Local Buckling of High Strength Steel W

Free Web Hosting Provider - Web Hosting - E-commerce - High Speed Internet - Free Web Page

The design of most steel structural systems typically involve the use of rolled I-shaped members which are generally referred to as W-shapes. W-shaped steel sections are widely used as beam members since their shape offers an efficient resistance to bending moments. W shapes are also used as columns to transfer pure axial load and as beam-columns in situations where both axial load and bending moment may exist.

The current steel design specification related to local and global buckling, thus the ductility and strength of those members, are based on the constitutive nature of structural mild carbon steel, which, when tested uniaxially, possesses yield stresses between 220 MPa and 450 MPa and exhibits a well defined yield plateau followed by region of strain hardening. Recently, the class of High Performance Steel (HPS) is appealing to the designers due to its high strength-to-weight ratio. HPS is the designation given to any kind of new steel that has relatively high yield strength (typically greater than 453 MPa), exhibits good notch toughness, possesses good weldability and weathers with an atmospheric corrosion resistance. The material properties of HPS tend to be dramatically different from mild carbon steel in several key areas of uniaxial stress strain relationship. HPS has neither a well defined yield plateau nor a substantial strain hardening slope when compared with mild carbon steel. HPS also has somewhat less material ductility than mild carbon steel

Predicting the ultimate flexural response of wide flange members manufactured from HPS has been one area of recent research. Research along these lines has been predominantly focused on demonstrating the ability of HPS steel members to resist the axial and flexural loading in a ductile manner as quantified by axial shortening and rotation capacity, respectively. This study is concerned with influence of the HPS, and the associated material characteristics, on the compactness and bracing limits for the strength and ductility of W shaped structural members such as stub column, short beam and long beam.

Depending on the amount of chemical and alloy constituents in steel and on the production practices such as cooling rate and subsequent heat treatment, several steel grades with different material properties can be produced. Some applications such as bridge construction require certain material characteristics such as weldability, good toughness and corrosion resistance. Thus, in addition to a desired strength, a structural designer may request a specific type of steel that possess these particular attributes. For this reason, each steel grades is distinguished by a number and a letter, such as 350W, where the number represents the nominal yield strength in MegaPascals and the letter refers to the type of steel. Here W means weldable in this example. The relationship between the stress and strain of steel can be obtained from testing a standard steel coupon in tension. The Stress-Strain relationship of mild carbon steel can be divided into four distinct ranges. The first range is an elastic range where the steel obeys the Hooke's law and the strain is directly proportional to the stress. The end of the elastic range is marked by a definite yield point, denoted as FY. In the elastic range, the applied force overcomes the attractive force due to the relative displacement between the atomic planes and the plane will return to their original position when applied load is released. In the second range, material develops slip planes as yielding develops. At the precise location of the plane, the strain can be thought of having slipped from y to st in one single jump, thus under no perceptible variation in the applied stress, the number of slip planes that have jumped from y to st will progressively increase until the entire length of coupon strain harden to a strain st. At this stage, the slip planes are lined up in a preferred orientation, thus the specimen is capable of resisting more loads, resulting in the strain hardening stage until the ultimate load is reached. The strain hardening range is the third range. When the specimen begins to strain-harden, the load carrying capacity of the specimen increases until the ultimate strength is attained. Beyond the ultimate strength point, the cross sectional area of the tensile coupon begins to decrease rapidly and results in a drop off of the loads. This fourth range of behaviour is referred to as necking range and is terminated by the rupture of the specimen. It must be realized that because the stress is calculated as engineering stress, where load is divided by the original cross sectional area, thus the stress appears to be decreasing in this range. The true stress, however, as calculated by using the actual cross sectional area, continues to increase despite the drop off in load occurring during the necking. A measure of material ductility may be obtained using the relationship between stress and strain by measuring the percent elongation that occurs over certain prescribed gauge length. Structural mild carbon steel is a very ductile material since there is a large total strain that develops before rupture. For structural mild carbon steel, this strain is (typically 0.2-0.24) well in excess of the strain of the elastic range.

A basic assumption made for the plastic design of steel structure is that the flexural members shall have sufficient plastic deformation capacity while maintaining the plastic moment resistance. The need for plastic deformation capacity is even greater for seismic design because it is expected not only that the yield mechanism will form, but also that the structure may displace beyond the point of incipient collapse and cycle back and forth under severe earthquake shaking. A large proportion of members used in steel structures are I-shaped members which are generally referred to as W shape sections. W shape sections are widely used as beams for resisting bending moments. W shape sections are also used as columns for resisting pure axial compression and as beam-columns for resisting combined axial compression and bending moment. When a W shape steel column is subjected to an increasing axial load, its stiffness decreases gradually as the maximum load carrying capacity is approached. The attainment of maximum load carrying capacity may be a result of local buckling failure in the plate element (flange and web) making up the section or overall buckling failure. W shape steel beams can efficiently carry large bending moments; however, these members are particularly susceptible to local buckling in the plates making up the cross section and global lateral-torsional buckling. This onset of local and global buckling limits the load carrying capacity of the members in the current specifications. In the beam-columns, the load carrying capacity may be limited by overall stability of the member or bending strength of a particular cross section. The overall stability of the member may be in-plane stability or lateral torsional buckling. The local buckling behaviour of the flange and web and the global buckling of member, thus the ductility and strength of those members, are largely determined by the slenderness and bracing provisions. The current specifications for such compactness and bracing requirements involve assumptions about the constitutive nature of the structural steel being used. If a similar uniaxial tensile test is performed on HPS, the material properties tend to be dramatically different from that of structural mild carbon steel in several key areas of the uniaxial stress-strain relationship. The HPS does not exhibit the well defined yield plateau that is a unique characteristic of mild carbon steel. Instead, HPS has gradual yielding which makes it difficult to determine the value of yield point. The yield stress is usually defined by the intersection of the stress-strain relation with a line drawn parallel to initial elastic range and at an offset of 0.2% strain. But for mild carbon steel, the yield stress is defined at the point where the stress actually exceeds Fy for a small strain range prior to the long flat plateau. A second notable difference is the lack of long flat plateau in HPS. In the mild carbon steel, there is a perfect long plastic range before the occurrence of the strain hardening, whereas in HPS, the elastic range is immediately followed by the strain hardening range. Also the strain hardening slope of HPS is considerably less than that of mild carbon steel. The reserve capacity defined by the ratio of ultimate strength, Fu, to yield strength, Fy, is very low for HPS compared with mild carbon steel. A further observation that can be made through the stress-strain relationship is with regard to material ductility. HPS has the tendency to rupture at a smaller elongation which shows that HPS does not have excellent deformation capacity for which mild carbon steel is noted. Above comparisons indicate that HPS have inelastic material characteristics that are inferior to that of mild carbon steel. These differences are expected to have effects on the strength and deformation capacity of structural members as determined by local and global buckling instabilities. Hence, it is questioned how HPS may be used in plastic and seismic design applications, where substantial deformation capacity is expected. There is also a question as to whether the current notional decoupling of so-called local and global lateral buckling phenomena is applicable for HPS members. In predicting structural ductility correctly, in addition to cross sectional compactness and unbraced length, it is also believed that one must consider certain important material parameter in which HPS and mild carbon steel differs.