Page 296 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 6 APPLICATIONS OF BRIDGE DESIGN AND RATING METHODS 271

6.8.6 Deck Replacement of Slab Composite with Repeated Beams
Empirical design: Alternate method to traditional method (9.7.2.4)
Flexure in longitudinal direction: Design as a series of longitudinal strips. Idealize width of
strip 3 Width of wheel 4 2 8 Effective depth of slab
Max. BM due to dead load 3 w (L /8)
2
d
2
Max. BM due to lane load 3 w (L /8)
L
Max. BM due to truck load is calculated from influence lines. For deck slabs, Puncher’s

influence line diagrams were used.

AASHTO analytical method: Analyze as a strip of unit width continuous over beam
fl anges.
Many states in the U.S. have ready-made simplified structural solutions for deck thickness

and rebars based on AASHTO empirical methods. These are frequently used in practice and
were developed by examination of repeated drawing details and calculations performed for a
large number of spans. Some of the approximations in current AASHTO or state code design
methods are:
1. Boundary effects of skew and curved decks not considered.
2. Arching action at supports (9.7.2.1) arising from reverse bending curvature: Planar or
membrane forces will be generated in addition to bending. Three dimensional modeling
and analysis will be required.
3. Conventional methods do not consider additional thickness for transverse deck drainage,
camber thickness, thickness of concrete haunches on top of flanges, or any groove forma-

tions. In addition,
• Added stiffness due to stay-in-place folded steel or aluminum formwork
• Secondary stresses such as resulting from creep and shrinkage stresses contributing to
cracking
• Daily thermal stress variation during summer and winter months
• Composite behavior of wearing surface thickness, using special concrete (such as latex
modified or corrosion inhibitor aggregates for forming defense against tire friction and

braking forces).
4. Effects of shear deflection are neglected.

5. Applications of fracture mechanics formulae for deck cracking.
6. Approach slab analysis: Approach slab behind integral abutments is itemized as a structural
member during construction. For analysis, it needs to be idealized as slab on grade and
acts as a plate on elastic foundations. Geotechnical properties of subgrade material will be
required.
More accurate analytical method: Deck slab is idealized using FEM. Ultimate load behav-
ior of RC elements needs nonlinear analysis. For concrete, stress-strain curve is nonlinear
during cracking stage. The tangential stiffness method is used.

6.8.7 LRFD Design Methods for Deck Slab Design
Use Mathcad or spreadsheets to program equations, load, and resistance factors to comply
with AASHTO LRFD code (refer to Chapter 7).
The following steps are provided for ready reference. Prior to developing a software or for
solving the equations using hand calculations, equations need to be checked against the latest
version of applicable AASHTO LRFD Specifications or LRFR Manual.