温度应力筋（精选7篇）
温度应力筋 篇1
随着国民经济的迅猛发展,结构形式正朝着大型化、复杂化方面发展,随着泵送混凝土技术的普及,超长混凝土结构在温度与混凝土收缩徐变等作用下的裂缝控制成为重要问题。本文从抵抗使混凝土开裂的拉应力出发,运用预应力技术在结构中建立有效的预压应力可以有效改善混凝土的抗裂性能。从工程实践的经验看,这种方法是较经济的。我公司在榆次一中教学楼工程、四川茂县高中部教学楼、山西博物馆等多项工程中采用此项技术均取得了良好的效果。
1主要特点
本工艺的主要特点在于针对工程超长结构采取设置后浇带方法及布设温度预应力筋技术,而达到抗裂的效果。结合后浇带与预应力技术的优势,在后浇带封闭以前,张拉预应力筋,使得预应力效应得以充分建立;后浇带封闭以后,结构形成受力整体,对确保工程质量起到了重要作用。适用于超长结构不设置伸缩缝的结构形式。
2主要施工技术
2.1 施工工艺流程
施工工艺流程见图1。
2.2 操作要点
2.2.1 预应力筋搭接布置
一般工程建筑平面尺寸较大,框架梁预应力配筋数量较多,孔道数量也较多,梁内预应力筋主要为曲线布置方式。且工程属超长、超大混凝土结构,预应力筋为了满足截面有效预压应力的要求,必须在一定长度内进行分段处理。根据施工具体情况,我们一般考虑后浇带的分布位置及施工段的施工先后顺序以及建筑周边张拉条件的限制。
一般根据施工的具体条件,搭接可采用集中搭接和交叉搭接两种方式,见图2,图3。
2.2.2 预应力节点构造
一般在工程中主要采用以下几种典型的节点形式:
1)柱边张拉节点,需注意锚具的排列必须满足构造要求,柱纵筋排列时须为张拉端垫板的安装留出一定的间距,张拉端采用外置式时,锚具凸出柱边不大于200 mm～500 mm。
2)中间搭接节点,根据工程中预应力梁截面及配筋较密的特点,搭接节点处张拉端的设置无法采用梁面张拉的方法,故采用梁侧加腋和梁侧内凹张拉两种节点。板底局部竖向双排张拉端,若上排张拉端距离板底较近不利于千斤顶放置时采用变角张拉方法(见图4)。
预应力搭接节点处两侧预应力孔道均由柱侧绕出,此处梁面纵筋与柱纵筋相交较密集,施工时应注意柱纵筋的绑扎位置。柱纵筋绑扎时应预先留出波纹管的穿设位置及足够的净间距,排列时应做出深化节点图。
2.2.3 预应力张拉节点做法
1)在梁柱间加腋、对称设置的方案。
在柱与框架梁或框架主、次梁的交界处,框架梁两侧对称设置两个长方体,为方便支模,高度同附近最小梁高。为保证预应力施工质量以及建筑要求,张拉端采用内置式,在预应力施工完成后,采用细石混凝土或防水砂浆封闭。
2)梁侧内凹张拉方案。
由于梁截面较大,在梁侧无法加腋的地方采用内凹的方式进行张拉。此方案转角度较小,因此摩阻损失减少,不影响土建施工普通钢筋的铺设。由于无须开槽,对预应力梁的截面无削弱,施工质量有保证(内凹处钢筋需截断待张拉后焊接),但是节点预埋处理有难度。
2.2.4 预应力张拉顺序及方法
确定合理的预应力施加过程及施加顺序,是预应力施工的关键所在。
1)每个施工段主梁的张拉应同步对称进行,张拉顺序为:a.对每个区而言,要求从中间向两边逐条梁依次进行,以保证整个区域变形一致。b.对每根预应力梁而言,当梁内预应力筋分段搭接时,应沿梁跨方向顺次张拉各跨预应力筋;不得在前一跨梁张拉未完成时,开始张拉后一跨梁,以防止未张拉跨内的梁截面产生受拉裂缝。
2)为防止张拉过程中梁产生较大的偏心受力,张拉次序一般为先中间后上下或两侧,当仅有两个平行孔时,可采取不对称张拉,先张拉其中一束,后张拉另一束。
2.2.5 预应力张拉
预应力筋张拉前应拆除端部模板并搭设相关张拉操作平台,张拉前编制详细的张拉方案。在各区段张拉前编制张拉通知单及张拉任务单,包括详细的针对各区张拉顺序及张拉伸长值等张拉控制。张拉时将详细填写张拉记录单和孔道灌浆记录。
1)张拉前准备工作。
a.设备标定。b.预应力主要材料复试检验。c.签署张拉通知单。d.张拉端端部清理和锚具安装。
2)预应力张拉控制。
a.对混凝土强度的要求。一般要求混凝土强度达到设计要求80%后才能张拉。只有混凝土强度试验报告表明混凝土强度达到要求后,才能开始张拉。b.张拉控制力。预应力束的张拉控制,以控制张拉力为主,同时用张拉伸长值作为校核依据。
3)理论伸长值的计算。
曲线预应力筋的理论张拉伸长值ΔLT按以下近似公式计算:
其中,Fj为预应力筋的张拉力;Ap为预应力筋的截面面积;Ep为预应力筋的弹性模量;LT为从张拉端至固定端的孔道长度,m;k为每米孔道局部偏差摩擦影响系数;u为预应力筋与孔道壁之间的摩擦系数;θ为从张拉端至固定端曲线孔道部分切线的总夹角,rad。
4)伸长值的实测和校核。
由于开始张拉时,预应力筋在孔道内自由放置,而且张拉端各个零件之间有一定的空隙,需要用一定的张拉力才能使之收紧。预应力筋张拉伸长值的量测,是在建立初应力之后进行。实际伸长值ΔL应用下式计算:
其中,ΔL1为从初应力至最大张拉力之间的实测伸长值;ΔL2为初应力以下的推算伸长值,可以根据初应力和二倍初应力的伸长值量测值之差求得;ΔLc为混凝土构件在张拉过程中的弹性压缩值(量值很小,可忽略)。
5)张拉顺序。
a.每个单元预应力梁的张拉应同步对称进行,张拉顺序为:从中间梁向两边梁依次进行,以保证整个区域变形一致。对每根预应力梁而言,当梁内预应力筋分段搭接时,应沿梁跨方向顺次张拉各跨的预应力束,以防止未张拉跨内的梁产生受拉裂缝。b.为防止张拉过程中梁产生较大的偏心受力,张拉次序一般为先中间后上下或两侧,当仅有两个平行孔时,可采取不对称张拉。
6)张拉流程。
a.一端张拉流程图:0→初应力(10%控制应力、读数量测伸长值)→20%控制应力(读数量测伸长值)→100%控制应力(读数量测伸长值,持荷2 min)→锚固→卸荷。b.两端张拉流程图:0→初应力(10%控制应力、读数量测伸长值)→20%控制应力(读数量测伸长值)→100%控制应力(读数量测伸长值,持荷2 min)→一端锚固→另一端补足张拉力后锚固→两端卸荷。
如果预应力筋的伸长值大于千斤顶的行程,可采用分级张拉,即第一级张拉到行程后锚固,千斤顶回程,再进行第二次张拉,直至达到张拉控制值。
3结语
裂缝关系到建筑物的使用功能以及结构耐久性问题,所以非常受人关注。采用后浇带结合预应力切实有效地防治了超长结构的裂缝现象,有利于结构耐久性。同时本工艺抗裂效果的有效性使得施工单位减少了裂缝形成带来的一系列质量纠纷、问题处理等种种不利的因素,且确保了结构的整体质量和耐久性,而且预应力施工的专业化使得工程技术含量明显增加,具有广泛的经济效益和社会效益。
摘要：针对工程超长结构裂缝问题,采取设置后浇带方法及布设温度预应力筋技术,结合后浇带与预应力技术的优势,在后浇带封闭以前,张拉预应力筋,使得预应力效应得以充分建立;后浇带封闭以后,结构形成受力整体,从而对确保工程质量起到了重要作用。
关键词：后浇带,温度预应力,张拉
参考文献
[1]季长征,李仰贤,崔土起.建筑工程常见裂缝的鉴定分析与处理[J].山西建筑,2010,36(4):154-155.
