Presentation on theme: "Trestle Rebuilds The Continuing Quest to Fill the Gap"— Presentation transcript:
1 Trestle Rebuilds The Continuing Quest to Fill the Gap By Howard Swanson PE SEEngineer StructuresNorfolk Southern
2 Out of Date Cultural Reference You can’t always get what you wantBut if you try sometimes, well you might findYou get what you needWritten by Mick Jagger and Keith RichardsRolling Stones 1969 album Let it BleedOver the last five years, I have had a number of recent college graduates work for me as Management Trainees, Engineering Associates and Bridge Supervisors. Every once and a while, the recent college graduate looks at me in a confused manner after I make what seems to me an appropriate culture reference. The fact that I referred to something that was popular while I was in college and they were born after I graduated from college may have something to do with the look.So, what do the Rolling Stones have to do with engineering?Engineering generally involves compromises. You have to decide what you want and what you need. Norfolk Southern has replaced somewhere in the neighborhood of 400 trestles in the 31 years it has been a company. We have made the decisions between what we want and what we need many times in that period. This is the story of getting what we need, keeping trains running safely.
3 What is a Trestle? Merriam – Webster definition A braced frame serving as a supportA braced framework of timbers, piling or steelwork for carrying a road or railroad over a depressionA bridge consisting of similar length short spans (under 35’ long spans)Supported by pile foundation that was exposed when builtMerriam Webster has this definition of what is a trestle. I would add to the definition that trestles consist of similar length short spans and that the spans are supported by a pile foundation that was exposed when built. If the spans are longer and supported on framework built on top of pedestals, I would call the structure a viaduct.
4 Parts of a Trestle Bridge Ties Backwall Stringers or Spans Cap DiagonalPilingThe assembly of the cap, piling and bracing is know as a bent. The end bents are sometimes know as the dump bents.Timber piling is round. When piling deteriorates above ground line to the point it needs to be replaced, the timber that replaces it are called posts.
5 Timber Trestle - Advantages Easy to work with material in remote locationsLight weight material that can easily be handledLong lasting material that does not need coatingEconomical materialEasy repair and replacement of componentsTimber can be easily worked in remote areas. Many manufacturers make gasoline powered chain saws and drills.Most timbers are light enough they can be handled by backhoes or small truck cranes.Creosote provides a long lasting coating on the surface of the timber. As long as the surface is not drilled or cut, the coating is very effectiveTimber is economical
6 Standard Timber Trestle This timber trestle is nearing the end of its useful life. During its lifetime, a second cap has been added to the bents to accommodate a track raise or settling bents. Most of the piling in the center of the bridge have rotted out or failed. The piling have been cut near the waterline and a beam called a sill has been placed on the pile stubs and new posts were placed on the sill. Additional diagonal bracing has been added. These component replacements added 20 to 30 years to the life of this trestle.
7 Timber Trestle - Disadvantages Timber splits and rotsTimber is flammableTimbers large enough to handle modern loads not readily availableMaximum span about 13’Difficult to obtain adequate penetration on timber piling to resisting longitudinal loadsOpen DeckBridge ties have to be replaced on a regular basis by non-mechanized forcesSurface problems at the end of the bridgeBallast DeckStringers separate allowing ballast to leakWe have not continued to build in timber trestles because of disadvantages with this type of construction.
8 Distressed PilingIn the top left picture, the fourth pile from the left has completely split apart. The cap has bent because the rest of the piling was unable to carry the rest of the load. The lower left picture shows the shell of the pilings have rotted out and have broken. The bent on the right is on a branchline going to a power plant. All of the loaded coal trains go to the right. The longitudinal traction forces has pushed the stringers to the left. Records show that the piling was only driven 5 to 7 feet deep. The piling was unable to resist the overturning force.
9 Crushed or Cracked Stringers The properties of timber change based on the direction of the grain. Timber can crush when a force is applied perpendicular to the grain. The picture on the left shows a stringer that has started to crush. Horizontal shear force is caused when a beam is loaded in bending. The horizontal shear force can be great enough to split a stringer along the grain. Eventually the stringer on the right will split into three separate beams. As three separate beams they are weaker than the original beam.
10 Stringers SeparatingStringers shrink with age as they dry out. If a gap forms between stringers, ballast can become lodged in the gap and act as a wedge. Over time the gap widens and becomes big enough for ballast to leak out. This causes depressions in the ballast. The track ties may loose support if the depression is in the middle of the track.
11 Steel Trestle - Advantages Readily available materialSelect welded connections can be made at remote locationsBolted structural connections of prefabricated members can be made anywhere.Easy construction in remote locationsWork can be done between trainsSteel piling can be splicedSteel is readily available. A controlled environment is required to obtain high quality tension welds in steel. However, if welds are not subject to forces that may cause cracking, welds can be made in less than idea circumstances. Bolted structural connections in prefabricated steel can be made in less than ideal conditions. Medium size cranes can lift steel trestle components. A skilled crew can remove a portion of an existing timber trestle and install a steel trestle span in four hours. Steel piling can be spliced. This allows a pile driving crew to cut a piling to clear up for a train. After the train has run, the crew can splice onto the piling and continue driving until the pile has reached capacity.
