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1、使用高级分析法的钢框架创新设计1导言在美国，钢结构设计方法包括允许应力设计法(ASD)，塑性设计法(PD)和荷载阻力系数设计法(LRFD)。在允许应力设计中，应力计算基于一阶弹性分析，而几何非线性影响则隐含在细部设计方程中。在塑性设计中，结构分析中使用的是一阶塑性铰分析。塑性设计使整个结构体系的弹性力重新分配。尽管几何非线性和逐步高产效应并不在塑性设计之中，但它们近似细部设计方程。在荷载和阻力系数设计中，含放大系数的一阶弹性分析或单纯的二阶弹性分析被用于几何非线性分析，而梁柱的极限强度隐藏在互动设计方程。所有三个设计方法需要独立进行检查，包括系数K计算。在下面，对荷载抗力系数设计法的特点进行了
2、简要介绍。结构系统内的内力及稳定性和它的构件是相关的，但目前美国钢结构协会（AISC）的荷载抗力系数规范把这种分开来处理的。在目前的实际应用中，结构体系和它构件的相互影响反映在有效长度这一因素上。这一点在社会科学研究技术备忘录第五录摘录中有描述。尽管结构最大内力和构件最大内力是相互依存的（但不一定共存），应当承认，严格考虑这种相互依存关系，很多结构是不实际的。与此同时，众所周知当遇到复杂框架设计中试图在柱设计时自动弥补整个结构的不稳定（例如通过调整柱的有效长度）是很困难的。因此，社会科学研究委员会建议在实际设计中，这两方面应单独考虑单独构件的稳定性和结构的基础及结构整体稳定性。图28.1就是这
3、种方法的间接分析和设计方法。在目前的美国钢结构协会荷载抗力系数规范中，分析结构体系的方法是一阶弹性分析或二阶弹性分析。在使用一阶弹性分析时，考虑到二阶效果，一阶力矩都是由B1,B2系数放大。在规范中，所有细部都是从结构体系中独立出来，他们通过细部内力曲线和规范给出的那些隐含二阶效应，非弹性，残余应力和挠度的相互作用设计的。理论解答和实验性数据的拟合曲线得到了柱曲线和梁曲线，同时Kanchanalai发现的所谓“精确”塑性区解决方案的拟合曲线确定了梁柱相互作用方程。为了证明单个细部内力对整个结构体系的影响，使用了有效长度系数，如图28.2所示。有效长度方法为框架结构提供了一个良好的设计。然而，有
4、效长度方法的使用存在着一些困难，如下所述：1、有效长度的方法不能准确核算的结构系统及其细部之间的互相影响。这是因为在一个大的结构体系中的相互作用太复杂不能简单地用有效长度系数K代表。因此，这种方法不能准确地测算框架单元实际需要的强度。2、有效长度的方法无法获取结构体系中内力非弹性再分配，因为带有B1、B2系数的一阶弹性分析只证明二阶影响，但不是非弹性内力再分配。有效长度的方法只是保守的估计了最终承载大型结构体系的能力。3、有效长度方法无法测算的结构体系受负荷载下的失效模式。这是因为荷载抗力系数相互作用方程不提供在任何负载下结构体系的失效模式的信息。4、有效长度的方法与计算机程序不兼容。5、有效
5、长度的方法在涉及系数K的单独构件能力检测时需要耗费比较长的时间。随着电脑技术的发展，细部结构的稳定性和整体结构的稳定性这两个方面，可以通过结构的最大强度测定来被严格对待。图28.1就是这种方法的间接分析和设计方法。直接设计方法的发展被称为高级分析，或者更具体地说，二阶弹性分析框架设计。用这种直接的方式，无须计算有效长度系数，因为不需要规范方程包含的单独构件能力检测。凭借目前现有的计算技术，直接使用高级分析法技术框架设计是可行的。这种方法过去在办公室设计使用时一直被认为是不切实际的。本章的目的是提出一个切实可行的，直接的钢框架设计方法，使用高级分析法产生跟荷载抗力系数法的相同的结果。利用高级设计
6、分析的优点概述如下：1、高级分析法是结构工程师进行钢结构设计的另一个工具，它的通过不是强制性的，而是为设计人员提供灵活的选择。2、高级分析法直接获取了整个结构体系和细部结构极限状态的强度和稳定性，这样就不需要规范方程包含的单独构件能力检测。3、相比荷载阻力系数设计法和允许应力设计法，高级分析法通过直接弹性二阶分析提供了更多结构性能的信息。4、高级分析法解决了常规荷载阻力系数设计法中由于不兼容弹性全球分析和单元极限状态设计的困难。5、高级分析法与计算机程序兼容性良好，但荷载阻力系数设计法和允许应力设计法则无法与计算机程序兼容，因为它们在过程中都需要有对系数K的单独构件能力检测的计算。6、高级分析
7、法可以得到整个结构体系弹性内力再分配的结果，并且节约高度不确定的钢框架的材料。7、过去在设计室使用高级分析法被认为不切实际，而现在则是可行的，因为个人电脑和工程工作站的能力正在迅速提高。8、通过高级分析法测定的各项数据都接近了荷载抗力系数法测定的那些数据，因为高级分析法对荷载抗力系数法的柱曲线和梁柱的相互作用方程进行了校准。因此，高级分析法替代了荷载抗力系数法。9、高级分析法比较高效，因为它完全消除了经常引起混淆的冗长的单独构件能力检测，包括荷载阻力系数设计法和允许应力设计法中的系数K的计算。在各种高级分析法中，包括塑性区准塑性铰法，弹性区塑性铰法，名义负荷塑性铰法和改进塑性铰法，推荐使用改进
8、塑性铰法，因为它保留了计算的效率和简便性及实际应用的准确度。这个方法是对简单的传统的弹塑性铰法的改进。其中包括一个简单的修改，证明在塑性铰位置截面刚度的逐步退化和包括细部两个塑性铰之间的逐步刚度退化。表28.1中对常规荷载抗力系数法和高级实用性分析方法的关键因素做了比较。荷载抗力系数方法用来证明主要影响隐含在其柱强度和梁柱相互作用方程之中，而高级分析法通过稳定性的功能，刚度退化的功能和几何缺陷方面来证明那些影响，在28.2中有详细讨论。高级分析法持有许多钢结构实际问题的答案，同样地，我们推荐寻找有效地合理地完成框架设计方法提供给工程师，但这要符合荷载抗力系数规范。在下面的章节里，我们将提出符合
9、荷载抗力系数钢框架结构设计的高级先进实用分析方法。该方法的有效性将通过比较基于精确塑性区解决方案和荷载抗力系数设计分析及设计结果的细部和框架的实际案例研究。大范围的案例研究和比较可以这种高级方法的有效性。2高级实用性分析本节介绍了一种消除规范单独构件能力检测的直接设计钢框架的高级实用性分析方法。改进后的塑性铰法是由简单的传统的弹塑性铰法发展调整而来，实现了简单和真实的反映了实际情况。下一节将提供了最终确认该方法的有效性的核查方法。高级分析能够验证连接的灵活性。常规分析和钢结构的设计通常在假设梁柱连接不是完全刚性或理想的固定下进行。然而，在大部分实际的连接是半刚性的并且它们的状态介于这两个极端的
