Appendix C. Rating Procedure for Existing Bridges

AbstractThis procedure can be used to standardize the method for determining maximum safe live load capacities of existing bridges at Company facilities. It provides a basis for computing the maximum loads that may be allowed on a bridge when materials are of good quality, members are acting normally, and deductions in size or area have been made for deteriorated members. It is assumed that the bridges are thoroughly inspected as often as the condition of the structures requires.

This procedure differs from the AASHTO requirements as it is more simplistic. However, the intent is to maintain safe bridge structures that can be used to their fullest capacity with the consideration that the bridges are used for low volume with vehicle speeds limited to reduce impact. This procedure is based on structural types typically used in process plants. It is important that a copy of Reference 1 be obtained, reviewed and used in any rating work.

Contents Page

C1.0 Rating Procedure C-2C2.0 Bridge Inspection Procedure C-4C3.0 References C-4Chevron Corporation C-1 April 1989

Appendix C Civil and Structural ManualC1.0 Rating Procedure1. Obtain design drawings or if possible as-built drawings of the bridge. The

bridge should be inspected to verify that the actual bridge details conform to the drawings.

2. If possible, the deck stringers, support girders and piers should be given a detailed inspection by an engineer or qualified inspector to determine the appropriate safety factors which should be applied. If a detailed inspection can not be made, the safety factors should be increased.

3. Determine stresses in bridge members as follows:

a. Loading:

1. All dead load should be included in stress calculations.

2. Live load should consist of a H or HS truck loading (Ref. 1 Sec. 3.7) or other live load specified by the operators of the facility. Only one maximum legal vehicle load should be placed on the bridge at a time if the bridge is less than 200 ft. long.

3. Impact load shall be applied (Ref. 1 Sec. 3.8.2). However, impact load can be reduced if speeds less than 10 mph are enforced on the bridge, and the unevenness of the approaches and bridge structure do not cause additional impact (Reference 2, Sec. 5.2.3). For typical bridge span lengths at Company facilities the impact allowance will be 30 percent. For enforced speed limits of less than 10 mph an impact allowance of 10 percent is suggested.

4. Other loads (e.g., Seismic/Wind) specified in Reference 1, Sec. 3 should be applied as appropriate to the structure without a truck/crane load.

b. Load Distribution and Load Factors:

1. Reference 1, Sec. 3.23 provides a method to distribute loads between a series of beams and stringers.

2. Reference 1, Sec. 3.22 provides load factors for designing bridge members using either service load method or the ultimate strength method.

c. Analysis of Stresses in Bridge Members:

1. Any recognized method may be used to find forces and stresses in members. If bridge piers are not pile supported, settlement of piers could cause overstress of longitudinal stringers. In this case it could April 1989 C-2 Chevron Corporation

Civil and Structural Manual Appendix Cbe best to use simple spans between piers and not assume continuity for continuous beams.

d. Timber Bridge Design Capacity:

1. Determine forces and stresses in timber bridges as discussed above. Allowable stresses are given in Reference 1, Sec. 13.

2. When checking the shear in a member locate the load as specified in Reference 1, Sec. 13.3.1. In many instances, shear from wheel loads will control the design of a timber bridge. Only use an increase in allowable shear stress if checks or cracks in the side of the member are very small as determined after a thorough inspection of each bridge member.

3. Deflection should be reviewed for long span members. If deflection is higher than L/240, the live load should be reduced or an increased impact factor applied as appropriate.

4. Reference 1, Section 13.2.5.1 allows an increase in allowable stresses for a short-duration load. It should be noted that the duration is cumu-lative over the life of the structure. Therefore, for a posted wheel load, this increased allowable probably will not apply.

e. Steel Structures Design Capacity:

1. Reference 1, Section 10 provides information for determining allow-able stress in steel structures.

2. Reference 2, Table 5.4.2.A and 5.4.2B provide the yield stress to be used for steel structures where the yield stress is not provided on the drawings.

3. If the inspection of the bridge shows that steel members have corroded, the section properties should be reduced as appropriate. The inspection should also verify that stress corrosion cracking has not occurred.

4. Reference 1, Section 3.27.3.1 discusses the distribution of wheel load to steel grating.

f. Concrete Structures (Reinforced and Prestressed) Design Capacity:1. From Reference 2, Section 5.4.6, A concrete bridge need not be

posted for restricted loading when it has been carrying normal traffic for an appreciable length of time and shows no distress. This general rule will apply to bridges for which details of the reinforcement are not known. However, the bridge should be inspected at frequent inter-vals for any distress signs which may develop until such time as the bridge is strengthened or replaced.

2. Distribution of load in concrete deck slabs can be determined by Reference 2, Section 5.3.3 or Reference 1, Section 3.24.Chevron Corporation C-3 April 1989

Appendix C Civil and Structural Manual3. Existing bridges will possibly not conform to the latest requirements of Reference 1 in at least two conditions:

a. Section 3.24.8 requires an edge beam on new structures. For existing structures, if there is no edge beam, assure that edge failure will not occur if the wheel is put 1'-0 from the curb.

b. Section 8.16.8.4 requires a distribution of tension reinforcement that will limit the size of cracks. The applicability of this section should be determined on a case-by-case basis.

g. Abutments and Piers

1. Check the overturning/sliding stability and soil pressure under piers and abutments. Many of these structures are not pile supported. There-fore, this could limit the rating of the bridge.

2. Lateral overburden pressure should be put on the wall from vehicle wheel loads. For low retaining walls, apply 60 percent of the vertical load, horizontally to the wall at a distance from the base of 60 percent of the height.

C2.0 Bridge Inspection ProcedureA Chevron Bridge Inspection Procedure is available from the Civil and Structural Division of the Engineering Technology Department. It is adapted from Reference 2, and covers:

Responsibilities and Qualifications of Inspection Personnel Frequency and Level of Inspection Inspection Procedures and Reports Bridge Inspection Reports

C3.0 References1. Standard Specification for Highway Bridges, 1986 or latest edition by Amer-

ican Association of State Highway and Transportation Officials (AASHTO).2. Manual for Maintenance Inspection of Bridges, 1986 AASHTO.

3. Manual of Steel Construction, American Institute of Steel Construction (AISC), 8th Edition.

4. American Institute of Timber Construction (AITC), 2nd Edition.5. Building Code Requirements for Reinforced Concrete (ACI 318), American

Concrete Institute.April 1989 C-4 Chevron Corporation

Manual ContentsApp. C ContentsC1.0 Rating ProcedureC2.0 Bridge Inspection ProcedureC3.0 ReferencesEngineering SpecificationsStandard Drawings & Forms