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Structural Investigation of the January 20, 2014
Plant Collapse at International Nutrition Facility
in Omaha, NE

U.S. Department of Labor
Occupational Safety and Health Administration
Directorate of Construction

July, 2014

Structural Investigation of the January 20, 2014 Plant Collapse at International Nutrition Facility in Omaha, NE - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Structural Investigation of the January 20, 2014 Plant Collapse at International Nutrition Facility in Omaha, NE

July 2014

  • This report was prepared by
  • Mohammad Ayub, P.E., S.E.
  • Office of Engineering Services
  • Directorate of Construction
Table of Contents
  1. Introduction
  2. The facility
  3. The bins
  4. The bin supporting structure
  5. Observation of the collapsed supporting structure
  6. Structural analyses
  7. Conclusions
Introduction

On Monday, January 20, 2014 at about 9:40 a.m. a massive collapse occurred at a plant in Omaha, Nebraska producing nutritional supplements for animal feed. Two employees were killed and thirteen others were injured. The facility was located at 4444 S. 76th Circle, Omaha, NE. It is owned by International Nutrition, Inc. of Omaha, NE. The offices of International Nutrition, Inc. are located at the adjoining property at 7706 I Plaza, Omaha, NE. Bodies of the dead employees were retrieved within two days. The injured employees were able to get back to work after a few days. The structural collapse was so massive that it rendered the entire plant inoperable, and it had to be shut down. The central area of the building containing nine bins over the roof of the building and their supporting structure collapsed in a northerly direction and could be seen resting on the failed structure at an angle. See Figures 1 thru 3, below. The remaining structure was in an unstable and precarious condition, and, therefore, had to be immediately abandoned. No one was permitted in the facility except under strenuous conditions.

Figure 1 – Aerial view of collapse - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 1 – Aerial view of collapse

Figure 2 – Aerial view of collapse - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 2- Aerial view of collapse

Figure 3 – Aerial view of collapse - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 3 – Aerial view of collapse

The arduous task of investigating and determining the cause of the collapse began. Initially, it was surmised that the collapse occurred due to an explosion but this was soon ruled out by the experts from the OSHA Salt Lake Technical Center due to the lack of a debris field, and a source of the explosion. Mr. Lee Hathon and Jedd Hill from SLTC visited the site multiple times. The focus then shifted to a structural collapse. Immediately efforts were made to locate the original structural framing plan for the bins, their supporting structure, and the building. A partial framing plan for the building was located but the plans for the bins could not be found. Considerable efforts were made to locate the manufacturer and fabricator of the bins which were completed in around 1972. Neither the owner nor the City Hall had any records. This resulted in a total lack of information on how the bins were manufactured and how were they supported over the framed buildings. This lack of information impeded the structural investigation because it was considered unsafe to step into the collapsed portion of the building to obtain any information. Demolition began on or about March 22, 2014 in a controlled manner, and approximately six weeks after the demolition began, the structural pieces began to be retrieved from the massive debris. It took approximately 3½ months before a serious structural investigation could begin.

A few days after the collapse, the OSHA Regional Administrator for Region VII asked the Directorate of Construction (DOC) in OSHA's National Office for engineering assistance in investigating the incident which had attracted considerable media attention. A structural engineer visited the site multiple times to gather information, observe the collapse, perform field measurements, and discuss the failure with the participants. Talks were held with the owner of the facility to obtain documents, to learn about the activities at the plant, the methods of operation, the process of manufacturing nutritional supplements for the animal feeds, and to take photographs. Following the location of documents, and performing field measurements of the failed structure, we conducted structural analyses of the failed structure, and our report follows. This report would not have been possible without the tireless efforts of Mr. Scott Jacobson, the lead compliance officer, and Ms. Bonita Winingham, Director of the Omaha OSHA Area Office.

The facility

The facility was originally constructed around 1972, and it was owned by Vitamin Premixers of Omaha, Inc. Then around 1983, SmithKline Beecham Animal Health Company purchased the facility. The facility was then acquired by U.S. Pharmaceutical Company, Pfizer, in 1994. Finally, the present owner, International Nutrition Inc. (INI) acquired the facility around 1997. During the ownership of SmithKline, new legs were installed by PMI Nebraska Inc. around 1986 with a new conveyor at the top. See Figures 4 thru 7.

Figure 4 – Original facility - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 4 – Original facility

Figure 5 – Original facility - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 5 – Original facility

Figure 6 – Facility after modification - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 6 – Facility after modification

Figure 7 – Two outside bins being added - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 7 – Two outside bins being added

When INI acquired the property, it engaged Ken Bratney Company (KBC) of Des Moines, IA for major modification of the facility. The following were the major changes made:

  1. Adding two new mixers.
  2. New bagging machine was added.
  3. Installing a new receiving pit and tunnel with a new enclosure.
  4. New elevator legs and their supports added.
  5. New drag conveyor at the top platform added.
  6. New 10-outlet electric distributor added.
  7. Adding three additional pneumatic lines.
  8. Installing platforms at and above the electric distributor.

