COLLAPSE OF STEEL SILOS

8/7/2019 COLLAPSE OF STEEL SILOS

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by Rubin M. Zallen, P.E.

Steel silos are ubiquitous in manu-facturing plants wherever dry granu-lated or powdered materials need tobe stored. These silos vary in size

and are usually supported by a steelframe on a reinforced concrete foun-dation. The silos are loaded at thetop by pumping the materials in us-ing air pressure or using a conveyor.The silos discharge the materials to atruck or a freight car through a hop-per at the bottom.Silo materials do not act like a fluid; dry

materials have frictional or cohesive resis-tance and tend to form domes or bridges with thewalls of the silo that prevent it from flowing freelydownward. The lateral pressures on the silo shell fromthe dry materials are of a different character than thelateral pressures on a retaining wall from soil at theback of the wall. The vertical pressures on the bottomof the silo from the dry material are affected by thebridging action. In order to discharge the hoppers atthe bottom, a vibratory system, called a bridge breaker,that temporarily destroys the frictional resistance orcohesive bonds is required.Figure 2 (on page 2) is a schematic view of the silosand silo frame that were involved in the collapse that isreported in this paper. The purpose of the silos was totemporarily hold powdered gelatin, until it could bedischarged into trucks for shipping. Trucks drove in

No. 22 August 2008

2008 Zallen Engineerin

Collapse of Steel Silo

through the end of the frame, and were positioned un-der the conical hopper of the silo to be discharged.Each of the silos had a nominal capacity of 300 tons ofgelatin; however, the volume of each was sufficient tohold 333 tons.The silos were new and were in use for only ten daysbefore Silo No. 1 collapsed. See Figure 1, and Figures3 and 4 on page 3. The collapse occurred within 4hours of the completion of the filling of Silo No. 1with a reported 190 tons of gelatin powder.There were many theories posited as to the cause ofthe collapse, such as an inadequate silo shell design, aninadequate conical hopper design, impact loading froma collapsing dome of gelatin that had been created by

(Continued on page 2)

Figure 1 View of Silo No. 1 after it collapsed. Beam E is the top beam onthe left (see Figure 5 for location); Beam A is the top beam on the right.


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Forensic Engineering in Construction

Figure 2 Schematic views of three 300 Ton steel silos and their supporting frame.The front view is on the left; the end view at Silo No. 1 is on the right.

(Continued from page 1)

the bridging of the gelatin, and an improperly designedor constructed support frame. After a detailed exami-nation of the remains of Silo No. 1 and the part of thesupport frame that collapsed, and after some prelimi-nary structural analysis, the evidence pointed to thecollapse being initiated by a failure in the supportframe.Figure 5 (on page 4) shows a structural framing plan ofthe top deck of the silo support frame. The location ofthe silo skirt is shown dotted in green. Figure 6 showsSection A-A taken on the plan of Figure 5, whichrepresents the conditions at both Beams F and E. Thissection shows the silo skirt, which is an extension ofthe silo cylinder, the connection of the conical hopperto the silo cylinder, and the support of both the hop-per and cylinder skirts on top of the support frame,

prior to the collapse. The silo detail (on top and abovethe support plate) is also typical at the top of Beams K,L, M, N, O, and P, where the silo skirt and cone skirtrest directly on the top of those beams.Figure 7 (on page 5) shows the top of Beam K, andrepresents the conditions at the four diagonal beams,Beams K, M, N, and P. The skirt of the silo has beencut off just above the top of the beam and the topflange of the beam had been cut during erection of thesilo to accommodate the conical discharge hopper.The ends of Beams A and B have end plates which arebolted to the flanges of the columns. The ends ofBeams E and F are connected to the webs of the col-umns with standard double angle connections. See



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Figure 3 Rear view of Silo No. 1 after it collapsed. Thetwisted beam is Beam F.

Figure 4 View of the silo frame after Silo No. 1and the part of the frame supporting the silo have

been removed. The silo in front is Silo No. 2; the

one in the rear is Silo No. 3. The beam at the top is

Beam B.


Figure 4. None of these connections failed as a resultof the collapse.Figure 8 (on page 7) shows a typical diagonal connec-tion, consisting of a 3/8 inch bent plate welded to thebeam being supported and bolted to the web of thesupporting beam. There were variations among con-nections in bolt diameters, bolt hole diameters, and thedeveloped length of the bent plates.

