Investigation of the September 24, 2002, Collapse of the
1965-foot High KDUH-TV Antenna Tower in
U.S. Department of Labor
Occupational Safety and Health Administration
Directorate of Construction
Investigation of the September 24, 2002, Collapse of the
1965-foot High KDUH-TV Antenna Tower in
Report prepared by
Mohammad Ayub, PE
Dinesh Shah, PE
The Directorate of Construction, National OSHA Office, was requested to provide assistance in the investigation and causal determination of the September 24, 2002, collapse of the 1965-foot high KDUH-TV Tower in Hemingford, NE. The incident occurred when workers were replacing the diagonals of the sections of the tower. Two structural engineers from the Office of Engineering, Directorate of Construction, accompanied by personnel from the Omaha OSHA Area Office, visited the incident site on September 30, 2002. One of the structural engineers also visited the storage yard at Scottsbluff, NE, at a later date where the selected retrieved sections of the collapsed tower were stored.
The incident occurred around 11:45 a.m. as a work crew was replacing the existing diagonals with diagonals of larger diameter in the panels of section 10 and the bottom panel of section 47. The entire project consisted of upgrading the existing KDUH-TV antenna tower to support the load of a new high-definition TV antenna and to replace aviation lights. The tower is located in a remote field, 22 miles northwest of Alliance, NE. There were two fatalities and three injuries when the tower collapsed. The collapse crushed a nearby car and pick up truck. A small grass fire, ignited by electrical wires severed in the collapse, was quickly extinguished after rescue crews arrived.
Description of the Project:
The 1965-foot high -KDUH-TV antenna tower was under contract to replace certain tower diagonals and struts to support the high-definition TV antenna and to replace aviation lights as mandated by the Federal Aviation Administration (FAA). The owner of the tower was Duhamel Broadcasting Enterprises of Rapid City, SD, which used the tower to broadcast signals from KDUH-TV in Scottsbluff, NE. Structural Systems Technology, Inc. (SST) of McLean, VA, was the structural engineer of record for the tower modification. The subcontract for roofing work on the building at the tower's base was awarded to Weathercraft Roofing Co, of Scottsbluff, NE. This work was nearly completed prior to tower collapse. Mid Central Tower of Illinois (Mid Central) was under contract to replace the diagonal and strut members of the tower.
The tower, triangular in plan, was composed of legs, guys, struts, diagonals, and redundant members. The tower consisted of 63 sections, with each 30-foot high section composed of three 10-foot high panels. The sections were numbered in ascending order from the top to the base. The top section was numbered as 1; the bottom section was numbered 63. The panels of each section were numbered in ascending order from the bottom to the top. The bottom, middle, and top panel of the sections were numbered as 1, 2 and 3, respectively. The tower was guyed at the three corners of the triangle at eight elevations of 218'-6", 428'-6", 648'-6", 878'-6", 1118'-6", 1358'-6", 1608'-6", and 1875'-6" (Figure 1). The modifications of the tower (figures 2, & 3) consisted of:
- Adding redundant members (double angles 2 x 2 x 1/4) to section 1, panels 2 & 3
- Replacing existing 1" diameter diagonals with new 1 1/8" diameter diagonals in section 10, panels 1 & 2
- Replacing existing 3/4" diameter diagonals with new 7/8" diameter diagonals in section 47, panel 1
The antenna tower (Figure 1) was originally erected in 1968. To support the new High Definition TV antenna, the structural engineer of record determined that certain structural members be replaced. The tower had been previously modified in 2001 by replacing guy wires with larger guy wires. The new modifications required replacement of the structural members in sections 1, 10, & 47. The subcontractor, Mid Central, planned to begin work simultaneously on lower section 47, and upper section 10; and later on top section 1. On September 24, 2002, two employees of Mid Central were working at the site, one on section 10 and another at section 47. An employee of the Weathercraft Roofing Co, was working at the base of the tower. The tower collapsed around 11:45 a.m., when the workers were replacing structural members at section 10 and 47, killing the two workers of Mid Central. The third Weathercraft Roofing Co. employee on the ground suffered minor injuries.
The tower fell apart in three pieces. The two base sections remained standing vertically. The next 14 sections (section 62 through 48) fell in one piece. The remaining sections fell in a Z-shape (figure 4). Section 47 was the most destressed and was twisted and crumpled into the ground. Other sections were also twisted, tangled, and deformed to varying degrees. During the clean up operation, section 47 had to be cut into pieces and could not be salvaged for later examination. Wind data from National Weather Report indicated the wind speed at the time of collapse was approximately 8 miles per hour.
Selected sections of the collapsed tower were removed and stored in the warehouse of R & C Welding of Gering, NE, for further technical evaluation. OSHA representatives examined sections 10 and 48 in the warehouse. Our conclusions are identified in figures 5 and 6.
