论文设计基于概率极限状态设计的复合桩基分项系数研究42786

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1、专业好文档基于概率极限状态设计的复合桩基分项系数研究吴 巍(泉州市工程建设监理事务所 362000)摘要：本文在可靠性分析的基础上，对复合桩基承载力极限状态概率统计方法进行了研究，计算了与目标可靠性指标相对应的各抗力和荷载效应的分项系数，使得复合桩基的可靠度设计能方便地应用到工程实践。关键词：复合桩基 可靠度 分项系数中图分类号：TU473.1Study on the Partial Coefficients of the Composite Pile Foundation Based on the Limit State Probability DesignWu wei （Quanzhou

2、Construction Works Supervision Agency 362000）Abstract Based on the analysis of reliability, the paper researched into the limit state probability design method for the composite pile foundations bearing capacity, and calculated the partial coefficients of each resistance and load effect, correspondi

3、ng with the reliability index. Thus the design of the composite pile foundation, based on the reliability, can be applied conveniently to project practice.Key words composite pile foundation reliability partial coefficients复合桩基承载力极限状态设计的基本思路是采用给定的目标可靠性指标，作为复合桩基能否完成预定功能的度量, 这种设二阶矩模式的概率极限状态设计方法，对设计工作提

4、出了极为严格的要求。因而也就制约了可靠度设计方法的工程应用。基于可靠指标的分项系数法是一种由传统的总安全系数法向二阶矩模式过渡的方法，它继承了传统总安全系数设计法的习惯，易于为广大工程技术人员接受，在设计内容上，反映了随机变量参数的变异性及可靠性分析成果，并能够体现目标可靠性指标的要求。1 复合桩基概率极限状态实用设计表达式根据单桩设计的实用设计表达式建立原则和复合桩基由桩和承台共同承担荷载机理，可建立复合桩基以基本变量标准值和分项系数表示的竖向承载力概率极限状态设计表达式为1： （1）式中：、为永久荷载分项系数、可变荷载分项系数；、分别为桩侧阻群桩效应系数、桩端阻群桩效应系数、承台底土阻力群

5、桩效应系数；、分别为桩身截面周长、面积；、分别为桩周第i层土的极限摩阻力、土层的厚度及桩端处土层的极限端阻力；为基础形状系数；是与抗剪指标有关的承载力系数， 下标k表示式中基本变量的值均为标准值。2 复合桩基竖向承载力分项系数在可靠度理论分析中，得出的设计验算点的坐标满足承载力极限状态方程，因而把设计验算点的均值代入极限状态方程得： （2）比较式（1）和（2）有： （3）式（3）即为复合桩基概率极限状态设计中分项系数的定义式。式中：、分别为抗力分项系数、荷载分项系数；、分别为第i个抗力变量的标准值及其验算点坐标；、分别为第i个荷载变量的标准值及其验算点坐标。从上面分项系数的定义式中可知：由于设

6、计验算点的坐标与可靠指标有关，所以，抗力分项系数和荷载分项系数，不仅与复合桩基极限状态方程中所包含的全部基本变量的统计参数有关，而且还随目标可靠指标的改变而改变，这与定值设计法中的分项系数有本质的区别。3工程实例计算某综合楼，建筑面积约为3600，4层框架结构，主要柱网，场地土层分别为杂填土、粉质粘土、淤泥质粉质粘土、粉土（夹粉砂）、粉质粘土。地下水位，采用稀疏布置的低承台锚杆静压摩擦群桩与承台底土体共同承载的复合桩基的基础。现考虑该结构其中一柱下复合桩基极限承载力的可靠度计算，该桩下复合桩基承台截面尺寸，承台高,承台截面如图1。下设静压锚杆方桩7根，桩穿过淤泥土层到粉土夹砂层。桩截面，桩长，

7、桩距。作用于承台底荷载效应的均值为，承台底土层为粉质土层，上覆厚的杂填土，承台底土层的和各土层的的统计参数见表1。表1 基本变量的概率分布及统计参数基本变量概率分布均值变异系数正态正态正态正态正态对数正态正态极值型141320.012.034.0800.8344160.200.200.180.180.150.220.070.30 图1 复合桩基示意图4 分项系数影响因素分析本节分析一些主要因素如活载效应变异系数、承台底土层的变异系数、桩端阻的变异系数、桩端阻的均值、承台群桩效应系数、荷载效应比等对分项系数的影响。考虑上述各变量按不同尺度变化，计算所得的分项系数列于表2。从表中可以看出，随着活载

8、变异系数的增大，桩侧阻、端阻和承台底土反力分项系数均减少，恒载分项系数也相应减小，而活载分项系数则随之增大。随着的增大，桩侧阻、端阻、恒载效应、表2 各主要参量变化对分项系数的影响 参量变化k=0.50.11.081.041.201.321.202.711.131.084.102.000.21.061.031.151.261.152.321.101.513.882.000.31.041.021.111.191.111.951.082.053.642.00=0.50.051.091.051.231.351.231.971.151.605.232.000.101.081.051.221.341.2