Figure 9 shows the fracture through the bolt holes ofthe connection at Joint 1 on beam B, before the con-nection plate remnant and the bolts were removed.Figure 10 (on page 8) shows the fracture through thebolt holes of the connection at Joint 14. (The isolatedplate and bolts therein were removed from Beam Bwhen the collapse was dismantled). Figure 11 showsthe fracture of the connection at Joint 13; the weldsbetween the connection plate and the web of Beam Pfailed and part of the connection plate failed. (The

connection plate and bolts were removed from BeamF when the collapse was dismantled).Cause of CollapseThe silo shell is essentially rigid with respect to thesupporting frame and thus controls the distribution of

the silo loads to the frame. Further, as shown on Fig-ure 7, the silo shell was welded to the tops of the sup-porting beams, which restrained the supporting beamsfrom bending in the vertical plane. As shown on Fig-ure 6, the silo shell is also welded to the top of theadded plate on the sides of Beams E and F; however,the silo shell is eccentric to these beams, causing thebeams to twist, resulting in these supports being rela-

tively soft. Thus Beams E and F received only a smallportion of the total silo load.As the force or stress on a steel member is increased, itreaches a level where the steel yields plastically, calledthe yield point; at this level, the load on the member



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Figure 5

Structural plan of top

deck of silo support

frame.

Figure 6

Section A-A taken on

the plan of Figure 5,

showing silo support

detail on Beam F. De-

tail at Beam E is the

same but opposite hand


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Figure 7 Top of Beam K. The silo skirt, which hasbeen cut off, was welded to the top of the beam, a

condition which was typical at all the support

beams. The torch cut at the top flange was made to

accommodate the conical hopper. This torch cut

was made at all the diagonal beams.


remains relatively constant as the plastic deformationincreases. This yielding can cause redistribution of in-ternal forces. In the case of the silo frame a small yield-

ing deformation of a connection of a magnitude of0.02 inches (0.5 millimeters) will cause redistribution ofload to other connections.A structural analysis accounting for these behavioralaspects was made to determine the load distribution tothe various structural elements. The total design loadof the silo was 700 kips (1 kip = 1000 lbs) of which667 kips was the weight of the contents with the silofull, and 33 kips was the weight of the silo and the topdeck of the steel frame. The calculated connectionloads and the loads from the silo to Beams E and F aregiven in Table 1 (on Page 6).An analysis of the capacity of the as-built bent plateconnections was also made. These capacities are alsogiven in Table 1. The controlling calculated failuremode was fracture through the first line of bolt holesfrom the bend of the connection plate. See Figure 8.Referring to Elevation 1-1 of Figure 8, the connectionload from the beam (supported by the weld) creates adirect vertical load on the bolts (called shear), but alsocauses a rotational load in the plane of the connection

plate (called moment). The larger the distance from thebend to the first line of bolts, the larger is the moment.The critical section through the connection plate forthis combined shear and moment is through the threebolt holes of the first line of bolts from the bend. Thetypical distance between the bend to the first line ofbolts is 1.8 inches, but that same distance for Joint 1-1is 2.5 inches, which explains the lower capacity of theconnection of Joint 1 as compared to the capacity ofthe other joints.

In Table 1, the lowest ratio of connection strength tothe calculated load at a joint occurs at Joint 1. This ra-tio is 0.48. Thus the total capacity of the silo frame is0.48 x 700 kips = 336 kips. Subtracting the weight ofthe silo and top deck of the support frame, 33 kips,leaves 303 kips or 152 tons of product. The failure oc-

curred with a reported 190 tons of product inside;adding the weight of the silo and top deck of thesupport frame gives a total failure load of 413 kips.The discrepancy between the calculated and actual

failure loads is probably due to stronger steelstrengths than assumed in the calculations. Theminimum yield strength for the steel used in thesupport frame is specified at 36,000 psi; howeverexperience has shown that the yield strength of ac-tual steels at the time of this collapse (1979) variedfrom 1.1 to 1.3 times the specified minimum yield



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Table 1

Connection Loads and Capacities


strength. (The ratio 413/336 = 1.2). The discrepancyin part could also be due to an inaccurate measure-ment of the product inside of the silo after filling.