See Figure 6 showing the new legs, new platforms, conveyor, etc. All such modifications were performed around 1997. It must be noted that no changes were made to the original nine bins and their supporting structure. INI also retained PMI Nebraska, LLC of Grand Islands, NE to perform a variety of maintenance work at the plant from 1997 to 2014. But none of this work involved any structural modifications or major repairs to the bins or their support structure. KBC produced a set of drawings showing equipment layout, but to our knowledge no structural drawings were produced. By adding several platforms, a new electric distributor and a drag conveyor, KBC added substantial loads to the bins without modifying the bins' supporting structure.

It is interesting to note that on one of the KBC drawings, F1.8 dated July 2, 1996, there is a sketch showing nine bins with limestone in two diagonally opposite bins, Microlite in one corner bin, supple-K in the fourth corner bin, with the rest of the bins having rice hulls (see Figure 8, below).

Figure 8 – Bin layout (Taken from drawing F1.8 by Ken Bratney Company, 1996) - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 8 – Bin layout (Taken from drawing F1.8 by Ken Bratney Company, 1996)

We understand as per KBC that the drawing merely indicated what KBC believed to be the current practice at that time. Microlite and Supple-K generally weighs 75 and 85 pounds per cubic foot (pcf), respectively. Limestone weighs approximately 90-95 pcf, and Rice Hull is approximately 20 pcf.

In 2006-2007, INI contracted with PMI to put a roof over the delivery pit, and to install two new outside storage bins, later identified as bins No. 10 and 11, see Figure 7. In the summer of 2011, to meet the demands of a client, INI was planning to store limestone in bins No. 10 and 11 but decided against this on the advice of PMI who stated in an email that the outside bins could not support the heavier loads of limestone. Then attention turned to the existing nine bins on the roof where limestone, to some degree, was already being placed in bins No. 1 and 9 through existing pneumatic pipes. INI asked PMI to explore whether limestone could be stored in other bins on the roof. PMI consulted with a local engineering consultant, Reznicek Engineering Inc., to determine if that could be accomplished. Mark Reznicek visited the site a couple of times to take some field measurements of the supporting structure. He stated that he asked for additional information from INI to complete his analysis but that additional information never came. Reznicek said that then he abandoned the project. After this, things became murky. INI believes that PMI advised them at least verbally that other corner bins on the roof could support limestone. There is no e-mail or any document from PMI in existence that could substantiate this claim.

Limestone could only be transported to the bins through pneumatic pipes, and not through legs containing buckets because limestone is heavier. In 2012, PMI proposed to replace the existing pneumatic lines to bins No. 1 and 9, and install new pneumatic lines to bins 3 and 7 to deliver limestone in the bins. INI approved the proposal, and work was completed in the fall of 2012. It is not known whether PMI installed new pneumatic lines to bins No. 3 and 7 at the specific instructions of INI or PMI installed the lines because PMI believed that the bins 3 and 7 could safely support lime stone. In any event, PMI did not advise INI against installing pneumatic lines to two additional bins because a structural evaluation was not done. PMI installed the lines anyway, and INI managers stated that they believed that PMI must have determined that the bins could safely support limestone otherwise PMI would not have installed the pneumatic lines.

Existing facility:

The facility consisted of a three story precast concrete structure with precast columns, inverted tee beams and double tees. There were three bays in the north south direction, and four bays in the east west direction. Typically, the bays were 20 ft. by 20 ft. except for the "mill opening" which was 24 ft. x 24 ft. The overhead bins were located over the mill opening. Generally the columns were 16"x16" except for the mill opening rows which were 18"x18". All columns were precast, and were provided with corbels for the inverted tee beams to sit on. The inverted tee beams ran in the north-south direction. Double tees (12-20" deep) were provided in the east-west direction with 2 ½" topping on the second and third floors. Topping was not provided at the roof level. Floor-to-floor height between the first, second and third floor was 12 ft. each. The third floor to roof height was 15 ft. There was no basement and the first floor was slab on grade. Figure 9 indicates the original framing plans.

Figure 9 – Original structural drawing of the concrete framings - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 9 – Original structural drawing of the concrete framings

The bins

All nine bins were part of the original structure believed to have been constructed in 1972-74. The bins were not pre-fabricated but were originally put together at the roof over the bin supporting structure. The 8'x8' square bins, 27' high, consisted of ⅛" steel plates welded together. The intersecting walls of the bins were connected by welding the steel plates to corner steel angles, see Figures 10 thru 13, below.

