Page 193 - Bridge and Highway Structure Rehabilitation and Repair
P. 193

168            SECTION 2                                        STRENGTHENING AND REPAIR WORK



                        Table 4.7  Comparison of permit loads in three states.
                        Comparisons of           NJ DOT
                        permit vehicles          permit vehicle   Caltran 9 axle (P 9)  PennDOT P-82
                        Distance between axles   16.0 m           22.64 m             16.65 m
                        Total weight             890 KN           973 KN              910 KN


                        Table 4.8  Comparison of maximum live loads with NJDOT permit vehicles.
                        Short spans       Factored AASHTO ML governs
                        Medium spans      NJ DOT Permit vehicle governs
                        Long spans        Factored AASHTO HL-93 governs


                            Steel, due to its superior strength and long-term performance, is best suited for medium and
                        long spans. The old practice for medium span lengths for bridges was to weld cover plates on
                        rolled sections in high-tension areas, such as at the midspan. Due to repetitive loads, fatigue of
                        tension welds has been a problem, since it reduces the working life of expensive steel bridges.
                        Also, regular monitoring of fatigue-prone details increases maintenance costs.
                        4.12.2  Selection of Girder Depth for Defl ection Control

                            AASHTO requirements based on limiting maximum live load defl ection:
                            AASHTO Table 25263-1 provides guidelines for minimum depth as follows:

                            Minimum overall depth of composite I 6 girder 3 0.04 L for simple spans
                                                                     3 0.032 L for continuous spans.
                            Minimum depth of steel truss             3 0.1 L for simple or continuous spans.

                            Minimum depth of prestressed concrete I 6 girder and CIP box beams
                                                                     3 0.045 L for simple span and
                                                                     3 0.04 L for continuous spans.
                            Minimum depth of adjacent box beams      3 0.03 L for simple spans and
                                                                     3 0.025 L for continuous spans.

                            Any deviation for use of a shallower depth needs to be justified by detailed calculations.

                            Other structural requirements:
                        1. Repetitive deflections cause fatigue, debonding of reinforcing bars, cracking in deck slabs

                            and wearing surfaces, and affects durability adversely.
                        2. High instantaneous deflections cause discomfort for motorists.

                        3. Bearing rotations during construction result in large deflections. Camber needs to be provided

                            in girders to minimize dead load defl ection.

                            Limiting live load deflections on long spans:
                        1. All design lanes shall be loaded. Lane reduction factors are applicable for two or more
                            lanes.
                        2. For straight girders, distribution factor for defl ection 3 Number of lanes/Number of girders
                        3. Dynamic load allowance (impact factor) shall be applied.
                        4. For vehicular loads, maximum computed live load defl ection : L/800.

                        5. For vehicular and pedestrian loads, maximum computed deflection is 25 percent lower, i.e.,
   188   189   190   191   192   193   194   195   196   197   198