hopper level to 1.5 at the top were used to increase the hoop tensions due
to service conditions; the Cd factors account for additional effects generated
during grain withdrawal. For the determination of the largest downdrag forces
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FIG. 10. Ring Plate at Stirrup Connection: Both Stirrup Legs Fractured, Note
Welded Remnants

FIG. 11. Fractured Ring Plate at Improperly Welded Connection between Seg-
ments

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FIG. 12. Fracture Surface of Ring Plate Welded Connection; Note Minimal Weld
Penetration at Less than 1/8 in.

in the wall which, although unimportant for concrete silos, is vital for the
design of steel silos against buckling (Gurfinkel 1988) the greatest possible
value of u>' was used. Pertinent equations that are helpful in understanding
the relationship between these parameters are given in the current literature
(Amer. Concrete Inst. 1977; Gaylord and Gaylord 1984; Gurfinkel 1979;
Safarian and Harris 1985).
The presence of two 14 in. X 14 in. (0.36 X 0.36 m) outlets in the silo
wall, that were situated about 135° apart and 21 ft (6.40 m) above the silo
floor, could cause a 15-ft (4.57-m) eccentricity in the flow of grain because
of side withdrawal; this effect could generate horizontal bending moments
in the silo wall (Deutsches Inst, fiir Normung 1984). Design would be better
served if these moments were calculated and the wall were reinforced using

FIG. 13. Splicing of Hoop Bars in Concrete Wall Was Not Staggered

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 220
I
b m fro na
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210' i i

200'- ' n
^Design (Effective)

i i
190'

^j I ^ D e s i g n (Specified)

180'-

170'
l.
-Required (ACI Std.313-77)

160'

-Deficiency in Hoop-Steel
Reinforcement
150'

140'-

130'-

I O I K5,(Topof
l21
,
- Hopper)
120'
0.5 1.0 1.5 2.0
2
Hoop Steel, A s , in. /ft

FIG. 14. Variations of Area of Hoop-Steel Reinforcement (Required, Specified,

and Effective) with Height of Silo Wall

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 a double layer of hoop bars. However, American practice at the time (Amer.
Concrete Inst. 1977) called only for the calculation of additional hoop ten-
sions and provision of additional one-layer hoop steel to the silo wall; this
was considered equivalent to the effects of eccentric withdrawal. The cal-
culated increase in radial pressures ranged from 25% at side-withdrawal level
to zero at the top; these effects were not increased by the overpressure fac-
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tors, Cd.
To determine ultimate hoop tensions, F,„ a 1.7 load factor was used. The
required hoop steel reinforcement, As, was determined from As = T„/<\>fy,
where fy = 60 ksi (4,227 kg/cm2) and cj> = 0.9 is the capacity reduction
factor. The variation of As with wall height above hopper level is shown in
Fig. 14. This must be compared to the designer's specification for As with
which, assumedly, the contractor complied, although this fact could not be
ascertained in the field. Observation of Fig. 14 would indicate that the amount
specified exceeded the amount required for hoop steel throughout the height,
except at a small portion of the silo wall above the hopper, where a slight
deficiency took place.
The situation would be different, however, if one compared the required
and the effective areas of hoop steel. Because of necessary splicing the hoop
steel would not be 100% effective unless the length of splice were sufficient
to develop the yield strength of the steel. The designer specified 30- and 46-
in. (0.76- and 1.17-m) splice lengths for Nos. 6 and 8 bars, respectively;
compare these to the 43- and 71-in. (1.09- and 1.80-m) required values (Amer.
Concrete Inst. 1983), respectively. The latter were obtained by calculation
using/; = 4 ksi (282 k g / c m 2 ) , / = 60 ksi (4,227 kg/cm 2 ), and factors of
1.4 and 1.7 for top reinforcement and class C splices (no staggering), re-
spectively. In view of the shorter splice lengths specified by the designer,
effectiveness factors of 70% and 65% were used for Nos. 6 and 8 bars,
respectively, in obtaining the variation labeled design (effective) in Fig. 14.
A comparison between the effective and required areas of hoop steel indi-
cates that there is a deficiency in the first 55 ft (16.76 m) of wall height
above hopper level; note the shaded area in Fig. 14.
The deficiency in hoop-steel reinforcement of the silo wall could have
been even larger if any of the following departures from the specifications
had taken place; (1) Shorter lap splices; (2) larger spacing of hoop bars; (3)
lower concrete strength; (4) less concrete cover for hoop bars; and (5) lower
yield strength of steel. These items were not actually verified because of the
major amount of field and laboratory work required; they quickly became
irrelevant as progress in the investigation of the suspended hopper pointed
to the actual cause of the collapse.

