Lecture 1 Introduction_2072

Design of steel and prestressed concrete structures*LECTURE 1INTRODUCTION PART 1

January 30, 2012

Design of steel and prestressed concrete structures

Design of steel and prestressed concrete structures*IntroductionIntroductionConceptual design of building- studies earlierDesign Codes1)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.ActionsTributary areas studied earlierMaterial behaviour/ Properties of materials studied earlier


Design of steel and prestressed concrete structures*IntroductionEngineering Design consists of Two stagesFeasibility Study/ Conceptual design :Involves comparison of the alternative forms of structure and selection of most suitable typeThe success of this stage relies to a large extent on the engineering judgement and instinct, both of which are the outcome of many years 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 EC3These 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*IntroductionWhat is Structural steel?Steel - man made metal containing 98% or more ironsmall amounts of elements derived from raw materials and also elements added to improve certain properties..C, Si, Mn, P,S, Niobium, VanadiumCarbon improves strength and hardness but reduces ductility and toughness. Restricted between 0.2 and 0.25% to produce steel that is weldable and not brittlesmaller 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*Multi-storey steel building framefoundationcolumnbeam


Design of steel and prestressed concrete structures*columnbeamconnection


Design of steel and prestressed concrete structures*


Design of steel and prestressed concrete structures*Advantages of Steel:

High strength to low weight - good for long span bridges, tall buildingsUniformity-properties do not change with time unlike concreteElasticity behaves closer to design assumptions than most materials follows Hookes law to fairly high stressDuctility withstand extensive deformation without failure under high tensile stress free from sudden failureAdditions to existing structuresTime savingFlexibility in fabricationReuse on demolitionDisadvantages of Steel: Maintenance cost corrosion requires periodic treatmentFire proofing strength tremendously reduced at high temperature high cost of fire proofingSusceptibility to buckling for long slender membersFatigue strength reduced if large number of stress reversals


Design of steel and prestressed concrete structures*General Steel PropertiesThe important characteristics of steel for design purposes are:yield stress (Fy)ultimate stress (Fu).tensile strengthmodulus of elasticity (E)percent elongation ()coefficient of thermal expansion ()


Design of steel and prestressed concrete structures*The Tension test


Design of steel and prestressed concrete structures*Grade of Steel and Design Strength (table 3.1 EN 1993-1-1)page 26

Grade of SteelYield Strength or Design Strength(N/mm2)Ultimate strengthfuGrade 55S450440550Grade 50S355355510Grade 43S275275430Grade 36S 235235360


Design of steel and prestressed concrete structures*The four grades are S235,S275, S355, S460S460 is the strongest, but the lower grades are most commonly used in structural applications.S stands for StructuralThe number indicates the yield strength of the material in N/mm2.


Design of steel and prestressed concrete structures*Conceptual Design of building

Design process by which an optimum solution is obtained. In any design, certain criteria must be established to evaluate whether or not an optimum has been achievedDesign: Determination of overall proportions and dimensions of the supporting framework and the selection of individual members.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. The design should facilitate safe fabrication, transport, handling and erection- account future maintenance, final demolition, recycling and reuse of materials Responsibility: The structural engineer, within the constraints imposed by the architect (number of stories, floor plan,..) is responsible for structural design.Philosophies/ Theories used for design: Elastic design, Plastic design and Limit State Design


Design of steel and prestressed concrete structures*Object of Structural DesignSafety (the structure doesnt fall down during lifetime)Serviceability (how well the structure performs in term of appearance and deflection)Fulfill requirements of clientEconomy (an efficient use of materials and labor)

AlternativesSeveral alternative designs should be prepared and their costs compared


Design of steel and prestressed concrete structures*Plastic DesignUtilises strength of steel beyond yield pointThe 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 fibresThose parts of the structure that remain in the elastic-stress range are capable of supporting this incremental loadThe ultimate load is reached when these conditions cease to exist and thus the structure collapsesPlastic design is concerned with an allowable load, which equals the ultimate load divided by an appropriate factor called the load factor.


Design of steel and prestressed concrete structures*Limit State Concept in Design Stated in cl 2.2 EN 1993-1-1 2005 :Eurocode 3 Design of Steel Structures Part 1-1: General rules and rules for buildings, British Standards The standard gives recommendations for the design of structural steel work using hot rolled sections, flats, plates, 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*Limit statesExamples of limit states relevant to steel structures are given in Table 1.

