State St. Bridge is (Still) Falling Down
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February 12, 2009
Michael Nearing, P.E.
City of Ann Arbor – Project Management Unit
100 N. Fifth Ave.
Ann Arbor, MI 48104Re: Stadium Blvd. over State St. Bridge Condition
Dear Mr. Nearing:
On February 10, 2009 I met with the City’s maintenance crew and you at the East Stadium Boulevard Bridge over South State Street. You had expressed concerns over the condition of the 5th beam in from the southern side of the bridge. This beam has been under close observation since January of 2008 when a large chunk of concrete broke loose, exposing/breaking 7 prestressing strands. Your specific concern at this time was that you felt the beam was sagging lower than the adjacent beams. Once we were able to get up close and use a tape measurer with a straight edge we were able to see that the beam has indeed deflected 7/8” more than the adjacent beams.
On October 22, 2008 NCI completed a bridge safety inspection of this structure. As part of this inspection we brought in a manlift to get close access to the bottom of the beams. Special attention was given to beam #5 due to the large chunk of missing concrete and damaged prestressing strands. At that time we did not observe any deflection of this beam relative to the adjacent beams. Thus, I am of the opinion that this is a relatively recent development.
The 7/8” of additional deflection found on this beam is a significant problem which will require precautionary measures to be taken. Excessive deflection is one of the primary warnings of impending beam failure. Of additional concern is how fast this deflection has developed. If traffic continues to drive over this beam I would expect the deflection to continue to grow, eventually leading to beam failure. Therefore, my recommendation to you is that traffic be removed from over top of this beam. This can be accomplished by reducing Stadium Boulevard to 2 lanes over the bridge, and shifting these lanes to the north side of the road. I’ve attached a sketch showing how this can be accomplished.
The Load Factor Rating (LFR) Method utilizes live load distribution factors from the AASHTO Standard Specifications for Highway Bridges. According to Section 3.23.4.2, “In calculating bending moments in multi-beam precast concrete bridges, conventional or pre-stressed, no longitudinal distribution of wheel load shall be assumed.” AASHTO is telling us that a live load placed directly above a box beam will be carried by that beam alone, without assistance from the adjacent beams. Therefore in theory, by removing the traffic loads from directly above Beam #5 we should be able to keep the condition from getting worse. In reality though, I believe that Beam #5 will still see some load (albeit a significantly reduced load) from traffic over other beams. In recognition of the differences between theory and reality I would recommend that your maintenance crews continue to measure the relative deflection of Beam #5 as often as possible to ensure that the condition doesn’t get any worse. If the beam continues to sag or deteriorate please let me know and we can discuss further safety measures.
If I can be of any other assistance, or if you have any additional questions please let me know.
Sincerely,
NORTHWEST CONSULTANTS, INC.
Jonathan Drummond, P.E.
Bridge Project Managercc: File
