Abstract: On October 13, 1989, a second-level subcontractor and a first-level subcontractor for installing the Hambro D-500 Concrete Floor Roof System were pouring and finishing one wing of the fifth floor on a five-story office building. This wing was 140 ft by 80 ft. The columns and reams for the roof were set dividing the area into two bays and two outside rows of seven bays 20 ft by 30 ft and a center row of seven bays 20 ft by 20 ft. The subcontractors were using a concrete pump truck with boom and hose from above. Before the pour started, the other second-level subcontractors, ironworkers, had set all the roof bar joists. The workers spaced and locked the roof bar joists with a Hambro roll bar at each end but they did not weld them to the support beams. The joists were mostly spaced 4 ft thus plumbing and welding could have preceded the pour. The ironworkers declined to work above them. On the morning of the pour, the first-level subcontractor removed enough roll bars to move one joist in each bay to increase the opening for the concrete hose, which had a flanged coupling 10 ft from the discharge end. The boom operator was on the same floor but at a distance from the concrete crew. Moving the hose discharge end for about two hours, the pour had reached the center of the floor. The hose either bounced or was being raised and, as a result, three adjacent 20-feet joists were dislodged, 140 lb each, and one end dropped the other partly supported by roll bars. Three subcontractors sustained light injuries and one subcontractor sustained a bit more serious injury, so the work stopped. The concrete set before concrete removal began on another day. The ironworkers reset, plumbed, and welded all the joists in about two hours.