SUSPENDED HOPPER

The vertical forces acting on the suspended hopper were its selfweight,
Ws, the weight of the actual grain contained therein, Wh, and the load gen-
erated by the vertical pressure of the grain above, Wq. Calculations yielded
the following service loads: Wg = 10.4 k (4.7 Mg), Wh = 176.7 k (80.3
Mg), and W„ = 1,933 k (879 Mg). The latter value is the vertical force on
the hopper induced by the grain above it and.is equivalent to the effect of
a 55-ft (16.76-m) head of grain. The total vertical load on the hopper, W,
would be 2,120 k (964 Mg).
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 While the values of load components Wg and Wh are not subject to change,
even if the friction coefficient u/ increased, that is not the case for com-
ponent Wq. If a/ were taken .at the higher end of the range, i.e., u/ = 0.47
instead of 0.29, calculations would render Wq = 1,361 k (619 Mg), instead
of 1,933 k (879 Mg), and a total vertical load W = 1,548 k (704 Mg),
instead of 2,120 k (964 Mg). Although the actual value of Wq on the hopper
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may never be know with certainty, it is reasonable to conclude that it must

have been larger in the collapsed silo than in its twin. This is true because
of the smoothening effect that the constant storage of soybeans had on the
surface roughness of the collapsed silo, compared to that of the twin silo
which only stored corn.
The values of meridional and tangential forces in the hopper were cal-
culated for W = 2,120 k (964 Mg) at 31.8 and 41.7 k/ft (47.4 and 62.2
Mg/m), respectively. The maximum tensile stresses in the meridional and
tangential directions for the 1/4-in. (0.00635-m) thick steel hopper were
10.6 and 13.9 ksi (747 and 979 kg/cm 2 ), respectively. These values are low
enough to be acceptable for any kind of structural steel. It was concluded
that the silo did not collapse because of initial failure of the hopper under
load.
Consider now the actual attachment of the suspended hopper to the con-
crete walls (see Fig. 4). The critical element here was not the connection of
the hopper to the ring plate (to which it was continuously welded throughout
the perimeter) but the connection of the ring plate to the concrete wall. Note
that 5/8-in. (0.016-m) diameter stirrups, spaced 8.5 in. (0.216 m) apart cir-
cumferentially, were used for this purpose.
Analysis of the typical stirrup showed tensile and shear forces in the upper
leg generated by meridional forces in the hopper membrane. Calculations
showed tensile and average shear stresses equal to 49 and 24 ksi (3,450 and
1,690 kg/cm 2 ), respectively, acting simultaneously on the upper leg of the
stirrup. These stresses exceeded von Mises' yield criterion (Amer. Soc. of
Civ. Engrs. 1971; Hodge 1959; von Mises 1913), i.e.,CT2+ 3 T2 S* or2, 492
+ 3 X 242 5* 602, thereby producing an unacceptable state of generalized
yielding in the steel stirrups.
Using 5/8-in. (0.016-m) diameter fy = 60-ksi (4,227-kg/cm2) steel bars
for the stirrups, as specified by the designer, was not a wise decision. There
is less ductility in 60-ksi (4,227-kg/cm2) reinforcing steel (as compared to
40-ksi 2,818-kg/cm2) reinforcing steel) and, therefore, a greater probability
for prefracturing the bar when bending it to the short radius of curvature
required for fabricating the stirrup legs. This effect, in combination with the
high stresses generated in the upper leg of a typical stirrup by the service
load on the hopper, may have caused one of them to fracture. Such an event
would then overload and fail adjacent stirrups, thereby setting in motion the
progressive failure that resulted in the collapse of the silo.
Welding the stirrup legs to the ring plate should not be considered ac-
ceptable unless the designer specified, and the contractor provided, weldable
A706 (Amer. Soc. for Testing and Materials 1982) steel bars. Using standard
A615 Gr. 60 bars would have called for knowledge of their carbon equiv-
alent to figure out the required preheating and interpass temperatures (Amer.
Welding Soc. 1975). In the absence of this information it would only be
reasonable to: (1) Assume that the carbon equivalent of the steel exceeds
0.75; and (2) specify 500° F (260° C) minimum preheat and interpass tem-
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 peratures to prevent cracking of the welding in the heat-affected zone. Al-
though examination of the field evidence pointed to many more failures in
the cross section of the stirrup legs, rather than at the welds, the matter of
welding conventional reinforcing bars should not have been taken casually.
What should the designer have specified instead of the stirrups shown in
Fig. 4? The design would have been acceptable had two layers of 8-in. (0.20-
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m) long, 7/8-in. (0.022-m) diameter, shop-welded, conventional studs, made

out of weldable A108 steel, been specified instead of the stirrups. Other
designs for attachment of the suspended hopper are discussed in the literature
(Gaylord and Gaylord 1984; Gurfinkel 1979; Safarian and Harris 1985).

