Dynamics of Highway Bridges, HR-67, 1961

The AASHO specifications for highway bridges require that in designing a bridge, the live load must be multiplied by an impact factor for which a formula is given, dependent only upon the length of the bridge. This formula is a result of August Wohler's tests on fatigue in metals, in which he determined that metals which are subjected to large alternating loads will ultimately fail at lower stresses than those which are subjected only to continuous static loads. It is felt by some investigators that this present impact factor is not realistic, and it is suggested that a consideration of the increased stress due to vibrations caused by vehicles traversing the span would result in a more realistic impact factor than now exists. Since the current highway program requires a large number of bridges to be built, the need for data on dynamic behavior of bridges is apparent. Much excellent material has already been gathered on the subject, but many questions remain unanswered. This work is designed to investigate further a specific corner of that subject, and it is hoped that some useful light may be shed on the subject. Specifically this study hopes to correlate, by experiment on a small scale test bridge, the upper limits of impact utilizing a stationary, oscillating load to represent axle loads moving past a given point. The experiments were performed on a small scale bridge which is located in the basement of the Iowa Engineering Experiment Station. The bridge is a 25 foot simply supported span, 10 feet wide, supported by four beams with a composite concrete slab. It is assumed that the magnitude of the predominant forcing function is the same as the magnitude of the dynamic force produced by a smoothly rolling load, which has a frequency determined by the passage of axles. The frequency of passage of axles is defined as the speed of the vehicle divided by the axle spacing. Factors affecting the response of the bridge to this forcing function are the bridge stiffness and mass, which determine the natural frequency, and the effects of solid damping due to internal structural energy dissipation.

Feasibility Study of Dynamic Overload and Ultimate Load Tests of Full-Scale Highway Bridges, HR-160, 1973

As a result of the construction of the Saylorville Dam and Reservoir on the Des Moines River, six highway bridges are scheduled for removal. Five of these are old high-truss single-lane bridges, each bridge having several simple spans. The other bridge is a fairly modern (1955) double 4-span continuous beam-and-slab composite highway bridge. The availability of these bridges affords an unusual opportunity for study of the behavior of full-scale bridges. Because of the magnitude of the potential testing program, a feasibility study was initiated and the results are presented in this two-part final report. Part I summarizes the findings and Part II presents the supporting detailed information.

Ultimate Load Behavior of Full-Scale Highway Truss Bridges, Summary Report, HR-169, 1975

As a result of the construction of the Saylorville Dam and Reservoir on the Des Moines River, six highway bridges crossing the river were scheduled for removal. One of these, an old pin-connected, high-truss, single-lane bridge, was selected for a comprehensive testing program which included ultimate load tests, service load tests, and a supplementary test program. A second bridge was used for a limited service load test program. The results of the research are detailed in two interim reports. The first interim report outlines the ultimate load tests and the second interim report details the results of the service load and supplementary test program. This report presents a summary of these findings along with recommendations for implementation of the findings.

Field/Laboratory Testing of Damaged Prestressed Concrete Girder Bridges, HR-397, 1999

Due to frequent accidental damage to prestressed concrete (P/C) bridges caused by impact from overheight vehicles, a project was initiated to evaluate the strength and load distribution characteristics of damaged P/C bridges. A comprehensive literature review was conducted. It was concluded that only a few references pertain to the assessment and repair of damaged P/C beams. No reference was found that involves testing of a damaged bridge(s) as well as the damaged beams following their removal. Structural testing of two bridges was conducted in the field. The first bridge tested, damaged by accidental impact, was the westbound (WB) I-680 bridge in Beebeetown, Iowa. This bridge had significant damage to the first and second beams consisting of extensive loss of section and the exposure of numerous strands. The second bridge, the adjacent eastbound (EB) structure, was used as a baseline of the behavior of an undamaged bridge. Load testing concluded that a redistribution of load away from the damaged beams of the WB bridge was occurring. Subsequent to these tests, the damaged beams in the WB bridge were replaced and the bridge retested. The repaired WB bridge behaved, for the most part, like the undamaged EB bridge indicating that the beam replacement restored the original live load distribution patterns. A large-scale bridge model constructed for a previous project was tested to study the changes in behavior due to incrementally applied damage consisting initially of only concrete removal and then concrete removal and strand damage. A total of 180 tests were conducted with the general conclusion that for exterior beam damage, the bridge load distribution characteristics were relatively unchanged until significant portions of the bottom flange were removed along with several strands. A large amount of the total applied moment to the exterior beam was redistributed to the interior beam of the model. Four isolated P/C beams were tested, two removed from the Beebeetown bridge and two from the aforementioned bridge model. For the Beebeetown beams, the first beam, Beam 1W, was tested in an "as removed" condition to obtain the baseline characteristics of a damaged beam. The second beam, Beam 2W, was retrofit with carbon fiber reinforced polymer (CFRP) longitudinal plates and transverse stirrups to strengthen the section. The strengthened Beam was 12% stronger than Beam 1W. Beams 1 and 2 from the bridge model were also tested. Beam 1 was not damaged and served as the baseline behavior of a "new" beam while Beam 2 was damaged and repaired again using CFRP plates. Prior to debonding of the plates from the beam, the behavior of both Beams 1 and 2 was similar. The retrofit beam attained a capacity greater than a theoretically undamaged beam prior to plate debonding. Analytical models were created for the undamaged and damaged center spans of the WB bridge; stiffened plate and refined grillage models were used. Both models were accurate at predicting the deflections in the tested bridge and should be similarly accurate in modeling other P/C bridges. The moment fractions per beam were computed using both models for the undamaged and damaged bridges. The damaged model indicates a significant decrease in moment in the damaged beams and a redistribution of load to the adjacent curb and rail as well as to the undamaged beam lines.

