Analysis sheds new light on possibility of West Seattle Bridge repairs.

Photo by Chun Kwan

Analysis sheds new light on possibility of West Seattle High-Rise Bridge repairs. We continue to assess how much repairs would cost, how long they would take, and whether they would last long enough to be a worthwhile investment.

New analysis indicates that repairing the West Seattle High-Rise Bridge may be possible, but we still do not know how long any repairs would last or whether useful repairs allowing us to reopen the bridge, even partially, for the time it would take to plan, design and construct a replacement would be feasible.  

The West Seattle High-Rise Bridge is a complex structure that uses a post-tensioned segmental box girder system to span between four sets of columns. In simple terms, there are two structural systems that work together:  

  • The concrete structure, which is visible to the naked eye 
  • An internal steel structural system, called a “post-tensioning system,” contained within the concrete

Over the past few months, we have been hard at work conducting more than 100 scientific tests to analyze the structural stability of the West Seattle High-Rise Bridge. The tests completed so far have not found indications of problems with the post-tensioning system, comprising the steel strands running through the structure like a skeleton. Problems with this system would mean repairs would be much less likely to succeed.  

We are continuing to analyze how long repairs would take, how much they would cost, whether or not repairs would allow traffic to return to previous levels, and how long and in what capacity the bridge could remain open after potential repairs were completed so that we can tell whether or not fixing the bridge is a worthwhile investment.  

We have also installed an intelligent monitoring system to track movement and growth of cracks in the bridge. While cracks in the bridge have slowed, they have not stopped growing entirely and we are continuing to plan for every possibility.  

We are keeping all options open and are still moving forward with our search for a team to design a replacement for the bridge in case repairs are not a feasible option. Meanwhile, we have begun assembling our construction equipment to stabilize the bridge, which will be a necessary step in every possible scenario.  

The full analysis of the structural stability of the bridge should be complete in early July. We are currently working on a cost-benefit analysis to answer bigger questions about the trade offs of repairing or replacing the bridge. This will be an iterative process, but we expect to have a much fuller picture of the options in fall 2020.  

Photo of West Seattle High-Rise Bridge being built
West Seattle High-Rise Bridge being constructed. Photo courtesy of Jack Scott USACE

Here is a look at how the steel post-tension system works and the types of scientific tests we performed to analyze the structural stability of it.  

All modern concrete bridges have steel reinforcements that increase strength and control cracking. Smaller conventional concrete bridges often use steel reinforcing bars, or rebar. Longer-span bridges require something in addition to rebar to prevent cracking, which is called prestressing.  

These “Prestressed” bridges use high-strength cables to compress the concrete, so that the bridge can hold a heavier load. It is called prestressing because it compresses the concrete prior to applying the weight of vehicle traffic to the concrete. The steel cables used to accomplish the prestressing are installed and stressed prior to pouring the concrete or installed and stressed after the concrete has been poured and hardened. The former is called pre-tensioning and the latter is called post-tensioning.  

Post-tension ducts with steel post-tension tendons running through them that were used on the West Seattle High-Rise Bridge.
Post-tension ducts with steel post-tension tendons running through them that were used on the West Seattle High-Rise Bridge. Photo courtesy of Jack Scott USACE

This post-tensioning method was used when building the West Seattle High-Rise Bridge. 

Post-tensioning is commonly used for bridges that are cast-in-place (where the concrete is poured on-site) because the steel tendons can be used to hold the whole structure together.  

In a post-tensioned system, like the West Seattle High-Rise Bridge, anchorage points and hollow tube-like ducts are placed in the concrete casting bed before concrete is poured. The steel tendons are run through the hollow ducts and then pulled tight at the anchor points once the concrete has cured. This type of tension system is especially useful when connecting multiple concrete segments in a segmental box girder bridge like the West Seattle High-Rise Bridge.  

Post-tension ducts running through the West Seattle High-Rise Bridge as the bridge was being constructed. The ducts do not yet have many post-tension steel tendons running through them. Photo courtesy of Jack Scott USACE

Because the steel is introduced to the structure through hollow ducts rather than implanted into the concrete itself, it is not bonded to the bridge or protected from the elements. To make sure that post-tensioned steel tendons are protected, grout is poured through the ducts after the steel has been pulled tight and anchored in place. This bonds the post-tensioning system to the bridge and protects the steel from corrosion. 

Sometimes, the grouting of the ducts is not entirely successful during construction. This can leave the steel tendons exposed to water, and they can corrode faster than if they were completely encased in grout as intended, which can harm the structural integrity of the bridge and weaken the structure. This is especially true in bridges near saltwater. Cracks that extend down to the ducts can also expose the tendons and makes it easier for the elements to corrode the steel. 

