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  Technical Reference Manual 2.4.2 Steel Design per AISC/LRFD Specification The 2nd and 3rd editions of the American LRFD code have been implemented. The commands to access those respective codes are:For the 3rd edition code:PARAMETER CODE LRFDor PARAMETER CODE LRFD3For the 2nd Edition:PARAMETER CODE LRFD22.4.2.1 General Comments2.4.2.2 LRFD Fundamentals2.4.2.3 Analysis Requirements2.4.2.4 Section Classification2.4.2.5 Limit States2.4.2.6 Design Parameters2.4.2.7 Code Checking and Member Selection2.4.2.8 Tabulated Results of Steel Design2.4.2.9 Composite Beam Design   2.14 Steel Design per AISC/...Page 1file:///C:/Users/Aurva.K/App...6/15/201711:58:44 AM  Technical Reference Manual 2.4.2.1 General Comments The design philosophy embodied in the Load and Resistance Factor Design (LRFD) Specification is built around the concept of limit state design, the current state-of-the-art in structural engineering. Structures are designed and proportioned taking into consideration the limit states at which they would become unfit for their intended use. Two major categories of limit-state are recognized--ultimate and serviceability. The primary considerations in ultimate limit state design are strength and stability, while that in serviceability is deflection. Appropriate load and resistance factors are used so that a uniform reliability is achieved for all steel structures under various loading conditions and at the same time the chances of limits being surpassed are acceptably remote.In the STAAD implementation of LRFD, members are proportioned to resist the design loads without exceeding the limit states of strength, stability and serviceability. Accordingly, the most economic section is selected on the basis of the least weight criteria as augmented by the designer in specification of allowable member depths, desired section type, or other such  parameters. The code checking portion of the program checks that code requirements for each selected section are met and identifies the governing criteria.The following sections describe the salient features of the LRFD specifications as implemented in STAAD steel design. A detailed description of the design process along with its underlying concepts and assumptions is available in the LRFD manual. However, since the design  philosophy is drastically different from the conventional Allowable Stress Design (ASD), a brief description of the fundamental concepts is presented here to initiate the user into the design  process.Technical Reference Manual 2.4.2.2 LRFD Fundamentals The primary objective of the LRFD Specification is to provide a uniform reliability for all steel structures under various loading conditions. This uniformity can not be obtained with the allowable stress design (ASD) format.The ASD method can be represented by the inequality Σ Q i  < R  n /F.S.The left side is the required strength, which is the summation of the load effects, Qi (forces and 2.14 Steel Design per AISC/...Page 2file:///C:/Users/Aurva.K/App...6/15/201711:58:44 AM  moments). The right side, the design strength, is the nominal strength or resistance, R n, divided  by a factor of safety. When divided by the appropriate section property (area or section modulus), the two sides of the inequality become the actual stress and allowable stress respectively. ASD, then, is characterized by the use of unfactored working loads in conjunction with a single factor of safety applied to the resistance. Because of the greater variability and, hence, unpredictability of the live load and other loads in comparison with the dead load, a uniform reliability is not possible.LRFD, as its name implies, uses separate factors for each load and resistance. Because the different factors reflect the degree of uncertainty of different loads and combinations of loads and of the accuracy of predicted strength, a more uniform reliability is possible. The LRFD method may be summarized by the inequalityy i Q i  < R  n ØOn the left side of the inequality, the required strength is the summation of the various load effects, Qi, multiplied by their respective load factors, yi. The design strength, on the right side, is the nominal strength or resistance, R n, multiplied by a resistance factor, Ø.In the STAAD implementation of LRFD, it is assumed that the user will use appropriate load factors and create the load combinations necessary for analysis. The design portion of the  program will take into consideration the load effects (forces and moments) obtained from analysis. In calculation of resistances of various elements (beams, columns etc.), resistance (nominal strength) and applicable resistance factor will be automatically considered.Technical Reference Manual 2.4.2.3 Analysis Requirements The types of construction recognized by AISC specification have not changed, except that both simple framing (formerly Type 2) and semi-rigid framing (formerly Type 3) have been combined into the same category, Type PR (partially restrained). Rigid Framing (formerly Type 1) is now Type FR (fully restrained). Type FR construction is permitted unconditionally. Type PR construction may necessitate some inelastic, but self-limiting, deformation of a structural steel element. Thus, when specifying Type PR construction, the designer should take into consideration the effects of partial restraint on the stability of the structure, lateral deflections and second order bending moments. As stated in Sect. C1 of the LRFD specification, an analysis of second order effects is required. Thus, when using LRFD code for steel design, the user must use the P-Delta analysis feature of STAAD.Technical Reference Manual2.14 Steel Design per AISC/...Page 3file:///C:/Users/Aurva.K/App...6/15/201711:58:44 AM  2.4.2.4 Section Classification The LRFD specification allows inelastic deformation of section elements. Thus local buckling  becomes an important criterion. Steel sections are classified as compact, noncompact or slender element sections depending upon their local buckling characteristics. This classification is a function of the geometric properties of the section. The design procedures are different depending on the section class. STAAD is capable of determining the section classification for the standard shapes and user specified shapes and design accordingly.Technical Reference Manual 2.4.2.5 Limit States 2.4.2.5.1 Axial Tension2.4.2.5.2 Axial Compression2.4.2.5.3 Flexural Design Strength2.4.2.5.4 Combined Axial Force and Bending2.4.2.5.5 Design for Shear Technical Reference Manual 2.4.2.5.1 Axial Tension The criteria governing the capacity of tension members is based on two limit states. The limit state of yielding in the gross section is intended to prevent excessive elongation of the member. The second limit state involves fracture at the section with the minimum effective net area. The net section area may be specified by the user through the use of the parameter NSF  (see Table 2.6). STAAD calculates the tension capacity of a given member based on these two limit states and proceeds with member selection or code check accordingly.Technical Reference Manual 2.4.2.5.2 Axial Compression The column strength equations have been revised in LRFD to take into account inelastic deformation and other recent research in column behavior. Two equations governing column   2.14 Steel Design per AISC/...Page 4file:///C:/Users/Aurva.K/App...6/15/201711:58:44 AM
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