More bridge repairs

Old pony bent, caps and roller nest on

the east pier of bridge 35.

It started out as a small project for the 1891-built Bridge 35: replace several timbers under the Pratt truss that had detectable deterioration or evidence of crushing.  Simultaneously, bearing pads - in this case, a nest of steel rollers - that are designed to allow the bridge to freely expand and contract were frozen and were also scheduled for replacement. In preparing for the work, it was noted that the pony truss that supports the timber trestle approach to the steel span was not properly pinned and had unequal spacing of the posts so it was also scheduled for replacement.

Performing comparatively minor bridge work is expensive for many of the reasons noted in the last blog post, but also for the cost of mobilizing the specialized equipment to perform the work. Any opportunity to combine two or more tasks into one project generally yields substantial cost savings.  So the scope of work was set: change timber caps and girders, replace bearing pads, and replace pony bent. For bridge 35, this should have been four days work but unfortunately not everything goes according to plan.

Work began on a typical spring day in the Northwest: wet and cool. Initially, the bridge lifted without incident and the old roller nests were removed.  Timber replacement began and then something started to go wrong: a weld in the steel added to support the jacking arrangement failed and two steel angles near the jacking area began to fail.  The bridge slowly descended approximately 6 inches onto the pier.  Fortunately no one was hurt and there was no serious damage.  However this was another timely reminder about how challenging it can be working with a structure designed and built more than 125 years ago.  And because it is "safety first," this minor damage will be repaired before any passenger trains operate over the bridge, even though this will affect the first trains of the year.

Imhoff's 65 ton crane lifts the end of

bridge 35.

The next step was to get a crane on site to lift the bridge up. (Special thanks to King County for granting permission to drive up the levee on just six hours notice!)  Imhoff's 65 ton Pierce American answered the call and arrived first thing the next morning.  Owner/operators Scott and Tammy Imhoff are familiar with the Museum and its unique needs - this is the same crane that was used to build bridge 31.3 at Snoqualmie Falls, lift the chapel car and Great Northern caboose off heavy haul truck decks, and change wheels or trucks on a number of Museum cars and locomotives. 

New copper-treated timbers are in

place and now workers are installing

the new base, soul plate, and bearing

pad on the south side of the east pier.

The east pier consists of large timbers typically 14 inches square and a length to suit their purpose.  Timber girders are eight feet long and caps are longer.  Originally, the caps were 26 feet long, but the replacements will be split, using two 12-foot timbers to perform the same function.  This is more economical and they are easier to handle. Timbers are pinned in place, but only as required because each hole is an opportunity for moisture and oxygen to get into the wood.  All the new wood is treated with copper naphthenate and was supplied by Wheeler Lumber in South Dakota.  While creosote is preferred by the rail industry, permitting agencies in Washington State generally make it difficult (or impossible) to use because they are concerned about residue that might contaminate the river sediment.  Copper napthenate is comparable in performance to creosote and is now accepted by the Railway Tie Association for timber ties.

New soul plate is inserted

on top of the new

Fabreeka pad under bridge

35 in North Bend.

An important part of the project scope was replacement of the bridge bearing pads.  The original steel roller nest was probably troublesome for much of the bridge's history.  Modern bridges often use a stainless steel and Teflon interface to address this need.  On the advice of the bridge engineer, this design was adopted.  In addition, bridges often sit on a fabric pad to help absorb vibration.  Fabreeka pads consist of cotton duck laminations impregnated with neoprene which in a similar form have been in use since 1918.  The Museum elected to have a Teflon coating laminated to the top of the Fabreeka pad.

With all the work except installation of the new pony bent complete, repair of the damaged steel angles is now the focus.  New angle irons are being drilled to match the existing holes.  They will be incorporated into the bridge as soon as their fabrication is complete.  After these minor repairs are completed and the bridge is "double checked," train service to North Bend will resume.

Bridge repairs

So you think you can repair a bridge?  Well great, you start tomorrow!  Now if only it were that simple . . .

Damaged sections are obvious but
the method for replacement is not.

Most bridges are near water and at a minimum require a hydraulic permit before work can begin, and this usually adds conditions to a project.  Railway bridge work - by law - must be supervised by someone experienced in the maintenance and repair of railway bridges.  And any modifications must be reviewed by a qualified railway bridge engineer.  So even the damage caused by a tree striking a bridge triggers a variety of additional requirements besides just ordering new timber.

A tree striking a bridge?  Yes, that was the subject of a recent blog post. A very large tree struck and damaged formerly Northern Pacific's Bridge 35's trestle during a recent windstorm. Bridge inspectors allowed several trains to pass but stipulated that permanent repairs had to be undertaken right away. And as one of the largest objects in the Museum collection, how those repairs are performed is also critically important for collection care standards. So the Museum committed to completion prior to the start of the 2014 operating season using an experienced contractor and licensed engineer.

First of the timbers required to repair
Bridge 35 arrived and were moved to
the site using the Speedswing.

Step one: given the urgency of the bridge repair, the Northwest Railway Museum was able to secure an emergency hydraulic permit. Step two: pressure-treated railway bridge timbers were located in South Dakota (ironically, made from Douglas fir cut near Tacoma) and were ordered. Step three: a qualified contractor, foreman and engineer were identified and hired.  Step four: real work begins!

Stringers under the timber deck are
exposed during the pile cap replace-
ment.

To help control costs, some of the site preparation was performed by community work crews. They removed bolts and shovel railway ballast off the deck. In addition, Museum volunteers cleaned, chased threads, and primed the bolts to allow reuse. And the Museum agreed to supply the materials to avoid handling charges from the Contractor.

