Stakeholders’ Advisory Working Groups
Bridge Use Meeting #2, July 10, 2007

The Land Use SAWG meeting, on the Existing TZB Conditions topic, was held on July 10, 2007 at Crowne Plaza Hotel, White Plains, New York. View minutes of the meeting here (PDF, 38 KB).

The presentation can be viewed in the following formats:

Image of Tappan Zee Bridge and the Hudson River.

The suggested meeting topics with today’s meeting highlighted “Condition of existing Tappan Zee Bridge.”

The agenda for today’s presentation.

Recap of last presentation. The purpose of the previous presentation was to put the bridge into the context of its surroundings. This slide shows the Hudson River in yellow with the main channel in blue and green. The slide shows that main span of Tappan Zee Bridge (TZB) is located, as expected, over main channel.

A movie (not shown here) showing the highlights the five different segments of the Tappan Zee Bridge.

1. West trestle spans (Causeway);
2. West deck truss spans;
3. Main spans;
4. East deck truss spans; and
5. East trestle spans.

Cut-away image of the geology below the river bed.
The five sections of the bridge correspond to the different founding conditions. These three slides show how the bridge is part of and influenced by a larger physical environment.

Thanks to Milton Hoffman and John Messina for providing information and thanks to all attendees for their contribution to the working group.

This is the main part of the presentation and includes numerous photographs from the biennial inspections. The aim is to explain how the bridge condition is determined, that the current rate of deterioration requires significant maintenance effort and that continuous deterioration and repair cycles can be expected.

The purpose of the next few slides is to introduce "condition rating" and to show how it is used to identify trends in deterioration and plan maintenance on the bridge.
This graph shows the condition rating of the bridge against time. The horizontal axis is time from 1975 to 2004. The vertical axis is a condition rating from 1 to 7 where 7 is a new bridge, 5 is minimal deterioration 3 is serious deterioration and 1 indicates failure. A rating of 3 or 4 usually indicates that maintenance is required. Normally bridges oscillate between 3 and 5. A rating of 5 is desirable. The solid black line shows the trend in the general condition rating with a 5 rating until 1987. With repair contracts instigated by NYSTA the rating returned to the desirable 5 rating in 1994 but was again reduced to 4 in 2002. The current rating for the TZB is 4 (2006 inspection) and the NYSTA has a number of repair contracts in development to continue with the intention of bringing the overall rating back to 5. This cyclical nature of deterioration and repair is a feature of older bridges and would be expected to continue. The purple line shows the bridge rating using more precise data recorded in the inspections and its source is described in the next slides.

The reports that are the product of the biennial inspection instigated by the NYSTA. The condition rating and year of rating is indicated on the cover sheet. The inspections are usually carried out by independent engineering consultants. The NYSTA tends to vary the consultants used to get a broad range of experience and interpretation of condition.

The next few slides show inspectors gathering data on different parts of the bridge.
Inspector examining the concrete deck underneath the bridge. Inspectors must get within 2-feet of the bridge component being inspected and usually use hammers and other tools to examine the structure.

Inspectors in an Under Bridge Inspection Unit (UBIU) These units allow the inspector to gain access and examine each structural member, joint and rivet.

Inspector measuring the thickness of the steel.

Inspector inside a box girder.

Inspector examining truss beam beneath the west deck truss.

Inspector on top of the main span truss. The rust visible is only superficial. In this location the paint would be rated 3 but the structural member would be rated 5.

Inspectors turn field notes and sketches into inspection records and diagrams for each structural member and element.

Results from the inspection are summarized as a rating for each particular component on the bridge and are transferred to the inspection table (yellow on slide). Each bridge span is broken up into deck, superstructure, piers and utilities which are in turn divided into relevant subcategories. This table provides a comprehensive report on the condition of the bridge and provides a detailed overview.

This table can be used in different ways; for instance,  isolating certain spans or looking at the rating of specific element. Each row represents a different span of the bridge. Each column represents a different component of the bridge. As part of the inspection all 4000 entries are filled by the inspectors at the end of the inspection based on the details of the inspection.