SILO COLLAPSE EXPLAINED

Structural analysis and field observations showed that failure initiated nei-
ther in the upper 6-in. (0.015-m) thick concrete walls of the silo (which,
although overstressed in hoop tension, were not close to their limit strength)
nor in the suspended hopper. Instead, the evidence points out to failure ini-
tiation at the connection of the suspended hopper to the 10-in. (0.25-m) thick
concrete wall of the silo.
Field observations showed that a large number of the 5/8-in. (0.016-m)
diameter stirrups used for the hopper-silo connection had fractured; analysis
verified that the stresses generated in the stirrup legs were large enough to
cause generalized yielding of the steel. In addition, it is likely that the sharp
bending of the stirrup legs, for attachment to the hopper ring plate, caused
prefracturing of the hard-grade low-ductility steel bars.
Fracture of the stirrup legs was followed by separation of the hopper ring
plate from the concrete wall. This caused the suspended hopper to lose sup-
port and fall down instantly as the full mass of grain overhead was set in
motion. Upon reaching the floor the grain mass flattened the conical hopper
and then, by expanding laterally, split open the concrete walls of the silo.
Failure of the lower silo walls occurred instantly and spread vertically up-
ward. As the grain spilled outwards, pushing the lower walls into adjacent
structures, the upper portion of the silo severed off and toppled over.
It may be concluded that the silos had been designed and erected with an
inherent propensity for collapse. The eventual collapse of one of them, four
years after being placed in service, was an incident waiting to happen. The
pending questions on why hadn't the silo failed sooner, why had its twin
remained standing, and whether or not the latter required strengthening, may
now be answered. One must only recognize that the initial surface roughness
of the concrete walls diminished gradually with the passage of time. As a
result, reduced friction of the grain with the tank wall increased lateral pres-
sures against the wall, reduced downdrag friction forces on the wall and
increased vertical pressures on the grain. Because the silo had been provided
with a suspended hopper, the increased vertical pressures on the grain had
particular significance in the collapse; they substantially increased the load
on the hopper and its attachment to the silo wall. The process was faster in
the collapsed silo than in its twin, probably because of a shorter "turn," and
the fact that the former was used for a long time to store soybeans while
the latter was not.
It took four years of wall smoothening, and the heaviest corn in storage
in the last two years, to increase the vertical load on the suspended hopper
257

J. Perform. Constr. Facil. 1989.3:243-264.

 to an intensity that had not been reached previously, but was still enough
to trigger collapse of the silo. It was clear that the same fate awaited the
twin silo unless it were strengthened as required.

TWIN SILO REPAIRED

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Repair of the remaining silo focused on the need to prevent the meridional

X
<
LLI Exist. Wall Opening
^

lOin. Thick RC
Wall, Exist.

•Exist. Wall
Opening

Plan

FIG. 15. Additional Steel Columns Supporting Suspended Hopper Are Placed
around Inside-Wall Perimeter of Twin Silo

258

J. Perform. Constr. Facil. 1989.3:243-264.

 - 3 - # 8 at 8 O.C., Exist.
Top S Bottom of

-Detail
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A36 Steel

-Detail C

#8at24"O.C.Horiz, Exist.

#8at24"O.C. Vert, Exist.

3"_
Clear

-Detail D

*-Fill as Req'd

*" .•« st^AL_S~6"WelI Compacted Gravel

SiSv/SjA—^ or Stone as Req'd
#8x10-0" at 9"0.C.

FIG. 16. Steel Columns Support Suspended Hopper and Are Attached to Con-
crete Wall and Foundation

forces exerted by the hopper from being supported by the ring plate and set
of stiiTups. (Direct strengthening of either one was hardly feasible and def-
initely impractical.) Thus, to avoid eventual repetition of the fracture mech-
anism that triggered collapse of the silo, it was only reasonable to bypass
the existing hopper-support system and provide a new one instead.
The design was based on the largest possible load on the suspended hop-
per. The load was determined using the grain-withdrawal condition, and the
lowest possible values of the parameters jo-' and p, to account for the effects
259

J. Perform. Constr. Facil. 1989.3:243-264.

 10 in. R/C Wall, Exist.

