Institute for Transportation

Go! Magazine

posted on June 18, 2015

Love locks are a symbol of love and commitment. Inspired by an ancient custom, which is believed to have originated in China, lovers lock a padlock on a chain or gate and then throw away the key—symbolically locking their love forever.

A historic pedestrian bridge in Paris, called the Pont des Arts (also known as the first love-lock bridge), has become a popular destination for couples to place their love lock and then throw the key into the Seine River. Although a romantic gesture, the locks each weigh about 0.17 pounds—not so bad, right? But what about the weight of all the love locks? Today, there are nearly 375,000 padlocks on the bridge. ¹ That means that the padlocks are adding 62,000 pounds of weight to the bridge (the equivalent of more than 450 people!) That is four times the permitted weight allowed on the Pont des Arts!

So now what? Well, Parisian officials have agreed to take all of the locks down for two reasons:safety and aesthetics (the love locks block the view of the river!) The impetus for taking down the locks is due to the fact that part of the bridge broke off in 2014 under the weight of the love locks.

But how bad would it be if Paris had decided to leave some of the locks on the bridge? Could something else be done that could protect the safety of pedestrians, but still maintain the love locks? In order to answer my questions, I talked with the Director of the Bridge Engineering Center at the Institute for Transportation, Brent Phares.

A conversation with Brent Phares

Can you talk a little bit about the research that you do?

That’s really broad. Almost everything we do is bridge related. Our work is primarily around bridge construction and bridge condition assessments. One of the hottest topics is accelerated bridge construction, which means instead of taking say six months to build a bridge or impact mobility around the bridge, construction can just include days of mobility impact.

Also, one of the areas we have worked on is using better tools to assess bridge capacity. These tools allow us to more accurately determine when a bridge can handle more or less load.

How do you estimate how much weight a bridge needs to hold when constructing a bridge?

Well, there’s two important aspects when determining capacity: how the bridge resists the loads (so where do the loads flow throughout the three-dimensional structure) and looking at the capacity of individual components of the bridge. There’s no real good way of assessing capacity of individual components of a bridge. One thing code documents (guiding documents) can’t really tell us very accurately is how loads get distributed throughout the bridge. So by doing physical testing of a bridge, we can determine how that actual bridge distributes the load throughout.

How would you describe to the general public the importance of weight bearing on a bridge?

Well, the number one thing on a bridge designer’s mind is ensuring the safety of the traveling public. That is the number one priority, but it takes a number of different forms. Bridge capacity is high on that list to make sure it can resist reasonable loads anticipated on that bridge. There are other aspects to consider as well. One example is if a bridge is constructed over a waterway, you want to make sure that the bridge opening is large enough to accommodate the biggest reasonable flood so the water can get under the bridge without compromising safety. In all, safety is the number one priority.

Have you done bridge research around the country? Around the world?

Mainly around the country—we’ve done some outside the country. In the United States, we have done bridge research in around 30-35 states.

How much does it cost to build a bridge in general?

It’s extremely difficult to say because the cost is very state and region dependent. In the state of Iowa, it typically costs between $100 and $110/per square foot to build a bridge, which includes labor, materials, etc. But if you go to a place like New York City, that number is not true. Even if you go to Kansas, that number could be very different.

What professions are involved in bridge building, from start to finish?

So obviously structural engineers/civil engineers, draftsman (who create constructible documents to say what a bridge needs to look like), you’ve got construction workers, construction managers. And if you want to go broad, you’ve got legal people, accountants. Everything that makes a business run is needed. If it’s a major bridge being constructed, there are even public relations people involved in introducing the bridge to the public. And there are a lot of activities that go into bridge construction, all the way down to the person that answers the phone.

What are the risks of having extra weight on a bridge?

Well, that you will exceed its load carrying capacity and its’ going to fall down.

In the case of the love locks bridge, it’s a pedestrian bridge. Is it possible you could add elements to allow the bridge to hold more weight?

Yeah, and that’s not uncommon. Even highway bridges designed 50 years ago are still in service. And 50 years ago the design criteria were different than they are now:trucks were lighter, there weren’t as many of them. So to bring a bridge up to current standards, strengthening is something that is routinely done. To do so, you add additional components, such as an additional steel plate to a bottom flange. You add additional girders to the whole system. It’s just about getting more capacity, more elements in there.

Is that expensive?

It’s less expensive than building a new bridge. It also depends on the extent of the strengthening that needs to take place. If you’re under capacity by 10 percent that will be less expensive than if you’re under capacity by 80 percent. So it depends on the extent of the deficiency.

Will a new bridge last longer versus a bridge that’s been strengthened?

Probably, that just makes sense. Just because you put new tires on your car, doesn’t mean it’ll last longer than buying a new car. However it doesn’t guarantee that a new car will last longer than your old car. There is no guarantee either way.

What are some of the biggest issues facing bridges around the world today?

Finances—having enough money to repair or replace them.

What is your opinion on the balance between structural soundness of a bridge and its’ aesthetics or cultural importance?

Safety is obviously the most important item for any engineer. However, you can’t discount other intangibles, such as how long a bridge has been there or if a bridge shows up on a city’s logo. The Des Moines’s flag even shows bridges. But you can’t replace safety for aesthetics in building a bridge.

But is there room in the budget for aesthetics?

Well again, it comes at a cost. There are places where the form of the bridge is so important that it’s worth paying extra for. It almost always cost extra to ensure some sort of aesthetic view. Again, we come back to ”˜do we want to replace this arch bridge with another arch bridge and pay 20 percent more but the expense is we’re not going to replace the bridge over there?’ That’s a difficult decision to make, but that does happen and it does happen frequently.

You can see an example of this if you drive over I-235 in Des Moines (Iowa) and see the basket-handle arch pedestrian bridge. Did we need a basket-handle arch there? No. But there was enough aesthetic reasons:like creating a gateway to the City of Des Moines that it warranted the extra cost.

Can love endure?

Though many are upset about the loss of the love locks, it was ultimately a decision to ensure the safety of pedestrians crossing the bridge. For newlyweds that had hoped to travel to Pont des Arts, another trend has started where couples take a picture of themselves on the bridge using the Twitter hashtag “#lovewithoutlinks.” ² In place of the locks, panels of artwork will be placed along the bridge.

Citations

1. http://www.newstatesman.com/future-proof/2014/06/how-break-bridge-weight-too-many-padlocks
2. http://www.abc.net.au/news/2015-06-11/romantic-street-art-replaces-love-locks-on-paris-bridge/6537016

By Jackie Nester, Go! Staff Writer

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