温度应力筋 篇2
The ultimate stress of external prestresed tendons is one of important parameters in calculation of ultimate limit states about external prestressing structure.It is complicated to have a accurate calculation of ultimate stress for external prestressed tendons,and usually it needs to consider the iterative process of material nonlinearity and geometric nonlinearity.To solve this problem,many scholars have proposed some different simplified formulas[1,2,3,4,5,6,7,8,9].At present,various simplified formulas are set up mainly based on two ways:1)calculation models are established according to calculation theory,and then calculation parameters are amended compared with experimental results;2)to identify main factors which affect the ultimate stress,then to make a statistical analysis according to experimental results,and then to build empirical formulas
External prestressing technique has received a va-riety application in bridge reinforcement.Due to the particularity and risky of bridge reinforcement projects,it is still a question that existing computational models can be able to meet the requirements of this kind of projects.Therefore,a kind of methods for calculating ultimate stress of external prestressed tendons need to be built,which is suitable for Chinese highway bridge strengthening and can be used for guiding the calculation of prestressing reinforcement in design of highway bridges.The author once in the documents[10,11]made an evaluation and analysis on the calculation equations of nine kinds of ultimate stress of external prestressed tendons in the way of statistical analysis,combining the data of 48 test beams within the external prestressed concrete both at home and abroad.In this paper,based on this work,in accordance with the compiling work in Specification for Strengthening Design of Highway Bridges(JTG/T J22—2008),after the selection of basic computing model for the characteristics of Highway Bridge Strengthening in China,and the improvement of the relevant parameters with the reference of the relevant standards and rules in highway industry,the author puts forward to a simple and practical formu-la applied in the calculation of the ultimate stress of the external prestressed tendons,laying a foundation for the measure of the ultimate bending load capacity in the external prestressing bridge reinforcement structure.
1 Basic Calculation Model
Although calculation methods of effective prestress in overseas and inland specifications are not identical[8,9,12,13,14],the ultimate stress of external prestressed tendons can be described as the form of effective prestress plus the ultimate stress increment regardless of what kind of computational theory is adopted,
Where:σpuis ultimate stress of external prestressed tendons;σpeis effective prestress of external prestressed tendons;Δσpeis ultimate stress increment of external prestressed tendons.
Up to now,among the understanding of the above formula in China and foreign countries,there is a more unified understanding about the first item,that is,about the effective prestres in the external prestressed tendons,while the second one,that is,the expression form and its impact factors of the ultimate stress increment,there are some different opinions on its recognition.ACI318-02[8],Harajli strand stress calculation model[3]and《Technical Specification for Concrete Structures Prestressed with Unbonded Tendons》(JGJ 92—93)are all established based on the relationship between ultimate stress and sectional reinforcement;Chakrabrti(U.S.)formula[7]is modified based on ACI318—02 formula,taking into account some influencing factors,such as effective prestress,height-to-span ratio,strength of concrete,common reinforced;AASH-TO—2004 and BS8100 formula is founded on the basis of the theory of equivalent plastic zone;others such as the formula by Harbin Institute of Technology based onplasticity theory as well as quadratic effect[2],an experimental calculation formula based on test statistical analysis by Tongji University,is about to be incorporated into"the Guide for Prestressed Concrete Bridge Design"(sent for screeningdraft)[4],and Du Jinsheng's calculation formula based on the ultimate stress of structural deformation[1],all these calculation formula are different in terms of its factors considered and research priority.
How to evaluate the validity and accuracy of the above mentioned calculation methods,and to choose one which is fit for bridge safety in China?The author in the document[10,11],in accordance with collected domestic and foreign statistical analysis on the data of completed 48 pieces of prestressed concrete beams,analyses and reviews the above nine kinds of formulas,the statistical results can be seen in table 1.
According to table 1,the statistical analysis results show that AASHTO—04 formula,Harajli formula,the formula by Harbin Institute of Technology based on plasticity theory as well as the quadratic effects,AASH-TO—04 modified formula and the calculation formula based on test statistical analysis by Tongji University are in good agreement with the experimental results,with the average ratio of the calculated value and the experimental value between 0.88 to 1.07.Among which calculation formula by Harbin Institute of Technology has a relatively high accuracy with x=0.95,the dispersion coefficientcv=0.04,but because of its more complicated calculation methods and only suitable for simple supported beam bridges,it is not convenient to be used.Tongji formula is a regression formula established on basis of section property and reinforcement index.From the perspective of analysis results,the average of the ratio is 0.08,but the dispersion coefficientcv=0.05 is relatively slightly larger.Although AASHTO—04 is slightly different form with its modified formula,their calculation accuracy are almost thesame,the average ratio of both are 0.88,0.85 respectively,and their dispersion coefficient are 0.02,0.01.By comprehensive consideration,AASHTO—04 modified formula has property of accuracy calculation,is simple and suitable and can be used for continuous beam bridge.
Since 1998,Specifications for American Highway Bridges has adopted the theory of plastic hinges to calculate the ultimate stress,and to determine equivalent plastic zone length according to Pannell's model related to neutral axis depth of key sections.In other words,the number of plastic hinges when the member breaks is taken into account and it is fit for the calculation of simple supported beam and continuous beam bridges.The following is the basic model:
Where:leis effective length of external prestressd
tressed tendons between two ends anchorages,Nsis the number of plastic hinges when members break,and other symbolic meaning remain the same as the former.
