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1、Some Practical Aspects of Column Base Selection DAVID T. RICKER The following three methods can be used effectively to prepare a landing site for the erection of a column: 1. Leveling plates (see Fig. 1) 2. Leveling nuts (see Fig. 2) 3. Preset base plates (see Figs. 3 and 4) LEVELING PLATES For smal
2、l- to medium-sized base plates, say up to 22 in., the use of leveling plates is probably the most effective method to prepare for column erection. The leveling plates are usually in. thick and are sheared to the same size as the base plates. Sometimes the leveling plates are made about 1 in. larger
3、in each direction than the base plate, but this is not necessary. It is not necessary to remove the edge burr left by the shear. Shearing may cause the plate to curl and if so the plate should be flattened to within standard plate flatness tolerances. The holes in the leveling plates are usually mad
4、e 3/16 in. larger than the anchor bolt diameter, but this is not a firm figure and may vary among fabricators. Leveling plates are sent to the field in advance of the main column and grouted in place, usually by the general contractor or foundation subcontractor. Since this work is done in advance i
5、t permits time for an accuracy check. Once set, plates are relatively tamper proof. If a leveling plate is found to be out of level or at the wrong elevation, it is easily removed, the grout broken up and cleaned away, and the leveling process repeated correctly. When a column is first “stood-up“ an
6、d the hook let go, there is a short period of time when it must stand alone before being tied in with beams or guy cables. During this interval the column may be subject to wind loads; jostled by other members being hoisted into place; or accidentally hooked by an errant choker or the eccentric load
7、ing of a float and the connecting erector. The column base, therefore, should offer some moment resistance. The leveling plate method provides a solid contact surface; safety is one of its prime attributes. Leveling plates can be used with any number of anchor bolts and for square, rectangular, ell-
8、shaped, or offset base plates. Some designers express concern that the leveling plates may not be set flat; yet these same individuals do not hesitate David T. Ricker is vice president, engineering, The Berlin Steel Construction Company, Inc., Berlin, Connecticut. to land a wall bearing beam on a pr
9、eset bearing plate or to set a column on top of a beam whose flanges may be slightly out of parallel. Loss of contact between leveling plate and base plate may also be the result of inaccurate milling of the column shaft, curling of the column base plate caused by the heat of welding, or the column
10、shaft being slightly out of plumb during the erection process. If the gap persists after plumbing, it should be treated the same as gaps at column splices.1 Tests have proven that columns with initial gap at a splice have essentially the same load capacity as columns without splices.2 There is no re
11、ason to believe this would not be true at column bases. LEVELING NUTS Fortunately, when the size of the base plate becomes so large as to make the use of a leveling plate impractical, there is another method available for setting column base plates. In this procedure, generally, four or more anchor
12、bolts are utilized, each bolt having two nuts and two heavy washe- Figure 1. Column base w/ leveling plate THIRD QUARTER / 198981 rs. The anchor bolts must be set far enough apart to be able to develop an effective resisting moment to overturning. The base plate is generally large enough so that the
13、 bolts can be located outside the perimeter of the column near the corners of the base plate. Avoid a closely spaced cluster of anchor bolts which will act as a fulcrum about which the column might rotate and topple. Also the leveling nut method is not recommended if the bolts must be closely spaced
14、 in one direction, as on a narrow wall. The bolts will not develop good restraint in that one direction. In this situation it would be better to use a leveling plate or to design the wall with a pilaster so as to utilize a more nearly square base plate. When the leveling nut method is used, usually
