] [Thread] [Date] [Author]

["Chad Druffel" <CDruffel(--nospam--at)AHBL.com>]

This is a follow up from a topic I brought up a few weeks ago:


RESPONSES TO CHARLIE:

A)  
I appreciate your detailed description of the 1.7 factor, this helps to
clarify the intent of the code provisions.  I DO think that this should
be described within the commentary to AISC 341-02, so that everyone can
understand the intent (while on the subject, wouldn't it be nice to have
a commentary for the IBC and ASCE-7 published along side each of the
sections like the ACI does?  Although these resources would probably
then triple in size).

B)
There is conflict/confusion when you compare IBC 2003, ASCE-7 and AISC
341-02 regarding the "1.7 factor" and also the use of a phi factor with
ASD in the amplified seismic equations.  IBC section 1617.1.1.2, which
relates to the simplified analysis procedure only, states that "Where
ASD methodologies are used with the special load combinations of Section
1605.4, design strengths are permitted to be determined using an
allowable stress increase of 1.7 and a resistance factor phi=1.0..."

The corresponding ASCE-7 Section 9.5.2.7.1, which relates to the
equivalent lateral force procedure, states that "Where ASD methodologies
are used with the special load of this Section applied in load
combinations 3 or 5 of Section 2.4.1, allowable stresses are permitted
to be determined using an allowable stress increase of 1.2..." (no
mention of the phi factor)

AISC 341-02 refers to the 1.7 increase, and variable phi factors
depending on what you are designing.

You can see the confusion that these three references create when
compared with one another.  IBC says to use phi=1.0, AISC says it's
variable, and ASCE-7 says nothing at all.  And where in the world did
this 1.2 increase come from in ASCE-7?  Bottom line, is there a right
answer?


MY OWN GENERAL VENTING:

In general, I am disappointed with the IBC 2003 / ASCE 7-02
relationship.  IBC creates confusion by throwing you into and out of
ASCE-7.  ASCE-7 should have been more in-line with the IBC so that it
could have been fully adopted with only minor modifications.

I am also disillusioned with ASD Steel Design.  ASD is essentially being
made more and more difficult and confusing, in part due to the issues I
previously discussed.  With the inclusion of variable phi factors with
the allowable stress increase, they have basically permitted you to
continue to use stress based design, but both the load and resistance
sides of the equation are going to be factored.  This creates a new
level of difficulty when using design programs such as RISA, that do not
take into account a phi factor when performing ASD; thus you have to
increase the load side of the equation instead, and create new load
combinations, and these load combinations will vary with the phi factor
depending on what portion of the braced frame you are designing.  One
way to get around this complexity is to use the most conservative phi
factor everywhere, but what a waste.

The ASD "exceptions" that are interlaced throughout the code and other
references have created too much confusion and I find myself getting
"lost in the numbers" more and more.  So I have reluctantly decided to
make the switch to LRFD for steel design.  I feel that I might as well
make the switch now because that is the direction we will continuously
be "nudged". 

Sincerely,
Chad P. Druffel, P.E.




From: "Carter, Charlie" <carter(--nospam--at)aisc.org>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: 1.7 allowable increase gone? 2003 IBC

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>IBC Section 1617.1.1.2 indicates that an allowable stress increase of 
>1.7 can be used with the amplified seismic forces, which is not a
change for
>me...great. Except, this only applies when you are using the simplified
analysis
>procedure, which is only allowed to be used for relatively small
structures ...
>When using the equivalent lateral force procedure, you are directed to
ASCE
>7-02, which in turn only permits the use of a 1.2 allowable increase
for the
>amplified loads!  Is this correct?... This is a whopping 42% increase 
>in force.