AASHTO 2004 modified formula and AASHTO2004 formula share the same boot,only coefficient of6 300 with the representation of 0.03Ep.This form indicates a more universal meaning to different types of external prestressing steel
Symbol meanings of the above formula are the same as former.
To sum up:through statistical analysis on data of test beams,it is thought that AASHTO—04 modified formula is better simple、practical and can be used for continuous beam bridge calculation;therefore formula(4)is selected as the basic calculating model.
According to this kind of model,taking into account the conversion of concrete strength index,making a statistical analysis of ultimate load capacity of 48 test beams at home and abroad,the results show that the average of ratio between calculation value and experimental value is 0.96 and variance is 0.02,dispersion coefficient is 0.02,so the degree of agreement is quite ideal.
Annotation:σjis calculated stress,σsis measured stress.Datas in the above table are statistical results of the ratio between calculated value and experimental measured value of ultimate stress of external prestressed tendons,on basis of experimental data of 48 test beams.
2Calculation methods and safety factors of the ultimate stress of external prestressed tendons in Bridge Strengthening Design
By combing the above mentioned,the formula(4)is accurate and reliable as the calculation formula on the ultimate stress of external tendons(beams),comparing the theoretical results with the actual test values,they are in good agreement with higher precision.Based on the formula,in needs to complete the following tasks to change it into the one which can guide the design of bridge strengthening in China:1)the completion of transformation from the strength index of material in the U.S.regulation to that of Chinese existing regulation;2)transformation from calculating the stress based on the test to the design value.The specific process is below:
Modified formula in AASHTO—04 uses strength of concrete cylinder f'c,that is the cylinder of 150×300 mm.Taking into account differences in the shape and size for 15 cm×15 cm×15 cm cube strength in China,the strength of cylinder should be replaced by the standard value of cubic compressive strength of China fcu,kor standard value of axial compressive strength fck.Through conversion basic formula can be got,that is,0.85f'c=0.752fcu,k=1.124fck.That is,0.85f'c=0.75fcu,kis applied and substituted into the modified formula of AASHTO—04,using as the parameters of reinforcement for flexural strength.
In the calculation of ultimate stress of external prestressed tendons,the first requirement is that the position of section neutral axis c need to be established.Specifications of AASHTO—04 adopts the design method based on the destructive strength,and a single reduction factorфis used to reduce the ultimate resistance of the structure,and consequently the steel stress in original beams and stress of external prestressed ten-dons are respectively expressed by its yield strength,fy and fpy.In Formula(4)the physical meaning of the second item can be understood as stress increment of external prestressed tendons on the basis of effective stress fpe,reached when the appearance of plastic hinge near control sections.
Ultimate limit state design method,expressed by multi-factors,is used in China's current"Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts"(JTG D62—2004).Thus the use of AASHTO—04 modified formula should be in accordance with the way of Chinese bridge design,taking into account material safety factorγpin the ultimate stress of prestress steel.In the process of determining the safety factor,two main factors should considered:firstly,(JTG D62—2004)in table 3.2.3-2the ratio of strength design value and strength standard value of high-strength steel wire,steel strand and fine rolled twisted bar;secondly,when the structure has a plastic failure(which is the basis for the establishment of the formula),it should have more safety room than that when reinforced of controlled section yield.There are two means to determine the forms of safety factorγp:one is to establish the safety factor of ultimate stress fps;the other is to establish the safety factor ofultimate stress increment Considering ef-fective stress necessarily presents to the elastic stage,to ensure safety there should be a control to ultimate stress increment,that is the second meaning.By comprehensive consideration and much numerical simulation,it is got thatγp=2.2.Reducing ultimate stress increment of external prestressed tendons by using this safety factor,and plus the effective prestress fpe,then to take a less value as the ultimate sterssσpu,eof external prestressed tendons compared with tensile strength design value fpdof external prestressed tendons.
Based on the above considerations and unification of symbolic expression,calculating formula of externalprestressed tendons is finally established,which is also brought into Specifications for Strengthening Design of Highway Bridges(JTG/T J22—2008).
Whereσpu,eis the calculated value of ultimate stress for external prestressed tendons;σpe,eis existing prestressing force of external prestressed tendons;hp,eis distance between resultant force action points of external prestressed tendons and top of the section;Ep,eis elastic modulus of external prestressed tendons;leis the effective length of external tendons at calculated span;Where liis the length of external tendons between the end anchorages,Nsis the number of plastic hinges;γpis safety factor of external prestressing steel,andγp=2.2;fpd.eis tensile strength design value of external prestressed tendons;c is the distance between section neutral axis and the top of concrete compressive region.
For T-section:
For rectangular section:
Whereβis reduction coefficient of depth of concrete compression,andβ=0.80,Ap,eand Apis section area of external and internal prestressed tendons respectively;fpkis tensile strength standard value of internal prestressed tendons;Asand A'sis section area of common reinforced in tensile and compression area respectively;fskand f'skis tensile strength standard value of common reinforced in tensile and compression area respectively;fcu,kis concrete axial compressive strength standard value;b'fis effective width of compressed flange;b is width of rectangular section or web width of T-section;h'fis depth of compressed flange.
In the solving progress forσpu,e,due to relate to two unknown quantityσpu,eand c and many other calculation parameters,the procedure is too complicated which is to solve a simultaneous equations of formulas(5)and(6)or formulas(5)and(7).In order to convenient for calculation and application,the author introduces three intermediate variables X、Y、Z,and gives a final expression of calculation results as reference.
For T-section and rectangular section,solvingσpu,e and c by direct substitution into expression group(8)、(9)、(10).
Where calculation expressions of parameters X and Y are as follows:
Calculating of Z is related to section type:
For the second type of T-section:
For the first type of T-section or rectangular:
Paying attention in calculation that section width b of the first type of T-section should be the effective flange width b'f.
In order to simplify the procedure of solving equations,takingσpu,e=σpe,eapproximately in preliminary design of bridge strengthening by external prestressing.And the section ultimate flexural capacity calculated by this method is simply safe.
