chapter4 concrete structure
英文原文来源：《土木工程英语》陈瑛 邵永波主编
4.3 bond,anchorage,and development length
4.3.1 fundamentals of flexural bond
in the reinforced concrete beam of figure 4.17a was constructed using round reinforcing bars,and,furthermore,if those bars were greased or lubricated before the concrete were cast,the beam would be only little stronger than if it were built of plain concrete without reinforcement.if a load was applied,as shown in 4.17b,the bars would tend to mainntain their original length as the beam deflects.the bars would slip longitudinally with respect to the adjacent concrete,which would experience tensile strain due to flexure.for reinforced concrete,it is essential that bond forces be developed on the interface between concrete and steel,such as to prevent significant slip from occuring at that interface.
如图4.17a所示的钢筋混凝土梁中使用光圆钢筋构建，再者，当在混凝土浇筑之前润滑钢筋，梁的强度仅比没有配筋的素混凝土梁强度稍微高一点。如果施加荷载，如图4.17b所示，钢筋将随着梁变形而保持原长。钢筋将就临近的混凝土而言纵向滑动，这将由于弯曲在钢筋中产生拉应力。对于钢筋混凝土，在钢筋和混凝土截面上保证足够的粘结力是很重要的，如防止在界面上产生大的相对滑动。
figure 4.17c shows the bond forces that act on the concrete at the interface as a result of bending,while figure 4.17d shows the equal and opposite bond forces acting on the reinforcement.it is through the action of these interforce bond forces that the slip indicated in figure 4.17b is prevented.
图4.17c中在界面上由弯曲产生作用在混凝土上的粘结力，而4.17d图中作用在钢筋中的等大反向的粘结力。通过截面粘结力防止如图4.17b中的滑动。
some years ago,when plian bars without surface deformations were used,initial bond strength was provided only by the relatively weak chemical adhesion and mechanical friction between steel and concrete.once adhesion and static friction were overcome at larger loads,small amount of slip led to interlocking of the natural roughness of the bar with the concrete.however,the natural bond strength is so low that in beams reinforced with plain bars,the bond between steel and concrete was frequently broken.such a beam will collapse as the bar is pulled through the concrete.to prevent this,end anchorage is provided,chiefly in the form of hooks.if the anchorage is adequate,such a beam will not collapse,even if the bond is broken over the entire length between anchorages.this is so because the member acts as a tied arch,as shown in figure 4.18,with the uncracked concrete shown shaded representing the arch and the anchored bars representing the tied rod.in this case,over the length in which the bond is broken,bond forces are zero.this means that over the entire unbonded length the force in the steel is constant and equal to T=Mmax/z.as a consequence,the total steel elongation in such a beam is larger than in beams in which bond is preserved,resulting in larger deflection and greater crack widths.
很多年前，当使用没有表面变形的光面钢筋，初始粘结强度近由钢筋与混凝土之间相对微弱的化学胶结力和机械摩擦力。一旦在较大荷载下胶结力和静摩擦力被克服，小滑动致使在钢筋与混凝土之间由粗糙度形成的机械咬合力。但是，自然粘结强度是如此低以至于在配置光圆钢筋的梁中，钢筋和混凝土之间的粘结频繁失效。这样的梁由于钢筋在混凝土中拔出而破坏。为了防止上述情况的发生，将配置钢筋末端锚固，主要是以弯钩形式。如果锚固足够，这样的梁将不会倒塌，即使在锚固之间沿着全长的粘结已失效。这是由于将构件视为拉杆拱，如图4.18所示，阴影部分的未开裂混凝土表示拱而锚固的钢筋表示拉杆。在这种情况下，按着钢筋全长的粘结破坏，粘结力为零。这意味着在全部无粘结长度内，钢筋中的力是连续的并且等于T=Mmax/z。因此，这样的梁中钢筋的全部伸长量比粘结有效的梁的全部伸长量大，这将导致更大的挠度和裂缝宽度。
to improve this situation,deformed bars are now universally used.with such bars,the shoulders of the projecting ribs bear on the surrounding concrete and result in greatly increased strength.it is then possible in most cases to dispense with special anchorage devices such as hooks.in addition, crack widths as well as deflections are reduced.
为改善这一情况，现在普遍使用变形钢筋。这样的钢筋中，突出肋的肩作用在周围的混凝土上并且使得强度大幅度提升。在大多数情况下将免除比如弯钩的特殊锚固措施。而且，裂缝宽度和挠度会减小。
4.3.2 bond strength and development length
for reinfored bars in tension,two types of bond failure have been observed.the first is direct pullout of the bars,which occurs when ample confinement is provided by the surrounding concrete.this could be expected when relatively small diameter bars are used with sufficiently large concrete cover thickness and bar spacing.the second type of failure is splitting of the concrete along the bar when cover confinement or bar spacing is insufficient to resist the lateral concrete tension resulting from the wedging effect of the bar deformations.present-day design methods require both possible failure modes be accounted for.
