Main Functions of Buildings • Provide a safe and comfortable environment for work and rest. (Protect us from wind, rain and the wild animals) • Two types of loads act on a building structure： 1. Vertical force (Load from the weight of human, furniture or building itself) 2. Lateral force (Load from wind or earthquake etc.) • Basic requirements in building architecture： 1. For safety, a building will not collapse under great stress 2. For comfort, a building will not move excessively under great pressure
Stress • Load acting on a structure will be shared by each component. eg. For a load of 200,000 kg shared by 4 pillars, each pillar has to bear 50,000 kg. • Stress = Force/Area In the above case, if the area increases and the load remains unchanged, the stress will decrease. • The maximum stress that a material can bear is called the material strength.
Beam L • A component used to bear the bending forces in a structure. • When stress is exerted, it has to bear a pressure from above and a pulling force from below. (see the figure above) • Under constant load (W), if the beam’s span (L) increases, it’s vertical displacement will increase and the stress it bears will increase. • The beam’s vertical displacement and stress will decrease if the thickness of the beam (H) increases. H W
Building Material 1. Timber： • Light; cheap; easy to link up; has a high availability. • Used for buildings of one or two levels (common in Europe and America). • Low strength; easy to deform under stress or pressure; cannot be used in tall buildings or long bridges. 2. Steel bar： • High strength; high resistance to deformation; suitable for constructing tall buildings and long bridges. • Expensive • Difficult to link up (components are prepared in factory, rather in the building site.)
Steel-reinforced Concrete • Concrete can bear a very high pressure but is weak in bearing tension. • Steel lines are used to bear tension. • Cheaper than steel bars. • Can apply directly in working place; can be made into components for combination.
Building Structure • Vertical Force • Passes from the floor to the beam, then to the pillar, and finally to the base. • Lateral Force • The framework can bear the lateral force. • A shear wall is added to the structure of building with over twenty levels and thus the building can bear a greater lateral force.
A higher building suffers a greater lateral force, and therefore a tall building has to face a much greater lateral force than the vertical force. • Buildings having 40 to 50 levels (or more) require special designs : 1. Reduce the distances between the pillars and thicken the beams. =>To increase the structural efficiency.
However, when the structural efficiency maximizes, the frame will become a box with many small holes on it. eg. Jardine House
2. Super Frame - Most of the external forces are taken by the super-sized structural components. eg. Hong Kong and Shanghai Bank Building
Acknowledgment： • Hong Kong University of Science and Technology (HKUST) • Dr. Christopher K.Y. LEUNG Associate Professor, Department of Civil Engineering, Hong Kong University of Science and Technology