Continuous Prestressed Post-tensioned Segmental Concrete Box Girders

Last week we inspected four ramps in Boston and Cambridge Mass near Leverett Circle and the Boston Sand and Gravel.

The ramps take traffic from the Tobin Bridge to I-93, Zakim BridgeLeonard P. Zakim Bunker Hill Memorial Bridge, Leverett Circle or Storrow Drive. The ramps consist of continuous prestressed post-tensioned segmental concrete box girders.

To inspect the exterior of the box girders an 80' man lift was used since most of the structures were no accessible with smaller lifts due to on going construction and a park. The exterior inspection consisted of visually inspecting the box girders, piers and measuring the bearings for movement.

The interior of the box girders were inspected with three man teams since they are considered a confined space. When inside the box girder the inspection team needs an air meter, rescue air tank and radio, luckily we have only had to carry these once down the interior since there is a hatch at both ends. The interior inspection consisted of visually inspecting (even though we were always within an arms reach)the box girders even though we were always within an arms reach, utilities and measuring the pintle slots for movement. 

Prestressed Concrete I-Beam Rating

Last February I worked on a rating for MassDOT on two simple span prestressed concrete I-beam bridge with an 8" composite deck. The bridges were built in 1982 and carries route 146 through Sutton MA. The bridges span 78'-4" with a 8°-22'-09" skew.

I used Virtis to perform the rating of the prestressed concrete I-beams.  The rating of the reinforced concrete deck was computed by hand in accordance with the AASHTO Standard Specifications and MassHighway Bridge Manual.

The prestressed concrete I-beams have a set of 16 straight low relaxation strands and 7 harped or draped relaxation strands. At first I had trouble modeling the beams in Virtis and would come up with rating factors lower than 0.05. At first I thought the strand pattern may have been incorrect or a material was entered wrong. After exhausting all the typical mind fart errors, we were able to figure out a problem with the live load modeling.

When we inspected the bridge to confirm measurements and gather other information we noticed that the majority of the prestressed beam ends had areas of rust staining with minor cracking throughout but nothing serious enough to effect the rating.