The records from past inspections are available. The rating of any group of elements or spans can be examined over time to get a feel for the rate of deterioration. Identifying and understanding trends is a useful tool for the NYSTA to plan maintenance.

The purple trend line on the graph is the average rating of all the 4000 entries in the inspection records for different years, showing a deterioration and repair cycle that lasted for 20 years between 1980 and 2000. As will be seen later in the presentation the NYSTA instigated repair contracts in the mid-1980»s and it took 15 years to bring the condition rating back to almost 5 in the year 2000. Since 2000 it is evident that a further deterioration cycle has commenced and a further series of repair contracts has been planned by NYSTA.

The overall bridge rating for the past 30 years for each of the five bridge sections: The East Trestle, East Deck Truss, Main Span, West Deck Truss and the Causeway. From the graphs it is possible to see local trends for each of the segments. At various times the causeway spans, and deck truss spans had the lowest rating. Those areas on the graphs that show an improvement in condition correspond to periods of repair by the NYSTA. Repair periods follow closely after periods of deterioration.

The bridge rating divided into components: The Deck, Superstructure and Piers. These records show notable deck deteriorated between 1982 and 1990 despite maintenance in the mid 1990s. NYSTA is currently implementing another major deck replacement contract. Both the superstructure and piers are shown as deteriorating in the 1990´s requiring substantial repair contracts to return the overall ratings to 5.

What does the thruway do with this data?
A list of all the contracts on the bridge, the majority of them maintenance and repairs, between 1970 and today. The number and value associated with the contracts is increasing.

The cumulative cost of all expenditure on the bridge from original construction to 2004 (in 2004 dollars). Since the early 1980´s expenditure on the bridge has been increasing. Further contracts not included in this graph are in development by the NYSTA.

Agenda Item 2 - A list of the individual  bridge components: Deck, superstructure, piers and foundations.

This section is part two of Agenda Item 2 – A detailed look at the condition of the individual components of the bridge. These four areas are: Deck, Superstructure, Piers and Foundations.
This slide shows the bridge deck from above and is the first of the four individual components.

Video (not shown here) of deck condition from above and below showing potential defects in color, showing the extent of the deterioration on the deck across the whole crossing.

The following 9 slides show photos of the deck from below where cracking and other defects have been noted by the inspectors. On this slide a cracked area of concrete has been circled and labeled ‘H’ by the inspector. ‘H’ indicates that the concrete sounds hollow when hit by the inspector with a hammer.

Map cracking is where the cracks show the reinforcement pattern indicating potential corrosion of the rebar.

A leak through a deck joint that has caused corrosion of the steel beam below.

Water seepage through the deck.
This type of cracking is caused by water ingress into small cracks followed by a freeze thaw cycle.

Water seepage through the concrete deck and visible corrosion of the steel girder and steel reinforcement.
The question here is about what can’t be seen: the Girder is made up of multiple plates and interfaces typical to construction in the 50’s. Many of these are hidden and could be corroding out of sight. Inspection and maintenance of these hidden areas is difficult and time consuming. In modern construction girders would tend to be manufactured in one piece eliminating the interfaces and the associated risks.

Seepage close to the diaphragm beam which represents a stiff point under the deck.
This area tends to be pounded by the wheels of trucks on the deck above.

Eventually the combination of cracking, seepage and corrosion will lead to a punch-through (hole in the deck).

A punch through seen from above the deck.
The punch through is next to a deck joint which is a stiff point as described above and also a source of water leakage as the seal between the decks has a short life span. Water leaking through the joint will sit on the steel beams and drip on to the surrounding members. Deck joints on the existing Tappan Zee Bridge are an inherent weakness of the bridge and have to be replaced about every 10 years.

A bottom view of the 1995 replacement deck.
There are signs of leakage and corrosion on the new deck – The repairs need repairs. The point of this slide is to show that due to weaknesses and vulnerabilities inherent in the design of the existing bridge the repair cycle can be very short.

How do you get the overall view of the deck condition shown on the video? The next few slides will describe how the deck is rated.
The sketch on this slide is the inspector’s report on the concrete condition on the underside of the deck (the top is not accessible for inspection). This section of deck is for the 50’ long and 92’ wide section between piers 31 and 32. There are nearly 200 spans altogether.

