Engineering

The Evolution of Mount Analysis

August 20, 2018

“Many 4G-enabled cell towers pose a safety threat today because of improperly mounted LTE antennas.” In April of 2013, Brandon Chapman, the engineering manager for Valmont Site Pro 1, told this to the audience at AGL’s Wireless Infrastructure Conference in Irvine, California. He went on to say that, “With the introduction of LTE and smartphones, many remote radio heads have been installed on towers using the same mounts that the 3G antennas used, which are insufficient because they are rated for much lighter and smaller antennas.”

This first wave of LTE was when the industry first recognized a widespread need for mount analysis. Since 2013, mount analysis in our industry has become commonplace. TIA Rev-H and the TIA-5053 Mounting System Classification have increased the prevalence and accuracy of mount analyses, but we still have work to do. Tieback orientation is the next big item we need to address as an industry, and Site Pro 1 has already completed an extensive amount of research on this topic.

Enforcing Engineering

In the end, the mount analysis doesn’t matter if the correct mount doesn’t end up on the tower. Historically, mounts have not been scrutinized in our industry. In addition, many mounts are specified with the “or equivalent” nomenclature. Since engineers are not involved in the purchase, sometimes these “equivalent” mounts are chosen by the contractor instead of the engineer of record (EOR).

The engineer of record needs to review the alternate mount to ensure it meets not only the form and function, but also the structural needs of the mount. The EOR should include an audit and close-out recommendation, and it is important that the carrier follows through with this. This will ensure that the mount that ends up on the tower is designed to handle the antenna loads.

Figure 1 below illustrates what can happen if audits and close-out inspections are not performed. The entire face of the antenna mount was made out of channel strut, which is not intended for structural purposes. This created a dangerous and costly situation to fix.

TIA Rev H

TIA Rev H was finalized in the fall of 2017 and will be incorporated in the next run of IBC 2018. Some carriers and tower owners are moving to enforce this sooner, so we are already seeing Rev H being enforced more quickly than Rev G.

TIA Rev G provided a method to analyze mounts, and the analysis procedures in Rev H does not differ much. Rev H now has an entire chapter, Chapter 16, dedicated to mount analysis. The code states that a full detailed structural analysis is needed with a 5 percent loading change, but there is an exception for changed conditions that are within the documented capacity of the mount. This means that the TIA-5053 Mounting System Classification will help avoid some of these mount analyses when there is a loading change.

Section 16.5.2 of Rev H recommends using pinned tower connections in the analysis of mounts that are not able to transfer a moment. Many connections are able to transfer moments, but most t-arms will require pinned connections. Because of this, some t-arms arms that passed under Rev G may not pass under the pinned boundary condition required in Rev H.

Finally, TIA Rev H now requires seismic considerations. We have not seen the seismic loads govern a design, but the new engineering letters we are providing allow you to check the seismic loads as well. Overall, TIA Rev H emphasizes mount analysis more than Rev G, but the TIA-5053 Mounting System Classification is having a larger immediate impact than TIA Rev H.

Figure 2 depicts two small pipe-to-pipe clamps being used as a structural mount to support a large microwave dish. These clamps are intended for applications with small loading and are definitely not adequate support the dish. A mount analysis, close out inspection and an audit would have prevented this.

TIA-5053 Mounting System Classification

The TIA-5053 Mounting System Classification was created to provide a standardized method to rate a mount’s capacity. Per Rev H, the classification will allow you to avoid a full mount analysis if you can show the site loading is within the documented capacity of the mount. TIA-5053 will make it easier to compare mount ratings from different manufacturers and move away from the “or equivalent” verbiage, but as I will discuss later, the analysis procedure can greatly affect the mount rating.

TIA-5053 classifies a mount by intended use. Category R mounts are mounts that are analyzed with equal front and side wind forces. This is meant to simulate the situation where RRUs are installed behind the antennas. Category R mounts are, for the most part, all that is being called out right now. Category A mounts are analyzed with a front wind force that is larger than the side wind force. This would simulate single panel antennas with no equipment loaded behind. Finally, Category L mounts are for optional services, where a delay in service is acceptable. These are not analyzed for maintenance loads.

