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为了模拟建筑物主体结构在载荷发生明显变化条件下的应力场变化,实验针对常见的方形钢和工形钢模拟测试,分别搭建成果类似建筑物主体结构中的支撑结构。测试结构长度2.0 m,载荷施加于目标梁的中间位置,宽度0.8 m。方形钢和工形钢长2.0 m,梁面宽度为150 mm,厚度10 mm。光纤传感网络分布主要有两个方向组成,一组是与梁方向平行,分布于梁上部支撑板的下表面,另一组是与梁方向垂直,分布于垂直支撑架与梁相接的位置及每间隔0.5 m位置的梁体上。分布设计如图3所示。
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光源采用1630~1660 nm激光,通过波长调制完成对整个FBG网络的扫描,由于建筑物应变场属于缓变信号,故采用10 Hz工作频域。解调部分才与之相应的滤波方式得到FBG中心波长偏移量。
测试时在两组测试体结构中分布的FBG获取载荷从200~1000 N时其对应的波长响应值,其中按照分布位置主要分为三大类:一是在交叉位置的敏感区域(FBG1,FBG2),二是在施力平面位置与竖直平面位置的稳定区域(FBG3,FBG4),三是在应力施加位置的反向区域(FBG5)。对于方形钢测试结果如图4(a)所示,对于工形钢测试结果如图4(b)所示。
由图4(a)可知,对于方形钢而言,FBG1在其水平梁与垂直梁的交叉位置处,其响应效果最为明显,在整个载荷递增过程中,波长偏移约25 nm。相比之下,随着应力场梯度位置不同,距离约0.5 m位置处的FBG2的波长偏移量减小,总偏移量约为16 nm。而在平面位置与垂直平面位置的测试值并不具备单调性,而是存在一定的波动,波长基本没有变化,可见这两个位置的应力场敏感程度很低,这和FBG的位置是有关系的。最后,在应力施加位置的反向区域,波长偏移量为负值,与应力位置相反,其说明形变方式与FBG1和FBG2正好相反,即压缩与拉伸的关系。由图4(b)可知,工形钢的应力响应曲线斜率大于方形钢,说明其在相同载荷的施力条件下变形程度更明显。但从其三个分布类型的响应曲线变化趋势可以看出,其规律与方形钢上FBG的回波特征是一致的。再有了FBG传感网络对待测件的应力-波长测试数据后,就可以根据上文中采用的验算方法完成对模拟建筑物在受到载荷时的位置偏移进行解算了,从而为建筑物健康状态提供数据支撑。
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实验中为了对比该系统的测试效果,采用ODISI6100型分布式解调仪在临近位置进行测试,其空间最小采样间隔优于1 mm,应变测试范围±12000 με,应变测试精度±30 με。与此同时,将两组测试结果与ANSYS仿真值进行对比,测试数据如表1所示。
Stress value/N Position deviation/mm Error ANSYS ODISI System 200 0.0425 0.034 0.037 8.8% 400 0.0854 0.081 0.090 11.1% 600 0.1256 0.120 0.115 4.2% 800 0.1637 0.169 0.154 8.3% 1000 0.2024 0.212 0.202 4.9% Table 1. Comparison of square steel test data
由表1可知,当载荷逐渐增大时,三种方法的位置偏移都呈增大趋势,从测试数据的总体分布中可以看出,仿真分析计算值普遍大于实际测试值,分析认为由于仿真分析是通过理论计算得到的,而实际测试的过程中有噪声的引入,再抑制噪声的过程中,会对信号强度有一定的抑制。对比ODISI系统和本系统的测试结果可以看出,分布规律基本一致,测试误差除400 N应力加载时都优于10%,符合设计要求。表2与表1的测试数据分布规律相似,但单位应力对应的偏移程度有所不同,相比之下,工型钢比方形钢的测试结果更明显,应力响应效果更大,这与其结构有关。对比两表之间的数据可知,模拟方形钢条件下应力与位置偏移量的平均比率是1.99×10−7 m,模拟工型钢条件下应力与位置偏移量的平均比率是2.79×10−7 m。综上所述,该模拟实验验证了测试系统可以获取在受到外界额外载荷条件下的应力场分布,并依据此完成主体结构的位置偏移计算,为建筑物结构健康提供风险分析的数据依据。
Stress value/N Position deviation/mm Error ANSYS ODISI System 200 0.0624 0.063 0.059 6.3% 400 0.1151 0.121 0.111 9.9% 600 0.1658 0.172 0.164 4.7% 800 0.2139 0.208 0.217 4.3% 1000 0.2627 0.269 0.287 6.5% Table 2. Comparison of I-shaped steel test data
Research on optical fiber sensor network monitoring system for building structural health
doi: 10.3788/IRLA20210263
- Received Date: 2021-04-23
- Rev Recd Date: 2021-05-10
- Publish Date: 2021-08-25
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Key words:
- optical fiber sensing /
- structural health /
- bending calculation /
- state analysis
Abstract: The analysis of the overall stress field distribution and state parameters of the main structure in building health monitoring is a bottleneck problem in the online state assessment technology. In order to comprehensively and consistently reflect the overall state information of the building, an optical fiber sensor network system was designed for the main structure of the building. A mapping algorithm was proposed, which established the relationship between the stress field information in the main structure and the position offset data of the overall structure. According to the theory of material mechanics and deflection, the deflection matrix was used for the conversion of stress field and displacement field. The simulation analyzed the distribution law of the stress field of the horizontal beam and the vertical beam in the main structure when the external load was applied, so as to provide a quantitative basis for the laying of the sensing unit. In the experiment, square steel and I-shaped steel were used to simulate the main structure. The load external stress range was 200-1000 N, fiber grating strain sensors were selected as sensing unit, and the same type of temperature sensors were used to compensate for temperature drift. The test results show that the intersection of the horizontal beam and the vertical beam is the most sensitive. The wavelength shift can well reflect the stress field distribution. The total amount of wavelength shift at this position is about 25 nm. At a gradient distance of 0.5 m, the total wavelength shift is 16 nm. The average error of the position offset of the main structure calculated is 7.46% and 6.34% based on the wavelength offset data. The average ratios of the corresponding stress to position offset for square steel and I-shaped steel are 1.99×10−7 m and 2.79×10−7 m, respectively. It can be seen that the system can test and calculate the overall health of the structure.