The Japanese in Burma are now compelled to resort increasingly to delaying tactics. In this connection, the first two bridge demolitions of any importance performed by Japanese forces in this theater are of interest. These were partial demolitions, and showed signs of defective technique. One bridge was built of masonry and is referred to in this article as Bridge A. The other bridge, referred to as Bridge B, was of steel-girder construction. Both bridges had been designed to carry a narrow-gauge railway.

TWO-SPAN MASONRY BRIDGE

Bridge A (see fig. la) was a two-span, masonry bridge, which had one span of 15 feet and one span of 40 feet. The two spans, or arches, each 4 bricks thick, were supported at the center by a masonry pier rising from the watercourse below. The Japanese demolished only the 40-foot span, leaving intact the two bridge-end abutments, a pier between the spans, and the 15-foot span.

After examining the bridge, military observers report that six pressure demolition charges probably were employed by the Japanese to destroy the long span. The condition of the arch stumps and the nature of the debris indicate that two charges were placed near the abutment, two at the span's center, and two close to the pier. That charges were placed at the span's center may be deduced from the fact that most of the rubble under the bridge was in comparatively small pieces. If no center charge had been used, the debris would have contained larger chunks.

Figure 1a. Bridge A. The 40-ft masonry arch was destroyed, but the short span, the center pier, and the two bridge-end abutments were left intact. Figure 1b. Bridge B. The far I-beam (65 ft.) was damaged by a charge which blew out approximately 16 ft. of its vertical section. This beam buckled, but remained in place. One end of the near beam was forced from the abutment and slid down the hillside; its pier end remained in place.

TWO-SPAN STEEL-GIRDER BRIDGE

Bridge B (see fig. 1b) contained two steel-girder spans, each 65 feet long. Each span consisted of two steel I-beams, 4 inches deep and with horizontal members 15 inches wide. At the center of the bridge, the two spans rested on a high masonry pier. Only one span was demolished by the Japanese. The second span, the center pier, and the two abutments were left intact.

This demolition (according to the observers) was effected in a rather unusual manner. Charges were so placed on the span that portions of the vertical panels in both I-beams were blown out, but the upper and lower horizontal members of the beams remained intact. Furthermore, charges were not placed in the same relative positions on the two beams. One beam, with approximately 16 feet of its vertical panel destroyed, merely buckled and remained in position. The other beam was damaged by a charge which blew out a few feet of the vertical section next to the abutment. The end of the beam was forced off the abutment and slid several feet down the hillside, while the end resting on the pier remained in place.

The technique employed by the Japanese in these demolitions was poor. Subsequently, both bridges were reconstructed by British engineers, who were able to utilize the piers and other elements of the bridges left standing by the enemy. According to an observer, "Bridge A could have been rendered far more useless if the pier had been destroyed and one or both abutments partially destroyed. If a demolition charge had been placed near the top of the center pier of Bridge B, the entire bridge would have collapsed and the obstacle would have been more formidable. The comparative ineffectiveness of the demolitions probably was the result of inexperience and improper appreciation of demolition technique."