Figure 10 – Bin construction - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 10 – Bin construction

Figure 11 – Bin construction - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 11 – Bin construction

Figure 12 – NE concrete column in bin No. 1 - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 12 – NE concrete column in bin No. 1

Figure 13 – NE concrete column in bin No. 1 - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 13 – NE concrete column in bin No. 1

The walls of the bins were reinforced by welding 5x3x¼" steel angles to the steel plate at 4' o.c., vertically. There was a common wall between the bins. The hoppers were welded to the bins, and were integrated to each other with a lip bearing over the top flange of the top chords of the external and internal trusses. Thus, the bins and the hoppers were one integrated structure resting over the steel beams, see Figures 96 thru 98.

At the top of the bins were two roofs (horizontal diaphragms) consisting of ⅛" plates. There was a space of 6-8" between the two diaphragms. The lower diaphragm covered the top of the bins and was the roof of the bins. The upper diaphragm provided a platform for walking and maintenance work. See Figures 14 & 15 below for the framing of the upper and lower platforms.

Figure 14 – Roof bin - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 14 – Roof bin

Figure 15 – Roof bin - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 15 – Roof bin

Around 1997, a major renovation was done to the facility when a new electric distributor and new platforms were added over the bins. Due to these renovations, an additional load of approximately 20,000 pounds were imposed on the bins (e.g., weight of the electric distributor, new platform, and new conveyor belts, etc.).

The bin supporting structure

The structure supporting the bins and the hoppers rested on four corner steel columns, W8x40, approximately 10' high supported over 18x18" concrete columns identified as NE, NW, SE and SW columns. The steel column's base plates were welded to the steel plates embedded in these concrete columns. The columns were precast concrete reinforced with four #8 reinforcing bars with lateral ties placed at 18"o.c. In addition to supporting the bins, the concrete columns were part of the building frame supporting a roof, and the third and second floor framings. The bin supporting structure consisted of four exterior and four interior trusses, see Figure 106 on next page.

Figure 106 – Bin supporting structure - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 106 – Bin supporting structure

Two exterior trusses ran in an east-west direction and were directly supported over the steel columns resting on the concrete columns at the east and west ends of the bins. The other two exterior trusses ran in a north-south direction, and were framed to the steel columns through seated connections welded to the flange of the columns. Two interior trusses ran in the east-west direction framed to the east and west primary trusses. Similarly, two interior trusses ran in the north-south direction and were framed to the north and south primary trusses. The connections of the interior trusses to the exterior trusses were through the seated connections projecting from the top chord of the exterior trusses.

Unfortunately, original structural drawings of the framing prepared in around 1972 or any related fabrication drawings were not available, and so field measurements had to be done after the damaged trusses were retrieved during the demolition. The exterior trusses were 9 ft. deep whereas the interior ones were 6 ft. deep.

Observation of the collapsed supporting structure:

The top and bottom chords of the exterior and interior trusses were W6x15. Diagonals and vertical members were W6x20. The cross bracings were 3x3x¼". The four supporting columns were W8x40 with a base plate which was field welded to an embedded steel plate in the concrete column. As stated earlier, the exterior and interior trusses were 9 ft. and 6 ft. deep, respectively. The top chords of north and south trusses rested over W8x40 columns at each end, and were connected with four bolts. The reaction from the north and south trusses were transferred directly to the column centerline. The top chords were provided with web stiffeners. The east and west trusses were, however, framed to the columns differently. The top chords of the east and west trusses rested over the column cap plate which was extended some 6 inches beyond the flange of the column, and were connected by four bolts, see Figure 93. This produced an eccentricity of approximately 7 inches to the center of the supporting column about its major axis. The ½" extended cap plate was provided with one ⅜" stiffener, as shown below. The top chords were also provided with web stiffeners.

All four beam-bearing connections on the east and west exterior trusses failed when the ⅜" stiffener plate buckled. The four connecting bolts remained mostly intact, see Figures 34, 39 and 44. The north and south trusses are shown in Figures 86 and 87 respectively. Their joints are shown in Figures 16 thru 33 below.

Figure 86 – North Truss. Figure 87 - South Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 86 – North Truss (left); Figure 87 - South Truss (right)

Figure 16 – Joint 1 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 16 – Joint 1 – North truss

Figure 17 – Joint 2 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 17 – Joint 2 – North truss

Figure 18 – Joint 3 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 18 – Joint 3 – North truss

Figure 19 – Joint 4 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 19 – Joint 4 – North truss

Figure 20 – Joint 5 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 20 – Joint 5 – North truss

Figure 21 – Joint 6 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 21 – Joint 6 – North truss

Figure 22 – Joint 7 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 22 – Joint 7 – North truss

Figure 23 – Joint 8 – North truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 23 – Joint 8 – North truss

Figure 24 – Joint 1 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 24 – Joint 1 – South truss

Figure 25 – Joint 1 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 25 – Joint 1 – South truss

Figure 26 – Joint 2 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 26 – Joint 2 – South truss

Figure 27 – Joint 3 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 27 – Joint 3 – South truss

Figure 28 – Joint 4 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 28 – Joint 4 – South truss

Figure 29 – Joint 4 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 29 – Joint 4 – South truss

Figure 30 – Joint 5 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 30 – Joint 5 – South truss

Figure 31 – Joint 6 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 31 – Joint 6 – South truss

Figure 32 – Joint 7 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 32 – Joint 7 – South truss

Figure 33 – Joint 8 – South truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 33 – Joint 8 – South truss

The east and west trusses are shown in Figures 88 and 89 respectively. Their joints are shown in Figures 34 thru 51 below.