Field observations indicated that the bolts connecting the diagonals and redundant members, hereafter called lug bolts, were missing in a number of panels of sections 10, 47 and 48. Visual examination indicated that the lug bolts were missing prior to the collapse because the lug boltholes did not exhibit any deformation and/or elongation normally expected if they had failed during collapse. The lug boltholes appeared "clean".
In panel 1 of section 10, all six diagonals were replaced with new diagonals and bolted to the gusset plates at each end by high strength bolts, as required by the construction documents. However, the lug bolts were not placed in any of the three faces of panel 1. In panel 2 of section 10, four out of six diagonals were replaced with new diagonals and bolted at each end. Two old diagonals remained. A few of the bolts connecting the old diagonals sheared off during the collapse (see Figure 6). Again, the lug bolts were missing in all three faces of panel 2, including the face where the existing diagonals had not yet been replaced.
Of all the sections of the failed tower, Section 47 was the most distorted and wrinkled, and partially buried in the ground. However, panel 1 of section 47 was examined though with difficulty due to distortions of its members. Examination at the scene revealed that the lug bolts of panel 1 were missing on all three faces as the holes were devoid of any deformations or elongations. Also, observation of section 48 in the warehouse indicated that the lug bolts were missing on two faces of panel 3 of section 48. Lug boltholes did not exhibit any sign of distortion or deformation that led to our conclusion that the bolts were missing prior to the collapse. It was also discovered that where panel 3 of section 48 joined panel I of section 47 at least one diagonal and one strut member were unbolted at the gusset plate connection of the legs of section 48. Figure 5 indicates the missing bolts and missing members. As the removal of the bolts of the diagonal and the strut was critical to the stability of the tower and this investigation, the OSHA Salt Lake Technical Center (SLTC) visually examined the holes to confirm whether the bolted connections of the diagonal and strut were removed prior to the collapse. A metallurgist from SLTC visited the warehouse and examined the holes in question of section 48 confirming that the connections of a diagonal and a strut member at section 48 were removed prior to the collapse (see appendix A for SLTC report).
See figures 7 to 27 for the collapsed tower.
Professor Hugh Bradburn of the Department of Civil Engineering, University of South Carolina, Columbia, SC, was contracted to perform the following structural analyses of the antenna tower:
- To determine the adequacy of the structural design of the KDUH-TV tower with new diagonals and new guy wires, as specified by the structural engineer of record, to support the new loads in accordance with the applicable TIA/EIA standards for steel antenna towers.
- To perform structural analyses of the KDUH-TV tower with the conditions that existed immediately prior to the collapse of the tower, as best determined by OSHA, i.e. with bolts and members in certain tower sections removed.
A three dimensional finite element program based on a stiffness matrix was used to model and analyze the structure. The program recognizes large displacements and large rotations of the members whose shears and moments are computed in deformed positions. The guys are modeled by three dimensional, geometrically nonlinear finite element cable stiffness elements. The tower is divided into elements corresponding roughly to a tower segment. Truss freedoms of each element are reduced and transformed to equivalent beam type elements, then assembled into a tower stiffness matrix. The program reduces compression in slender diagonals instead of eliminating them completely which produces a much more realistic distribution of forces in the structure. This program was written, developed and run by Dr. Hugh Bradbum. Many nationally recognized engineering firms involved in the analysis and design of cable supported tower structures have verified the validity of these computer programs.
Five analyses were preformed, as follows:
- To determine the gross results with existing members with no wind;
- To determine member forces in the tower with existing members with no wind;
- To determine gross result with the proposed new members of the tower with a wind pressure of 65 psf;
- To compare maximum member force with allowable force for all tower members (based on AISC/ASD) with wind pressure of 65 psf;
- To determine member forces in the tower with proposed new members with wind pressure of 65 psf.
The applicable design standard for the tower was considered to be TIA/EIA RS-222-C. The objective of the analysis was to determine causal factors that may have contributed to the collapse of the antenna tower and to determine whether the HDTV antenna loads would have overstressed the tower section members when completed.
The properties of the tower members of interest were taken from original contract documents prepared by SST of McLean, VA. It was assumed that the drawing reflected as-built: conditions so no verification of the member sizes and properties was done except in isolated locations.
The wind speed was taken as 65 mph and the resulting wind pressure was uniformly applied for the entire height, as recommended by the EIA standard. Wind was applied in three directions, direction 1 is normal to a tower face; direction 2 is 180 degrees from direction 1; and direction 3 is parallel to a tower face.
The following documents provided the basis for the structural analyses:
- Contract drawings dated 1977, 1993, and 2002.
- A report "Rigorous Computer Structural Analysis and Evaluation of the KDUH-TV 1965-foot Guyed Tower Hemingford, Nebraska" prepared by SST, structural engineer of record.