Table 1 below breaks down the TIA-5053 Mount Rating Nomenclature using an M1600R(2800)-4[6] rated mount.

Maintenance Loads, Tying Off on Mounts and “Man” Rating

TIA-5053 does provide standard maintenance loads to use for the mount, but it is important to note that the 5,000 pound OSHA anchorage is not considered as part of the code. This has been a hot topic in the industry, and we have been frequently asked if it is acceptable to tie off on our antenna mounts. We never recommend tying off on a mount, and there are both safety and financial reasons for this.

First off, it can be considered an OSHA violation to tie-off on a mount under construction. An engineer can verify if a mount can be tied off on, however, it is difficult to verify the mount’s existing structural integrity. Extreme wind events, improper construction procedures and contractors rigging off mounts can greatly affect the stability and performance of a mount. In the end, you never know the existing condition of a mount, and it is always safer to tie off to the tower.

The mount can be used for worker positioning and support at any time. The lifeline, however, should be connected to an independent tower member above the dorsal attachment point, which is between the climber’s shoulders. It is extremely difficult for the climber to achieve this proper connection when tying off to a mount he is working on.

In addition, if a contractor falls and the mount does arrests him, then the mount should be fully inspected and potentially replaced. Rather than tying off on a mount, Valmont Site Pro 1 recommends the use of secondary tie-off points that are dedicated to fall arrest. These allow for proper tie off positioning, and they are inexpensive to replace in the event of a fall.

Figure 3 below depicts Site Pro 1’s new Halo Maintenance Tie-Off Point. This is a dedicated 5,000 lb maintenance tie-off point that can be installed in the ideal position above the installers head.

Best Practices White Paper

TIA-5053 provides a standardized method to rate the structural capacity of a mount, but the analysis procedures used can have a huge effect on the rated capacity. The Best Practices White Paper Committee was formed after the completion of TIA-5053 to standardize analysis procedures for our industry.

To start off, the A&E firms involved reached out to the various mount manufacturers for our mount manufacturing drawings. The A&E firms then independently used identical information to model, analyze and rate the mount per TIA-5053. In the end, the rated capacities differed by as much as 200%. This was among A&E firms that are actively involved in the TIA Committees, so industry wide, there must be an even greater discrepancy.

This illustrates that the mount rating a manufacturer or A&E firm provides can be subjective. If proper methods are not used, mount manufacturers can provide a mount rating that is much higher than what the mount can actually handle. Out of everything the White Paper Committee is addressing, tieback orientation is something we need to address as an industry immediately.

Tieback Orientation

The tieback location and orientation has a great effect on the strength of an antenna mount and its corresponding mount rating. Most mount manufacturers’ mount ratings are based on ideal tieback configurations. If the installed tieback orientation differs from what the manufacturer assumed, then the mount is not as strong as the mount rating describes.

Rather than only providing the ideal tieback mount rating, Site Pro 1 provides three standard tieback orientations and publishes the worst case mount rating for each configuration. We will be following up with an in-depth blog post on how to take tieback orientation into account when analyzing a mount.

Conclusion

We have come a long way since 2013 when Brandon Chapman and Site Pro 1 were among the first to see a need for widespread mount analysis. Now mount analysis is so common that many carriers and tower owners are finding it difficult to get all the mount analyses completed. TIA Rev H has increased the number of mount analyses performed, and the TIA-5053 Mounting System Classification is helping to streamline this process. It is important to ensure we are all using the same analysis procedures, and the Best Practices White Paper will bring standardization to the mount analysis process.

Until the Best Practices White Paper is released, tieback orientation is the main issue the industry should address immediately. Just like mount analysis was almost unheard of in 2013, including tieback orientation in the mount analysis is almost unheard of today. Keep posted for a follow-up blog post on how to take tieback orientation into consideration on your mount analysis.

Please feel free to contact our engineering team with any questions or comments you have on this post.

Luke Ferguson

Luke Ferguson graduated with his Bachelors in Mechanical Engineering from the University of Nebraska-Lincoln in 2010. The first five years of his career were spend performing structural analyses on Boeing 737 fuselages, lighting and traffic equipment and utility structures. Learn More