对受拉钢筋，已经观测到了两种粘结破坏形式。第一种形式是钢筋的直接拔出，这发生在混凝土保护层足够时。当使用相对较小的钢筋直径并且保护层厚度和钢筋间距是足够的。第二种形式是当混凝土保护层或者钢筋间距不足以抵抗由钢筋肋产生的楔入作用的横向混凝土拉力，混凝土沿着钢筋的劈裂。当下的设计方法要求两种破坏模式都要计入计算。
-
bond strength
if the bar is sufficiently confined by a mass of surrounding concrete,then,as the tensile force on the bar is increased,adhesive bond and friction are overcome,the concrete eventually crushes locally ahead of the ribs immediately adjacent to the bar interface,as shown in figure 4.19,and bar pullout results.the surrounding concrete remains intact.for modern deformed bars,adhesion and friction are much less important than the mechanical interlock of the deformations with surrounding concrete.
当钢筋周围的保护层足够时，随着钢筋中的拉力增加，胶结力和摩擦力都被克服，钢筋肋前面、临近钢筋面的混凝土最终局部压碎，如图4.19所示，并且钢筋拔出。周围混凝土保持完好。对现在的变形钢筋，化学胶结力和摩擦力还不如肋和混凝土之间的机械咬合作用更重要。
bond failure resulting from splitting of the concrete is more common in beams than direct pullout.such splitting comes mainly from weding action when the ribs of the deformed bars bear against the concrete.it may occur either in a vertical plane as in figure 4.20a or horizonally in the plane of the bars as in figure 4.20b.the horizonal type of splitting frequently begins at a diagonal crack.the dowel action increases the tendency to splitting.this indicates that shear and bond failure are often intricately interrelated.
梁中混凝土劈裂导致的粘结破坏比拔出破坏更普遍。当变形钢筋肋作用在混凝土上时劈裂破坏主要来自楔入作用。它可能发生在如图4.20a的垂直面上或者如图4.20b的钢筋水平面上。劈裂破坏的水平形式始于一道斜裂缝。销栓作用增加了劈裂作用的趋势。这表示斜截面破坏和粘结破坏通常是复杂相关的。
when pullout resistance is overcome or when splitting has spread all the way to the end of an unanchored bar,complete failure occurs.sliding of the steel relative to the concrete leads to immediate collapse of the beam.
当拔出抵抗力被克服或者当劈裂已经沿着全长开展到未锚固钢筋的末端，发生完全破坏。钢筋相对于混凝土的滑动导致梁的立刻破坏。
2.development length
the development length is defined as that length of embedment necessary to develop the full tensile strength of the bar,controlled by either pullout or splitting.when reference to figure 4.21,the moment,and therefore the steel stress,is maximum at point a (neglecting the weight of the beam)and zero at the supports.if the bar stress is σs at a,then the total tension force Asσs must be transfered from the bar to the concrete in the distance l by bond forces.to fully develop the strength of the bar,Asfy,the distance l must be at least equal to the development length of the bar ,established by pull-out tests.in the beam of figure 4.21,if the actual length c is equal to or greater than the development length ld,no premature bond failure will occur.that is ,the beam will fail in bending or shear rather than by bond failure.this will be so even if in the vicinity of cracks loacl slip may have occured over small region along the beam.
基本锚固长度定义为必要的嵌入长度以充分利用钢筋的抗拉强度，不受拔出或者劈裂的影响。如图4.21所示，弯矩以及产生的钢筋应力，在a点达到最大值（忽略梁的自重）以及在支座处为零。如果在a点钢筋应力为σs，全部拉力为Asσs必须通过粘结力在长度l内从钢筋传递到混凝土。为充分利用钢筋的强度，距离l必须至少等于由拔出试验确定的基本锚固长度。在如图4.21所示的梁中，如果实际长度c大于或者等于基本锚固长度ld，不会发生脆性破坏。这就是说，梁发生弯曲破坏或者剪切破坏而不是粘结破坏。即使在裂缝附近一些沿着梁长的位置处可能发生局部滑动。
it is seen that the main requirement for safety against bond failure is this :the length of the bar,from any point of given steel stress(σs or at most fy) to its nearby free end must be at least equal to its development length.however,if the actual available length is inadequate for full development,special anchorage such as by hooks, must be provided.
可知防止发生粘结破坏的安全性要求为：钢筋的长度，从任意给定钢筋应力位置到临近自由端的距离必须大于等于钢筋的基本锚固长度。但是，当实际可利用长度对全部钢筋不足时，必须设置如弯钩形式的特殊锚固。
3.factors influencing development length
experimental research has identified the factors that influence development length,and analysis of the test data has resulted in the experimental equations used in present design practice.the most basic factors include concrete tensile strength,cover thickness,spacing of the reinforcing bars,and the presence of transverse steel reinforcement.
