The Cubocta Bridge is designed to connect ‘stairs’ to an adjacent building. A simple function elevated to an artistic solution. Walking through the bridge, your experience will be like passing through a series of rhombuses pinned on a single corner vertically; making it obvious that you can only walk on this bridge by design.

The steel, the material the Cubocta is totally made of, is obscured overwhelmed by the feeling of the enclosure created by the form. The material is devoid of its lightness and rather appears as solid from the outside and completely porous from the inside. This duality of appearance creates a feeling of immateriality in the architecture.

The Cubocta like the cube packs on to itself to form a continuous row both horizontally and vertically. Four of these solids were lined up to form the bridge. In reality, you walk inside a row of cubocta’s.

The bridge extends about nine meters horizontally to the adjacent building without a need to employ any special support, taking advantage of the three-dimensional nature of the structure. You actually walk in a 3D structure, which was also the reason why it ended up being super light.

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By Nazrawi Ephrem

In the heart of Addis Ababa an iconic mega-building project is rising, consisting of a tower and podium with 44 and 5 stories respectively. The tower rises 180m above the ground level and functions as an office for Oromia Insurance Company. However, for the past six months, the tower part of the building development has been stuck on the first floor.

The client’s structural reviewers have notified the consultant to carefully inspect the front shear wall in a diverse and too conservative design approach. The tip of the front share wall was detailed by the architect cutting out part of the structure, which has resulted in the primary structure being stressed. This issue should be addressed either through architectural or structural means.

The preferred solution is to come up with a structural solution. The shear wall (P2), which features chamfers arranged in a zigzag pattern, looks great but makes a weak point for the structure. The wall effective length is only 3.6m. Unfortunately, this makes it insufficient to resist the lateral forces, to ensure the safety of the building. Two possible solutions were identified: either enhance the concrete grade to C-70/85 and keep the chamfered part of the wall, or close up the wall up to the second floor. But the second solution would ruin the building and was highly resisted by the architect. The option of raising the concrete strength was accepted by the reviewers.

The challenge was where to get a plant that could supply a C70/85 concrete strength. Such a concrete strength is never reached in the construction practice in the country. The two approved suppliers of the project were unable to achieve the stated strength. They made multiple trial mix designs by controlling the quality of ingredients, including fiberglass, enhancing fly ash, applying more cement content, utilizing artificial sand, chemical admixture, micro silica (UAE market), and so on, but couldn’t achieve the desired strength. By this time about three months have already lapsed. The floor has been waiting to be concreted, except for the P2.

P2 has become very famous. Nobody knew what P2 was, except for the engineers who work on the design. Now P2 is even known by the CEO of the Insurance Company and the Board.

When everything looked dark, we got information that there is a supplier who had already tested and achieved such a strength. We rushed in to contacting this supplier and made multiple samples. Unfortunately, the ones taken in the presence of all parties failed to achieve the required strength. The client threatened us to close the wall feature and proceed with the work. It was indeed a super frustrating moment. The team started contemplating if closing up the wall would be the fate of the building. We approached the Addis Ababa University for possible expert who could help this last supplier achieve a C70/85 strength. A long process started. Four months lapsed.

Even after all this effort, the push from the client side to close up the wall and move forward never stopped. We wondered, what would the meaning of the whole exercise be if you would ruin your face. The resistance never stopped.

It was again in this super dark moment that the structural team sorted out a detail that could provide additional support to the structure and allow the building to use a lesser-strength concrete. A spark of hope came out that could bring a solution. The structural engineers took on the assignment of checking the concept overnight, and the next day it proved to be fruitful. The analysis was submitted to the structural review team.  They too were surprised why that option was missed from the beginning and endorsed it. This time employing a C55/67 concrete which seemed achievable and the contractor was encouraged to resume work on P2. That was an exciting news for the architect.

After a super long arduous and frustrating process, after six months of pause, P2 has finally been casted as envisioned by the architect. It is indeed an exhilarating experience witnessing the culmination of a dedication finally coming to fruition, bringing to life the manifestation of a vision.

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The main concept of our proposal is connecting this north direct road naturally to the front street with an internal alley that crosses the building at the ground level. This is an interesting element that resolves the level difference between the upper most rear edge and the lower main street (about 1.5 m below). Though it is unlikely that the cul-de-sac proposed at the back will materialize, yet the setback left for off-street parking entertains that possibility.