Ultimate limit states (ULS)Serviceability limit states(SLS)Strength cl6.1DeflectionStability against overturning and sway stabilityVibrationFatigueWind induced oscillationBrittle fracture cl3.2.3Durability, cl.4


Design of steel and prestressed concrete structures*General principlesThis course discusses Ultimate limit state of strengthServiceability limit state of deflection.Stability aspect of complete structures or sub-structures. Structures must be robust enough not to overturn or sway excessively under wind or other sideways loadingFatigue taken care by the provision of adequate safety factors to prevent the occurrence of high stresses associated with fatigue.Brittle fracture avoided by selecting the correct grade of steel for the expected ambient conditions.Excessive vibrations and oscillations subject of structural dynamicsCorrosion- 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.


Design of steel and prestressed concrete structures*Different types of load have different probabilities of occurrence and different degrees of variability, and that the probabilities associated with these loads change in different ways as the degree of overload considered increases. Because of this different load factors should be used for the different load types.

Load partial factorsF, G, QPartial factor for variability of strengthM


Design of steel and prestressed concrete structures*Limit State DesignAlso called LRFD (Load and Resistance Factor Design) in USA.The structure is deemed to be satisfactory if its design load effect does not exceed its design resistanceDesign load effect Design resistance(effect of specified loads x g,Q) specified resistance / M factorThough 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 capacityEach load effect (DL, LL, ..)has a different load factor which its value depends on the combination of loads under consideration.


Design of steel and prestressed concrete structures*Characteristic and Design Material StrengthThe material strength may be less than intended because (a) of its variable composition, and (b) because of the variability of the manufacturing conditions , and other effects such as corrosion.Item (a) is allowed by using the characteristic value.The characteristic strength is the value below which the strength lies in only small percentage of cases.The characteristic value is determined from test results using statistical principles , and is normally defined as the value below which not more than 5% of the test results fall. The overall effect of items under (b) is allowed for using a partial safety factor : m for strength Design Strength is obtained by dividing the characteristic strength by the partial safety factor for strengthThe value of m depends upon the properties of the actual construction materials being used.


Design of steel and prestressed concrete structures*ACTIONSBS EN 1990:2002 : ACTIONS ARE A SET OF FORCES (LOADS) applied to a structure ,or/and deformations produced by temperature , settlement or earthquakesValues of actions are obtained by determining characteristic or representative values of loads or forcesIdeally, loads applied to a structure during its working life, should be analysed statistically and a characteristic load is determined.Characteristic Load: is the representation of the real load, which is defined as the load with 95% probability of not being exceeded throughout its lifetimeCharacteristic Load = Average Load +1.64 X Standard deviation


Design of steel and prestressed concrete structures*Classification of ActionsPERMANENT ACTIONS (G)are due to weight of the structure i.e. walls, permanent partitions, floors, roofs, finishes and servicesThe actual weights of materials (Gk) should be used in design calculations; but if not known use density in kN/m3 from EN 1991-1:2002.Also included in this group are water and soil pressures, forces due to settlement etcVARIABLE ACTIONS (Q)Imposed floor Loads (Qk) are variable actions; given for various dwellings in EN 1991-1-1:2002.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. The loads are usually given as distributed loads or an alternative concentrated loadWind Actions (Wk) : Are variable but for convenience are expressed as static pressures in EN 1991-1-4(2002). Thermal effects need to be considered for chimneys, cooling towers, tanks and cold storage services. Classified as indirect variable actions.


Actions to be taken for adequate performance in fireACCIDENTAL ACTIONS(A)Accidental actions during execution include scaffolding, props and bracing (EN 1991-1-6:2002). These may involve consideration of construction loads, instability and collapse prior to completion of the projectEarthquake Loads (the effects of ground motion are simulated by a system of horizontal forces):EN1998-8(2004)Actions induced by cranes and machinery : EN 1991-3(2004)Impact and Explosions covered in EN 1991-1-7(2004).