Figure 88 – East Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 88 – East Truss

Figure 89 – West Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 89 – West Truss

Figure 34 – Joint 1 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 34 – Joint 1 – East truss

Figure 35 – Joint 1 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 35 – Joint 1 – East truss

Figure 36 – Joint 2 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 36 – Joint 2 – East truss

Figure 37 – Joint 3 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 37 – Joint 3 – East truss

Figure 38 – Joint 4 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 38 – Joint 4 – East truss

Figure 39 – Joint 4 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 39 – Joint 4 – East truss

Figure 40 – Joint 5 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 40 – Joint 5 – East truss

Figure 41 – Joint 6 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 41 – Joint 6 – East truss

Figure 42 – Joint 7 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 42 – Joint 7 – East truss

Figure 43 – Joint 8 – East truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 43 – Joint 8 – East truss

Figure 44 – Joint 1 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 44 – Joint 1 – West truss

Figure 45 – Joint 2 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 45 – Joint 2 – West truss

Figure 46 – Joint 3 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 46 – Joint 3 – West truss

Figure 47 – Joint 4 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 47 – Joint 4 – West truss

Figure 48 – Joint 5 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 48 – Joint 5 – West truss

Figure 49 – Joint 6 –West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 49 – Joint 6 –West truss

Figure 50 – Joint 7 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 50 – Joint 7 – West truss

Figure 51 – Joint 8 – West truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 51 – Joint 8 – West truss

The east and west interior trusses are shown in Figures 108 and 109 respectively. Their joints are shown in Figures 52 thru 63 below.

Figure 108 – East Interior Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 108 – East Interior Truss

Figure 109 – West Interior Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 109 – West Interior Truss

Figure 52 – Joint 1 – East interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 52 – Joint 1 – East interior

Figure 53 – Joint 2 – East interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 53 – Joint 2 – East interior

Figure 54 – Joint 3 – East interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 54 – Joint 3 – East interior

Figure 55 – Joint 4 – East interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 55 – Joint 4 – East interior

Figure 56 – Joint 5 – East interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 56 – Joint 5 – East interior

Figure 57 – Joint 6 – East interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 57 – Joint 6 – East interior

Figure 58 – Joint 7 – West interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 58 – Joint 7 – West interior

Figure 59 – Joint 8 – West interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 59 – Joint 8 – West interior

Figure 60 – Joint 9 – West interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 60 – Joint 9 – West interior

Figure 61 – Joint 10 – West interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 61 – Joint 10 – West interior

Figure 62 – Joint 11 – West interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 62 – Joint 11 – West interior

Figure 63 – Joint 12 – West Interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 63 – Joint 12 – West Interior

The north and south interior trusses are shown in Figures 110 and 111 respectively. Their joints are shown in Figures 64 thru 77 below.

Figure 110 – North Interior Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 110 – North Interior Truss

Figure 111 – South Interior Truss - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 111 – South Interior Truss

Figure 64 – Joint 1 – North interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 64 – Joint 1 – North interior

Figure 65 – Joint 2 – North interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 65 – Joint 2 – North interior

Figure 66 – Joint 3 – North interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 66 – Joint 3 – North interior

Figure 67 – Joint 4 – North interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 67 – Joint 4 – North interior

Figure 68 – Joint 5 – North interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 68 – Joint 5 – North interior

Figure 69 – Joint 6 – North interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 69 – Joint 6 – North interior

Figure 70 – Joint 7 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 70 – Joint 7 – South interior

Figure 71 – Joint 7 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 71 – Joint 7 – South interior

Figure 72 – Joint 8 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 72 – Joint 8 – South interior

Figure 73 – Joint 8 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 73 – Joint 8 – South interior

Figure 74 – Joint 9 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 74 – Joint 9 – South interior

Figure 75 – Joint 10 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 75 – Joint 10 – South interior

Figure 76 – Joint 11 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 76 – Joint 11 – South interior

Figure 77 – Joint 12 – South interior - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 77 – Joint 12 – South interior

The four seated connections of the north and south trusses over the column cap plate did not exhibit any signs of failure except the damage sustained during the collapse. The top chords of all exterior trusses did not exhibit severe buckling except at some panel points, although it cannot be determined whether the buckling occurred pre- or post-collapse. The bottom chords of all the exterior trusses contained severe distortions; see Figures 21, 22, 32, 41, 42, 49, 50, and 107 believed to have occurred during collapse. The connection plates at the bottom chord of all the exterior trusses to W8x40 remained intact, and the bottom chords near the connections remained relatively undamaged.