实验研究已经证实影响基本锚固长度的因素，以及对实验数据的分析得出了现在工程实践中的经验公式。最主要的因素包括混凝土抗拉强度，混凝土保护层，钢筋间距，以及箍筋的配置情况。
the tensile strength of the concrete is important because the most common type of bond failure in beams is the type of splitting shown in figure 4.20.the development length is inverse proportional to the axis tensile strength of concrete,ft,but is not inverse proportional to the cube compressive strength of concrete,fcu.
混凝土的抗拉强度是很重要的，由于粘结破坏的一般形式为如图4.20所示的劈裂破坏。基本锚固长度是与混凝土轴心抗拉强度成反比的，但是不与混凝土立方体抗压强度成反比。
cover thickness also influences splitting.clearly,if the horizontal or vertical cover is increased,more concrete is available to resist the tension resulting from the wedging effect of the deformed bars,resistance to splitting is improved,and development length is less.
保护层厚度也影响劈裂。明显地，如果横向或者纵向的保护层增加，更多地混凝土将用于抵抗由于变形钢筋肋楔入作用产生的拉力，提高抗劈裂能力，基本锚固长度减少。
similarly,figure 4.20b illustrates that the bar spacing is increased (eg,if only two instead of three bars are used),more concrete per bar would be available to resist horizontal splitting.in beams,bars are typically spaced about one or two bar diameters apart.on the other hand,for slabs,footings,and certain types of members,bar spacing are typically much greater,and the required development length is less.
相类似的，4.20b图中钢筋间距增加（比如，仅配置两根钢筋而不是三根钢筋），每根钢筋周围有更多的混凝土用于抵抗水平劈裂。在梁中，钢筋通常以一倍或者两倍钢筋直径配置。另一方面，对板、基础、和特定形式的构件，钢筋间距通常更大，并且所需的基本锚固长度更小。
transverse reinforcement improves the resistance of tensile bars to both vertical failure or horizontal splitting failure because the tensile force in the transverse steel tends to prevent opening of the actual or potential crack.
箍筋提高抵抗受拉钢筋水平或者垂直劈裂破坏的能力，因为箍筋中拉力趋于防止实际或者潜在裂缝的开展。
based on the result of a statistical analysis of pull-out texts data,with appropriate simplifictions,the length ld needed to develop stress fy,in a reinforcing bar may be expressed as ld=α*fy/ft*d,where fy=design value of tensile strength of steel bar,ft=design value of tensile strength of concrete,d=bar diameter,α=apparent coefficient of steel bar
基于拔出试验的数据分析结果，进行适当的简化，利用钢筋强度fy的长度ld，在钢筋中可以表示为 ld=α*fy/ft*d，其中fy为钢筋的抗拉强度设计值，ft为混凝土的抗拉强度设计值，d是钢筋直径，α为钢筋的形状系数。
4.3.3 anchorage of tension bars by hooks
1.standard dimensions
in the event that the desired tensile stress in a bar cannot be developed by bond alone,it is necessary to provide special anchorage at the end of the bar,usually by means of a 90du or a 180du hook.the dimensions and the bend radii for such hooks have been standardized in code as follows:
在钢筋中所需的拉应力不能单独由粘结产生，有必要在钢筋末端设置特殊的锚固，通常是90度或者180度的弯钩。弯钩的尺寸和弯曲半径在设计规范中如下规定：
(1)a 180du bend plus an extension of at least 3 bar diameters at the free end of the bar (2)a 90du bend plus an extension of at least 15 bar diameters at the free end of the bar (3)for stirrup and tie anchorage ,a 135du bend plus an extension of at least 5 bar diameters,but not less than 50mm,or a 90du bend plus an extension of at least 10 bar diameters at the free end of the bar,as shown in figure 4.23.
（1）180弯钩加上钢筋末端至少3倍钢筋直径长度的直线段（2）90度弯钩加上钢筋末端至少15倍钢筋直径长度的直线段（3）对箍筋和拉结筋，135度弯钩加上至少5倍钢筋直径的直线段，并且不少于50mm，或者90度弯钩加上钢筋末端至少10倍钢筋直径的直线段，如图4.23所示。
2.mechanical anchorage
for some special cases,e'g,at the ends of main flexural reinforcement in deep beams,there is no room for hooks or the necessary confinement steel,and special mechanical anchorage devices must be used.they may consist of welded short transverse bar,welded plate,or T-headed bar.
