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c. If the collapse was due to a failure of the upstream chain in the Ohio side span, why did the West Virginia side span rotate in a clockwise direction viewed from the west, with the downstream edge falling first?
d. What explains the presence of the numerous fractures in the entire lower chord system, many of which are distinctly brittle in nature?
e. How did the hanger connection system at U13N get pulled out of the truss if the chain had already failed?
f. In addition, the following observations by one or more eyewitnesses and survivors did not appear to be adequately explained:
(1) The presence of a bolt and/or a retaining caplike object on the deck prior to the collapse.
(2) The rather inconsistent reports of unusual vibrations just prior to the collapse.
(3) The loud "cracking" and "sonic boom" sounds reported just prior to or during the initial period of the collapse.
(4) The experience on the part of many of the survivors that the bridge dropped a short distance and then there was a momentary hesitation prior to the remainder of the collapse.
4. *Logical Framework for the Conduct of the Investigation*
The logical framework on which this investigation has been conducted is shown in Figure 3. The logic "tree" includes the work conducted by the Bridge Design Review and History Group as well as that of the Structural Analysis and Tests Working Group.
The possible causes of failure may be conveniently classified as those due to substructure failure, overload or overstress, superstructure defect, wind excitation, and external events including sabotage, vehicle collisions with the structures, and "sonic booms."
The substructure failures may be further divided into those due to scour around the base of the piers, anchorage movement or pier displacement which may have been caused by foundation failures or by collision of water craft with the piers. At the time of the interim report it was possible to discard scour as a cause of failure in that the divers had found no abnormal holes or lack of support at the base of the piers during their work on the salvage