Evaluation of Steel Bridges, Volumes I & II, TR-493, 2007

This report is divided into two volumes. This volume (Volume I) summarizes a structural health monitoring (SHM) system that was developed for the Iowa DOT to remotely and continuously monitor fatigue critical bridges (FCB) to aid in the detection of crack formation. The developed FCB SHM system enables bridge owners to remotely monitor FCB for gradual or sudden damage formation. The SHM system utilizes fiber bragg grating (FBG) fiber optic sensors (FOSs) to measure strains at critical locations. The strain-based SHM system is trained with measured performance data to identify typical bridge response when subjected to ambient traffic loads, and that knowledge is used to evaluate newly collected data. At specified intervals, the SHM system autonomously generates evaluation reports that summarize the current behavior of the bridge. The evaluation reports are collected and distributed to the bridge owner for interpretation and decision making. Volume II summarizes the development and demonstration of an autonomous, continuous SHM system that can be used to monitor typical girder bridges. The developed SHM system can be grouped into two main categories: an office component and a field component. The office component is a structural analysis software program that can be used to generate thresholds which are used for identifying isolated events. The field component includes hardware and field monitoring software which performs data processing and evaluation. The hardware system consists of sensors, data acquisition equipment, and a communication system backbone. The field monitoring software has been developed such that, once started, it will operate autonomously with minimal user interaction. In general, the SHM system features two key uses. First, the system can be integrated into an active bridge management system that tracks usage and structural changes. Second, the system helps owners to identify damage and deterioration.

Standards for Single Span Prefabricated Bridges, Phase I - Concept Development, TR-663, 2014

In coordination with a Technical Advisory Committee (TAC) consisting of County Engineers and Iowa DOT representatives, the Iowa DOT has proposed to develop a set of standards for a single span prefabricated bridge system for use on the local road system. The purpose of the bridge system is to improve bridge construction, accelerate project delivery, improve worker safety, be cost effective, reduce impacts to the travelling public by reducing traffic disruptions and the duration of detours, and allow local forces to construct the bridges. HDR Inc. was selected by the Iowa DOT to perform the initial concept screening of the bridge system. This Final Report summarizes the initial conceptual effort to investigate potential systems, make recommendations for a preferred system and propose initial details to be tested in the laboratory in Phase 2 of the project. The prefabricated bridge components were to be based on the following preliminary criteria set forth by the TAC. The criteria were to be verified and/ or modified as part of the conceptual development. - 24’ and 30’ roadway widths - Skews of 0o, 15o, and 30o - Span lengths of 30’ – 70’ in 10’ increments using precast concrete beams - Voided box beams could be considered - Limit precast element weight to 45,000 pounds for movement and placement of beams - Beams could be joined transversely with threaded rods - Abutment concepts may included precast as well as an option for cast-in-place abutments with pile foundations In addition to the above criteria, there was an interest to use a single-width prefabricated bridge component to simplify fabrication as well as a desire to utilize non-prestressed concrete systems where possible to allow for precasting of the beam modules by local forces or local precast plants. The SL-1 modular steel bridge rail was identified for use with this single span prefabricated bridge system.

New Method for Breaking High Strength Prestress Cable, MLR-97-6, 1997

A new method was developed for breaking high strength prestressed cable. The old method used an aluminum oxide grit packed into a special gripping jaw. The new method uses aluminum shims wrapped around the cable and then is gripped with a V-grip. The new method gives nearly 100% "good breaks" on the cable compared to approximately 10% good breaks with the old method. In addition, the new cable breaking method gives higher ultimate tensile strengths, is more reproducible, is quicker, cleaner and easier on equipment.