3 Calculation Demonstrations
3.1 Reinforced concrete simply-supported beam bridges
A reinforced concrete simply-supported beam bridge with span 16 m,the superstructure of which is formed by four T beams,and with symmetrical arrangement in direction cross bridge.The height of beams is130 cm,and effective flange width of T-beams is b'f=210 cm with average thickness h'f=16 cm.Web width at mid-span is b=18 cm.Concrete of C30 is used in original beams.The compression reinforcement are two pieces of HRB400 steel in diameter 22 mm,tensile reinforcement are 10 pieces of HRB335 steel in diameter28 mm.The original bridge design load was vehicle-15,trailer-80,now by strengthening of external prestressing,the bridge design load has been improved to highway-I level,using 1 860 MPa high strength steel wire as prestressing tendons,the standard value of its tensile strength is fpk=1 860 MPa.The area of the strand is Ap,e=1.4×4=5.6 cm2,and the arrangement of its external tendons can be seen in graph 1.Through the calculation,after deducting prestress loss,the effective prestress force of external prestressed tendonsσpe,e=1 061.1 MPa,the total length of external tendons between both ends anchorage is le=1 627.2 cm.
Assuming that the section is the first type of T-section,by simultaneous of formulas(5)and(7),to solve ultimate stress of horizontal strengthened bars of external tendonsσpu,e:
Every parameters were substitution into equation,and was solved as follows:
c=6.53 cm<h'f=16 cm.
Proofing that the assumption is correct and the section is the first type of T-section.
3.2Three-span prestressed concrete continuousbeam bridge
A three span prestressed concrete continuous beam bridge with span of 5 590 cm+9 000 cm+5 590cm which was built some years ago,was strengthened by external prestress technique because of less bridge bearing capacity than highway-I standard and down deflection at mid-span zone at present.Arrangement ofexternal tendons can be seen in fig.2.8 pieces of low relaxation steel wire of 19Ф515.2 mm were arranged in the mid span as external tendons,and 24 pieces of low relaxation steel wire of 12Ф515.2 mm were arranged in the mid span as internal tendons,and average length of tendons between both ends anchorages at mid-span is li=9 718 cm.By calculating,after deducting prestress loss,the effective prestress of external prestressed tendons at mid-span isσpe,e=995.2 MPa,hp,e=181.2 cm.Concrete of C50 was used in original beams.Effective flange width of box girder is1 200 cm,after conversion to I shaped cross section based on equivalent principle of flexural strength,depth of flange is 32.9 cm,web width is 80 cm.
According to simplified formula group(8)、(9)、(10),to solve ultimate stress of external prestressed tendons at mid-span sectionσpu,e.Firstly assuming that the section is the first type of T-section,parameters were substitution into formula(10),it can be got that Z=20.83 cm;then to solve intermediate variables parameter X and Y by substitution of from parameter Z into formula(9),and it can be got that X=1.003 2,Y=27.30 cm.
According to formula(8),to solve c andσpu,eafter getting the value of variables X and Y.
c=Y/X=27.30/1.003 2=27.2 cm<h'f=32.9 cm.
Proofing that the assumption is correct and the section is the first type of T-section
4 Conclusion
1)According to analysis of test beams data and theory,it shows that formula(4)established by structural deformation and theory of plastic hinges has property of accurate calculation,concision,practicality,and be suitable for calculating of continuous structure
2)On the basis of formula(4),through corresponding changes including the transformation from strength index of material in the U.S.regulation to that of Chinese existing regulation;the transformation from experiment calculation stress to the design value,adjusting some parameters with reference to standards and criteria of China's highway bridge construction industry,so a calculation method of the ultimate stress for external prestressing tendons is obtained.
3)This paper makes a simulation and verification on the above mentioned calculation method through two calculating demonstrations about reinforced concrete simply-supported beam bridge and a three-span prestressed concrete continuous beam bridge.The results show that the proposed calculation method for the ultimate stress of external prestressed tendons is feasible and reliable,and can be used for the calculation of the reinforcement design for China's highway bridges.
