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Tapered Steel Beam

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Bill S. Marczewski wrote:

"A local steel fabricator has asked me to approve a
beam spanning 56'-0"
(w21x147) that was sized by others and needs to have a
pie-shaped section of the web removed.  The web will
be cut down in height to as little as 12" remaining at
the end, with a diagonal slope extending for 74" along
the longitudinal axis of the beam.  The fabricator
would like to cut this pie-shaped section out of the
web, fold the top flange down along the new diagonal
web and weld it in this location." 

Check the shear force, obviously, but it may well not
govern.  With the top flange higher out in the span
than at the ends, bracing becomes even more critical
than usual.

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?


Mike Hemstad, P.E.
St. Paul, Minnesota 



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