Marsh rainbow arch bridges in Iowa, 1997

The Marsh Rainbow Arch Bridge is a patented bridge design by James Barney Marsh, a graduate of Iowa State College of Agriculture and Mechanic Arts (now Iowa State University). Around the turn of the 20th Century, reinforced concrete was introduced in Iowa as an important new bridge construction material. Marsh used the new technology to encased steel truss arches in concrete to produce a sturdy yet esthetic arch bridge. This booklet touches on the important aspects of Marsh's life, business and industrial contributions.

Design of Piles for Integral Abutment Bridges, HR-252, 1984

More and more, integral abutment bridges are being used in place of the more traditional bridge designs with expansion releases. In this study, states which use integral abutment bridges were surveyed to determine their current practice in the design of these structures. To study piles in integral abutment bridges, a finite element program for the soil-pile system was developed (1) with materially and geometrically nonlinear, two and three dimensional beam elements and (2) with a nonlinear, Winkler soil model with vertical, horizontal, and pile tip springs. The model was verified by comparison to several analytical and experimental examples. A simplified design model for analyzing piles in integral abutment bridges is also presented. This model grew from previous analytical models and observations of pile behavior. The design model correctly describes the essential behavioral characteristics of the pile and conservatively predicts the vertical load-carrying capacity. Analytical examples are presented to illustrate the effects of lateral displacements on the ultimate load capacity of a pile. These examples include friction and end-bearing piles; steel, concrete, and timber piles; and bending about the weak, strong, and 45° axes for H piles. The effects of cyclic loading are shown for skewed and nonskewed bridges. The results show that the capacity of friction piles is not significantly affected by lateral displacements, but the capacity of end-bearing piles is reduced. Further results show that the longitudinal expansion of the bridge can introduce a vertical preload on the pile.

Pile Design and Tests for Integral Abutment Bridges, HR-273, Addendum, 1987

Addendum to HR-273

Heavy Agricultural Loads on Pavements and Bridges, HR-1073, 1999

During the harvest season in Iowa, it is common to have single axle loads on secondary roads and bridges that are excessive (typical examples are grain carts) and well beyond normal load limits. Even though these excessive loads occur only during a short time of the year, they may do significant damage to pavements and bridges. In addition, the safety of some bridges may be compromised because of the excessive loads, and sometimes there may be little indication to the users that damage may be imminent. At this time there are no Iowa laws regulating axle loads allowed for agricultural equipment. This study looks at the potential problems this may cause on secondary roads and timber stringer bridges. Both highway pavement and timber bridges are evaluated in this report. A section (panel) of Iowa PCC paved county road was chosen to study the effects of heavy agricultural loads on pavements. Instrumentation was applied to the panel and a heavily loaded grain cart was rolled across. The collected data were analyzed for any indication of excessive stresses of the concrete. The second study, concerning excessive loads on timber stringer bridges, was conducted in the laboratory. Four bridge sections were constructed and tested. Two of the sections contained five stringers and two sections had three stringers. Timber for the bridges came from a dismantled bridge, and deck panels were cut from new stock. All timber was treated with creosote. A hydraulic load was applied at the deck mid-span using a foot print representing a tire from a typical grain cart. Force was applied until failure of the system resulted. The collected data were evaluated to provide indications of load distribution and for comparison with expected wheel loads for a typical heavily loaded single axle grain cart. Results of the pavement tests showed that the potential of over-stressing the pavement is a possibility. Even though most of the tension stress levels recorded were below the rupture strength of the concrete, there were a few instances where the indicated tension stress level exceeded the concrete rupture strength. Results of the bridge tests showed that when the static ultimate load capacity of the timber stringer bridge sections was reached, there was sudden loss of capacity. Prior to reaching this ultimate capacity, the load sharing between the stringers was very uniform. The failure was characterized by loss of flexural capacity of the stringers. In all tests, the ultimate test load exceeded the wheel load that would be applied by an 875 bushel single axle grain cart.

Demonstration Project using Railroad Flatcars for Low-Volume Road Bridges, TR-444, 2003

