Large Ferris wheels are very complex structures that require considerable attention during design and construction
DESIGN DETAILS THAT HAVE AN IMPACT ON THE SAFETY OF LARGE STRUCTURES
by Enrico Fabbri
It is commonly believed that the design and construction of a Ferris wheel is quite simple. This opinion may be partly true when building a small Ferris wheel, but is absolutely not true when dealing with large wheels. In this article we will examine the important critical points that need to be taken into consideration on large fixed Ferris wheels, i.e. anchored to the ground with concrete foundations.
FORCES AT WORK
Unlike many other attractions, Ferris wheels may be subjected to significant wind and earthquake loads. These external factors are significant due to the large surface area exposed to the wind, and the mass of the rotating structure, as concerns earthquakes. In most cases, the sizing of the structure depends on these factors rather than on the calculation of the structure’s fatigue strength during routine operation.
A good designer therefore needs to appropriately determine the action of wind and earthquakes and design all the structural details needed to distribute these forces down to the ride’s foundations. The image below shows an example of how a force acting at the top creates stress on the entire structure, represented by the various colors. Now try and imagine strong winds or intense earthquake forces acting on the entire rotating structure of a Ferris wheel, pushing against it and all of its components.
These forces are then exerted on the main axle and therefore on the masts that support the Ferris wheel. A customer who purchases a large Ferris wheel naturally expects that it can be used for several decades (around 3), without major maintenance work on the structure. We know that installation costs are quite high and therefore it is fundamental that the design of the structure and the structural calculation are extremely accurate: if, for example, after 10 years major defects are found that require the structure to be dismantled, this would mean economic disaster for the operator.
Consequently, carefully evaluating the manufacturer’s construction procedures and the specific experience of the engineering firm that carries out the structural calculations is the first step towards obtaining an attraction that will stand the test of time. In particular, the manufacturer’s designers and the engineering firm that carries out the structural calculations must use advanced software that indicate to reinforce the structure only where necessary and no more, so as to obtain a structure that is both resistant and light, and at the same time streamlined and attractive. If the wheel is installed between very high buildings that can create turbulence and whirlwinds, the action of the wind can be simulated using special software or alternatively models placed in a wind tunnel.
The foundations are a likewise important element that are often not given due consideration. I have already spoken in a previous article (see G&PI February 2016) about the importance of the design of a ride’s foundations, therefore refer to this article for further information. In this case, however, the anchoring of the Ferris wheel’s masts to the concrete foundations is even more important. Usually these are anchored to base plates embedded in the concrete foundations using anchor bolts.
The masts are fixed to the top of these plates, which is where the problems start. Some manufacturers weld the masts directly to the plates on site, others use flanges with bolts, and others use adjustable anchor bolts. All of these systems are suitable for achieving the purpose, however specifying that: each weld and anchor bolt must always be visible, even after assembling the ride, so as to be able to perform periodical inspections; each weld must be flawless and inspected using non-destructive testing; a system is needed to level the wheel’s structure and compensate for errors that always occur in the level of the concrete foundations; the certifying body that inspects the Ferris wheel must also include certification of the systems used to anchor it to the ground.
Another important aspect is that, during an earthquake, the foundations may move in relation to one another, causing further stress on the structure; the foundations must therefore be suitably joined together.
The masts of a Ferris wheel represent a relatively simple part from a constructional point of view; usually they are made from round tubular steel for aesthetical reasons. The larger a wheel, the bigger diameter the masts need to be, and in some cases need to be especially made to measure. The various components of the masts are bolted together, and here it is worth remembering that it is always preferable to use a large number of small bolts rather than few large ones.
The larger a bolt, the harder it is to tighten it to the right torque and therefore also carry out periodical checks. The accessories that are fastened to the masts are also important, such as the ladders for maintenance and the platforms that are used to access the drive system. These accessories are essential and must be designed taking into account local regulations and not only the general requirements of EN-13814, as this type of structure resembles more a building than a funfair ride. In fact, in some cases emergency lighting, fire-fighting equipment and safety signs for the maintenance operators are required.