- Miscellaneous photographs and newspaper articles concerning the collapse.
- Weather data for Alliance, NE, for September 23-24, 2002.
- Miscellaneous information.
Appendix B contains the report on the structural analysis
Prior to the tower collapse, the workers were replacing existing diagonals with new diagonals of larger diameter in sections 10 and 47. National Weather Report data indicates that the wind speed at the time of collapse was approximately 8 miles per hour, considered inconsequential.
Towers are designed to carry gravity, wind, and ice loads. All members, legs, diagonals, struts and redundant members provide structural stability to the tower with little redundancy. Absence or removal of a member can trigger catastrophic failure unless a substitute is provided before removal. Legs carry axial loads and derive their strength from shorter unbraced lengths by the presence of redundant members and their connection to the diagonals with lug bolts. If the lug bolts are removed without providing any substitute to the frame, the leg is subjected to a higher unbraced length that immediately reduces its load carrying capacity. If a diagonal is also removed in addition to the lug bolts, the unbraced length of the leg is greatly increased, jeopardizing its load carrying capacity. Failure of one leg can result in a tower collapse because there is no redundancy. Redundant members and diagonals are critical to the stability of the tower.
The Structural Engineer of Record (SER) had stated that he had provided instructions (Appendix C) to the contractor to use Come-A-Longs before removing diagonals. It is believed that the Come-A-Longs were not used. The diagonals cannot be removed without removing the center lug bolts. There is a presumption in the instructions issued by the SER that removing the lug bolt will not substantially reduce the load carrying capacity of the leg. Structural analysis indicated that for the KDUH-TV tower, such was the case.
Structural analyses indicated that the actual load carried by each leg at section 10 was approximately 90 kips at the time of the incident. As mentioned above, wind was not considered a factor. The capacity of the legs was determined to be far greater than 90 kips. If the lug bolts of section 10 were removed on all three faces without disconnecting the diagonals at their ends, the capacity of the legs would be reduced but would be more than adequate to support the loads. However, if one diagonal was removed in addition to the lug bolts, the capacity is significantly reduced to between 137 and 150 kips. This reduced capacity is still higher than the actual loads carried by the legs. It was therefore concluded that the lack of use of Come-A-Longs in replacing diagonals was not critical to Section 10.
The actual loads carried by the legs at section 47 was determined to be 260 kips without wind. The capacity of each leg was determined to be 970 kips which provided adequate factor of safety. However, if the lug bolts were removed, the capacity of the legs would be reduced to 690 kips, based upon inelastic buckling, but still well above the actual load. If a diagonal on one face is removed in addition to the lug bolts, the capacity will be significantly reduced to between 260 and 285 kips, equal or close to the actual load the leg must support. With such conditions, the collapse of the tower could be imminent. If a horizontal strut member was removed in addition of a diagonal, as was the case in this collapse, the failure would be inevitable.
An analysis of the proposed upgrade was performed using Telecommunications Industry Association (TIA) and Electronic Industry Association (EIA) Standard RS-222-C for a uniform wind pressure of 65 pounds per square foot with no ice. The results of this analysis indicated that the proposed upgrade met all requirements of EIA Standard RS-222-C.
Based upon the above findings, it is concluded that:
- Construction activities at tower section 47 immediately before the incident caused the tower collapse. Simultaneous construction activities at section 10 did not contribute to the collapse.
- A worker removed the bolts connecting the diagonals and redundant members of panel 1, Section 47, and a diagonal of one face of panel 1, Section 47, resulting in a severe reduction of load carrying capacity of the tower legs, which eventually led to the tower collapse. The worker also removed a horizontal strut of panel 1, Section 47 that contributed to the failure.
- Removing the bolts at the intersection of the diagonal and the redundant members of panel 1, Section 47, would not have been critical to the structural integrity of the tower. However, the lower legs in Section 47 lost their capacity to support the loads when the diagonal was also removed.
- Though similar construction procedures were used at Section 10, the capacity of the legs of Section 10 did not diminish to the extent that they were unable to support the loads.
- Come-A-Longs or other devices were not used prior to removing the diagonal. The contractor did not follow the generally accepted industry practice of installing a temporary bracing member before removing a diagonal. The structural engineer of record stated that he provided instructions to the contractor to use a Come-A-Long. It is, however, believed that the structural engineer did not adequately warn the contractor of the critical nature of the diagonals to the stability of the entire tower.
- Wind was not a causal factor in the tower collapse.
- The proposed design by the structural engineer of record was determined to be adequate to support new loads.
Appendix A (for the complete appendix, see PDF)
Appendix B (for the complete appendix, see PDF)
Exhibits (for the complete exhibit, see PDF)
Appendix C (for the complete appendix, see PDF)
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