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RE: Light gage steel web stiffener

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> From: "Rich Lewis" <seaint03(--nospam--at)lewisengineering.com>

> supposed to be inside the flange.  One detail shows one screw with some
> arrows for others, but doesn't specify how many screws are required where
> the arrow points.  I talked to another tech rep. and he said the design
> engineer decides how many screws are needed, based on what, he couldn't tell

Yes, this is a missing bit in the standard.

If it is truly just a stiffener at a bearing point and not a bearing
element, fastening is a very minimal requirement.

AISI Clause C3.4 (see the commentary as well - nice graphics of failure
modes)
Web crippling occurs for many factors, including; rotation restraint of
the bottom flange; eccentricity due to the radius of transition from
flange to web.

So, most of the crippling is a result of effects near the bearing
surface. If you install a stiffener/fastener close to the bearing
flange, you effectively reduce h/t to something that approaches the
distance from flange to fastener. If you put the stiffener on the side
of the inside of the radius it is more effective (bend <= 90 deg,
approx.) but may not be the easiest detail.

Think about typical bracing -  stiffness is inversely proportional to
force development. So, a stiff stiffener (almost anything is stiffer
than a flat CF web) sees virtually no out of plane force. So the
fastener can be minimal. 

There will be some shear transfer but it will not control (web strains,
fasteners shear/yield a tiny bit, web reaches new equilibrium, life goes
on).

Don't forget the reaction fastener at the opposite end of the stiffener.
You will probably be fine with a SD fastener at 3" spacing or bolts at
ends only. Seat of the pants, still.

If you really want to be picky, assume new h from top flange to bottom
fastener but ignore the Cr reduction. This would check for crippling
above the first fastener.

Alternately, use a bearing "stiffener" and lots of fastners to take the
load above web crippling.

If there's a budget for it ... test. (Bottom line for most CF conditions
not covered by the spec.) 
  
-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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