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RE: Tapered Steel Beam

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Mike:

 

Thanks for your input.  I don’t believe this beam will be subject to fatigue as it clear spans in the roof of an aquatic center.  I also asked why the original design engineer doesn’t approve this modification.  Here’s the response I was given.  The steel fabricator is under contract from a pre-engineered wood truss company who thought they could span the 56’ with a glue-lam beam.  Guess it didn’t work and the truss company hired a local steel/welding company who then contacted me.  Since my first posting, I have now been asked to verify the W21x147.  I live in a relatively rural county that has some odd construction practices, and sometimes frightening design solutions are presented by a variety of local consultants (PE’s and Non-PE’s).  I’m the first structural engineer that has settled in this community, so I expect to have some headaches associated with construction means, methods, procedures and techniques. 

 

 

 

Bill S. Marczewski, P.E.

BSM Structural, LLC

Astoria, Oregon 97103

 

 

 

 

Many 2-span bridges have been built using a similar

technique to make a deeper section over the pier--the fabricator slits the web near the bottom flange, pulls the flange down at the centerline of the pier, welds in a tapered plate to fill in the web.  These beams are visually attractive and work fine at first, but have proven over time to be subject to fatigue at the ends of the slit (typically about the fifth point of the span either side of the pier).  I don't know if the fatigue cracks initiate in the web at the end of the slit (large residual stresses due to the welding) or in the flange (residual stresses due to bending); I would guess the former.  Now, the quarter or fifth point near the center support of a two span beam has a much higher live load stress range than 74 inches from the end of your (presumably simple span) 56 foot beam.  Still, if your beam is subject to fatigue, I'd stay away from it.  If not, check flexure at mid-taper and shear at the end, but it's probably OK.

 

Out of curiosity, why doesn't the original design

engineer approve it?

 

HTH

 

Mike Hemstad, P.E.

TKDA

St. Paul, Minnesota