Timothy A. Wood

Evaluation of Production Models for Load Rating Reinforced Concrete Box Culverts

Analyses of two production-oriented culvert load-rating demand models were performed using live-load test data from three instrumented RC box culverts under four cover soil depths. The demand models were a two-dimensional (2D) structural-frame model and a 2D soil-structure interaction model. As expected, increased sophistication in the soil-structure model compared with the structural-frame model resulted in higher precision and accuracy for predicted moments. The impact of modeling accuracy for sections in a culvert where the demand moments approach zero was deemed practically insignificant. When evaluating model accuracy, it is of first importance that the models predict meaningful load magnitudes. Variations in predicted moment accuracy and precision were not uniform but were a function of the location of the critical section in the culvert structure. Improvements in modeling prediction associated with increased modeling sophistication were seen only when the structural-frame model was very imprecise.

Influence of Cover Soil Depth on the Load Rating of Reinforced Concrete Box Culverts

This paper describes the influence of cover soil depth on the load rating of the designs of multibarrel, cast-in-place (CIP) reinforced concrete (RC) box culverts and highlights implications for the load rating and design of culvert structures. The basics of culvert load rating are discussed and are followed by a history of culvert design policy and the challenges created by the use of culvert standard designs. A population of Texas Department of Transportation CIP RC standard culvert designs developed between 1930 and 1980 was load rated by using AASHTO policy guidance and a two-dimensional model of direct stiffness structural demand for a full range of cover soil depths. This analysis resulted in a set of 1,081 relationships of load rating versus cover soil depth. Three typical relationships of rating versus depth are illustrated and described in detail. The distribution of characteristic relationships of rating versus depth on the basis of culvert geometry, design cover soil depth, and design era is explored. Cover soil depth is shown to be a critical parameter that must be explicitly considered for the intelligent load rating and design of RC box culverts.

Pullout Resistance Factors for Steel MSE Reinforcements Embedded in Gravelly Backfill

Abstract This paper presents results from a laboratory program of 287 pullout tests of galvanized steel reinforcements used in the construction of mechanically stabilized earth (MSE) walls. Results focus on the evaluation of pullout resistance factors for ribbed steel strip and welded steel grid reinforcement embedded in gravelly backfill. This project used Texas Tech University’s large-scale MSE test box with dimensions of 3.6 × 3.6 × 1.2 m and an applied overburden capacity equivalent to 12 m of soil fill. The research design evaluated pullout resistance factors for both ribbed strip and welded grid reinforcements for a variety of independent variables, including overburden pressure, reinforcement length, grid bar size, and grid geometry including both transverse and longitudinal bar spacing. Appropriate statistical analyses were used to interpret the data within the context of published design guidance for inextensible MSE reinforcements. The results show that pullout resistance factors for both ribb...

Improved Load Rating of Reinforced-Concrete Box Culverts Using Depth-Calibrated Live-Load Attenuation

AbstractThis paper describes depth-calibrated live-load attenuation for the load rating of reinforced-concrete box culverts using production-simplified models. In-plane depth calibration is accomplished using a production-simplified, two-dimensional, linear-elastic, finite-element, soil-structure interaction model with results compared with those from the recommended direct-stiffness, structural-frame model. Out-of-plane live-load attenuation considers each potential critical section depth rather than the cover soil depth only. The effectiveness of depth calibration is assessed by comparing predicted live-load moments obtained from the models versus measured live-load moments obtained from full-scale culvert load tests. A load rating case study illustrates the potential for improved alignment between load rating and observed performance. Findings show that depth calibration improves current load rating practice by increasing the accuracy and precision of live-load demand predictions, particularly in culve...

Pullout Resistance Factors for Steel Reinforcements Used in TxDOT MSE Walls

This paper presents results from an extensive laboratory program of pullout testing of steel reinforcements used for Mechanically Stabilized Earth (MSE) walls constructed in Texas. Results focus on evaluation of pullout resistance factors for sandy backfill and MSE reinforcement combinations used by the Texas Department of Transportation (TxDOT). A unique aspect of this study is that this project uses Texas Tech Universitys large-scale MSE Test Box, one of the largest such devices in the world, with dimensions of 3.7m x by 3.7m x 1.2m (12ft x 12ft x 4ft) and an applied overburden capacity of 12.2m (40ft) of backfill. This test box facilitates pullout testing at a scale not unlike typical field construction. Results consist of pullout resistance factors for both ribbed strip and welded grid reinforcements for a variety of test variables including overburden pressure, reinforcement length, level of compaction, grid wire size, and grid geometry. We evaluate the data within the context of published AASHTO design guidance for inextensible MSE reinforcements.

