Monday, February 27, 2012

Two-cents on the Sabo bridge deal.

The Sabo bridge crossing Hiawatha Avenue at 28th Street was probably designed with the intent to handle ample loading at an infinite lifespan with minimal maintenance - I didn't design it so I cannot comment on the specifics, but basic engineering practices state that the design is at least twice as strong as any worst case scenario.

After seeing the emergency vehicles on the bridge 72-hours before the cables broke, I right away wondered to myself what the actual weight limit of the bridge is, as I've never seen a posted notice that are routinely seen on bridges designed for motor vehicles.  Heck, a lot of pedestrian and bicycle bridges have obstacles that prevent vehicles from crossing them at all.  

Note that I did not see the firetruck drive across the bridge, only approach it from the west ramp.  The police and HCMC ambulance did drive across the length of the suspension, however.  Also note that cop cars are routinely seen driving across the Sabo bridge.  The fact that the two cables snapped roughly 72 hours later is most likely a mere coincidence.  

My guess is that we will likely see failure in the construction methods; that is deviations from the original design.  See the 1981 Hyatt hotel balcony collapse that killed 114 people - though there are also much bigger issues in the Hyatt tragedy relating to communication issues between what the design engineers thought were just rough ideas and what the steel suppliers thought were final designs.  Luckily the Sabo bridge closing is merely a nuisance and not a tragedy.

Workers on Saturday shoring up the suspended section with missing cables:
Shoring up the bridge

Inspectors high at work:
Bucket brigade

A reminder of the various shots I've taken over the years of the Sabo bridge during various phases of construction and completion.  You can see that they used concrete pumpers to get concrete to the bridge span and did not park concrete trucks on the bridge itself.

That's it, let's sit back and see what the engineers find.


pete said...

I watched the whole thing, the fire truck stayed on the west side and on the road/approach. The cop car was the first up, the ambulance the 2nd up. I thought the same thing. I also wonder about the plow trucks/etc. that run that route. The bridge was built to handle these so it shouldn't be an issue. This will be interesting to see what the problem was.

danbikes said...

What does a cop car weight? at least 4000 lbs? Ambulance 6000+ lbs? A tight pack of cyclists in the same space may weigh 1500 lbs (6 x 250 lbs). So the Ambulance is a factor of 4x the max cyclist weight. Why would the bridge be built to handle that much weight? Designing a bridge for large cars is obviously much more expensive then cyclists.