15、one of the lower nuts is brought to the proper elevation, allowing for the thickness of the heavy washer which must be placed on top of the nut and below the base plate. This bolt and nut is then spray painted to identify it as the nut with the proper elevation. The other nuts are brought to the sam
16、e elevation. If someone bent on mischief attempts to change the elevation of the key nut, the broken paint will expose the misdeed and help to re-establish the proper elevation. When it comes time to erect the column, it can be dropped into place very quickly and efficiently and the upper washers an
17、d nuts installed. One of the major advantages of the leveling nut method is that it can accommodate a base plate slightly out of level or a base plate curled by the heat of welding. Leveling nuts are best used for base plates ranging up to about 36 in. in size. Beyond this size, bending of the base
18、plate may become a problem, and shipping the base plate separately should be considered. PRESET BASE PLATES Large-sized base plates (36 in. and larger) are often shipped to the job site and set in advance of the start of erection. This is done because these large plates are often so heavy and cumber
19、some that they make shipping and handling of the column very difficult if not impossible. These large base plates are usually furnished with some kind of leveling devices in the form of bolts or threaded rods. Shims and wedges can safely be used in this situation because there is not an attached col
20、umn shaft waving around in the sky. A three-point support (like a milking stool) is satisfactory. If leveling bolts are provided, small steel plates must be placed under the points of the bolts so they wont dig into the concrete. When colossal-sized (say over four tons in weight) base plates are req
21、uired, an angle frame is often supplied in advance. This angle frame is carefully leveled and filled with concrete which is screeded off accurately and results in a Figure 2. Column base w/ leveling nuts Figure 3. Heavy column base 82ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION lev
22、el concrete pad of proper elevation on which the column base plate is directly placed (see Fig. 4). ANCHOR BOLTS The selection of the column base type is determined primarily by the geometry of the foundation and the nature of the loads which influence the base. The geometry consists of the shape an
23、d location of the foundationwhether it is a square or rectangular footing, pile cap, a narrow wall, a pier or a pilaster, isolated or part of a wall, or at a corner of a wall. The loads may consist of vertical gravity loads, uplift, shear, moment, or combinations of any of these. Erection loads, for
24、 example, may be a combination of gravity load and moment (see Fig. 14). Columns subject to gravity loading alone, theoretically, would not need any base anchorage. During the erecting of a column, however, there is a brief period of time, before the column is stabilized with beams or guys, when a c
25、olumn must stand on its own. For example, a 14 in. wideflange freestanding column 31 ft long, being scaled by an erector on a breezy day, will require a resisting base moment of approximately 5 ft kips. Some kind of anchorage is required to hold the base plate to the foundationusually anchor bolts o
26、r rods. Anchor bolt and anchor rod mean the same thing in this text, and the terms are used interchangeably as they are in the trade. Anchor bolts are primarily a tension device. To prevent the anchors from pulling out of the concreteshould the bond stresses be exceededhooks, plates, or other shapes
27、 are added to the embedded portion of the anchors (see Fig. 10). Anchor bolts vary in size from approximately in. diameter to 2 in. diameter with in., 7/8 in., 1 in., 1 in., and 1 in. being the most common diameters. Avoid specifying bolt diameters in sixteenths and eighths (except 7/8 in. and 1 1/8
28、 in.) as these sizes may not be readily available. Anchor bolts less than in. diameter may lose section due to corrosion and result in less than anticipated service life. Anchor bolts greater than 2 in. diameter may be difficult to find nuts for and wrenches to fit. Anchor bolts, subject to corrosiv
29、e conditions, may be galvanized. When ordering galvanized bolts, specify that the threads be “chased“ so the nuts will work freely. If anchor bolts must be galvanized, it is best to specify A307 and A36 material to avoid the embrittlement sometimes resulting when high-strength steels are galvanized.
