Tuesday, February 25, 2020

A very short primer discussing how blast interacts with structure

You may remember this image from an earlier post.

This short essay attempts to give the reader a sense of how a blast interacts with the bridge structure. Important caveat, these physics only exist in Capiche and surround areas and should not be used to pad your resume.

Brown is dirt and/or concrete. They have similar mass densities and act in similar ways in the initial stages of the blast. Medium gray is bridge span. For the sake of this example we will consider it to be steel reinforced concrete.  Red cross-hairs are the location of the charge. The small, solid red circle between the bridge span and the dirt is the roller-nest. Assume the span is symmetric.
Work is (force X distance). The force is equal around the center of the blast but the energy is deposited where there is the most movement, that is, the least resistance. The blast has lobes. A smart demo will have the charge closer to the vertical face of the abutment than to the horizontal plane of the pavement to ensure a major lobe disables the abutment. Pavement can be repaired with loose fill. Abutments require concrete.
Fill displaced by the blast will form an above-grade ring around the crater. The shadow of the blast will lift the bridge span, shatter the end closest to the blast, put large bending-moments on the span and compress it.

The compressive strain causes the span to spring back toward the center of the blast after it hits the travel limits on the roller nest that was not destroyed. The span may, or may not, end up in the stream depending on the amount of rebound.

A likely outcome of a well executed demo on one end of a steel reinforced, concrete bridge.

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