LECTURE 1INTRODUCTION – PART 1 January 30, 2012 Design of steel and prestressed concrete structures 1 Introduction •Introduction •Conceptual design of building- studies earlier •Design Codes 1)BS EN 1993-1-1:2005 Eurocode 3 Design of Steel Structures Part 1-1: General rules and rules for buildings, British Standards. 2)BS EN 1993-1-8:2005, Eurocode 3: Design of Steel structures — Part 1-8: Design of joints, British Standards. •Actions •Tributary areas – studied earlier •Material behaviour/ Properties of materials – studied earlier Design of steel and prestressed concrete structures 2 Introduction • • • • • • Engineering Design consists of Two stages Feasibility Study/ Conceptual design :Involves comparison of the alternative forms of structure and selection of most suitable type The success of this stage relies to a large extent on the engineering judgement and instinct, both of which are the outcome of many year’s experience of designing structures. Detailed design: involves detailed design of the chosen structure The detailed also requires these attributes but is usually more dependent upon a thorough understanding of the codes of practice for structural design namely EC2 and EC3 These documents are based on the experience of many generations of engineers, and the results of research. They help to ensure safety and economy of construction, and that mistakes are not repeated. Design of steel and prestressed concrete structures 3 Introduction • What is Structural steel? • Steel - man made metal – containing 98% or more iron – small amounts of elements derived from raw materials and also elements added to improve certain properties..C, Si, Mn, P,S, Niobium, Vanadium – Carbon improves strength and hardness but reduces ductility and toughness. Restricted between 0.2 and 0.25% to produce steel that is weldable and not brittle – smaller amounts of manganese, nickel etc • Structural steel – steel available in various shapes and forms – utilised to support loads and resist the various forces to which a structure is subjected. Design of steel and prestressed concrete structures 4 column beam Multi-storey steel building frame Design of steel and prestressed concrete structures foundation 5 . beam connection column Design of steel and prestressed concrete structures 6 . Design of steel and prestressed concrete structures 7 . good for long span bridges.Advantages of Steel: • • • • • • • High strength to low weight . tall buildings Uniformity-properties do not change with time unlike concrete Elasticity –behaves closer to design assumptions than most materials – follows Hooke’s law to fairly high stress Ductility – withstand extensive deformation without failure under high tensile stress – free from sudden failure Additions to existing structures Time saving Flexibility in fabrication Reuse on demolition • Disadvantages of Steel: • • • • Maintenance cost – corrosion – requires periodic treatment Fire proofing – strength tremendously reduced at high temperature – high cost of fire proofing Susceptibility to buckling for long slender members Fatigue – strength reduced if large number of stress reversals Design of steel and prestressed concrete structures 8 . General Steel Properties • The important characteristics of steel for design purposes are: – yield stress (Fy) – ultimate stress (Fu)….tensile strength – modulus of elasticity (E) – percent elongation () – coefficient of thermal expansion () Design of steel and prestressed concrete structures 9 . The Tension test Design of steel and prestressed concrete structures 10 . Design of steel and prestressed concrete structures 11 . Design of steel and prestressed concrete structures 12 . Design of steel and prestressed concrete structures 13 . 1 EN 1993-1-1)…page 26 Grade of Steel Yield Strength Ultimate strength or Design fu Strength(N/m m2) Grade 55 S450 440 550 Grade 50 S355 355 510 Grade 43 S275 275 430 Grade 36 S 235 235 360 Design of steel and prestressed concrete structures 14 .Grade of Steel and Design Strength (table 3. Design of steel and prestressed concrete structures 15 . S275. • S stands for “Structural” • The number indicates the yield strength of the material in N/mm2. Design of steel and prestressed concrete structures 16 . S355. but the lower grades are most commonly used in structural applications. S460 • S460 is the strongest.