Prevention within Military Engineering. Proposal of an observation system A modular and portable observation tower

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1 Prevention within Military Engineering. Proposal of an observation system A modular and portable observation tower Hugo Alberto Correia Soares Abstract The prevention of a safe environment means having the army s commitment to act in a strategic mode mainly by the use of observation techniques. In this context, the conception and use of a watch tower may be considered as a proficient option. The design of the tower was done considering an optimal solution which began with the knowledge of similar structures. The chosen solution was a modular and portable structure that uses a hexagonal cabin able to operate at heights between 2 and 12 meters. The design criteria relating to the Ultimate Limit State and to the Serviceability Limit State was taken into consideration to guarantee structural safety and comfort. Key words: Metallic Structure, Watchtower, Modular, Portable, Portuguese Army 1 Introduction The Portuguese army's engineering corps is active during both peace and wartime, often working together, on a national level with civil protection forces, and on an international level with other allied and friendly nation's armies. Due to the varied spectrum of missions, it needs to be capable of participating in a large number of activities. With the objective of contributing towards improving the organic means and capacities of the Portuguese Military Engineering in completing their various missions, Soares' (2015) dissertation aimed to put forward a modular and portable observation tower. These two characteristics are meant to enable the observation tower to successfully adapt to mission criteria. The observation tower could be used in numerous situations such as: in cases of catastrophe; refugee camp surveillance; itinerary control and surveillance of areas and buildings from an upper height that ensures a greater security distance; 1

2 defensive perimeter surveillance; control tower for improvised aviation runways in battlefields; border surveillance; checkpoint towers for early fire detection (see Figure 1). Figure 1: Checkpoint Tower. (Source: David Shankbone,2007) 2 General framework It is essential to create a framework for a modular and portable observation tower. To be able to do this it is first necessary to understand concepts that are referred to herein and secondly present existing structures that will help understanding the proposed solution. 2.1 Observation Tower The term "observation tower" is associated to a type of tower that allows a specific area, location or object to be observed. According to Eurocode EN (2006), the term "tower is defined as a self-supporting cantilevered steel lattice structure of triangular, square or rectangular plan form, or circular and polygonal monopoles. 2.2 Skywatch Frontier Skywatch Frontier (Flir, 2014), is a mobile elevated tower created by Flir, and its main objective is to improve the surveillance capacity of armed forces. 2

3 The structure has various characteristics which are useful for designing a modular and portable observation tower, namely its multi-functionality, equipment coupling capacity, tinted windows and armour. Skywatch does, however, have a serious security flaw, as whenever an element within the cabin needs to be replaced the cabin must be lowered, meaning that during this period the mission is compromised. 2.3 United Nations Tower This structure, presented in Figure 2, is, according to LT. Col. Eng. Rocha Afonso, available to FND whenever missions are under United Nations (UN) mandate. Consisting essentially of a modular and portable structure, it was developed with the aim of helping armed forces completing their missions. This structure has two main elements, but has the disadvantage of requiring preliminary foundation preparation, having environmental terrain impacts and increasing structure assembly time, thus reducing the advantages of being a modular structure. This structure does have characteristics that will be considered in the proposal of a modular and portable observation tower, specifically its modular constitution, its transport capacity, its part assembly process and the cabin format. Figure 2: United Nations Tower. (Photograph courtesy Lt. Col. Eng. Rocha Afonso) 2.4 Description of the proposed solution Soares (2015) pictured an observation tower to increase the response capacity of the Portuguese Army to a variety of missions, reason why the structure would need to have characteristics such as: portability, structural reliability, multi-functionality, adaptability to its surrounding as well as reduced terrain occupation. The tower should be portable in so much as to be able to ease its transportation. To achieve this it is necessary that the structure be light and that its assembly and disassembly be quick and easy, given 3

4 that within a military context it is not always possible to have personnel available that are exclusively there to assemble the tower. Structural reliability stems from the need for characteristics related to structural resistance, use and durability. The structure should be able to be used both temporarily and repeatedly, therefore requiring appropriate structural reliability. Multi-functionality is important as it will increase the variety of applications that it can be used in. The capacity to adapt to the surroundings and reduced terrain occupation helps to increase the number of locations that are appropriate for assembling the observation tower. To be able to correspond to these requirements, a structure with the following characteristics is proposed: Hexagonal armoured cabin, measuring 1.15m (sides) by 2.50m (height), enough to accommodate two adults and their respective equipment (Figure 3); Heights that the cabin base can be allocated at: 2, 4, 6, 8, 10 and 12 metres; Reduced terrain occupation; Reduced terrain intervention; Modules that are easy to transport and use; Modular construction; A cabin with windows on all sides and inverted openings. Figure 3: Cabin dimensions [m] The dimensions and heights for the cabin are justified by transport restrictions, specifically naval containers and heavy tactical vehicles used by the Armed Forces. 3 Dimensioning Criteria 3.1 Structural Safety Regulation In analysing and dimensioning the structure, the criteria defined in the Portuguese Eurocode for safety verification were applied (to ULS and SLS). 4

