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.
(1) Discuss why the extra load effect from AASTHO GDF’s and other equations provides additional load carrying capacity to enhance load rating of bridges;
(2) Review the development of a fatigue consumption formula to calculate fatigue damage cause by the state-specific traffic loads;
(3) Explain how to use a FEM model to capture the distribution of loads across a bridge width;
(4) Summarize procedures to evaluation live load distribution and perform load ratings using digital image measurements.
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.
Effects of Barriers on Load Distribution in a Concrete Slab Span Bridge
Presented By: Kendall Hill
Affiliation: University of Minnesota Duluth
Description: Characterization of load distribution is useful for determining the load rating of bridges, but results in the literature have shown that the structure type and presence of secondary elements both impact this distribution. The focus of this study was to determine how the load distribution in a concrete slab-span bridge was affected by the presence of concrete barriers. There are minimal studies reported in the literature that address this phenomenon in slab-span bridges. Field live load tests and a validated finite element modeling technique were used to study how the behavior of a slab-span structure was influenced by the presence of a concrete barrier. Field testing utilized a suite of instrumentation that included displacement potentiometers and tiltmeters. A three-dimensional solid-element finite element model was used to determine an expected range of behaviors and corroborate the field data regarding how load distributed when placed near and away from a barrier. Results indicated that the barrier had an impact on load distribution in concrete slab-span bridges. This impact could be quantified to determine the live load demand more accurately for use in load rating a concrete slab-span bridge.
Exploring Effects of Freight and Emergency Vehicles on Distribution Factors
of Concrete T Beam Bridges Using Refined Analysis and Machine Learning
Presented By: Abdou Ndong
Affiliation: University of Virginia
Description: Recent revisions to federal guidelines require state departments of transportation (DOTs) to rate bridges in their inventory with special hauling vehicles (SHVs) and emergency vehicles (EVs). SHVs refers to single unit trucks with closely spaced multiple axles, typically ranging from four to seven. EVs are designed for use under emergency conditions such as fires or other hazardous conditions. They can have considerably higher axle weight and gross weight than standard rating vehicles. It is recognized that the load effects (bending moment and shear) produced by SHVs and EVs on certain bridge types and spans might be greater than those caused by the previous rating vehicles. This presents a challenge to state DOTs as some bridges may require posting when rated with these specialized vehicles. This research evaluates the distribution factors for concrete T-beam bridges under different truck loads using refined analysis. In particular, a total of 25 in-service T-beam bridges are modeled and analyzed to compute the moment and shear load distribution factors for exterior and interior girders under one-lane and multiple-lane loaded conditions, and the results are compared with those computed from the code-specified equations. In addition, a support vector machine (SVM) was trained using numerical data to identify the governing truck type for distribution factor based on bridge parameters such as span length and spacing. Using the data obtained from these numerical simulations, a series of multi-parameter linear regression models are also developed to predict the percent change in distribution factor for T-beam bridges with different geometrical characteristics if a refined method analysis is implemented.
Evaluation of In Service Reinforced Concrete Box Culverts through Diagnostic Load Testing and Refined Analysis
Presented By: Amir Gheitasi
Description: According to the Texas Department of Transportation (TxDOT) Inventory, there are more than 19,000 reinforced concrete box culverts in service. The majority of these culverts have been carrying unrestricted traffic for many years with no significant signs of structural distress or deteriorations. However, recent load rating analysis of a select population of these culverts indicated insufficient capacity in carrying design and legal loads, which would trigger posting requirements as well as traffic restrictions. WSP USA was tasked with comprehensive performance evaluation, including load testing and refined analysis, to improve the load ratings of forty representative in-service culverts. The select culverts were in different districts across the state and deemed to be representative of the state’s broad culvert inventory, covering a wide range of geometrical configurations, material properties, design loads, and design standards. Diagnostic load testing was conducted on ten out of the forty culverts to valuate in-service performance and gain a better understanding of the actual load distribution behavior (i.e., load attenuation) in this class of culverts. The results of load testing were then used to calibrate a series of full 3D Finite Element (FE) models including the effects of soil-structure interaction. Calibrated 3D FE models were leveraged to adjust key assumptions and attributes in a series 2D FE models for load rating purposes on all forty culverts. This paper describes details of load testing and FE modeling to refine the load ratings of the select forty culverts. Results from this investigation indicated significant improvement in load ratings of the evaluated culverts in the presence of more realistic modeling and consideration of soil-structure interaction and its impacts on live load distribution behavior of reinforced concrete box culverts.