The four interior trusses escaped serious damage and emerged relatively intact from the debris see Figures 104 and 105. The top chords of the north and south interior trusses were continuous and were connected to the east and west exterior trusses by four bolts on an extended gusset plate. The top chords of the east and west interior trusses were simple spans and were connected by two bolts to the north and south interior trusses. It is noteworthy that the bottom panel joints of all four interior trusses were practically undamaged; see Figures 54, 55, 59, 60, 66, 67, 74 and 75.

The four corner columns, W8x40, remained intact and did not exhibit any signs of distress. The failure occurred at the bottom of the columns where the welds to the column base plate failed, see Figures 78 thru 85. The column base plate remained connected to the embed plates of the concrete columns; see Figures 78 thru 85 and 95. For typical hoppers pictures after they have been retrieved from the debris, see Figures 96 thru 98.

Figure 78 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 78 – W8x40 base plate

Figure 79 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 79 – W8x40 base plate

Figure 80 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 80 – W8x40 base plate

Figure 81 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 81 – W8x40 base plate

Figure 82 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 82 – W8x40 base plate

Figure 83 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 83 – W8x40 base plate

Figure 84 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 84 – W8x40 base plate

Figure 85 – W8x40 base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 85 – W8x40 base plate

During the collapse, all four concrete columns remained intact except the NE column which was sheared at approximately the level of the inverted tee beam at the third floor, and was found inside bin No.1 during the demolition and recovery. The reason of the shear at that location was the lack of adequate lap splice of the longitudinal bars on one face of the column, see Figures 99-103.

Structural analyses

The most significant factor in evaluating bin structure is to determine the ratio of the loads carried by the bins, and by the underlying supporting structure. This factor depends upon the angle of the internal friction of the granular material, the coefficient of friction between the material and the bin walls and the ratio of the area to the perimeter of horizontal cross section of the bins. For this analysis we have taken a factor of 30% indicating that 70% of the gravity loads of the material stored in the bins would be directly transferred through the walls of the bins to the four supporting columns (W8x40) directly, and only 30% of the loads would be transferred through the bottom hopper to the trusses. The distribution between the frictional force and the net vertical force was estimated using the Janssen's method which is well recognized by the industry for the design of bins for storing granular materials.

The supporting structure of the bins consisted of two exterior and two interior trusses spanning in a north-south direction, and two exterior and two interior trusses in the east-west direction. Exterior trusses were supported on four steel columns which were connected to the four concrete columns, identified as NE, NW, SE and SW columns. Framing for all four exterior trusses were identical, see Figures 86-89, except for the end connections. The framing for four interior trusses was also identical. The connections of the four corner steel columns to the concrete columns were also identical.

As the facility is believed to have been constructed during 1972-1974, the grade of steel for all wide flange shapes and plates was assumed to be ASTM A-36. The sizes of the top and bottom chords, diagonals, verticals, and the stiffened beam connections were field-measured, as shown in Figure 86. The top and bottom chords were W6x15. Diagonals and verticals were W6x20. The four primary columns over the top of concrete columns were W8x40. The steel bins were constructed with ⅛" steel plates welded together with stiffeners and corner angles. The bins had two layers of roof plates 6" apart, also consisting of ⅛" plates. It was estimated that an additional load of 20,000 pounds consisting of distributor, platforms, drag conveyors, etc. were imposed on the bins, but were directly transferred to the four corner columns. The trusses were assumed not to share any load coming from the roof of the bins. It is noteworthy that the bins were many times stiffer than the trusses, and they would receive a much greater share of the loads than the trusses.

For the purpose of analysis, limestone was regarded as weighing 90 pcf, and the rice hulls as 20 pcf.

Two models were created to assess the stresses in different members of the trusses under different loading conditions. The first model provided the forces imposed on the stiffened seated connection by the east and west trusses at the north and south ends, and the second model provided the forces and stresses in the truss members. The first model more accurately reflected the existing framing conditions at the stiffened seat connection. Analyses were done under the Load Resistance and Factored Design (LRFD), and under Allowable Stress Design (ASD). Under the LRFD, no increase in loads was assumed by using a load factor of 1.0, and no capacity reduction was taken by using the phi factor as 1.0. The intent under the LRFD evaluation was to compute the failure load at which a collapse could occur. ASD evaluations provided the usual factors of safety in accord with the industry practice. A finite element analysis and hand computations were performed to compute the load that the stiffened seat connection could carry. Three dimensional views of the two models, and the finite element plate analysis are shown below, see Figures 90 thru 94.

