3403.2.3.2 has two limits; 5% reduced strength and 10% increase of
seismic force.
A modified element is no longer an existing condition. Include in the
modification reinforcement to prevent a more than 5% reduction in
strength and to prevent more than a 10% increase of force to it and
elsewhere.
If another element does increase more than 10%, then make it part of the
new by reinforcing it also. This becomes more of a concern if the
stiffness of the first element is reduced by about 20%.
I think what might be more likely to kick in a full upgrade is when one
adds an average seismic weight to a whole floor or building. Or a change
in use.
David Merrick, SE
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