Model Making Workshop Structure of Tall Buildings and Towers

Transcription

1 Science Teaching Kit for Senior Secondary Curriculum Force and Motion Model Making Workshop Structure of Tall Buildings and Towers [Teacher notes] Organizer Sponsor Research Team

2 Contents Preamble Teaching plan Lesson 1 : Model Making Workshop - Structure of Tall Buildings and Towers 1.1 Introduction to Tall Buildings 1.2 Loads and Forces on Buildings Vertical Forces Horizontal Forces Internal Forces Exercise: Forces on the Structure 1.3 Typical Structural Systems in Tall Buildings Project Brief on Tower-Making Workshop Summary, Key words and Further reading i Disclaimer Create Hong Kong of the Government of the Hong Kong Special Administrative Region provides funding support to the project only, and does not otherwise take part in the project. Any opinions, findings, conclusions or recommendations expressed in these materials/events (or by members of the project team) do not reflect the views of the Government of the Hong Kong Special Administrative Region Hong Kong Institute of Architects

3 Topic 03 Model Making Workshop Structure of Tall Buildings and Towers Major teaching areas Physics: Chapter II Force and Motion Force and motion Related teaching areas Physics: Chapter X Investigative Study in Physics Learning objectives To understand the forces acting on a stable structure To learn the major structural systems for buildings To apply new knowledge in a hands-on exercise Teaching plan Lesson Lesson 1 Model Making Workshop Structure of Tall Buildings and Towers Contents Exercise 1.3 Project Interdisciplinary teaching areas Design and Applied Technology Strand 1 Design and Innovation Strand 2 Technological Principles Introduction to tall buildings Explanation of the forces that act on buildings Understanding of internal and shear forces, structural elements, and the relationship between action and reaction Examples of typical structural systems for high-rise buildings Brief and assessment methods for tower-making workshop i

4 Lesson 1 Model Making Workshop Structure of Tall Buildings and Towers 01

5 Lesson 1 Model Making Workshop Structure of Tall Buildings and Towers 1.1 Introduction to Tall Buildings Tall buildings are symbolic elements within any city, carrying significant political, social, cultural and even religious meanings. Today cities compete to produce the tallest building in the world as a way of showcasing financial and economic power. Understanding the structures of these buildings, and how they support themselves as well as the loads imposed on them by the environment, is a fascinating way to see the real-life applications of physics. q The Eiffel Tower is a 320-m-high steel structure that was completed in 1889 as the entrance arch for that year s World s Fair. p The Leaning Tower of Pisa (55.86 m) was built in 1372, using marble stone in a Romanesque style. Its current leaning appearance is due to sub-soil settlement. 02

6 In the early 20 th century, cities became bigger and denser. Urban populations were growing but land supply was limited. High-rise buildings became an essential solution to the problem. New technologies and building materials, such as industrial reinforced concrete, steel and elevators, made high-rise structures feasible and drove innovation. t The 90-m-high Royal Liver Building in Liverpool was one of the first concrete buildings in the world. It was completed in 1911 after a Neoclassical design by Walter A. Thomas. Chowells - Wikipedia User p The modernist Wainwright Building in St. Louis was completed in 1891 by architects Dankmar Adler & Louis Sullivan. Its 10 storeys are supported by an early steel framing system. p Equitable Life Assurance Building in New York was completed in It was the first building equipped with elevators. It was destroyed by the fire in The 36-storey Equitable Building in New York was completed in Its architect, Ernest R. Graham, used a Neoclassical style despite the building s modernity. It triggered the implementation of height limits and setbacks for tall buildings to allow sunlight to reach street level. 03

7 1.2 Loads and Forces on Buildings The statics of a building deal with its structural stability. When an object is in equilibrium, the sum of all forces equals zero. Various forces from the environment, the weight of the building elements, furniture and equipment installations, users and other sources act on the building structure. There are three types of loads generally: Dead Load Dead loads are the loads of the structure and fixed components. It is a permanent force that is relatively constant for a extended period of time. The force is gravitational. Live Load Live load is a changing force generated by mobile objects inside the building, such as people within the building or stock in a warehouse. The force is gravitational. Environmental Load Environmental loads are forces acting on the building from its environment and may include wind, rain, earthquakes and temperature changes. The forces created can be either horizontal or vertical, positive or negative Vertical Forces Dead loads and live loads contribute to the vertical forces on the structure of buildings. Vertical loads are transferred from the floors to the columns and walls, and eventually to the soil or bedrock. At times, environmental loads also act vertically Horizontal Forces Environmental loads contribute most of the horizontal forces acting on the structure of a building, with loads from wind being the most common. Architects refer to these horizontal forces as shear force. Adding cross bracing or shear walls can improve structural resistance to shear forces Internal Forces The internal strength of the entire structure must be equal to or larger than the total forces applied on the building in order to stay in equilibrium. The ability to withstand all forces depends on the structural component s dimensions and the solidity and elasticity of the material. Internal forces include compressive force, tensile force and torque. Compressive and Tensile Forces According to Newton s Third Law, forces act in pairs. In structural terms, tensile force pulls a structural element apart while compressive force compresses it. Torque If opposing forces are applied at different points, a structural element may become twisted. This is called torque in the building industry. Dead loads e.g. Weight of the building Environmental Loads e.g. Wind Live loads e.g. Weight of people at the building p Live loads, dead loads and environmental loads are the three major types of forces on the Bank of China Tower. The loads are transferred to the ground via columns and pilings. Compressive force p Internal forces in a structural element Tensile force Torque 04

