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1、Anchorage of Steel Building Components to Concrete M. LEE MARSH and EDWIN G. BURDETTE Anchorage of steel building components to concrete is a fact of life. It has been used, and will continue to be used, in essentially all steel industrial structures. It would seem logical that the behavior and cons
2、equently the design principles concerning such anchorages are as well understood, for instance, as those concerning flexure of the components themselves. However, such has not always been the case. Because of this situation, recent work has been undertaken to enhance understanding of anchorage behav
3、ior. Much of this work has been brought about as a result of the stringent quality assurance programs of the nuclear power industry with the corresponding stringent design requirements for anchorage to concrete. While the nuclear industrys design requirements are rather rigorous relative to the need
4、s of the industrial building industry, much of the information obtained to meet these requirements can be utilized in industrial building applications. The purpose of this paper is to describe various types of anchorage devices, discuss their behavior and present appropriate design guidelines for im
5、plementation in industrial building construction. ANCHORAGE TYPES Anchorage of steel attachments and structural elements to concrete has been accomplished by a variety of methods. In the past a gap in both knowledge and standardization of anchorage devices has existed. Utilization and design of anch
6、orages have been accomplished by precedent, code information and manufacturers data. However, recent work has done much to advance understanding of anchorage behavior, and this increased understanding provides a more rational basis for their design. Some basic anchor types have evolved which fall in
7、to two categoriescast-in-place and drilled-in anchors. Cast- in-place anchors, as the name implies, are set before the M. Lee Marsh is Structural Engineer, Midwest Technical Inc., Oak Ridge, Tennessee. Edwin G. Burdette is Professor of Civil Engineering, The University of Tennessee, Knoxville, Tenne
8、ssee. concrete is placed or are inserted while the concrete is still fresh. On the other hand, drilled-in anchors are set after the concrete is fully hardened. Cast-in-Place Anchors Cast-in-place anchors are available primarily in the following forms: wire-form inserts, studs, common bolts, smooth a
9、nd deformed bars which may be straight or bent and structural shapes. Typically, embedded anchors have formed heads as illustrated in Fig. 1. These heads provide a bearing surface between the embedment and the concrete, thus Fig. 1. Illustrations of anchor types FIRST QUARTER / 198533 enhancing the
10、anchors resistance to pull out. Deformed bars may be used without such a positive bearing surface, provided proper anchorage is achieved through adequate development lengths. Development lengths should conform with those specified in the ACI 318 design code.1 Smooth bars quite often are hooked on th
11、eir embedment end to assure proper anchorage. However, it is the authors opinion that a bearing head should be used with these type anchorages since a hooked, smooth bar will straighten and pull out unless an extremely long embedment length is provided. A smooth bar offers much less development of s
12、trength along its length than a deformed bar does. Welded studs, common bolts and deformed bars possess a positive bearing surface. In fact, studs and bolts offer enough bearing surface, as they are, to develop the full strength of the anchor, provided adequate embedment depth and edge distance are
13、available. The use of washers and plates above the bolt head to increase pull-out strength generally should be avoided.2 The failure mechanism for an adequately embedded bolt or stud is pull-out of a cone of concrete radiating outward from the head of the bolt to the concrete surface. The presence o
14、f a washer or plate over the head only serves to spread the pull-out cone outward from the bolt centerline. It does little to enhance the strength of the embedment and can cause severe problems with edge distances as well as with adjacent anchorages. Cast-in-place anchorages with bearing heads have
15、the distinct advantage of being able to positively engage the concrete in confined bearing. These anchorages can usually be detailed to develop the full strength of the steel embedment. They are typically very stiff since the bearing surface cannot slip. The strength and stiffness advantages of cast
16、-in-place anchorages may be offset by the inherent difficulty of accurately locating and maintaining alignment of the anchorage configuration before and during concrete placement. It may also be difficult and quite often is impossible to anticipate future required embedments. For these reasons, dril
17、led-in anchors have evolved and are in wide usage. These are placed in hardened concrete and offer flexibility in precisely locating the anchorage positions. And they allow future, unforeseen anchorages to be placed with relative ease. Drilled-in Anchors Drilled-in anchors come in a myriad of types.
