The top chord is under compression. So adding a load at top is just how the truss/joist was designed to take additional loads as long as you don't exceed capacities. The bottom chord is under tension. The truss/joist is not designed to take on much tension. For example, imagine trying to crush a joist by adding weight vs. trying to pull the two chords apart from each other. It takes a whole lot less weight to pull them apart than it does to crush them. Finally, if you attach a load to the side of the bottom chord, you essentially create a torque on the truss/joist. Once again, these components are not meant to take on torqued loads. Their principle duty is to take on compressive loads.
Duane Johnson Strickland Fire Protection -----Original Message----- From: Sprinklerforum [mailto:[email protected]] On Behalf Of Travis Mack Sent: Wednesday, January 21, 2015 5:30 PM To: [email protected] Subject: Re: Hanging off botom of steel bar joists I was told by a structural engineer many years ago that joists are designed to bear on the top and not on the bottom chord. Therefore, his opinion was that all hanging should be from the top. Since then, I have always shot for hanging to the top chord of a joist. NFPA 13 doesn't specify either way. Travis Mack, SET MFP Design, LLC 2508 E Lodgepole Drive Gilbert, AZ 85298 480-505-9271 fax: 866-430-6107 email:[email protected] http://www.mfpdesign.com https://www.facebook.com/pages/MFP-Design-LLC/92218417692 Send large files to us via: https://www.hightail.com/u/MFPDesign On 1/20/2015 9:19 AM, Gregory Lindholm wrote: > I know there was a discussion on when you can hang off the bottom of bar > joists, but for reason, I can't get into the archives. > Can somebody refresh my memory? (I pretty much always hang off the > tops, but there are times when that is not reasonable possible, and am > trying to find out if, or when, you can hang of the bottoms.) Thanks > > Greg Lindholm > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkl > er.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