1Du Jinsheng,Lu Wenliang,Ji Wenyu.Discussion of stress in the exter-
预应力碳纤维筋损失试验 篇3
关键词：碳纤维筋,预应力,损失,计算公式
1 碳纤维筋材料特性
1.1 试件制作规范
试验中所采用的CFRP筋的直径为9.5 mm,共制作了3根套筒灌胶粘结锚具拉伸试件,其中3根试件两端钢套筒长度为110 mm,根据日本JSCE-1997和ACI440.3R-04的建议[1,2,3,4],纤维塑料筋自由段长度与直径的比值l/d在40～70之间,本文的碳纤维筋的l/d值为47。在CFRP拉伸试件的制作过程中,首先,调和适量树脂;其次,必须设置对中螺栓和对中胶圈,保证CFRP筋试件拉伸时只受到单向轴力的作用[5,6,7]。
1.2 试件制作
CFRP筋为各向异性材料,其抗剪能力较弱,本试验采用粘结型锚具,这种锚具的套筒内外表面均带有螺纹,在套筒内注入粘结剂实现碳纤维筋的锚固,再利用固定螺母将套筒固定到构件上。参考本课题组前期试验结果,本试验采用环氧树脂作为粘结剂,由环氧树脂、固化剂和金刚砂按一定质量配合比配制而成,制作后的试件如图1所示。
为增强粘结材料与CFRP筋材之间的摩擦力,其内表面加工有螺纹;为使套筒牢固的固定于纤维筋试验仪螺栓之上,其外表面也加工有螺纹通过固定螺帽使其不发生相对滑移,两端纤维筋插入无缝钢管的入口处加了对中胶圈,使纤维筋都处于无缝钢管的中部,降低实验误差,对中胶圈见图2,金刚砂试样如图3所示。本次试验中自主研发的“简易适用于纤维筋拉伸强度试验的装置”已经获得国家专利委员会的批准,专利号:201220335176.6。
2 材料性能试验结果分析
2.1 CFRP筋拉伸试件的破坏形态
本试验中3根9.5的CFRP筋试件均为纤维拉断破坏,破坏时可以清晰的听到纤维丝被拉断的声响,拉断后呈爆炸放射状。破坏形态如图4所示。各拉伸试件均未发生锚固段的粘结滑移破坏,说明试验采取的锚固措施稳定可靠。
2.2 CFRP筋的材料性能
CFRP筋的抗拉弹性模量按下式计算:
其中,Δσ,Δε分别为CRFP筋的应力增量和应变增量;ΔF为荷载增量;Af为CFRP筋的横截面积。CFRP筋的应力—应变曲线,如图5所示。由图5可知,碳纤维筋应力—应变图形基本呈线性关系,说明了CFRP筋线弹性的材料特点。拉伸试件的最终破坏形式表现为承载力突然丧失,故CFRP筋混凝土受弯构件将表现为脆性破坏。CFRP筋的材料性能指标如表1所示。
3 预应力损失分析
由于本试验采用先张法进行预应力的张拉,故取摩擦损失σl2=0;本项目预应力梁在实验室常温下试验,忽略温度变化带来的预应力损失,取温度损失σl3=0;当试件锚固在张拉体系上时,CFRP筋在锚具内发生滑移,致使被拉紧的CFRP筋内缩α引起了预应力损失σl1,混凝土结硬时体积收缩,或混凝土在预应力产生预压力方向上的徐变,引起构件长度的变短,则预应力CFRP筋也会发生内缩,因此会出现预应力收缩与徐变引起的损失。
3.1 预应力损失试验
本试验主要针对预应力短期损失和中长期损失,共设计6组试件,预应力短期损失试验的碳纤维筋长4 000 mm,在其两端分别制作套灌胶粘结型锚具,锚固端伸出350 mm,张拉端伸出650 mm,中间自由段长2 600 mm。两端各嵌入木模220 mm,梁长3 000 mm,见图6。
预应力碳纤维筋短期应力损失试件共4组,J-1和J-4为一个张拉锚固体系;J-2与J-3张拉锚固在另一个台座体系上。将预应力CFRP筋张拉至31 k N,持荷9 d后有效预应力如表2所示。
各预应力碳纤维筋持荷9 d的损失试验结果如表3所示。
碳纤维筋在长期荷载下,预应力筋松弛试验设备如图7所示。
如图7所示,本试验超张拉5%,形成稳定的锚固体系,待读数稳定后记录荷载值。监测并记录234 d。试验室温度处于8℃～18℃之间,保持一个相对稳定环境,定期记录压力传感器的力的荷载值,并记录CFRP筋上的应变值是否稳定,松弛试验结果见表4。
3.2 预应力损失计算
3.2.1 锚具损失
参考钢筋混凝土锚具损失计算方法,由锚具引起的损失按下式计算:
其中,α为张拉端锚具变形和纤维筋内缩值,mm;l为张拉端至锚固端之间的距离,mm;Ef为预应力CFRP筋的弹性模量,N/mm2。
由试验及理论得具体预应力损失值σl1见表5。
MPa
3.2.2 混凝土收缩与徐变引起的损失
本试验参照钢筋混凝土梁由收缩与徐变引起的预应力筋损失σl5,本试验梁中的收缩与徐变引起的损失可按下列公式计算:
其中,σpc为预应力筋在受拉合力点对混凝土法向产生的压应力;fcu'为混凝土立方体抗压强度;ρ为受拉区预应力筋的配筋率。
3.2.3 应力松弛引起的预应力损失
通过监测并记录234 d,室内温度处于8℃～18℃之间,基本上保持一个相对稳定环境。碳纤维筋松弛试验结果见表3,表4。碳纤维筋的松弛损失率是时间对数一元线性函数,CFRP筋的松弛损失较小,通过一组试验数据发现松弛率与时间对数值有良好的线性关系。由试验数据进行线性回归得到松弛损失拟合计算公式,并推算出在500 d后松弛损失率为7.13%,1 000 d后松弛损失率为7.61%,见表6。
4 结语
1)自主研发碳纤维筋拉伸试验仪,测试得到了碳纤维筋的强度和弹模。2)通过分析得知试验梁无摩擦损失与温度损失,本试验梁预应力损失主要有锚固损失、混凝土收缩与徐变引起的损失和应力松弛损失。3)本文给出了先张法混凝土的锚固损失、混凝土收缩损失和混凝土徐变损失三种情况下的建议公式,通过长期监测应力松弛并由一元线性回归,推导得到了碳纤维筋在长期荷载作用下的松弛率随时间对数变化的建议公式。
参考文献
[1]王鹏.CFRP筋(碳纤维筋)产品及其工程探析[J].公路交通科技,2011(6):12-19.
[2]薛景伟.碳纤维筋加固在桥面铺装中的应用[J].公路交通科技,2012(11):6-15.
[3]冯丽.我国碳纤维的发展现状及建议[J].纺织科技进展,2012(1):23-27.
[4]张新元.高性能碳纤维的性能及其应用[J].棉纺织技术,2011(4):13-19.
[5]郝庆多,王勃,欧进萍.纤维增强塑料筋在土木工程中的应用[J].混凝土,2006(9):38-44.
[6]杨小平.碳纤维复合材料在新能源产业中的应用进展[J].新材料产业,2012(2):17-23.
温度应力筋 篇4
在预应力框架梁的设计中, 预应力筋应选用钢绞线或碳素钢丝, 非预应力筋一般采用Ⅱ级钢筋, 其数量估算可按抗裂验算要求或按受弯承载力进行。
1 按抗裂验算要求估算
对处于室内正常环境, 跨度为一跨、二跨及三跨框架通常配置的预应力筋, 其数量由最大弯矩截面的裂缝验算要求, 分别采用荷载的短期效应组合和长期效应组合进行估算, 取其计算结果的较大值, 它常由荷载的短期效应组合计算结果所确定。其余截面预应力筋的配筋量则采用相同的数量:
式中
M——按均布荷载的短期效应组合或长期效应组合计算的弯矩设计值;
σpe——预应力筋的有效预应力, 对单跨框架梁, 可取0.8σcon;对二跨和三跨框架梁的内支座截面, 可取0.7σcon, 边跨跨中及边支座截面, 可取0.8σcon, 三跨内跨中, 可取0.6σcon;
αct——混凝土的拉应力限制系数, 根据裂缝控制等级、荷载短期效应组合和长期荷载效应组合的要求选用;
γ——截面抵抗矩塑型影响系数;
ftk——混凝土轴心抗拉强度标准值;
ep——预应力筋重心至截面重心轴的距离。
预应力混凝土受弯构件中, 预应力筋常放在截面受拉区, 但对于截面较大的构件, 受拉区要配置较多的预应力筋, 而梁的自重往往不足以抵消偏心预应力在梁顶面所产生的预拉应力。所以在梁的顶部要配置预应力筋, 在受拉区往往也要配置部分非预应力筋。
求出所需的预应力筋Ap后, 可按下式求得As:
式中
Md——外荷载效应组合引起的弯矩设计值;
Ac——框架梁计算截面的混凝土截面面积;
fRy——预应力筋的抗拉强度设计值, 无粘结筋取其极限应力设计值M;
fy——非预应力筋的抗拉强度设计值;
hP、hS——预应力和非预应力筋截面形心至混凝土受拉区最外边缘的距离;
x——混凝土受压区高度;
A、W——混凝土截面面积及荷载作用下受拉区最大纤维处的抗弯模量, 估算阶段可用毛面积计算。
如果按式 (2) 计算得到的AS为负值, 则说明设计的框架梁截面过小, 应调整后重新设计计算。
2 按受弯承载力要求估算
式中
λ——预应力度, 应根据环境条件及永久荷载与可变荷载的比值确定。通常可在0.55~0.75之间选用。对于室内正常条件的屋面梁, 因活载占的比例相对比较小, 故应选用0.75为宜。
x——按式 (5) 计算。
预应力筋AP求出后, 即可按式 (2) 计算AS。当计算结果小于构造配筋时, 应按构造要求, 并综合考虑预应力度λ及延性要求配置AS。
在上述两种估算预应力筋的计算方法中, 为考虑次弯矩对支座和跨中截面的有利和不利作用, 可采用对支座截面和跨中截面分别采用0.9和1.2的系数, 对弯矩设计值进行调整。
小结
预应力筋的设计是预应力结构设计的重要环节。按不同的设计要求和设计阶段, 可选择不同的预应力筋估算方法。按裂缝控制要求估算预应力筋的面积, 能较直观地体现预应力筋的作用, 估算结果较准确, 但计算相对比较繁琐, 当裂缝控制要求比较高时 (一级、二级) , 采用这种方法相对合适;当结构的裂缝控制要求一般时, 适合采用按受弯承载力要求估算法, 而且此方法需要用外荷载作用下的内力, 故在设计阶段使用比较方便。
参考文献
[1]林同炎.预应力混凝土结构设计[M].北京:中国铁道出版社, 1983:233-251.
