Friday, October 27, 2000

PCEF Design Parameters Subcommittee

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**