Bridge deck is supported
with blocking and a wide
flange beam.

Late in March 2014 Pivetta Brothers Construction began work on the bridge under the supervision of Muth Consulting Engineers.  Best practices including a silt fence were established to prevent any silt, dirt, rock or pieces of wood from getting in the river. Concrete ecology blocks and timber jacking pads were situated under the damaged section. Damaged deck and curb timbers were removed along the the stringers. Bracing and bolts were removed from the pile cap. Then blocking was used to support a wide flange beam that in turn was used to support the bridge deck. This allowed the damaged pile cap (the end was lightly crushed when the tree hit) to be removed and replaced.

Two new stringers were installed
above the new pile cap.

Two stringer sections each 28 feet long, 9 inches wide and 18 inches deep were damaged by the tree strike.  These were replaced with two new Douglas fir stingers of similar (slightly wider) dimension that were pressure treated with copper naphthenate preservative. The timbers had to be drilled for indexing pins, and for bolts to tie the stringers to the deck and ballast curb.

Total working time for the trestle repair was five days and was completed before the end of March; costs exceeded $25,000.  Ideally, this would have been the extent of bridge work for 2014.  However that was not to be and will be the subject of an upcoming blog post.

Damaged section is repaired!  New ties have been installed
and the track is about to receive new ballast.  The new and
existing ballast curb is shown on either side of the deck.

A bridge not far enough!

A recent wind storm has had a devastating impact on the Northwest Railway Museum's bridge 35.  A large cottonwood tree that measures more than five feet at the base has blown over and landed on the structure.  Estimates are that the tree weighs more than 12,000 pounds and the bridge was subjected to the entire force.

While the tree was intertwined with the bridge structure, damage appeared to be minor. Unfortunately, serious damage became obvious as soon as the tree was removed. Significant damage was sustained by the outer stringers, a pile cap, and some of the deck boards that support the ballast.

Bridge 35 brings the railroad into downtown North Bend and consists of a through-pin-connected Pratt truss and two segments of conventional wood trestle structure. The Pratt truss was first erected over the Yellowstone River in Montana in 1891, and moved to North Bend in 1924. However the damaged section is constructed of timber and dates from 1923.  The last major work on this section was performed in 1964.

A bridge inspection and cost estimates are driving the repairs.  The Museum blog will feature another post while repair work is underway.

Bridging the gap

Bridges are vitally important to railroads.  They allow trains to cross rivers and gullies, swamps and streams, roads and highways, and sometimes other railroads, too.  It was evolving bridge technology that allowed steel bridges to be built in remote areas of the west, and for the train and locomotive mass to rise dramatically.

Railroad bridges are different than other kinds of bridges because they are subjected to unique dynamic forces.  An interesting historical fact is that even today railroad bridges are designed using the Coopers loading system, a system developed in 1894 and based on the loading of two consolidation-type steam locomotives. 

The Northwest Railway Museum has a collection of bridges representative of those that transformed the west.  Timber trestles, open and ballasted decks, a pin connected truss and even a voided-slab concrete span are all critical structures on the Museum's railroad.  All these structures are inspected annually by an independent railroad bridge inspector, and periodically by Museum staff.

During bridge inspections components are inspected and rated.  Elements that remain effective but show any signs of deterioration are placed on a watch list.  On each subsequent inspection, the "watch" parts can be closely examined and an informed decision can be made about when to replace a part.

To change a cap, the stringers and

bridge deck are jacked and supported

from adjacent pile caps.  This removes

the weight from the affected cap and

allows it to be changed.

During the 2013 inspection, two pile caps were identified for replacement.  Caps are large timbers that sit on top of the piles and support the stringers, the long beams that run from one pile cap to the next and support the deck.  The inspection team estimated that they each had less than two years of life remaining.  So in Fall 2013, two recycled old growth timber caps each 14 inches by 15.5 inches and 14 feet long were purchased from a dealer and installed.  Each cap took approximately 7 hours to exchange and work was completed without delay or incident.



With the weight off the cap, the old one is removed and a new one is inserted. 

Shims are inserted to make up any difference in height between the old and

new cap.  Then the weight is placed back on the cap and everything is bolted

together again.

 

Trains are running again!

The Jackson tamper was introduced in

the 1950s on the Northern Pacific

Railway and continues in a new role

at the Northwest Railway Museum.

It has been an unusually long winter in the Northwest but spring weather has finally arrived.  Track machines and crews have been working on the railroad and trains are running again between North Bend and Snoqualmie Falls.  Check out this season’s schedule here; regular trains are operating on Saturday and Sunday through the end of October.

Vibrating work heads pack ballast

(rock) under the ties.

There are many aspects to a operating a railroad that are not apparent to the casual observer. Whether historic or contemporary, railroads have a right of way to maintain and it is a significant responsibility that consumes the majority of a railroad's resources. Bridges are "big ticket" items but even the ubiquitous tie (at the Museum there are more than 3,000 per mile) are valued at more than $100 each to purchase and install.  Signals and railroad crossings also require significant resources to inspect and maintain in compliance with Federal regulations.

A hyrail excavator from RailWorks

installs a new stringer in one of the
Museum's bridges.  Each timber is
9 inches x 18 inches and can be over
30 feet long.

Over the winter, volunteer and community work crews changed more than 150 ties and tamped them, performed maintenance along the right of way including brush cutting, and performed annual, periodic and monthly maintenance on signals. Bridges were inspected and bridge maintenance performed. In addition, the locomotives were check out, oiled and fueled.  Coaches had minor repairs and were cleaned. Now the Museum is ready for an estimated 50,000 guests who are expected to ride the railroad this year.  Come up and visit this weekend and take a trip back in time!