• The four grades are S235. Aim of Structural Design – To provide with due regard to economy a structure capable of fulfilling its intended function and sustaining the specified loads for its intended life. transport. The design should facilitate safe fabrication. Plastic design and Limit State Design Design of steel and prestressed concrete structures 17 . Philosophies/ Theories used for design: Elastic design.account future maintenance. within the constraints imposed by the architect (number of stories. certain criteria must be established to evaluate whether or not an optimum has been achieved Design: Determination of overall proportions and dimensions of the supporting framework and the selection of individual members. final demolition. handling and erection.) is responsible for structural design. In any design. recycling and reuse of materials Responsibility: The structural engineer. floor plan..Conceptual Design of building • • • • • Design – process by which an optimum solution is obtained.. Object of Structural Design • Safety (the structure doesn’t fall down during lifetime) • Serviceability (how well the structure performs in term of appearance and deflection) • Fulfill requirements of client • Economy (an efficient use of materials and labor) Alternatives • Several alternative designs should be prepared and their costs compared Design of steel and prestressed concrete structures 18 . Design of steel and prestressed concrete structures 19 .Plastic Design • Utilises strength of steel beyond yield point • The structure may be loaded beyond the yield point if: • The tendency of the fibre at the yield point stress toward plastic deformation is resisted by the adjacent fibres • Those parts of the structure that remain in the elasticstress range are capable of supporting this incremental load • The ultimate load is reached when these conditions cease to exist and thus the structure collapses • Plastic design is concerned with an allowable load. which equals the ultimate load divided by an appropriate factor called the load factor. plates. British Standards • The standard gives recommendations for the design of structural steel work using hot rolled sections.Limit State Concept in Design • Stated in cl 2. flats. hot finished structural hollow sections and cold formed structural hollow sections. in buildings and allied structures • Structures should be designed by considering the limit states beyond which they would become unfit for their intended use Design of steel and prestressed concrete structures 20 .2 EN 1993-1-1 2005 :Eurocode 3 Design of Steel Structures Part 1-1: General rules and rules for buildings. cl. Ultimate limit states (ULS) Serviceability limit states(SLS) Strength cl6.Limit states • Examples of limit states relevant to steel structures are given in Table 1.2.4 Design of steel and prestressed concrete structures 21 .3 Durability.1 Deflection Stability against overturning and sway stability Vibration Fatigue Wind induced oscillation Brittle fracture cl3. Excessive vibrations and oscillations – subject of structural dynamics Corrosion.General principles • This course discusses – Ultimate limit state of strength – Serviceability limit state of deflection. Design of steel and prestressed concrete structures 22 .serious problem for exposed steelwork – correct preparation and painting of the steel will ensure maximum durability and minimum maintenance during the life of the structure. Or else weather resistant steels can be used. Brittle fracture – avoided by selecting the correct grade of steel for the expected ambient conditions. Structures must be robust enough not to overturn or sway excessively under wind or other sideways loading Fatigue – taken care by the provision of adequate safety factors to prevent the occurrence of high stresses associated with fatigue. • • • • • Stability – aspect of complete structures or sub-structures. and that the probabilities associated with these loads change in different ways as the degree of overload considered increases. Load partial factors γF.Different types of load have different probabilities of occurrence and different degrees of variability. γQ Partial factor for variability of strength γM Design of steel and prestressed concrete structures 23 . γG. Because of this different load factors should be used for the different load types. Design of steel and prestressed concrete structures 24 . The structure is deemed to be satisfactory if its design load effect does not exceed its design resistance Design load effect ≤ Design resistance (effect of specified loads x γg.)has a different load factor which its value depends on the combination of loads under consideration.Q) specified resistance / γM factor Though limit state design method is presented in a deterministic format. the partial factors are obtained using probabilistic models based on statistical distributions of loads and structural capacity Each load effect (DL. . LL.Limit State Design • • • • • Also called LRFD (Load and Resistance Factor Design) in USA.. and other effects such as corrosion.Characteristic and Design Material Strength • • • • • • • The material strength may be less than intended because (a) of its variable composition. Item (a) is allowed by using the characteristic value. and is normally defined as the value below which not more than 5% of the test results fall. The characteristic value is determined from test results using statistical principles . The overall effect of items under (b) is allowed for using a partial safety factor : γm for strength Design Strength is obtained by dividing the characteristic strength by the partial safety factor for strength The value of γm depends upon the properties of the actual construction materials being