18、 Three general types which are widely used are discussed here. These include self-drilling anchors, wedge bolts and undercut anchors (see Fig. 2). The mechanism by which the first two types of anchors work is basically the same, but small differences in each type cause their behavior to be highly va
19、ried. Drilled-in anchors are placed in holes drilled into hardened concrete. Some type of mechanism is used to draw a mandrel between pieces of metal which engage the side ofFig. 2. Illustrations of anchor types 34ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION the hole. Then, frictio
20、n between the metal and the sides of the hole resists pull-out of the anchor. Self-drilling anchors are unique in that their casings are used to drill the holes. Thus the need for a separate drill bit is eliminated. However, a special tool is required to connect the anchor shell or casing with an ap
21、propriate hammer drill. Once the hole is drilled, the shell is removed and all debris and dust removed from the hole. The shell is then reinserted with the mandrel in place. The hammer drill is then used to hammer the shell down over the mandrel, causing the outer edges of the shell to engage the si
22、des of the hole. Thus, tension on the anchorage is resisted by friction. The self- driller has the inherent weakness of never becoming any tighter than it is when it is seated, since the direction in which force is applied to the anchor is precisely opposite to that in which it is seated. However, t
23、he usual mode of failure of self-drilling anchors is pull-out of a small cone of concrete because of its shallow embedment. Accordingly, this anchor should only be used for light to medium loads and where no preload is required. Wedge bolts are somewhat better than self-drilling anchors in terms bot
24、h of preload and the load levels they can sustain. They are set by simply drilling a proper size hole into hardened concrete and driving the wedge bolt into it to the appropriate depth. For a wedge bolt the hole depth is not critical as long as it is at least as large as the embedment depth. The anc
25、hor is seated by applying torque to a nut on the end of the anchor. This pulls the anchors mandrel up through the wedges, forcing the wedges to engage the side of the hole. The anchorage resists pull-out by friction developed between the wedges and the side of the hole. The wedge bolt has several di
26、stinct and important advantages over self-drilling anchors. First, the direction in which a tensile load is applied is also the same as that required to seat the anchor. Thus, the application of additional tensile load tends to tighten the anchor. Second, the depth of embedment of wedge bolts is sub
27、stantially greater than that of self-drillers. The added depth allows more force to be developed in the anchor since the surrounding concrete does not control capacity. The two advantages just cited tend to make the wedge bolt a very tough anchor in terms of potential failure by pull-out. The anchor
28、 does, by nature, slip, which reduces its stiffness, and failure typically occurs by slip. It does exhibit a relatively high degree of ductility before failure. Where existing reinforcement, edge distances and neighboring anchors do not interfere with the wedge bolt, it can be used to support substa
29、ntial loads. The undercut anchor provides still greater strength and toughness for a drilled-in anchor. They are in a somewhat different category from self-drilling and wedge-bolt anchors since they possess a positive bearing surface against confined concrete. This feature makes them comparable to e
30、mbedded bolts and studs. Undercut anchors are installed by drilling a proper size hole into hardened concrete. A special tool is then used to flare or undercut the hole at a predetermined depth. This depth is usually set by the undercutting tool which in turn is matched to the particular anchor bein
31、g installed. Once the hole is undercut, the anchor itself is dropped into place and the bolt torqued, thereby drawing the mandrel up into the bolts shell. This causes the shell to expand into the undercut part of the hole. The anchor is thus seated and possesses a positive bearing surface to resist
32、pull-out. The undercut anchor has the advantage of having been subjected to a proof load. In other words, when the anchor is set, it is also proofed as the shell expands into the flare or undercut at the bottom of the hole. BEHAVIOR Tension Anchors which have embedment heads directly bearing on conc
33、rete may fail under tensile loading by pulling out a cone of concrete. This type failure occurs when embedment depth is inadequate to develop the tensile strength of the anchor itself. The concrete is then the weak link, thus fails first. The anchor types to which this behavior primarily applies are