1.7 is not really an allowable stress increase. I think the IBC and AISC
Seismic Provisions have parallel and complementary provisions, which
ensure that the ASCE/IBC load provisions and AISC steel strength
provisions are properly used together. That is, strength-level loads are
compared to design strength-level capacities, and ASD loads to
working-stress-level capacities. The AISC Seismic Provisions are based
upon strength-level loads whether you do LRFD or ASD, so the IBC always
put you to the strength-level loads when it requires you to design in
accordance with the AISC Seismic Provisions.
 
In AISC Seismic Part III (ASD), when you multiply by 1.7, you are
offsetting the effects of the factor of safety buried in the ASD
capacities -- you are multiplying the working-stress-level capacities by
the value of the least factor of safety used in ASD (5/3, which is
roughly equal to 1.7). In effect, this converts the ASD design equation
from the working stress level to the nominal strength level. Then, the
appropriate phi factor in Part III of the AISC Seismic Provisions is
used to reduce the calculated strength to the design value. The
variability of loading was already taken care of in the calculation of
the loads and the phi factor takes care of the variability in the
strength.

 
>A conflict is that the seismic provisions for steel structures
(AISC341-02) specifically
>indicate that the 1.7 increase may be used.

As explained above, I think they are coordinated. But please let me know
if after reading that you still think they conflict.

 
>What has happened to the 1/3 stress increase for steel design when used

>with non-amplified seismic or wind loads?  The 9th Edition AISC had it 
>in there, but it was then removed by Supplement #1 (December 17, 2001),

>and instead put the responsibility on the applicable building code.  
>IBC 2003 does not reference a 1.33 allowable increase for steel design 
>as far as I have seen (they in turn put the responsibility back on 
>AISC), except in the case of the amplified seismic load.

The ASCE 7 load combinations, which form the basis for IBC (and NFPA)
load combinations, specifically address how load reductions for
combinations of multiple transient loads are to be addressed. Their
treatment is more advanced than AISC's treatment of it. To remove the
conflict, and in recognition that AISC does not have the authority to
address load combinations (in LRFD or ASD) since that is the job of ASCE
7, the one-third stress increase is now deleted from the AISC
Specification.
 
The one-third stress increase has evolved almost continuously from its
early uses in material design specifications, through the days of ANSI
A58.1, which is the load document that preceded and became ASCE 7, and
now ASCE 7. My colleague Keith Mueller and I have written a summary of
how ASCE 7 (1998 and 2002) and current model building codes (including
IBC, NFPA 5000, UBC, BOCA, and SBC allow -- or do not allow -- the
one-third stress increase. This summary will be published as an
installment of the feature SteelWise in the October 2003 issue of AISC's
Modern Steel Construction magazine.
 
Charlie 





List:
 
IBC Section 1617.1.1.2 indicates that an allowable stress increase of
1.7 can be used with the amplified seismic forces, which is not a change
for me...great.  Except, this only applies when you are using the
simplified analysis procedure, which is only allowed to be used for
relatively small structures (and I'm assuming that the simplified
analysis is overly conservative anyways, if this code follows how the 97
UBC worked).
 
When using the equivalent lateral force procedure, you are directed to
ASCE 7-02, which in turn only permits the use of a 1.2 allowable
increase for the amplified loads!  Is this correct?  I thought that it
must be a typo so I searched for an errata to the 7-02, but nothing is
out there.  This is a whopping 42% increase in force.  Does anyone have
input?  Looks like I'm almost being forced to switch to strength design.
 
A conflict is that the seismic provisions for steel structures (AISC
341-02) specifically indicate that the 1.7 increase may be used.
 
This brings up an additional point.  What has happened to the 1/3 stress
increase for steel design when used with non-amplified seismic or wind
loads?  The 9th Addition AISC had it in there, but it was then removed
by Supplement #1 (December 17, 2001), and instead put the responsibility
on the applicable building code.  IBC 2003 does not reference a 1.33
allowable increase for steel design as far as I have seen (they in turn
put the responsibility back on AISC), except in the case of the
amplified seismic load.
  
Chad P. Druffel, P.E.

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