The weaknesses which can be improved include:
1) The present collision posts, made of channel members, are attached to the underframe, on only one side, in an unbalanced manner so that an impact force along the axis of the car can be converted into a twisting force (torque) at the post attachment point. The attachment point is not optimally designed to resist torque, and is only minimally able to resist lateral bending which is produced by twisting or by any deflection of impact to left or right.
2) The underframe design contains a number of single bevel welds which do not develop the full theoretical strength, are unpredictable, and not intended to resist impact optimally. Single bevel welds are warned against in the current edition of the Welding Handbook of the American Welding Society for impact-resisting purposes.
3) So-called "skip welds" were used in the attachment of the side sheets of the car to the underframe in the car involved in this accident. The welds developed only about one-third of the strength of a fully welded joint. Full welded joints would not have strengthened the walls sufficiently to prevent the penetration by the opposing car in the crash, but the destruction of full welds would have absorbed more crash energy and probably somewhat reduced the distance of telescoping penetration. The later cars in the previous highliner series included 100 percent welding of the side sheets to underframe, and the full welding is technically feasible.
For these reasons the Safety Board recommends that the following areas be included in specifications for the 15 additional highliner cars to be funded by UMTA:
1. Design specifications to require that all weld designs in the center sill area and in the underframe at ends of cars comply with specified current recommendations of engineering practices, and that single bevel welds not be employed.
2. Design specifications to require that welds, or other fasteners which join side walls to underframe and side walls to roof, develop a high proportion of the strength of the parent metal.
