TIMBER BRIDGES AND 286 LOADING

Transcription

1 TIMBER BRIDGES AND 286 LOADING AREMA TECHNICAL CONFERENCE 2002 Presented on behalf of Committee 10, Structures Maintenance & Construction by: David Franz, Area Engineer Osmose Inc., Railroad Division P.O. Box 501 Kearney, MO Phone Fax

2 INTRODUCTION OVERVIEW OF PROBLEM Many railroads are facing or have already faced an increase in loading from 263,000 lb. cars to 286,000 lb. cars. Many of the products that are moved in these heavier cars are bulk commodities such as coal, grain, rock, ash, etc., that are moved in unit train consists of 25 to 100 cars or more. The combined effect of heavy axle loads and unit train consists are showing wear and tear on timber bridges. In addition to the heavier axle loads and increased use of unit trains, many MOW personnel are facing increased pressure to increase track speeds. Higher speeds increase the impact on bridges and, particularly in the case of timber bridges, has an adverse effect on overall stability. This lack of stability is due to the inability to drive timber piles to high bearing capacities as well as the flexibility of the individual timber members and their joint fastenings. Timber bridges are a significant percentage of the total bridge population on the Class I railroads, and a large percentage of the bridges on most short lines and regionals. How will all these timber bridges fare with the increased loading? In order to better deal with this question, it helps to understand a little about timber and how it behaves.

3 STRUCTURE CAPACITIES When evaluating a timber bridge for capacity, the individual making that evaluation should be familiar with timber bridge materials and construction in general, as well as with the individual structure. Most timber trestles were built to a standard developed by the individual railroad that constructed them. Therefore, many railroads have a standard or several standards that cover most or all of their timber bridges. The standard for the industry relating to timber bridges is Chapter 7 of the AREMA Manual for Railway Engineering. This chapter identifies allowable stresses to be used for the design of timber structures as well as requirements for rating of existing structures. Often time a single design or rating calculation will provide the information necessary to evaluate a large number of bridges due to the standardization previously mentioned. These calculations may have to be modified for the actual conditions at any specific bridge, but theoretically, if all the bridges are well maintained, they will all have a similar capacity. Timber is a grown product. It is not manufactured to certain specifications, but rather graded visually to establish relative quality. When lumber is cut to size, it is carefully inspected for a number of characteristics to establish what grade of lumber it will be. Lumber used for railroad bridges is the highest quality readily available in the large sizes required. Due to the fact that lumber is a non-homogeneous product, there is a wide

4 variability in the strength of different pieces of lumber even within a grading classification. Another important factor in the evaluation of lumber is that much of the newer lumber is fast growth lumber. The quality, while still good, is not that of some of the older lumber still found in many structures. Current design values for timber bridges must, by necessity, be based on the weakest lumber that can be expected to pass the visual grading requirements for a particular type and grade of lumber including a safety factor. From a design perspective this requirement is a must to ensure the safety of bridges. What this means in the field is that many bridges are constructed of lumber that is considerably stronger than the design stresses that govern it. When evaluating a timber bridge for 286,000 lb. loading, the bridge should be carefully inspected to see how well it is holding up under the existing 263,000 lb. or 268,000 lb. loading. If it is already experiencing significant problems it is not likely that the situation is going to get any better by increasing the loads. On the other hand, if the bridge is in good condition, it is not likely that a 7% to 9% load increase will cause the bridge to fall down. An experienced, and I emphasize experienced, railroad bridge engineer who is familiar with timber structures should be able to evaluate the structure for 286,000 lb. loading based on the rating analysis and the inspection results. Caps and, piling in particular, on timber bridges should be evaluated for possible voids and section loss. Because these members have high compressive forces, they are

