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Live Load Distribution on Concrete Bridges: Design, Evaluation, Construction, Innovation, Part 1 of 2

Monday, October 18, 2021  1:00 PM - 3:00 PM

The session will focus on live load distribution issues for concrete bridge decks and supporting girders. In addition to discussing the AASHTO methodology, other available codes/specification provisions will be reviewed. Potential topics related to live load distribution include, but are not limited to, simplification of the AASHTO methodology, traffic non-parallel to girders, construction stage issues, partial composite deck-girder systems, long-span girders, slab-span structures, and bridges with missing as-built plans.
Learning Objectives:
(1) Understand the basis of AASHTO live load distribution factors and simplification options for girder bridges;
(2) Determine AASHTO knowledge gaps and ways to plug them for girder bridges;
(3) Consider real-life implications of design, evaluation, construction, and maintenance issues for girder bridges;
(4) Identify advanced numerical analysis or experimental load testing techniques used to determine live load distribution for girder bridges.

This session has been approved by AIA and ICC for 2 PDHs (0.2 CEUs). Please note: You must attend the live session for the entire duration to receive credit. On-demand sessions do not qualify for PDH/CEU credit.


Live Load Distributions on a Slab-on-Girder Bridge Subjected to Corrosion and Differential Settlement

Presented By: Jun Wang
Affiliation: University of Colorado Denver
Description: This presentation discusses the distribution of live load on a slab-on-girder bridge subjected to corrosion and differential settlement. A finite element model is developed to predict the flexural behavior of a conventional reinforced concrete deck supported by prestressed concrete girders under the synergistic distress, including a comparative study against the performance of a deck reinforced with glass fiber reinforced polymer (GFRP) reinforcement. Of interest are the detrimental implications of corrosion in altering live load distributions and the efficacy of the non-corrosive reinforcement in preserving load levels within the tolerable boundary of the undamaged state. Furthermore, the susceptibility of the low modulus GFRP to the differential settlement is examined. Parametric investigations are conducted to evaluate the effects of bridge configurations (e.g., skew) and the degree of damage. All findings are appraised using the live load distribution equations stated in the American Association of State Highway Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications.


Load Rating of Damaged Double-Tee Girder Bridges

Presented By: Mostafa Tazarv
Affiliation: South Dakota State University
Description: Precast prestressed double-tee (DT) girder bridges have been frequently used in South Dakota and the neighboring states especially on local roads. DT bridges are cost-effective and are easy to construct and install. Nevertheless, many of these bridges are deteriorating due to insufficient detailing and environmental conditions. Current AASHTO specifications generally include the bridge superstructure damage into load rating and yet to include the component-level damage in the assessment. A damage-based component-level load rating is needed to successfully estimate the safe capacity of damaged/old DT bridges. To attain this goal, field testing, laboratory testing, and an extensive analytical study were performed. First, two DT bridges (more than 30-yr old) with deteriorated girder-to-girder joints were field tested to determine the live load distribution. Subsequently, two 45-year-old DT girders, one 30-ft (9.14-m) and another 50-ft (15.24-m) long, were extracted from a bridge in Rapid City, SD, and were tested to failure at the Lohr Structures Laboratory at South Dakota State University. The salvaged girders, as received, had extensive damage such as exposure of tendons, loss of stem concrete, and flange concrete spalling. Furthermore, more than 1500 analyses were performed to determine the capacity of DT girders with different damage types and condition states. Finally, based on the experimental and analytical studies, a new method was proposed to load rate damaged DT girder bridges. The presentation highlights the findings of the study and discusses the proposed damaged-based load rating methodology.


Live Load Testing of Prestressed Concrete Girder Bridge with Missing As-Built Plans in Cali, Colombia

