1 CE – 3180 DESIGN - PowerPoint Presentation

1. 1 CE – 3180DESIGN OF STEEL STRUCTURESLecture-1

2. L – T – P: 3 – 1 – 0 CE318N Credits: 4.0Unit 1 Properties of Structural Steel, I. S. Rolled Sections, I. S. Specifications,   Built up sections, Design philosophy, Introduction to Plastic analysis; Simple cases of beams and frames .Unit 2, Type of Connections, Riveted, Bolted and Welded Connections, Strength, Efficiency  and Design of Joints, Modes of Failure of a Joint, Advantages and  Disadvantages of Welded Joints, Eccentric Connections.Unit 3 Design of tension members, splicing of tension member, concept of shear lag, use of lug angles. Design of compression members, Beam-column connections.Unit 4 .Design of flexure members, Plate girder, Gantry Girder

3. Text BookS S Duggal “ Design of steel structures”Kazmi S. M. A. and Jindal, R.S. “Design of Steel Structures” PHI, New Delhi, India  Reference BooksArya and Ajmani “Design of Steel Structure”, NCB, Roorkee IndiaRamamrutham “Design of Steel Structures” Dhanpat Rai, Delhi, India. Selected B.I.S CodesI. S.:800-2007-Code of Practice for General Construction in Steel, BIS, New Delhi, India.I. S. Handbook No 1 –list the properties of structural rolled sectionsIS 875- provides guidelines for estimating loads.

4. To introduce to students the theory and application of analysis and design of steel structures. 2. To develop students with an understanding of the behaviour and design of steel members and systems. 3. To prepare students for the effective use of the latest tables, design aids and computer software in the design of steel members.Course Objectives:

5. 1. recognize the material properties of steel products [POs: e]2. recognize the design philosophy of steel structures and have concept on limit state design [POs:a, e] 3. ability to design bolted and welded connections for tension and compression members and beams. [POs: a,e] 4. apply the principles, procedures and current code requirements to the analysis and design of steel tension members, beams, columns, beam-columns and connections, Girders [POs: a,c,d,e, i] 5. ability to obtain basic knowledge about the failure mode of steel structure. [POs: a,c,d]Course Outcome

6. 1. Understanding of the behaviour of structural steel2. Ability to analyze and design simple bolted and welded connections3. Ability to perform plastic analysis of simple structures4. Ability to analyze and design Gantry GirdersCourse Outcome

7. 1. Assignments and Quizzes (15%)2. Mid-Semester Examination (25%)- 1 Hour3. End Semester Examination (60%)- 2 HoursCourse Assessment Method

8. Attendance policy - Students are expected to attend all the lectures. They are also expected to perform all the work assigned by the teacher.Tardy policy - All the assigned work must be submitted by the due date and time. Submissions that are 0 - 24 hours late will be penalized for 25% of the marks. Submissions that are 24 - 48 hours late will be penalized for 50% of the marks. After 48 hours, submissions will neither be accepted nor graded. Exceptions can be made for students with emergencies or special circumstances. COURSE POLICIES

9. STRUCTURE OF A BUILDINGBuilding structure is an assemblage of a group of members expected to support and transmit the loads and forces to the ground. “Tracing the Loads” or “Chasing the Loads” eg. Bridges, Towers, Multi-storey buildings, Storage tanks and Industrial buildings, etc

10. CARBON STEELMost popular and effective building material.Carbon Steel is an alloy made by combining iron and other elements the most common of these being carbon. And traces of  manganese (1.65% max), silicon (0.60% max), and copper (0.60% max).Carbon Steel is Intermediate stage between wrought iron and cast iron wrought iron- Carbon content less than 0.05% steel - Carbon content: 0.05% to 1.5% cast iron - Carbon content :2% to 4%10

11. VARIETIES OF CARBON STEELBased on carbon contentMild steel – 0.05 to 0.25%(steel sections)Medium carbon steel - 0.25 to 0.6% ( rail wheels)High carbon steel - 0.6 to 1.5%(cutlery hammers)Increase in carbon contentIncrease strengthDecrease ductility and becomes more difficult to weld & lowers the melting point and its temperature resistance in general.11

