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RE: Reinforcing existing steel members

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>For the bending member case, take as an example
>a simple span bridge girder where the top flange
>is not laterally braced by the deck system, but
>only at discrete points by the transverse diaphragms.
>The geometry results in stability of the compression
>flange being  the controlling limit state in this
>case.  Say the section modulus is 500 in3, the DL
>moment is 500 ft-k, and the DL stress at the top
>flange is 12 ksi.  To add capacity to the bridge,
>the tension flange is reinforced, while the full
>dead load stress is maintained on the original
>section.  The new allowable stress or section
>strength is determined by evaluation of the composite
>section.  This is not a function of the load history.
>Suppose the new allowable stress based upon stability
>of the top flange in the now-reinforced section is 17
>ksi, and the new section modulus to the top flange is
>600 in3.  But what is the remaining usable capacity of
>the now composite section - can this be determined apart
>from the knowledge of the state of stress (or demand) at
>the time the reinforcement was added?   Or is it
>(17-12)*600/12 = 250 ft-k, for this simplistic case?

You can literally look at the reinforced cross-section as though it had
never been loaded before for the purposes of determining strength. As a
simple common example, consider a composite beam. The wet concrete loads the
"unreinforced" beam and then becomes reinforcement after it cures. Good
thing too, because if you did have to know the state of stress, the engineer
regularly would have to become involved in means and methods of construction
to ensure adequate strength in each and every composite beam ever used.

For deflection, though, it's a cumulative process.


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