MPC
Research Projects (2000-01)

Identifying Number

MPC-193

Project Title

Rigorous Computer Modeling of Timber Trestle Railroad Bridges

University

Colorado State University

Principal Investigator(s)

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

Jeno Balogh, Visiting Researcher
Department of Civil Engineering
Colorado State University
(970)491-8291 or Fax (970)491-2788
jbalogh@engr.colostate.edu

External Project Contact

N/A

Project Objective

The objective of this research is to utilize available commercial 3-D finite element software to rigorously model standard open deck timber trestle railroad bridges. Ordinary structural mode ling has proven inadequate, due to the presence of "discontinuities" such as loosened connections, gaps due to shrinkage to moisture/drying cycles; support movement, differential bearing of members, etc. There is a need to reflect such features into an improved structural analysis modeling for accurate assessment of structural response to train loads.

Project Abstract

Nationally, the structural condition of short span timber trestle railroad bridges is one of concerning circumstances. Reports by the Association of American Railroads (AAR) indicate that degradation has been occurring with material failure evident at some sites. Railway car weights and trainloads have increased considerably. Deterioration due to this heavy, frequent loading and aging and exposure is an important issue. Loosened connections, gaps due to shrinkage to moisture/drying cycles; support movement, differential bearing of members, etc. develop over time and are present in older bridges. These alter load capacity and stiffness from the original condition and significantly affect behavior. To improve understanding of the load paths and structural behavior it is necessary to accurately analytically model the geometry and member condition and properties of actual bridges. A recently developed, leading edge commercial software (Axis VM) will be employed to simulate (model) the standard open deck, timber trestle bridge configuration, including representation of actual condition. The resulting model will be used to predict the displacement response of the bridge under static loadings. Predicted response will be verified by comparison with the results of past load tests of such bridges.

Task Descriptions

An extensive database of measured load-deflection and load-strain data exists from prior load tests of open deck, timber trestle bridge s. In particular CSU researchers have conducted recent load test studies (see section 11 below). Empirical examination of load distribution in stringers of chords was included in that work. Current AREA code analytical modeling techniques are based on a simplified, idealized representation of the standard open deck, timber trestle bridge. Evidence exists from field load tests to suggest that this modeling leads to inaccurate calculated lo ad distributions within members. The aim of this research is to improve upon current analytical modeling through the use of contemporary advanced commercial software for structural analysis. The planned tasks are:

• Task 1 – Examine the available geometry, condition and structural response data from past load tests for general and specific implications to the rigorous modeling.
• Task 2 – Develop a detailed computer model using the AxisVM software.
• Task 3 – For selected loadings used in past field and laboratory tests, predict the field (and laboratory) response on the bridges (and laboratory specimens).
• Task 4 – Compare the analytically predicted structural responses for each loading with the counter part physically measured responses.
• Task 5 – Assess the accuracy of the computer model.
• Task 6 – If necessary, refine the computer model to improve the analytical predictions.
• Task 7 – Compare the computer model results with those implied by the current AREA design code provisions.
• Task 8 – If appropriate, make recommendations concerning possible changes in current code provisions.

The following deliverables will be provided based on the research findings:

A computer modeling technique to accurately model standard open deck, timber trestle bridges. A final report documenting the outcome of the research, including recommendations on appropriate design code changes, if warranted.

Milestones, Dates

• Starting Date: July 1, 2000
• Project Milestones:
  • Computer modeling technique April 30, 2001
  • Draft research report May 31, 2001
  • Final research report June 30, 2001
• Ending Date: June 30, 2001

Yearly and Total Budget

This is a one-year project. The attached budget is for July 1, 2000-June 30, 2001. The amount requested from USDOT is $10,404 and is augmented by $14,647 as anticipated CSU and external match. The total budget is $25,051.

Student Involvement

A part-time student (undergraduate or graduate level) hourly research aide will be active on the project. He/she will be involved in examination of the past load test data for use in this study, as well as in application of the computer model and assessment of the analytical results vs. actual measured behavior and existing AREA design code provisions.

Relationship to Other Research Projects

CSU researchers have been cooperating with the AAR on load testing for Structural assessment of condition of existing older open deck, timber trestle railroad bridges. Several bridges have been load tested in the field in a past MPC/AAR funded multi-year project. Later, a full-scale chord of three span timber trestle bridge was subjected to laboratory load testing in a Year 12 MPC project. It replicates the chord of an existing bridge previously field-tested before and after strengthening. These project outcomes provide an extensive database of actual measured geometry and structural response under real and simulated train axle loadings. This data will be used for verification of the accuracy of the computer modeling to be undertaken in this project. The past projects also strongly suggest that dynamic impact of moving trains is not a major factor in the displacement and strain response of the bridges for the loads and train speeds utilized. Hence, the modeling will focus on static loads for fixed positions of the axle loads actually used in the past load tests.

Technology Transfer Attributes

Techniques for advanced computer modeling will result that are usable in Commercial software. The final report will reflect the input from the AAR bridge research staff and be disseminated via the various modes of the MPC itself and those within the UTCP overall.

Potential Benefits of the Project

Railroad bridge failures or closures can be devastating to local, state and regional agricultural and freight economies. Increased train axle loads have been occurring in recent decades. Also, increased trains are resulting in consideration of a 30% increase in the axle loads used in design code provisions. Consequently, some existing bridges need to be strengthened to more safely carry these increased loads to avoid potential structural problems leading to costly replacements. Improved analysis will improve safety of old bridges by way of more accurate prediction of structural behavior. The aim is to determine correct analytical procedures vis-à-vis current code design procedures. The AAR is implementing such research to develop strategic plans for the next and intermediate term retrofit needs for their large inventory of timber bridges. The outcomes are also directly pertinent to recommendations being developed by the AAR for updating applicable railroad bridge design code procedures and provisions.

TRB Keywords

Bridges, computer modeling, railroad, timber, trestle