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SUMMARY OF THE INVESTIGATION OF A COLLAPSE AT UNOWEN PLAN

Internal document; not for external release. Written by Michael Rhodes.

On Thursday, December 11, 2008, at approximately 10 a.m., the collapse of an indoors bridge crane occurred at the UNOWEN plant in Richmond, Florida. UNOWEN manufactures marble plates using marble blocks and sells them throughout the state. Our firm was contracted by the plant owner to carry out an independent investigation on the causes of failure.

BACKGROUND INFORMATION

The bridge crane was a single truss crane designed and constructed by a local firm. Each truss was formed by 23 tubular members of the same material (HSS). The diagonal members are HSS 2 × 2 × 3/16; the vertical elements are HSS 2 × 2 × 3/16; and the horizontal elements are HSS 3 × 3 × 3/16. The purpose of the crane was to move heavy weights inside a storage building; specifically, marble blocks. It was completed less than a month before the collapse, and this was the first time that the crane bridge was used to move heavy objects different than marble blocks.


ACTIVITIES

We visited the plant and the sites of the collapse before the debris were cleared. We had discussions with colleagues and contractors and obtained the geometry of the truss together with material specifications. We were informed that the material had modulus of elasticity E = 200 GPa, Fy = 317 MPa (tensile strength).

EXAMINATION OF POSSIBLE CAUSES OF FAILURE

The visual inspection and an examination of the documentation obtained allowed eliminating several possible causes of failure:
1. There was no evidence of foundation failure, because the supports of the truss were in place even after the collapse, showing minor damage.
2. The crane bridge was part of an enclosed building, so that wind was ruled out as a possible action. Further, there was no seismic movement reported on that day.
3. This was a new bridge crane, and corrosion or aging could not be possible factor influencing the collapse.
4. Failure happened without the occurrence of dynamic action from operation of the crane or noticeable events reported in the building.
Based on the above, we concluded that the cause of the collapse should be excessive loading being carried by the bridge.

CALCULATIONS

Structural analysis was carried out using commercial software. The model of the truss adopted to carry out calculations is shown in Figure 2. All connections were constructed as pin joints, so that truss hypotheses were considered acceptable for the calculations in this Report. Elastic behavior was assumed to identify the level of stresses in the trusses.

The HSS 3 × 3 × 3/16 horizontal members have a cross section 12.1935 cm^2, moment of inertia 102.3929 cm^4, radius of gyration 2.8978 cm, length 125 cm. The tensile load is 387 KN, whereas the compressive load is dominated by buckling and given by 341 KN.

The HSS 2 × 2 × 3/16 vertical members have a cross section 7.6774 cm^2, moment of inertia 26.6804 cm^4, radius of gyration 1.8642 cm, length 75 cm. The tensile load is 243 KN, whereas the compressive load is dominated by buckling and given by 218 KN.

The HSS 2 × 2 × 3/16 diagonal members have a cross section 7.6774 cm^2, moment of inertia 26.6804 cm^4, radius of gyration 1.8642 cm, length 145.7738 cm. The tensile load is 243 KN, whereas the compressive load is dominated by buckling and given by 161 KN.


A load P was placed at node J (which was the position of the pulley system at the time of collapse).
Based on my calculations, failure of the truss should have occurred for a load P = 11.6 Tons, which is less than the load that was present at the moment of the collapse.

   
This research was supported by NSF-CCLI, DUE-0736828: "A Computer-Based Simulated Environment to Learn on Structural Failures in Engineering"