I believe what we are seeing is the wooden forms and supporting wooden and steel supporting “false works” so that a reinforced concrete fly-over viaduct can be built in one giant monolithic seamless concrete placement.
The idea was that the falseworks supports the entire bridge form-work until the concrete placement is ready to occur. At that time they would have started placing concrete with pumps around the clock for two days while traffic is diverted. The falsework substructure would then be pulled out after the concrete reached design strength and the forms removed from below a side at a time.
This procedure is sometimes used when the option to divert traffic is considered too constrictive.
There was probably no “blow torch” but instead a cutting torch was being used to cut some steel or a welding torch was being used to weld an item in place and a small fire was started in windy conditions and the welder had failed to have a fire extinguisher nearby as required in his daily Task Hazard Analysis, his overall Activity Hazard Analysis, his Hot Work Permit procedure and his overall Safety Plan all of which were required procedures for this type of work.
It looks a lot like the way we pour “quake proof” floors in multi-story buildings in Seattle, except about ten times thicker in the bridge.