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RE: 1.7 allowable increase gone? 2003 IBC
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: 1.7 allowable increase gone? 2003 IBC
- From: "Carter, Charlie" <carter(--nospam--at)aisc.org>
- Date: Sun, 14 Sep 2003 12:13:31 -0500
16126.96.36.199 indicates that an allowable stress increase of 1.7
>be used with
the amplified seismic forces, which is not a change for
only applies when you are using the simplified analysis
>which is only
allowed to be used for relatively small structures ...
the equivalent lateral force procedure, you are directed to
in turn only permits the use of a 1.2 allowable increase for
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
>A conflict is
that the seismic provisions for steel structures (AISC 341-02)
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
>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
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.