It's nice having you back again on my blog. The last time it was about food, but this time, it's a different ball game.
This question has been a question engineering students ask frequently from lecturers and they get different opinions from them, especially the civil engineering students. During my days in the ordinary national diploma (ND) this same question was asked in strength of materials class. Our lecturer then told us that ‘its not possible to have a cantilever more than 3meters’. Everyone accepted and took to that because he has been in the field for quite a number of years, until when one of the students (after the class) showed us a video of the construction process of Burj alrab having a cantilever of 27meters long …….. how come? That’s the big question, I made further research and get to find out that there are even longer cantilevers than that, which is the Busen cinema Centre in korea having a cantilever of 85meter long. So here we have a great puzzle which will be figured out in this post, which I will be very brief and precise.
I might have been boring some who don’t know what cantilever is, so as to carry everyone along; I will start by defining cantilever. Cantilever is a beam anchored at one end and projecting into space, such as long bracket projecting from a wall to support a balcony. Smaller cantilevers are simple beams, larger ones use trusses from structural steel. Cantilevers can be in various form; cantilever roof, cantilever bridge, simple beam cantilever.
Brief history
Now that we know what a cantilever is, let us enlighten ourselves on where cantilevers originated from and who was behind it.
Heinirich Gerber was one of the engineers to obtain a patent for a hinged girder and that was 1866, he is recognized as the first to build one. The Hassfurt Bridge over the main river in Germany with a central span of 38meters(124feet) was completed in 1867 and is recognized as the first modern cantilever bridge.
Couldn't get a free image, but if you want to have a view. Hassfurt Cantilever Bridge
From the definition of cantilever above, if you are following me closely, it was stated clearly that we have small and large cantilevers. In which the small once are like simple beams and the large once are made from trusses. Certainly our professor then must be referring to the smaller cantilevers of simple beams - can’t be longer than 3meters which is very correct because if its longer than that deflection sets in as a result of the point load at the tip of the beam, as a result of the weight of the concrete and steel which is used to make up a beam. When deflection is in place then the structure fails.
However when it comes to large cantilevers which are made from trusses the span is not restricted, you only have to design well, and not make it fail. There are certain factors that would be put into consideration before you can successfully determine the span of a cantilever made from trusses, which will be discussed in the concluding part of this post.
I was privileged to watch some part of the making of the Burj alrab hotel in dubai, and the mindset was to have a restaurant like you are eating in space (as if you are eating on water). The architect came up with the drawing of the cantilever and it was worked on by the structural engineer and they used very long trusses and coupled together tightly and having a c-shape, because its over 50meters from the ground level.
As at today, Busan Cinema in korea has the longest cantilever roof in the world. The roof is 163 meters long and 60meters wide with an 85meter cantilever portion. The roof is made of steel which made the cantilever span longer, and from the pitch, it has a large depth also which was among the factors to determine span of a cantilever.
Conclusion
What should be the maximum span of a cantilever?
For a small cantilever which are simple beam, maximum of 3m. for large cantilever, there are factors that will be put into consideration;
The depth of the cantilever.
The quality of the material.
The magnitude.
Successfully transferring the load from the cantilever to the ground.
These are the main factors to check before the span of a cantilever can be determined.
References
Tekla structures
Amr Ismail /B.sc. civil engingeering, Alexandria university 2017
wikipedia
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