- 5 /8 in. Diam. Stirrups

at 8'/2in. Spacing, Exist.
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Ring Plate, Exist.

Web Stiff., 3/8 PI.

Existing Hopper

2-|l/2"Diam. A325
Bolts, 6"Apart

PI. 24" x 16" x3/ 4 "

Use E60I8
Weld Electrodes

(JLCol.

FIG. 17. Anchor Bolts Attach Steel Column to Compression Ring in Concrete
Wall

of time in smoothening the inside surface of the silo walls.

To resist the vertical component of the meridional forces generated along
the perimeter of the hopper, a set of 16 W12 X 53 A36-steel columns was
provided (see Figs. 15 and 16). The columns were made to bear directly
underneath the hopper (see Fig. 17) and were supported by, and anchored
to, the existing silo foundation (see Fig. 18).
Details of the column connection to the steel hopper, and to the ad-hoc
reinforced-concrete compression ring in the 10-in. (0.25-m)-thick silo wall,
260

J. Perform. Constr. Facil. 1989.3:243-264.

 WI2x53~
vDemolish and Rebuild
\Concrete Fl.about WI2's L4"RC Floor
< 10 Thick
•Mm
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RC W a l l ~ %- No. 4 at
12" - f
(Exist.)
-Embed WI2's
in Concrete to
Floor Level ~

Grout 7000 psi (Non-

Shrink, Embeco or
Equivalent)

2-ll/2"Diam.xl,IO"A325
-18" Thick RC K Anchor Bolts Set in POR-
ROK in 2"Diam. Holes
Ring Foundation • Drilled into Existing Concrete
(Exist.) Spaced 12 in. Apart.

J_tLJ

FIG. 18. Steel Columns Bear Directly on Existing Ring Foundation and Are At-
tached to It

are given in Fig. 17. Note that a steel plate 24 X 16 x 3/4 in. (0.61 X
0.41 x 0.019 m) was welded to both the hopper and the flanges of each
W12 X 53 column in order to improve the transfer of hopper meridional
forces to the new steel columns. The horizontal component of the force is
transferred to the original compression ring in the 10-in. (0.25-m) thick wall
by means of two 1 1/2-in. (0.038-m) diameter A325 bolts that attach each
steel column to the wall at hopper level (see Fig. 17).
Because of the discrete number of columns supporting the hopper, hori-
zontal bending moments are generated in the composite compression ring at
the wall. The combined action of these moments and the hoop-compression
force was checked against the thrust-moment interaction diagram of the com-
posite compression ring and was found acceptable. Finally, lateral stability
of the steel columns was secured by bolting them to the silo walls at mid-
height (see Fig. 19).
261

J. Perform. Constr. Facil. 1989.3:243-264.

 a.
a.

o
0)
m «
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*-.2>
*-
0 1
o

o o

z 2-%"Diam.
A325 Bolts ~ Inside
Silo -
6 in. Apart

•Outside
Silo ~
~IOin.Thick
RC Wall, ~ W 12x53
Existing ~ A36 Steel
Column ~

FIG. 19. Steel Columns Are Attached to Silo Wall at Midheight for Lateral Sta-
bility

The twin silo was strengthened and subsequently returned to service by

the owner in late 1986; the writer's services were not requested during this
period. In late summer of 1988 the writer visited the site, at his own initia-
tive, and found that the required strengthening had been only partially im-
plemented. Although the steel columns and overhead plates had been pro-
vided as required, none of the anchor bolts specified in Figs. 17 and 19 had
been placed. Instead, the contractor tied all columns together at midheight
by welding horizontal segments of a W8 X 31 beam to the webs of the W12
X 53 columns, thereby forming a 16-sided polygonal ring. The owner was
notified by the writer of these discrepancies. He was also informed of the
following determinations: (1) Because the polygonal ring would provide lat-
eral restraint to the columns equivalent to the midheight anchor bolts (see
Fig. 19), the latter were no longer required and need not be installed; and
(2) placement of the anchor bolts at the upper end of the steel columns (see
Fig. 17) was still very much required.
At the present time the writer has been informed by the owner that mea-
sures are being taken to ensure compliance.
262

