Floating Planter: an architectural illusion

Introduction: My friend who is a landscape architect and aspiring structural engineer called one evening to inform me that his client wants to float a planter in the air. I was confused and wondered how it is possible. I thought, his client, who is a top executive in a private firm, doesn’t know the concept of gravitational force. A lot of thoughts started pouring into my mind, It was then my friend who convinced me, that this small task will be interesting and would need my support in executing it. We met after a couple of days and he explained to me in detail about the problem.

Existing Planter Condition and Reinforcement Cage (Left)

Problem: After a couple of days, I and my friend were discussing the on-site situation. There are two existing planters already on the ground, inside a small pool. Planters will be surrounded by water.  But one fine day, when both the planters (i.e. planter A and planter B) were already constructed, and the only pending work was final finishing. The client informs the architect that he wants the third planter ( planter C ) and it should look like the planter is floating in the air. The client was kind enough to propose that we can take support at the corner of planter A.   I had to brainstorm the ideas, because, the size of the planter was comparatively larger in size. which would be filled with wet soil and should be supported on one corner of planter A. It was challenging, as there was a space constraint, and plus we didn't want to break the existing planters to make a monolithic structure. If it was a monolithic structure then it would have been easy. The problem arose when we had to support on top of the existing structure. I took a day time to brainstorm ideas, It was really fun. Though the task was easy, I had a lot of learnings from it.

Plan (Proposed)

Elevation (Proposed)

Solution: So next day I proposed to my friend three solutions,

    Solution 1: Tension pile:  one corner of planter C will rest on the pile and the pile will be in tension. But this is not a feasible solution, because for such a small-scale project renting a pile boring machine and constructing a pile will be very expensive, and there was space constraint.

Solution 1: Tension Pile

    Solution 2: Tension cable: Corner of the planter C will rest on the corner of planter A. Where the corner of planter C will be tied to the bottom of planter A using a tension cable. But once again this solution was rejected because the tension cable will be in constant contact with wet soil, which will lead to easy corrosion. Plus cable will not provide adequate resistance to lateral load, which planter C may experience.

Solution 2: Tension Cable

    Solution 3: Tension column: Same concept as the first and second was used here, but with slight modification.  An RCC column was proposed at the corner of planter C. The column will be in the tension, as concrete is a dummy in the tension hence only the steel cage inside the column will act as tension carrying member.  But concrete is used as cover against the soil contact.  To hold the tension force of planter C, the column was started from the big chunk of a concrete block below planter A whose weight is more than the weight of planter C.  This solution was affordable and can be constructed easily.

Solution 3: Tension Column

Engineering concept:  In this mini project, the technical concept is very simple.  Basically, I have tried to hold planter C at one corner using a tension column. Also, I have made sure that the column doesn't take moment by placing planter C on planter A. So planter C is simply supported on planter A. Hence column is purely in the tension. This column is designed in such a way that all the tension force is taken by the main reinforcement. A minimum stirrup has been given as per Indian code requirements. Detailing has been done in such a way that the main reinforcement is continuous without any splices or laps. This main reinforcement is going inside the concrete block which is below planter A. Almost the length of 2 times development length is extended into the concrete block to have enough bond strength between the main reinforcement and concrete. This bond strength has been verified and made sure that it is 4 times the tension force. The concrete block weight is twice the weight of planter C . Concrete block has been provided with min side face reinforcement on all faces to resist the shrinkage cracks. Since all the moments are converted into a tension force in the column, hence no overturning moment will be experienced by the concrete block.  Also, it is assumed that very minimal lateral load will be experienced by the planters. The size of the column has been given enough to take those loads. This was the concept that I used to design this planter, if you find any observations then please do comment below.

Proposed Reinforcement Detailing

During Construction

Conclusion: A small unique and innovative idea led to a lot of braining storming.  Bringing an idea to reality always feels good. Though it was a small project, we had to use a lot of engineering knowledge to keep the planter stable. Also, we had restrictions on aesthetics, plus restrictions on cost. We could not use high-end engineering technologies for this small-scale project. Without compromising technical aspects, satisfying the client and architect in both aesthetic and financial terms is very important whether it is a small project or a large project.  A challenge may be small or large but we have to attend to it with the same responsibility. Having an attitude of learning and enjoying the work will lead to success, have passion and do what you like, whether it is corporate, government, or freelancing, do it will full passion.

Looking forward to bigger challenges in the future. 

>>>Thank You<<<

Note: Will add more pictures when the water is filled in a mini pool