12. Advantages of Steel as Structural MaterialHandled by trained persons in the factory - better quality control than concrete structuresHigh Strength to weight ratio and can reduce foundation costsHigh DuctilityEnvironment-friendly-Easily recycled and hence greener than concreteSpeed of erection more, Rapid construction in all weathersDemount ability12

13. 13Least Disturbance to CommunityLarge spans and bay sizes possible, providing more flexibility for ownersEasier to modify and reinforce if architectural changes are made to a facility over its lifeAdvantages of Steel as Structural Material

14. DISADVANTAGES OF STEEL AS STRUCTURAL MATERIALMaintenance cost high due to paintingFire-proofing costsSusceptibility to BucklingReduction in strength due to Fatigue loadsHighly skilled labour required14

15. STRUCTURAL STEEL MECHANICAL PROPERTIES15

16. Material propertiesProperties of steel required for engg design may be classified as1. Physical properties2. Mechanical properties3. Chemical properties

17. Physical properties of structural steel, as detailed by cl.2.2.4.1 of IS 800:2007, irrespective of its grade may be taken as:a) Unit mass of steel, p = 7850 kg/m3b) Modulus of elasticity, E = 2.0x105 N/mm2 (MPa) c) Poisson ratio, p = 0.3d) Modulus of rigidity, G = 0.769x105 N/mm2 (MPa) e) Coefficient of thermal expansion cx.=12x10 -6/0C Physical properties

18. MECHANICAL PROPERTIES AS PER IS 800:200718

19. Mechanical Properties of Structural Steel 2.2.4.2Principal mechanical properties important to designer are yield stress, ultimate tensile stress, percentage elongation and notch toughness. All except notch toughness are determined from Tensile stress test of specimen. Commonly used properties are summarised in (Table1 P13)In general properties that need to be considered by designers when specifying steel construction products are: StrengthToughnessDuctilityWeldabilityDurability.

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21. Stress-Strain Relationship in Structural Steel

22. If a piece of ductile structural steel is subjected to tensile force it will begin elongate.The amount of elongation will increase linearly within certain limits (Hooke’s law).When tensile stress reaches roughly equal to three-fourths of the ultimate strength the elongation will begin to increase at a greater rate without a corresponding increase in the stress

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24. Cold Working & Strain Hardening

25. 25c07f25

26. Structural Steel CharacteristicsElasticity: Ability of metal to return to its original shape after loading and subsequent unloading High elasticityDuctility: Ductility is a measure of the degree to which a material can strain or elongate between the onset of yield and eventual fracture under tensile loading High DuctilityToughness: Combination of strength and ductility. If the steel is insufficiently tough, the 'crack' can propagate rapidly, without plastic deformation and result in a 'brittle fracture'. High toughness

27. Structural Steel CharacteristicsWeldability: Measured by carbon equivalent, Ceq, as per IS: 2062, 1992, Where C = carbon, Mn = manganese, Cr = chromium, Mo = molybdenum, V = vanadium, Ni = nickel and Cu = copperIf carbon content < 0.12 %, then Ceq can be tolerated up to 0.45 %

28. Structural Steel CharacteristicsCorrosion: Susceptible to Corrosion when exposed to air and water 0.075 mm/year of the thickness may be lost due to corrosion. Use paints, or weathering steelsFireproofing: Strength reduces with increased temperature - Fireproofing required (Section 16 of code)Fatigue: The damage and failure of materials under cyclic loads is called fatigue damage. Section 13 of code

29. Structural Steel Characteristicshardness – the property of being rigid and resistant to pressure; not easily scratchedmachinability – the property of a material that can be shaped by hammering, pressing, rolling

30. MECHANICAL PROPERTIESThe number after Fe is characteristic ultimate tensile strength in MPa; Letter A,B,C indicate the grade of steel.Grade A to be used in structures subjected to normal conditionsGrade B to be used in structures subjected to non critical applications, where temp does not fall below zero degree celcius. Prone to brittle failure.Grade C for structures at low temp ( upto minus 40 degree celcius) and impact properties

31. DESIGN PHILOSPHY

32. A design philosophy is a set of assumptions and procedures which are used to meet the conditions of serviceability, safety, economy and functionality of the structureWorking stress Method(WSM)/ Allowable Stress Design (ASD)Ultimate Load Method (ULM)Limit State Method(LSM)Three Major Design Philosophies

33. Working Stress Method/ Allowable Stress DesignThe main assumption in the WSM is that the behaviour of structural material is restricted with in linear-elastic region and the safety of it is ensured by restricting the stresses coming on the members by working loads. Thus the allowable stresses will come in the linear portion (i.e., initial phase) of the stress-strain curve. Thus a factor of safety was introduced to the design.