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We are sharing a recent experience of a record volume of concrete casted at a time, to a raft foundation for a high-rise building at Mexico square. We felt the lessons learned are unique and would be of interest to architects and engineers.

Our firm obtained the Oromia Insurance headquarters building project in an architectural design competition held in 2019. The site is located in a prominent junction at Mexico Square, in front of Wabeshebele Hotel, adjacent to the Ethiopian Coffee and Tea Authority Building.

There are a lot of interesting architectural stories about this project, which will be shared in the future. But what prompted us to write this article is the volume of concrete we had pumped into the foundation for this high-rise building at a time. The foundation of this 40 story building is a 3m thick raft foundation. Being only a raft, that is without foundation piles, itself is unique. The foundation sits on a rock 20m below ground providing five basement floors. A record 6000m3 of concrete was pumped to the foundation at one time; or a 3,000,000 kg of cement (30,000 quintals of cement).

The thought of casting 6000m3 at a time was a nightmare for the design team. There was no experience of managing this size of concrete at a time in the country. And there is also the issue of the site, a location super busy with people and vehicular traffic – with a very limited access to pour concrete.

As a design team, we have been contemplating over this challenge for long time. We were concerned that with the limited space and pumping possibility, how such volume of concrete could be casted without forming a cold joint. We knew it would take a few days to cast such volume of concrete.

We requested the contractor to provide us a methodology how they would accomplish such an extensive volume of concrete at a time. But did not respond to our requests. The contractor, China Jiangxi, is a Chinese company with an extensive experience. The time came close and we called for a meeting. By that time the contractor has already identified an experienced concrete supplier by the name Gentium Concrete Technology. Gentium is also a company with a good experience in Dubai. We held a series of meetings and shared our concerns and demanded both teams for a methodology. But they don’t seem to be worried about it. When the contractor did not provide a methodology, we came up with one and shared it in one of our joint pre-concrete meetings. It seems both the contractor and the concrete supplier had a methodology in mind. We came to understand later that in their experience such concrete would be laid in layers. We argued with the limited number of concrete pumps they would be able to deploy that this strategy would obviously form cold joints. And there is also a shortage of cement in the country which the supplier had plans to supply only about 1,200m3 per day. This would mean it would take five days to complete.

Our team insisted that the methodology should be something that would start from one end of the site and that would quickly build up to reach the top of the raft, the 3m. Then we would follow it down. Luckily our floor plan is of an elongated rectangle of about 30x90m and with a sharp tip. Theoretically, this idea looked logical to quickly pile up the intended 3m level. We proposed to start the casting at that tapering tip.

We made a simple physical model to explain the idea to the contractor. On the day of the casting, we found out that the concrete pumps were set differently. We thought at first it was because of the challenge of the site. But when we started pumping concrete the contractor team was fixed for the same traditional layering method. We argued right on site, as the concrete was advancing, to return to the methodology already agreed upon. After a fierce argument, the agreed strategy was adhered. We managed to reach the 3m top level at the corner after about 12 hours of frustrating concrete pumping exercise; and finally got a relief.

Another challenge was that the concrete was being cast in the midst of a rainy season. The tight site had obliged us to build first a concrete trimmer, which acted as a drainage ditch including during casting. It is like doing the backfill first and building your foundation afterward. The trimmer acted as a formwork for the raft foundation.

We installed thermometers in three layers at 10 locations and gauged the inner temperature of the raft concrete. In the early days the heat of the middle layer reached over 90o C. But what was important was that the temperature difference between the top and the middle layer is maintained within the recommended temperature range to avoid surface cracks.

The concrete supplier had already modeled the concrete and provided us a recommendation. They had also advised us to seal the concrete top from the outside for about seven days until the concrete sets, and before curing begins.

The concrete work took one more day from what was originally planned due to a shortage of cement. The first concrete truck left the batching plan on Thursday afternoon at 01:37 PM, 30 June 2022; and the last truck arrived on site on Wednesday evening at 7:48 PM, 06 July 2022. All in all, took about six days of a round the clock work – our team working in a 12-hour shift of 3 to 3 AM/PM. It ended up being a success story.

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