The axle is the most important part of a Ferris wheel, as it supports the entire rotating structure. Any major problems involving the axle may mean the ride needs to be dismantled, at a very high cost. In this case too, it is worth remembering the value of a good designer and an excellent engineering firm to achieve the objective of long operating life. The first decisions concern the system used for the rotation of the wheel. Usually bearings are used, however the larger the wheel the bigger and stronger these need to be.
For certain designs, standard bearings may not be readily available, and consequently may need to be specially built, meaning much longer delivery times. Above all when wind or earthquake forces are quite significant, it may be more suitable to use large bushings rather than bearings. These, in fact, offer a larger support surface area than bearings and therefore longer life.
The choice of product quality and expected lifespan is important, as it will not be possible to dismantle these components for maintenance. The shaft structure must be designed taking into account the possibility to easily periodically inspect both the structure and the welding, all the pins, bolts and safety pins. All the shaft’s components need to be designed and calculated considering a lifespan that is longer than that specified by EN-13814 or ISO-17842.
SPOKES AND RING BEAM
Even if the spokes and ring beam are quite simple parts to construct during production, they are nonetheless sensitive elements. The most important elements that are often neglected are the tension system that fastens the spokes together.
On a large wheel there are significant wind forces (as already mentioned in Part 1), which cause repeated small movements (vibrations) that in turn affect the durability of these components. Consequently, it is worth oversizing these parts, designing them in a way that the forces can be suitably transferred and paying the utmost attention to the welding.
On the entire structure of the spokes there must be no part where water can accumulate, so as to prevent rust from forming. These aspects are important above all on the tension fixing system and the lighting.
The main concern of any Ferris wheel operator is to prevent falling objects that may injure the operators themselves, the passengers or the public nearby. This aspect is important both during assembly and in operation and maintenance.
All components that are dismantled for any reason must be equipped with rings and chains to secure them and prevent them from falling. Not just plates and covers but also pins and bolts of any size. It should never be forgotten in fact that also even a small object falling from a great height can be a risk to safety.
This applies above all to the lighting fixing system. As the lighting is usually fixed to the rotating part of the wheel, clearly this too is subjected to stress over time and the safety pins may break, creating a significant risk of falling objects.
We know that there have been cases in which lighting fixtures have detached from the spokes, causing serious injuries. For this reason, even if EN-13814 does not expressly require such, proper risk assessment must also include these aspects, determining the most suitable solutions.
The fixing system must therefore be double and independent, so as to prevent the breakage of a component from becoming a general safety risk.
The cars are another important component in terms of safety. We can start by examining the system for securing the cars to the ring beam. The most common solution is a shaft that supports the car, which at the top is connected to the two ring beams of the rotating structure. This shaft must be designed in such a way as to avoid any structural indentations and places where breakages may occur; the process for fixing the bearings must not overly reduce the load resistant section and this must not become fragile due to unsuitable mechanical processing.
Not many accidents have ever occurred regarding these components (I can recall just one, in Argentina); nonetheless the new design criteria, until now adopted by only very few manufacturers, recommends the addition of emergency support plates that can sustain the car’s shaft even if the main supporting joint breaks. This is a very simple yet effective idea and above all has a minor impact on overall production costs.
Let’s now look at the access doors to the passenger car or cabin. These should always open towards the inside, or alternatively be built in such a way as to never interfere with any part of the wheel’s rotating structure, either when open or during the opening and closing movements. When the doors are closed, they must not be able to be opened by passengers, and they must feature double (redundant) locking. A fault in one of the components of the locking systems must not allow the door to open.
For large wheels, it is good practice to also check that any openable panels on the car, even small ones, cannot be accessed by children, so as to prevent the objects from falling out of the windows and causing injuries. In wheels over 100 meters high, some countries also require a defibrillator in the cars in the event of passenger heart attacks, given that the time needed for evacuation may exceed 30 minutes.
The design of the station platform is also worthy of attention. When a car arrives at the station and stops, there is a gap between the same car and the platform. If this is too large, passengers may be exposed to risk when embarking/disembarking by stepping between the car and the platform. The design of the gap between these two elements is always a major point of discussion between manufacturers and certifying bodies with regard to local regulations.