34. Working Stress Method/ Allowable Stress DesignWSM cannot account for loads acting simultaneously, but has different degrees of uncertainty. It cannot account for the loads having counteracting effects, such as dead load and wind load. The above will lead to non-conservative design. Working Stress method will lead to large FOS and over-sized sections, thus reducing the design economy.

35. This is also known as load factor method In this we make use of the nonlinear region of stress strain curves of steel and concrete. The safety is ensured by introducing load factor.The ULM makes it possible to consider the effects of different loads acting simultaneously thus solving the shortcomings of WSM. As the ultimate strength of the material is considered we will get much slender sections for columns and beams compared to WSM method. Ultimate Load Method

36. But the serviceability criteria is not met because of large deflections and cracks in the sections. The fall-back in the method was that even though the nonlinear stress strain behaviour of was considered sections but the nonlinear analysis of the structural was not carried out for the load effects. Thus the stress distribution at ultimate load was just the magnification of service load by load factor following the linear elastic theory.Ultimate Load Method

37. Limit state is the state of impending failure, beyond which a structure ceases to perform its intended function satisfactorily, in terms of either safety or serviceability.”There are 2 types of limit statesUltimate Limit State: It considers strength, overturning, fatigue, sliding etc.Serviceability Limit State: It considers crack width, deflection, vibration etc.Limit State Mewthod

38. Limit state method will be used- The objective of the design is to achieve a structure that will remain fit for use during its life with acceptable target reliability. In other words, the probability of a limit state being reached during its lifetime should be very low. In the limit state method reliability based analysis is performed and based on a particular dependability safety factors are calculated.Two types of safety factors are usedPartial Safety factor-LoadsPartial Safety factor-materials

39. Table 4 Partial Safety factor-Loads, fPARTIAL SAFETY FACTORS FOR LOADS ACCOUNTS FOR possibility of Deviation of load from characteristic values. possibility of Inaccurate assessment of loads. possibility in Uncertainty in assessment of load effects. (static/ dynamic) Uncertainity in the lmit state being considered (servicibility

40. Partial Safety factor-Material, mPARTIAL SAFETY FACTOR FOR MATERIAL strength ACCOUNTS FOR Possibility of unfavorable deviation of material strength from the characteristic value,Possibility of unfavorable variation of member sizes,Possibility of unfavorable reduction in member strength due to fabrication andtolerances, andUncertainty in the calculation of strength of the members.

41. The following combination of loads with appropriate partial safety factors (see Table 4) maybe considered.a) Dead load + imposed load,b) Dead load + imposed load + wind orearthquake load,c) Dead load + wind or earthquake load, andd) Dead load+ erection load.NOTE — In the case of structures supporting crmres, imposed loads shall include the crane effects as given in 3.5.4.3.5 Load Combinations

42. In general consider the 8- load combinations: (1) 1.5 (DL + IL) + 1.05(CL or SL)(2) 1.2 (DL + IL) + 1.05(CL or SL) ± 0.6(WL or EL)(3) 1.2 (DL + IL ± WL or EL) + 0.53 (CL or SL)(4) 1.5(DL ± WL or EL)(5) 0.9 DL ± 1.5 (WL or EL)(6) 1.2 (DL + ER)(7) 0.9DL + 1.2 ER(8) DL + 0.35(IL + CL or SL) + ALWhere, DL = Dead load, IL = imposed load (live load), WL = wind load, SL = snow load, CL = crane load (vertical / horizontal), AL = accidental load, ER = erection load and EL = earthquake load.42Load Combinations

43. A load value obtained by multiplying the characteristic loads by the relevant factors, given in Table 4, to get the design loads or design load effects.Design Load/Factored Load

44. The Design Strength, is obtained from ultimate strength, and partial safety factors for materials, given in Table 5 as Sd= Sa(ultimate strength)/ Υmwhere partial safety factor for materials,5.4.1 Design Strength