12 Standard Steel Open Deck Trestle This is a standard steel open deck trestle. This trestle has steel spans, caps and piling. The backwalls have also been fabricated out of steel. These trestles are quick and easy to build. These trestles were mainly built on lighter density lines.
13 Section of Steel Open Deck Trestle This is a cross section of a typical steel open deck trestle. 12” or 14” H pile or 12 ¾ outside diameter pipe pile is generally used to construct the bents for a steel open deck trestle. Scrap H pile is used to brace the bents. The piling is driven through holes cut in the timber trestle being replaced.The caps are shop fabricated out of W14x159 rolled beams. These beams are strong enough to transmit the forces from the spans into the piling. The flanges are also wide enough to provide room to weld completely around the piling. The depth of the span is designed to allow the cap to be installed below the existing timber stringers. This allows the caps to be installed before the old timber stringers are torn out.The spans typically have only one beam per rail. This makes the beams fracture critical. A higher quality of steel and additional testing is required for fracture critical beams. The spans are shop fabricated as one unit.
14 Standard Steel Ballast Deck Trestle This is a standard steel ballast deck trestle. This bridge also has steel spans, caps and piling. The ballast and track are supported by a steel deck plate. The backwall is also fabricated out of steel. This was Norfolk Southern’s standard trestle before we switched to concrete.
15 Section of Steel Ballast Deck Trestle This is a cross section of a typical steel ballast deck trestle. Like the steel open deck trestle, the piling are either 12” or 14” H piling or 12 ¾” outside diameter pipe piling. The procedure for driving piling for a steel ballast deck trestle is the same as the steel open deck trestle.The same rolled shape, a W14x159 are used for the caps for a steel ballast deck trestle. The caps are shop fabricated. Similar to the open deck trestle, the depth of the span is designed to allow the cap to be installed below the existing timber stringers. The new caps are installed before the old timber stringers are torn out.The spans typically have two beam per rail. The beams are not fracture critical and additional performance testing is not required. The two load beams per rail have beam diaphragms and stiffeners welded in by a fabricator. Similarly the outside beams have stiffeners shop installed on the ends. When setting the spans, the beams are set on the sole plates. A welder then welds the span to the sole plates. Steel deck plate is welded to the top flange of the beams. Waterproofing material is applied to the top of the plate. Temporary timber sleepers are placed under the track ties. Ballast is dumped and the sleepers are pulled out.
16 Steel Trestle - Disadvantages Steel needs to be coatedSteel piling rustsOpen DeckBridge ties have to be replaced on a regular basis with non-mechanized forcesTrack surface issues at the ends of the bridgeBallast DeckSteel deck plates rustSteel deck plates leakSteel corrodes faster when it is in water that contains corrosive chemicals. These chemicals are found in most swamps.Norfolk Southern has developed an efficient system for maintaining and replacing track ties. The ties on open deck bridges cannot be changed out by the efficient mechanized gangs that change out track ties. Open deck ties have to be replaced or sole plates shimmed to make adjustments for track raises.Installing the waterproofing is the last step before placing the new span in service. At times the waterproofing did not sufficiently cover the deck plate. The seams between the deck plates leak and rust out.
17 Pile ReductionThe picture on the left shows piling in brackish water. This water contains chemicals that accelerate the corrosion process. The process can also be accelerated by fertilizer runoff and industrial chemicals. The picture on the right shows a hole that has rusted through a pile. In addition to rusting through the piling has cracked
18 CorrosionIn some locations, water rises above the level of the caps. If the water is brackish or if it carries fertilizer runoff, it can accelerate the corrosion process. On the left a steel cap is severely corroded. The bolt in the middle of the picture has necked down or rusted to a smaller diameter.The picture on the right shows the span to cap connect. The corrosion has caused a crack in the web of the beam adjacent to the anchor bolt that goes through the sole plate.
20 Prestressed Concrete Spans AdvantagesDisadvantagesLong lasting material that does not need coatingCan be economical if produced in large quantitiesSpans can be designed to handle expected loadsPrecast components are heavyConcrete Piling difficult to spliceCast-in-place concrete difficulties in remote locations
21 Span Design Considerations Spans have to be uniform over many bridges and span lengthsSpan has to be stable when not in the bridgeDeck has to be ready for track soon after span is set
23 Cap and Piling Considerations Cap and BackwallsPilingConnections that can be made under field conditionsWeight limitationsFabrication ConsiderationsLong LifeInstallation between trainsInstallation in remote areas
28 Current Generation New Concerns New Solution Walkways for trainmen have to be attached to the outside of the bridgeBallast gets stuck in gratingCurbs too close to track to allow for mechanized replacement of tiesNew SolutionWiden deck to four boxesAdd handrail