8 [Exercise] Forces on the Structure 1 Illustrate the action and reaction pairs, and distinguish dead loads and live loads acting on the following structure. Assume the weight of the mass is F A and the weight of each building block is F B, and the structure is in equilibrium. Possible perspectives The weight of mass ( F A ) is the live load and the total weight of the structural blocks ( 3F B ) is the dead load. 1. Action by the mass on the block = F A Reaction by the block on the mass = F A 2 Can you identify the internal forces acting on each piece of the structure? What paths do they take? Compression Tension Loads Stress points 2. Action by the horizontal block on each vertical block = 0.5 (F A + F B ) Reaction by each vertical block on the horizontal block = 0.5 (F A + F B ) 3. Action by each vertical block on the ground = 0.5 (F A + F B ) + F B Reaction by the ground on each vertical block = 0.5 (F A + F B ) + F B Possible perspectives The horizontal structural block is supported at two points. When a force is applied on the member between the two points, the upper surface of the member is in compression where the lower surface is in tension. On the other hand, vertical structural blocks are in compression mainly. The loads are then transferred to the earth. This shows the difference between the load-supporting behaviours of vertical members (e.g. columns and wall) and horizontal members (e.g. floor and ceiling). When the internal strength of a structural member cannot withstand the loads imposed on it, it will buckle or even break at the most stressed point. 3 How would you modify the below structure to resist shear force? Possible perspectives Cross bracing can be added at the joints of the horizontal and vertical block. Cross bracing is usually constructed of diagonal supports between structural members. It can resist both compression and tension forces, depending on the direction of the shear forces acting on the building. Shear Force 05

9 1.3 Typical Structural Systems in Tall Buildings Outrigger Core Columns p Installation of outriggers at the International Commerce Centre Raymond Wong Core and Outrigger structure The International Commerce Centre is built using a Core and Outrigger concept. The core at the centre of the building bears most of the vertical load, while columns at the perimeter carry less weight and are thus smaller in dimension. Loads are transferred to the core through steel outriggers that balance the lateral forces on the whole building. p Outrigger connecting the core and the columns s Columns at the perimeter carry less weight. Weight is centralised to the core. Outriggers help balancing lateral forces. p Plan of International Commerce Centre Steel Steel is a common construction material for tall buildings because it has good performance in withstanding compressive and tensile forces, as opposed to concrete s low tensile strength in compression. Steel bars can be used to reinforce concrete to add extra structural performance. However, steel is relatively weak in fire-resistance. An extra layer of fire-resisting coating is often put onto the steel surface. The Bank of China Tower is a steel trussed-tube structure. The whole building acts as a single tubular truss, with the diagonals wrapping the building to transfer loads. Teaching Tips Field trips to Central can be organized to facilitate the learning of high-rise structure, can refer to Science Topic 02: VISIT: Central - Structure of Skyscrapers. p Bank of China Tower Teaching Tips More information on the construction process, can refer to Design and Applied Technology Topic 02: Construction Process - Victoria Park Swimming Pool Complex. 06

10 Truss Trusses are a very common structural element in architecture. Steel members are joined together into triangular shapes, which are very strong and able to resist external forces. When joined together, these triangles can form large truss systems that can span long distances. External force External force result result External force External force Compression Compression Tension q Common types of truss Structural Building Components Association 07

11 Project Model Making Workshop Tower

12 Project Model Making Workshop Tower Divide the class into groups of four to five students. Each group is required to build a tower that should be: structurally stable and aesthetically pleasing tall lightweight resistant to horizontal forces able to support a heavy load Submit a laboratory group report after the workshop. Tools needed Sketching papers and pencil Scissors, cutters, tape, glue Different weights (10 g/ 50 g/ 100 g/ 500 g/ 1 kg) Weight scale Measuring tape Electric fans Suggested materials Cardboard Bamboo sticks Recycled cans Recycled plastic bottles Fishing line p Using rope to join the components Assessment criterias: Tests Descriptions Score Structural stability The tower should be free-standing without external supports. 20 Aesthetics The teacher will judge the beauty of the tower and will give a score Height Measure the height of the tower from the ground to the highest point. The tallest tower gets the highest score. Weight-height ratio Weigh the tower and find out the weight-height ratio. The tower with the smallest ratio will get the highest score R = W H Resistance to wind Blow the tower from the side with an electric fan. The tower that does not fall will get the highest score. 10 Load supporting Test the maximum weight that the tower can support. The weight needs to be placed above the ground. Students should be able to explain how load is transferred to the ground. Total score 100 Teacher tips: To make the workshop more informative, teachers are recommended to summarize critical factors for a light-weight and stable tower. Evaluations are useful to point out the reasons for structural failure. 09

13 Summary 1. Although humans have long attempted to build tall structures, skyscrapers began to appear in our cities in the late 19th century as a result of technological breakthroughs in building materials and methods, including reinforced concrete, steel and elevators. 2. Buildings bear three types of loads: dead loads, live loads and environmental loads. 3. All loads are resolved into vertical and horizontal forces on the structure. 4. Typical structural systems used in tall buildings include core and outrigger structures, steel frames and trusses. Key words High-rise buildings Skyscraper Tower Structure Force Equilibrium Further reading 1. Foster, Jack Stroud, Raymond Harington, and Roger Greeno. Structure and Fabric. 7th ed. Harlow: Pearson Prentice Hall, Brotrueck, Tanja, Basic: Roof construction. Basel: Birhaeuser, Organizer Sponsor Research Team