Modern Tests Demonstrate Soundness Of Old Iron Bridge

An unusual bowstring truss iron bridge that carried traffic across Roaring Run in Bedford County, Va. for almost 100 years is now a picturesque footbridge at the I-81 Ironto, Va. rest stop. Built in 1878, it is the oldest standing metal bridge in Virginia. In early December, a Virginia Tech undergraduate conducted a load-bearing analysis of the structure.

It may have been the first such test on the bridge. "There was no documentation of a structural analysis from when the bridge was designed," said Elaine Huffman of Bowie, Md., a student in civil engineering. As part of her research project, she did a historical survey of the bridge through a literature review, developed a computer-based structural analysis, and recently verified the computer model with an actual load test.

The bowstring truss design was patented by Z. King in 1859 under the name "Tubular Arch Bridge," Huffman learned. When the bridge was bypassed by a pipe culvert in the 1970s, it began to fall into disrepair. "The Virginia Transportation Research Council recognized the importance of the bridge as a historical landmark and worked to preserve it by restoring it and putting it into use elsewhere," said Huffman. "Much work was put into determining the original paint scheme and recreating it once the bridge was relocated. The new site was selected to maintain the function of the bridge." Huffman noted a number of unique features of the wrought iron bridge. For example, "there is a unique bracing system perpendicular to the truss that restrains lateral movement of the arch," she said. "Cross braces prevent longitudinal motion of the bridge deck as it hangs from the vertical cables."

In her computer model stress analysis, Huffman applied three different loads, two of them from the era of iron bridges. One test came from the 1893 Practical Treatise on the Construction of Iron Highway Bridges, which suggested that a uniform distributed load of 75 pounds per square foot (psf) be applied to ordinary country bridges 60 feet and shorter to represent a typical load. For a vehicular point load, the 1898 work, De Prontibus, suggested using a six-foot by eight-foot wagon load of five tons distributed equally between all four wheels. The third load was representative of the three-ton truck that would be used in the load test.

The deflected shape of the truss with the 3.5-ton wagon load was the same as the five-ton wagon load, Huffman determined. The uniform load created the highest stresses and highest deflections. "Generally, stresses are limited in a modern bridge design to 60 percent of the yield stress in service," Huffman said. "But in its current location, the bridge will most likely never see such high stresses because pedestrian traffic over it is neither constant nor high enough."

On December 3, Huffman carried out a load test to verify the accuracy of the computer model. Dial gages, which turn small linear movements into readable increments on a dial, were set up below the center of each truss. Then, a truck weighing three tons was driven across the bridge, pausing every five feet to record the deflection. "The bridge behaved as expected for the most part. The maximum deflection recorded for one truss was 0.14 inches, 70 percent of the result predicted by the model. However, the second truss deflected a smaller amount," Huffman said. "Preliminary analysis suggests that the diagonal cable members have loosened over time and are supporting the bridge loads unevenly, allowing one truss to deflect more than the other,"

The results from this test will be contributed to the Adaptive Bridge Use Project based at the University of Massachusetts Amherst and supported by the National Science Foundation (www.ecs.umass.edu/adaptive_bridge_use/). The program aims to restore and study historic bridges to enhance structural engineering curriculum and preserve examples of bridge designs from the past, said Huffman's advisor, Cris Moen, assistant professor of civil and environmental engineering. The Ironto Wayside footbridge, the last remaining bowstring arch-truss in the state, is a significant landmark in Virginia. "It is useful to study historic landmarks, as they can guide us in the design of future structures," said Huffman.

Her computer model can be used as an example for creating structural models to test other bridges, said Moen. "Perhaps the analysis will aid in the future assessment of the bridge's condition as it continues to be preserved as a historic landmark," said Huffman.

Cobham Software Performs Lightning Strike Tests

Cobham Technical Services has announced that its Opera electromagnetic design software has been used in an aircraft lightning strike simulation project. Lightning strike tests of a helicopter at Eurocopter's Donauworth facility have verified the accuracy of finite element analysis techniques for characterising the electromagnetic behaviour of complete and custom-cabled modern aircraft structures constructed using advanced composite materials.

The exercise was performed using the Opera software as a final element of the company's work for the ILDAS (In-flight Lightning Strike Damage Assessment System) project. Simulation of the ILDAS tests highlighted how finite element techniques can generate accurate models of complex assembled airframes and simulate the effects of lightning strikes rapidly - in around a day on a standard office PC - to help developers evaluate and optimise lightning protection measures during the design cycle. The industry's current certification against lightning is based on threat levels derived from measurements of cloud-to-ground strikes.

While this approach has served well for traditional airframes with good metallic conduction, modern aircraft are incorporating increasing amounts of lightweight composite materials. This makes them more susceptible to direct damage at lightning entry and exit points, and potentially to indirect energy coupling effects into the electrical systems as current flows through the aircraft. As a result, it becomes increasingly important to understand the exact nature of the threat by accruing data on actual in-flight strikes. Modelling the current flow patterns within complete assembled airframes with validated software can also reduce costly testing procedures.