The inspector’s sketch is shown color coded: Map cracking (dark green), hollow concrete (light green) and spalling Concrete (yellow). The worse condition is where the concrete is spalling; this tends to occur under the drains on either side of the deck or near deck joints or historic openings.

The inspection results are available over several years showing the progression of the deterioration. The marked deterioration between 2000 and 2004 is very clear.

The previous slides only show defects on the soffit of the deck. To determine if there were other defects present the NYSTA commissioned a special radar inspection to look through the deck. The deck surface is inspected by means of a Ground Penetrating Radar (GPR). Ground penetrating radar only gives a very indirect appreciation of the concrete condition.

The results of the ground penetrating radar.
Purple indicates deck areas where there was a notable discontinuity in the radar signal – an indication of possible area of concern. Pink areas indicate areas where there was a major discontinuity in the radar signal – an indication of areas likely to have more significant defects.

A compilation of the inspection results from the bottom of the deck with the GPR results from the top of the deck and giving a full representation of the condition of the deck.
There was some correlation in the data showing bad deterioration at either side of the deck where the bridge drains are located.

Concrete cores through the deck were taken at the locations indicated in pink in the previous slide to determine if the predictions were actually indicative of deck deterioration. As can be seen the concrete in this core has crack near the top. The presence of many cracks in all the sample cores taken provided some confidence in the GPR survey results. The cracked section is a significant issue as this affects the capacity of the deck to carry cars and trucks.

A view into the deck after the concrete core was taken out. In this area the crack within the depth of the deck was clearly visible.

Deck cracks lead to punch through failure.
An area of the deck being repaired.
The repair involves enlarging the hole back to sound concrete, cleaning of the steel reinforcement and replacement with new concrete.

A video (not shown here) showing the deck condition.
90% of the pink areas from the radar inspection occur in the outside 2 lanes (1000 total). There is a current Thruway contract to replace the 2 outside lanes of the deck in both directions. Once this contract is completed the remaining center lanes are scheduled for replacement.

Deck replacement during night shift. Limited access and time for construction make deck replacement a slow laborious process.

The number of deck punch-through failures in each lane over a period of 18 months. 44 of the 45 punch through failures occurred in the outside two lanes.

The structural steel of the bridge.
Having looked at the bridge deck in some detail in this next section we will look at the steelwork, which comprises of primary steel (members that do the work and hold up the deck, and secondary steel (members that hold the primary steel in place).

View of the steel underneath the deck of East Deck Truss Spans.

View of steel underneath deck of East Deck Truss Spans.

A detail of steel underneath deck of West Deck Truss Spans.
The purpose of this slide is to show the complexity of the steelwork.

View under the deck of the main span.

Under the main spans showing the track for the maintenance platform that hangs below the bridge.

The following slides look at the condition of the primary steel horizontal members. The members are divided in to the following categories: Horizontal members, Vertical and inclined members, Connections, Connectors and Key Details.

Corrosion of a horizontal member.
In this case the corrosion is superficial, the paint would get a low rating (e.g. 3) and the steel would get a rating of 5. This probably occurred in the 2 years since the last inspection illustrating the rapid rate of deterioration. The rapid corrosion on the horizontal members is caused by water (mixed with salt) dripping down from the deck above and collecting on the horizontal surface.

Corrosion affects structural steel with potential reduction in the steel thickness. Loss of thickness means that the capacity of the structural member is reduced. This does not mean that the member will fail but that the member has a reduced factor of safety. To determine if the loss of thickness of the bridge member is significant the inspectors will conduct a structural analysis of the bridge.

Rust pitting and reduced thickness of steel on a horizontal member.
Repair of this section might be anticipated but the pitting is adjacent to a hole in the steel that was part of the original construction that may indicate that the pitting is acceptable. The inspectors’ knowledge of how the steel functions within the overall bridge will lead to a decision to repair as a priority or as part of repairs to be conducted at a later date. These holes in the steel members that were part of the original construction are a vulnerability of the bridge as ware is can easily get inside the steel members.

A more serious loss of section.

Stagnant water trapped inside a box girder.
Corrosion is very visible. Locations such as these are vulnerabilities inherent to the design and construction methods used on the existing bridge.

