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RE: Reinforcing existing steel members

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

The paper is correct. Elastic superposition of stresses is generally not
necessary because of the redistribution (inelasticity) that will occur well
before failure.

This is true whether you use LRFD or ASD. The column curves for both methods
are based upon the same data (as they should be -- the column does not know
which method was used to design it -- it will behave as it will behave).

This same thing holds for beams and other reinforced members. One caveat:
deflections and deformations are accumulative. So the existing member
deflection/deformation is the the starting point for the additional
deflection/deformation of the reinforced cross section due to the additional
loads.

Charlie










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