10、例子之间。在允许应力设计-荷载抗力系数规范，有两类特定的建筑：FR（完全受限）结构和PR（部分受限）结构。荷载抗力系数规范允许通过“合理途径”连接灵活性评估。瞬间旋转的关系代表了连接的状态，已经完成多方面的试点连接工作和收集大批的瞬时旋转数据。有了这个数据库，研究人员已经开发了数个连接模型，包括线性，多项式，B曲线，动力和指数。鉴于此，Kishi和Chen提出的三参数幂函数模型被采用了。在使用高级分析时，几何缺陷必须由框架单元加以塑造。几何缺陷在构造或架设过程中导致不可避免的错误。对于建筑结构的结构构件，几何缺陷的种类属于非线性和非垂直的。明确建模和等效名义载荷被研究人员用来证明几何缺陷。在这
11、一章节中，发展了基于进一步减小构件切线刚度的新方法。这种方法提供了一种简易的途径用来证明没有输入名义载荷或明确几何缺陷的不完善的影响。本节中描述的高级实用性分析方法仅限于受静载的两维支撑，无支撑，和半刚架。不考虑结构的空间状态，并且假定有足够的侧向支撑防止侧扭屈曲。假设W节就是这样的节可以在无局部屈曲情况下发挥全塑性时刻能力。强轴和弱轴弯曲宽凸缘部分的研究都采用高级实用性分析方法。该方法可被视为介于现在广泛使用的常规荷载抗力系数方法和像在未来实际应用中塑性区的制定方法等的更严谨的高级分析/设计方法之间的一个临时的分析设计方法。An Innovative Design for Steel Fra
12、meUsing Advanced AnalysisIntroduction The steel design methods used in the U.S. are allowable stress design (ASD), plastic design (PD), and load and resistance factor design (LRFD). In ASD, the stress computation is based on a first-order elastic analysis, and the geometric nonlinear effects are imp
13、licitly accounted for in the member design equations. In PD, a first-order plastic-hinge analysis is used in the structural analysis. PD allows inelastic force redistribution throughout the structural system. Since geometric nonlinearity and gradual yielding effects are not accounted for in the anal
14、ysis of plastic design, they are approximated in member design equations. In LRFD, a first-order elastic analysis with amplification factors or a direct second-order elastic analysis is used to account for geometric nonlinearity, and the ultimate strength of beam-column members is implicitly reflect
15、ed in the design interaction equations. All three design methods require separate member capacity checks including the calculation of the K factor. In the following, the characteristics of the LRFD method are briefly described.The strength and stability of a structural system and its members are rel
16、ated, but the interaction is treated separately in the current American Institute of Steel Construction (AISC)-LRFD specification 2. In current practice, the interaction between the structural system and its members is represented by the effective length factor. This aspect is described in the follo
17、wing excerpt from SSRC Technical Memorandum No. 5 28:Although the maximum strength of frames and the maximum strength of component members are interdependent (but not necessarily coexistent), it is recognized that in many structures it is not practical to take this interdependence into account rigor
18、ously. At the same time, it is known that difficulties are encountered in complex frameworks when attempting to compensate automatically in column design for the instability of the entire frame (for example, by adjustment of column effective length). Therefore, SSRC recommends that, in design practi
19、ce, the two aspects, stability of separate members and elements of the structure and stability of the structure as a whole, be considered separately.This design approach is marked in Figure 28.1 as the indirect analysis and design method.In the current AISC-LRFD specification 2, first-order elastic
