AISC aminmansour2000Q2.pdf
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1、ABSTRACT Design aids are developed using the AISC-LRFD Specifica- tions for design of steel beam-columns. These aids give designers more options in design and often allow selection of sections that are more efficient for a given set of conditions than the current design aids do. Several examples are
2、 worked out using the current method of practice as well as the pro- posed method developed in this paper. INTRODUCTION The design of steel beam-columns consists of a trial-and-error process in which the designer selects a trial section and checks the selection for compliance with the appropriate eq
3、uation in the AISC Specifications. The trial section is normally selected from the Column Load Tables in the AISC LRFD Manual of Steel Construction using Pueq, an “equivalent” axial load cal- culated based on the applied axial load as well as the equiva- lent effects of bending moments about the x-
4、and y-axes. A number of methods have been used in the past for determining Pueq. Generally, these methods use multiplier co- efficients to convert bending moments about the two axes to equivalent axial loads. These equivalent axial loads will then be added to the applied axial load to obtain Pueq. B
5、urgett (1973) introduced helpful aids for preliminary design of beam-col- umns based on the Allowable Stress Design method. Later, Uang, Wattar, and Leet (1990) recommended similar values for design based on the LRFD method. Both of these methods were utilized in the subsequent AISC Manuals. While s
6、uch methods are easy to follow and produce reason- able results, the process of selecting a trial “column” section based on an equivalent load for a beam-column and checking it for compliance with the AISC Specifications introduces limitations in the process. Further, the work involved in checking t
7、rial sections may be reduced by developing design aids for this purpose. This paper presents development and application of design aids that, while consistent with the AISC-LRFD Specifica- tions, provide more ease, speed, and flexibility for the de- signer. They also avoid some of the limitations th
8、at exist with the current method of practice. It should be noted that while the basic concepts utilized in this process apply to any pris- matic singly or doubly symmetric shape subjected to flexure and axial compression, the focus of the discussions as well as design aids developed in this paper ap
9、ply to W-shapes only. CURRENT PRACTICE Over the years, the design of beam-columns has gone through a number of major changes (Sputo, 1993). However, the method currently recommended in Part 3 of the AISC LRFD Manual of Steel Construction (subsequently referred to as the AISC Manual) (AISC, 1994) to
10、design a beam-column con- sists of the following steps: 1. With the known value of KL (effective length), select a first approximate value of m from Table 3-2 in the AISC Manual. Let u equal 2. 2. Solve for Pueq using the following equation. Pueq = Pu + Muxm + Muymu ( 1) where Pu = actual factored a
11、xial load, kips Mux = factored bending moment about the strong axis, kip-ft Muy= factored bending moment about the weak axis, kip-ft m = factor taken from Table 3-2 of the AISC Manual u= factor taken from Column Load Tables in the AISC Manual 3. From the appropriate Column Load Table, select a ten-
12、tative section to support Pueq. 4. Based on the section selected in Step 3, select a “sub- sequent approximate” value of m from Table 3-2 and a u value from the Column Load Tables. 5. With the values selected in Step 4, solve for Pueq. 6. Repeat Steps 3 and 4 until the values of m and u stabilize. 7
13、. Check section obtained in Step 6 per AISC Equation H1-1a or H1-1b, as applicable. It is not clearly pointed out in the AISC Manual whether KL, the effective length, used in step 1 should be with respect to the x- or y-axis. However, it appears from the referenced source of the tabulated values tha
14、t (KL)y is to be used for this A New Approach for Design of Steel Beam-Columns ABBAS AMINMANSOUR Abbas Aminmansour is assistant professor, structures divi- sion, school of architecture, University of Illinois at Urbana- Champaign. ENGINEERING JOURNAL / SECOND QUARTER / 2000 41 2003 by American Insti
15、tute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. purpose. If flexural buckling about the strong axis controls, the equivalent effective length, (KLy), should be used instead.
16、 In the development of this method, the following assump- tions were made. Lb = (KL)y This is usually a conservative assumption. Mn = Mp = 0.9ZFy 12 This assumption is correct if the section is compact (most sections are), and Lb Lp, and the condition Mp 1.5SFy is satisfied about both axes, as per A
17、ISC Specifications requirements. For almost all W-sections, however, the condition Mp 1.5SFy is not satisfied for bending about the weak axis and the 1.5SyFy limit should be used for Mny instead. Further discussion of this issue will follow in later sections. DISCUSSION OF THE AISC-LRFD SPECIFICATIO
18、NS In this paper, AISC Equations H1-1a and H1-1b are labeled as Equations 2 and 3, respectively as follows. (a) For Pu Pn 0.2 Pu Pn + 8 9 Mux b Mnx + Muy b Mny 1.0 (2) (b) For Pu Pn Lp, or if the section is not listed in the Column Load Tables. Having to repeat the process of checking trial sections
19、 this way could become a drawn-out endeavor. It is noted that it is not suggested here that these considerations are not necessary. Rather, the point is that design aids may be developed so that some of these steps may be simplified or shortened and to avoid the designer having to go through these c
20、alculations repeatedly. Selections Limited to “Columns” Sections: The current recommended method of selecting a trial beam-column sec- tion restricts the designer to the sections listed in the Column Load Tables of the AISC Manual. The W-sections listed in these tables are limited to those W4 to W14
21、s that are more appropriate for use as columns. In fact, not all sections within each of these nominal depths are included in the tables. This situation eliminates consideration of other sections, which may be more effective as beam-columns, unless the designer chooses to consider them on his/her ow
22、n. At times, deeper sections such as W16, W18, and W21 are more efficient and more economical as beam-columns, particularly when rela- tively large bending moments are present about the x-axis and considerations such as architectural and other criteria do not limit the designers selection to a certa
23、in nominal depth. Beam and column sections have different cross sectional characteristics. Column sections often have fairly stocky flanges while beam sections have more slender flanges. Beam sec- tions are generally relatively deep, whereas, column sections are typically shallower. Further, column
24、sections have rx/ry ratios in the range of about 1.6 to about 3.0, while this range for beam sections is usually much higher. Consequently, what may be considered as a good “beam” section, is not necessar- ily a good “column” section. Yet, what is more efficient as a beam-column, depends on the rela
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