Page 271 - Bridge and Highway Structure Rehabilitation and Repair

246 SECTION 2 STRENGTHENING AND REPAIR WORK

3. The measurements of end rotations are by mechanical tiltmeters.
4. Optical methods use surveying tools or laser methods.
5. Electrical methods use linear variable differential transducers that transform displacements
to voltage.
6. Dynamic characteristics of the bridge, longitudinal and torsional mode shapes, frequency
of vibrations and damping effects can be effectively studied.
7. For bridges located in severe earthquake zones, earthquake response can be studied by
measuring bridge frequency, vibration and damping.
8. Vibration tests are usually performed by portable sinusoidal shakers, impulsive devices such

as hammers, sudden release of applied deflections and sudden braking of vehicles. For modal
frequency, mode shapes and damping ratios, accelerometers are used.
9. Weigh in motion testing (WIM) is commonly used to survey the truck volume and weight
spectra. By using axle sensors, WIM tests provide data on vehicle arrivals, speed and axle
loads.

6.3.11 Increasing Remaining Fatigue Life
Fatigue life shall be evaluated for stress reversal (as per flow diagram developed by the

author). Governing truck weight, effective stress range, traffic count for correct value of ADTT

and number of cycles per truck passage shall be considered for an accurate fatigue analysis.
If remaining fatigue life of a steel bridge is not acceptable, the following steps are appli-
cable:
1. Load restriction
2. Identifying fatigue prone details, retrofitting critical details to change AASHTO detail cat-

egory.
3. Accepting greater risk due to redundancy and increased inspection.

6.4 SIMPLIFIED FORMULA

6.4.1 Use of Infl uence Lines
1. Reciprocal theorem and Muller-Breslau Method for elastic deflections and forces are ap-

plicable. Effect of all unit moving loads at specific sections of beams is computed. At that
section the envelope of all axle loads is generated. Location and magnitude of absolute
maximum positive bending moment and shear force or reactions is determined.
2. This method is particularly useful for redundant or continuous beams with equal spans,
where absolute negative bending moment and shear maximum values for trucks with 3 or
more axles are required. The raw values need to be multiplied by load factors, distribution
factors, multiple lane reduction factor and impact factor etc to obtain design moment and
shear force.
3. However, it gets cumbersome when one or more spans are unequal. With the advent of fast
computers and stiffness matrix software it is much quicker to resort to computer solution.
4. Majority of bridges in practice is of single span for which the author has developed approxi-
mate simplified formulae for maximum bending moment and shear forces listed below for

HS, H and S trucks. The formula can be used for preliminary design. For HS-20 formula
see Chapter 4.