Figure 90 – STAAD MODEL 2. Figure 91 - STAAD MODEL 1 - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 90 – STAAD MODEL 2 (left); Figure 91 - STAAD MODEL 1 (right)

Figure 92 – Elevation (Finite element model) - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 92 – Elevation (Finite element model)

Figure 93 – Elevation (Seated beam connection of the east and west truss) - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 93 – Elevation (Seated beam connection of the east and west truss)

Figure 94 – Finite element model - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 94 – Finite element model

Figure 95 – Intact column base plate - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 95 – Intact column base plate

Figure 96 – Typical hopper - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 96 – Typical hopper

Figure 97 – Typical hopper - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 97 – Typical hopper

Figure 98 – Typical hopper - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 98 – Typical hopper

Figure 99 – NE corner column lying in bin No. 1 - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 99 – NE corner column lying in bin No. 1

Figure 100 – Fracture of NE corner column at 3rd floor - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 100 – Fracture of NE corner column at 3rd floor

Figure 101 – Another view of fracture of NE column at 3rd floor - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 101 – Another view of fracture of NE column at 3rd floor

Figure 102 – NE corner column showing inadequate lap splice - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 102 – NE corner column showing inadequate lap splice

Figure 103 – NE corner column showing inadequate lap splice - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 104 – NE corner column showing inadequate lap splice

Figure 104 – Interior trusses lying in debris - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 104 – Interior trusses lying in debris

Figure 105 – Interior trusses being retrieved - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 105 – Interior trusses being retrieved

Figure 107 – Exterior truss after being retrieved - For problems with accessibility in using figures and illustrations, please contact the DOC at 202-693-2020.

Figure 107 – Exterior truss after being retrieved

The analysis was conducted using the following criteria:

  1. The steel conformed to ASTM A-36, as the bins were constructed around 1972 at which time high strength steel was just beginning to emerge into the market, although at a premium price, and was generally not used in conventional framing until the 1980s.
  2. The structure was analyzed under the Load Resistance Factored Design (LRFD) code established by the American Institute of Steel Construction (AISC); except the load factor and the capacity reduction factors were assumed as 1.0 to eliminate any factor of safety.
  3. The structure was also analyzed using Allowable Stress Method with the usual factors of safety.
  4. No allowance for any corrosion was provided.
  5. The bins were constructed with ⅛" thick steel plate, and reinforced with horizontal stiffeners of unequal 5x3x¼" steel angles at 4'-0" on centers vertically.
  6. The hopper (aka cone) was an integral part of the bins, and they were supported over steel beams on all four sides.
  7. Regardless of the quantity of the material in the bins, the hoppers were assumed to be full.
  8. Scales, mixers, and conveyor belts below the hoppers were supported independently of the bins and hoppers, and had their own supporting structure.
  9. The truss members were field-measured and generally were comprised of W6x15 except for the diagonals and vertical members which were W6x20. The four corner steel columns conformed to W8x40. The cross bracings in the trusses were comprised of steel angles 3x3x¼".
  10. Rice hulls weighed 20 pounds per cubic foot (pcf).
  11. There were two types of limestone powder stored in the bins, PureCal from Cerne Calcium Company of Fort Dodge, IA weighing 95 pcf, and Unical from Cilc Resources of Urbandale, IA weighing 84 to 92 pcf. For the analysis, 90pcf has been taken for all limestone.
  12. Dry Distilled Grain (DDG), aka Solulac, weighed 20 pcf similar to the rice hulls.
  13. Bin 9 was partitioned into two equal bins, but contained the same material in both parts but of varying quantities.
  14. At the time of collapse, the loads in the bins were as follows:
    • Bins 1,3 and 7 combined approximately 375,000 pounds (Each bin assumed to carry approximately 125,000 pounds)
    • Bin 2: 33,000 pounds
    • Bin 4: 30,000 pounds
    • Bin 5: 28,000 pounds
    • Bin 6: 20,000 pounds
    • Bin 8: 26,000 pounds
    • Bin 9: 87,000 pounds

A three-dimensional frame analysis was conducted to determine the stresses in the bin supporting frame under varying load conditions. The following are the 24 different cases considered in the analyses.

  1. Case 1: To determine whether the existing structure could support limestone in the four corner bins, and rice hulls in the remaining five bins under LRFD method (no factor of safety) using model No.1. All bins were assumed to be filled up to 85% of the capacity. Absence of factor of safety is a violation of industry standards.

    Result: All truss members passed but the seated beam connections failed.

  2. Case 2: Same as Case No.1 but using model No. 2. Again no factor of safety.

    Result: All truss members passed but the seated connections failed

  3. Case 3: To determine whether the existing structure could safely support limestone in four corner bins, and rice hulls in the remaining five bins with the required factor of safety (using ASD method), as per industry standards, using model No.1. All bins were filled up to 85% of capacity.