Crew member suspended by ropes and safety harnesses descended from the edge of the West Seattle High-Rise bridge to drill precision holes in the concrete and collect core samples.
Crew member suspended by ropes and safety harnesses descended from the edge of the West Seattle High-Rise bridge to drill precision holes in the concrete and collect core samples.

To better determine the West Seattle Bridge’s structural integrity, we needed to look beyond the visible cracking to the steel tendons deep inside the bridge. 

We did this through non-destructive tests where we used different methods to examine the bridge’s materials and components without destroying or changing their usefulness. This allows us to locate and identify flaws and defects that are not visible on the surface. To do this, we used ultrasonic technologies like ground-penetrating radar, MIRA ultrasonic tomography, and impact echo. Each of these methods use sound and light waves to look for voids beneath the surface. Subtle differences in the methods are described in the images below.  

We used these tests to look for voids in the grout that might indicate that the steel has been exposed to water and is corroding. When these initial tests showed evidence that there might be a void, we took a small core sample from the bridge to test whether or not there really is a void.  

On the West Seattle High-Rise Bridge, we performed more than 140 scanning tests and took core samples in 14 different locations.  

While test results are not yet finalized, we found only one suspected void in a post-tensioned tendon, but the strand appeared to be in good condition. When we took a core sample to confirm, we found that the grout was still in place all the way around the tendons, so there was no evidence of corrosion.  

In each location where we took a core sample, we also sent grout samples to a lab and sprayed with an acidic mist of sodium-chloride solution to create a highly corrosive “salt fog” to determine how resistant the concrete is to corrosion. This tests whether there are higher-than-normal levels of calcium chloride which would indicate that the steel at greater risk of corrosion.  

All samples came back negative for corrosion. While test results are not yet complete, results so far have indicated that highest calcium chloride concentration we found was similar to the amount that would naturally occur in concrete, so the results show that post-tensioning steel corrosion and deterioration is not a problem, and therefore unlikely to be contributing to cracking. 

In addition to using non-destructive tests to examine post-tension steel tendons, we used a variety of methods to measure horizontal cracks. 

While we have a monitoring and instrumentation system that monitors surface level crack growth and width, we need to use non-destructive tests to measure existing crack depths. We used these tools to investigate depth for cracks between the deck and the web, and in the middle of the web. These are not the cracks that led us to close the bridge, some of which are full thickness cracks,  but have been observed as we continue our inspection and monitoring. 

The testing on the West Seattle High-Rise Bridge has indicated that the average depth of these cracks is in the shallow range (less than 5”), extending down primarily to just the first mat of reinforcing steel (rebar). Had these cracks extended through the thickness of the web walls, these cracks could also have undermined the structural integrity of the bridge.  

Crack depth was primarily measured through a process called Ultrasonic Pulse Velocity (UPV) testing. There are several methods for detecting cracks by sending and receiving sound energy waves with a “transducer”. Variations in wave reflections indicate the presence of a crack by identifying the change in timing when the sound wave returns, and the wave signal is then displayed on a screen for the engineers to read and interpret. To measure the depth of the cracks, a more sophisticated method was employed, with two transducers placed on each side of the visible crack. UPV tests were performed with the transducers placed at two different distances. We determined crack depth by examining the travel time – this told us the distance the sound wave traveled. In a few cases, small holes were drilled into the cracks to visually verify the results of the UPV. Image by WSP.

The good news is that preliminary non-destructive testing results indicate that the observed cracking that led us to close the bridge are the primary source of deterioration. We have not identified any other significant defects, such as corrosion in the post-tensioning or voids in the grout.  

Though these preliminary results indicate that repair may be possible, it is still vital that the bridge remain closed. The cracks that led us to close the bridge are still growing, though at a slower rate, and there is risk of further degradation, and even failure, if traffic is returned now.   

We are finishing up the comprehensive analysis of the West Seattle High-Rise Bridge’s structural condition, which we expect to complete in the near future.  

Then, we will know more if repairs to reopen the bridge are technically feasible. From there, it will then be a decision as to whether or not repair is advisable, when compared to other replacement options. 

A cost-benefit analysis is already well underway, an effort fed and informed by the critical data we’ve been collecting for weeks now. This will help us and our Technical Advisory Panel (TAP) better understand the tradeoffs of repairs compared to several replacement options. This will also allow the TAP and our team to assess potential repair techniques, construction options, and sequencing. All the while, we are ensuring all pieces are in place for a rapid pivot, and no time lost, if a decision is made to commit to the replacement pathway. We will continue to work closely with elected leaders, partner agencies, the Technical Advisory Panel, and the Community Task Force on all major decisions about the future of the bridge. 

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