Design of steel and prestressed concrete structures*Characteristic and Design LoadWhen checking the safety of a member, the designer cannot be certain about the load the member must carry because (a) of the variability of the occupancy or environmental loading, and (b) because of unforeseen circumstances which may lead to an increase in the general level of loading, errors in analysis, errors during construction etcItem (a) is allowed by using the characteristic value.The characteristic load is the value above which the load lies in only small percentage of cases.Statistical principles cannot be used at present to determine characteristic loads because sufficient data is not available.Therefore the characteristic loads are normally taken to be the design loads from other codes of practice : BS 648 and BS 6399. 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*Combinations of Design ActionsFOR THE ULTIMATE LIMIT STATE, three alternative combinations of actions, modified by appropriate partial safety factors (), must be investigated(a) Fundamental: a combination of all permanent actions including self weight(Gk), the dominant variable action (Qk) and combination values of all other variable actions(0Qk)(b) A combination of the dominant variable actions(0Qk). This combination assumes that accidents of short duration have a low probability of occurrence(c)Seismic:reduces the permanent action partial safety factor(G)with a reduction factor ()between 0.85 and 1FOR 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*Properties of materialsDesign strengthBS 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, including ductility and weldability.The design strength py should be taken as 1.0Ys 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. For the more commonly used grades and thicknesses of steel the value of py may be obtained from Table 3.1.


Design of steel and prestressed concrete structures*Standard Cross-Sectional Shapes


Design of steel and prestressed concrete structures*Compound SectionsCompound sections are formed by:Strengthening a rolled section (say UB) by welding a cover plateCombining 2 separate rolled sections like in crane girderConnecting two members to form a combined strong member. Example: laced and braced members


Design of steel and prestressed concrete structures*Fabricated sections/ Built-up sectionsFabricated sections can be welded or bolted


Design of steel and prestressed concrete structures*Cold rolled sectionsCold formed Rectangular Hollow sections


Design of steel and prestressed concrete structures*Differences between cold formed and hot rolled sectionsCold-formed steel has been widely used in building construction, from residential houses to industrial buildings. Cold-formed steel offers versatility in building because of its lightweight and ease of handling and use. Cold-formed steel represents over 45 percent of the steel construction market in US, and this share is increasingThe hot-rolled steel shapes are formed at elevated temperatures while the cold-formed steel shapes are formed at room temperature. Cold-formed steel structural members are shapes commonly manufactured from steel plate, sheet or strip material.The manufacturing process involves forming the material by either press-braking or cold roll-forming to achieve the desired shape. Examples of the cold-formed steel are corrugated steel roof and floor decks, steel wall panels, storage racks and steel wall studs.


Design of steel and prestressed concrete structures*Press-braking is often used for production of small quantity of simple shapes. Cold roll-forming is the most widely used method for production of roof, floor and wall panels. It is also used for the production of structural components such as Cees, Zees, and hat sections. Sections can usually be made from sheet up to 60 inches (1.5m) wide and from coils more than 3,000 feet (1,000m) long.During cold roll-forming, sheet stock is fed longitudinally through a series of rolls, each of which works the sheet progressively until it reaches the desired shape. A simple section may require as few as six pairs of roll, but a complex shape can require as many as 24 to 30. The thickness of material that can be formed generally ranges between 0.004 (0.10mm) up to 0.312 inches (7.7mm), although heavy duty cold forming mills can handle steel up to of an inch (19mm) thick.


Design of steel and prestressed concrete structures*Cold rolling Mill


Design of steel and prestressed concrete structures*Cold rolled shapes


Design of steel and prestressed concrete structures*Differences between cold formed and hot rolled steelthickness shapes. Since cold-formed steel members are formed at room temperature, the material becomes harder and stronger. Its lightweight makes it easier and more economical to mass-produce, transport and install.One of the main differences between designing with cold-formed steel shapes and with hot-rolled structural shapes is that with the hot-rolled, one is primarily concerned about two types of instability: column buckling and lateral buckling of unbraced beams. The dimensions of hot-rolled shapes are such that local buckling of individual constituent elements generally will not occur before yielding.This is not the case with cold-formed members. Here local buckling must also be considered because, in most cases, the material used is thin relative to its width. This means that the individual flat, or plate, elements of the section often have width to thickness ratios that will permit buckling at stresses well below the yield point.


Design of steel and prestressed concrete structures*EXAMPLE 1Determine the properties Iyy, Zy, Sy of 610 x 229 UB 125 section with a 300mm x 20 mm plate welded to each flangeBecause of symmetry of the section the centroid of the plated UB is at the web centre


Design of steel and prestressed concrete structures*Ixx = (IGG+Ar2)

= 98500+2 x 300 x 20X{(611.9+20)/2]2/10000 = 218290 CM4


Design of steel and prestressed concrete structures*Shape factorShape factor is defined as EXAMPLE 3Determine 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


Determine the shape factor for 610 x 229 UB 125 section



Documents

Lecture 1 Introduction_2072