https://www.seattletimes.com/seattle-ne ... t-repairs/

Another chapter in the often ill-fated bridges over to West Seattle happened again over two years ago, as the center span was in dire danger of collapsing. Engineers determined it could be repaired and restored to its original lifespan. Miles of post-tensioned cables have been added. The complications of doing all of that along with temporary repairs to stave off collapse make for interesting reading in this modestly technical article.

The addition of post-tensioning this late in the life of a bridge, is highly unusual, but is a good solution for this hollow bridge. The critical step is balancing of the compression movements at the piers. I see they had already reinforced the joint diaphragms in earlier modifications.

It's good that they didn't ignore the rapid crack growth, as occurred with the FIU bridge in Florida, which subsequently collapsed. Also unlike that bridge, they recognized the pre-built tensioning was inadequate and added more, instead of just trying to tighten the existing tendons. The Seattle bridge would have collapsed as well, had they tried that.

Random question, if a bridge can last 80 years at original specification, why not 120 years…

The US and Europe and Japan have a whole bunch of infrastructure (post WW2) that is nearing 80. The US also has a bunch of pipelines already going beyond 120 years. So long term maintenance of old infrastructure is a looming question.

LCDFlight wrote:

Random question, if a bridge can last 80 years at original specification, why not 120 years…

It has to do with fatigue and corrosion life of the structure, which are part of the design decisions, and a trade off with costs.

The structure is designed to withstand so many loading cycles before the materials become fatigued and can no longer support the design loads. You can compensate for that by heavier design, at greater cost. For bridges and highways, the fatigue life can be shortened by increases in traffic volume, which create a greater number of cycles at higher loading.

Similarly corrosion life is one of the design factors, but particularly in salt-air environments, structures often end up having a shortened corrosion life, such that additional protection is added during the life of the structure. Like fatigue, corrosion degrades the load-carrying ability of the structure.

For these reasons, structures are inspected and their remaining life is constantly evaluated and updated.

LCDFlight wrote:

Random question, if a bridge can last 80 years at original specification, why not 120 years…

The US and Europe and Japan have a whole bunch of infrastructure (post WW2) that is nearing 80. The US also has a bunch of pipelines already going beyond 120 years. So long term maintenance of old infrastructure is a looming question.

It basically comes down to cost. Most jurisdictions cannot justify the added cost to the taxpayers as that infrastructure might not be adequate in much less time.

I have seen some bridges in Canada that are designed for a lifespan of 100 years.

StarAC17 wrote:

LCDFlight wrote:

Random question, if a bridge can last 80 years at original specification, why not 120 years…

The US and Europe and Japan have a whole bunch of infrastructure (post WW2) that is nearing 80. The US also has a bunch of pipelines already going beyond 120 years. So long term maintenance of old infrastructure is a looming question.

It basically comes down to cost. Most jurisdictions cannot justify the added cost to the taxpayers as that infrastructure might not be adequate in much less time.

I have seen some bridges in Canada that are designed for a lifespan of 100 years.

For sure it is impressive to see a bridge last 100 yrs.

What I am hearing is, the long term cost may be lower replacing rather than repairing bridges. Golden Gate and Brooklyn Bridge might be sentimental bridges? With added cost, they can be kept up indefinitely?

LCDFlight wrote:

For sure it is impressive to see a bridge last 100 yrs.

What I am hearing is, the long term cost may be lower replacing rather than repairing bridges. Golden Gate and Brooklyn Bridge might be sentimental bridges? With added cost, they can be kept up indefinitely?

The Brooklyn Bridge is a good example of the cost trade-offs. It was built for 100 year life, but due to the large uncertainties undertaken in the project, as well as in the available design methods, it was built with a safety factor of 6.

The Roeblings who designed and built the bridge, estimated that the span was strong enough to stand on its own, without the cables, if they should fail. (Although of course it could not carry a load in that state, but it would not collapse from its own weight).

The result of that has been a far longer life than planned, with the bridge easily handling modern traffic loads. There is not really an end in sight. It's in much better shape than many newer bridges. Even without the iconic & historic significance, there is not an engineering reason to replace it.

Today, it would be impossible to fund a bridge with a safety factor of 6, or to overbuild a bridge in that way. The most you will see now is 2.5, with many smaller bridges built to 1.5, which is the minimum engineering standard.

LCDFlight wrote:

StarAC17 wrote:

LCDFlight wrote:

Random question, if a bridge can last 80 years at original specification, why not 120 years…

The US and Europe and Japan have a whole bunch of infrastructure (post WW2) that is nearing 80. The US also has a bunch of pipelines already going beyond 120 years. So long term maintenance of old infrastructure is a looming question.

It basically comes down to cost. Most jurisdictions cannot justify the added cost to the taxpayers as that infrastructure might not be adequate in much less time.

I have seen some bridges in Canada that are designed for a lifespan of 100 years.

For sure it is impressive to see a bridge last 100 yrs.

What I am hearing is, the long term cost may be lower replacing rather than repairing bridges. Golden Gate and Brooklyn Bridge might be sentimental bridges? With added cost, they can be kept up indefinitely?

I should have specified the type of bridges I was speaking to. These would be your standard freeway/river overpass that would have a factor of safety of 3-4. Not big span bridges like the Brooklyn or Golden Gate bridge. I am talking the ones you don't even notice when driving on the highway.

StarAC17 wrote:

LCDFlight wrote:

Random question, if a bridge can last 80 years at original specification, why not 120 years…

The US and Europe and Japan have a whole bunch of infrastructure (post WW2) that is nearing 80. The US also has a bunch of pipelines already going beyond 120 years. So long term maintenance of old infrastructure is a looming question.

It basically comes down to cost. Most jurisdictions cannot justify the added cost to the taxpayers as that infrastructure might not be adequate in much less time.

I have seen some bridges in Canada that are designed for a lifespan of 100 years.

Ya know what's a good bridge in Canada, is the High Level Bridge in Edmonton.

(I also enjoyed the Angus MacDonald Bridge in Halifax but it was dark out and rainy & foggy-like so I didn't really get a good look at it :/ )