Time-variant reliability of timber beams according to Eurocodes considering long-term deflections

ABSTRACT

In order to achieve a consistent level of failure probability, structural design codes are optimized using probabilistic methods. This optimization process traditionally focuses on the ultimate limit states (ULS). However, in the design of timber structures the performance of the structural members is often governed by the serviceability limit state (SLS) associated with different load levels than applied in the ULS. The probability of serviceability failure is strongly dependent on the loading sequence and the time-dependent response of timber; therefore, a time-variant probabilistic model is recommended to estimate them properly. This study aims to investigate the time-dependent reliability for long-term deflections of timber office and residential floor beams according to the specifications of the Eurocodes. A simple creep model is used to calculate the deflections and Monte Carlo simulation is carried out to determine the reliability index. It was found that the creep factor and the suggested deflection limits given in Eurocode 5 might not be appropriate to achieve the expected target reliabilities. To obtain a more consistent reliability, more suitable values for the mentioned parameters were suggested. However, the primary aim was to present a framework to determine appropriate deflection limits for structural codes.     

番茄果实蠕变特性表征的Burger's修正模型

针对经典Burger’s模型在蠕变特性表达中的不足,进行了绿熟期、变色期、红熟前期和红熟中期番茄果实的蠕变试验,并在Burger’s模型和常用指数型与幂函数型修正模型分析的基础上构造提出了4元件6参数修正模型.该模型对不同成熟期蠕变段试验数据拟合的平均决定系数与和方差分别达0.9975~0.9994和0.04047~0.07633,蠕变变形率和弹性度的拟合平均相对误差分别为2.54%~3.04%和2.89%~7.12%,蠕变量预测的平均相对误差为0.29%~0.46%,蠕变速率也与实际更加吻合,其曲线拟合精度、关键指标表达精度和蠕变预测精度大大优于Burger’s模型和常用修正模型.研究表明该模型实现了蠕变行为的更准确表征与预测,从而为果实收获、储运、分选的减损提供了更好的模型依据.     

Experimental Analysis on the Decomposition-Creep Settlement Properties of Municipal Solid Waste

In order to study the creep-degradation settlement properties of municipal solid waste, the change of static-creep-settlement for fresh municipal solid wastes which collected from the Chongqing landfill was observed elaborately during the 330 days Laboratory test, and the PTH Model which would fit the settlement properties better was funded. By analyzing leachate-degradation-strain-time relational curves, the number of total creep settlement caused by external load and inner degradation in the waste can reach 33.2% of the sample height. The leachate production can be controlled by the external load and organic content. The tests results indicate that the degradation rule of MSW can be expressed as the Richards model,and the relation of cumulative sellenment with cumulative leachate fit the Exponential function model. The corresponding section for organic content that suit for the initial compression deformation and the creep-degradation settlement could be ranged from 29.1% to 36.47%. During the monitoring for settlement and internal temperature field in the anaerobic reaction stage, an optimum temperature zones that varied from 20℃ to 41℃ are found, which could accelerate degradation speed of organic matters in the waste. Moreover, degradation speed of organic matters becomes maximum when the temperature reaches 41℃.     

木材静曲蠕变成套测试系统的研发

为解决木材变湿蠕变测试普遍存在的环境参数(温湿度和风速)控制方式和控制精度不足、蠕变变形量测试灵敏度和精度不足、数据智能采集和处理以及系统安全防护不足等问题,在集成现有技术的基础上研发出一套木材蠕变测试系统。该系统包括环境气候箱、蠕变测试机架、木材蠕变变形量及干缩湿胀测试单元、荷载同时装卸单元、数据采集和处理单元、系统保障和防护单元6个部分。该系统可在温度为0~70℃、相对湿度为10%~98%、风速为0.3~5.6 m/s时对木材或木基材料的普通蠕变和机械吸附蠕变进行长时间稳定的测试、记录,蠕变变形量及木材厚度方向的干缩湿胀检测精度为±0.01 mm,数据采样间隔在1 s~24 h范围内可调。与现有木材蠕变测试系统相比,本系统在多参数自动同步获取、数据采样精度、运行稳定性、安全性和环境风速可调性等方面有独特优势。通过前期对木材112 d循环变湿蠕变量、干缩湿胀量及环境温湿度参数的观测和测试结果分析,证实该套检测系统可对木材的蠕变挠度、干缩湿胀量、环境温湿度等参数进行长期连续稳定的检测、记录和显示,所有测试指标均能达到设计预期。同时,由于该系统具有多参数协同检测和精度较高等特性,使得实测木材变湿蠕变比采用喷蒸变湿处理试件所获蠕变测试结果在局部地方存在明显差异,如吸湿过程中的木材静曲挠度并不都是反向减小,而是取决于吸湿速率和外荷载的竞争关系,从而为木材静曲变湿蠕变机理的揭示提供了有力证据。该系统的研制为精确可控变温变湿环境下木材蠕变机理的研究提供了新的平台。     