[2]吕志涛, 孟少平.现代预应力设计[M].北京:中国建筑工业出版社, 1998.
[3]李晨光, 刘航, 段建华等.体外预应力结构技术与工程应用[M].北京:中国建筑工业出版社, 2008:99.
温度应力筋 篇5
北京某大厦工程为框架剪力墙结构, 总建筑面积约12万m2。地上部分板中布置后张无粘结预应力筋, 预应力钢筋采用1860级钢绞线, 张拉控制应力为1 302 N/mm2。本工程的预应力筋已经张拉完毕, 部分楼板需要开洞。为了避免切断预应力钢筋, 造成预应力失效和断筋产生安全隐患, 需要对结构进行改造。
2施工准备
由于预应力钢筋一般是连续布置, 而开洞只是在局部位置, 所以开洞时应注意结构中预应力筋的布置, 合理安排张拉端、固定端位置。在施工之前需要制定完善详细的施工方案, 方案中应考虑被切断预应力筋重新张拉的张拉力、洞口对局部温度应力的改变作用, 洞口较大时需采用在洞口周边设置暗梁、附加钢筋等补强措施。对混凝土的剔凿、预应力筋的切断、放张、施工改造过程中的每一道工序和每一根预应力钢筋如何处理都必须交待清楚。
3施工流程
标记预应力筋→人工剔凿开洞, 找出预应力筋→锚具临时固定→切断预应力筋, 放张、卸锚→大面积剔凿、补强→预应力筋二次张拉→封堵。
4施工关键步骤
1) 标记需要切断的预应力筋。
a.在已浇筑楼板上重新开洞时, 根据新开洞口位置并参考结构预应力施工图纸及预应力铺放时标示位置, 并结合一些探测仪器, 确定预应力钢筋的准确位置以及预应力节点的位置, 并做出标记, 为下一步剔凿混凝土找出预应力钢筋做准备。b.原有预留“虚洞” (主要包括强电井、弱电井、风机盘管、空调水管等管道井) 开洞时, 由于预应力筋的位置与水电管、暖通管道位置矛盾, 需要水电、暖通等专业提前吊线, 根据管道吊线结果确定需要切断的预应力筋。
2) 人工剔凿找出预应力钢筋。
由于在剔凿过程中对预应力筋的铺放位置难以准确定位, 剔凿时必须先人工点剔凿, 待露出预应力筋且经过处理后再大面积剔凿, 并严格按所给流程图施工, 保证预应力筋不受任何损伤。
3) 预应力筋临时固定。
在虚洞中找到预应力筋后使用特种锚具 (如开口锚具、C形锚具) 夹持住预应力筋, 需根据不同情况采用不同的开洞和临时固定方法。
4) 预应力筋切断、放张、卸锚。
采用机械切割 (砂轮锯或氧炔焰) 的方法将洞口内的预应力钢筋切断, 将预加应力慢慢释放掉, 进行放张、卸锚操作。施工过程中应注意以下问题:切断前在该预应力筋张拉端做可靠的防护, 以免张拉端预应力筋及锚具飞出伤人。施工过程中张拉端后禁止站人, 避免出现安全事故。切断预应力钢筋时预留足够的张拉工作长度, 切断普通钢筋时预留足够的锚固长度。放张过程中应缓慢放张, 放张前必须制定技术措施, 切筋和放张过程必须按照一定顺序进行。
5) 混凝土剔凿、补强加固。
预应力筋放张、卸锚完成后, 对开洞改造位置的混凝土进行大面积剔凿, 剔凿过程要注意保护预应力钢筋, 剔除后应确保开洞位置周边混凝土表面平整, 并需用高标号水泥砂浆抹平。混凝土拆除完毕后需要重新浇筑一部分混凝土安放承压板, 并安置螺旋钢筋, 以承担预应力钢筋局部承压的作用。如果洞口过大, 需要增设钢板等进行补强加固。
6) 预应力筋二次张拉。
依据预应力筋的长短, 对预应力筋的二次张拉采取如下三种不同的处理方法:
a.对两端进行二次张拉, 适用于放张截断后两端较长的预应力筋。切断、锚固、张拉位置说明详见图1～图3。虚洞内Ⅰ轴两侧的预应力筋切断和二次张拉顺序按照1, 2, 3…依次进行 (见图4) 。b.仅对一端进行二次张拉, 适用于放张截断后一端较短的预应力筋。 其切断、锚固、张拉位置说明详见图5～图7。 由于断筋后洞口右方剩余预应力筋较短, 该处温度应力较小, 可不进行二次张拉。虚洞区域内预应力筋按照1, 2, 3…的顺序依次切断和二次张拉虚洞内预应力筋 (见图8) 。c.对于放张截断后两端较短的预应力筋, 不再进行二次张拉。预应力筋用特种锚具卡住固定后, 使用砂轮锯或氧炔焰慢慢将虚洞中的部分预应力筋切断或烧断, 将预加应力慢慢释放掉。特种锚具永久性卡住预应力筋, 之后不再对预应力筋放张, 也不对预应力筋二次张拉。虚洞区域内预应力筋按照1, 2, 3…的顺序依次切断 (见图9) 。
7) 微膨胀混凝土封堵。
在预应力筋张拉、灌浆完成并经验收合格后, 用切割机切断外露的预应力筋, 剩余30 mm～50 mm, 对张拉端头做防腐防锈处理, 使用C40微膨胀混凝土封堵, 封堵孔洞要求充填密实, 防止孔内有蜂窝, 表面不得露筋。
摘要：介绍了北京某大厦工程预应力混凝土板预应力筋改造工程的施工准备和施工流程, 重点阐述了七个施工关键步骤中对洞口周边混凝土、需处理的预应力筋的处理措施, 从而达到了满足原设计要求的目的, 保证了预应力混凝土板的施工质量安全。
关键词：预应力,钢绞线,张拉,放张,卸锚
参考文献
柱中预应力筋的合理锚固位置 篇6
1 框架的尺寸及配筋
1.1 梁的尺寸及配筋
框架梁采用600 mm×1 200 mm的截面,跨度为18 m,预应力钢筋为12根7Φj5,fp,k=1 860 MPa,张拉控制应力σcon=0.75fptk。线型为由两段具有相反曲率的抛物线组成。梁的尺寸及配筋如图1所示。
1.2 柱的尺寸及配筋
框架柱采用600 mm×800 mm的截面,高度为6 m,预应力钢筋为8根7φj5,fptk=1 860 MPa,张拉控制应力σcon=0.75fptk。线型为三次抛物线。柱的尺寸及配筋如图2所示。框架结构的尺寸和荷载如图3所示。
