Page 222 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 197

3. Maximum live load deflection of slab and beam system 3 deflection of composite beam

due to fl exure 4 shear deflection due to impact 6 deflection due to arching action.

Maximum live load deflection would occur under the heaviest axle of a permit load.
4. The National Cooperative Highway Research Program (NCHRP) published a comprehen-

sive report, “Improved Live Load Deflection Criteria for Steel Bridges.” An important task
under this study was a review of existing literature and the state of practice for steel bridge

deflection control. It included:
• Overview and historical perspective of the subject.

• Review of effect of bridge deflections on structural performance.

• Review of effect of bridge deflection on superstructure bridge vibration (human response,
field studies, analytical studies).

• Review of alternate live load deflection serviceability criteria.

• Survey of state bridge engineers to determine how AASHTO deflection limits are actually
applied in bridge design.
5. Sause and Fisher, in “Application of High Performance Steel in Bridges” (Figure 5.3) have
shown that an increase of yield strength between 50 ksi to 70 ksi decreases the composite
girder weight by 14 to 19 percent. However, the AASHTO deflection limit of L/800 can

be reached earlier for the lighter HPS beam. Hence, there is a need to revisit the defl ection
limit for HPS 70W and higher yield strength steel girders.
Deflection limitation for 100W steel: Some states have initiated the use of 100W fl ange

plates resulting in shallower girders. In addition to live load deflections, vibration studies
need to be carried out since it is not always possible to reduce girder spacing due to the need
for multiple utility pipes to be located between the girders.
From the in-depth studies above, it is envisaged that graphs and tabulated results of span
versus live load deflections can be plotted for a variety of girder depths, high performance

materials, and skew boundary conditions. These easy and ready to use results will enable the

AASHTO

Figure 5.3 Defl ection versus yield strength for the Lehigh Street Bridge in PA (Homma and Sauce 1995).