    Result: Truss members failed and connections also failed

  4. Case 4: Same as Case 3 but using model No. 2.

    Result: Truss members failed. All connections also failed.

  5. Case 5: To determine whether the existing structure could support rice hulls in all nine bins under LRFD (no factor of safety) using model No.1. All bins were filled up to 85% of capacity.

    Result: All members and connections passed.

  6. Case 6: Same as Case 5 but using model No. 2.

    Result: All members and connections passed.

  7. Case 7: Same as Case 5 but with the required factor of safety (using ASD method) using model No.1

    Result: All members and connections passed.

  8. Case 8: Same as Case 7 but using model No. 2.

    Result: All members and connections passed.

  9. Case 9: To determine whether the existing structure could support the loads of the material believed to be in the bins at the time of collapse using LRFD method (without any required factor of safety) using model No. 1.

    Result: All members passed but the connections failed.

  10. Case 10: Same as Case 9 but using model No. 2.

    Result: All members passed but connections failed.

  11. Case 11: Same as Case 9 but with the required factor of safety (using ASD method) using model No. 1

    Result: All members passed but connections failed.

  12. Case 12: Same as case 11 but using model No. 2

    Result: All members passed but connections failed.

  13. Case 13: To determine whether the existing structure could support four corner bins filled with limestone up to 65% of the capacity with the rest of the five bins filled with rice hulls up to 85% of the capacity using LRFD method (without any factor of safety) using model No. 1.

    Result: All members passed but the connections failed.

  14. Case 14: Same as Case 13 but using model No. 2

    Result: All members passed but the connections failed.

  15. Case 15: Same as Case 13 but with the required factor of safety using (using ASD method) model No. 1.

    Result: All members passed but the connections failed

  16. Case 16: Same as Case 15 but using model No. 2

    Result: All members passed but the connections failed.

  17. Case 17: To determine whether the existing structure could support limestone in two corner bins filled up to 85% capacity with the rest of the bin filled with rice hulls up to 85% capacity using LRFD method (without any factor of safety) using model No. 1

    Result: All members passed, and the connections barely passed.

  18. Case 18: Same as Case 17 but using model No. 2

    Result: All members passed, and the connections barely passed.

  19. Same as Case17 but with the required factor of safety (using ASD method) using model No.1

    Result: All members pass but the connections fail.

  20. Same as case 19 but using model No. 2.

    Result: All members passed but the connections failed.

  21. To determine whether the four corner bins could support lime stone filled up to 55% of the capacity with the rest of the bins filled with rice hulls up to 85% of capacity using LRFD method (without any factor of safety) using model No. 1.

    Result: All members and connection passed.

  22. Same as Case 21 but using model No. 2.

    Result: All members and connections passed

  23. To determine whether the existing structure could support lime stone in the four corner bins filled up to 20% of the capacity with the rest of the bins filled with rice hulls up to 85% of the capacity using ASD method (with the required factor of safety) using model No. 1.

    Result: All members and connections passed.

  24. Same as Case 23 but using model No. 2.

    Result: All members and connections passed.

Above analyses' results are summarized in Table 1 on next page.

These tables are best viewed on tablets, notebooks, or desktop computer screens.

TABLE 1

Summary of the analyses results of the bin supporting truss structure

No. STAAD FILE No. No. of supports No. of corner bins with LS Rest of Bins with RH Load coefficient % of height fill LS density RH density ASD or LRFD method Truss results Seated beam connection
1 1 Model 1 4 5 0.3 85% LS
85% RH
90 20 LRFD Pass Fails
2 1A Model 2 4 5 0.3 85% LS
85% RH
90 20 LRFD Pass Fails
3 3 Model 1 4 5 0.3 85% LS
85% RH
90 20 ASD Fails Fails
4 3A Model 2 4 5 0.3 85% LS
85% RH
90 20 ASD Fails Fails
5 2 Model 1 0 9 0.3 85% RH No limestone 20 LRFD Pass Pass
6 2A Model 2 0 9 0.3 85% RH No limestone 20 LRFD Pass Pass
7 4 Model 1 0 9 0.3 85% RH No limestone 20 ASD Pass Pass
8 4A Model 2 0 9 0.3 85% RH No limestone 20 ASD Pass Pass
9 5 Model 1 4 5 0.3 Actual loads 90 20 LFRD Pass Fails
10 5C Model 2 4 5 0.3 Actual loads 90 20 LFRD Pass Fails
11 5B Model 1 4 5 0.3 Actual loads 90 20 ASD Pass Fails
12 5A Model 2 4 5 0.3 Actual loads 90 20 ASD Pass Fails
13 6 Model 1 4 5 0.3 65% LS
85%RH
90 20 LRFD Pass Fails
14 6A Model 2 4 5 0.3 65% LS
85%RH
90 20 LRFD Pass Fails
15 7 Model 1 4 5 0.3 65% LS
85%RH
90 20 ASD Pass Fails
16 7A Model 2 4 5 0.3 65% LS
85%RH
90 20 ASD Pass Fail
17 8 Model 1 2 bins at NE & SW 7 0.3 85% LS
85% RH
90 20 LRFD Pass Barely passes
18 8A Model 2 2 bins at NE & SW 7 0.3 85% LS
85% RH
90 20 LRFD Pass Barely passes
19 9 Model 1 2 bins at NE & SW 7 0.3 85% LS
85% RH
90 20 ASD Pass Fails
20 9A Model 2 2 bins at NE & SW 7 0.3 85% LS
85% RH
90 20 ASD Pass Fails
21 10 Model 1 4 5 0.3 55% LS
85% RH
90 20 LRFD Pass Pass
22 10A Model 2 4 5 0.3 55% LS
85% RH
90 20 LRFD Pass Pass
23 11 Model 1 4 5 0.3 20% LS
85% RH
90 20 ASD Pass Pass
24 11A Model 2 4 5 0.3 20% LS
85% RH
90 20 ASD Pass Pass

