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RE: Old Metal Building - Tapered Moment Frame

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Not sure as to the brand (might be able to tell from the outside ridge caps if metal walls were used) but here is some info.  If no ridge cap you might look for some specific detail of the design.  If the splices have "hammerhead" connections (Butler... i.e. extended endplate with a stiffener in line with the web that is straight for a section and then tapers to the frame), knee connection with or without diagonal stiffeners (horizontal, vertical, sloped same as roof beam, or diagonal), how the eave strut sits on the frame (i.e. roof beam extended, seat under eave strut), whether cable or rod bracing is used, girt connections and orientation, etc. should also help determine the manufacturer.
 
Typically the roof beams would be designed as continuous over the interior columns with a rigid connections to the sidewall columns and pinned connections to the interior columns. 
 
From what I can tell sometime in the mid-to-late 70's the typical steel was 50 ksi.  prior to that was most likely 42 or 36 ksi depending upon supplier.
 
Depending on purlins and RTU weight, you will most likely need support beams that run between frames to support the units.  Most likely a fairly small unit but if you have a large unit > 15 ft dimension in a horizontal direction then don't forget to add snow drift loads.
 
Yes, typically these buildings are designed "gnat's ass" unless the design specifics are vague such as RTUs, cranes, etc. then some extra is designed in.  For best results locate the RTU near columns and that should help minimize any frame reinforcing that is required.

Thanks,
Greg Effland, P.E.
Project Engineer, Product Development
Butler Manufacturing Co. Research & Development
13500  Botts Road
Grandview, MO 64030
Phone: (816) 968-5714
Fax:     (816) 968-5750

-----Original Message-----
From: Alden Manipula [mailto:amanipula(--nospam--at)novagroupinc.net]
Sent: Wednesday, March 28, 2001 9:53 AM
To: SEAINT Listserve
Subject: Old Metal Building - Tapered Moment Frame

Hello everyone, 
 
I'm looking at an old metal building (~20 years old, this is a guess) to which new rooftop units need to be placed upon.  The building system consists of tapared beams ranging from 24" deep to 31" deep w/ 6" wide flanges (tf = .3" and tw=.25").  4 bolt stiffened moment connections at midspan (max span = 46'-0") and over the columns and full welds connecting the tapered sections at about 8'-10' from the columns.  Z-sections at 4' o.c.
 
Can I analyze the beam lines as a continous beam w/ supports at the column?  Which would mean the largest moments should be at midspan and at the columns.  This makes sense to me and confirms the 4 bolt stiffened connections at those locations. 
 
My main question is did they use 33/46/50 ksi steel for these things?  I tried looking for a shop stamp on the beams, no luck.
 
These metal buildings sure are designed to the gnat's ass, aren't they?  I'm guessing that these beams will have to be reinforced w/ plates.
 
Alden Manipula
Structural E.I.T.
www.novagroupinc.net