A loss of thickness on the lower plate of a horizontal member.

A loss of a section – complete hole in the plate.

Significant loss of section on the lower plate of a horizontal steel member. Salt water gathered on the horizontal section from wind and spray and has resulted in the full loss of the steel thickness.

A repair at another location that had similar corrosion to that on the previous slides.

The following slides look at the condition of the primary steel vertical and inclined member.

Surface rust on an inclined member.

Minor pitting is visible on this inclined member with a reduction in the steel thickness.
This reduction in thickness reduces the safety factor for the member but may be acceptable based on analysis of the bridge. In those places where reduction in thickness is acceptable no repair is warranted but the reduction in thickness may limit the ability of the bridge to be expanded possibly to carry additional transit or shoulders as this would increase the forces on the member.

Plate buckling due to corrosion.

Plate buckling due to corrosion.
Water ingress was causing rusting between the individual plates that made up the steel member. This again illustrates the problem of corrosion that can»t be initially seen. This section had to be opened in order to determine which plates needed to be replaced. Repair to any primary member must be done with extreme caution. The member can not be taken out and replaced with another as all primary members are required to carry the loads on the bridge. Instead plates must be replaced individually.

Corrosion of plate.
The underneath of the bridge deck is visible. This member is exposed to dripping and spray from the roadway and from spray from the river.

The inspector has highlighted orange areas where there is notable reduction in steel thickness. As part of the inspection the inspector will determine if this loss of steel section is structurally significant and the Thruway will determine if repair is warranted in the near term.

Buckling of local steel members due to rusting of interface is clearly visible.

Inspector notes 50% section loss.

The following slides look at the condition connections between all the primary members. Connections are usual made up of a series of individual steel plates specially shaped to connect members.

Corrosion on a connection.
Three primary members are connected to one plate. Only the external plate in this connection is shown. There are many other plates on the other side of the main members and at intermediate locations that are not shown but that must be inspected.

Red circles show areas where there is a loss of steel thickness. Note number of locations the inspector has highlighted.

The interface between the plates is another area to be inspected. At the interface between the steel plates shown local rusting is evident. The number of interfaces between steel plates is substantial with the potential need for much repair in the future. The presence of these kinds of details that are prone to deterioration is another vulnerability of the TZB. A vulnerability is an area of potential weakness or a specific detail on a bridge whose presence may be indicative of the need for future repair and expenditure. The extent of vulnerabilities can be a useful measure for the Thruway to establish priorities and to predict the extent of future repairs and allocated budget accordingly.

Corrosion at the interface between plates.

More corrosion at the interface between plates.

Corrosion at the interface between plates has become unacceptable and was repaired.

Corrosion in this key location on the main span was not immediately visible. Because the extent and structural significance of the corrosion was difficult to establish a temporary support was introduced until the steel was repaired.

An example of corrosion inside a pin connection.

The following slides look at the condition of the primary steel connectors (rivets, bolts and welds).

Not only do the interfaces and connections need to be thoroughly inspected but the rivets, nuts, bolts and welds do too. As seen in this example of a connection plate some rivets which hold the steel plates together, in the area outlined in red, have become loose. These rivets hold the steel plates together.

Bolt securing truss to concrete pier shows a significant loss of steel. In this image the thickness of the steel on one side of the bolt is much smaller that that on the other side.

A loss of thickness in the steel plates and also in the circular bar.

Failure of a weld joining two pieces of steel.

The following slides look at key areas of the bridge.

This connection shows a gap between plates on the bottom left hand side. The next slide shows a close up view. The gap is only on one side of the connection.

A close up view of the gap between the plates.
The gap was not present when the bridge was constructed. The gap is indicative of movement of the suspended spans of the bridge. The bridge is constantly moving and shifting due to temperature, foundation movements and external forces such as wind. The bridge also moves slightly under each vehicle that crosses it.

One of the “seating stools” that joins the deck truss to the road stringers above.
The top plate of the stool is twisted even though it was level when constructed. The stool is being moved by the deck above and the truss below which move separately in response to truck loading and temperature. The stool does not allow for this relative movement and will eventually fail. On all bridges there is a need to allow the bridge to move slightly in response to various loads. Typically this movement allowance is provided by bearings.