5 susceptible to failure without many prior signs of visible distress. Stringers are a bending member and, because of the flexibility of timber and its short term capacity to carry overloads, will show signs of distress long before failure. What does this mean to railroads facing 286,000 lb. loading? Do all of your timber bridges have to be replaced? Not likely. Do you need to be on top of your bridge evaluation, inspection, and maintenance? You bet!! Certainly everyone needs to understand that while 286 loading will not require the replacement of all timber bridges, it will mean higher maintenance costs and a shorter life cycle. It is important to note that heavier loads on timber bridges will magnify existing problems and probably find some new ones. Let s look at some of the conditions to be found as well as some solutions. PROBLEM AREAS AND HOW THEY ARE IDENTIFIED Track Conditions Harmful track conditions are easily identified by visual observation. A poor track surface (or vertical profile) will lead to large impact loads on the bridge, especially when operating at higher speeds. Although AREMA Chapter 7 does not calculate impact loading into the capacity of a bridge, these loads definitely cause an increase in mechanical wear and must be considered in the overall life expectancy of a structure.

6 Defects that cause poor track surface include low dumps or approaches, battered or open rail joints, flat spots in rail such as engine burns, poor tie condition, special trackwork such as expansion or moveable bridge castings, and problems in the bridge foundation, substructure, or superstructure. One particular problem on short lines is the tie condition around joints where the combination of running rail and oversized angle bars will often cause heavy tie damage. Just as poor track surface is of concern, so is the track alignment on timber bridges. Poor track alignment places significant lateral loads on the bridge. Bolted timber connections do not respond well to the lateral loads. The causes of poor track alignment include poor lining during deck installation, single shoulder and undersized tie plates that just aren t holding up to the loads, poor anchorage of the rails and tie deck, or misalignment of the structure itself. Misalignment of the structure itself can be attributed to a number of factors, and is often a combination of these: Crushing or failure of structural members, pile settlement or inadequate pile penetration, soil instability or erosion, external forces acting on the piles such as ice or drift accumulation, or inadequate bracing.

7 Stringer Conditions Stringer problems can be broken into three basic categories: cracked or broken stringers, horizontal shear cracks, and crushing at the cap bearing. Broken stringers can be an indication of overload on the structure, but they are often isolated and relate more to the quality and grading of the lumber or internal decay. Excessive deflection of stringers will often cause a horizontal shear crack to develop middepth on a stringer. Eventually this crack will run the entire length of the stringer with the ultimate effect that the member is now acting as two smaller stringers with considerably less capacity than as originally designed. Another very common problem found on timber bridges is crushing where the stringer rests on the bent cap. This crushing is often a result of mechanical wear at the stringercap interface usually caused by excessive movement of the bridge under traffic. Other causes of crushing include internal or external section loss due to decay or an overload of the structure. Cap Conditions Caps are one of the more crucial elements of a timber bridge because so much load is concentrated here and they can fail quite suddenly. There are several distinct types of problems that can be identified with caps. One of the more common defects is crushing

8 of the cap at either the stringer or pile bearing locations. This is usually caused by internal section loss due to internal decay. Vertical splits are often seen through the middle of caps. Longitudinal forces transmitted through the caps and deflection of the stringers pry on the cap and tend to split it through the drift pin holes. Installing drift pins without predrilling can also aggravate this condition. Lumber quality is obviously a factor as well. Horizontal cracks or splits are caused by inadequate support due to failed or pumping piles or poor pile cutoffs resulting in gaps between the pile top and cap. Cornering or splitting off of the corners of caps is normally associated with excessive longitudinal movement or deflection of stringers. Another common cause for cornering is ring separations in the cap which is most common in newer, fast growth lumber. Piles / Foundations Pile problems are related to both the structural condition of the pile itself as well as the capacity of the pile foundation as driven or as dictated by present day conditions. The most common pile failure is due to decay. Piles normally decay from the inside out and when there is no longer enough wood section remaining in the shell, it may start to broom or split up into splinters prior to failure. Large splits in piling are normally not caused by

