Civil Engineers Make Historic Bridge Safer

ALGOR SOFTWARE HELPS CIVIL ENGINEERS MAKE HISTORIC BRIDGE SAFER

This Photo of the Smithfield Street Bridge was taken in 1889. It shows the unique lenticular truss structure. (Photo courtesy of the Carnegie Library of Pittsburgh.)

Mackin Engineering Company Re-engineers Pittsburgh's Famous Smithfield Street Bridge with Algor Software.

The Smithfield Street Bridge links Pittsburgh's corporate "Golden Triangle" downtown area with the city's South Side, a bustling commercial, entertainment and residential district. The bridge, more than 100 years old, is located a quarter mile from where the Monongahela and Allegheny rivers meet to form the Ohio River.

A Piece of History

Owned by the Pennsylvania Department of transportation, the bridge has a number of historic designations including National Historic Landmark, City of Pittsburgh Historic Landmark and National Historic Civil Engineering Landmark.

Designed by internationally-renowned civil engineer Gustav Lindenthal, the unique "lenticular truss" structure of the bridge causes it to vibrate, which could lead to dangerous structural fatigue and failure. When the bridge opened in 1883, it was designed to carry trolley and pedestrian traffic. Lane expansions and other renovations were performed in 1891, 1911, 1933 and 1975. The most recent renovation brought the bridge to one lane of traffic in each direction with the original trolley tracks still in place.

First FEA Experience

The renovation undertaken by Mackin Engineering includes removing the trolley tracks and providing for three lanes of traffic, with the center lane changing directions depending on traffic volume. Since the bridge experiences considerable vibration, Mackin engineers needed to work with finite element analysis software, which they had never done before. In addition, they needed to perform extensive modeling along with dynamic and modal (natural frequency) analyses to test possible replacement deck designs. Mackin selected Algor software after an exhaustive search of engineering software products on the market.

"A number of academic and industry sources referred us to Algor due to the high quality of Algor products. As first-time users of FEA software, it was of great importance to us that we deal with an accessible supplier who was ready to work with us. Algor provided support throughout every step of the project," explains Thomas Riester, chief structural engineer for Mackin.

Many vibration analyses like the one shown here were run by engineers from mackin Engineering to make the bridge safer.

A Doubting "Thomas" Convinced

"Because this was our first experience with FEA software, we were somewhat skeptical about how it could help civil engineers, admits Riester. "But we know that Algor software was a winner when we performed actual load testing on the bridge for strain measurement and then put Algor software to the same test. The software matched and corroborated our field results.

As the Mackin engineers learned, instead of relying solely on building or structural code, an engineering team using Algor software can translate the uses, actions and potential vulnerabilities of a planned structure into a set of specifications that will, in most cases, go far beyond the code. The engineering team can employ advanced technology to design and test every critical aspect of the structure.

This photo shows construction work underway on the Smithfield Street Bridge. When completed, the bridge will carry three lanes of traffic.

Models with Multiple Element Types

Steve Keller, the Mackin engineer performing the design analysis, had to examine vibration and natural frequencies. "I needed FEA software that could perform dynamic analysis to determine the bridge's behavior under highway loads due to live loads and frequency of vibration. I used Algor's Beam Design Editor to model portions of the bridge constructed with beams and trusses. The deck was modeled using plate/shell elements. The two models were combined using Algor's COMBSST program to create a single FEA model," says Mr. Keller.

Two-Phase Analysis

During the first phase, design analysis to determine the bridge's natural frequencies was performed on the structural models, including the existing structure and three alternatives. the frequencies for the first and second modes of vibration (in-plane) were identified to determine the effect of the proposed heavier floor systems on the structure frequencies.

The second phase of analysis, a direct integration analysis using time varying loads, was performed on the structure models to determine maximum stresses for selected truss members and maximum truss deflection. A single time-dependent forcing function was developed to simulate the test truck used during actual load testing. As a result, the existing structure model member stresses could be compared to stresses determined from strain gage measurements to verify model accuracy.

Finding the Best Solution

By using Algor software, Mackin engineers were able to efficiently look at the existing flooring system and possible replacement options. In selecting a replacement for the aluminum flooring system and aluminum orthotropic deck, dating to 1933, Mackin engineers found that the dynamics of the bridge would be affected little by the type of flooring system chosen. Therefore, they were able to select the least expensive system feasible - a filled steel grid deck - which also required the lowest future maintenance.

Mackin has offices in both Pittsburgh and Mechanicsburg, PA. Its client base includes the Pennsylvania Department of Transportation and the Pennsylvania Turnpike commission. With a staff of 100, Mackin is involved in major highway and bridge products throughout Pennsylvania.