MPC
Research Projects (1999-00)

Identifying Number

MPC-178

Project Title

Experimental Wood-Concrete Railroad Bridge

University

Colorado State University

Project Investigator

Dr. Richard M. Gutkowski
Department of Civil Engineering
Colorado State University
(970)491-8291 or Fax (970)491-2788
gutkowsk@engr.colostat.edu

External Project Contact

Not Yet Established

Project Objective

The objective is to conduct pilot full-scale experimental tests on a composite, layered wood-concrete beam specimen for use as caps and chords in timber trestle bridge repair and upgrading. Year 1 will involve studies of configurations tested under ramp loads. Year 2 will examine repeated loading and tests after exposure to accelerated conditions of temperature and humidity. A prototype member configuration will result.

Project Abstract

Due to increased trainloads in recent decades, a 30 percent increase in design load requirements for timber trestle railroad bridges is imminent in the AREA design code. Existing bridges are being upgraded to more safely carry increased loads and to avoid potential structural problems leading to costly replacements. Large size solid timbers used in current configurations are increasingly difficult to obtain. Higher design loads will lead to even larger required sizes. The end result of the research is expected to be a prototype two-layer wood-concrete member, with the wood member being half the size of currently used fully solid wood members, thus obviating the industry's concern about lack of available timber sizes.

Task Descriptions

This project focuses on configuration of and pilot load testing of a mixed wood-concrete layered beam for use as timber trestle bridge caps and main chords. A set of smaller solid sawn wood members (much smaller than typical sizes used for caps and chords) will be augmented by a concrete layer to create the composite member. By inter-connection with a notched-shear key anchor detail it is potentially possible to create a high degree of composite interaction between the two layers. In this way the upper layer of concrete acts in compression (its strongest stress resisting mode) and the lower wood layer acts in tension parallel to grain (its strongest stress resisting mode). If a high degree of composite action is achieved, this is even superior to solid reinforced concrete beams as the lower half of concrete in solid concrete beams, cracks and serves only to hold expensive steel rebar in place. This material waste in solid reinforced concrete beams leads to cost inefficiency. Cracking also subjects steel rebar to corrosion and the concrete to spalling possibilities. Thus, improved material durability performance as well as cost savings are expected for the envisioned mixed wood-concrete member vis-a-vis (a) all wood caps and hords and (b) solid reinforced concrete caps and chords.

• Task 1 – Configure a set of 4-5 notched shear key anchor connection specimens with the feasibility to effect the needed interlayer load transfer. (R. Gutkowski - December 15, 1999)
• Task 2 – Acquire material for the connection specimens and construct the specimens. (R. Gutkowski - February 28, 2000)
• Task 3 – Conduct load-slip tests on the connection specimens. (R. Gutkowski - April 15, 2000)
• Task 4 – Configure the geometry of the layered wood-concrete beam specimens. (R. Gutkowski - May 15, 2000)
• Task 5 – Acquire material for 3-4 replicated beam specimens and construct the specimens. (R. Gutkowski - July 31, 2000)
• Task 6 – Conduct the beam load-deflection tests under ramp and repeated loading and evaluate the data and performance of the specimens. (R. Gutkowski - September 15, 2000)
• Task 7 – Prepare an MPC interim letter report on the findings and recommendations. (R. Gutkowski - October 31, 2000)

If successful in configuring an effective pilot beam member configuration in Year 1, additional studies are needed and planned in Year 2. These will focus on the durability issues of repeated exposure to extremes of temperature, moisture and humidity. A state-of-the-art environmental test chamber is available (at CSU) to conduct such conditioning under exposure to accelerated cycles of such hydrothermal conditions. The conditioned specimens will then be load tested to examine performance. This work will be proposed for renewal via the annual MPC project proposal process, as described in the MPC Strategic Plan recently approved by the federal agency.

