ResearchPod Summary
This dissertation investigates the shear behavior of post-tensioned bridge girders, specifically focusing on the impact of internal bonded and unbonded tendons located within the web. As bridge design evolves to include flexible fillers for easier inspection and strand replacement, understanding the shear performance of these unbonded systems has become critical. The research aims to fill the gap in current design guidance by evaluating how different bond types, duct geometries, and reinforcement ratios influence shear strength and failure mechanisms.
To address these questions, the study conducted two comprehensive test series at Purdue University. Test Series 1 involved 25 moderate-scale girder specimens (I-shaped and box-shaped) to isolate variables such as tendon bond type, transverse reinforcement ratios, and duct diameter-to-web width ratios. Test Series 2 validated these findings with four full-scale girder tests representative of field conditions. The specimens were subjected to monotonic loading until failure, with extensive instrumentation—including strain gauges on stirrups and prestressing strands—to monitor internal force distribution and crack development.
The experimental results reveal that the bond type of web tendons has a negligible impact on overall shear capacity. While bonded tendons provide a slight increase in strength, the failure mechanism for both bonded and unbonded cases is consistently characterized by localized concrete crushing near the duct level. The study found that increasing the duct diameter-to-web width ratio decreases shear capacity, while increasing transverse reinforcement effectively enhances it. Based on these observations, the author proposes refinements to the AASHTO LRFD Bridge Design Specifications. These include using the full web width for concrete contribution calculations and removing the reduction factor for transverse reinforcement efficiency, providing a more consistent and accurate design approach for both grouted and ungrouted systems.
[[RP_SECTION:shear-failure-mechanisms|Shear failure mechanisms]]
Alex: [steady, analytical] For post-tensioned concrete bridge girders, shear failure is dominated by localized web crushing at the duct level—and that holds whether the tendon is grouted or unbonded. Which means current design codes are treating two structurally equivalent situations very differently.
Sam: This is from Sarah Bowlin's dissertation at Purdue. So the implication is that AASHTO's current provisions are systematically overestimating shear capacity for at least one of those cases.
Alex: That's the concern. And what makes Bowlin's work significant is that it's the first comprehensive experimental dataset specifically for unbonded tendons. That was a genuine blind spot—design provisions for unbonded systems were essentially extrapolated from bonded data, without direct experimental backing.
Sam: So walk me through the mechanism. If the duct is creating a plane of weakness, what's actually happening at failure? [[RP_SECTION:duct-geometry-and-failure|Duct geometry and failure]]
Alex: Think of the web as a series of diagonal compression struts—a concrete truss carrying shear. When you insert a duct, you're punching a void through those struts. That reduces the effective cross-sectional area resisting compression and concentrates stress around the duct wall. The result is that the concrete crushes locally at the duct before the stirrups ever reach yield. The transverse reinforcement doesn't get to do its job.
Sam: So it's not a reinforcement problem—it's a geometry problem. The steel capacity is irrelevant if the concrete fails first.
Alex: Exactly. And that's why bond condition turns out to be secondary. Whether the duct is grouted or filled with flexible filler, the void geometry is what governs. The grout's contribution to shear resistance is negligible. The failure mode is the same either way.
Sam: And the experimental program confirmed this directly?
Alex: Across 29 girders with varying bond types and duct-to-web ratios, the failure mode remained consistent: localized crushing at the duct. Bond condition didn't shift the mechanism. What did matter was the ratio of duct diameter to web width—that's the parameter that scales the capacity reduction. [[RP_SECTION:aashto-code-limitations|AASHTO code limitations]]
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Sam: So how does current AASHTO handle this, and where does it go wrong?
Alex: The code currently assumes bonded tendons have full effective web width—no reduction applied. Unbonded tendons do get a reduction, but it's not consistently derived from this failure model. So you have a situation where the bonded case is almost certainly non-conservative, and the unbonded case may be inconsistently specified depending on configuration. [[RP_SECTION:proposed-unified-model|Proposed unified model]]
Sam: And the fix Bowlin proposes is to apply a consistent reduction to both—subtracting the duct diameter directly from the effective web width, regardless of bond condition.
Alex: That's the unified model. The duct diameter is treated as a direct deduction from the web width when calculating nominal shear resistance. The mechanism is the same for both bond conditions, so the design equation should reflect that. And when you validate this against the broader literature, the correlation holds well across configurations. [[RP_SECTION:upper-limit-on-strength|Upper limit on strength]]
Sam: You mentioned an upper limit on shear strength. What's the role of that?
Alex: Without an upper bound, the model can overestimate capacity in heavily reinforced sections—cases where you have dense stirrups but the concrete compressive strength is the actual bottleneck. Bowlin evaluated several candidate limits, and the one tied to concrete compressive strength is the most practical. It's consistent with existing AASHTO logic, and it catches the unconservative outliers without disrupting the rest of the design space.
Sam: So the full proposal is a unified web width reduction based on duct geometry, plus a compressive-strength-based upper limit on nominal shear resistance. That's a fairly targeted set of changes to push into a code revision cycle.
Alex: And that's the point. These aren't sweeping changes to the code framework—they're specific, data-driven corrections to provisions that were built on an incomplete experimental base. The segmental box girder market has been growing, and engineers choosing between grouted and unbonded systems deserve design equations that actually reflect the physics.
Sam: It's a good example of how a single well-designed experimental program can resolve a question that's been sitting in the literature as an assumption. The mechanism was always plausible—now there's direct evidence behind it.
Alex: And the practical consequence is real. If you're designing a segmental bridge and your shear capacity estimate assumes full web width for bonded tendons, you may be operating with less margin than you think. Bowlin's framework gives engineers a more defensible basis—and gives code committees a dataset to act on.
Sam: Thanks for walking through the mechanics, Alex. And thanks for listening to ResearchPod.