It is clear that the cause of the failure was diagonalconnections that were substantially under-strength.From the view of accepted engineering practice,where the minimum yield strength is used for design,and where a safety factor of 1.67 is also used, thecritical connection had 29% of the strength theyshould have had.

Sequence of CollapseThe silo shell was welded to the tops of thesupporting beams, and thus the relativelyrigid silo, in the vertical direction, restrainedthe bending of the supporting beams andredistributed load as the silo frame connec-tions failed. The most likely sequence is asfollows:1. The connection plate at Joint 1, the

weakest connection plate, failed throughthe bolt holes. See Figure 9.

2. Load is transferred to Joints 2 and 13a.

The connection plate of Joint 13a failed

through the bend in the plate. The con-nection at Joint 1a did not fail.

3. The connection plate at Joint 14 failed

through the bolt holes. See Figure 10.4. Connections at Joints 2 and 13 failed,

allowing Silo No. 1 to rotate toward SiloNo. 2 about a horizontal axis throughJoints 4 and 10. At Joint 2, the connec-tion bolts were pulled out of the web of

Beam E. At Joint 13, the welds from theconnection plate to Beam P and part ofthe connection plate fractured; see Fig-ure 11.

5. Connections at Joints 4 and 10 failed.

The connection at Joint 4 failed throughthe weld connecting the bent connec-tion plate to Beam K. The connectionplate at Joint 10 sheared through thebend of the plate. At the same time as

these connections failed, the eccentricityof the load on Beams E and F (see Fig-ure 6) caused Beams E and F to twistinward. See Figures 1 and 3.

6. Connections at Joints 7 and 9 failed by

the bolts tearing out of Beam A. Con-nections 4a and 9a did not fail.

See discussion of responsibilities

for the collapse on page 8


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Figure 8 Typical diagonal connection.

Figure 9 Connection 1 on Beam B.


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Figure 11 Connection 13. Beam F is at the top; Beam P isat the bottom left. The connection plate and bolts were re-

moved from Beam F when the collapse was dismantled.

Responsibilities For the CollapseThe contract to supply the three silos and the support framewas a design-build contract between the owner of the plantand the manufacturer of the silos. The manufacturer hired a

structural engineer to design the support frame and its foun-dations, a construction contractor to build the foundations,a steel fabricator to fabricate and erect the support frame,and a millwright to assemble and erect the silos. There wasno single individual designated by the manufacturer to re-view the design and supervise construction.The structural engineer testified at his deposition that hisonly involvement was to provide the structural design, andto visit the site at the request of the director of the manufac-turing plant to inspect the erection of the additional platesand stiffeners at Beams E and F. (See Figure 6). When hewas at the site, he did not inspect any other parts of theerected work.The capacity of the diagonal connections as-designed by thestructural engineer is 54 kips, greater than the capacities ofthe as-built connections (see Table 1), but still substantiallyunderstrength. Also, the steel framing was substantially un-derstrength. From the evidence, it appears that the structuralengineer did not make an independent check of the siloloads.There is evidence that the steel erector burned connectionholes, used oversized holes, and undersized bolts in some ofthe connections. The fracture of the welds from the connec-tion plates to the supported beams and the fracture of the

connection plates at the bends of the connection plates, re-ported in the sidebar on the sequence of the collapse, areclear indicators of improper fabrication and erection.There is evidence that the manufacturer of the silo providedincorrect diameters of the silos to the structural engineer,requiring retrofit in the field by the erector, which was doneimproperly.The party most responsible for the collapse was the manu-facturer of the silo who was the design-build contractor,because it failed to provide a competent person to superviseand coordinate all aspects of the work, and to review thedesign by the structural engineer.

The structural engineer contributed to the failure by not insome way verifying that the loads from the silos that he usedwere correct. The erectors work was substandard andwould have led to decreased capacity of the support framehad it been properly designed; however the substandardwork alone did not cause the collapse.

Forensic Engineering in Construction is

published by Zallen Engineering, 1101

Worcester Road, Framingham, MA 01701.

www.zallenegineering.com.

Rubin M. Zallen, P.E., principal ofZallen Engineering, lead the

investigation of this collapse.

Figure 10 Connection 14. The isolated plate and boltswere removed from Beam B when the collapse was disman-

tled. The beam on left is Beam P.

COLLAPSE OF STEEL SILOS

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