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Re: Steel HP piles and the IBC

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The slenderness ratio limitations on ASTM A36 HP shapes is a known
"unintended problem".  The 2003 IBC removes the slenderness limitation.
The 2002 AISC Seismic Provisions raises the slenderness limit to permit the
use of common HP shapes in ASTM A36 steel.

Starting with the 1999 LRFD Specification, AISC now expresses all
slenderness limits in dimensional consistent form.  The E value which used
to be "hidden" or "included" in the numerical part of the slenderness
ratios is now "pulled out".  This eliminates the need to express equations
in multiple forms.

Rick Drake, SE
Fluor, Aliso Viejo, CA
rick.drake(--nospam--at)fluor.com
949-349-5791

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                      Rick Burch                                                                                                       
                      <rburch(--nospam--at)conterra.co      To: seaint list server <seaint(--nospam--at)seaint.org>                                              
                      m>                                                                                                               
                      06/08/03 06:12 PM        cc:                                                                                     
                      Please respond to                                                                                                
                      seaint                   Subject:  Steel HP piles and the IBC                                                    
                                                                                                                                       
                                                                                                                                 ..... 
                                                                                                                                       




I just noticed some strange things in the IBC regarding steel piles.  In
1808.3.5, the width/thickness ratio of the flanges of piles in Seismic
Design Category D and above is limited.  It looks to me like there are
mistakes in equation 18-11 (and 18-12 for steel pipe piles), but I don't
see any mention of them in the IBC errata up through the sixth printing
of March 12, 2003.

1. The first apparent mistake is that the width/thickness of flanges is
limited to 52*square root of Fy, however I feel sure this should have
said 52/(square root of Fy), the same limit as in the AISC Seismic
Provisions. Then, strangely enough, the equation in SI units is
0.317*square root (E/Fy).  In other words, the SI equation is not just
converted to different units, it has a whole new variable (E) in it. Fy
is in the denominator as it should be however.

2. The same problems exist in the 18-12 equation for steel pipe piles.

3. Here is the real kicker. Lets say that the equation is really meant
to be 52/(square root of Fy) same as the AISC Seismic Provisions. For
A36, which HP piles are made of, this gives a ratio of 8.67.  If you use
b/2tf, also as the AISC does, there is not a single HP section listed in
the AISC manual that meets this criteria.

Since the IBC uses the same width/thickness ratio limits as the AISC
Seismic Provisions (that is, if you assume the IBC equations are just
wrong), you would assume the applicable ratio is b/2tf, also the same as
in the AISC.  However, the IBC uses the term "unsupported flange to
thickness ratio", so maybe you could justify using (b/2-k1)/tf, in which
case a few of the HP sections do meet the criteria.

Can anyone shed any light on this situation? Are these equations wrong,
and has the IBC essentially eliminated the use of steel HP piles for
Seismic Design Category D and above? Or is the use of (b/2-k1)/tf
reasonable?

Rick Burch
Columbia, SC

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