Tower Engineering

Twisted lattice telecom towers

Architecturally particular, but not intentionally designed "twisted" lattice 3-legged towers in Mozambique.

Balázs Kisfali
Balázs Kisfali
Jun 23, 2020
min read
Twisted lattice telecom towers

This post will show you what can happen when assembly drawings are not read properly on site and connection members are swapped unintentionally. The story is from my own experience as the designer of the towers and technical lead of the investigation team. The towers are located in Mozambique, Sub-Sahara, Africa.

Issues register and details

Swapped members on RTB 35m Cat C tower

During the investigation the objected towers have swapped diagonals at section C. The C5 and C6, C7 and C8, D10 and D11, D12 and D13 bracings have been inverted with each other. The investigated sites are according to our previous report. During this inspection two sites have been investigated and the same issues were occurred.
Bracings C10&C11, C12&C13, D5&D6 and D7&D8 are correctly installed.
The twist is evident at that level where the swapped bracings have been
placed. (see photos no. 07, 08 and 09.)

Climbing on the tower and marking members.

Assembly drawing review

RTB 35m Cat C tower

By reviewing the assembly drawings it shows a slightly different length of X-bracings at Section C, D, E etc. The inner or outer bracings have a different length due to the different shape of connection plats to the main legs. The difference between the correspondent bracing members 15mm in terms of their center to center distance of the edge holes of the angles. Since the connection plates have a same difference, thus the diagonal’s length longer or shorter by a 30mm. The members C7 and C8 have a same deviation.

Shop drawings

The shop drawings have been generated from the assembly drawings and they do
show the correct dimensions of the members.

Production accuracy check at warehouse

Methodology of the production accuracy

In order to be able to make a reliable and confident inspection we have been done an investigation in 5 layers. These are the following.

  • Tower Key Plan (TKP)
  • Assembly drawings
  • Shop drawings
  • Manufacturing
  • Roll out

Since the Assembly drawings has been approved and quality checked by our technical department, our investigation has been on focus for the last 3 steps.

  • We have been checked all of the members of the Section C and D that the shop drawings are showing the correspondent measurements against the assembly drawings. Those are correct.
  • Then we have been checked all of the manufactured elements for these sections at the client's warehouse in Maputo. We documented all of the measurements, which is available in our internal library. The manufacturing has been according to the shop drawings, the deviation on these members under the industry limit.
  • As a final step we have been checked several sites in order to be aware of the installation is either according to the assembly drawings or not. We have found out that the X-bracings have been swapped at section C and D, the assembly drawings have not been followed accurately.
Shop drawings and connection plates.

Trial assembly at warehouse

One face of a 35m exposure C tower

In order to make sure that the tower can be erected properly according to the assembly drawings, we has been assembled one of the face of a 3-legged 35m Exposure C tower, for sections D, C and 1B.
Even all of the towers used to be erected at the factory, our purpose was double check if the manufactured members might be assembled properly on site. We have created a leveled supports for the each section underneath, like section 1B was horizontal.

Conclusion for trial

The manufactured tower members connect to each other without any external
force on any member. The tower (its one face) shape is straight, accurate and showed the demanded layout.
The measured diagonals were equal, the main legs were parallel and the bracings were able to connect to the connection plates properly.

Horizontal assembly of one face.

Generated twist on the assembled tower part

By swapped members

Our purpose was established the same situation as it is on sites. In order to be able do this we swapped the member C5 with C6 and C7 with C8 exactly how they are on the erected and twisted towers.

Since the trial parts were lying horizontally the bracing connections were in need to get external force in order to be able to connect each other and to the connection plates. Notes: since the deviation in length is 15+15mm, in vertically this kind of situation easily can be created, even it is not noticeable for the riggers.

Before swapping the members.
After swapping the members.
Evident deformation on the main leg after swapping members.
Deviation by a 50mm at top of the section C.

Summary of result

Twisted geometry

In order to be able to see the reason for the experienced twist on sites we made all of the steps from the drawings until the assembled tower. A correct layout can be reached in a truss system by their bracings. Our focus was on these members around those sections. After carefully checking the assembly drawings, the shop drawings and inspection of the manufacturing accuracy, we can state by confidence that the reason for the twist is definitely the incorrect diagonal length. This situation can be created by swapping members due to the asymmetric connection plates. In fully correspondence with the drawing the tower shape has to be correct. As soon as the diagonals has changed the tower shape will be deformed, the main legs start to follow the positions dictated by the bracings. It is easily visible to take a look along the main leg from down to up. The more swapped bracings the more twisted, deflected the tower.

Swapped X-bracings

According to our previous site reports all of the investigated towers have been having a swapped X-bracings, most of the cases at Section C and D. Usually the connection plates are placed according to the assembly drawing, only in few cases we have found swapped plates on the erected towers.

Approximately 6 degree twist.

Assessment and recommendation

Rectifying actions

In order to the towers could get back their symmetric shape the swapped members have to be reverted. The assembly drawings have to be followed, the individual members have to be on the correspondent position on the tower.

Methodology of replacement

Under temporary bracings, 2 couple of X-bracings can be dissembled in one face at a time. It is not allowed to remove the bracings from a whole section or all bracings at the same level of the tower on all faces. Face by face, and half section by half section can be done at a time.

The temporary bracings should be adjustable since the new tower shape will be in need to be straight, which means the bracings bolt connections might not be matched at the first sight. By a proper external axial force in the the diagonal, the main legs could get their straight position back and the bracings will be able to connect by their bolts.

These corrective actions has to be done according to the assembly drawings.

The rectifying actions can be done only low-wind condition, at a maximum of $5\frac{m}{s}$ wind speed.

Lesson learned

The root cause of the tower twist has been found to be the swapped members by not following the assembly drawings. However, where ever problems occur I always try to see how the process can be done in a better way, at least how I can better contribute to it. In this case I've learned that simplification should have been priority than tower weight in a certain extent. In a competitive market every kg matters, so sometimes it is difficult to find the "sweet spot" in the optimization/practicality landscape. With modern CAD programs its easy to generate any kind of shape of steel what a CNC laser cutter can cut. With some adjustment, manual overwrites and certain level of standardization can certainly reduce the number of different elements and at the end of the day, the likelihood of installation issues.

Maputo, Mozambique, 2013.

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