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Re: GL beam strengthening

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Joe,
 
The 16 1/2 inch beam takes a larger portion of the load, based on a percentage of EI for both beams, or a proportion of h^3 if b and E are equal.  You need to transfer that portion of shear that the 16 1/2" takes back into the 12" GLB with bolts or hangers.  You need the bolts to take all of the shear very near the ends or transfer it continuously back into the 12" beam.  How about placing the 2nd beam underneath the first beam with a saddle hanger over the top beam, much like a beam hinge hanger?  Use a saddle over the top beam with a field welded or bolted steel connection to the bearing plate under the larger bottom beam.  You might have to remove one sheet of plywood above each end to insert the saddle.  The combined beam would be 28 1/2 inches deep.
 
If the new saddle hanger fix is hung close enough to the end, the shear problem in the smaller beam may go away because the distance from the new hanger to the bearing point may be less than the d distance of the smaller beam.  Don't dap the top beam for the saddle or you'll reduce the thickness.
 
You can't add the second beam and expect it to take it's share of the dead load unless you shore the beam that is in place, which is taking dead loads already.  You have to take the loads off of the first beam before adding the second beam by shoring it up straight or to the original camber.
 
If the first beam is 12" and second beam has to be 16 1/2", what was the size of the beam that was intended for this location?  The combined I is 3499 in^4 and S is 468.3 in^3.  Was the originally designed beam a 6 3/4 x 19 1/2 or 6 3/4 x 21 ?
 
Steel channels bolted on both sides of the 12" beam could also work.
 
I hope that helps.
Dave Gaines
Gaines Engineering
Pasadena, CA
----- Original Message -----
From: Joe Grill
Sent: Friday, June 02, 2006 2:50 PM
Subject: GL beam strengthening

A client has installed a glue-lam beam of the incorrect size (too small).  The beam installed is a 6 ¾? x 12? GL.  It is early enough in the project to do a fix, but they don?t want to remove and replace.  I have sized an additional GL to be installed next to and bolted to the existing bm.  The new beam is 16 ½? deep.  The original beam is bearing on top of a masonry wall, therefore the new beam can?t bear on top of the wall due to the deeper section.  The original beam can take all the shear by itself, but just barely.  The new beam takes about 60% of the load. Bolting along the beam is designed to transfer that 60% across the joint.  If I cut the new beam off at the face of the wall can I provide bolting at the end of the new beam to transfer its 60% back into the 12? deep beam?  Or, am I missing something?

 

This would be the easiest installation, then I don?t have to design any hangers at the wall.

 

Thanks,

Joe

 

Joseph R. Grill, P.E. (Structural)

Shephard - Wesnitzer, Inc.

Civil Engineering and Surveying