used. Design of steel and prestressed concrete structures 25 . and (b) because of the variability of the manufacturing conditions . The characteristic strength is the value below which the strength lies in only small percentage of cases. which is defined as the load with 95% probability of not being exceeded throughout its lifetime • Characteristic Load = Average Load +1. settlement or earthquakes • Values of actions are obtained by determining characteristic or representative values of loads or forces • Ideally.ACTIONS • BS EN 1990:2002 : ACTIONS ARE A SET OF FORCES (LOADS) applied to a structure . • Characteristic Load: is the representation of the real load.or/and deformations produced by temperature . should be analysed statistically and a characteristic load is determined. loads applied to a structure during its working life.64 X Standard deviation Design of steel and prestressed concrete structures 26 . finishes and services The actual weights of materials (Gk) should be used in design calculations. tanks and cold storage services.Classification of Actions • PERMANENT ACTIONS (G) are due to weight of the structure i. Classified as indirect variable actions. permanent partitions. walls. cooling towers. floors. but if not known use density in kN/m3 from EN 1991-1:2002. Thermal effects need to be considered for chimneys. given for various dwellings in EN 1991-1-1:2002. The loads are usually given as distributed loads or an alternative concentrated load Wind Actions (Wk) : Are variable but for convenience are expressed as static pressures in EN 1991-1-4(2002). These loads include a small allowance for impact and other dynamic effects that may occur in normal occupancy. Do not include forces resulting from the acceleration and braking of vehicles or movement of crowds.e. Also included in this group are water and soil pressures. forces due to settlement etc • • • • VARIABLE ACTIONS (Q) Imposed floor Loads (Qk) are variable actions. roofs. Design of steel and prestressed concrete structures 27 . These may involve consideration of construction loads. props and bracing (EN 1991-1-6:2002).• Actions to be taken for adequate performance in fire • ACCIDENTAL ACTIONS(A) • Accidental actions during execution include scaffolding. Design of steel and prestressed concrete structures 28 . instability and collapse prior to completion of the project • Earthquake Loads (the effects of ground motion are simulated by a system of horizontal forces):EN1998-8(2004) • Actions induced by cranes and machinery : EN 19913(2004) • Impact and Explosions covered in EN 1991-1-7(2004). Statistical principles cannot be used at present to determine characteristic loads because sufficient data is not available. the designer cannot be certain about the load the member must carry because (a) of the variability of the occupancy or environmental loading. errors in analysis.Characteristic and Design Load • • • • • • When checking the safety of a member. Design Load is the value used in design calculations – product of characteristic load and partial safety factors in order to increase reliability Design of steel and prestressed concrete structures 29 . The characteristic load is the value above which the load lies in only small percentage of cases. errors during construction etc Item (a) is allowed by using the characteristic value. and (b) because of unforeseen circumstances which may lead to an increase in the general level of loading. Therefore the characteristic loads are normally taken to be the design loads from other codes of practice : BS 648 and BS 6399. Combinations of Design Actions • FOR THE ULTIMATE LIMIT STATE. three alternative combinations of actions. must be investigated • (a) Fundamental: a combination of all permanent actions including self weight(Gk). This combination assumes that accidents of short duration have a low probability of occurrence • (c)Seismic:reduces the permanent action partial safety factor(γG)with a reduction factor (ξ)between 0. modified by appropriate partial safety factors (γ). the dominant variable action (Qk) and combination values of all other variable actions(ψ0Qk) • (b) A combination of the dominant variable actions(ψ0Qk).85 and 1 • FOR SERVICEABILITY LIMIT STATE : 3 alternative combination of actions must be investigated • (A) The characteristic rare combination occurring in cases exceeding limit state causes permanent local damage or deformation Design of steel and prestressed concrete structures 30 . • For the more commonly used grades and thicknesses of steel the value of py may be obtained from Table 3. including ductility and weldability.Properties of materials • Design strength • BS EN 1993-1-1(2005) covers the design of structures fabricated from structural steels conforming to the grades and product standards specified. If other steels are used. due allowance should be made for variations in properties. • The design strength py should be taken as 1.1. Design of steel and prestressed concrete structures 31 .0Ys but not greater than Us /1.2 where Ys and Us are respectively