FROM: Bijan Khaleghi
DATE: July 10, 2007
SUBJECT: Damaged Prestressed Girder Replacement Protocol
TO: All Design Section Staff
The purpose of this protocol is to develop criteria to evaluate whether to repair or to replace a prestressed concrete girder damaged by a vehicular impact.
Several factors need to be considered when evaluating whether to repair or to replace a damaged girder. Among them are the level of concrete damage, number of broken strands, location and magnitude of web damage, permanent offset of the original girder alignment, and overall structural integrity. Other considerations include fresh damage to previously damaged girders, damage to adjacent girders, and cost of repair versus replacement. Ultimately, the evaluation hinges on whether the girder can be restored to its original capacity and whether the girder can be repaired sufficiently to carry its share of the original load.
The following guidelines describe damaged girder conditions which require replacement:
- Strand Damage: More than 25% of prestressing strands are damaged/severed.
If over 25% of the strands have been severed, replacement is required. Splicing is routinely done to repair severed strands. However, there are practical limits as to the number of couplers that can be installed in the damaged area.
- Girder Displacements: The bottom flange is displaced from the horizontal position more than ½ per 10 of girder length. If the alignment of the girder has been permanently altered by the impact, replacement is required. Examples of non-repairable girder displacement include cracks at the web/flange interface that remain open. Abrupt lateral offsets may indicate that stirrups have yielded. A girder that is permanently offset may not be restorable to its original geometric tolerance by practical and cost-effective means.
- Concrete Damage at harping point: Concrete damage at harping point resulting in permanent loss of prestress. Extreme cracking or major loss of concrete near the harping point may indicate a change in strand geometry and loss in prestress force. Such loss of prestress force in the existing damaged girder cannot be restored by practical and cost-effective means, and requires girder replacement.
- Concrete Damage at girder ends: Severe concrete damage at girder ends resulting in permanent loss of prestress. Extreme cracking or major loss of concrete near the end of a girder may indicate unbonding of strands and loss in prestress force. Such loss of prestress force in the existing damaged girder cannot be restored by practical and cost-effective means, and requires girder replacement.
There are other situations as listed below which do not automatically trigger replacement, but require further consideration and analysis.
- Adjacent Girders: Capacity of adjacent undamaged girders.
Consideration must be given as to whether dead load from the damaged girder has been shed to the adjacent girders and whether the adjacent girders can accommodate the additional load.
- Damaged Adjacent Girders: Damage to adjacent girders.
Replacement may also be warranted if the adjacent girders have been damaged from this or previous impact and have reduced capacity.
- Previously-damaged Girders: Damage to a previously damaged girder.
An impact to a girder that has been previously repaired may not be able to be restored to sufficient capacity. (Otherwise it sounds like it a fabricated defect and hence was a contributing factor to its inability for re-use or repair)
- Cost: Cost of repair versus replacement.
Replacement may be warranted if the cost of repair reaches 70% of the replacement project cost.
If you have any questions regarding these guidelines please contact Joe Merth at 705-7165, Harvey Coffman at 570-2556, or Bijan Khaleghi at 705-7181.
cc: Mohammad Sheikhizadeh, Bridge Construction Engineer - 47354
Finn Posner, Bridge and Structures Office 47340