5 3.2 Structural loads Structural loads are those which can lead to stress or strain deformation in any given structural element, be it permanent, variable or accidental. The observation tower is considered a temporary structure which has a service life of 10 years (NP EN 1990, 2009), but as it is a structure that should be assembled within a maximum period of two years, the structural load limits were defined so as to not be exceeded during this time. The only exception to this is wind, as according to regulation the reference period is one year. Dead loads include the structures own weight (dead load - DL) and the remaining permanent weight (live load - LL). Variable loads are those that are relevant despite being variable over time and the overloads (SC) considered are wind and those on the horizontal elements. Snow was not considered given the reduced exposed area (3.5 m 2 ) and to consider it would actually be illogical, considering that where snow is most like to fall is also where the wind is a conditioning factor, and if snow is considered it is actually a stabilising factor. To be able to quantify SC category B Office of the NP EN (2009) was used. The considered loads are presented on Table 1: Table 1: Considered loads Steel (γ aço ) 77.0 kn/m 3 Filler Material (γ enc ) 18.0 kn/m 3 DL Cabin weight 0.4 kn/m 2 Cabin roof 0.4 kn/m 2 Armour 1.2 kn/m 2 Glass class BR6 S 1.3 kn/m 2 LL Category B 2.0 kn/m 2 Adult 1.0 kn As for the variable wind load, the reference value used was 30 m/s, which corresponds to the windiest regions (archipelagos and places on the mainland that are located on coasts that are 5 km wide or 600m above sea level). 3.3 Materials The materials to be used on the observation tower are: Steel EN S 355 H; Steel dowel pins, class 8.8; Defensive barriers type HESCO. The foundations of a structure are meant to transmit loads to the ground so as to ensure structural stability, and usually involve substantial terrain preparation. However, one of the objectives of the proposed observation tower is to reduce this intervention as much as possible. Within this context, therefore, arises the use of the defensive barriers type HESCO, which act as counterweights. The defensive barriers will be installed on the lower part of the observation tower, and whenever the structure 5

6 is subject to a destabilising load; the counterweight works to oppose this force helping to ensure the structure's global stability. The defensive barriers type HESCO (HESCO, 2015), consist of a system of barriers that protect military forces in the field where there is a high threat level, and consist of an array of soldered steel and zinc-aluminium cords that are covered with a polypropylene geotextile. Figure 4 shows an example of these defensive barriers used by the Portuguese Army. Figure 4: Use of defensive barriers type HESCO, example 1. (Photograph courtesy Lt. Col. Eng. Rocha Afonso) The barriers can be recoverable or not, and come in various sizes, adaptable to the terrain requirements and the threat level. Protection is ensured by the filler material, usually consisting of varied granulated inert. 3.4 Mast The solution adopted includes three vertical elements and it was chosen because with this configuration, cables are not necessary. These elements are CHS profiles, ranging from 88.9*5.0 and 60.3* Foundations The foundations are meant to transmit loads from the observation tower to the terrain, ensuring that structure's global stability is not compromised. Figure 6 illustrates the chosen design. 3.6 Cabin Structure The cabin structure is the element that transmits loads from the cabin to the poles, as shown in Figure 5. Circular profiles CHS 76.1*5.0 are used to connect vertical elements, and rectangular profiles RHS 150*100*10 to support the lower part of the cabin's structure. 6

7 Figure 5: Cabin support structure 3.7 Final Structure After dimensioning all of the elements, namely the materials, mast, foundations and cabin structure, the structure was analysed as a whole. Two models were considered: one in terms of the cabin structure and the other in terms of the mast and foundations. Figure 6 illustrates the model used during the structures' final dimensioning. Figure 6: Model of the Observation Tower 7

8 4 Load analysis and verification of structural elements After modelling the structure, it was necessary to identify the conditioning factors that affected safety verification. As such changes were made to the elements, namely the connections between the vertical elements the mast and the structure of the foundation. Figure 7 illustrates the connections using dowel pins. Figure 7: Connection with pins - detail [m] Given the welded connections presented, the methodology used was: identification of the most conditioning location; safety condition verification so as to be able to choose the thickness of the filet; adoption of this thickness throughout, with the minimum thickness being 3 millimetres. The location that was most conditioning for filet design was in the connections, between vertical elements of the mast and the support structure, leading to an adopted effective thickness of 4 millimetres. 8

