AASHTO-LRFD Camber and Deflection Approach II
AASHTO-LRFD Camber and Deflection Approach II for superstructure tutorial’s content is;
- camber and deflection,
- calculations using approach II,
- for fully bonded straight strands,
- for debonded straight strands,
- downward deflections and,
- long-term deflection.
Camber and Deflection ;
Firstly, designers need to quantified camber and deflection of superstructure at different stages of construction. Because maintaining a certain passenger comfort and to providing information for construction detailing are important for design. Furthermore, contractors need to use these values during erection for proper construction. Also, designers also need to engineer the bridge to have a smooth ride without much up and downs between adjacent spans in long term. The permanent undesired deflections can be minimized either by use of haunches and different slab thickness along the girder. In case of camber up, the haunch or slab will reach to its minimum depth at mid-span and maximum depth at end zone.
Calculations Using Approach II ;
Secondly, usually bridge owner decides on computation method including long-term effects. Designer could use the approach introduced in PCI paper (Tadros et al 2011) “JL-11-Winter Precast Prestressed Girder Camber Variability. This approach includes the camber deflection calculations.
Long-Term Deflections ;
Furthermore, long-term deflections can be determined by magnifying instantaneous deflections with a specified factor. Tadros et al (2011) suggests to use the following approximate multipliers. Creep effects developing due to sustained loads will be considered thru creep coefficient, 𝚿. Also, sustained loads represent all dead loads and initial applied prestressing forces. Long-term prestress losses will gradually develop with time reducing sustained loads. Creep coefficient can be reduced by the aging coefficient of 0.7. Than the coefficient will be equal to 0.7𝚿. Moreover, losses until the deck casting due to creep, shrinkage and relaxation of steel are included in computations. Long term losses will be expressed in terms of deflection, taking the instantaneous deflection at prestressing as a reference state.
Downward Deflections ;
Finally, designers could compute the downward deflection due to girder self weight on the clear span length of the member. The same equation has been used to compute the downward deflection due to slab weight. Furthermore, the downward deflection for superimposed dead load need to be computed on the composite section properties of the girder.