Page 120 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 3 BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING 95

the fracture. Hence, they occur without warning andthe details are essentially non-inspect-
able. The following preemptive retrofit strategies appear to be highly desirable:

• Avoid use of high carbon brittle steel
• Avoid poorly executed welding leading to high residual stress level
• Avoid bad detailing
• Avoid dynamic loads that cause high strain rates and reversal of stress

• CAD drawings should have sufficient details. Emphasize increasing the strength of joints
by adding bolts and the strength of the girder web and flanges by adding plates, etc.

• QA/QC procedures should ensure adequate checking of criteria, method of analysis, design
details, and technical specifi cations.
3.8 CONSTRUCTION DEFICIENCY AND SUGGESTED PREVENTIVE ACTIONS
3.8.1 Lack of Quality Control or Construction Supervision

Failure or early demolition of a bridge needs to be avoided at all costs. The most vulnerable
stage is during construction. During construction, critical items are inadvertently overlooked,
thereby leading to failure. Critical items include:
1. Inadequate design of formwork or its premature removal.
2. Inadequate bracing.
3. Improper sequence of concrete placement.
4. Improper sequence of erection.
5. Improper placement of reinforcing bars.

6. Incorrect profiles of post-tensioning tendons.
7. Welding deficiencies in steel connections.

8. Incorrect thickness of gusset plates.
Table 3.6 shows reasons for failures during construction or by negligence in the field. It is

an area of weakness where expertise in construction techniques is desirable.

Many failures seem to happen during construction. One of the difficulties is that construction
practice varies from state to state and from job site to job site. Site organization is based on selec-
tion of one general contractor, who in turn selects several sub-contractors who have specialized
in a particular trade such as concreting, formwork, steel fabrication, bearings, reinforcing steel,
etc. Construction procedures need to be streamlined.

3.8.2 Method of Construction
1. Current specifications do not adequately cover construction related design and temporary loads.

Future construction codes should address issues created by the use of the latest technology
such as new construction loads. Technical specifications may also be made comprehensive

to give minutest details of construction procedure.
2. Accelerated bridge construction: Modern construction technology seems to be pulling the
train on design methods. Precast technology is a world apart from traditional wet construc-
tion methods. Self-propelled modular transportation (SPMT) has enabled the transportation
of long span assembled girders without the need for splices, resulting in increased factory
production.
The connection design used for precast construction is different than that used in traditional
construction. Joint strength must be be tested in a structures laboratory.

3.8.3 Suggested Preventive Action against Construction Failures
1. Design of temporary works such as scaffolding supports and formwork supporting a deck
during concreting needs to be in accordance with AASHTO temporary works design manual