Page 217 - Bridge and Highway Structure Rehabilitation and Repair

192 SECTION 2 STRENGTHENING AND REPAIR WORK

2. The uniform service levels and bridge reliability resulting from using LRFD should ensure

superior serviceability and long-term maintainability. These specifications were calibrated
using structural reliability techniques that employ the probability theory. New specifi cations
are probability-based and incorporate serviceability design while accounting for “extreme
events” such as ship collisions and earthquakes. They can help produce more reliable struc-
tures.
3. Significant differences between the standard and LRFD specifications include the live load

model, the dynamic load (impact) factor, live load distribution factors (DF), and the load

combinations. The standard specifications use the greater of the HS 20-44 truck or the design

lane loading for live load. The LRFD specifications use the HL-93 model truck.
4. The standard specifications provide a simple expression for DF for girder bridges in S/D

format, where, for example, D 3 5.5 for a bridge constructed with a concrete deck on pre-
stressed concrete girders carrying two or more lanes of traffic, and S is the girder spacing

in feet. The effects of various parameters such as skew, continuity, and deck stiffness are
ignored. There was a need to develop DF formulas that apply to a broad range of beam and

slab bridges. The live-load DF formulas in the LRFD specifications consider the effects of
different parameters, including skew, deck stiffness, and span length.
5. The fatigue and fracture limit state, which applies primarily to steel bridges, is not applicable
to the design of wood components under current design practices. Provisions of the LRFD

specifications that apply are:
• Use of load and resistance factors.
• Inclusion of a dynamic load allowance for static truck loads.
• New live load defl ection criteria.
• Revised load combinations.
• Live load distribution requirements.
• New values for material strength (base resistance).
• The extreme event limit state, which is intended to ensure structural survival in major
earthquakes, floods, and vehicle collisions, will generally not control the design of most

wooden bridges. For wood bridge design, the most applicable of these limit states are the
strength limit state, which is intended to ensure that the structure will provide the required
strength and stability over the design life, and the service limit state, which restricts stress
and deformation under regular service conditions.
6. Concrete bridges: The majority of U.S. bridges in the past were designed using AASHTO
standard specifications. Several differences exist between the standard and LRFD specifi ca-

tions with respect to ultimate strength design in prestressed concrete beams:

• The LRFD load factors for the ultimate flexural strength design load combinations, strength
I, are less than those provided by the standard specifi cations.
• Overall, the LRFD shear design provisions were found to yield conservative results as
compared with the standard specs. LRFD shear design provisions overestimate the shear
strength of girders with span-to-depth ratios of 1.5 and below and underestimate the shear
strength of girders with span-to-depth ratios of 2.0 and above.
• The design of an AASHTO Type III girder bridge design was similar in most respects for

both the specifi cations. The most significant changes observed were in the shear design,
where the LRFD skew factor and reinforcement requirements increased the required
concrete strength and reinforcement.
• In a study presented at the 2006 Concrete Bridge Conference, Adil, Hueste, and Keating
investigated that the LRFD designs tend to have a slight reduction in the maximum span