Another example of a seating stool where movements have been present.
The original plates have been replaced and a repair using bolts has been previously installed. As can be seen one of the new bolts has been sheared (broken). The bridge movement was enough to break the bolt. Future repair of this detail may need to allow for the movement of the bridge rather than try and prevent the movement.

A member on the top of the main span.
This member was straight when the bridge was originally constructed. The inspector is measuring how far the member has moved from the original straight line. This is another example of how the bridge moves under the action of the forces applied. These movements are not a major concern but rather indicate that the bridge is working to carry to the applied loads.

The following slides look at the condition of the secondary steel members and the fascia members.
Secondary steel is best defined as the steel members that do not carry the main load. Rather this steel is present for example to hold the primary steel in position during construction or is present to frame a joint or a drain.
The fascia beam is the edge member along the outside of the bridge. It is the member that we see on the edge as we drive along the bridge.

Video (not shown here) showing primary (grey) and secondary (red) steel members.

View of fascia steel beam showing rust.

Some of the secondary steel members behind the fascia beam.
Significant corrosion of the secondary steel that supports the edge drainage and sidewalk is apparent. This degree of corrosion is significant but because the beam does not carry the primary loads of the bridge there is no danger. The cause of this deterioration is the road salts that are spread on to the deck during cold periods. These road salts have been washed through the drains and on to the beams shown.

An example of a repaired sidewalk beam.
Note that since the repair further corrosion of the bridge has occurred and further repair is required.

Steel corrosion behind the fascia beam at the deck truss spans.
The presence of bolts indicates that this area has been previously repaired. Further repair may be warranted when the rust is cleaned and the reduction in the steel work thickness is determined. A constant theme on the TZB is the need to repair previous repairs.

The area behind the edge member on the main spans.
Corrosion under a drain opening is present.

An example of the drainage outlet at the edge of the road.
This open drainage is responsible for much of the steelwork corrosion on the bridge and is considered a vulnerability.

The opening frame of a movie of the main spans of the TZB (movie not shown here). The movie zooms into the detail at the edge of the bridge similar to that shown on the main span.

The path for water draining off the road deck and through the open drains.

The edge of the deck truss spans particularly the area behind the fascia beam.
Drainage from the deck and through the open drains falls through the steelwork where the wind blows the water and road salts on to the steelwork.

The next few slides look at a structural problem associated with the 200 deck joints.
The road joints are a source of water leaks on to the steelwork below. Though the NYSTA continues to replace and upgrade the joints the leakage since construction has had some notable affects on the bridge components below the deck including the piers and bearings which are discussed later in this presentation.

View of the deck joint at the intersection of Westchester and Rockland Counties.

A view of one of secondary steel members (known as a diaphragm) below one of the joints in the causeway structure.
This member has a long crack in the diaphragm that is approximately six-feet long. This defect was a surprise to the inspectors as there were no visible signs of corrosion.

The location of diaphragm beam shown in the previous slide. The diaphragm beam is in red (secondary steel member). The member is directly below the joint on the deck. There are two diaphragm beams side by side – one on each of the joints. Inspectors could not inspect the area between the two beams because the area was full of concrete.

Another view of the cracked beam.
The crack was caused by corrosion behind the beam in the area that could not be inspected. The corrosion wasn’t visible until the web cracked. This type of defect was found in many places and is another vulnerability inherent in the TZB structure. Having looked at all the different things happening to the steel work, these next few slides look at the overall trends in the condition of the steelwork.

Having looked at all the different things happening to the steel work this next few slides look at the overall trends in the condition of the steelwork.
A graph of the condition rating for the primary steelwork over last 30 years for the five different bridge segments.
The graph shows an overall deterioration in rating to a low point of 3 in the early 1990s for the east and west deck truss spans and the main spans. A repair program instigated by NYSTA through the 1980’s and 1990’s brought the back to 5 by the year 2000. Since 2000 further deterioration observed in the latest inspection has reduced the condition rating some segments and the NYSTA has again instigated further repair contracts. This cyclical nature of deterioration and repair is expected to continue and the NYSTA is budgeting accordingly.

