In the PGSplice Time Step Details Report the table that reports the incremental forces due to external and restrained component forces, it reports both composite and individual component results. At the abutments for the two span bridge I am analyzing, PGSplice is reporting force effects for CR, SH, and RE in both the girder and the deck but the forces in the composite section are being reported as zero. It appears as thought the composite output matches the restrained section forces, which makes sense that there would be not be any restraint due to continuity here but it seams like there should be addition forces due to the shrinkage of the deck itself. I'm having a difficult time reconciling the fact that the individual components are experiencing a force effect but the composite section does not. Any guidance or clarification that can be provided would be much appreciated.
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 Time Step Rpt - Interval 31 | 248.79 KB |
The results can be checked by simple statics. Sum of internal forces at a section is equal to sum of external forces at a section. Since this is a free end section, the sum of external forces is zero. Therefore, the sum of internal forces must be zero.
If you add up all the axial force and moment in the individual components of the cross section, they sum to zero. This summing of individual component forces is reported as the resultant force on the composite section.
Consider a rectangular concrete prism with a square cross section and a single piece of rebar at the center. The member is unrestrained. The concrete shrinks. This causes compression on the rebar. For force equilibrium, tension must develop in the concrete. The internal tension and compression are equal and opposite, sum to zero, and are in equilibrium with the external forces (which are zero).
The same thinking is applied here. There is no external force on the section because it is unrestrained. All of the time-dependent effects of the individual components of the cross section must sum to zero.