Expert: Stephen Major - 1/17/2012

Question
I want to hang an steel I beam from some roof joist. in a commercial property. Its standard steel x joist about two ft tall they are every 5 feet  the I beam would be for a trolley system and hold about 600 lbs across a 20 ft span would i need to have this approved or anything i would use beam clamp to attach to the joist and 1/2 in to  5/8 all thread rod. Also the beam would be about 3 ft from the bottom of the joist. I am a pipe fitter so some of the stuff is easy to come by

Answer
Dean,

You have two structural concerns to deal with here.  The first is to be sure that the I-beam is strong enough to support the trolley and the greatest anticipated added load.  If you are truly going to span 20 feet, this means that you are supporting the beam at its ends only.  If you are able to support the beam in additional locations, such as every five feet (as would be the case if it is perpendicular to the joists), then your span is five feet, not 20 feet.  An engineer or perhaps your steel supplier can provide load data for various I-beam shapes.  But typically dealers do not give structural advice for liability reasons.

The second concern is the ability of the joists to carry the additional load.  Most roof bar joists are sized to handle the dead and live loads of the building as it was designed.  When you add additional load, and especially a moving load that introduces torques and other forces, you can easily over-stress the joists and possibly cause failure or excessive deflection.  I would make the assumption that the existing joists are not designed to handle this additional load, and you should add appropriate support such as columns with footings at either end, or add proper reinforcement to the joists.  In this case you should have a professional engineer do the load calculations and suggest correct alternatives.