J. Perform. Constr. Facil. 1989.3:243-264.

 SUMMARY AND CONCLUSIONS

1. A tall reinforced-concrete silo full of corn collapsed and toppled over on

a sunny, windless day. A twin adjacent silo, also full of corn, remained standing.
2. Examination of the wreckage disclosed a completely flattened-out hopper
that used to be suspended from the concrete wall of the silo.
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3. Observation of the plans and specifications disclosed an awkward design

for the hopper-supporting system that involved using stirrups made out of 5/8-
in. (0.016-m) diameter, 60-ksi (4,227-kg/cm 2 ) yield-strength steel bars, bent at
a sharp curvature, and welded to structural steel without any preheating require-
ments.
4. Further examination showed that the hopper-supporting system had failed,
in that all stirrups used to anchor the hopper to the concrete walls had fractured,
or had otherwise failed at their welded connections.
5. Analysis indicated that major overstressing of the stirrups would have caused
generalized yielding of the steel under service loads. Fracture should follow in
the presence of an existing crack, such as one caused by sharply bending a
5/8-in. (0.016-m) diameter low-ductility hard-steel bar into the shape of a stir-
rup.
6. Bad welding of the component segments of the hopper ring plate was
found. Penetration of the welding was minimal; the intended full-penetration butt
welding of the cross section ended up as a weld of cosmetic value only.
7. Collapse of the silo was triggered by the fracture of the stirrup legs sup-
porting the suspended hopper.
8. Although the hopper-supporting system was designed and built with in-
herent defects, it took four years of service before it failed. This is attributed to
a gradual reduction in the surface roughness of the concrete walls that eventually
increased the vertical load on the hopper to an amount that exceeded the strength
of its supporting elements.
9. Strengthening was required because the twin silo was bound to fail even-
tually given more service time. A new hopper-supporting system, that com-
pletely by-passed the existing one, was designed and detailed as shown in the
paper. Design was based on the largest possible load on the hopper.
10. Inexplicably, not all of the required strengthening was accomplished be-
fore returning the twin silo to service. At the present time the owner has been
made aware of the unacceptability of this situation and further action is expected.

APPENDIX. REFERENCES

"Building code requirements for reinforced concrete." (1977). ACI 313-83, Amer-
ican Concrete Institute, P.O. Box 19150, Redford Station, Detroit, Mich.
Gaylord, E. H., and Gaylord, C. N. (1984). Design of steel bins for storage of bulk
solids, Prentice-Hall Inc., Englewood Cliffs, N.J.
Gurfinkel, G. R. (1979). "Reinforced concrete silos and bunkers." Structural en-
gineering handbook, E. H. Gaylord and C. N. Gaylord, eds., 2nd Ed. McGraw-
Hill Book Co., Inc., New York, N.Y., 221-237.
Gurfinkel, G. R. (1988). "Tall steel tanks: Failure design and repair." J. Perf. Constr.
Fac, ASCE, 2(2), 99-110.
Hodge, P. G. (1959). Plastic analysis of structures. McGraw Hill Book Co., Inc.,
New York, N.Y.
"Lastannahmen fur Bauten, Lasten in Silozellen, Entwurf und Erlauterungen." (1984).
DIN 1055, Sheet 6, Deutsches Inst, fur Normung, ev. Burggrafenstrasse 4-10,
1000 Berlin 30, West Germany, Feb.

263

J. Perform. Constr. Facil. 1989.3:243-264.

 Mises, R. von. (1913). "Mechanik der festen Korper in plastisch deformablen Zu-
stand." Nachr. Gottingen Akad. wiss. Math-physik Kl. 582-592.
Part 4, Steel-structural, reinforcing, pressure vessel, railway; fasteners. (1982).
American Society for Testing and Materials, Philadelphia, Penn., Mar.
"Plastic design in steel—A guide and commentary." (1971). ASCE Manual of En-
gineering Practice, No. 52, ASCE, New York, N.Y.
"Recommended practice for design and construction of concrete bins, silos, and bunkers
Downloaded from ascelibrary.org by New York University on 05/12/15. Copyright ASCE. For personal use only; all rights reserved.

for storing granular materials (ACI 313-77) and commentary." (1977). Revision—
ACI 313-R77, ACI J. May-June 1983, 250-252.
"Reinforcing steel welding code." (1975). AWSD12.1-75, Table 5.2 by Amer. Welding
Soc, Inc., 2501 NW 7th St., Miami, Fla.
Safarian, S. S., and Harris, E. C. (1985). Design and construction of silos and
bunkers. Van Nostrand Reinhold Co., New York, N.Y.

264

J. Perform. Constr. Facil. 1989.3:243-264.