Pullout Resistance Factors for Inextensible Mechanically Stabilized Earth Reinforcements in Sandy Backfill

This paper presents results from a laboratory program of 402 pullout tests of inextensible reinforcements used for walls of mechanically stabilized earth (MSE). Results focus on the evaluation of pullout resistance factors for ribbed-steel strip and welded-steel grid reinforcements embedded in sandy backfill that marginally met AASHTO requirements for select granular fill. This project used Texas Tech Universitys large-scale MSE test box with dimensions of 12 3 12 3 4 ft and an applied overburden capacity of 40 ft of backfill. This test box facilitated pullout testing at a scale not unlike typical field construction. The research design evaluated pullout resistance factors for both ribbed-strip and welded-grid reinforcements for a variety of independent variables, including overburden pressure, reinforcement length, level of compaction, grid wire size, and grid geometry, such as transverse and longitudinal wire spacing. Appropriate statistical analyses were used to interpret the data within the context of published AASHTO design guidance for inextensible MSE reinforcements. The results show that pullout behaviors of both ribbed strips and welded grids in properly compacted sandy backfill are conservative compared with the default pullout resistance factors provided by AASHTO. The data also suggest that the current AASHTO equations for pullout resistance factors for grid reinforcement do not accurately capture the influence of transverse and longitudinal bar spacings.

Instrumentation and Monitoring of an MSE/Soil Nail Hybrid Retaining Wall

MSE/soil nail hybrid earth retaining walls provide a more economical design for applications in cut/fill situations than the traditionally used full height MSE and drilled shaft retaining walls. MSE/soil nail hybrid earth retaining walls use a soil nailed wall in the cut section and an MSE wall in the fill section. In spite of the significant cost savings they offer, hybrid walls have not seen widespread use primarily because of lack of understanding on wall design and performance. This paper describes an instrumentation and monitoring effort that was undertaken with the objective of improving our understanding of hybrid wall design and performance. In this project, two separate panels of a hybrid wall constructed in San Antonio, Texas were selected for instrumentation and monitoring. The first wall panel consisted of a 4.0m soil nail wall and a 5.4m MSE wall while the second wall panel consisted of 5.0m soil nail wall and a 4.4m MSE Wall. The instrumentation scheme for the wall included vibrating wire strain gages, vertical inclinometers, horizontal inclinometers, and tiltmeters. The data collected from these two instrumented wall sections provide valuable insight to the mechanisms controlling the performance of MSE/Soil Nail Hybrid Wall systems.

Evaluation of AASHTO Default Values for Pullout Friction Factor, F*, for Steel Grid Mat Reinforcement

This paper presents an evaluation of default AASHTO values for pullout friction factor, F*, for steel grid reinforcement versus measured pullout behavior determined from a laboratory program of 397 pullout tests on welded steel grid mats of various geometries. Within the context of internal stability analyses associated with mechanically stabilized earth wall design, the laboratory test program for this study established pullout friction factors for grid mats relative to several independent variables including backfill type, overburden pressure, reinforcement length, bar size, and grid geometry that included both transverse and longitudinal bar spacing. Appropriate statistical analyses were used to interpret the pullout test data relative to published AASHTO design guidance for inextensible reinforcements. As expected, the results show that pullout capacities for steel grid mats in properly compacted sandy and gravelly backfill are conservative compared with the default pullout friction factors provided by AASHTO. The data suggest that current AASHTO equations for pullout friction factor for steel grid reinforcement may not adequately capture the influence of transverse bar spacing, longitudinal bar spacing, and backfill particle size.

Comparison of Culvert Load Ratings Calculated by Three Methods

This paper explores the influence of the choice of structural analysis method on inventory and operating rating factors calculated using American Association of State Highway and Transportation Officials (AASHTO) load rating equations for bridge class culverts. The authors used three increasingly sophisticated, production-oriented structural analysis methods to load rate 86 reinforced concrete box culverts selected to represent the full population of 1855 culvert designs maintained by the Texas Department of Transportation (TxDOT). It is presumed that more complex soil structure interaction models will generate more accurate and less conservative load ratings than simpler models. Results indicate that increasing the model sophistication by using soil springs does not significantly change the inventory rating when compared to the first method. Increasing sophistication further by modeling soil using linear-elastic finite elements yields mixed results, showing an increase in the rating factor for sufficiently high soil moduli of elasticity but a slight decrease in the rating factor for lower soil moduli of elasticity.

Effect of Skewing and Splaying on Pullout Capacity of Steel Reinforcements in Mechanically Stabilized Earth Structures

Structures embedded in the reinforced fill of mechanically stabilized earth (MSE) retaining walls often prevent the soil reinforcements from being placed in their proper design configurations. Such conflicts are generally addressed with alternative reinforcement layouts that circumvent the obstructions. The most commonly used strategies include the skewing of strip-type reinforcements and the cutting and splaying of grid-type reinforcements. This research investigated the impact of skewing and splaying on the pullout resistance capacity of these two types of reinforcement. The research involved a series of pullout tests that were conducted in a specially designed large-scale pullout resistance test system. This MSE pullout load test system included an MSE test box with dimensions of 12 by 12 ft (3.66 by 3.66 m) in area and 4 ft (1.22 m) in depth. In this test system, the action of the soil overburden pressures on the embedded earth reinforcement were simulated with a reaction frame assembly that consisted of nine 4- by 4-ft (1.22- by 1.22-m) pressure plates that were hydraulically jacked against three wide-flange cross beams. The reaction frame assembly allowed the simulation of overburden pressures up to 40 ft (12.2 m) of fill. The pullout test program included (a) tests conducted on strip-type reinforcements with skew angles of 0°, 15°, and 30° and (b) tests conducted on grid-type reinforcements with splay angles of 0°, 15°, and 30°. The pullout loads obtained were then compared to determine the impact of skewing and splaying on the pullout resistance.