the minimum yield strength and the minimum tensile strength specified in the relevant product standard. Design of steel and prestressed concrete structures 32 . Design of steel and prestressed concrete structures 33 . Standard Cross-Sectional Shapes Design of steel and prestressed concrete structures 34 . Design of steel and prestressed concrete structures 35 . Example: laced and braced members Design of steel and prestressed concrete structures 36 .Compound Sections Compound sections are formed by: •Strengthening a rolled section (say UB) by welding a cover plate •Combining 2 separate rolled sections like in crane girder •Connecting two members to form a combined strong member. Fabricated sections/ Built-up sections Fabricated sections can be welded or bolted Design of steel and prestressed concrete structures 37 . Cold rolled sections Cold formed Rectangular Hollow sections Design of steel and prestressed concrete structures 38 . Examples of the cold-formed steel are corrugated steel roof and floor decks. The manufacturing process involves forming the material by either press-braking or cold roll-forming to achieve the desired shape. Cold-formed steel offers versatility in building because of its lightweight and ease of handling and use. Cold-formed steel structural members are shapes commonly manufactured from steel plate. and this share is increasing The hot-rolled steel shapes are formed at elevated temperatures while the cold-formed steel shapes are formed at room temperature. sheet or strip material. steel wall panels. Cold-formed steel represents over 45 percent of the steel construction market in US. storage racks and steel wall studs. Design of steel and prestressed concrete structures 39 .Differences between cold formed and hot rolled sections • • • • • • Cold-formed steel has been widely used in building construction. from residential houses to industrial buildings. A simple section may require as few as six pairs of roll.312 inches (7. floor and wall panels. Zees.000m) long.5m) wide and from coils more than 3. each of which works the sheet progressively until it reaches the desired shape.10mm) up to 0. Cold roll-forming is the most widely used method for production of roof.• • • Press-braking is often used for production of small quantity of simple shapes. The thickness of material that can be formed generally ranges between 0. Sections can usually be made from sheet up to 60 inches (1. but a complex shape can require as many as 24 to 30. and hat sections. It is also used for the production of structural components such as Cees. although heavy duty cold forming mills can handle steel up to ¾ of an inch (19mm) thick.000 feet (1. sheet stock is fed longitudinally through a series of rolls. During cold roll-forming.7mm). Design of steel and prestressed concrete structures 40 .004 (0. Cold rolling Mill Design of steel and prestressed concrete structures 41 . Cold rolled shapes Design of steel and prestressed concrete structures 42 . Design of steel and prestressed concrete structures 43 . the material used is thin relative to its width. The dimensions of hot-rolled shapes are such that local buckling of individual constituent elements generally will not occur before yielding. Since cold-formed steel members are formed at room temperature. one is primarily concerned about two types of instability: column buckling and lateral buckling of unbraced beams. This means that the individual flat. One of the main differences between designing with cold-formed steel shapes and with hot-rolled structural shapes is that with the hot-rolled. transport and install. Its lightweight makes it easier and more economical to mass-produce. This is not the case with cold-formed members.Differences between cold formed and hot rolled steel • • • • • • thickness shapes. the material becomes harder and stronger. Design of steel and prestressed concrete structures 44 . in most cases. elements of the section often have width to thickness ratios that will permit buckling at stresses well below the yield point. or plate. Here local buckling must also be considered because. EXAMPLE 1 Determine the properties Iyy. Zy. Sy of 610 x 229 UB 125 section with a 300mm x 20 mm plate welded to each flange • Because of symmetry of the section the centroid of the plated UB is at the web centre Design of steel and prestressed concrete structures 45 . 9+20)/2]2/10000 = 218290 CM4 218290 Zx 6697cm4 611.Ixx = Σ(IGG+Ar2) = 98500+2 x 300 x 20X{(611.9 2 20 /(2 10 Design of steel and prestressed concrete structures 46 . Shape factor • Shape factor is defined as plastic mod ulus S xx Elastic mod ulus Z xx EXAMPLE 3 Determine the shape factor for a rectangular section of width 10 mm and depth 500 mm. Zxx = bd2/6=10 x 5002/6 Sxx = bd2/4 = 10 x 5002/4 Therefore shape factor = Sxx/Zxx = 6/4 = 1.5 Design of steel and prestressed concrete structures 47 . 14 Elastic mod ulus Z xx 3221 Design of steel and prestressed concrete structures 48 .• Determine the shape factor for 610 x 229 UB 125 section plastic mod ulus S xx 3676 1.