Page 228 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 203

Table 5.2 Human effect versus acceleration (as reported in literature).
Effect Acceleration(m/sec2)
Humans cannot perceive motions :0.05
Sensitive people can perceive motion, hanging objects may move slightly 0.05–0.10
Majority of people will perceive motion or desk work becomes diffi cult or 0.1–0.4
almost impossible
People strongly perceive motion, diffi cult to walk naturally, standing people 0.4–0.6
may lose balance; most people cannot tolerate motion and are unable to
walk naturally
People cannot walk or tolerate motion 0.6–0.7
Objects begin to fall and people may be injured 90.85

employs both 2-D and 3-D models. 2-D models are more efficient computationally and will

be used to better identify important parameters that will be further studied with the 3-D
models. Typical 2-D models include beam elements to represent both the superstructure and
substructure. The superstructure composite properties are typically used in modeling the
deck since the shear studs are headed and it is well established that there is perfect bonding
between the concrete deck and the steel girder.

5.3.10 Accelerations from Moving Loads
The acceleration that affects human reaction is due to overall superstructure response, which a
2-D model can easily simulate. It is the spatial effects (such as load distribution or relative girder
vibration) that is not represented with a 2-D model and requires application of a 3-D model.
Use of compatible eccentric beam elements will ensure proper modeling of the cross-sectional
geometry while enhancing computational efficiency. A vibration analysis will be needed, espe-

cially for longer spans of pony truss-through girders. There are many possible varying locations
of moving loads and load patterns. One such method, which considers the vertical acceleration
as the most important parameter affecting human comfort, was proposed by Wright and Walker
for steel bridges as published by AISI:

1. Determine static defl ection, , due to live load.
s
2
Ensure the acceleration, a, does not exceed 100 in/sec or 0.25g.
2. Among limitations of the above method are: The frequency equation is suitable only for
simply supported bridges, and there is a need for estimating the frequency of practical
bridges; the DI factor might not necessarily be accurate for existing system and loading;
and lack of consideration to the relationship between the bridge natural mode and dynamic

load. Relaxing L/D limits or not considering deflection and L/D limits will not necessarily
result in designs with large defl ections.

3. For flexible girders with higher L/D ratios:
• Loads are better distributed to other girders (decreased distribution factor and load per
girder).
• Reduced live load moment due to a decrease in distribution factor.
• Reduced peak negative live load moment that can aggravate deck cracking.
• Better accommodation of shrinkage strains due to higher deck-to-girder stiffness.
The latter advantage can minimize transverse deck cracking potential as recommended
by Saadeghvaziri and Hadidi.
4. Vibrations may lead to overstress. Analytical grid modeling of deck, girders, and diaphragms
for vibration shall consider: