The Mechanical Properties Of Steel 6a2q4v

Brief On The Chemical And Mechanical Properties Of Steel Different steels have different values of strength and toughness depending on the alloys made and the heat treatments used in production. Therefore the testing methods are important to determine values and to ensure that standards are adhered to. Methods of testing are normally to determine the yield strength, ductility and stiffness through tensile testing, toughness through impact testing, and hardness through resistance to penetration of the surface by a hard object. The mechanical properties of structural steel are also fundamental to its classification and hence, application. Even though chemical composition is a dominant factor of the mechanical properties of steel, it is also very important to understand the minimum standards for the mechanical Properties. Critically even though the major constituent of steel is iron the addition of very small quantities of other elements can have a significant effect on the properties of the structural steel. For example the strength can be increased by the addition of alloys such as manganese, niobium and vanadium. However, these alloy additions can also adversely affect other properties, such as ductility , toughness and weldability . Consequently minimizing the level of sulphur can enhance ductility , and toughness can be improved by the addition of nickel. The alloying elements also produce a different response when the material is subjected to heat treatments involving cooling at a prescribed rate from a particular peak temperature. The manufacturing process may involve combinations of heat treatment and mechanical working that are of critical importance to the performance of the steel. Mechanical working takes place as the steel is being rolled or formed. The more steel is rolled, the stronger it becomes. This effect is apparent in the material standards, which tend to specify reducing levels of yield strength with increasing material thickness. The major types of structural steel are classified according to chemical composition and processing characteristics; Carbon or carbon-manganese steels, High strength, low alloy steels, High strength, quenched and tempered alloy steels, High strength, quenched and self-tempered alloy steels 

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Carbon or carbon-manganese steels are referred to as mild structural steels. In addition to iron, the primary chemical elements are carbon and manganese. Content restrictions are also placed on the amounts of various chemicals, particularly phosphorus and sulphur which have significant detrimental effects on the ductility and weldability of the steel. High strength, low alloy (HSLA) the higher strength is achieved by lowering the carbon content and by adding certain alloying elements that provide for increased strength, ductility and toughness. All of these steels are weldable; some have increased corrosion resistance, through larger copper content, for example.

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Quenched and tempered (QT) steels comprise a small group of materials with a specified minimum yield stress of 90 to 100 ksi, depending on the governing thickness of the delivered product. The high strength is achieved through a combination of lower carbon content and a rapid cooling sequence Quenched and self-tempered (QST) Their high strength is gained through selective quenching of certain regions of a shape, but in addition, the heat that is stored in the material from the rolling procedure is utilized to provide the tempering effect.

Comparison Between A36 and A992 Structural Steel A36 Structural steel A36 Steel is the American Society for Testing and Materials (ASTM) designation for carbon steel. It is one of the most widely accepted and available general purpose structural steel. It is primarily used in the steel plate fabrication industry. It's very versatile and used for most building components, including columns, beams, decking, finish elements, bridges and other structures by means of welding, bolting riveting. One of the prime properties of A36 steel is its ductility. This ductility allows buildings to stand longer after the limits of a structure have been met allowing inhabitants to exit safely before collapse. The ductility doesn't allow the alloy to be used as cable, and it should not be used as reinforcing bar. Chemical Properties of A36 Steel Under ASTM standards for carbon or low alloy steel, allowable iron content is as high as 99 percent. Common alloys found in carbon steel and the maximum percentage of the alloy allowed by ASTM follows:

Carbon ,Max% Manganese, ax% Phosphorus ,Max% Sulfur ,% Silicon, % Copper,%

0.27 1.20 0.04 0.05 0.150.40 0.20

Mechanical Properties of A36 Steel The mechanical properties of A36 steel are basically a product of yield and tensile strength. ASTM requires the following: Yield strength Tensile strength Modulus of elasticity Density Shear modulus Poisson’s ratio Corrosion resistance Weldability

36,000ksi minimum. 58,000ksi - 80,000ksi. 200 GPa (29,000 ksi). 7.85 grams per cubic centimeter 79.3 GPa (11,500 ksi). Ratio of lateral longitudinal strain, 0.269. Same atmospheric corrosion resistance as plain carbon steels and can be enhanced by specifying copper additions. Easily welded, using good shop or field practices by all of the usual methods; shielded metal-arc, submerged-arc, gas metal-arc and resistance welding

A992 Structural Steel ASTMA992 is intended for building construction usage, and applies mostly to wide-flange shapes. For all practical purposes A992 steel is A572 Grade 50 with additional requirements. The major advantage of A992 is its better material definition, it has a specified minimum yield stress of 50 ksi, A992 also gives an upper limit for F, of 65 ksi. The yield ratio, F,/Fu, cannot be larger than 0.85, and the carbon equivalent cannot exceed 0.50. These maximum limits are considered desirable attributes for seismic design. A992 offers excellent weldability and ductility characteristics. Chemical Properties of A992 Steel Under ASTM standards for carbon or low alloy steel, allowable iron content is as high as 99 percent. Common alloys found in carbon steel and the maximum percentage of the alloy allowed by ASTM follows: Carbon ,max % Manganese Phosphorus ,max % Silicon ,max % Sulfur ,max %