The secondary steelwork condition rating for the last 30 years for the five different bridge segments. The trends shown are similar to the primary steel with low ratings in the 1980’s and many repair periods. One notable point on the graph is the 2004 condition rating of the main spans. The rating was notably lower (at 4) than the previous inspection in 2002 (at about 4.8). This trend was observed by NYSTA who again instigated a repair program.

The condition rating for the sidewalks and fascia beam for the past 30 years. The trend shown is notably different from the primary and secondary steelwork shown in the previous slides. As seen on this graph the condition rating for the sidewalk and fascia reduced from 5 (acceptable) to 3 and below (serious deterioration). Though some repairs were conducted to these bridge components the overall rating has not change much since 1990. In the 1990’s the NYSTA concentrated on improving the condition rating for the most important components of the bridge – the primary and secondary steelwork. Priority was not given to the sidewalks and fascia beams. Since the 2004 inspection, the NYSTA has implemented a major repair program for the fascia members and a notable improvement in condition is anticipated in the next bridge inspection.

Graph of painting contracts on the 5 bridge segments as red dots over the last 30 years.
The graph shows that the painting has been a constant activity usually focused on areas with the greatest deterioration.

The next slides will look at the next component of the bridge: The bridge piers under the causeway, the deck truss and the supporting bearings.

The concrete piers below the causeway deck.
Leaking of water through the joint above the pier is evident. Water is dripping on to the pier at a number of locations across the full width of the bridge. The water would likely contain road salts in the winter whose chemical constituents would instigate corrosion in the steel reinforcement inside the concrete. Expansion of the reinforcement as it rusts would cause cranking in the concrete which would eventually spall.

Concrete repairs made by the NYSTA on concrete in the piers that had deteriorated.

Two photographs of the same column in one of the piers in the causeway spans.
The photo on the left shows a serious crack in a concrete column at the top under a drainage outlet. The photo on the right shows the bracing that was installed during the inspection by the NYSTA maintenance crew prior to a full repair. Repair of this defect would occupy a major proportion of the NYSTA maintenance crew for 2 weeks. Because the time required to make repairs can be substantial, the NYSTA typically combines many repairs into sizeable repair contracts using specialist contractors that supplement the NYSTA maintenance crew.

A crack in the concrete pier below the bearing.
This concrete was repaired in 1995 but has cracked again. The point of this slide is to show that repair of repairs is often required. In this case the cause was unwarranted reaction between the new and old concrete compounded by the water leakage from the deck above.

Another pier column where new cracks were observed in previous recent repairs.

Another crack in a repaired piece of concrete. This type of crack was found in over 50 piers.

All the slides of the piers so far have focused on the top of the piers on the causeway where defects are caused by the water runoff from the deck through the joints and the edge drainage. But there is also deterioration at the base of the columns in some piers. To establish the source of this crack the concrete around the crack was removed to determine how far the crack penetrated through.

The area of the concrete that was removed.
See the next slide for a close up of the exposed concrete.

A close up of the exposed concrete.
As seen at the top of the opening the crack continues to through the concrete to the depth of the steel reinforcement and in particular the crack is seen to start at the reinforcement bar on the left. The reinforcement bar on the left is corroding and is the source of the concrete cracking. As steel rusts the result is an expansion of the area of the reinforcement. This expansion pushes the concrete away from the bar and results in cracking of the concrete.
Why is the reinforcement corroding? The Hudson River water is saline and constitutes a fairly aggressive environment for reinforced concrete. The base of the pier is exposed to the tidal drying and wetting cycles as well as the freeze thaw cycles in winter. This saline environment and the actions of freeze-thaw has probably allowed water to penetrate to the reinforcement and initiated corrosion.
The point of this slide is to highlight that the concrete and steel in the piers is deteriorating but for different reasons. One positive observation from the exposed concrete is the large distance from the reinforcement to the outside face of the concrete. This distance, known as the reinforcement cover, is high and therefore is a good protective layer for the reinforcement.

A concrete pier supporting one of the deck truss spans.
The image shows large areas of repair on the face of the concrete. These were repairs of cracks in the concrete observed during inspections in the 1990’s. The source of the cracking was similar to that at the top of the causeway piers.