9 decay and should be treated appropriately. These large splits often start at the drift pin when there is excessive movement of the bridge, particularly in the longitudinal direction. Even though the pile as a stand alone structural element may be sound, there are other problems related to a pile foundation that may affect the condition and load capacity of the bridge. Vertical settlement of a pile foundation can indicate one or more of several potential problems. The pile may have been driven with inadequate penetration for the applied loads, both vertical and lateral. Inadequate penetration due to insufficient hammer energy, poor soils for pile driving, or a less than diligent pile driving crew can lead to settlement of the bridge. A hard, impenetrable soil or stone layer at a shallow elevation will often provide adequate load capacity, but very little lateral stability. Over time, the capacity of a pile foundation can be reduced from a loss of pile penetration due to erosion or excavation of soil adjacent to the bent. If original pile driving records or other plans are not available, this condition can often be identified in the field by observing bent bracing or pile discoloration. Even where pile penetration is adequate, there can be bent settlement due to a larger, deep seated slope failure. This type of failure occurs on banks and side slopes and is usually identified by a lateral or translational movement of the bent along with the settlement.

10 Decay Preservative treating of large structural members does not fully penetrate the lumber. Decay of treated wood members usually begins at locations where the protective creosote barrier has been compromised due to drilling or cutting. Locations to look for decay are fasteners (drift pins, bolts, drive screws, etc.), sawcuts such as ends of stringers, and at the groundline. Decay below the groundline usually occurs within the top two to three feet where oxygen is readily available, but in certain soil and climatic conditions, the decay can extend considerably further below the surface. REPAIR / UPGRADE / REPLACEMENT OF INDIVIDUAL COMPONENTS The repair or upgrade of individual bridge components will extend the life of a bridge and can often increase its effective capacity to handle both higher loads and increased speeds. Track Eliminating or reducing the number of rail joints on a bridge will help to reduce impact and lateral loads. Where joints are located on bridges, try to eliminate excessive gaps as well as end batter, especially on open deck bridges. Look at heavier rail sections and larger tie plates. Anchor the rail and ties on open deck bridges to eliminate longitudinal

11 movement. Replace poor or defective ties. And maintain the bridge dumps. It is apparent from the tie condition on the ends of most open deck timber bridges that low dumps cause considerable damage to bridges. Many railroads have gone to placing several longer 10 cross ties off the ends of a bridge with good results. Stringers If existing stringers are not adequate to handle the desired loads, several alternatives are available to increase capacity. Additional plies of stringers can be added to the chords. Existing stringers should be carefully measured so that when additional ply are added, the load is evenly distributed among the plies. Laminated wood stringers are available for railroad loading and are a viable alternative to full section sawcut lumber. Laminated stringers are made by gluing together smaller pieces of lumber to make the desired section. The structural properties are quite good as well as the dimensional tolerances. Steel stringers are an additional alternative that can be utilized to replace the timber stringers and strengthen a bridge. These stringers are quite stiff so that they help to eliminate deflection and movement of the bridge. There are enough standard depths available to fit almost any dimensional need and they are readily available.

12 Caps Simply double capping a bent will often provide significant benefit where there is a little pile settlement or poor pile bearing. Other alternatives to strengthening or stiffening a cap are laminated timber as described previously or prestressed concrete caps. There is a standard detail for a prestressed concrete cap in the AREMA Manual, Chapter 8. Because it is so stiff, a concrete cap will often help reduce the mechanical wear that occurs at the cap/stringer interface. When installing concrete caps be sure to monitor the condition of the batter piles. Because these caps are so much stiffer than a timber cap, there is usually more load distributed to the batter piles and if they are in marginal condition, problems may develop. Piles Piles that are failing can be posted in kind. When cutting off a pile prior to posting, be sure to dig down far enough to get to good wood below the decay zone. It is best to utilize some type of splice detail that will develop the bending capacity of the pile in addition to its vertical load capacity. When several piles in a bent are posted or require posting, a number of railroads prefer to frame out the entire bent. This is accomplished by cutting off all piles at the same elevation and placing a framed bent consisting of a sill, posts, and a cap.

