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RE: Steel beam--change of depth

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Mike:
 
The stress in the flange has to flow from the one flange at the 14" depth to the one flange at the 27" depth. I am not aware of a specified procedure for the design other than the overlap length must be sufficiently long to:
Another approach could be to connect the two flanges at approx. 30 degrees with full pen welds and provide vertical stiffeners at each end where they connect with the horizontal flanges.
 
Jim K.
 
-----Original Message-----
From: Michael Hemstad [mailto:mhemstad(--nospam--at)mbjeng.com]
Sent: Wednesday, September 06, 2006 12:35 PM
To: seaint(--nospam--at)seaint.org
Subject: Steel beam--change of depth

All,
I have a project with existing steel roof girders (W27).  Due to vertical clearance requirements, we are placing a column at midspan, cutting off the bottom half of the girders, welding a new bottom flange plate on, and turning the W27's into heavy W14's.  As originally intended, this was to be full length of the girders; but the greater clearance is not required everywhere, and now the contractor is asking us to create details needed to scarf the bottom flange only in certain areas.  Thus, we will need to put a new bottom flange on in these areas which will overlap the existing bottom flange.  My question is, what should be this overlap distance?
 
I sized the new bottom flange to match the existing top flange; the stresses are relatively low (about 16 ksi of service-level bending stress).  Deflection is acceptable; shear stresses are low.  Can anyone direct me to a reference discussing the required overlap?  With 12 inches clear between the new and existing bottom flanges, I currently have settled on 24 inches of overlap as providing a gradual enough transition that I hope to avoid a vertical stiffener.  Does anyone know of an accepted technique to actually calculate this length?
 
Thanks,
Mike Hemstad
MBJ
Minneapolis, Minnesota