AASHTO-LRFD Camber and Deflection Approach I

AASHTO-LRFD Camber and Deflection Approach I for superstructure tutorial’s content is ;

  • camber and deflection,
  • calculations using approach I,
  • 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 I ;

Secondly, usually bridge owner decides on computation method including long-term effects. In approach I, camber deflection formulas will be derived using basic moment-area theorem. Total instantaneous deflection due to prestressing is summation of fully bonded and debonded strands. In general, moment area method is used to compute deflections under each effect that can be superimposed in a later stage. Basic steps of  moment area method, applied  on a conjugate beam are determining;

  • curvature of the beam by dividing moment diagram by EI,
  • reactions for M/ EI shape loading and,
  • deflection at point of interest by computing moment equilibrium using the M/ EI shape loading.
For Fully Bonded Straight Strands ;

Additionally, taking moment of this M/EI diagram of the fully bonded strands about mid-span obtains the deviation from mid-span to support.

For Debonded Straight Strands ;

Similarly, taking moment of this M/EI diagram of the debonded strands about mid-span obtains the deviation from mid-span to support.

Downward Deflections ;

Moreover, the downward deflection due to girder self-weight is computed based on the clear span length of the member. The same equation has been used to compute the downward deflection due to slab weight. Designers need to compute the downward deflection for superimposed dead load on the composite section properties of the girder.

Long-Term Deflections;

Lastly, the long-term deflections can be determined by simply multiplying the instantaneous deflections by a certain factors that are given in table.

Reference;
AASHTO LRFD Bridge Construction Specifications, 4th Edition