WIM-Based State-Specific Load Spectra
Presented By: Sylwia Stawska
Affiliation: Auburn University
Description: Live load specified in AASHTO is applicable for the design of bridges in all states. Extensive weigh-in-motion (WIM) data indicates that truck loads are strongly not only state-specific but also site-specific. The objective of this paper is to review the available recent WIM records, compare them and identify the differences and trends. For many WIM stations, the data is available for several years, so it is possible to observe the traffic changes over time. Bridges are affected by load effects rather than gross vehicle weight (GVW). Therefore, the recorded vehicles, axle loads, and axle spacings are used to calculate the bending moments and shear forces. The results are plotted in form of cumulative distribution functions (CDF) for easier comparison. The statistical parameters of live load can then be calculated and compared for different states, road categories, and also for different classes of vehicles. Also, the long-term effect of traffic-induced loads is considered. The fatigue damage is caused by a repeated number of passages of vehicles across a bridge, which may create one or more stress cycles in the structural components. It can result in the accumulation of fatigue consumption over time. The percentage of consumption depends on load magnitude and can be calculated using the available formulas. Knowing the statistical parameters of traffic from weigh-in-motion measurements, traffic volume (ADTT), the economic life of a bridge, and bridge-specific parameters, the total fatigue load spectrum is to be determined. A fatigue consumption formula was developed to calculate fatigue damage caused by the state-specific traffic loads.
Using Finite Element Analysis for Assessing the Load Distribution for the Evaluation and Design of Solid Concrete Slab Bridges
Presented By: Bruno Massicotte
Affiliation: Polytechnique Montreal
Description: One-way simply supported or continuous slab bridges are generally considered as simple constructions that are just an extension of the beam theory. However, as revealed in some recent studies, important issues regarding the behavior of these simple structures have been raised. The dramatic collapse of Concorde Boulevard Bridge in Laval (Canada) in 2006 raised important questions on the cracking of concrete structures and its impact on safety. The studies that were carried out following this event showed the usefulness of using refined analysis, from simple grillage models to detailed nonlinear finite element models, to understand the dead and live load distribution in both service and ultimate conditions. The objective of this paper is to illustrate how linear and nonlinear finite element analysis can contribute to increase the engineering knowledge on the actual behavior of these apparently simple concrete structures. Concrete slab bridges are modelled using linear and nonlinear finite element models to highlight the shear forces that develop in slab bridges, particularly at the corners, allowing quantifying the consequence of these forces. Various types of loads are applied to understand the elastic and nonlinear behavior of the corner force in service conditions and near ultimate. Indications on how simple grillage models can be used in design and evaluation conditions are given.
Experimental Investigation and Refined Load Rating of a Concrete Pan Girder Bridge
Presented By: Nuzhat Kabir
Affiliation: Texas A&M University College Station
Description: In the load rating process, bridges that do not have sufficient capacity to carry the legal loads are load posted. Rating procedures can include conservative assumptions, which may result in load posting bridges that have sufficient load carrying capacity. Such load postings can lead to additional costs to the traveling public due to the time required to detour around a posted bridge. This paper aims to investigate the accuracy of load distribution factors and determine potential refinements to the load rating process for simple span concrete pan girder bridges.
The AASHTO Manual for Bridge Evaluation (MBE) provides guidelines for load rating bridges in the United States using Load and Resistance Factor Rating (LRFR), Load Factor Rating (LFR), and Allowable Stress Rating (ASR). These procedures are intended to be conservative and can have varying degrees of accuracy as compared to the in-situ behavior of specific bridges. A typical load-posted simple-span concrete pan girder bridge, without as-built drawings, was selected as a representative case study to further investigate the distribution of live load between the girders and potential refinements for load rating. Ultrasonic pulse velocity testing and rebound number testing were used to determine the in-situ concrete compressive strength onsite. Information regarding the steel reinforcement, such as spacing and cover, was also determined on site using ground penetrating radar. The behavior of the bridge, including live load distribution, was studied through FEM modeling and field testing. The results obtained from the field tests were compared with recommendations from AASHTO specifications and guidelines and were used to calibrate the FEM model. The FEM model was found to accurately capture the distribution of loads across the bridge width.