Page 219 - Bridge and Highway Structure Rehabilitation and Repair

194 SECTION 2 STRENGTHENING AND REPAIR WORK

7. Scour design (local or contraction scour): For new bridges, scour depth as calculated may
require deep foundations. For existing foundations subject to scour, adequate countermea-
sures such as sheet piling or stone riprap protection of the riverbed and foundations are
required.
8. Seismic design.

9. Modified compression field theory for shear design of concrete.
10. Use of segmental concrete bridges for longer spans.
5.2.6 Experimental Verification of Plastic Behavior

1. Experimental verification of plastic behavior is desirable since the number of parameters

affecting failure is large. Both the yield line theory for slabs and the plastic hinge theory
for beams and columns are based on experimental verifi cation.
2. For more complex structural systems, the application of the plastic theory is not fully de-
veloped, and approximate applied factors are assumed.
3. When the plastic theory is used, it is important to anticipate the failure mechanism, which
should be based on observed response of extreme force effects. The location of plastic
hinges in a theoretical model should coincide with those observed in well-instrumented

experimental laboratory models or observed in the field after seismic events.
4. If flexural failure is identified, failure should not occur due to deficient shear design, buck-

ling, or bond between concrete and steel.
5.2.7 Use of in-Built Ductility in Materials
The response of material beyond the elastic limit can be either ductile or brittle. Ductile

behavior can be defined as members displaying significant inelastic deformation and energy dis-
sipation, without any loss of load carrying capacity. Failure is not sudden, and there is warning
in terms of excessive deformations prior to collapse.
Brittle behavior implies sudden loss of load carrying capacity after elastic limit is reached.
To prevent failure, the system should have more ductility as opposed to more brittleness. Greater
ductility will lead to greater economy in design.

5.2.8 Comparisons of LRFD Design Method with ASD and LFD Methods
Using appropriate load factors, strength load combinations for typical dead and live loads
may only be expressed as:

ASD 6 1.0 DL 4 1.0 (LL 4 I) Resistance safety factor
LFD 6 1.3 DL 4 2.17 (LL 4 I) 7 R n
LRFD 6 1.25 DC 4 1.25 DC 4 1.50 DW 4 1.75 (LL 4 I) 7 R n
2
1
5.3 LRFD SERVICE LOAD REQUIREMENTS
5.3.1 Defl ection Control
1. AASHTO LRFD live load deflection criteria is based on arbitrary limits:

Minimum single span length 3 20 feet. Maximum single span length 3 300 feet.
2. For the design or rehabilitation of bridge girders, the two criteria are:
• Providing adequate strength

• Deflection control—Live load deflection should be less than:
L/800 for bridges without sidewalks
L/1000 for bridges with sidewalks