2 有限元模型的建立
2.1 单元划分
本章中的预应力混凝土框架形状很规则,因此在ANSYS程序中采用了映射划分,所有实体单元都是正六面体单元,由于只对框架进行弹性分析,故采用Solid45单元来模拟混凝土材料,用Link8单元来模拟预应力钢筋和普通钢筋。
2.2 约束条件
根据对称性,可取图3中的1/2模型进行有限元分析。相应的在ANSYS程序模型中的约束条件见图4。
3 计算结果及分析
有限元分析按柱中预应力筋的锚固位置不同分为三种方案,如图5所示,A:柱中预应力筋锚固在梁柱节点的正中部;B:柱中预应力筋锚固在距离梁下边缘0.6 m处;C:柱中预应力筋锚固在距离梁下边缘1.0 m处。为了更便于分析柱中预应力筋的锚固位置对梁柱节点受力性能的差异,首先对框架进行整体分析,然后对梁柱节点进行局部分析,在进行局部分析时对其单元进行细化,单元尺寸取为0.05 m。其方法是将整体分析得到的结点力和结点位移加载在梁柱节点局部有限元模型相应的结点上,梁柱节点局部有限元模型如图6所示。
1)方案A的分析结果图7～图12分别为方案A的各个应力分量的应力云图。
2)方案B的分析结果
图13～图18分别为方案A的各个应力分量的应力云图。
3)方案C的分析结果图19～图24分别为方案A的各个应力分量的应力云图。
4)各种方案结果的比较
此处应力比K是指梁柱节点处最大应力值与正的最小应力值之比,即
A、B和C三种方案的SX、SY、SZ三个方向的应力分量的应力比K如表1所示。
从表1可以看出SX、SY、SZ三个方向的应力分量的应力比K,方案B比其他两种方案都要小。
表2、3中UX、UY、UZ、SX、SY、SZ分别表示板体x、y、和z方向的位移(m)及应力(N/m2);SXY、SXZ、SYZ表示相应的剪应力(N/m2)。由表中可以看出,由于柱中预应力筋锚固位置的不同,梁柱节点的位移、应力和应变具有较大的差别。从应力云图上可以看出方案B的应力分布比其它两种方案更为均匀一些,从表2看出方案B的梁柱节点处柱子的位移比方案A和方案C都要小,这说明由于柱子侧移所引起的附加弯矩方案B比方案A和方案C都小,方案B中框架结构的受力更为有利。从表3看出方案B的梁柱节点处各个方向的正应力和剪应力都要比方案A和方案C都小,特别是x、y、和z三个方向的拉应力方案B比方案A和方案C都小,这说明方案B能有效减小梁和柱处的拉应力,对提高梁柱节点的抗裂性能和防止梁柱节点过早出现裂缝具有重要的意义。综上所述可知方案B是柱中预应力筋较为合理的锚固位置。
4 结语
通过对柱中预应力筋的锚固位置的有限元计算分析比较,可以得知,方案B即柱中预应力筋锚固在距离梁下边缘0.6 m处计算所得到的梁柱节点的位移、应力及应力比K都是最小的,且节点应力分别较为均匀,对梁柱节点的受力性能是三种方案中最好的,所以方案B是柱中预应力筋较为合理的锚固位置。
摘要：由于多层工业厂房预应力混凝土框架结构的跨度大,荷载大,且梁柱采用刚接,使顶层边柱的偏心弯矩很大。因此,如果将顶层边柱设计成普通钢筋混凝土柱,则需要很多的纵向主筋,造成配筋密集,钢材用量大,施工困难。如果将顶层边柱设计成预应力混凝土柱,则可以有效地解决柱中配筋过多过密的难题。柱中配置预应力筋的一个重要问题是柱中预应力筋的锚固位置,它会对整个框架结构尤其是梁柱节点的受力性能产生一定程度的影响。为了探讨和研究这个问题,本章利用有限元程序ANSYS比较分析了柱中预应力筋三种不同的锚固位置,比较了其对梁柱节点受力性能的影响,从而提出了柱中预应力筋的合理锚固位置。
关键词：预应力混凝土,框架,施工工艺,锚固位置,ANSYS
参考文献
[1]混凝土结构设计规范,GB 50010-2010
[2]杜拱辰.现代预应力混凝土结构.北京:中国建筑工业出版社,1988
[3]陈惠玲.高效预应力混凝土工程实践.北京:中国建筑工业出版社,1993
后张法预应力束筋伸长量误差分析 篇7
后张法预应力混凝土施工是通过千斤顶施加给预应力束筋预加张拉力,预加张拉力的大小由经过标定的油泵仪表测量,油泵仪表上显示的张拉力数据是束筋张拉力的基本数值,但由于千斤顶在使用过程中可能出现误差以及发生部件的损坏,还有束筋发生破坏等因素存在,因此油泵仪表读数不能作为施加预应力施工的唯一控制标准。为了保证施工顺利进行,一般在张拉作业时,还必须对预应力束筋张拉伸长量进行校核,二者结合起来,才能综合反映预应力束筋张拉力是否满足设计要求。本文将根据多年来的后张法预应力施工体会,对后张法预应力束筋伸长量误差予以分析讨论。
1 预应力束筋理论伸长量的计算
后张法预应力束筋伸长量的计算公式如下:
其中,P为预应力筋的张拉力;A为预应力筋的横截面面积;Es为预应力筋的弹性模量;LT为从张拉端至计算截面的孔道长度;k为每米孔道局部偏差对摩擦影响的系数,即孔道偏差系数;μ为预应力筋与孔道壁之间的摩擦系数,即孔道摩擦系数;θ为从张拉端至计算截面曲线孔道部分切线的夹角;e为自然对数底数,e=2.718 28。
公路施工技术规范提供的预应力束筋伸长量的计算公式如下:
其中,
《后张法预应力钢筋混凝土铁路简支梁现场预制施工工艺》提供了预应力束筋伸长量的计算公式为:
其中,L1为预应力筋在孔道内直线长度的1/2;L2为预应力在孔道内曲线长度的1/2;L3为预应力筋孔道出口处至千斤顶契块锚固处的长度;6k为张拉时锚下控制应力;σ0为预应力筋的初始应力。
