Truss Lifting Guidelines outlined in BCSI offer some very practical and clear guidance for an activity that I hope most builders will be doing a lot of in the busy summer months ahead. These lifting guidelines are summarized in this post, but the BCSI booklet is a very good document to be familiar with for all things related to wood trusses: handling, lifting, installing, bracing, and restraining. I discussed this great document in a previous Structural Integrity post, so let's put our hard helmets on and get into some specifics on truss lifting!
As shown in the first truss lifting diagram above, the truss lifting guidelines warn that lifting by a single pick point at the peak can cause truss damage. It doesn't guarantee damage if you fail to heed this warning, just as I'll say that a building is not guaranteed to fall down if it is built slightly with a little less strength than I design it for. The BCSI guidelines, as with good structural engineering, make every effort to ensure success with their provisions, rather than threatening failure if the provisions are not followed.
The recommended lifting method for small trusses, here defined as trusses less than 30 feet from out-to-out of the bearing point locations, is shown above with two pick points. To avoid the truss being pulled out of plane by its own weight, limitations are put on the amount of compression that is placed on the truss component by limiting the reach of the two pick points to 1/2 of the truss span. Also, keeping the horizontal pull-in force of the lifting reaction low is accomplished by ensuring that the angle between the two straps or chains is 60° or less.For Slightly Longer Trusses...
For trusses 30' or longer (up to 60' long), a spreader bar is recommended to make sure that there is an overall tension force placed on the truss as it is lifted into place. This tension force is the major force in keeping these larger trusses flat during lifting. The truss' own stiffness to resist bending is not very large, as many of you may have observed during lifting and placement of floor or roof trusses.
Long Span Trusses...
For trusses of any length, including those that may be over 60' in length, a spreader bar or stiffback may be lashed to the truss while lifting. This is a good method for a few reasons: 1) The truss lifting forces are transmitted from the lifting cable to multiple locations along the truss length, decreasing the localized stress on each truss to bar connection. 2) The truss reaction from the stiffback is vertical only, which is the trusses strongest direction to resist loads... the horizontal compressive reaction from the lifting straps is absorbed by the stiffback itself. 3) Control of the truss during movement is improved and the hook height can be maintained lower compared to using a spreader bar above the truss.
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