Frederick R. Rutz

Alternate Load Paths in Historic Truss Bridges: New Approaches for Preservation

Engineers who work on historic truss rehabilitation encounter projects for conversion of former vehicular bridges to pedestria n usage. The engineer finds today’s design vertical live loads to be on the same order as those used by the designer from a century ago. Material allowable stresses are also similar (or better). But design wind load is significantly higher. This paper examines non-traditional load paths as an alternative to conventional “skeleton” approach to analysis. The authors’ conclude that alternate load paths do exist and could be utilized by designers using commonly available computer software. Modern Wind Load Criteria One avenue for preservation is conversion of such bridges to pedestrian use. This kind of adaptive reuse permits ready access to historic structures and has the added advantage of providing incentives for continuing maintenance. But the prev ailing code, AASHTO Guide Specifications for the Design of Pedestrian Bridges (AASHTO, 1997) mandates higher design wind load than was used originally. Structural engineers attempting to convert historic bridges to pedestrian use often discover that the o ld structures don’t have the lateral capability to resist today’s AASHTO wind load criteria. This contributes to either a “heavy -handed” design approach, which is both expensive and detrimental to the historic character being preserved in the first place, or to condemnation of the bridge.

Structural Health Monitoring with Interferometric Radar

Changes in fundamental frequency in structures can reveal if a loss of stiffness has occurred, which may be an indication of critical deterioration. This paper presents a radar-based method that measures member deflection as a function of time - providing data which can be used to determine fundamental frequencies - for virtually all structural elements in the line of sight and in a non-contact manner. This paper summarizes interferometric radar theory, presents various applications, and reports on lab testing for verification of utilizing radar to monitor vibrations in cables. While interferometric radar has been used to monitor vibrations in light poles, wind towers, buildings, and other structures, particular attention is given herein to cable stayed bridges. Limitations are also discussed.

Ultrasonic Study at Cimarron Narrow-Gauge Railroad Bridge

Ultrasonic testing can be useful in evaluating components of historic metal bridges, although obtaining meaningful data can be hampered by field conditions. Results from ultrasonic testing on the pins of an historic wrought iron bridge near Cimarron, Colorado are presented. This paper discusses the checkered history of the bridge, the current structural assessment, field problems of obtaining meaningful ultrasonic data, and recommendations for their solution

Wind Load Analysis of a Truss Bridge at Rifle Colorado

Current AASHTO requirements for pedestrian bridges may prove some historic truss bridges to be under-strength when applying wind loads to simple skeleton models. The Rifle Bridge over the Colorado River at Rifle, Colorado, is a historic steel truss structure that was one of five in a study to analyze actual wind loads on existing structures. This paper discusses the effects of including stiffening elements in 3D models by comparing actual and calculated wind loads. During the six week wind study period, maximum wind loads measured were in excess of 60 mph, which resulted in easily measured strains. Analytical models include the metal deck with asphalt as a stiffening element, which is treated as plate elements with a modulus representative of the composite section. Recommendations for modeling the deck are provided.

Field Testing and Data Acquisition of Historical Truss Bridges using Modular Strain Transducers

Teby V. Herrero, Engineer III, Brown and Caldwell, Suite 200, 1697 Cole Blvd., Golden, CO 80401; PH 303/239-5400; therrero@brwncald.com Frederick R. Rutz, Senior Project Manager, J.R. Harris and Co., 1776 Lincoln Street, Suite 1100, Denver, CO 80203-1080; PH 303/860-9021; fred.rutz@jrharrisandco.com Kevin L. Rens, Associate Professor, University of Colorado at Denver, Department of civil engineering, Campus box 113, Denver, CO 80217-3364.

Prowers Bridge Study: Experimental and Analytical Techniques for Wind Loading Analysis at an Historic Truss Bridge

The University of Colorado at Denver has been studying the relationship between wind loading and structural response in historic truss bridges adapted to pedestrian use. Currently, many historic truss bridges with traditional timber decks would be inadequate for pedestrian conversion using the traditional skeleton method of modeling and the current American Association of State Highway and Transportation Officials (AASHTO) Guide Specifications for Design of Pedestrian Bridges for lateral (wind) design loads (AASHTO 1997) on the windward bottom chord members (eyebars). An experimental and analytical study was completed on the Prowers Bridge over the Arkansas River, constructed in 1909, which is located near Lamar, Colorado. The experimental study utilized data from anemometers and clamp-on modular strain transducers to provide verification of an analytical deck model of the current Prowers Bridge. This paper presents the equipment, results with methodology, and engineering applications based on the experimental and analytical response to the lateral (wind) loads at the Prowers Bridge. The overall results indicate that increasing the dead load of the deck and accounting for the stiffening effect of the deck in the analytical model allows the windward bottom chord eyebars to satisfy AASHTO lateral (wind) loading requirements. In summary, this research provides useful applications to aid rehabilitation and restoration of historic vehicular truss bridges for pedestrian use.