20、analysis or second-order elastic analysis is used to analyze a structural system. In using first-order elastic analysis, the first-order moment is amplified by B1 and B2 factors to account for second-order effects. In the specification, the members are isolated from a structural system, and they are
21、 then designed by the member strength curves and interaction equations as given by the specifications, which implicitly account for second-order effects, inelasticity, residual stresses, and geometric imperfections 8. The column curve and beam curve were developed by a curve-fit to both theoretical
22、solutions and experimental data, while the beam-column interaction equations were determined by a curve-fit to the so-called “exact” plastic-zone solutions generated by Kanchanalai 14.In order to account for the influence of a structural system on the strength of individual members, the effective le
23、ngth factor is used, as illustrated in Figure 28.2. The effective length method generally provides a good design of framed structures. However, several difficulties are associated with the use of the effective length method, as follows:1. The effective length approach cannot accurately account for t
24、he interaction between the structural system and its members. This is because the interaction in a large structural system is too complex to be represented by the simple effective length factor K. As a result, this method cannot accurately predict the actual required strengths of its framed members.
25、2. The effective length method cannot capture the inelastic redistributions of internal forces in a structural system, since the first-order elastic analysis with B1 and B2 factors accounts only for second-order effects but not the inelastic redistribution of internal forces. The effective length me
26、thod provides a conservative estimation of the ultimate load-carrying capacity of a large structural system.3. The effective length method cannot predict the failure modes of a structural system subject to a given load. This is because the LRFD interaction equation does not provide any information a
27、bout failure modes of a structural system at the factored loads.4. The effective length method is not user friendly for a computer-based design.5. The effective length method requires a time-consuming process of separate member capacity checks involving the calculation of K factors.With the developm
28、ent of computer technology, two aspects, the stability of separate members and the stability of the structure as a whole, can be treated rigorously for the determination of the maximum strength of the structures. This design approach is marked in Figure 28.1 as the direct analysis and design method.
