The inspection process of timber and steel bridges follow material-specific procedures. The AREMA manual recommended practice provides information on the conditions to report, and how to record findings for the inspection of railroad bridges. Inspections of any structure are best completed using a formalized method, including an 1) initial pre-inspection data gathering phase to find out general information, 2) a thorough visual inspection in the field and 3) standardized reporting, as outlined below (Ritter, 1992).
1. Pre-inspection evaluation: any existing document with information of construction date, design drawings, as-built drawings, loading history, maintenance, and previous inspection reports should be reviewed in order to familiarize with the structure and potential areas of
deterioration. A thorough pre-inspection evaluation helps the inspector perform a more effective field inspection. A third party consultant provided yearly inspection reports to WVSRA
28
dating back to 2011 up to 2013 on the Dailey Branch bridges, and these were available to WVU for reference.
2. Field inspection: The track condition should be noted both at the approaches and throughout the length of the bridge, checking for alignment, condition of track rails, position of rail in reference to the center line of the stringer chords, bearing on ballast, and proper amount of ballast. The ties condition should be inspected, recording the size, spacing, uniformity of bearing, and deterioration. Areas susceptible to decay are usually near connections and locations where mechanical damage has created an opening for water to collect. The most common mechanical damage is induced by loading timber bridges above the designed capacity over long periods of time, which may induce shear or bending cracks visible in the side faces of beams and stringers. Another sign of overloading are crushed surfaces where stringer ends bear on beams, or beams bear on posts. Foundation settlement is another cause of mechanical damage, affecting proper load distribution among adjacent members, and proper bearing of substructure members. Component inspection is necessary when deterioration is identified; the location of the component and the extent of rust or rot need to be noted. Substructure
components usually experience decay due to contact with soil and fluctuating water elevations. Debris may also impact bridge components due to stream channel overflow, including
components both in the superstructure and substructure. Locations where members connect such as cap/column joints, bracing bolts, backwall/wingwall intersections are highly susceptible to deterioration. The superstructure components are inspected last, looking for locations where water and debris tend to be trapped. Non-structural components, such as tie spacers, need to also be inspected for decay. Section 3.2 of this report was performed following
recommendations from this section.
3. Reports and records: for each individual bridge, the inspector shall record the bridge number, name of nearest station, lowest mile-post number, Global Position System (GPS) location, age and type of structure, total length of spans, height of spans, and number of spans. Per AREMA, all bridge components and spans along the length of the bridge shall be numbered in increasing order in the direction that the mile posts increase. Transverse members, such as stringers and posts, shall be numbered in increasing order from left to right when looking from bent number 1 towards the last numbered bent. A summary of inspection findings including strength reducing factors that may lower the capacity of the bridge needs to be documented. Drawings, pictures, and sketches depicting the location and extent of deterioration need to be included with the
29
inspection report, as provided in Appendix 2. Dimensions and quantities need to be verified and updated if any discrepancies between as-built drawings and the current condition of the
structure exist. Recommendations need to be provided in the report with respect to repair, replacement, capacity reduction, or closure of the bridge and bridge components. An inspection report should provide means of determining a cost-benefit analysis for future work on the inspected structure.
AREMA recommends that a periodic inspection is completed on all bridges at least once a year. Periodic inspections help track any change in the structural condition of the bridge through its service life, and any major findings are noted for further investigations. AREMA further recommends special inspections when periodic inspections demand further investigation, when determining the current capacity rating of the bridge, when preparing repair plans, and whenever fracture-critical members (FCMs) are encountered specifically in steel structures. Emergency inspections are special inspections in situations where a bridge undergoes an event that may have affected its capacity to resist design loads. These events can be floods, derailments, collisions, fires, and earthquakes. (AREMA, 2014). The load rating procedure in chapter 4 falls under the category of special inspections, since it provides further information regarding the bridge current load carrying capacity.
