Page 174 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 4 AN ANALYTICAL APPROACH TO FRACTURE AND FAILURE 149

2. Continued cost benefits due to savings in reinforcement.
3. Structural slab thickness can be less.
4. Improved detailing procedure will result.
Description of full composite action:
1. The curvature of the slab in transverse and longitudinal directions is governed by relative
stiffness at the boundary of the slab and girder. The greater the eccentricity of the slab and
beam, the greater the arching or dome effect.
The load path for gravity dead and live loads is from slab to beam. The slab defl ects in

the transverse direction, and the beam deflects in the longitudinal direction. The resulting
deflected shape of the slab is similar to a shallow arch, with the highest point on the beam

centroid and lowest point on the slab centroid.
2. Arching action or dome boundary effects would occur due to full composite action between
slab and supporting beam. The phenomenon has been referred to as arching action, mem-
brane action, or shear lag effects in slabs (Figure 4.6). It is due to structural mechanics but
is not related to the material composite behavior that may result from the use of dissimilar
or composite materials of the slab and beam.
Boundary effects: Due to the defl ected shape of an arch, compressive forces additional to
primary bending are introduced at slab boundaries as planar forces (Figures 4.7). For a com-
prehensive analytical approach, the following effects at the boundaries of the slab need to be
considered:

1. Dual deflections of the slab and supporting beam at a right angle.
2. Reverse curvature of the slab near the boundaries, with sagging at midspan and hogging at
supports.
3. At midspan, the top part of the beam above neutral axis is in a longitudinal compressive zone.
Slab longitudinal edge, which is connected to the top of the beam, will be fully immersed
in the much deeper compressive zone of the beam. Due to compatibility, the longitudinal
force will cause a compressive planar force in the slab.
4. Slab curvature at the support (which is generally neglected) will depend upon the eccentric-
ity of the beam and slab centroids. If centroids of the beam and slab coincide (for example
in a through girder type connection) arching action is negligible.

5. In hybrid steel girders, flange steel and web steel strengths would alter the shape of the beam

Transition

Figure 4.6 Progressive distribution of compressive stress across slab thickness due to arching action.