以上3个计算公式计算方法基本相同,公式(1)为通用公式,式(2)和式(3)只是公式(1)的简化,公式(1)是对预应力束筋分段计算,将孔道的曲线段和直线段分开计算预应力束筋的伸长量,然后叠加在一起,较为精确,公式(2)计算束筋的伸长量是采用预应力束筋的平均张拉力,有一定的计算误差存在,公式(3)考虑了千斤顶段内预应力束筋的伸长量,它只适用于后张法构件预应力束筋布置沿中截面对称的工程结构。总之,三个计算公式各有特色,在施工时,应针对施工的具体情况,选用预应力束筋伸长量的计算公式。
2 理论伸长量计算误差的影响因素
2.1 预应力束筋的面积和弹性模量
当预应力钢绞线的钢丝在拉拔时,生产厂家在生产时规定了钢丝的直径,按照规范要求允许有一定程度的偏差,钢丝拉拔操作时,最后一道模子的孔径会随钢丝的拉拔磨损而逐渐增大,直至达到最大允许误差后才停止使用。对于270级钢绞线测得的公称直径尺寸允许偏差为+0.66 mm～-0.15 mm,则生产的钢绞线公称面积允许偏差约为+4%～-1%(与名义公称面积相比较)。现在许多预应力技术规范还没有确定测试钢绞线弹性模量的统一方法和标准,测试弹性模量的计算公式为:
其中,A,L分别为测试的钢绞线的面积和长度;P为试验拉力;ΔL为测试长度L段的变化值,显而易见,测试时绝对不可以使用钢绞线的名义工程面积,而只能使用钢绞线的实测面积,长度L的取值对Es亦有一定影响,主要原因是钢绞线的捻向和捻距有了变化,试验时在钢绞线上粘贴应变片的方向对测量的ΔL结果值也有较大的影响。
以上分析表明,钢绞线的面积和弹性模量对预应力束筋伸长量的计算有较大的影响,采用精确的原材料数据甚为重要,否则在计算预应力束筋伸长量时便已有±3.5%的误差,在施工时要满足伸长量误差在±6%范围之内相对而言就特别困难,将会给施工带来许多不必要的麻烦。
2.2 孔道摩阻系数μ和孔道偏摆系数k
公路桥涵施工技术规范根据预应力筋的成孔方式以及预应力束筋的类型,分别给出了后张法预应力束筋孔道的摩阻系数μ和偏摆系数k。规范所给系数虽然很大程度上具有一定的可靠性和代表性,但它不可能简单地同某一工程施工时预应力束筋孔道的μ和k值切然相吻合,由于μ和k均是由具体的工程实例中测量出来的参数,是波动的,在一定幅度范围内发生变化。
1)在同一孔道中,摩阻系数不是一常量,随预应力束筋应力值的变化而变化。2)摩阻系数的测量值与规范给定值有明显区别。3)从以上3组计算预应力筋伸长量的计算公式可以看出,预应力束筋受到摩阻力的大小对束筋伸长值的影响十分显著。
从以上分析可知,后张法预应力束筋在张拉前,应对预应力束筋截面面积、弹性模量以及孔道摩阻系数进行测试,必要时还要进行适当的修正,这样才能比较准确地计算出各施工阶段预应力束筋的伸长量。另外施工过程中有时为了减小孔道摩阻力的影响,可采用石墨或肥皂液作为润滑剂。
3结语
1)钢绞线面积不能以名义公称面积为准,计算伸长量时必须按生产厂家(试验室实测)提供的钢绞线实测面积为计算依据。2)注意弹性模量的真实性和可靠性,试验室测设弹性模量时一定使用预应力筋的实测面积,建议施工单位在订购预应力筋材料合同时要求生产厂家提供弹性模量数据。3)计算预应力束筋的伸长量时要合理选用计算公式,使之与施工实际相符合。充分考虑施工中各种因素对预应力筋的影响,例如,采用穿心式千斤顶时,不要漏算在千斤顶段范围的预应力筋的伸长值。4)设计图提供的预应力伸长量只能作为一组参考数据,在施工现场,施工前应对原材料参数取值的精确程度加以修改,并在施工中分析测量数据,提供正确的预应力束筋的伸长量,正确指导施工。5)建议设计人员在提供设计图时,一并提供预应力束筋各阶段的预应力要求和混凝土构件预加张力的满足值,以便在施工时处理出现的问题。6)加强现场量测工作,做到不缺项,不漏项,认真记录,认真整理,及时发现问题,及时分析处理问题。7)摩阻系数μ和偏摆系数k对测量预应力束筋伸长量偏差要比弹性模量对它的影响大得多,测得的μ一般不会是一常量,当加载不同时会发生不同的变化,我们使用的摩阻和偏摆系数是预应力束筋处于控制应力状态时的参数。8)当预应力束筋伸长量偏差超出规定的负偏差时,在排除了以上论述的因素影响后,要考虑改善预留孔道的性能,采用石墨、肥皂液或可溶性油以减少摩阻力的影响,必要时可增加预张力(在预应力束筋弹性范围内)。同理,当极少量预应力束筋伸长量偏差超出规定的正偏差范围,该束预应力筋总应变εp≤1%时,应控制锚下应力不要超过规定的最大允许应力,如果没有发现预应力束筋断丝,可以认定该构件是合格的,必要时可以增大锚具回缩时的回缩量。
摘要：结合后张法预应力的施工体会,详细分析了对束筋伸长量影响较大的物理参数,提出了相应的修正方法,并通过具体算例予以验证,为后张法预应力混凝土结构的设计和施工提供了有意义的指导。
关键词：后张法预应力,伸长量,误差分析
参考文献
[1]周志祥.预应力混凝土桥梁新技术———探索与实践[M].北京:人民交通出版社,2005.
[2]JTJ 041-2000,公路桥涵施工技术规范[S].
[3]施岚青.预应力混凝土实用技术[M].北京:中国建筑工业出版社,2004.
[4]房贞政.预应力结构理论与应用[M].北京:中国建筑工业出版社,2005.
[5]吕李清,杨意安,仝为民,等.大跨度体内预应力张弦梁结构的施工技术[J].施工技术,2008(4):61-63.
[6]黄建跃,王树林,刘成龙,等.大跨度连续刚构桥施工主梁变形监测的必要性与方法[J].桥梁建设,2003(1):52-55.