IDAC Case Study

Case Study

South Midlands Communications Ltd

Macro Development for a Structural Analysis of a Telescopic Mast



	Company Profile
	Established in 1958, South Midlands Communications Ltd (SMC) initially specialised in antenna support masts, towers and HF antennas. SMC is now recognised globally as a specialist communications enabling company. The last 50 years have seen the SMC product range grow to include pneumatic telescopic masts, aluminium lattice masts, high-level photography solutions, Mil' specification antenna positioners and two way radio products including GPS, AVLS and data transfer. SMC are able to advise, design, supply and install mobile communications systems, from the simple to more complex and integrated networks.
	Background

SMC are regularly required to design and manufacture telescopic communication masts, which are customised to handle customer specified load configurations. The deflections of the mast must be kept within a very tight tolerance for proper operation. Stresses also need to be kept within specified limits.

SMC required an automated procedure to analyse different design configurations based on a number of predefined design variables under a given set of loads. This would require a set of input design variables to be supplied by a 'User' to the ANSYS Software which would then create the Finite Element (FE) Model to be analysed, solve the FE Model and post-process the results into a set of text files and graphical plots for final assessment by an SMC.

IDAC were required to develop a macro that would allow SMC Engineers to predict the deflections and stresses seen by the communication masts they manufacture. The project to create this automatic procedure was split into the following phases:

Generation of the parametric ANSYS macro.
Generation of the design variables input file
Generation of the automatic post-processing macro

Analysis

A macro to generate the parametric model was written, so that a model consisting of a set of concentric tubes overlapping at the ends would be created in the ANSYS Software for analysis. Examples of the variables required for input by the SMC Engineers for each tube were as follows:

Length
Thickness
Diameter
Overlap distance

Each mast tube also had the option of being guyed. The tube guy sets could be either 3 or 4 guy ropes arrangements. The guy ropes could be solid (where tension and compression could be supported) or cable like (supporting tension only).

The geometry was meshed with 1D pipe elements, element type PIPE16 and the guys were meshed with either LINK8 (tension and compression) or LINK10 (tension only) elements.
The overlapping of the tubes was modelled with coupling restraints between the tubes.

A three-dimensional nonlinear (large deflection) structural analysis was carried out. The mast will be fixed at its base and the free ends of the guys will also be fixed. The guys, if defined, were specified an initial pretension load. External loads can be defined as follows:

Horizontal and Vertical point loads at the top of the mast with an offset
Axial torque applied at the top of the mast
Internal pressure in mast
Wind pressure applied up the mast (defined by SMC based on an equation
dependent on height).

Material input was required for each tube; this consisted of Young's Modulus, Poisson's Ratio and failure criteria for safety factor calculations.

Click for larger image

The post-processing procedure was automated by means of a macro. Running of the macro generated a tabulated text file containing the tip deflection, the deflected angle and safety factors as well as all the input variables. A set of stress plots was also created. The figures to the left and right show typical deflection and stress plot graphical outputs.

Click for larger image

Design Benefit

IDAC have created an ANSYS application for SMC enabling their Engineers and Project Managers to run full and rapid three-dimensional nonlinear (large deflection) structural analyses, by entering mast parameter values as prompted. The application extends the use of ANSYS structural analysis software within SMC to a rapid assessment tool allowing them to access the impact of various customer driven loading scenarios on proposed mast configurations. This capability will improve SMC's ability to recommend the most reliable, safe and economical mast arrangements to satisfy the customer's various 'in-field' requirements in the shortest possible time. This makes SMC's engineering and sales department more responsive to design requirements and/or requests by clients to asses the impact of changes to the payloads imposed on an existing mast.