Inspection Detail

Inspection: 300564051 - Diamond Builders

Inspection Information - Office: Madison Area Office

Investigation Summary

Employees #1 and #2 were on a tubular welded-frame scaffold finishing the installation of the last of several 1,100 lb trusses, when one of them gave way, causing the others to fall. The collapsing landed on the scaffold, knocking off the two workers. Employees #1 and #2 sustained injuries, for which they were hospitalized. The two-piece scissor trusses were connected by a metal plate and joined together at the midpoint by two, 2 by 8s (one on either side of the midpoint) secured together with eight bolts. The trusses were also reinforced at the apex by plywood nailed into the 2 by 8 top plate and the two, 2 by 6s connecting the top and bottom cords. Altogether, the span of the trusses was 60 ft, and they were set 24 in. on center. The top cord had a slope of 10-in-12 and the bottom cord had a pitch of 5-in-12. According to the foreman, temporary continuous lateral bracing was installed in two places along both sides of the top plate, at about the center of the 40 ft long top plate. The foreman claimed that diagonal bracing had been installed along the top plate between the top cord lateral bracing. Permanent bracing was installed along the bottom cord and on both sides along the 2 by 8 intersecting and connecting the top and bottom cords. It was observed that top cord temporary bracing was installed using only one 16-penny nail. The foreman stated that there was some ground bracing on the south side of the structure, but no ground bracing along the north side. The Truss Plate Institute's guide recommended that at least 10 lengths of continuous lateral bracing be applied to the top cord of the size being set. The first of the continuous lateral bracing should have been installed on the top cord at the bearing point of the trusses along the side walls, and then at intervals of between 5 and 7 ft up to the peak. Top cord diagonal brace spacing should have been every five trusses, and all temporary bracing should have been installed using two fully seated 16-penny nails. According to the Truss Plate Institute's guidelines, grounding of both interior and exterior bracing is required. Other ground bracing of other erected trusses showed that the splices were connected with only three 16-penny nails. A minimum of ten, 3 1/4 in. nails was recommended for such splices. It was concluded that although permanent bracing (other than roof sheeting) had been installed along the interior cords and webs, temporary bracing was inadequate because of insufficient top cord lateral and diagonal bracing. The temporary braces were not nailed according to the Truss Plate Institute's recommendations. There was no ground bracing on the north side of the structure. A precipitating factor was the actual collapse and failure of one of the trusses. Questions have been raised on the engineering, design, and manufacture of the trusses being used. Analysis of the failed truss indicated that the metal king-post connection may not have been placed in an area which provided optimum support of the 2 by 12 bottom cord. Other trusses salvaged from the collapse revealed characteristics similar to those of the failed truss.

Keywords: ERECTION PROCESS, TRUSS, COLLAPSE, UNSECURED, WORK RULES, CONSTRUCTION, BRACE, FALL, BRACING, TUBULAR SCAFFOLD