The use of Railroad Flatcars (RRFCs) as the superstructure on low-volume county bridges has been investigated in a research project conducted by the Bridge Engineering Center at Iowa State University. These bridges enable county engineers to replace old, inadequate county bridge superstructures for less than half the cost and in a shorter construction time than required for a conventional bridge. To illustrate their constructability, adequacy, and economy, two RRFC demonstration bridges were designed, constructed, and tested: one in Buchanan County and the other in Winnebago County. The Buchanan County Bridge was constructed as a single span with 56-ft-long flatcars supported at their ends by new, concrete abutments. The use of concrete in the substructure allowed for an integral abutment at one end of the bridge with an expansion joint at the other end. Reinforced concrete beams (serving as longitudinal connections between the three adjacent flatcars) were installed to distribute live loads among the RRFCs. Guardrails and an asphalt milling driving surface completed the bridge. The Winnebago County Bridge was constructed using 89-ft-long flatcars. Preliminary calculations determined that they were not adequate to span 89 ft as a simple span. Therefore, the flatcars were supported by new, steel-capped piers and abutments at the RRFCs' bolsters and ends, resulting in a 66-ft main span and two 10-ft end spans. Due to the RRFC geometry, the longitudinal connections between adjacent RRFCs were inadequate to support significant loads; therefore, transverse, recycled timber planks were utilized to effectively distribute live loads to all three RRFCs. A gravel driving surface was placed on top of the timber planks, and a guardrail system was installed to complete the bridge. Bridge behavior predicted by grillage models for each bridge was validated by strain and deflection data from field tests; it was found that the engineered RRFC bridges have live load stresses significantly below the AASHTO Bridge Design Specification limits. To assist in future RRFC bridge projects, RRFC selection criteria were established for visual inspection and selection of structurally adequate RRFCs. In addition, design recommendations have been developed to simplify live load distribution calculations for the design of the bridges. Based on the results of this research, it has been determined that through proper RRFC selection, construction, and engineering, RRFC bridges are a viable, economic replacement system for low-volume road bridges.

Field Testing of Railroad Flatcar Bridges Volume I: Single Spans, TR-498, 2007

Based on the conclusions of IHRB Project TR-444, Demonstration Project Using Railroad Flat Car Bridges for Low Volume Road Bridges, additional research on the use of RRFC bridges was undertaken. This portion of the project investigated the following: (1) Different design and rating procedures; (2) Additional single span configurations plus multiple span configurations; (3) Different mechanisms for connecting adjacent RRFCs and the resulting lateral load distribution factors; (4) Sheet pile abutments; and (5) Behavior RRFCs that had been strengthened so that they could be used on existing abutments. A total of eight RRFC bridges were tested (five single span bridges, two two-span bridges, and one three-span bridge). Based on the results of this study a simplified design and rating procedure has been developed for the economical replacement bridge alternative. In Volume 1, this volume, the results from the testing of four single span RRFC bridges are presented, while in Volume 2 the results from the testing of the strengthened single span bridge plus the three multiple span bridges are presented.

Field Testing of Railroad Flatcar Bridges Volume II: Multiple Spans, TR-498, 2007

Based on the conclusions of IHRB Project TR-444, Demonstration Project Using Railroad Flat Car Bridges for Low Volume Road Bridges, additional research on the use of RRFC bridges was undertaken. This portion of the project investigated the following: (1) Different design and rating procedures; (2) Additional single span configurations plus multiple span configurations; (3) Different mechanisms for connecting adjacent RRFCs and the resulting lateral load distribution factors; (4) Sheet pile abutments; and (5) Behavior RRFCs that had been strengthened so that they could be used on existing abutments. A total of eight RRFC bridges were tested (five single span bridges, two two-span bridges, and one three-span bridge). Based on the results of this study a simplified design and rating procedure has been developed for the economical replacement bridge alternative. In Volume 1, the results from the testing of four single span RRFC bridges are presented, while in Volume 2,this volume, the results from the testing of the strengthened single span bridge plus the three multiple span bridges are presented.

Investigation of Steel-Stringer Bridges: Superstructures and Substructures, Volume I, TR-522, 2007

There are hundreds of structurally deficient or functionally obsolete bridges in the state of Iowa. With the majority of these bridges located on rural county roads where there is limited funding available to replace the bridges, diagnostic load testing can be utilized to determine the actual load carrying capacity of the bridge. One particular family or fleet of bridges that has been determined to be desirable for load testing consists of single-span bridges with non-composite, cast-in-place concrete decks, steel stringers, and timber substructures. Six bridges with poor performing superstructure and substructure from the aforementioned family of bridges were selected to be load tested. The six bridges were located on rural roads in five different counties in Iowa: Boone, Carroll, Humboldt, Mahaska, and Marshall. Volume I of this report focuses on evaluating the superstructure for this family of bridges. This volume discusses the behavior characteristics that influence the load carrying capacity of this fleet of bridges. In particular, the live load distribution, partial composite action, and bearing restraint were investigated as potential factors that could influence the bridge ratings. Implementing fleet management practices, the bridges were analyzed to determine if the load test results could be predicted to better analyze previously untested bridges. For this family of bridges it was found that the ratings increased as a result of the load testing demonstrating a greater capacity than determined analytically. Volume II of this report focuses on evaluating the timber substructure for this family of bridges. In this volume, procedures for detecting pile internal decay using nondestructive ultrasonic stress wave techniques, correlating nondestructive ultrasonic stress wave techniques to axial compression tests to estimate deteriorated pile residual strength, and evaluating load distribution through poor performing timber substructure elements by instrumenting and load testing the abutments of the six selected bridges are discussed. Also, in this volume pile repair methods for restoring axial and bending capacities of pile are developed and evaluated.