Presented By: Sebastian Castellanos-Toro
Affiliation: Universidad Del Valle Cali
Description: In this study, a prestressed girder bridge without plans and severe levels of deterioration, located in Cali, Colombia, was load-tested to quantify, experimentally, its live-load behavior. The bridge consists of seven prestressed I-girders covered with a reinforced concrete deck slab and four diaphragm beams. A geometric survey was performed to obtain the dimensions for a shell-based linear finite-element model (LFEM) representing the bridge superstructure and an initial basic finite-element analysis of deformations and modal properties. In this survey, it was observed that the diaphragm beams in the span are geometrically inadequate to contribute to the structural system. Based on the experimental modal properties and the design regulations in force at the time of bridge design and construction a first update was made. Modifying the effective stiffness of some elements to model the girder's deterioration, a second update was performed based on strain gauges information from three load tests and visual inspection (VI) of the elements. The three models (basic, modal updated, and load-test/VI updated) were compared with the distribution load factor (LDF) obtained from the load test and AASHTO distribution factors estimations. Visual inspection, dynamic characterization, and load testing response of this structure indicated some severely deteriorated girder stems and null effect of transverse beams. The results show that AASHTO recommendations overestimate the LDF in comparison with the LFEM without girder’s deterioration. When the girder’s deterioration is included in the model, the LDFs change drastically showing that AASHTO estimations are not in line with the experimental results. As such, for cases of bridges with severe levels of deterioration, it is recommended to use field data to estimate the distribution factors.


Evaluation of Live Load Distribution Factors for Existing Prestressed Concrete Bridges

Presented By: He Zhang
Affiliation: Rutgers University
Description: The live load girder distribution factors (GDF) were derived to simplify the three-dimensional analysis of the bridge system. This simplification led to using the “S-over” distribution equations in the AASHTO Standard Design Specification since 1930 for more than 70 years until the AASHTO-LRFD was published. While the GDF in the Standard specifications were considered to be conservative but simple, the LRFD are more accurate but are based on regression of multiple variables. Both GDF’s are developed to ensure the design outcome is satisfactory. Given the number of prestressed concrete bridges in the NBI database, it is critical to investigate the performance of various existing bridges with regard to their load rating performance. New Jersey owns around 1,500 prestressed concrete bridges, and 66 percent of them were built before 1990. Depending on the design year and specifications, the required load effects from LRFD and LFD of these existing prestressed concrete bridges may have been overestimated. To evaluate the actual load distribution, a load test is performed on a sixty-year-old prestressed concrete I-girder bridge. The field-testing live load distribution factors are determined for both interior and exterior girders using strain sensors. The three-dimensional finite element model (FEM) is developed for the comparison of equations for girder distribution factors. Moreover, the bridge database containing various prestressed concrete bridges from different design years is analytically assessed using the FEM model in terms of the distribution factors. Based on the comparison between the AASHTO GDF’s and other equations with more rigorous methods and field measurements, the extra load effect resulting from these equations provides additional load carrying capacity that would enhance the load rating of these bridges.


Live Load Distribution Factors in an I Girder Concrete Bridge with Excessive Vibration and Partial Deck Girder Composite Action

Presented By: Ikram Efaz
Affiliation: University of Texas Arlington
Description: The current AASHTO provision for live load distribution factor (LLDF) of prestressed concrete I-girder bridges depends on several factors, such as span length, spacing and stiffness of girder. However, it does not take into account the effect of partially composite deck-girder interaction. The current study evaluated the LLDF of a newly constructed partially composite prestressed concrete I-girder bridge with excessive vibration issue on SH 75 in Dallas, TX. Impact Echo (IE) scans of the top of the deck revealed severe delamination at the cast-in-place (CIP)-precast deck panels and deck-girders interfaces. In addition, an initial and follow-up static load tests as well as vibration measurements showed that the excessive vibration was due to delamination-induced loss of composite action. Even though the theoretical calculated LLDF per AASHTO LRFD Bridge Design Specification for one lane loaded condition was 0.52, the LLDF values obtained from the load test were in the range of 0.26 to 0.43. It was concluded that the 21% to 50% significant reduction in the LLDF compared to the theoretical value was the result of stiffness degradation due to partial composite action. This finding necessitates developing a LLDF provision for partially composite prestressed concrete I-girder bridges that can be readily used for load rating.


Live Load Distribution of Slab on Girder Bridges Using Vision Based Measurements

Presented By: Monique Head
Affiliation: University of Delaware
Description: In this study, vision-based measurements and digital imaging techniques are deployed during a controlled load test to compute live load distribution factors. Results are compared to AASHTO live load distribution factors. Data from mounted sensors like strain transducers and string potentiometers are also collected to verify the vision-based measurements during the field test. Using the bridge plans and data collected, a finite element bridge model is generated, calibrated, and used to conduct a bridge load rating per the AASHTO LRFR method. Based on the findings, a procedure to evaluate live load distribution and perform load ratings using digital image measurements in an accurate and contactless way is proposed.

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