Introduction. Structures may be classified on the basis of materials used for construction, as follows: Steel structures. Aluminium structures

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1 Steel Structures 1

2 Introduction Structures may be classified on the basis of materials used for construction, as follows: Steel structures Aluminium structures Concrete structures Composite structures 2

3 Steel Steel, as a building material has been extensively used in various structures in Singapore. To facilitate safe and economical design, standards and codes are prepared. 3

4 Advantages of Steel Steel has many advantages as a structural material. Steel members weight. have high strength per unit Being light, steel members can be conveniently handled and transported. Properly maintained steel structures have a long life. 4

5 Advantages of Steel (Contd) The properties of steel mostly do not change with time. Steel, being a ductile material, does not fail suddenly, but gives visible evidence of impending failure by large deflections. Additions and alterations can be made easily to steel structures. 5

6 Advantages of Steel (Contd) They can be erected at a faster rate. Steel has the highest scrap value amongst all building materials. 6

7 Disadvantages of Steel Steel has the following disadvantages as a structural material. Steel structures when placed in exposed conditions are subjected to corrosion. Therefore, they require frequent painting. Steel structures need fire proof treatment, which increases cost. 7

8 Uses of Structural Steel Use of steelwork in building construction is gaining popularity in view of: - speed of construction - less labour intensive - cleaner work site - economy in terms of construction cost - allow design flexibility due to excellent strength and predictable behaviour under loading 8

9 Projects in Singapore Singapore National Library The Esplanade Capital Tower Gardens by the Bay Singapore Sports Hub 9

10 Elements Present in Steel Carbon Silicon Phosphorus Sulphur Manganese 10

11 Mild Steel Composition: Carbon % Sulphur 0.055% Phosphorus 0.055% 11

12 Properties of Mild Steel Its structure is fibrous with dark bluish colour. It is malleable and ductile It is difficult to harden and temper. Its specific gravity is 7.8. It can easily be welded, riveted and forged. 12

13 Properties of Mild Steel (Contd) It is more tough and elastic than cast iron and wrought iron. It is equally strong in compression, tension and in shear. 13

14 Uses of Mild Steel As rolled structural sections like I-section, T- section, channel section, angle iron, plates, round and square rods in construction works. Mild steel round bars are extensively used as reinforcements in Reinforced Cement Concrete (RCC). Mild steel tubes are finding much use in structures. 14

15 Uses of Mild Steel (Contd) Plain and corrugated sheets of mild steel are being used as roof coverings. Mild steel is also used in the manufacture of various tools and equipments, machine parts, towers and industrial buildings. 15

16 High Tensile Steel Composition Carbon 0.8% Manganese 0.6% Silicon 0.2% Sulphur 0.05% Phosphorus 0.05% 16

17 Uses of High Tensile Steel It is essentially a low carbon steel and the percentage of carbon is kept less than 0.15%. It is also known as high strength steel. High tensile steel is normally used in prestressed concrete 17

18 Types of Steel Steel is given different names based on its alloyed composition: Carbon Steel Alloy Steel Structural Steel Sheet Steel 18

19 Mechanical Properties of Steel The most important mechanical properties of steel are its strength, elasticity and plasticity, characterised by stresses and elongations as well as its tendency to brittle failure. 19

20 Behaviour of Steel under Tension Stress-strain diagram for mild steel and high strength steel is shown below: 20

21 Stress-Strain Diagram The load-deformation or conventional stressstrain diagram is very important, as it is the basis for accepting the steel for most of the structural requirements. The total curve can be divided into three distinct regions: Elastic Region Region of Pure Plasticity Region of Plasticity with Work Hardening 21

22 Design Objectives for Steelwork Structural Steel work can be single member an assembly of a number of sections connected together & perform a specified function Three Main Design Criteria: - Safety - Economy - Appearance 22

23 Factors Influencing Design of Steel The spans involved The vertical loading The horizontal loading The services required The ground conditions 23

24 Design Methods 1. Elastic (allowable stress) Design The process involves: Determine the stress of members subject to service load on the structure & thereafter. Design the member so as not to exceed the allowable strength of material used. In view of possible inaccuracies in the loading and material behaviour, the material factor used is normally high. 24

25 Design Methods (Contd) 2. Limit State (load factor) Design The process involves: Ultimate limit state which assess the strength of material, stability against overturning, sway and fatigue. Serviceability limits states which assess the deflection, durability, corrosion & vibration. Determine the stress of member subject to the ultimate load. 25

26 Limit State Approach Two limit states considered in the design of structural steel work: 1. Ultimate Limit State - strength - stability against overturning - fracture due to fatigue - brittle fracture 26

27 Limit State Approach (Contd) 2. Serviceability Limit State - deflection - vibration - durability - corrosion 27

28 Load Factors For the limit state, the service or actual loads are multiplied by load factors and the products are referred to as ultimate loads. For the serviceability limit state, load factor is unity (1). Partial safety factor for dead load = 1.4 Partial safety factor for live load =

29 Categories of Steel Building Construction The majority of steel building fits within one of the following categories: Type Main Use Bearing Wall Low rise, lightly loaded Steel Frame Wide variety of types and sizes of building Long Span Coverage of long column-free areas High Rise Tall buildings i.e. more than 20 storeys 29

30 Types of Load Dead Load (DL) - Point load (kn) - Uniformly distributed load, UDL (kn/m) Refers to self-weight of steel member and other permanent parts of the building. Dead Load of a structure can be computed based on the density of the material. For eg: Density of Steel is 7850 kg/m 3 30

31 Types of Load (Contd) Imposed Load (Live Load) - Point load (kn) - Uniformly distributed imposed load UDL (kn/m) Refers to temporary load. eg: people, furniture Imposed load varies with functional usage of space / room BS 6399 Part 1 Code of Practice for DL & LL 31

32 Types of Load (Contd) Wind Load (WL) BS 6399 Part 2 Code of Practice for Wind Load Wind speed is selected based on the location of building, topography, surrounding buildings, height above the ground level, component size and period of exposure. 32

33 Load Combination In design of structural steel building, the following principal combination of loads should be taken into account: Load Combination 1: 1.4 Dead Load Imposed Load Load Combination 2: 1.4 Dead Load Wind Load Load Combination 3: 1.2 Dead Load Imposed Load WL 33

34 Design Procedure Design problems start with: Selection of type and layout of structure Estimate the loading Conduct analysis to determine the maximum moments, forces and shear 34

35 Design Procedure (Contd) Select the material and proportioning of members and connection Check the performance of the structure under service load such as deflection, vibration, corrosion, etc. Production of detailed drawing 35

36 Fabrication & Erection Simple connection Quality welding should be done at the factory Bolting is preferred at the site Avoid expensive weld fillet weld is cheaper than butt weld Design joints based on calculated forces rather than section capacity Consider installation sequence in design Design for simple construction 36

37 Material Economy Use material only when necessary High strength & lightweight steel Cost per ton for SHS is 30% more than H or I-section Be reasonable in design 37

38 Buildability Consider an integrated system i.e. mechanical services, superstructure and foundation Use high strength lightweight design to reduce load on foundation Use fast track construction for early return of investment Use complete design to enhance strength and stiffness as well as for fire protection 38

39 Conclusion Steel is buildable compared to precast construction Materials can be recycled, reused and reduced Light, dry and fast construction Cost effective for: - commercial buildings - long span bridges - industrial buildings - sport and leisure buildings 39