0.23. 0.1-1.5 035 0.5 0.045

Vanadium ,max % Vanadium ,max % Copper ,max % Columbium, max %

0.11 0.45 0.60 0.05

Mechanical Properties of A992 Steel The mechanical properties of A36 steel are basically a product of yield and tensile strength. ASTM requires the following:

Yield strength Tensile strength Modulus of elasticity Elongation in 2in Elongation in 8in Density Weldability

50,000ksi - 60,000ksi. 65,000ksi 200 GPa (29,000 ksi). %21 %18 7.85 grams per cubic centimeter enhanced weldability

Figure 1 Stress-Strain Curve Comparison for A36 and A992 Structural Steel

Steel Subjected to high temperatures During fabrication and operation, steel structures can be exposed to different levels of temperatures, thus designers need to for the influence of temperature on the behavior of structural steels. The mechanical properties of a structural steel vary with temperature. In material behavior, changes in temperature can cause the following effects, namely: • Elastic constants (e.g., E, ν) of the material can change; • Strain can develop without mechanical loading; • Material yield strength decreases with increase in temperature; and • The material can lose ductility with decrease in temperature. At high enough temperatures, the stiffness and strength of structural steels may be reduced even if the ductility increases. At low temperature, the yield strength tends to increase while brittle fracture can more easily initiate and propagate since the ductility is reduced. For fire safety of steel structures, the capacity of structural should be adequate to resist the applied loads in a fire. While the behavior of steel in fire is affected by the heating rate, steel begins to lose strength at temperatures above 300 ◦C and reduces in strength at an approximately steady rate until around 800 ◦C. To avoid excessive deformations due to fire, regulations recommend strain limits. Some of the methods for fire proofing structural steel include: 1. Concrete Encasement: involves encasing the steel member in concrete. Concrete is a good thermal insulator (thermal conductivity of 1-3W/m.K). It therefore delays heat transmission to adjacent structural elements. (See figure 2 below) 2. Insulating Board System: slab-type materials made from calcium silicate, gypsum plaster or mineral fiberboard. They are typically attached to metal or wood framing, which is then attached to the structural member.(See figure 3)

3. Man Made Mineral Fiber System: typically made from fibers of melted rock (97-99% by weight), organic binders and oils. The board systems can be installed by various means. (See figure 4 below) 4. Spray Applied Fireproofing: cement-based products or gypsum with a light weight aggregate that have some type of cellulosic or glass fiber reinforcement. (Figure 5 below) 5. Intumescent Paint: Two key components: a resin binder and a mixture of chemicals that decomposes and releases a gas when heated. During a fire, the material melts. A gas-producing reaction is triggered at a temperature corresponding to an appropriate resin melt viscosity, and the release of gas causes the resin melt to foam developing an insulating layer. This then produces a thick char, which insulates the steel from fire. Intumescents may typically expand approximately 15 times to 30 times their initial thickness during a standard fire test.(See figure 6 below)

Figure 2: Concrete encasement of structural steelwork protection systems

Figure 3: Board

Figure 4: Mineral wool close-up. Cementitious spray on beams

Figure 5:

Figure6: Application of thin film intumescent coatings offsite

Design Philosophy The main purpose of structural design is to satisfy the criteria for strength, serviceability and economy. Strength in this context refers to the general integrity and safety of the structure under most loading conditions. In saying this, the structure is also expected to withstand occasional overloading comfortably without severe damage and failure during its intended lifetime. Serviceability refers to the way the structure functions under normal or service loads with respect to its appearance, maintainability and durability. Some of the design considerations associated with serviceability include deflection, vibrations, permanent deformation, cracking, corrosion. Economy deals with factors like labor, construction and material cost that are necessary to bringing the design to reality. This also includes fabrication, erection, and maintenance processes of the structure.

Design Procedures Allowable stress design (ASD) Allowable stress design (ASD), has been for years the first choice of design methods for steel design of buildings and bridges. In allowable stress design, member stresses are computed under service or working loads are compared to some pre-designated stresses called allowable stresses. The allowable stresses are often expressed as a function of the yield stress (Fy) or tensile stress (Fu) of the material divided by a factor of safety. The factor of safety is introduced to for the effects of overload, understrength, and approximations used in structural analysis. The general format for an allowable stress design has the form:

Rn F .S ≥

m

∑ Qni i=1

o Rn = the nominal resistance of the structural component expressed in unit of stress (i.e., the allowable stress) o Qni = the service, or working, stresses computed from the applied working load of type i o F.S. = the factor of safety, i is the load type (dead, live, wind, etc.) o m = the number of load type considered in the design Load and resistance factor design (LRFD) Load and resistance factor design (LRFD) is a probability-based limit state design procedure. In a limit state design, the structure or structural component is designed in accordance to its limits of usefulness, which may be strength related or serviceability related. In developing the LRFD method, both load effects and resistance are treated as random variables. Their variabilities and uncertainties can be represented by frequency distribution curves. A design is considered satisfactory according to the strength criterion if the resistance exceeds the load effects by a comfortable margin. The concept of safety is represented schematically in Fig. 1 below

