ITALIAN CONSULTING FIRM TURNS TO ALGOR FEA FOR CRITICAL ANTENNA DESIGN

The engineering consulting firm of Studio Tecnico Zocca in Bologna, Italy, has introduced customers in several industries to Finite Element Analysis (FEA). Recently, Ing. Alessandro Zocca, mechanical engineer and principal member of the firm, was asked to design, build and mount a 20-foot high group of rotating antennas atop an existing 85 foot steel tower. The tower required extensive modifications in order to support the group of antennas under a variety of conditions.

Protecting a Radiotelescope

The purpose of the project is to locate the source of illegal radio transmissions which are interfering with the work of a large radiotelescope in Medicina, near Bologna. The Northern Cross Radiotelescope (Croce del Nord) is operated by the Radioastronomy Institute of The University of Bologna. The telescope gets its name from its cross-like shape with long arms extending in four directions. Since the radiotelescope searches for very low intensity astronomical radio signals, the presence of even a small amount of interference can effectively put a halt to the research.

The rotating group of antennas includes 11 low frequency antennas, two 5.3-foot diameter high frequency parabolic radio receivers, and a large amount of electronics. Since the antennas are designed to be highly directional, precise mounting and positioning is critical. For example, the angular error acceptable for the project was ?.15^o. In practice, Mr. Zocca was able to achieve a low ?.08^o level of angular error.

Structurally, the group consists of a tubular framework welded to a steel base which is bolted to a rotating support assembly. An asynchronous motor driving a worm gear speed reduction system rotates the antennas at speeds of 0 to 1.45 rpm.

The completed 85 foot steel tower is shown with the antennas mounted

Tower Redesign

In order to reinforce the existing tower to accept the large group of antennas, Mr. Zocca began by making a close inspection of the structure. The tower is constructed of steel beams connected by bolted joints. It has a square base section of 5 x 5 feet, which terminates on a concrete slab and foundation. "Using Algor, we passed the loads and moments resulting from loads applied to the tower, to the engineer who designed the foundation," says Mr. Zocca, "He verified its integrity."

A series of panels with a small grid pattern covers the entire outside surface of the tower. Their purpose is to protect people using a circular staircase built into the tower's center. The presence of these panels significantly increases the force of the wind against the structure. At the top of the tower is a 3.9 x 3.9 foot platform where technicians work on the electronic equipment. Four non-reinforced, L-shaped beams are attached to the center of the platform. It is here that the antennas were to be mounted.

The Analyses

According to Mr. Zocca, "We began by designing and manually verifying the rotating antenna group. Then, we applied loads and moments, caused by wind and snow. We ran several analyses in order to properly reinforce the upper part of the tower while maintaining access to the internal staircase.

"The basic tower structure was drawn using AutoCad 10, then passed to Superdraw II where area properties were calculated," he continues. "The wireframe was passed to Beam Design Editor where I applied boundary conditions, gravity and uniform loads (to simulate the presence of people working on the tower) and finally forces and moments. I found it very easy to separate different beams with colors and orient them with group codes."

The model of the tower has 443 beam elements and 344 nodes. According to Mr. Zocca, "The tower was verified, in accordance with Italian Code, for a wind of 140 km/h and a snow load of 900 N/m². In all, we built four models of the tower, each with two load cases to simulate wind coming from different directions. After the analysis, we had a model of the reinforced tower with good stress distribution along the entire steel structure, without the stress peaks in the upper part of the tower found in the original design."

Mr. Zocca used Algor's Beam Design Editor to construct the model of the tower.

Modal Analysis

"On the final model," continues Mr. Zocca, "we also ran a dynamic modal analysis reaching the first six natural frequencies of the tower with the new antenna group on top. Next, we performed a time history, simulating an impulsive wind load acting in resonance with the lowest frequency. Finally, using Supercap, we produced a slide show of the various analyses for our customers. They liked it very much."

About Algor

On the subject of Algor FEA software Mr. Zocca says, "We decided to buy Algor because of its ease of use with no lack of performance in terms of analysis possibilities and model size limitations. We are still very happy with our choice." About the accuracy of Algor analysis results: "On the final model, we performed a manual calculation. We obtained the same stress results as Algor FEA, but we experienced a lot of human physical stress solving the manual equations!"

In addition to their work on structures such as the radio tower, Studio Tecnico Zocca uses Algor design and analysis software in the creation of high quality machine tools, spindles and tooling fixtures, using FEA in both static and crucial vibration analyses. The company works with customers in a wide variety of industries throughout Italy.