Interior beam torsion 1

[haynewp]

1. I have a situation with a 60 year old one-way slab and beam concrete building where the floor load in an area is being increased. It is causing torsion in an interior beam as the live load significantly changes on one side of it to the other. The beam has open stirrups and ACI requires closed stirrups to meet a certain level of torsion when it is compatibility based torsion. FRP would require fully enclosed wraps to resist torsion or partial wraps with anchors through the top portion of the beam connecting the FRP at each side, which I’m not real sure about. Are there any other options for torsion strengthening you have seen before?

2. Also, does anyone else skip the live load to see the effects of torsion on interior beams as a general practice?

 

[JAE]

Usually use skip loading for continuous concrete beam/slab designs but not for the purpose of checking torsion - or even dealing with torsion on interior beams.
The code states the requirement to "skip" load the live load but doesn't suggest that 100% of the LL be alternated. You could make the case that 50% to 75% should be alternated.

As far as strengthening an interior beam for torsion, I think it would be very difficult.
A couple of ideas that come to mind:
1. Is your f'c based on the stated concrete strength on the original plans or on the actual in-place current strength? i.e. core samples to justify a higher f'c?
2. Your analysis of the skip loading torsion may be based on very idealized one-way span concepts where the reality may be more complex - a two-way flow of loading that results in smaller torsion values.
3. Provide other supplemental structural elements to minimize slab deflections and thus reduce the resulting torsion.
4. Strengthen the beam by partial concrete removal and replacement per the images below....not sure how to account for the torsional longitudinal steel at the ends.

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