A close up of the top of one of the piers of the deck truss spans. The racks in this pier are still present and are widespread.

Another view of the repairs conducted on one of the piers of the deck truss spans. The extent of repairs is an indication of the extent of the previous cracking.

Similar to the causeway piers this slide shows cracking of the concrete at the base of one of the piers supporting the deck truss spans.

Arrows point to the bearings under between the deck and the piers in the causeway spans. There are 15 bearings per pier and 166 piers with bearings.

One of the bearings on the causeway spans. The bearings are located directly under the joints in the road deck of the causeway span and are subject to the water and road salt leakage mentioned previously. It was expected therefore that the bearings will show signs of deterioration. A crack in the concrete below the bearing is evident.

The concrete around the bearing has substantially spalled and is missing. A loss of the material around the bearing could lead to a loss of support for the deck above.

This image shows another bearing with corrosion evident under the bearing.
The bearing is tilted and concrete is missing.
Note the concrete underneath the bearing looks in good condition. This concrete had been repaired in a previous recent contract.

Corrosion around the bearing had resulted in failure of the holding down bolts. These bolts hold the deck steelwork in position to stop the bridge moving off of its bearings.

Graph of the trend in the condition of the piers over the last 30 years for each of the 5 bridge segments.
The graph shows how maintenance contracts over 15 years brought piers back up to a good condition rating. Similar to the previous graphs notable deterioration was first recorded in the inspection in the 1980’s which triggered repair contracts by the NYSTA that returned the bridge to good condition (a 5 rating) by the year 2000. Since 2000 the condition of some segments has again deteriorated and the NYSTA may need to instigate another repair contract for the piers in the near future.

The next slides will look at the bridge foundations.
As the majority of the foundations are not visible, therefore no condition rating is possible. However, we can see the pilecaps and some of the timber pilings.

Pilecaps under the causeway spans.
At low tide the timber piles under the causeway pilecaps are just visible.

The same area of under the causeway near full tide. The water level covers the pilecaps. The top of the timber piles are subjected to wet and dry conditions on a daily basis with the tidal flow of the river.

The top of one of the pilecaps.
The concrete of the pile cap is somewhat damaged however no rebar is exposed

The side of one of the pilecaps.
Cracking of the concrete is visible and may be a cause for concern. The timber pile shown is not a structural pile but nevertheless shows the type of deterioration possible for timber piles in the 50 years since the bridge was constructed.

Another view of the same pilecap showing the cracking and deteriorating timber pile.

This image was taken by a diver sent into the river to look closely at the timber piles. In this view some of the exterior piles are visible at low tide The timber piles are showing their age (barnacles and holes are visible) however they are in relatively good condition.

A close up of the timber piles below the concrete pile cap.
The timber piles were tested for presence of marine borers that would indicate potential deterioration. No evidence of marine borers was found though they have been found on similar timber piles nearby elsewhere on the Hudson River. Marine borers are a major concern of the NYSTA and testing is continuous conducted for their presence. As the Hudson River water quality improves, the extent of marine borers is expected to increase. Because of the potential future implications of marine borers on the timber piles the timber piles are categorized as another vulnerability of the TZB.

A close up of one of the timber piles under water The pile shown is split with a gap in the center. This split probably occurred during construction and was not a widespread defect for those piles could be seen.

The cofferdam foundations of the west deck truss spans.
Two separate foundations are shown, one corresponding to each column in the pier.

All that can be seen of the foundations under the deck truss spans.
When the foundations were constructed they were surrounded in steel. This steel encasing has rusted and as either been removed or has fallen away since construction. The foundations appear in good condition.

One of the foundations that still has the casing of steel sheet piles.
Some degrading of the surface concrete is visible.

In this close up of one of the foundation of the deck truss spans, some spalling of the concrete is visible.

A winter view of the piers and foundation of the main span.
The yellow and black ship protection around the floating foundations is visible. The floating caissons are below water level and cannot be seen in this view. The buoyancy provided by the floating foundations support up to two-thirds of the weight of the main span. The rest of the weight is carried by steel piles below the floating caissons.