13 If a pile is settling, i.e. it doesn t have adequate bearing capacity, posting or framing will not correct the situation. Short of redriving the bridge, helper piles can be driven between the existing piles to provide additional capacity. A short term solution to stabilize a bent can also be to add posts sitting on mudsills, or spread footings. This is not an effective repair in areas subject to scour or erosion. MAJOR UPGRADES AND REPLACEMENTS In some cases, the condition of a bridge is such that replacement of one or two components will not produce the desired results and a major upgrade or renewal is required. For timber trestles, this type of upgrade will usually fall into one of two categories: a cap-up renewal or complete replacement. Cap Up Renewal A cap-up renewal consists of replacement of all or most components above the piling. Pile driving is an expensive operation from both a labor and equipment standpoint. It also requires a significant amount of track time. Assuming the condition of the piling is good and the bearing capacity is adequate, it is often a very cost effective solution to make a cap-up renewal of a timber bridge. A cap-up renewal can consist of replacement in-kind or the use of some premium components to achieve an upgrade so to speak. Replacement in-kind needs no further

14 explanation. Upgrades include use of a laminated or prestressed cap along with laminated or steel stringers. An upgrade might also consist of converting from an open timber deck to a ballast structure. Another possibility is to utilize prestressed concrete spans on concrete caps. There are also other combinations of the above materials utilized for upgrades. As part of a cap-up renewal, consideration should be given to in-place preservative treating of piles to extend their life. Replacements If the decision is made that any type of upgrade is not sufficient or cost effective, then consideration must be given to a complete replacement. Special site conditions may require the construction of a totally different, long span structure but in most cases a concrete and/or steel trestle is utilized. Substructures are normally driven piles or drilled shafts. Common types of pile are prestressed concrete or steel (h-piles or pipe piles). Drilled shafts are cast-in-place reinforced concrete. Access, site conditions, and soils dictate the proper foundation. Steel spans can be used for both open deck and ballast deck applications and concrete slabs or girders are used in ballast deck applications.

15 DECISION PROCESS REPAIR, UPGRADE, OR REPLACE The decision process for what type of action is appropriate to prepare bridges for 286 loading should involve experienced railroad bridge personnel. First collect all the rating and inspection data you need. Find out what the future operating requirements are likely to be (in addition to 286 loading this could include higher speeds, unit trains, future maintenance budgets, etc.). If the bridge is in relatively good shape, maybe some routine maintenance and replacement in-kind of defective members is adequate. However, if numerous caps, stringers, and piles are showing distress, replacement or upgrade costs should be considered before deciding to spend large dollars on in-kind component replacements. If an upgrade such as a cap-up renewal is being considered, make sure that s a wise investment. If the piles are pumping or moving under load, you don t want to spend money to put a new superstructure on a weak foundation. Is the opening adequate to pass a large flood event? A new or upgraded structure may need to be lengthened. You may also want to consider drift accumulation and how much of a risk and expense that is for the railroad. It may be necessary to install at least one or two longer spans to pass most or all of the drift that comes downstream.

16 CONCLUSION 286,000 lb. traffic is here and it s here to stay. On some lines there are timber bridges out there that are not going to be up to the challenge. But there are large numbers of timber bridges out there that can and will be carrying 286,000 lb. traffic for years to come. This may require more frequent inspections, and it will require more maintenance dollars, but it is like most problems in the railroad industry. It needs to be properly managed.

17 ACKNOWLEDGEMENTS The author wishes to acknowledge the members of AREMA Committee 10 for their help in the review and editing of this paper. REFERENCES AREMA Manual for Railway Engineering Southern Pine Inspection Bureau Grading Rules West Coast Lumber Inspection Bureau Standard Grading Rules