|Table of Contents|

Research progress on geometric imperfections of cold-formed steel members(PDF)

《建筑科学与工程学报》[ISSN:1673-2049/CN:61-1442/TU]

Issue:
2025年02期
Page:
1-15
Research Field:
建筑结构
Publishing date:

Info

Title:
Research progress on geometric imperfections of cold-formed steel members
Author(s):
ZHANG Junfeng1;  ZHANG Jiangchuan2;  HAN Dongguang3;  REN Liqun3
(1. School of Water Conservancy and Transportation, Zhengzhou University, Zhengzhou 450001, Henan, China; 2. School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China; 3. The Second Construction Co., Ltd. of China Construction First Group, Beijing 102600, China)
Keywords:
cold-formed steel;  geometric imperfection;  data analysis;  measurement technique;  theory of simulation
PACS:
TU392.1
DOI:
10.19815/j.jace.2023.09130
Abstract:
In order to improve the geometric imperfections in the design system of cold-formed steel(CFS)members, the relevant experimental and theoretical achievements in different countries were analyzed through the classification of imperfections, measurement techniques, simulation theories and other aspects. The results show that at present, there is a lack of relevant provisions on distortional imperfections in Chinese standards, which needs to be further improved. In terms of measurement techniques, contact measurement techniques and non-contact measurement techniques have their own advantages and disadvantages in aspects such as cost, measurement accuracy and data post-processing. Among the numerical modeling methods, the finite strip method has a unique advantage in the decomposition of buckling modes and the solution of half-wavelength. The theories of simulation on geometric imperfections are related to buckling modes, among which the first buckling mode approach is the simplest, but its theoretical basis is insufficient and predicted results are not accurate. The traditional combined mode approach is not accurate enough to simulate the global imperfections, and the predicted results of middle long columns and long columns tend to be conservative. The one-dimensional modal spectral approach transforms geometric imperfections from time domain to frequency domain, and the magnitudes of imperfections show a certain periodicity, and the predicted results are more accurate. In addition, the two-dimensional spectral approach is complicated, and the simulation of distortional imperfections and global imperfections is limited. Machine learning provides a new research direction for stochastic probabilistic method, but it requires a lot of measured data about geometric imperfections to train the model.

References:

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图11 几何缺陷的模拟统计值与实测值对比
Fig.11 Comparison between simulated statistical
values and measured values of geometric
imperfections4.3 随机概率法
随机概率法通过随机模拟缺陷形状或缺陷幅值的方式表征未知的缺陷,近年来的创新方向为搭建机器学习框架,这需要大量的实测数据用以训练模型,才能得到接近实际的缺陷模拟结果。Farzanian等[25]研究发现,在使用常规的模拟理论对CFS构件进行非线性分析后,得到的构件极限强度或承载力的离散程度较高,因此基于实测几何缺陷数据进行机器学习训练,开发了一个能够生成随机场的几何缺陷概率模型。与许多建模方法不同的是,随机场内数据的概率分布并未被假定为任何特定形式,而是与实测数据相匹配,但部分模型的可靠性还有待验证。Zhao等[30]根据设计好的特征识别算法,对112个C形CFS构件的激光测量点云数据进行参数化处理,并使用多维高斯过程来表征沿纵向联合分布的局部几何参数和全局几何参数,将这些几何参数汇总成集,可作为基础数据,用来进行机器学习中最大似然估计的训练,最终可重构包含几何缺陷信息的CFS构件模型。对48个样本的验证结果表明,这种靠机器学习建立的高斯模型适用于大多数C形构件的几何缺陷表征。
在缺乏足够的缺陷信息时,通常会简化或假定几何缺陷的随机可变性来模拟最不利情况,以获得偏于安全的缺陷模拟结果。Lauterbach等[84]通过谱分解法分析了非常有限的缺陷数据,假设几何缺陷的随机可变性满足高斯分布以进行简化处理,凭此构建出几何缺陷随机场。
此外,还有一些其他的几何缺陷模拟理论,例如将虚拟的名义荷载视为横向荷载的虚拟荷载法。Chou等[85]使用垂直于构件表面的横向载荷模拟局部几何缺陷,而AISI规范[86]根据不同的设计方法规定了对应的虚拟荷载计算公式,并指出可利用虚拟荷载考虑初始缺陷。
5 结 语
(1)根据各国相关规范对于CFS构件制造尺寸的控制,将构件几何缺陷进行分类。从各国规范的对比中可以发现,美国规范的分类最为细致,而中国规范对整体缺陷的分类不够细致,同时缺乏畸变缺陷的相关规定,需要在未来进一步完善。
(2)几何缺陷可分为接触式测量法和非接触式测量法,两类缺陷测量方法在成本、操作难度、测量效率、数据后处理和测量精度方面都有各自的优缺点。接触式测量法对设备的要求低,操作和数据后处理简单,但在测量时效率较低,且数据精度不足。相较于接触式测量法,非接触式测量法具有精度和测量效率高的优势,得到了快速发展。然而由于对设备与环境的要求很高,非接触式测量法的运行成本很高,难以大范围推广。此外,点云数据过于庞杂,只有在处理冗余的异常数据后,才能定位构件表面的坐标,准确描绘构件轮廓,并与理想形状作对比。在未来,应简化非接触式测量法的设备布置以降低成本; 为提高分析结果的精确度与效率,应针对不同的测量设备去开发合适的点云数据算法。
(3)各类数值模拟方法能以较小的资金成本和时间成本代替部分试验,其中有限元法和有限条法最为主流。有限元法精度高,但建模复杂、计算效率低; 有限条法操作便捷、计算效率高,但无法准确模拟具有复杂截面的构件和包含螺钉等连接件的组合截面构件。为了更精确地预测构件的受力性能,应针对几何缺陷改进模拟方法并开发相关软件,它们的可靠性需要经过实测数据的验证。已有研究统计了冷弯型钢构件各类几何缺陷的测量值,使研究者可针对特定构件查询相关的几何缺陷信息。为构建完整的几何缺陷数据库,在未来要对更多截面类型的构件进行实测,丰富包括半波长在内的各项几何缺陷信息。
(4)得益于计算机的发展,研究者在提出与验证各种缺陷模拟理论时,分解提取缺陷形状和变换频域等复杂要求已能够被满足。常规的缺陷模拟理论涉及屈曲模态,其中一致缺陷模态法最为简便,但理论依据不足且预测结果不准确,在研究重点不在几何缺陷方面的冷弯型钢构件中有大量应用; 传统组合模态法考虑多种屈曲模态,但对整体缺陷的模拟不够准确,对于中长柱和长柱的预测结果趋于安全; 一维频谱模态法将几何缺陷由时域变换至频域,缺陷幅值表现出一定的周期性,预测结果较为准确,应用前景广。而二维频谱法理论复杂,对畸变缺陷和整体缺陷的模拟不够准确,存在一定的局限性。此外,根据几何缺陷实测数据开发机器学习框架为相关模拟提供了一种新途径,通过大量实测数据的训练,能形成充分考虑几何缺陷随机性的模型。目前,在开发适用于C形构件的模型方面已取得一定进展; 在未来,开发完成的机器学习框架应能针对CFS构件的不同类型,智能地调整分析历程,但要实现这样的前景,需要统计极大数量的缺陷数据并将它们用于模型的训练。

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