Truss Lifting Guidelines

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!

No use of the Single Pick Point?

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.

In post frame and other wood construction, the trusses play a significant role in the building's ability to resist the required loads applied through snow, wind, rain, gravity, and earthquakes. Their important role in the completed building and the high capabilities to carry loads has to be measured against the high vulnerability the truss components have to out-of-plane loading and deflection during installation, which is why this topic is so important.

For Trusses less than 30'

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.

If you have questions about truss lifting and placement, ask your qualified design professional sooner instead of wishing you had done so later. For a complete discussion of the BCSI provisions, get a copy of the BCSI book for yourself by visiting the WTCA or TPI web page (or following this link: http://www.sbcindustry.com/pubs/BCSIED2-D ).

BCSI - Guide to Good Truss Practices

The latest Building Components Safety Information (BCSI) book was published in October of 2006 and should be familiar to all contractors, builders, and designers using Metal Plate Connected (MPC) Wood Trusses. At the 2007 National Frame Builders Expo in Indianapolis, I gave a presentation with Brent Leatherman of TimberTech Engineering in which I stressed the importance of this document and the specific recommendations in BCSI-10 which are specific to Post Frame buildings.

The Subtitle is descriptive and accurate: "Guide to Good Practice for Handling, Installing, Restraining & Bracing of Metal Plate Connected Wood Trusses." The BCSI offers many diagrams and photos to illustrate the concepts and guidelines outlined in the text.

The document is available for order ONLINE for $10.20/book or less, depending on quantity ordered and your membership status with the Truss Plate Institute (TPI) or Wood Truss Council Association (WTCA).

This document does not have the force of the building code behind it, but it does create an accepted level of care within the industry which could be used in the event of litigation when bracing, handling, installation or restraint of trusses are involved. Having said that, however, I have heard of some building officials enforcing this guide within their jurisdictions as if they were part of the enforceable code, all the more reason to be familiar with this document!

I have created a new post category ("Wood Trusses") for this entry since I intend to write more about different aspects of this topic, including lifting guidelines, temporary bracing requirements, and permanent bracing practices. If you have any subjects you'd like to see explored here, please let me know!