2.3.1.1. Specific Inspection Needs for Timber Railroad Bridges
All timber members in any location of a bridge need to be visually inspected for cross grain, and for any sign of failure due to shear, bending or bearing. Signs of failure are usually due to overstressed
members, natural defects in wood and uneven bearing conditions. Such signs can be seen as significant cracks across or along members, and crushed wood at bearing areas. It is also important to note whether the timber member is treated or untreated. A timber expert assisted the inspection crew in locating and recording defects in the wood, such as cross grain, to then determine the structural grade of timber as shown in section 2.3.
On open-deck bridges, the condition of the anchorage of ties, guard timber and tie spacers should be noted. All stringers should be checked for any deterioration and natural defects, recording the location, type and size. The dimensions of each stringer, number of stringers per chord, and whether they are packed or spaced within the chord system should be noted. The bearing condition of the stringers on each bent need to be checked; including number of shims where used, uneven bearing among stringers in a chord, and any crushed surfaces due to bearing stress.
30
The substructure should be inspected last, recording the number of posts in each bent, their
dimensions, the spacing among posts, and if they are braced. The bearing conditions of the corbels (if present) on the cap beams, the cap beams on the posts, and the posts on the sills or blocking should be noted. Any deterioration found on all substructure members including lateral and longitudinal bracing should be noted. The wingwall and backwall components should be examined in a similar fashion. “Sketches, drawings, and photographs are invaluable for illustrating inspection results and should be used freely to locate, identify, and clarify the condition of the bridge components” (Ritter, 1992). A set of drawings including sketches and photographs is included in this report in Appendix 2 in order to provide clear and thorough information regarding inspection findings.
2.3.1.2. Specific Inspection Needs for Steel Bridges
Corrosion can appear in any member of a steel bridge, and any loss of section from corrosion needs to be noted including the location and extent of rust and section loss. Areas where dirt and water can collect, such as bearings and flanges, need to be checked for corrosion or potential future rust. Areas needing spot painting or repainting are locations for future rust to develop, thus recommendations for repainting are important for the longevity of the structure. Locations of expected higher stress should be inspected for cracks and signs of local buckling. For steel bridges, AREMA recommends the
inspection to focus on the following items.
- Anchors, bearings, and bridge seats: Type of bearing of superstructure on masonry below. Condition of expansion bearings, rollers, or rockers including proper functioning, cleanliness, correct positioning, and bearing.
- Expansion: The condition of the masonry at support bearings, abutment and piers, looking for apparent movement from original position.
- Straightness and alignment of members: whether the girders, beams, and stringers have any bends or kinks, and condition of alignment.
- Cracks and breaks: record any cracks and breaks in stringer connection angles, flange angles, ends of cover plates, and lateral bracing.
- Rivets, bolts, pin holes and nuts: record any loose rivets, corrosion heads of rivets, and wear of rivet holes.
- Corrosion: Any corrosion should be noted, including the size, location of corrosion in member, and location of corrosion in the bridge. Any loss of cross section due to corrosion should be recorded. Collection of rust at bearing locations and at bottom of stiffeners.
31
- Paint and cleanliness: the condition of the paint throughout the bridge including areas in need of spot painting. Note areas were dirt and debris collect on any surface.
For rating purposes, inspection findings should be taken into consideration, including any additional dead load or fixed loads since the structure was originally built. Loss of section due to corrosion should be measured as best possible in the field. All bracing that reduces the unbraced length of compression members are to be noted. Special attention needs to be paid when inspecting fracture-critical members, which are defined by AREMA as “tension members or tension components of members whose failure would be expected to result in collapse of the bridge or inability of the bridge to perform its design function”. When inspecting typical steel railway bridges, AREMA suggests a standardize nomenclature for each bridge component as shown in Figure 22 for deck girder bridges and Figure 23 for through girder bridges.
32
Figure 23 - Typical Through Girder Bridges (AREMA, 2014)