Design Information Of Observation Tower

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Introduction

The chosen design is of Observation Tower. The location of the tower would be adjacent to markeaton site which is in the School of Technology building. For the observation tower we are using the wood. The purpose of the report is to identify the information need to develop an observation tower and the feasibility of it. The report contains the preliminary design diagrams. The required data are gathered in the design process of the wooden observation tower. The project report also contains the economical and environmental issues regarding the observational tower. We have chosen the small diameter round wood for the project.

Objectives:

  1. To gain a deeper practical and academic knowledge in infrastructure engineering
  2. To justify the feasibility of the design project and other options
  3. To critically assess and to evaluate the simulated design process
  4. To identify all data which required in design i.e. design information

Observation Tower Design:

The design project is to build a 15 meter high public accessible wooden observation tower. The construction of the tower is made by slender Larch beams and having a structure based on hyperboloid geometry. The section of the beams is cylindrical. When making bolted or dowelled joints in such beams, the crack can lead to severe practical problems. Therefore an alternative jointing principal is suggested which can be less sensitive to the cracks. The special elements in the structure are the steel nodes that connect the round wooden beams. The detailing of the coupling in the nodes is such that it is possible to disassemble each round wood beam in the structure, can be replaced easily. The aspect of the nodes is important, as the wood in the tower is not treated with preservatives. A light steel staircase is attached which leads the person to the top level platform approx 15 m from the ground.

Structure of the Observation tower:

There are two cooperating structural systems, firstly the vertical columns and platform floors that support the stairs and secondly the hyperboloid diagonal structure at the outside of the tower. The floor column system has a pure vertical bearing constitution, while the outer diagonal structure has been designed such that bears its own weight.

The diameter of the poles at the ground level is only 140 mm, with length of 4 metre, while in the floors above the diameter is only 120 mm. [ENV (1995)] .The shape of the main load bearing system is based on the geometry of hyperboloid. By this choice, the beams that form the structure are exactly over the height of the tower.

A new node system is suggested to benefit from this straightness. In the nodes the beams are pinwise connected in lateral direction. This will make it possible to realize the axial connections between the poles in a standard way. The elements in a node are equal for all nodes in the structure. Using ring width, maximum knot diameter and maximum initial curvature as selection criteria, The 140~150 beams should be needed with length up to 4.5 m. The EC5 strength class of the round wood should be C24 and all beams are going to use should have the initial curvature less than or equal to (1/200).[ Chellis, 1961]

Wide cracks can lead to problems when dowel type fasteners are used. To cope with this problem a connection mechanism is suggested. This anchor block connection mechanism is based I load transfer through a threaded steel rod that is 'anchored' in the pole. This job results in a wooden beam with on both ends a threaded end. The steel anchor block has dimensions that are an order larger than the maximum width of a crack. This can ensure the anchorage of the rod to be less sensitive for the cracks. The principle of the load transfer mechanism is simple: compression forces are transferred directly by contact pressure through the head plate of the connection, while tensile forces are transferred to the anchorage block by the steel rod. [American Plywood Association, (1996)] From the anchorage block, the force is transferred to wood by contact pressure and ultimately through shear stress transferred to tension stresses in the centre part of the beam.

The elements of the connection should be assembled under prestress. This way all slip can be pulled out, and a stiff connection results, that is well suited to transfer changing tension and compression forces. To be sure of the prestress level in the connection, the prestress is applied externally, using auxiliary tensioning devices. While the external tension is acting on the connection, the head plate is tightened and the prestress is locked into the connection area of the beam. Certainly in time the prestress level would decrease by creep of wood, but only a minimal prestress level is enough to ensure a stiff reacting connection. [Cvetkovic, 2002]

Joints:

In the joints of the tower the beams should be connected in both lateral and axial direction. For this joint a chaining block should be designed. In one direction the block has openings to couple it in lateral direction; in the other direction the faces of the block have incisions, leading to a centre hole. [Graham, 1992] The faces with the incisions are used to connect the poles in axial direction. The incision enables assembly and disassembly of poles while chaining blocks are already laterally chained. The poles are slided in the slots and fixed to the chaining block using locking disks that exactly fit in the centre hole. The disks can be designed specially for this purpose. The chaining block can have 120*120*10 mm cube. The principal of joint can also be applicable in case of trusses.

