Open Discussion: 35W Bridge Reopens

Yep. I hope this one lasts a little longer (there are ancient Roman bridges that are still up).

The design is a bit plain, but I think that it was a rush job.

Will someone in the media ask Flatiron how much extra labor overtime and product premiums were paid over the orginal bid in return for that premium the state paid?

Sure hope this one will last but I am a bit concerned about that

I drove across this morning. Traffic flow was good, there is a nicer view of the downtown skyline on the new bridge.

Are there any safety concerns over the unsealed medians (the thing for which they have to shut down a lane or two, to finish)?

BRIDGE TO SOMEWHERE
Engineering, design, and good ol' fashioned American ingenuity mixed with a lot of hard work came together to bridge a river and to connect a city. A tip of the construction hat to all involved. Thanks.

Quoting Liz H. "Are there any safety concerns over the unsealed medians (the thing for which they have to shut down a lane or two, to finish)?"

The MnDot has not posted the plans nor the specifications for the new I 35W Bridge on their website. New concrete bridge decks have been "sealed" in the past with everything from linseed oil to polymer compounds in order to increase the concrete's resistance to water intrusion. These attempts have not been very successful since the non-skid wearing surface of the concrete deck wears off from traffic, removing the applied sealer. Good quality concrete is usually water-tight, one inch below the surface. The quality of the concrete depends on the materials and the environmental conditions underwhich it is placed and the cure of the concrete taking place during the month following its placement. Bridge decks are notorious for early replacement, repairs and overlays. The previous bridge (#9340) required an overlay within 10 years and partial replacement 40 years later. The top surface of this new bridge will be under constant attack by environmental forces and wear from traffic. It doesn't surprise me that some sort of sealer was used to try to mitigate those factors, however I doubt that this sealer will do much towards extending the life of the wearing surface of the bridge deck.

I wish the MnDot well in the maintenance of this bridge.

The old steel truss bridge #9340 has been described as "lacking redundancy" on this website and others. What this means is that the gravity loads, acting on any member in the truss, could collapse the structure if one member failed or broke. Bridges in the United States are currently required to have redundant load paths to avoid collapse. It can be argued that Bridge #9340 WAS redundant since the two main trusses had interconnecting trusses called "floor trusses" between them. So that if a member in one truss were to be severed, the gravity loads (weight of the bridge, traffic, and wind) would be transfered to the remaining unaffected truss and thus transfered to the foundation. In fact, the engineering firm URS did an extensive study of this bridge (9340) for MnDot which found that a single failure in one of the trusses would not collapse the bridge. This study was made and reported to MnDot in 2006. I would also point out that a reasonably intact concrete bridge deck would serve the same function. (The URS 2006 report considered the bridge without a concrete deck .)

A post-stressed concrete segmental bridge does have a longevity issue. The sealed tendons within the concrete have been difficult to repair when water infiltrates the sheathed strands. I assume that extra tendons were placed so as to back up the highly stressed tendons in the structure. Some bridges have provision for replacement of the tendons because of this vulnerability and our experience so far with this type of bridge.

My impression is that this type of bridge was chosen because it could be fabricated onsite and the majority of the materials are concrete rather than steel. Steel is in short supply and has doubled in cost in recent years. The engineers at MnDot know this and in order to reliably construct such a bridge, crossing the Mississippi, they chose the right one for this time in history.

Ok. Clearly Bill is an engineer, possibly a bridge engineer. My safety concern was more about shutting down lanes during full traffic to do the work. Or were they going to do the work at night (which has its own risks, primarily for the workers)?...