A closer view of the main spans foundations and shows the concrete plinths below the steel towers. Note the absence of the yellow and black shipping protection as this photograph was taking before the protection was constructed.
The NYSTA upgraded the ship protection around the floating caissons to reduce the risk of possible damage to caissons from shipping. On the tower on the right it is possible to see a small building which is the access to the floating caissons.

A recent image of the plinths at the base of the steel towers, showing the access building and gangway. Some cranking in the concrete of the plinth is visible.

A close up view of the outside of the plinth above the floating caissons. Some cracking in the plinth is evident.

The concrete on the top of the plinth.
Again some cracking is visible and the concrete is stained red from the rust that has fallen from the steel on the bridge above.

A view of a corner inside the floating caisson.
Inside the caisson there is very little water and they are generally dry. As seen here there are some cracks that allow water to seep in. The spots on the top in this view are water droplets formed from condensation.
In general the caissons are in good condition. The caissons are typically 40-feet deep and embedded into the river bed so that it is not possible to see the piles below the floating caissons.

Another view inside the floating caissons and another area of water ingress.

A view of pumping equipment inside floating caisson.
The pumps only need to operate at a low rate. No evidence of increased water ingress has been noted. There are water marks inside the caissons at some height. It is assumed that these are historic watermarks from the original construction as there are no records that indicate that the caissons flooded during there service life.

This view inside the caisson shows another view of pumping equipment inside.

A summary of agenda item 2.2 which was a detailed view of the condition of the various bridge segments and components.
What does all this mean? All that was presented in the previous slides is very important for the rehabilitation alternative. It is also important to take into account all of the above in the design of a new bridge.

Agenda Item 3 - Traffic accidents on the TZB are another vulnerability of the existing bridge.
The increasing number of incidents and the repercussions on bridge traffic are linked to the way the bridge is designed and to the high traffic volumes. This section aims to provide an overview of the cause and location of the accidents on the bridge.

An incident on the bridge near the Tarrytown shore. A car is on fire and is being extinguished by the fire department.

The aftermath of a traffic accident involving a car and a van on the causeway.
Emergency services and tow trucks are hampered in their ability to respond rapidly to incidents due to the lack of shoulders. Because of the limited width of the bridge some lanes are narrowed than the full 12-feet width desirable. The minimum lane width of the bridge is 11 feet and 5 inches.

A SUV has gone over the side of the bridge at the causeway spans. The SUV is being lifted out of the river and placed on the NYSTA maintenance barge.

A truck that lost its load on the bridge.
Trucks are involved in 20 to 30% of all accidents on the bridge. Trucks representing only 6-7% of vehicular traffic over the bridge.

The result of an accident between a car and a light van.
The narrow lanes and absence of shoulders leave little room for error by drivers.

A major car/truck accident on the bridge that included a fatality.

A recent accident on the bridge in July 2007 that again involved a car and a truck that also resulted in a fatality. Because of the fire the structural integrity of the bridge was in question and the bridge was closed for 9-hours while a full inspection and repairs were conducted.

Summary of some general data for a typical three-year period.
The NYSTA maintenance staff report several incidents a day on the bridge that have an impact on traffic.

Where accidents occur on the bridge.
The toll plaza has the highest number as might be expected due to the lane changes and toll booths. These accidents tend to be minor. The other locations are where there are changes in the roadway grade – at the top and bottom of the hill near the middle of the bridge. A the change in grade drivers may not compensate for the change in speed that results. Because traffic is following too close behind accidents can occur. These types of accidents are present at all changes in grade throughout the Thruway system. But because of the narrow lanes and lack of shoulders there is little room to compensate for the typical conditions.

The cause of accidents as recorded by the state troopers that patrol the bridge (Troop T).
The majority of the accidents are associated with speeding, following too closely and unsafe lane change. These are also the cause of the majority of the accidents throughout the whole of the Thruway. Note there are a significant number of accidents associated with debris which is not typical of the whole Thruway.

The suggested topics for the next meetings of the bridge group.
The next meeting will be the third on the existing bridge focused on preservation and rehabilitation. It will address the seismic capacity and the capacity to carry extra load such as transit.