Note: Model 1 is generated with 8 support condition to determine forces at the seated beam connections and to determine truss member forces. Model 2 is generated with 4 supports condition to compare forces in the truss members and seat beam connections with Model 1.

The weakest link in the structure was the four stiffened seated connections of the top chords of the east and west trusses at each end. The bottom flanges of the top chords of the east and west exterior trusses, W6x15, were bolted with four bolts to the ½" cap plate of the corner column projecting beyond the column flanges. The cap plate was welded to the column. A stiffener plate ⅜" thick was provided underneath the cap plate. The seated connection was subjected to a vertical load coming from the east and west trusses and a horizontal force equivalent to the compressive force in the truss top chords. Both the vertical and horizontal forces created eccentricity on the connection, and it was determined that a single stiffener plate would overstress the seated connection to failure. If either a north or south connection was provided with a sliding joint to relieve the horizontal force, the connection with only one stiffener plate would have been able to resist the loads satisfactorily. Alternatively, if two stiffener plates were provided as was done in the cases of the interior trusses, the stresses would be satisfactory. As stated earlier, all four stiffened seated connections failed, see Figures 34, 35, 38, 44 and 47.

Conclusions:

Based upon the above, we conclude that:

  1. The cause of the collapse was the failure of the four seated connections of the east and west exterior trusses at their north and south ends under the loads of limestone placed in the corner bins and other products in the remaining bins at the time of the collapse. The four seated connections were provided with only one stiffener plate unlike other similar connections of the interior trusses which were provided with two plates. If either two stiffener plates were provided instead of the one, or if one end of the east and west trusses was placed on a sliding pad, the collapse would not have occurred in spite of the limestone in the corner bins, and other products in other bins.
  2. Throughout the forty-plus years' history of the plant, there is no record available to establish that there ever was a structural evaluation of the bins' supporting structure to determine whether limestone could be placed either in the four corner bins or in two diagonally opposite corner bins.
  3. The weakest link in the structure was the load bearing capacity of the stiffened seated connection mentioned in conclusion #1, and was the controlling factor in determining load capacities of the bins. If a structural evaluation was done, this weak link would have been discovered, and this incident would not have occurred.
  4. International Nutrition, Inc. began to place limestone in the four corner bins without either receiving any document from the previous owners establishing the structural adequacy of the bins to support the weight of limestone or other heavier materials, or conducting an independent structural evaluation to verify the same. Therefore, Section 5(a)(1) of the OSH Act was violated
  5. The original intent of structural design of the bins' supporting structure appeared to store rice hulls (20 pcf) in the bins, and not limestone. The supporting steel structure can support rice hulls up to the 100% capacity in all nine bins with adequate factors of safety, as per industry standards.
  6. The structure could not support limestone (90 pcf) in the four corner bins and rice hulls (20 pcf) in the remaining five bins at 85% of the capacity, with or without any factors of safety. However, the four corner bins could support limestone up to a maximum of 55% of bin capacity with rice hulls in the remaining five bins filled to 85% capacity without any factor of safety, a violation of the industry standards.
  7. The structure could support limestone in four corner bins filled up to 20% of the capacity with the remaining bins filled with rice hulls up to 85% of the capacity with the required factor of safety.
  8. If the four corner bins were filled with limestone greater than 55% of the bin capacities with the remaining five bins with rice hulls filled up to 85% of capacity, the collapse of the structure would be imminent.
  9. The structure could support limestone up to 85% of the bin capacity in two diagonally opposite corner bins with the remaining bins filled with rice hulls up to 85% of capacity, but without any factor of safety which is a violation of industry standards.

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