West Coast Engineering Group Pioneers a New Approach for Analyzing Free-Standing Structures with ALGOR MES Software

West Coast Engineering Group, Ltd. (WCEG), headquartered in Delta, British Columbia, Canada, is a leading maker of steel and aluminum poles for numerous custom applications including traffic signal poles, ornamental street lamps, sporting event lighting rigs, highmast antennas, windmills and electrical power transmission towers. As part of its pole design and manufacturing process, WCEG uses finite element analysis (FEA) software from ALGOR, Inc. of Pittsburgh, Pennsylvania to virtually test and predict how its designs will respond to environmental and operational loads.

"For more than eight years, numerical simulation has been an integral part of our design process, helping us to build better products for the benefit of our customers," said Ioan Giosan, P.Eng. and Senior Design Engineer with WCEG. "In support of our research and development programs, we have set up a specialized 'virtual laboratory' for testing and optimizing our products – as well as our complex technological manufacturing processes – with state-of-the-art ALGOR FEA software."

In particular, Giosan has used ALGOR's Mechanical Event Simulation (MES) software for several projects including analysis of wind turbine towers, free-standing structures, earthquake simulation and a roll-forming technological process. "MES is one of the most advanced engineering technologies available," said Giosan. MES provides nonlinear, multi-body dynamics with large-scale motion, large deformation and large strain with body-to-body contact, which allows engineers to see motion and its results, such as impact, buckling and permanent deformation.

Investigating Dynamically Induced Loads in Free-Standing Structures

As part of research at WCEG, Giosan used ALGOR MES software to investigate how to more realistically simulate the effects of dynamically induced loads (caused by interaction with the environment such as ground motion, wind gusts or a broken transmission wire) in free-standing structures (such as poles, antennas, highmasts and transmission towers). "Dynamically induced loads are a common source of fatigue and fracture in free-standing structures," said Giosan.

These photographs show fatigue failure in an antenna tower, which occurred as a result of dynamically induced loads. (Photos provided by Morrison Hershfield consulting company.)

Typically, fatigue occurs in products where a material is repeatedly cycled through varying stresses (that either modulate in intensity or actually reverse). Over time (perhaps months or years depending on the frequency), the cyclic stresses cause a weakening of the material that results in fracturing and failure at significantly less than the material's yield strength. "It is widely recognized that approximately 80 to 90 percent or more of mechanical failures arise from fatigue and fracture problems and most fatigue failures can be traced to deficiencies in design rather than inadequacies in material or improper manufacturing or maintenance methods," said Giosan.

According to Giosan, using the methodologies presented in industry design standards (AASHTO, 2001; CAN/CSA-S6-00; NBC, 2005; and others), all forces will be approximated and applied in a static fashion; additionally, the damping coefficients will not fit properly for all natural frequency modes.¹ "Most of the design standards present methodologies to design the structures for fatigue, but lack methods for realistically accounting for the fatigue sources – these being the interactions between the analyzed system and the surrounding real-world elements, which in most cases are of a nonlinear nature," he said. "This approach distorts the system response by eliminating the most dangerous loads, the cyclic ones, which potentially generate fatigue in the system's critical connections. To avoid this inaccuracy, we use a multi-body, dynamic, fully nonlinear approach."

Giosan developed a new methodology using ALGOR MES software and custom-calculated damping coefficients to analyze critical connections in free-standing structures under dynamically induced loads. His custom procedure involves first performing a modal frequency analysis to determine a structure's natural frequencies and then calculating the Rayleigh damping coefficients. Giosan's procedure for calculating damping coefficients is applicable to a large range of tubular, multi-sided, tapered, free-standing structures with a height less than 50 meters. "Using this method, in conjunction with MES, the engineer no longer has to approximate the loads and apply them statically," said Giosan. "The simulated interaction will be a realistic one, triggering the fatigue sources, which are the cyclic loads."

Simulating a Transmission Tower

To test his method, Giosan used ALGOR MES to analyze a Y-shaped, 400-kV electrical power transmission tower. The tower had been designed and manufactured by WCEG for a client, BC Hydro (one of the largest electric utilities in Canada), nearly 10 years ago, and it was installed in Abbotsford, a city located approximately 40 kilometers southeast of Vancouver, British Columbia. "The ALGOR analyses were performed last year as an exercise to verify my new methodology," said Giosan. "I chose this tower because of the complexity of its welded and bolted connections."

This Y-shaped, 400-kV electrical power transmission tower in Abbotsford, British Columbia, Canada, was designed and manufactured by West Coast Engineering Group, Ltd. (WCEG) for a client, BC Hydro. The inset illustrations show close-up views of critical connections in the tower where dynamically induced loads can potentially cause fatigue and fracturing. WCEG Senior Design Engineer Ioan Giosan studied this tower with ALGOR Mechanical Event Simulation (MES) software.

Giosan created a CAD model of the tower geometry in SolidWorks and then opened the model in ALGOR MES software for analysis. With ALGOR's easy-to-use single user interface, FEMPRO, he entered the custom damping coefficients that he had calculated for the tower. Next, the MES analysis was performed to determine the dynamic behavior and stresses induced in the tower when a transmission wire breaks during a high wind load. Forces derived from the full tower assembly were subsequently applied to a detailed sub-model of the tower base, which was analyzed separately.

"The new design approach used by WCEG placed the system in direct contact with the surrounding environment and calculated the loads based on this interaction," said Giosan. "Using advanced nonlinear numerical simulations, we are able to predict, with a high degree of precision, the stress distribution in complicated, welded, flanged connections and around cutouts in the structure's shaft."

Results from analysis with ALGOR MES software show von Mises stress contours in the full model of the Y-shaped transmission tower (left) and the sub-model of the tower base (inset on right).

Future Plans for FEA

In addition to the research described above, Giosan also developed two related methodologies for: 1) analyzing a free-standing structure's response to dynamically induced stress by vortex shedding phenomenon; and 2) numerical simulation of seismic loads to analyze and optimize the structure's ability to withstand a potential earthquake. Giosan plans two additional phases to complete his goal of outlining methodologies and simulation procedures for designing free-standing structures: 1) investigate the wind drag coefficient for a large variety of cross sections and wind speeds; and 2) build a test stand and apply load tests on tubular, free-standing structures in order to validate the design procedures and experimentally determine the damping ratios.

Giosan added, "Our FEA research programs, using ALGOR software, have enabled us to establish WCEG as a leader in technological innovation within our industry."

As part of his research into outlining methodologies and simulation procedures for designing free-standing structures, Ioan Giosan, P.Eng. and Senior Design Engineer with West Coast Engineering Group, Ltd., used ALGOR MES software to realistically simulate the effects of dynamically induced loads on the critical connections of a Y-shaped transmission tower.

Ioan Giosan, P.Eng., earned a Ph.D. in Mechanical Engineering from University "Politehnica", Bucharest, Romania. For twelve years, he worked as a Research Engineer with "Research & Development Institute for Thermo-Power Equipment" in Bucharest. Since 1995, he has worked as a Senior Design Engineer with West Coast Engineering Group, Ltd. in Delta, British Columbia, Canada. For more information about WCEG, visit www.wceng.com.

¹ See "Dynamic Analysis with Damping for Free-Standing Structures Using Mechanical Event Simulation".