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Re: Reinforcing existing steel members

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It doesn't matter whether or not you design by ASD or LRFD.  The column is
too stupid to know the difference between ASD and LRFD,  and will reach the
critical buckling stress at some load, depending on its slenderness ratio.
If the critical buckling stress is in the slenderness ratio range that
elastic (Euler) buckling governs, none of the cross-section will reach
yield.  If the critical buckling stress is in the slenderness ratio range
that inelastic buckling governs, some of the cross-section will reach
yield, due to initial residual stresses, not due to external loads.

The column critical buckling stress will be a function of the slenderness
parameter (KL/r) of the modified column.  It is not a function of the load
history.

A similar thought process would follow for all buckling modes where
critical stresses are a function of slenderness, not yield stress; lateral
torsional buckling (L-sub-b / r-sub-y), flange local buckling (b / t), web
local buckling (h / t-sub-w), and shear (h / t-sub-w).

Rick Drake, SE
Fluor, Aliso Viejo, CA

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                      Roger Turk                                                                                                       
                      <73527.1356@compuse      To: seaint(--nospam--at)seaint.org                                                                   
                      rve.com>                                                                                                         
                      01/28/03 09:09 AM        cc:                                                                                     
                      Please respond to                                                                                                
                      seaint                   Subject:  Reinforcing existing steel members                                            
                                                                                                                                       
                                                                                                                                 ..... 
                                                                                                                                       




Keith,

This depends on whether you are designing in ASD or LRFD.  In ASD, the
stresses in the existing member and in the added reinforcing are going to
be
different, which has to be accounted for.

In LRFD, all of the steel is yielding (unless controlled by instability),
therefore the strength of the member reinforced with no load is the same as

the strength of the member reinforced with partial loading.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Keith Hyndman wrote:

. > Reference: AISC Journal 4th qtr 1990
. > "Reinforcing Steel members and the Effects of Welding"
. > by R.H.R Tide

. > This article and several written prior to it, states that the "strength
of
. > columns reinforced under load and under no load are identical."  This
. > brought me to question some of the practices on my own office where we
. > have sized reinforcement of steel members by subtracting out the
existing
. > (actual) stresses, resulting in an overly conservative design.

. > So I thought I'd write to the audience to see if this philosophy is
widely
. > held.

. > Also is this approach true for tension and bending members?

. > Ultimately I'd like to convince my "crusty cohort" to change his ways,
to
. > a more efficient approach.

. > Thanks,

. > Keith Hyndman, P.E.

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