Milestones, Dates

• Starting Data: November 1, 1999
• Project Milestones:
  • Configuration of an effective interlayer notched shear key anchor connection (see above action items).
  • Configuration of an effective layered wood-concrete beam geometry (see above action items).
  • Interim MPC letter report (see above action items)
  • A MPC research report is planned in Year 2.
  • A MS thesis is expected in Year 2.
  • A prototype member configuration will result in Year 2.
• Ending Date: October 31, 1999 (Year 1 phase)

Yearly and Total Budget

Student Involvement (e.g. Thesis, Assistantships, Paid Employment)

Partial support for a graduate research assistant will be provided in this project. If the project continues into a second year, it is expected that a thesis on the topic will result from the effort. Undergraduate or graduate student laborers will also be employed, as needed for the physical laboratory work.

Relationship to Other Research Projects

A successful Year 11 MPC project has shown composite wood/concrete decks for short span highway deck bridges with an efficiency (degree of composite action realized) of 80 to 90 percent can be achieved under ramp loads. Preliminary to the bridge deck tests, some exploratory small depth beams were also tested to determine likely efficient deck geometrics. They exhibited moderately high efficiency, too. Another Year 12 MPC project is being undertaken to examine such members under repeated loads. This project on railroad bridges addresses the extension of the highway bridge beam tests to an application in railroad bridge cap and chord members. Much larger (but still short span) beams and considerably heavier loads are involved in railroad bridges. Thus, the project is a pilot study to examine a significant extension beyond the specimens involved in the ongoing highway bridge study. It benefits from the outcomes of the ongoing studies of highway bridge specimens, but is dramatically different in challenge.

Past MPC projects in Year 10 and 11 have been conducted in cooperation with the Association of American Railroads (AAR) on field load testing of three existing timber trestle railroad bridges and follow-up field tests of one of the bridges after it was strengthened by adding traditional solid timber chord plies. A laboratory study of full-scale timber trestle bridge chord to examine that and other field repair methods is ongoing. Those studies related to the need to strengthen existing timber trestle bridges by traditional methods. This project complements those studies by seeking to develop a prototype wood-concrete member as an alternative to using large timber members to upgrade old bridges as well as construct new ones.

Technology Transfer Activities

The project involves an innovative idea for strengthening timber trestle railroad bridges and solving a construction material supply concern. Thus there is potential for partnering with the AAR. The technology transfer attributes of the project speak directly to the main mission of the AAR. While short line railroads have not traditionally been extensively involved with AAR research, the work described herein is valuable to all timber trestle bridges, whether on short line or main line bridges. Because of the nature of the bridge strengthening strategic planning by the AAR, these technology service providers are expected to be included as partners in the development effort for this proposal. This approach is consistent with other projects, which have been undertaken by the AAR such as those described above. Year 1 provides for the pilot studies to demonstrate an initial proof of concept outcome for the innovative idea. The results of Year 1 studies will be conveyed to the AAR to foster interest and support for Year 2.

Potential Benefits of the Project

The MPC project effort will focus on the needs for upgrading and maintaining timber trestle bridges on short line and spur trackage. As main line freight rail has become increasingly focused on the transportation of certain commodities and intermodal freight, short line railroads have developed with a more critical cost structure. These railroads often serve a small number or even a single important customer. The short line railroads are often critical to the transportation infrastructure of rural areas which may depend on a rail spur for transportation of a key commodity which is produced in the region. However, the fundamental differences in the cost structures of the short line railroads can potentially make maintenance a significant risk to the financial viability of a short line railroad. Any significant maintenance investment by the railroad could easily overwhelm the revenue potential of the railroad. Thus, cost containment is critical to remain in business. However, the safety of the same region the railroad serves may be dependent on proper bridge maintenance as well. Thus the development of an alternative member configuration for main bridge elements, in lieu of large solid sawn timbers can be an important cost saving approach.

TRB Keywords

Bridges, composite, laboratory tests, load tests, trestle, wood-concrete members