Leadcom: Why 1+1 doesn’t always equal 2 when designing and strengthening towers for multiple tenants

TowerXchange wanted a basic explanation of tower design and tower strengthening for readers who aren’t structural engineers, and our friends at Leadcom kindly obliged! Chief Engineer Hagai Admor’s interview takes us from Africa’s early over- and under-designed towers, with some towers overloaded and some with spare capacity, to today’s innovative tower designs that minimise initial capex but which are upgradeable for multiple tenants.

TowerXchange: Please tell us about the typical current state of Africa’s single tenant towers - were towers designed to accommodate additional tenants? Is there typically spare Equivalent Projected Area (EPA) for additional tenants?

In the early stages of the massive rollouts in Africa, providers could sell their own ready-made product towers as there was lack of standardisation. Telecom operators defined their towers in three basic levels:

• Standard - which was normally EIA/TIA 222

• Wind speed - which normally was 40m/s

• xx m² wind load in the top 10m

Based on these categories, customers defined and standardised their early towers.

A few years ago, a new version of the American standard was issued, better definitions for the different environmental factors were considered such as exposure area, importance factor, topographic factor, et cetera. This was given to the operators to be more accurate in their definitions and specifications.

The fact is that most of the “old” towers, sold based on “old” standards, were not a product of new, cutting edge analysis and engineering tools, so they weren’t accurately tailored to suit environmental factors. As a result, some older towers are under-designed and overloaded, while some of them are over-designed and can accommodate additional loading.

Another factor which has a major impact on the tower load is that with the technology evolution over the years (2G, 3G, PDH, SDH etc), operators added equipment to the towers without considering the designed capacity, and we have found some tower members that were over loaded by 300-400%!

With today’s know-how, we define towers based on exact loading. Unless it is requested by the customer, we do not mention the equivalent projected area at the top, but express the load as a function of the specific equipment to be mounted, such as antenna models, mounting apparatus, microwave dishes, coaxial feeders, vertical cable ladder, et cetera.

Expressing the tower capacity as a number of square meters at the top of the tower normally confuses the customer since some include feeders and some don’t, some express the load in a distributed method and some in a concentrated method, and some of the suppliers apply the load on one leg in one azimuth and some not.

With the size of recent equipment, we rarely find that additional tenants cannot be added due to real estate issues and we believe that there is always a solution that can be found.

Where once people would have talked about towers having 5, 10 or 15 square meters of capacity, towercos talk mainly about the number of tenants a tower has capacity for.

TowerXchange: How much additional capacity is required to add a second or third tenant? Does the load capacity increase proportionally; do you need double the capacity for a second tenant and three times the capacity for a third?

No, 1 + 1 does not always equal 2 when it comes to adding tenants to towers! Sometimes 1 + 1 equals 1.5 and in other cases 1 + 1 equals 2.5!

To explain it simply, if a 1kg force is acting on a 40m pole, then the moment is 40kgm, but if the same 1kg is acting on a 35m level (5m below top), the moment is 35kgm. Equipment for an additional tenant placed at the top of a 40m tower creates a greater moment, a greater load, than if the same equipment was placed 5m below the top. So you see adding additional tenants do not mean the load on the tower rises proportionally.

Additional equipment for the current or for a new tenant placed on top of a tower versus placing the same equipment 5m below the top will create different moments and forces along the tower. Loading the tower at different heights causes change in its static forces distribution. The tower body changes and members with initial utilisation might see additional force which will lead to exceeding the member’s capacity.

Operators want their equipment at a specific height to optimise coverage and capacity and to minimise interference. If the tower is already occupied, available slots may not be the optimum, and the space may or may not meet their needs. Some compromises may be possible with the location of RF equipment, but if they want to transmit data from one site to another via microwave then they need line of site, so if that’s not possible it tends to be a go / no go question.

TowerXchange: How does the tower location affect capacity, for example in areas with high wind speeds?

The new American standard takes into account environmental factors that directly influence tower performance. Which means that a tower in a sheltered urban area can be loaded more than the same tower if be located in open terrain. In addition, towers that are located on the top of a hill can carry lower loads then towers located in flat open terrain.

