Page 182 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 4 AN ANALYTICAL APPROACH TO FRACTURE AND FAILURE 157

B

t
D w d Max. stress

b/ 2 b/ 2

Figure 4.12 Shear stress distribution across an I-beam section.

Percent increase in this case, for shear deflection. Currently shear deflection is being

neglected by AASHTO for shallow beams.

2. Shear deflection at the cantilever end of hammerhead piers: Shear deflection of deep beams
is much higher than that of overhangs. The greater the depth, the greater the defl ection.
3. Predicting vibrations on lighter spans: Light, slender span designs are more popular and
easier to build, thanks to advances in material technology and construction techniques. As
modern-day designs of pedestrian bridges have become lighter in form, migrating away
from boxy, steel truss bridges to arch or cable-stayed spans with thinner decks, vibrations
have become more of a concern.
Typically, structural members are designed to vibrate at a frequency that would not interfere
with those caused by a person running or walking across the bridge. Studies have shown
that while pedestrians expect a bridge that looks lighter to exhibit more vertical movement,
lateral movements tend to be uncomfortable for the user.
V  B  BD 3 − bd 3 12M.y
2
2
In Figure 4.12 3  d + ( D − d 2 ) ,I3 , % 3
xy b 3 3
8I  t w    2 BD − bd

4. Engineers need to compute lateral forces from sway: Computational analyses and insight
from studies of pedestrian bridges with live loads exist. Member sizes used allow designers
to predict the dynamic behavior of a footbridge to a certain extent.
Damping effects: There are still many factors that make these predictions diffi cult. Damping
depends on a number of parameters such as:

1. Materials used.
2. Complexity of the structure.

w 3
slab

overhang
beam

Figure 4.13 Slab and beams load diagram.