29、 The development of the direct approach to design is called advanced analysis, or more specifically, second-order inelastic analysis for frame design. In this direct approach, there is no need to compute the effective length factor, since separate member capacity checks encompassed by the specificat
30、ion equations are not required. With the current available computing technology, it is feasible to employ advanced analysis techniques for direct frame design. This method has been considered impractical for design office use in the past. The purpose of this chapter is to present a practical, direct
31、 method of steel frame design, using advanced analysis that will produce almost identical member sizes as those of the LRFD method.The advantages of advanced analysis in design use are outlined as follows:1. Advanced analysis is another tool for structural engineers to use in steel design, and its a
32、doption is not mandatory but will provide a flexibility of options to the designer.2. Advanced analysis captures the limit state strength and stability of a structural system and its individual members directly, so separate member capacity checks encompassed by the specification equations are not re
33、quired.3. Compared to the LRFD and ASD, advanced analysis provides more information of structural behavior by direct inelastic second-order analysis.4. Advanced analysis overcomes the difficulties due to incompatibility between the elastic global analysis and the limit state member design in the con
34、ventional LRFD method.5. Advanced analysis is user friendly for a computer-based design, but the LRFD and ASD are not, since they require the calculation of K factor on the way from their analysis to separate member capacity checks.6. Advanced analysis captures the inelastic redistribution of intern
35、al forces throughout a structural system, and allows an economic use of material for highly indeterminate steel frames.7. It is now feasible to employ advanced analysis techniques that have been considered impractical for design office use in the past, since the power of personal computers and engin
36、eering workstations is rapidly increasing.8. Member sizes determined by advanced analysis are close to those determined by the LRFD method, since the advanced analysis method is calibrated against the LRFD column curve and beam-column interaction equations. As a result, advanced analysis provides an
37、 alternative to the LRFD.9. Advanced analysis is time effective since it completely eliminates tedious and often confused member capacity checks, including the calculation of K factors in the LRFD and ASD.Among various advanced analyses, including plastic-zone, quasi-plastic hinge, elastic-plastic h
38、inge, notional-load plastic-hinge, and refined plastic hinge methods, the refined plastic hinge method is recommended, since it retains the efficiency and simplicity of computation and accuracy for practical use. The method is developed by imposing simple modifications on the conventional elastic-pl
39、astic hinge method. These include a simple modification to account for the gradual sectional stiffness degradation at the plastic hinge locations and to include the gradual member stiffness degradation between two plastic hinges.The key considerations of the conventional LRFD method and the practica
40、l advanced analysis method are compared in Table 28.1. While the LRFD method does account for key behavioral effects implicitly in its column strength and beam-column interaction equations, the advanced analysis method accounts for these effects explicitly through stability functions, stiffness degr
41、adation functions, and geometric imperfections, to be discussed in detail in Section 28.2.Advanced analysis holds many answers to real behavior of steel structures and, as such, we recommend the proposed design method to engineers seeking to perform frame design in efficiency and rationality, yet co
42、nsistent with the present LRFD specification. In the following sections, we will present a practical advanced analysis method for the design of steel frame structures with LRFD. The validity of the approach will be demonstrated by comparing case studies of actual members and frames with the results
43、of analysis/design based on exact plastic-zone solutions and LRFD designs. The wide range of case studies and comparisons should confirm the validity of this advanced method.2Practical Advanced Analysis This section presents a practical advanced analysis method for the direct design of steel frames
44、by eliminating separate member capacity checks by the specification. The refined plastic hinge method was developed and refined by simply modifying the conventional elastic-plastic hinge method to achieve both simplicity and a realistic representation of actual behavior 15, 25. Verification of the m
45、ethod will be given in the next section to provide final confirmation of the validity of the method.Connection flexibility can be accounted for in advanced analysis. Conventional analysis and design of steel structures are usually carried out under the assumption that beam-to-column connections are
46、either fully rigid or ideally pinned. However, most connections in practice are semi-rigid and their behavior lies between these two extreme cases. In the AISC-LRFD specification 2, two types of construction are designated: Type FR (fully restrained) construction and Type PR (partially restrained) c
47、onstruction. The LRFD specification permits the evaluation of the flexibility of connections by “rational means”.Connection behavior is represented by its moment-rotation relationship. Extensive experimental work on connections has been performed, and a large body of moment-rotation data collected.
48、With this data base, researchers have developed several connection models, including linear, polynomial, B-spline, power, and exponential. Herein, the three-parameter power model proposed by Kishi and Chen 21 is adopted.Geometric imperfections should be modeled in frame members when using advanced a
49、nalysis. Geometric imperfections result from unavoidable error during fabrication or erection. For structural members in building frames, the types of geometric imperfections are out-of-straightness and out-of-plumbness. Explicit modeling and equivalent notional loads have been used to account for geometric imperfections by previous researchers. In this section, a new method based on further reduction of the tangent stiffness of members
