Thursday, April 19, 2018
Transportation

Questions continue on bouncy Bayside Bridge

Questions about the bouncing Bayside Bridge have been pouring into the Doc's inbox since we covered the issue in this column in May. Many readers insist that there are foundational issues with the construction of the bridge and have questioned the county's explanation that the rhythmic bouncing effect is due simply to cambers in the design of the bridge. Cambering allows individual concrete slabs to flex downward to a level position under heavy loads. The reasoning from the county was that bouncing is a common byproduct of bridge spans constructed with concrete slab spans.

This didn't hold water with readers, including several who identified themselves as engineers. The common theme of the emails and calls the Doc received is the perception that something went wrong with the construction of the bridge, which began in 1991 and was completed in 1993.

The Doc asked the county's engineering team to respond to four follow-up questions. We heard back from engineer Jorge M. Quintas, director of Pinellas County's division of engineering and technical support. Here is his response to our questions:

Was an error made in the construction process of the cement slab?

We are not aware of any errors in the construction process that can be directly attributed to the overcambering of some flat slab concrete segments at the south end. Cambering is a typical practice in bridge construction in which the superstructure is built with an upward deflection so that when the beam or slab is set in place it will deflect downward and assume a smooth riding surface in conjunction with the remaining spans. Overcambering took place on a portion of this bridge, and that is what drivers are feeling. Though this can result in an uncomfortable ride to a driver, it does not affect the structural integrity of the bridge.

Have corrective measures been taken to address the bounce? If so, what measures and when?

No corrective measures have been taken for the following reasons: This condition is extremely costly to correct, as the deck has to be milled, joints have to be replaced, and a deck overlay has to be poured. In most cases, a considerable improvement to the riding surface is not achieved after completing such costly corrective measures. Consequently, the cost-to-benefit ratio does not justify remediation. This condition is not wholly uncommon in bridges. As previously stated, the condition is primarily one of rider discomfort, and not one that generates structural concerns.

Was more than one contractor used in the construction of the bridge (hence the different feels)?

There are many subcontractors that work on large projects as these. However, they are all guided by the same contract plans and specifications. The contractor was Traylor Brothers Inc.

Why is the bridge so wavy/bouncy along one-quarter of the structure (south section of the northbound lanes) and not wavy/bouncy for the other three-fourths of the bridge?

We can only assume that, along the approximate bridge length of 2.65 miles, there was a nonuniform reaction to prestressed forces. A prestressed member will continue to deflect upward, or camber, for a considerable time after the prestressing strands are released at the casting yard. Not all prestressed members will react exactly the same, since their reaction can be affected by a multitude of factors. It would difficult, if at all possible, to ascertain what specific factor in 1991-1993 may have led to the uneven reaction of the prestressed members.

Email Dr. Delay at [email protected] to share your traffic concerns, comments and questions or follow Dr. Delay on Twitter @AskDrDelay.

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