Issue link: http://alpine.uberflip.com/i/493471
1 2 3 4 5 6 7 8 7.05 Rafting What is rafting? Rafting is a process where the installer uses complete trusses to assemble an entire roof, or section of a roof, on the ground and then lifts the completed assembly onto the building structure. The size of rafted sections is based on available space on-site, lift capacity of the available crane, or unique footprint of the roof system. Why use rafting? On suitable projects, rafting allows most of the roof framing and decking to be assembled on the ground, minimizing or eliminating the need for multiple lifts, scaffolding and fall protection systems. Less lifts means less crane time, which can translate into big savings on crane costs. On- ground assembly of entire roof sections, including permanent truss bracing, roof decks and mechanical systems, can save significant labor time and can allow the simultaneous construction of walls and roof systems. What are the special considerations for rafting? The design of the roof assembly to be rafted should consider the effect of an alternate load path, where the weight of the assembly is transferred through the lifting cables (or straps) to the ground at pick points instead of through the truss bearings. The number and location of pick points for rafting should be determined with attention to the following factors: • total weight of the assembly, • weight distribution in the assembly, • truss configuration, • crane capacity. In addition to design analysis for conventional roof loads, rafted trusses must also be analyzed for a case where the supports are at the pick points. In some instances, the permanent bearing members for the trusses (tube steel or I-beam, for example) could be included as a part of the assembly on the ground, and the entire assembly could be lifted from pick points located on the bearing members. Adequate bracing of trusses is needed for the stability of the roof system. Most of the roof decking, and almost all of the required truss permanent bracing for the webs and bottom chord, could be installed prior to rafting. Why raft with TrusSteel? TrusSteel trusses are light in weight (up to one- half the weight of trusses made from wood or "C" channel materials). Substantial roof sections can be assembled on the ground and then lifted with an average crane. With the exceptional lateral stability (stiffness) of TrusSteel trusses, roof assemblies can be built that will survive a lift without introducing significant extra bracing. How do I get an engineered raft? Rafts of trusses, no matter the brand or type, must be engineered so that they will lift safely and without causing damage to the trusses. The project Engineer of Record may perform this design service. Specialty engineering firms, such as BBD Engineering and Design, LLC, are available to perform these design services on a consulting basis. ENGINEERED BY ALPINE I N S TA L L AT I O N / B R AC I N G