anyone ever hear of this

huangyhg

anyone ever hear of this...
can you really get away with hanging lift hoists and beam trolleys from a bar joist roof?  what is the normal safety factor for a bar joist roof system?  i was in a plant last week that had a dozen or so beams and trolleys hanging from the bar joists.  they using them to move around 500 lb drums of material.  it was nice that they were labeled "1/2 ton capacity", but how do you now if the joists can carry the load.  
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depending on how you engineer it (assuming it really was engineered) it could work.  however, when the snow flies, that could be a different story.  perhaps they have a rule that there is no lifting during the winter?
this is a true story! i know of a ice skating rink that had hinged columns over the ice rink.  whenever it snowed, they swung the columns down to take up the snow load.  when it melted, they swung the columns back up into the timber roof trusses to provide a column free skating area! the building was designed in the 1930's and was in operation until the late 1980's when it burned down. file this under "strange, but true".
if you specify point load to joist manufacturer, he will design joist for this additional load with or without snow. joist is a truss, and may be designed for any load and load combination as long as you know what loads you need.
book "designing with steel joists, joist girders, steel deck" (published by nucor corporation) has chapter dealing with cranes and monorails, hangers, conveyors, roof top units, etc.
yes, bar joists can be designed for these loads, but the chances are very good that they were not.  bar joists are generally the cheapest roof framing for an industrial building.  once you start designing them for  hoists and trolleys-and their related impact loads- their economy diminishes.
often new owners come in, or tenants come in to a spec industrial building, and assume that the structure can take anything that they can put to it.  this is often done by in-house personnel without analyzing what is there.   the results can be frightening.
generally, we design industrial structures with the lightest, most efficient roof structure we can.   we then provide sub-framing between the columns, just under the roof framing, to accommodate future equipment and hanging loads.  the columns, of course, are overdesigned to accommodate these process/equipment loads.  by doing this, we give an extra measure of safety against do-it-yourself industrial loads, and also make it easier for future engineered projects: analysis is simpler, and playing around with the joists or adding/reinforcing columns is generally not required.
yes, it adds to the cost, but owners of built-to-suit projects almost always accept this sub-framing as an extra; they see its long-term value when we explain the flexibility they gain.  we have even had this system accepted by big-box industrial spec developers, who do not want a tenant bringing their building's roof down.