9、22.121.151.464.872.000.151.081.051.211.311.202.331.141.344.432.000.201.071.041.181.291.182.541.121.254.022.00=0.50.051.091.051.251.361.241.801.161.755.032.000.101.091.061.241.361.231.901.151.644.892.000.151.081.051.231.341.222.061.151.514.752.000.201.081.041.201.311.202.301.141.344.432.00=0.50.051.1

10、01.061.271.021.121.981.171.815.212.000.101.101.061.261.091.151.941.171.775.162.000.151.101.051.251.181.181.911.161.745.082.000.201.091.051.241.301.221.911.161.675.012.00(kPa)=0.56001.091.051.231.231.191871.151.564.681.937001.091.051.231.271.201.881.161.634.841.978001.091.051.241.301.221.901.161.675.

11、002.009001.091.051.251.341.231.901.161.755.152.0410001.091.051.251.371.251.921.161.785.302.08=0.50.21.031.021.071.121.081.261.031.331.591.250.31.041.021.101.151.101.491.031.662.591.500.41.041.031.111.171.121.681.041.963.311.750.51.041.031.111.171.111.821.042.273.882.010.61.051.031.121.181.122.021.04

12、2.424.362.270.11.181.101.581.521.441.961.201.116.472.000.251.161.091.471.461.381.841.151.736.382.000.501.131.071.361.401.321.701.111.885.902.000.751.091.051.251.321.221.481.072.035.352.001.001.071.041.201.241.221.581.052.205.022.00平均1.091.051.241.301.221.891.161.68方差0.0800.0670.1530.0440.1840.0800.0

13、670.153活载效应的分项系数均随之减少，承台底土反力分项系数相应增加。随着的增大，桩侧阻、端阻、恒载效应、活载效应的分项系数均随之减少，承台底土反力分项系数相应增加，但增加幅值小于变化的情况。这也说明当承台底土的摩擦角和粘滞力变异性大时，在设计中应增加考虑由于这种变异性对承台土反力的不确定性，因此，必须加大它的分项系数。由表中可以看出，端阻分项系数随着的增大而增大，而其余分项系数相应减少。随着的增大，各分项系数都略有增大，其中和增大较其他显著。对于本文实例改变, 分项系数和的改变远比其它分项系数显著得多，事实上承台效应系数的增大，意味着承台土反力有更大的安全度，从而也保证了能承受更大的荷载

14、效应值，因而，和势必要比其它值变化大得多。随着荷载效应比的增大，桩侧阻、端阻分项系数和承台土反力分项系数均减少，恒载分项系数也相应减少，而活载分项系数则随之增大。值得指出，在本例情况下，若把以上30种变化情况作为该工程的不同工况，取全部分项系数的平均值后可得桩端阻分项系数，桩侧阻分项系数，小于规范给出的；承台底土反力分项系数稍大于规范给出的。这说明了，本例的设计取值，使得桩的极限承载力比承台底土的极限承载力发挥更为充分。另一方面恒载分项系数平均值。而活载分项系数平均值。当考虑荷载项标准值和均值之间的统计关系，根据建筑结构设计统一标准（GB50068-2001），的修正后的恒载、活载的分项系数为

15、：/0.95=1.12/0.95=1.18, =/(1+1.5)=1.68/1.276=1.32。所得的结果与规范给定的和相当接近。5 结语分项系数的确定在概率极限状态设计中十分重要，不仅反映抗力各项的安全储备，而且反映了抗力各项对承载力的贡献大小。由于极限状态方程中设计验算点坐标与可靠指标有关，所以分项系数，不仅与桩基极限状态方程中所包含的全部基本变量的统计参数有关，而且还随目标可靠指标的改变而改变，与定值设计法的分项安全系数相比，已更新了内涵。参考文献1 李镜培，楼晓明，贾付波. 复合桩基的承载力安全度与可靠度分析 J,建筑结构学报 2003.42 熊启东复合桩基极限承载力的可靠度分析 J

16、,重庆建筑大学学报，1998，20（5）3建筑地基基础设计规范（GB50007-2002）,北京,中国建筑工业出版社，2002Editors note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what

17、 it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my ti

18、me.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but rig

19、ht now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man ta

20、ke a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, kn

21、owing that if they would just line up in a vertical straight line we would be go for launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy

22、 who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon a

23、s the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications),

24、 Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less

25、 than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifti

26、ng even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 ki

27、lometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be travel

28、ing fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top

29、of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he

30、plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach s

31、peeds.Even if everything goes as planned, it wont. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.6