4.6 Photographs showing the detail: debris field
The relatively clear platform of pier 3 is also visible in Figure 28. There are several pieces of shaped metal on the floor, at least one appears to be a bolt. A close-up of the floor, taken looking east, is shown in Figure 31. So, what are the fragments? At least three bolts are visible, but more significantly, there are nine broken lug ends. These are the pieces that correspond to broken lugs such as those in Figure 28 and are identifiable by:
a curved outer surface;
a bolt hole;
two shorter sides.
The two short sides must be the fracture surfaces, which originally were part of the complete intact lug. The curved surface would be the outer cast surface of the lug.
Several other metal fragments are more difficult to identify, but it is clear that lug ends are the most common metal fragments seen on the floor. As the broken lugs on the lowest tier can only have produced four such parts, we can infer the rest came from broken lugs on the upper tiers that collapsed during the accident.
Moreover, it is most likely the lug ends came from lugs on the north and south faces of the tiers, where the diagonal tie bars effectively held the two groups of three columns together. These so-called wind brace tie bars would have been stressed if there was any side-to-side (west-east or east-west) movement of the upper part of the bridge.
The other fragments comprise three part broken bolts, and a large bolt complete with its nut and two washers. There are also three nuts, two of which include parts of the broken threads they originally possessed. They could have come from either the flange joints, or the lug connections, and there is nothing to distinguish between the two possible origins.