对于一些特殊的情况，比如，深梁中主要受弯钢筋的末端，没有空间配置弯钩或者必要的箍筋，从而必须使用特殊的机械锚固装置。机械锚固装置包括焊接短的横向钢筋、焊接板、或者t形头钢筋。
4.3.4 bar splices
in general,reinforcing bars are stocked by suppliers in lengths of 9 or 12 meters for bars.for this reason,and because it is often more convenient to work with shorter bar lengths,it is frequently necessary to splice bars in the field.splices in the reinforcement at points of maximum stress should be avoided,and when splices are used they should be staggered,although niether condition is practical, for example,in compression splices in column.
通常来说，钢筋被供应者以9到12米的长度存储。出于这个原因，并且由于更短长度的钢筋在施工中较为方便，所以有必要在现场频繁地连接钢筋。钢筋的连接位置应当避免最大应力位置处，并且当多根钢筋连接时，尽管在实践操作都比较难实施，钢筋连接点应当错开，比如，柱中的受压钢筋连接位置。
there are three choices for joining bars together (1) mechanical splices,(2)welded splices,and (3)lap splices.
有三种连接钢筋的选择（1）机械连接（2）焊接连接（3）搭接连接
the machanical and welded splices must be tested to show the development in tension or compression of at least 125% of the specified yield strength fy of the bar.welded splices must conform to structural welding code for reinforcing steels.since splices introduce weak links into the structure,they should be loacted as much as possible away from points of maximum force and critical locations.
机械连接和焊接连接必须测试其强度应达到规定屈服强度的1.25倍。焊接连接必须遵守钢筋的结构焊接规范。尽管连接部位属于结构的薄弱部位，他们应当配置在与最大力位置、重要部位足够远的位置。
splices for bars smaller than 16mm in diameter are usually made simply by lapping the bars a sufficient distance to transfer stress by bonding from one bar to the other.the lapped bars are usually placed in contact and tightly wired so that they stay in position as the concrete is placed.design code prohibits the use of lapped splices for main reinforcement in axially loaded tension members and tension members with small eccentricity ,tension bars that are greater than 28mm in diameter,or compression bars that are greater than 32mm in diameter.
对直径小于16mm钢筋的连接通常是简单地搭接钢筋足够的距离以通过粘结作用将应力从一根钢筋传递到另外一根钢筋。搭接钢筋通常紧靠并绑扎以使当浇筑混凝土时钢筋相对位置不变。设计规范禁止在以下情况使用搭接连接：轴心受拉构件、小偏心手拉构件的主要受力钢筋，以及直径大于28毫米的受拉钢筋，或者直径大于32mm的受压钢筋。
generally,bars in tension need to be lapped over a distance of at least 1.2 times the develop length,and in no case be less than 300mm.the lap splice length in compression shall not be less than 70% of that in tension, not be less than 200mm.if any of the load combinations is expected to introduce tension in the column reinforcement,column bars should be lapped as tension splices.
通常来说，受拉钢筋的搭接长度至少为基本锚固长度的1.2倍，并且不应少于300mm。受压钢筋搭接连接长度不应该小于受拉钢筋的70%，并且不少于200mm。如果任一一种荷载组合在柱钢筋中产生拉力，柱筋应该视为受拉钢筋进行搭接连接。
no more than half of the total tension reinforcement be spliced within the required lap length.this may be achieved by staggering alternate bar splices at least a lap length.ties should be provided through the lap splice length with a maximum spacing of 5 times the minimum diameter of lapped bars or 100 mm,whichever is less.the spacing of ties provided through the compression lap splice length shall not exceed 10 times the minimum diameter of lapped bars,nor greater than 200mm.the diameter of ties shall not be less than 1/4 of the maximum diameter of lapped bars.
在规定的搭接长度内，有连接接头的钢筋数量不应该超过全部钢筋数量的50%。这可以通过交错相邻钢筋连接接头至少一倍搭接长度来实现。钢筋连接长度内配置箍筋的间距最大值为搭接钢筋最小直径的5倍或者100mm，取两者的较小值。受压钢筋连接长度内配置箍筋的间距不应超过搭接钢筋最小直径的10倍，并不超过200mm。箍筋的直径不应该小于搭接钢筋最大直径的1/4.
| chemical adhesion |
化学胶结力 |
| mechanical friction |
机械摩擦力 |
| natural roughness |
粗糙度 |
| bond strength |
粘结强度 |
|
projecting ribs |
突出的肋 |
| development length |
基本锚固长度 |
| wedging effect |
楔入作用 |
| dowel action |
销栓作用 |
| bond force |
粘结力 |
| mechanical anchorage |
机械锚固 |
| mechanical splices |
机械连接 |
| welded splices |
焊接连接 |
| lap splices |
搭接连接 |
|
plain bar |
光面钢筋 |
| deformed bar |
变形钢筋 |
| tied arch |
拉杆拱 |
| tie anchorage |
拉结筋 |
| in the event that |
在--情况下 |
发表评论