9 4.1 Global safety analysis - stability One of the criteria for safety verification of a structure is clearly its stability. For an observation tower this aspect is especially important as it is not fixed to the ground. Stability is ensured, as mentioned previously, through the use of modules type HESCO, and depending on the layout and height of the structure will require specific number of modules per support structure. Table 2 shows the number of modules that are required to ensure stability, when considering the destabilizing horizontal forces. The number of modules per foundation was obtained taking into account wind strength according to the height of the cabin and the structure's stability. Figure 8 illustrates the various module arrays. Table 2: Number of modules required per mast length Mast length [m] Nº modules per foundation Figure 8: Module array (per foundation) 5 Assembly and Disassembly Process 5.1 Necessary conditions for assembly and transportation When considering the assembly location, there is no structural conditioning. The observation tower was dimensioned so as to be placed and functional anywhere in Portugal. In the case of being assembled out of Portugal, it is necessary to re-verify structural stability. Depending on terrain conditions, it will need to be levelled and compacted, and corresponding to terrain category B. If the structure is to be used on terrain of lesser resistance, it is necessary to verify seismic action and terrain resistance. The area required to assemble and place the structure is 7x7 (m). During transportation, all of the elements should be secured so as to not suffer damage that may affect the assembly or stability of the structure. Before assembly all of the structure should be checked, as well as all equipment required. This verification consists of the following steps: identification of all of the elements; ensure that all elements meet the requirements necessary for use; check all equipment used for assemble. All of these steps should be done by a team with previous training. 9

10 5.2 Assembly and Disassembly manual The assembly manual consists of 13 stages as shown in Table 3, in which each respective material to be used is identified. According to the required layout some stages may be repeated. The disassembly process has, except for the first stage, the same stages as the assembly process, but they are processed in opposite order. Table 3: Assembly stages Stage Function Equipment 1 Levelling terrain [7x7 m] Excavator 2 Ground support structure array and alignment, Excavator aided by the cabin platform /Crane 3 Ground support structure fixation - 4 Removal of the cabin platform, defensive barrier type HESCO placement (according to height) and Excavator filling 5 Vertical element array (aided by the cabin platform) - 6 Placing of horizontal elements and fixing using dowel pins - 7 Counterweight placing and fixing using dowel pins, as well as safety dowel pins - 8 CHS 76.1*10.0 profile placing and fixing using dowel pins, as well as safety dowel pins - 9 Cabin platform removal and placing of vertical Excavator elements /Crane 10 Repeat stages 5/6/7/8/9 (depending on required height) Stair placement - 12 Mast elevation and connection to the support structures Crane 13 Cabin structure fixation and placement Crane 6 Conclusions During the elaboration of this project it was possible to conclude that there are currently various structures used for observation. The Portuguese Armed Forces use them in the FND when supplied by the international organizations, like for example the United Nations Tower. All of these have downsides, which the developed observation tower tries to counter. These disadvantages are mainly to do with portability, adaptability to different missions, and the need to have significant intervention in the field so as to ensure its appropriate service. Furthermore, it is also necessary to reduce FND dependency on other organizations when they require this type of infrastructure. The design of the structure began with defining the required characteristics: a cabin with the capacity for 2 people, a variable height, reduced terrain occupation and intervention, and an assembly and disassembly process that is as simple as possible. Through model analysis, it became apparent that the connecting elements require special attention. The reason for this is that these connections needed to not only be as simple as possible but also as few as possible. The analysis also showed that the most severe load on the structure was wind. Due to the array of elements along the mast, load transmission was primarily axial. 10

11 To reduce terrain intervention concrete footing for foundations was not considered. Therefore to ensure global stability the structure uses counterweights that consist of steel elements where defensive barriers type HESCO are placed. After the structure was modelled and the safety analysed, the assembly and disassembly process was developed. Only an area of 7x7 (m) levelled and compacted ground (of category B), is required for this process to be possible. The assembly process for the observation tower consists of 13 stages, and the equipment required is: an excavator, a crane and a vehicle for transportation. A vehicle for transportation is meant for moving the structure, the excavator to level the terrain and help during some of the assembly stages, and the crane is required to raise materials during the process. Bibliography David Shankbone, West Bank Checkpoint, 2007, Forces_checkpoint?file=West_Bank_checkpoint_by_David_Shankbone.jpg, consulted November Flir, Skywatch TM, 2014, consulted November HESCO, Recoverable Units, consulted February Eurocode Bases for structural projects (NP EN 1990), LNEC, December 2009 (Portuguese version). Eurocode 1 Structural loads Part 1-1: General loads, dead loads, overloads of buildings (NP EN ), LNEC, December 2009 (Portuguese version). Soares, Hugo Alberto Correia (2015), Prevention within Military Engineering. Proposal of an observation system A modular and portable observation tower, Master dissertation, Instituto Superior Técnico, Lisbon (in Portuguese). 11