The design of the joint is considered only to ensure possibility of beam replacement without taking the whole structure apart. This is essential, as the use of unpreserved wood may imply replacement of degraded pole during life cycle of the tower.

Structural Analysis:

The eccentricities in the joints of the round wood beams are of key importance on the structural behavior of the tower. The eccentricities in the joints and other connection detailing can cause bending moments in the round wood beams. The anchorage block joint connections should design essentially for axial loads and can have only limited bending moment capacity.[ Dolan, 1997]

Feasibility of the design and other options:

The wide base of the tower makes sense from a structural point of view. The large area of the upper floor is clearly beneficial for the user at the observation tower. From the waist of tower, the shape 'reaches out' to the surrounding. The orientation of the fence on the observation platform once again stresses this. The tower is situated in a silence area and can be reached by foot or bicycle. Some of the floor loading of the top platform and all horizontal forces(lateral and torsion), caused by both wind and horizontal load by persons) on the structure. The elements a node consists of are equal for all nodes in the structure. The drawback of the choice of nodes with crossing beams is internal eccentricity in the nodes. This means the transfer of vertical loads on a node leads to torsion forces on the node. Hence these forces have to be accommodated in the foundation of the structure and on the top level of the tower. [American Institute of Timber Construction, 1994]

The structural concept made it possible to build the tower with very slender beams. Light weight structures like this tower are sensitive to dynamic response of the horizontal loading by wind or by persons. To analyze this aspect of the structure, horizontal deformations of the structure should be studied carefully and calculations should be done to verify the dynamic behavior of the observation tower.

The location of the tower is adjacent to markeaton site which is in the School of Technology building. The length is shown in the figure-2, the area is approx 138m2 and the height is approx 15m. The area at the top is slightly more. The choice of material is no more important in this case as there are no more practical rules for these small diameter beams. The mode of transport near the tower should only by foot or cycle. Surroundings have a silent atmosphere. The life cycle cost is minimised by suggesting a way to pole replacement. The other option can possible to make a concrete based top floor on these small beams taken as columns.

Discussions and Conclusions:

This project report demonstrates that small diameter poles can be used for structural purposes. The connection that should be developed enables industrial production of high quality building elements, directly suited for assembly in a structure. The behavior and capacity of anchorage block joint is dependent on the prestress level in the connection. There are no such practical rules regarding the strength grading of small diameter poles hence the use of material for structural applications would remain marginal.

References

  1. ENV (1995)-1-1.Eurocode 5: Design of timber structures, Part 1.1: General rules and rules for building, European Committee for Standardisation, 1993
  2. Cvetkovic,R. (2002) Behaviour of Composite Timber-Concrete Structures with Bending Actions. Masters thesis, Department of Reinforced Concrete and Prestressed Concrete Structures, Ruhr University Bochum, Germany
  3. Chellis, Robert D.(1961), Pile Foundations, 2nd Edition, McGraw-Hill Book Co., New York,580-599
  4. Graham,James S. (1992) Treated Round Wood Piling Specifications, 17th annual members conference, New Orleans,LA,Deep Foundations Institute,169-191
  5. American Institute of Timber Construction. (1994). Timber Construction Manual. New York, New York: John Wiley and Sons, Inc.
  6. American Plywood Association. (1996). Proposed Cyclic Testing Standard for Shear Walls. Tacoma, Washington
  7. Dolan, J. D., and C. P. Heine. (1997). Monotonic tests of wood-frame shear walls with various openings and base restraint configurations, Timber Engineering Report TE-1997-001. Blacksburg: Virginia Polytechnic Institute and State University

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