Figure 2 Frequency distribution of load effect and resistance

Theoretically, the structure will not fail unless the load effect Q exceeds the resistance R, as shown by the shaded portion in the figure. The smaller this shaded area, the less likely that the structure will fail. In actual design, a resistance factor f is applied to the nominal resistance of the structural

component to for any uncertainties associated with the determination of its strength, and a load factor g is applied to each load type to for the uncertainties and difficulties associated with determining its actual load magnitude. Different load factors are used for different load types to reflect the varying degree of uncertainties associated with the determination of load magnitudes. In general, a lower load factor is used for a load that is more predictable, and a higher load factor is used for a load that is less predictable. Mathematically, the LRFD format takes the form m

Φ

where

Φ

Rn≥

∑ ΥiQni i=1

Rn represents the design (or usable) strength and ΣQni

represents the required strength or load effect for a given load combination.

Limit States A limit state is defined as a condition in which a structure or structural component becomes unsafe (i.e., a violation of the strength limit state) or unsuitable for its intended function (i.e., a violation of the serviceability limit state). There is also fatigue limit state and accidental limit state.

Serviceability Limit States (SLS) SLS conventionally represent failure for normal operations due to deterioration of routine functionality. The strength based limit state can be written in the general form: Actual Behavior < Allowable Behavior SLS Considerations in design may address: 

Local damage which reduces the durability of the structure or affects the efficiency of structural elements.

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Unacceptable deformation which affect the efficient use of structural elements orr the functioning of equipment relying on them; Excessive vibration or noise which can cause discomfort to people or affect the proper functioning of equipment ; and Deformations and deflections which may spoil the aesthetic appearance of the structure

Ultimate Limit State (ULS) Typically represents the collapse of the structure due to loss of structural stiffness and strength . Such loss of capacity may be related to :   

Loss of equilibrium in part or of entire structure ,often considered as a rigid body Attainment of the maximum resistance of structural regions , or connections by gross yielding , rupture of fracture ;and Instability in part or of the entire structure resulting from buckling and plastic collapse pf plating , stiffened s and

Fatigue Limit State (FLS) FLS represents fatigue crack occurrence of structural details due to stress concentration and damage accumulation (crack growth) under the action of repeated loading. The FLS design is carried out to ensure that the structure has an adequate fatigue life.

Accident Limit State ALS represents excessive structural damage as a consequence of accidents, e.g., collisions, grounding, explosion and fire, which affect the safety of the structure, environment and personnel.

ASD vs. LRFD There were two major differences between the two specifications: 1. The comparison of loads to either actual or ultimate strengths and 2. A difference in effective factors of safety.

Actual vs. Ultimate Strength

The first difference between ASD and LRFD, historically, has been that the old Allowable Stress Design compared actual and allowable stresses while

LRFD compares required strength to actual strengths. The difference between looking at strengths vs. stresses does not present much of a problem since the difference is normally just multiplying or dividing both sides of the limit state inequalities by a section property, depending on which way you are going. In fact, the new AISC Allowable Strength Design (ASD), which replaces the old allowable stress design, has now switched the old stress based terminology to a strength based terminology, virtually eliminating this difference between the philosophies.

Rn/ W = ASD

Capacity Φ Rn = LRFD Capacity Rn = Nominal Capacity

Figure 10: Comparison of LRFD/ASD Capacities On a Load vs. Displacement Diagram

Short comings of the ASD The disadvantages of the allowable stress design method are: 1. Since the limitation is on the total stress under service loads, there is no simple way to for different degrees of uncertainty of various kinds of loads. Live loads may have unknown and variable distribution. 2. Creep and shrinkage, which contribute major time dependent effects on a structure, are not easily ed for using the allowable stress design method. With allowable stress design the member is not allowed to reach the plastic stage. 3. Concrete stress is not proportional to strain up to its crushing strength, so that the inherent safety provided is unknown when a percentage of f'c is used as the allowable stress. 4. The ASD is more conservative which means that structures may not be as lightweight and economical as compared with the LRFD method.

References 

http://www.onealsteel.com/carbon-steel-beam-a36.html

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http://www.onealsteel.com/carbon-steel-beam-a992.html

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http://www.tatasteelconstruction.com/en/reference/teaching_resources/ architectural_studio_reference/technology/the_nature_of_steel/the_mec hanical_properties_of_steel/ Fundamentals of Structural Steel Design By GAMBHIR

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Structural Steel Selection Considerations: A Guide for Students, Educators ,designers and builders ;edited by Reidar Bjorhovde

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http://www.bgstructuralengineering.com/BGDesign/BGDesign05.htm

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http://www.steelconstruction.info/Fire_protecting_structural_steelwork