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FHWA Resource Center

STRUCTURES TEAM

PCEF Design Parameters Subcommittee


Friday, October 27, 2000

Issue #1 - Strand Pattern Types

Background

The strand pattern type (e.g. Draped, straight, or straight with unbonding) can have a significant effect on the production costs of pretensioned concrete girders. Although it is widely recognized that draped designs are more efficient than straight designs based purely on the number of strands required, this is typically not the critical point of comparison from a producer's point of view. Many issues, such as safety, cost or availability of plant tooling, setup times, bed utilization, and plant efficiency have significant cost implications.

In addition to unbonding used to control top tensile stresses ("designed unbonding", for lack of a better term), the use of short lengths of unbonding (typically a combination of 6" and 36" lengths known as "crack control unbonding") has been shown to be effective in Pennsylvania for the elimination of hairline cracks common to pretensioned beams caused by splitting forces due to strand expansion at the beam ends. Confining reinforcement is also used, however it is well known that reinforcement reduces crack size rather than contributing to the ultimate goal of elimination of the cracks.

A common misconception is that unbonding significantly reduces the shear strength of the girder. However, while it is commonly held that the prestressing force aids the development of shear resistance, it is also common that straight strand patterns require approximately 10% more prestress force than deflected strands for the same flexural capacity based on allowable tensile stresses at midspan. Therefore, while the unbonding does reduce the prestress force at the end of the girder, this is offset by an increase in the total force, giving a much smaller reduction than is immediately apparent. And, since the contribution of the vertical component of the draping force to shear capacity is small this is also not a compelling reason. In the event that shear resistance needs to be increased, it is more economical in most cases to add a few stirrups, or, in an extreme case, add strands to compensate for the reduced shear capacity.

Another issue commonly cited as a perceived problem with unbonding is the potential for contaminants containing chlorides to travel along the unbonded length, thus penetrating deeper into the girder than would otherwise be possible. However, given proper end treatment of the strands after removal from the casting bed, this potential can be eliminated.

Since the major goal of this committee is to recommend areas where regional uniformity could substantially improve quality and/or reduce costs for all producers and owners, and since the issue of strand type selection and modification during construction has a significant impact on costs, it is desirable to achieve a consensus among the participating states and producers on this issue.

From previous PCEF questionnaires, following is a listing of current practices for the participating states

CT Use either draped or unbonded strands.
DE Typically uses debonding and occasionally drapes strands.
MD No response. However, based on experience, current policy seems to be to only use draping, with no alternate for straight or unbonded strands allowed during the construction phase.
NJ Use both draped and debonded strands.
NC Uses either draped or debonded strands. In cases where either pattern could be designed, the straight strand pattern is used if the straight pattern requires less than 6 additional strands. However, debonding is not allowed in I-girders.
PA Uses either draped or debonded strands and allows changes during construction.
VA Only uses draped strands. No debonding is allowed.
WV Uses draped strands. Did not comment on debonding, or whether it is allowed.

Proposal ; Given the significant usage of straight strands with unbonding, for the states that currently do not allow unbonding as a design or construction option, the subcommittee requests that the rationale behind this limitation be explained so that means and methods might be proposed to alleviate the concerns.

Given a satisfactory resolution to #1, the subcommittee recommends that the full committee endorse draped, straight, and straight/unbonded strand patterns as potentially equal solutions to the same problem, and also the concept that alternative patterns may be proposed during construction.

The subcommittee welcomes comments and suggestions on this proposal. Comments should be received by December 31, 2001 so that a draft ballot can be prepared in time for our spring PCEF meeting.

Comments should be sent to: Joe Nagle at: jnagle@spibeams.com

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