The ILDAS project was conceived to develop an in-flight embedded system for measuring actual lightning strikes. This will help to better understand the threat, aid the design of lightning protection measures and streamline post-strike inspections and maintenance by capturing and communicating actual data on occurrences, intensity and strike points. Opera software was employed to predict lightning strike current flow patterns on structures with carbon-fibre composite materials. This knowledge helped ILDAS partners to select the best locations for sensors and then to compare current flow predictions against actual measurements.

To achieve this goal, Cobham Technical Services generated an electromagnetic design model of a specific airframe configuration for an EC135 helicopter using CAD files from Eurocopter. This part of the exercise mainly involved simplifying non-critical parts of the original design data in order to minimise simulation times, while maintaining good representations of critical elements such as metal space frames and surface panels, carbon composite panels, electrical bonding and cable harnesses - including those for client-specific equipment.

The modelling work took around two weeks, but this depended on experience gained from detailed modelling and analysis of typical composite structures, together with comparison with measurements and simulations performed by other partners in the project. With this supporting work in place, subsequent models of variations on the basic helicopter airframe would be much quicker to create - providing a simple means of evaluating aircraft construction programs. Once the model was ready, the simulation itself took a little more than a day to run on an office PC.

Real-life tests at Eurocopter's Munich facility then showed that the theoretical predictions of energy diffusion effects agreed very well with simulation predictions. 'Airframe structures making extensive use of composite materials have less natural protection against lightning,' said John Hardwick of Cobham Technical Services (Lightning Testing and Consultancy). 'As lightning protection measures such as conductive coatings or strips add weight it's important to optimise the design, and simulation provides an effective means of achieving this,' he added.

'These real-life tests of ILDAS's embedded monitoring system concept illustrate how airframe-specific lightning protection can now be accurately evaluated and optimised during the design cycle,' said John Simkin of Cobham Technical Services (Vector Fields Software). 'Finite element techniques make it easy to model complex airframe surfaces and important electrical details. 'The functionality of the Opera geometric modeller made it simple to accept CAD files and reduce the complexity of non-critical elements to ensure rapid simulation,' he added.

Ideal's 61-797 Tests Electrical Insulation

Ideal Industries has introduced the 61-797 Digital Insulation Meter, a rugged, industrial-grade tester designed for commissioning, routine troubleshooting and preventive maintenance. Tony Kumeta, general European manager of Ideal Industries (UK), said: 'The periodic testing of insulation for signs of deterioration will spot breakdowns in electrical systems, generators, switchgears and motor windings before failure occurs.

'More importantly, insulation testing can prevent the dangerous occurrence of short circuits or short to grounds.' The portable 61-797 can withstand the rigours of heavy commercial and industrial usage. Using test leads, the technician can quickly determine the integrity of insulation in new and existing wiring by identifying capacitive, absorption and leakage current at multiple test voltages (50, 100, 250, 500 and 1000V) and resistance up to 20G Ohms.

The 61-797 can also measure earth-bond resistance to assure verification of the continuity of the protective bonding. It automatically senses and displays AC/DC voltage to 600V and calculates the polarisation index (PI) and dielectric absorption ratio (DAR). Insulation testing is most effective when it is part of a scheduled maintenance programme. Repetitive testing is made easier with the 61-797's straightforward pass/fail function and its internal storage of values.

It also features twist-on alligator clips that fit over the test leads and a remote test-button for two-handed operation. The 61-797 was designed with a 600V Cat IV overvoltage rating and will safely disable itself if connected to a live circuit exceeding 30V. A live voltage hazard indictor is also prominently displayed on the front panel to warn the technician before use on an energised circuit.

Once testing is complete, the 61-797 automatically discharges capacitive voltage that may be a shock hazard from the equipment under test. The 61-797 comes with a carrying case, test leads and batteries.

GE Detector Tests Spot Welding

The Spotchecker from GE Sensing And Inspection Technologies is a hand-held flaw detector with application-specific software, which allows it to be used as a portable solution for spot weld testing. Contained within an environmentally protected housing, the instrument does not require intensive training and offers an 'expert' system for less qualified people in weld inspection.

Spotchecker will allow inspection to be carried out at the place of welding, rather than having to move welded parts to dedicated inspection points. Spotchecker is sealed to IP65 and weighs 3.5kg. Its ergonomically designed operator interface features an 8in LCD touch screen, six programmable function keys and twin track balls to allow ease of use, even in the most restricted of testing locations. It is battery operated and batteries can be 'hot swapped' to ensure continuous operation.

Battery charging can be done with batteries inside or outside the instrument, using an external charger. The instrument can also be remotely operated if required. The flaw detector can be connected to external periphery devices such as mouse, keyboard and printer either by USB or by WLAN. Data upload and download is performed wirelessly. The Ultralog application software included with Spotchecker is a tried-and-tested database program for the systematic inspection of spot-welded joints.

Combining ultrasonic expertise with electronic data processing, it provides a package for performing inspection, evaluation and documentation of results, while meeting strict quality management requirements. By setting out inspection and test plans, specifying the number of welds to be inspected, and describing the test location material data, test diagrams and ultrasonic settings, Ultralog allows Spotchecker to be used by relatively unskilled personnel.