Thursday, November 12, 2009

Re: Caltrans Sharp S-curve on the Bay Bridge claims first life

Abolhassan Astaneh-Asl wrote:
The problem here was to connect a straight bridge (the Eastern Spans of the Bay Bridge) to a Straight tunnel on the YerbaBuena Island. The entire activity was responsibility of bridge/structural engineer to design. Of course during the design, bridge/structural engineer had to get input from various sub fields such as geo, transportation, seismology, piling, etc..  It is the bridge/structural engineer who decides the radius of horizontal and vertical curves on the bridges by satisfying AASHTO provisions.
I find it hard to believe that the structural engineer decided the horizontal and vertical curves for this section of the roadway. and came up with the posted speed.
For a bridge of this magnitude, with 270,000 cars crossing it daily, these radii  should be such that the current speed limit of 50 m/h is maintained. In this case, the curves are far smaller than the AASHTO requirement. As a result, transportation engineers had to come in ( I am not sure even if they were consulted) and see if the properties of curves are appropriate and such a short radius curve at this location can be used given the volume of traffic. If they approve the curves to be appropriate for this application, then the proper speed limit is established given the radius of the curves , the tangent connector in the S-curve, driver expectation, etc. to ensure that with even this small curve, the level of safety , or risk of accidents is pretty much the same as what is in the rest of the bridge. In this case, there was  no reason for structural engineer to select such a small horizontal curves for the S curve. I have studied this bridge for past 20 years since Loma Prieta , with one month full time working on the bridge with 20+ of research associates investigating the Loma Prieta damage to this bridge. The entire intersection is on Yerba Buena Island with all the land below the S-curve available. As for 35 versus 40 m/h, until this fatal accident occurred, the speed limit was posted as 40 on the S-curve (50 on the rest of the bridge). Now it is not clear if it is 40 or 35. Caltrans has mentioned 35. I have to check it when I cross it next time.

One of the main reasons for these accidents , in addition to lack of engineering competence , is the unbelievable corruption of most members of the Caltrans Seismic Advisory Board , as I mentioned in my previous post and the Caltrans top brass. I have worked on Caltrans projects as researchers and consultant (not anymore!) and have known more than 100 engineers over these years. I have stated repeatedly in the past that I have highest respect for middle and junior level Caltrans engineers, but, when it comes to top brass, they definitely put preserving their jobs and public relation way ahead of public safety.
The supposed corruption of the Caltrans Seismic Advisory Board has nothing to do with the accident.  The driver was speeding and went over the edge.  While the posted speed limit may have been set too high, this is not an issue of the lack of engineering competence.  It's not unusual for speed limits to be adjusted after the road is opened.
Best wishes and thank you --gm for your very thoughtful post.

A. Astaneh

With Respect,

but why should structural engineers be looking at acceptable speed limits,
turning radius and traffic safety measures for this? I'm also not saying the
design is doesn't have its flaws. But realistically speaking, this is
probably the trickiest portion of the span replacement with a new bridge
next to an old bridge and a tunnel all in the same area.......

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