But Duane that's not how trusses work. Yes, top is in compression but that is what makes the bottom in tension, the forces are transferred. Add more weight to top and you get more tension in bottom or vice versa. You can't add a force to one member without effecting all members. The question is material properties and steel bears less in tension than compression but even that is dependent on the slenderness. All structural members don't have a principal duty, they resist gravity loads but also wind, eq and even uplift. Gravity with rain and snow might be the greater load but blow on a building and you get lateral loads that might change the orientation of some members (no pun intended).
I've spoken to many an structural engineer and all have said it doesn't matter provided the top cord and bottom cord are equal in size and shape and the point of connection can bear the point load. Also conveyed is sprinkler piping, especially branch lines, is so light weight relative to the loads they are design to it doesn't make a difference. Also, somewhat dependent is the size of the pipe. Start hanging 8" or a few 6" close to each other and you have to ask. Ultimately, don't take my words or the forum's words but ask the SE EOR. There are way too many variables to make a blanket statement especially as there probably are very few 'standard' designs of anything. Chris Cahill, PE* Associate Fire Protection Engineer Burns & McDonnell Phone: 952.656.3652 Fax: 952.229.2923 [email protected] www.burnsmcd.com *Registered in: MN Proud to be #14 on FORTUNE's 2014 List of 100 Best Companies to Work For -----Original Message----- From: Sprinklerforum [mailto:[email protected]] On Behalf Of Duane Johnson Sent: Thursday, January 22, 2015 7:48 AM To: [email protected] Subject: RE: Hanging off botom of steel bar joists 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 _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