热处理工艺对竹材蠕变性能的影响

采用不同热处理温度和时间对6年生毛竹进行改性处理,并分析其蠕变特性的变化规律。结果表明:当热处理温度为140℃时,蠕变柔量随热处理时间的增加而减少;热处理温度为220℃时,蠕变柔量随热处理时间的增加而增加;热处理温度180℃的竹材恒定蠕变阶段的蠕变速率大于140℃时的蠕变速率,蠕变柔量由低温时随热处理时间增加而降低,向高温时随热处理时间增加而增加转变过渡。热处理时间为2和4 h时,其蠕变柔量随热处理温度升高而降低,减速蠕变阶段变化较为接近;而热处理时间为6 h时,其塑性随热处理温度的升高而增大,蠕变柔量也随之增大。     

蠕变经历对落叶松材静强度影响

木材在长期重复载荷作用下的蠕变经历,即蠕变变形的发展和积累以及木材局部组织损伤和龟裂,使木材再次承载时的力学性质产生变异和劣化.试验研究表明,落叶松材在小于其比例极限应力的重复载荷作用下,经过3万次以上应力循环的蠕变经历,使其纤维方向弯曲弹性模量增大10％;顺纹抗剪强度和抗弯强度有减小趋势,这与蠕变变形的积累和木材局部组织损伤和龟裂程度密切相关,对于无明显塑性变形和局部损伤的试件其强度无明显影响.     

舍温对母猪行为与仔猪保温箱利用率的影响

为分析分娩猪舍温度与保温箱温度对母猪行为和哺乳仔猪保温箱利用率的影响,设置了高温(30.4℃)和低温( 15.3℃)2个组,选取12头约克夏母猪(每组6头)进行了现场试验.结果表明,低温组的仔猪保温箱利用率(62.4％)较高,而高温组的仔猪保温箱利用率仅为1％:低温与高温组母猪从站立到躺卧的行为转换时间分别为4.0和3.4 s(P＜0.05).低温组母猪坐立-腹卧或腹卧-坐立的行为转换频次比高温组母猪的低(P＜0.01).低温组未有仔猪被压死,而高温组的仔猪压死率达15.2％ (P＜0.001).低温组的仔猪增重也相对较快.维持保温箱与分娩舍间适宜温差有利于提高仔猪的保温箱利用率,实现降低仔猪压死率的目的.     

竹木复合工字梁静曲蠕变性能评价

为评价竹木复合工字梁（BWIB）的静曲蠕变性能和抗蠕变服役能力,借鉴美国标准ASTM D5055-04和ASTM D6815-02a检测分析BWIB 的快速蠕变性能和90 d 蠕变性能。结果表明, BWIB的静曲蠕变符合木质材料的一般蠕变特征;BWIB 90 d内发生的全程静曲蠕变挠度为5.33 mm,其中,初始段（第1天）内发生的蠕变挠度为1.38 mm,不可恢复的蠕变挠度为1.79 mm,分别占试验全程静曲蠕变挠度的25.9％和33.6％;BWIB的快速静曲蠕变回弹率和90 d静曲蠕变相对变形率分别为Φ＝91.05％和FD90＝1.55,分别符合美国标准ASTMD5055-04规定的Φ≥90.0％和ASTM D6815-02a规定的FD90＜2.0的要求;静曲蠕变对BWIB的静曲刚性无显著影响。     

Effects of temperature on mechano-sorptive creep of delignified wood

The effects of temperature on mechano-sorptive (MS) creep of delignified hinoki wood (Chamaecyparis obtusa Endl.) were investigated using longitudinal (L) and radial (R) specimens during adsorption and desorption over the temperature range of 20°–80°C. The results were compared with those of stepwise delignified specimens tested at a constant temperature of 20°C. It was found that the effects of temperature on the MS creep of delignified specimens are more remarkable than for untreated specimens. The tendencies of increasing MS creep with temperature, delignification, and their combination were observed. The increase in MS creep for L specimens was relatively small and almost equal in both adsorption and desorption processes, while for R specimens the MS creep was small in desorption, but significantly different in adsorption. In addition, good correlation was observed between the MS coefficient (K) and instantaneous compliance (J ₀). The increase in MS creep occurs as a result of temperature increase or decrease in lignin content, or their interacting effects. However, in the case of desorption for R specimens, the increase of MS creep was unexpectedly small due to a remarkably increased J ₀. Part of this report was presented at the 15th Annual Meeting of the Chubu Branch of the Japan Wood Research Society in Fukui, October 2005     

Creep of the beam in Japanese conventional structures composed of green and kiln-dried timber II: predictive model for relative deflections

We conducted creep tests to evaluate creep behaviors of conventional Japanese framing (jikugumi) structures as reported in a previous article. We measured beam deflections of two structures: one of them was composed of only green timbers (G) and the other with only kiln-dried timbers (D). Besides the two structures, we prepared green and kiln-dried beams to measure moisture content (MC), weight, and dynamic Young’s modulus (E _f) by the longitudinal vibration method. We attempted to predict deflections of beams in the structures by using experimental data for single beam specimens. The proposed simple predictive model was derived from two equations: a relation between MC and equilibrium moisture content calculated with temperature and relative humidity, and a relation between MC change and relative deflection change. Beam deflections were traced for 2.5 years, while the predictions were based on experimental data from loading to the 11th day of the test. It was assumed that sensitivity of deflection change to MC should differ during desorption or adsorption. Although annual cyclic changes were observed in E _f, there was no obvious relationship between E _f and beam deflection. Part of this article was presented at the Annual Meeting of the Architectural Institute of Japan, Kyushu, September 1998