Mast Wind-Loading & Survival

Time：2018-05-30 View：

 We often receive questions regarding wind load survival - “simply stated, what can it withstand?”  Unfortunately there is no simple answer to this question and anybody that advises otherwise does not have your best interest in mind.

Most of the masts designed for mobility solutions were never intended to handle today’s antenna loads. Developments of today’s huge multi-beam and spherical antennas, in conjunction with the need to deploy multiple technologies, has pushed most of today’s masts well beyond their designed capacities.  Although the mast manufacturers are behind the curve, new masts have been or are being developed in an effort to better support these needs.

Many factors must be considered to assure structural integrity of any deployment.  I think it’s a fair statement that no carrier wishes to make the 6 O’clock news due to a failed mast or overturned COW at a public event.  I have summarized the following factors an engineer would consider for structural integrity:

• Deployment Location - Area wind speed differs by location; one should not assume that a proven deployment in Arizona would also survive in Chicago or Miami.
• COW/COLT Chassis Weight - Obviously the heavier the chassis the greater ballast against overturn moment.
• Outrigger Footprint - A wider outrigger footprint increases overturn withstand, whereas a narrower footprint reduces overturn withstand.
• Mast Design Orientation On The Chassis – With relationship to weight and outrigger configuration.
• The Mast - Design materials, tube thickness, tube overlap distance, and downforce tolerance.  Generally the guying footprint is 80% of mast height i.e. 100’ Mast would equal an 80’ guying radius.  As the guying radius is reduced downforce pressure on the mast is increased.
• Antennas - Quantity, sail area, weight, rad center height, and azimuths.  Certainly 3 antennas covering one sector creates a larger sail area than the same 3 antennas covering three sectors.  Antenna rad center; the higher the antenna rad center the greater sail force against smaller mast tube sections.  The greater the sail area, the greater chance for mast failure and/or overturn moment.
• Antenna Mounting Structure - This structure increases weight and wind sail area.
• Coax - Coax type has an impact on weight.  Coax size/diameter increases/decreases weight and sail area.  Also coax quantity increases weight and sail area.
• Guying - A deployment may or may not require guying based on all the outlined factors.  Assuming it does, the earth auger’s size and depth needs to be determined. Or one could use concrete ballast blocks once the required weight is determined.  Guying radius also needs to be determined.  See above.
• Possibility or Presence of Ice - This is a big one!  Adding 1/4”-1/2” of radial ice changes the entire deployment.

In summary, no two deployments are the same therefore creating endless possible deployment configurations.  One should consider all deployment factors before assuming any deployment is structurally sound.  To date Sun West has hundreds of COWS/COLTS in service, which have been safely and successfully deployed.  To our knowledge we have not encountered an overturned chassis, or structural failure of a mast.  We attribute this to responsible engineering and honest guidance to our customers.