Moreover the latest version of the standard takes into account the level of importance of the structures, such as whether the tower is a switch site. In addition to that, better and more accurate data on wind speed in different regions can be identified today, not like in the past where a whole country was considered with one or maximum two wind zones. The effects of site-specific topographic parameters introduced in the latest version of the “EIA/TIA-222 G” version have a significant influence on the tower’s actual capacity. Towers that were procured and erected during early rollouts without site-specific design can now, in the re-analysis stage, show very poor results due to topographic/environmental conditions and lose most of their potential vacant capacity to increased wind loads.

TowerXchange: What ultimately defines the load capacity of a tower?

The capacity of the tower is defined by the most stressed element/member in the tower. It can be a leg in the bottom section, but also can be diagonal or horizontal in the middle section.

In some cases when analysing towers that have lower quality bolts, we find that the most stressed element/member is a bolt and the capacity of the entire tower is defined according to that.

The way you modulate the tower is of critical importance, on one hand the geometry and the screening of the members, and the other hand modulating the antennas and accessories in their exact location (height and azimuth and tower leg).

TowerXchange: What are the most common approaches to strengthening a tower structure?

The engagement of a turnkey provider can bring the towerco the most cost effective solution for strengthening towers. We are not bringing just z pure engineering approach, but we also add aspects from manufacturing, delivery lead-time, shipment and implementation which results in a tailor made solution for each specific site. We believe that our vast field experience with great engineering knowledge is a great benefit to the towerco.

The approach of replacing a leg with a bigger/thicker leg is not necessarily the right solution. Replacing a leg often seems to be a straight-forward, feasible solution, but the risks entailed can cause the tower to collapse if poorly executed. Our approach to tower strengthening is on a case by case basis since each tower has it own parameters in terms of loading, wind speed, and environmental factors. It is like a heart surgery where you have a procedure that you go by but each patient is unique.

A more sophisticated or innovative approach to tower strengthening, other than simply replacing failed members with stronger ones, is to change the static design and model geometry of the tower by installing additional members in a way that will better distribute the various forces within the tower main members, thus reducing the overstress in some of those members.

TowerXchange: When would you recommend replacing rather than upgrading a tower? How can service be maintained during transition to a replacement tower?

We do everything in our engineering capabilities to find solutions to keep the same tower and to reinforce it rather than replacing the tower.

The consequences of replacement not only involve the direct cost of the new tower but also lack of land (as normally the new tower will be tied in to the existing one), implementation of a new foundation, and telecom equipment cut-over which introduces a risk to the continuation of site performance. During the past three years I remember very few sites that we recommended dismantling. In most of the cases the reason was that our survey team found heavy rust on the structure. Replacement of towers because of inability to strengthen is rarely necessary and this is appreciated by our customers. One of the key elements that lead us when we design tower strengthening is the continuity of the site’s service; this is why we prefer to add members rather than replace members.

In some cases, specifically rooftop sites with severely deteriorated or poorly designed towers that must be replaced, we solve the downtime/service continuity problems by erecting a light-duty (very small footprint) temporary tower just outside the compound parcel, which is used during the construction period of the replacement rooftop tower, and then dismantled and reused on another location.

TowerXchange: Finally, how can tower operators control capital outlay on new towers whilst still having the flexibility to upgrade them for multiple tenants?

Since 2010 Leadcom has undertaken thousands of structural analysis surveys and strengthened an enormous amount of towers. From that experience, we devised a methodology to make a tower easily upgradable to accommodate extra tenants without interrupting service.

If the customer doesn’t want to deploy the capex to rollout towers with multi-tenant capacity now, we can install a light model tower with geometry capable of adding or upgrading members to achieve higher loading on the same tower without downtime or changes to the foundation. We install an MM Series tower (with medium load capacity) with the potential to upgrade to an MMTP site (suitable for harsh topographic environments, or additional tenants) using 100% the same geometry but with the addition of certain members to achieve a 30-40% higher load on the tower.

Clients seeking this kind of flexibility in capex deployment and upgradeable capacity are usually towercos, who can use these towers to mitigate the risk that it will take time to sell additional tenancies.