Another source of risk is the space between cars: passengers may accidentally miss the car and fall from the platform. For this reason, safety nets should be placed underneath the area where the cars transit.
We will now turn our attention to something that the public does not see, yet nonetheless is very important: the drive system and all the electrical systems required to make a Ferris wheel turn.
Each wheel has several motors used to move the rotating structure; these are normally installed on the two sides of the structure.
The total number of reduction drives installed must be such as to ensure the system keeps operating even if there are faults on one or two of these. They must also be easy to inspect via work platforms and must be able to be disconnected from the system in a few minutes.
I recall that several years ago a major Ferris wheel in Asia saw a fire break out on the main electrical panel, causing an extended emergency stop. Since that incident, on large wheels it is preferable to install two independent control panels, so that one can still operate correctly even if there is a fire on the other. In addition, all of the main electrical panels must be equipped with temperature sensors, smoke detectors and effective fire-fighting systems.
When something doesn’t go as planned, alarms are activated on the wheel and a procedure must commence to evacuate the passengers. The evacuation procedure implemented will depend on the seriousness of the fault. On large wheels, this becomes even more important.
In simple terms, these procedures can be classed as
The rise must have a sufficient number of sensors to identify correct operation of the critical safety components.
The term ‘routine evacuation procedure’ refers to faults that are not particularly serious. This category also includes faults that are statistically repetitive. An ordinary power outage is the most recurrent fault on these types of major attractions, accounting I believe for over 30-40% of all faults (the electronic systems, in fact, detect even very short power failures).
Other common faults may affect one drive unit or the electronic system that drives it. In these cases, the management system must gradually bring the attraction to a safe stop. This is followed by a check on what happened and a quick temporarily restart to evacuate the passengers.
Exceptional evacuation procedure’ refers to serious faults that generally +cannot be fixed by a specialist maintenance operator. The attraction always stops safely, but in order to restart it, even temporarily, a specialist technician needs to work on the ride, meaning it will be out of service for an extended period.
This category usually includes multiple faults on drive units or faults on the electrical panel that involve more than one power supply. For these reasons, each drive unit should have its own power supply and control unit that is separate from the others. In this case, the ride can restart temporarily, even if often at lower speed than usual.
Extreme evacuation procedure’, finally, covers events triggered when the fault or faults affect the operation of the entire attraction, even temporarily. For small Ferris wheels, the last-recourse evacuation system is quite simple and exploits the weight of the passengers to push the heavier part downwards due to gravity. As the height of the ride increases, friction in the system prevents the structure from turning to bring the passenger cars downwards, and consequently other systems must be designed to evacuate passengers.
If you are on a Ferris wheel 150 meters above the ground, which has stopped, and the drive system is not working, how do you get down? Have you ever asked yourself this question? There is in fact an answer. The spokes of the rotating part must be equipped with ladders, passageways and safety hooks to allow professional climbers to reach any of the cars; there must also be a manual emergency door opening system to access the inside of the cars.
Similarly to what happens when a mountain cable car stops, the passengers will be harnessed with safety straps and a system of winches controlled manually by experts will allow passengers to descend to a safe position at a lower level. This procedure obviously takes a lot of time, but currently there are no alternatives, and no other valid solutions are available on the market.
I hope to have covered many of the issues relating to the design, calculation and construction of a large Ferris wheel. If however you are still not convinced, I should remind you that a famous Ferris wheel installed in Australia, designed in Japan and manufactured in China, measuring 100 meters high, was dismantled after just two months due to serious problems with the welding. It was then redesigned and rebuilt.
Again, serious structural problems were found that led to the demolition of the attraction. Only the third time round, after several years, did they actually succeed; today that wheel is fully operating, albeit it is up for sale due to insufficient passenger numbers.
If this were not enough, I can also recall the case of another major Ferris wheel currently under construction in an Arab country. Installation work has been stopper for a year and it seems that there are serious design problems that may require the part built so far to be demolished.
The secret for the construction of complex designs is to have a good team of specialized professionals, and to verify that the purchaser of the attraction can organize a skilled and trained maintenance team.
Written by Mr. Enrico Fabbri firstname.lastname@example.org
Article originally published in Games Industry (Italy) magazine
Original date: March 2017
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