MPC
Research Projects (2003-04)

Identifying Number

MPC-215

Project Title

Structural Modeling of Substructure Resistance for Timber Trestle Railroad Bridges, Year 2

University

Colorado State University

Project Investigators

Wayne A. Charlie, Professor
Jeno Balogh, Research Associate
Department of Civil Engineering
Colorado State University
Fort Collins, CO 80523-1372
(970)491-5048 or Fax: (970)491-7727
wcharlie@engr.colostate.edu
jbalogh@engr.colostate.edu

External Project Contact

N/A

Project Objective

The objective of the project is to quantify the load-deformation (stiffness) response of pile foundations supporting timber trestle railroad bridges. The load-deformation response is needed to improve the accuracy of determining the load capacity of these bridges.

Project Abstract

Load capacity analysis and computer modeling of timber trestle railroad bridges does not generally account for support (pile foundations) movement. However, for statically indeterminate bridges including timber trestle railroad bridges, computer simulations conducted during Year 1 of this research indicated that support movements alter the bridge's stiffness and capacity and may significantly influence its behavior. A need exists to better represent the substructure. The proposed research (Year 2) will quantify the load-deformation response of timber pile foundations for a variety of pile lengths and subsurface soil conditions.

Task Descriptions

Load capacity analysis and computer modeling of indeterminate bridges with flexible supports requires that the vertical load-displacement characteristics (spring-constants) of the foundations be known. The need exists to empirically quantify this in a manner representative of the field response of such bridges.

The main goals of this proposed 2nd year research study are:

• Complete a literature search to determine typical vertical load-displacement characteristics of timber piles as a function of pile diameter, pile length (free standing and below ground), soil conditions, timber type, and other variables. A considerable amount of data and empirical correlations and theoretical analysis are available in the Geotechnical Engineering literature.
• Examine data from past field tests on timber trestle railroad bridges to determine load-displacement characteristics. An extensive database exists and some useful data was collected in Year 1.
• Compare the results obtained in 1 and 2 above and develop design curves, equations and/or tables for determining spring constants for piles as a function of the variables given in #1 above.
• Conduct laboratory tests on a three span scale model timber trestle bridge. The tests are designed to determine the influence of support movements on deflections and capacity.
• Conduct rigorous computer modeling of the model timber trestle bridge.
• Compare the results obtain in #4 and #5 above.
• Seek funding to apply results to actual bridges previously tested by CSU and other researchers.

Colorado State University has an extensive database from its past field tests of three timber trestle railroad bridges. The work plan for those tests did not include determination of spring constants for use in structural modeling. Some measurements of displacements were taken near or at supports in each bridge. From a study in progress, it appears that this data is not sufficient for the purpose of establishing spring-constants for the pile supports. Conducting actual field pile load tests involve an expense that is not favorable at this time. Hence, load tests of reduced size bridge specimens will be conducted at field sites. The data could be used to refine and improve a preliminary structural model developed in Year 1 of this project.

Milestones, Dates

Starting Date: July 1, 2003
Literature search: October 31, 2003
Refine structural model: December 31, 2003
Scale model lab tests: March 31, 2004
Refine structural model: April 30, 2004
Compare analytical and computer predictions with model lab tests: May 31, 2004
Ending Date: June 30, 2004

Student Involvement

N/A

Relationship to Other Research Projects

In Year 10 Dr. Gutkowski conducted an MPC project in conjunction with the Association of American Railroads (AAR). The researchers field-tested three timber trestle railroad bridges. In Year 11, a follow-up project was conducted to strengthen one of these bridges and retest it. In Year 12 a full scale chord of three span timber trestle bridges were subjected to laboratory load testing. It replicated the chord of an existing bridge previously field tested before and after strengthening. The purpose was to compare field vs. laboratory behavior. An on-going project has the objective utilizing available commercial 3-D finite element software to rigorously model this specimen. The research proposed is a continuation of a Year 11 research project (MPC-215).

Technology Transfer Activities

Technology transfer will be via an MPC technical report. If warranted, a technical journal and/or conference paper will be prepared for submittal and an MPC research seminar will be presented over the TEL8 system after completion of the project. Publicity about all of the MPC projects is available through the MPC website maintained at NDSU.

Potential Benefits of the Project

Structural condition of timber trestle railroad bridges is of national concern. Railway car weights and train loads have increased considerably in recent decades. In fact, a 30 percent increase in code loads is under consideration. Deterioration due to this heavier, frequent loading and aging and exposure is an important issue. Support movement, loosened connections, gaps due to shrinkage to moisture/drying cycles, differential bearing of members, deterioration and other variables develop over time and are present in older bridges. These alter load capacity and stiffness from the original condition and significantly influence structural behavior. Security of these bridges is imperative and improved structural modeling contributes to better understanding of response to these increasing loads and field conditions. The research proposed will lead to a better understanding of support movements of bridges leading to a more realistic assumption of a bridge's response to loading.

This project provides for a critical development related to accurately modeling existing timber trestle railroad bridges. Improved analysis will improve safety of old bridges by way of better prediction of structural behavior. The aim is to determine correct analytical procedures vis-a-vis old bridges and assess the improvements resulting from different rehabilitation techniques.

TRB Keywords

Bridge, computer model, pile, railroad, substructure, timber, trestle