U.S. Department of Labor
Occupational Safety and Health
Directorate of Construction
Report Prepared by
Mohammad Ayub, PE, SE
Office of Engineering Services
Directorate of Construction
Contributions to this report made by
Dinesh Shah, PE, Office of Engineering Services
Rodger Frey, CSHO, Avenel Area Office
On July 13, 2007 at approximately 2:10 p.m. the wood roof trusses suddenly collapsed and fell over the first floor of a new medical office building under construction at 32 Worlds Fair Drive, Township of Franklin, NJ. The trusses were erected that morning and employees were installing temporary and permanent bracings and purlins at the time of the incident. Most significantly, shortly before the incident, three bundles of 2x6s were placed over the top chord of the center trusses to be used as purlins and braces. At the time of the incident, approximately seven employees were reported to be working on the trusses near the center of the bay. Two employees were seriously injured.
The one-story building with basement is owned by Franklin Professional Park, LLC. The following were the key participants in the project:
This report is limited to the roof area where the incident occurred. The roof trusses spanned approximately 81' between supporting steel beams over steel columns. There were a total of 41 trusses consisting of 16 “Drop TC Hip" trusses and 25 trusses known as "Hip" trusses. On the top of the 25 hip trusses, 25 gable trusses were to be mounted at the top to meet the architectural roof configuration. At the time of the incident, gable trusses were yet to be erected but all 41 drop TC hip trusses and hip trusses were set in place, see Fig. 1.
On Thursday, July 12, 2007, the erectors consisting of five companies, namely Farrah Builders Inc., Pete Donoghue Construction, York Framing, AMH Contracting, Contaffa Construction Co. Inc., began setting the roof trusses and sloping rafters on the north and south side. First, they erected two triple trusses, HGIR, one on the north and one on the south side and then connected the sloping rafter trusses. They also erected five "Drop HC Hip" trusses marked H1SD1 thru H1SD5 on the north and south side (total of 10 trusses). During interviews with OSHA, the employees reported that the temporary braces for the top and bottom chords, and the webs were installed on the ten trusses as soon they were set in place.
On Friday, July 13, 2007, the truss erectors and their employees reported to the site at approximately 6:30 a.m. and began setting the remaining trusses called "Hip" trusses, marked H1SDBase. By lunchtime, all 25 hip trusses, weighing 718 pounds each, were set in place. The erectors reported that all temporary top chord, bottom chord and diagonal bracings were also installed on the hip trusses as soon as they were erected.
After lunch, the erectors began to install 2x6 purlins on the top chords of the hip trusses. The gable trusses were later to be erected over the hip trusses and nailed and supported over the 2x6s. Some of the crew members began to install temporary and permanent braces. Three bundles of 2x6x12' long pieces, each bundle containing 20 pieces, were hoisted by the crane and placed over the top chord of the hip trusses to be nailed to the top chord as purlins. Also, there were reported to be 50 pieces of 2x4x24' long pieces spread over the bottom chord. There were approximately seven employees on the trusses near the center of the trusses. Shortly after the three bundles were placed the employees observed, at approximately 2:10 p.m, that the roof trusses, except for the five trusses on the south side, suddenly collapsed at the center. The employees reported that the trusses fell just in the middle in a vertical direction, see Figures 2 to 5. The ends of the trusses remained generally connected to the supporting beams.
Abatare ordered the roof trusses from Builders General who contacted P.D.J. components to manufacture the trusses. Builders General provided a set of architectural and structural plans to P.D.J. P.D.J then designed the roof trusses using structural design software provided by MiTek and forwarded their design to MiTek to review and stamp the drawings with a seal of a professional engineer. After MiTek's review, PDJ manufactured the trusses in their facility and shipped them to the site on July 9-10, 2007. PDJ also provided standard bracing instructions to Abatare to follow during erection.
It is a standard industry practice to maintain structural stability of roof trusses by providing temporary bracings, i.e., top and bottom chord lateral and diagonal bracings, and web diagonal bracings during erection until the entire structure has been completed. The responsibility to provide these bracings rests solely with the erector. Roof trusses are light structural members, highly susceptible to instability and lateral torsional buckling without adequate temporary bracings. Due to the lack of adequate lateral and diagonal bracings of top chords, bottom chords, and web members, the installed trusses were in a state of instability, ready to collapse either due to buckling or under any significant lateral and gravity loads. Further, standard industry practice requires that materials should not be placed on trusses during erection unless proper evaluation is done, see Attachment B.
Details of the required bracings for trusses up to 60' in span are provided in "Building Component Safety Information - Guide to Good Practice for Handling, Installing, Restraining, & Bracing of Metal Plate Connected Wood Trusses i.e., BCSI -B1, 2006 edition (see attachment A), produced by Truss Plate Institute (TPI). For trusses over 60' in span, the industry requires that a professional engineer determine the type, size and location of temporary bracings.
Structural analyses were performed to determine:
The structural analysis of the roof's hip truss was performed using the commercially available software program STAAD.Pro 2007 to obtain the member forces. The following assumptions were used in the analysis.
The analyses indicated the following:
Post-incident examination revealed that the actual number of bracings provided to ensure the stability of the trusses was far less than is required by the industry practice, see Table 2.
SUMMARY OF MAXIMUM AXIAL FORCE IN TOP CHORD MEMBER
|Loading condition||Maximum axial compressive force without load factor||Ultimate axial capacity in compression* without strength reduction factor||Remarks
|Dead load (DL) of the truss
||804 pounds||2090 pounds||O.K.|
|DL of truss + minimum two employees||1360 pounds||2090 pounds||O.K.|
|DL of truss + minimum two employees + three bundles of 2x6 + loose 2x4 pieces||2167 pounds||2090 pounds||Failure imminent|
* Based on an effective unbraced length of 7'-2".
REQUIREMENTS OF TEMPORARY BRACING DURING TRUSS INSTALLATION AS
|Items|| No. of bracings required
| Top chord lateral bracing @ east face of the truss
|| 10, each @ 4'-0" apart maximum
| Top chord lateral bracing @ west face of the truss
|| 10, each @ 4'-0" apart maximum
| Top chord diagonal bracing @ east face and west face of the truss
|| 1st bracing covering five trusses and subsequent braces covering four trusses
| Bottom chord lateral bracing
|| 10'-0" to 15'-0" apart maximum
| Bottom chord diagonal bracing
|| 20'-0" apart maximum
| Web diagonal bracing
|| 10'-0" to 15'-0" apart (located at each of the bottom chord lateral bracings) and 20'0" apart in a transverse direction
| Ground bracing
||20 (to be located at each of the top chord lateral bracings )|
* Based on Attachment A.
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