34、 embedded bolts, studs and undercut anchors. Wedge bolts and other anchors which have a tendency to slip rarely cause sufficient tensile force to be developed in the concrete to produce this type failure. Proper embedment depth of a tensile anchor will avert failure by concrete pull-out. The simples
35、t case to consider is a single embedded anchor, shown in Fig. 3. Two separate strengths must be determined: (1) the tensile strength of the anchor itself; and (2) the tensile strength of the resisting concrete. For a ductile anchorage the tensile strength provided by the concrete must exceed the str
36、ength of the steel. This can be assured by providing sufficient embedment depth. Since concrete failure occurs by pull-out of a cone of concrete, the strength of the anchorage, as governed by the concrete, is determined by applying a nominal tensile stress perpendicular to the surface of the cone. T
37、he cone is defined Fig. 3. Failure cone of an embedded anchor FIRST QUARTER / 198535 by a failure surface radiating from the anchor head to the surface at an assumed angle of 45. In lieu of applying the nominal stress perpendicular to the cone, a simpler approach is to apply the nominal stress to th
38、e projected tensile area, as shown in Fig. 3. This resolves the area to be considered to a single plane, which makes the concrete strength calculation much simpler when multiple, overlapping stress cones are involved. Typically, the tensile strength of concrete may be taken as 6 f c . The distributi
39、on of tensile stress along the sides of the failure cone varies from a maximum at the embedment end to zero at the surface. For this reason, an average stress of 4 f c is assumed to act uniformly along the failure surface. By statics, this same stress can be applied to the projected area. Experiment
40、al studies have generally verified the predictions of this method. The 45-angle assumed with this method is reasonable except for shallow embedments. For these type embedments (5 in. or less), a wider angled cone will be pulled out. The method outlined using a 45- angle will thus underpredict the co
41、ncrete strength.3 If embedments are placed close enough to one another, the potential failure (stress) cones may overlap. When this occurs, the area of concrete effective in resisting pull-out is reduced. The tensile strength of the entire embedment group is likewise decreased. A rational way to cal
42、culate the tensile resistance of the concrete is to apply the nominal stress, 4f c , to the effective projected area of the group. The effective area accounts for a reduction in area due to overlapping. If an anchor is located close enough to a free edge, reduction of tensile resistance may again oc
43、cur. The effect of this must be accounted for. If the anchor lies within a certain distance of the side, a bursting or blowout failure may occur (see Fig. 4). This type failure arises from the high bearing stresses developed in the vicinity of the anchor head. The shape of the potential blow-out is
44、essentially conical like the tension stress cone. The Commentary to Appendix B of ACI 3492 offers a method for calculating the minimum side cover necessary to prevent lateral bursting failures. The preceding discussion neglects the effects of stress Fig. 4. Lateral blow-out cone caused by insufficie
45、nt edge distance present due to applied loads other than those on the anchorage. It is possible that tension or compression in the concrete supporting an anchor could be present, and this condition must be accounted for. For instance, if significant biaxial tension is present in the plane of the str
46、ucture, then the assumed 45-failure cone is not truly applicable. Reinforcement must be added to resist the effects of the tension. However, if reinforcement is added in accordance with ACI 318,1 the maximum crack width will be restricted. Thus, it has been stated that use of the 45-stress cone woul
47、d still be approximately valid.4 On the other hand, a state of biaxial compression would enhance an anchors strength since added confinement is present. The amount of preload applied to an anchorage has an important effect on anchorage behavior. The behavior of a preloaded anchor is discussed in an
48、appendix to this paper. Shear Transfer of shear from an anchor bolt into the surrounding concrete is accomplished primarily through bearing. The applied shear tries to bend the bolt away from the load. This bending of the bolt causes the concrete ahead of the bolt to crush or spall near the surface.
49、 Tests have shown that a wedge of concrete, roughly one-fourth of the bolt diameter in depth, may spall off2 (see Fig. 5). This behavior will almost certainly result if a base plate is not present to confine the concrete. The presence of a base or cover plate restricts the concrete wedge from moving. As the wedge tries to translate, it also tries to move upward. This upward movement cannot occur if a base plate is present. Thus, an upward thrust on the plate results rather than movement. This increases the tensile load in the bolt, which in turn increases the clamping force