30、 Weathering steels may also be used where anchor bolts are exposed to corrosive atmospheres, but with the understanding that they will rust and stain the foundation concrete if so exposed. Most anchor bolts are made from A36 material. Other materials used are A307, A325, A572, and A588. When Figure
31、5 Figure 4Figure 6 THIRD QUARTER / 198983 higher strength is required, 4140 steel is sometimes used.If it is necessary to identify the exact type of steel used, certified mill test reports should be requested from the supplier. Normally this is not required unless the anchors are subject to signific
32、ant tension. See Table 1C, page 4-4, AISC Manual of Steel Construction, 8th ed.1 for other rod steels. Most anchor bolts come with hex nuts. Occasionally a supplier may furnish square nuts. This is of little consequence. In tight situations where bolt projection is critical a half nut may be utilize
33、d. In rare cases, where the anchor bolt is not allowed to project above the top of the base plate, the plate can be counterbored so the nut will set into the depression. This is expensive and should not be used unless absolutely necessary. Of course, the plate must be thick enough to permit the requ
34、ired depth of counterboring. Generally, because most base plates will be provided with oversized holes, it is necessary to provide thick erection washers under the nuts. These are usually 5/16 to in. thick and must be accounted for when the bolt projection is established. The use of lock washers on
35、anchor bolts is seldom justified. Split-ring type lock washers have a relatively small outside diameter and have been known to suck down into the oversized holes resulting in an ineffective anchor bolt. It is better to spike the threads or use double nuts than to use lock washers. Anchor bolt projec
36、tion is shown in one of two ways, depending on the custom of the fabricator-erector. The top end of the anchor bolt may be dimensioned either down to the underside of the base plate, as in Fig. 11A, or down to the top of the concrete, as in Fig. 11B. During the concreting operation, it is very diffi
37、cult to get the concrete to the exact elevation. This must be recognized by those who choose to dimension to the theoretical top of concrete. (Theoretical and as-built usually differ.) However, both methods are used extensively and both seem to work. Provide plenty of extra threads on anchor bolts.
38、It is very frustrating for an erector to have a nut “shank out“ (run out of threads). In such an event the nut must be backed off and extra washers added. Stacked washers are not a problem on anchor bolts and special restrictions should not be imposed. When anchorage is required in concrete which is
39、 already poured, a hole is core drilled in the concrete and a straight anchor bolt with a swaged shank may be grouted in place. The swaging consists of dents pounded randomly in the shank. A similar result can be obtained by depositing little blobs of weld metal on the shank (see Fig. 12). Concrete
40、reinforcing rods with threaded ends have been successfully used for this application. In any event such bolts should be used with caution. If the dents or blobs are a bit sparse or the concrete is not properly cured, then the anchors may pull out of the concrete. Anchor bolts of any type should have
41、 proper concrete embedment. Avoid anchor bolts with upset threads unless absolutely necessary. The source of these rods are scarce and delivery times are long. Avoid specifying unusual threading requirements such as NF threads or left-hand threads. Anchor bolts, when used with the leveling nut metho
42、d of setting column bases, should have a nut and heavy erection washer both above and below the base plate. The washers are used to prevent the nuts from attempting to push through the oversized holes. Extra thread should be supplied for lower nut adjustment (see Fig. 2). Rods with full length threa
43、ding and nuts on each end can be used for anchor bolts. The lower nut must be welded to the rod to prevent it unwinding when the top nut is tightened (see Fig. 10C). Because of long-term relaxation of concrete, prestressing of anchor bolts is unreliable and hardly ever justified. Anchor bolts are th
44、e link between foundation and column. When a column base is subject to uplift, the uplift Figure 7 Figure 8 84ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION force must be resisted by a weight greater than the uplift force or the column base must be connected to the basic rock. Anchor
45、 bolts transmit this tension force. Several characteristics in anchor bolt design can be varied to suit the load requirements: the quantity of bolts, the diameter, the length, and type of material. Anchor bolts should not be used to resist shear forces in a column base. Shear forces can be resisted
46、by some device attached to the underside of the base plate and inserted into a groove or keyway in the concrete foundation (see Fig. 9). The shear forces can also be resisted by the encasement of concrete around a column base. More commonly, the shear resistance is developed by the friction develope
47、d between the bottom of the base plate and the top of the concrete foundation. (If the leveling plate method of setting column bases is used, the coefficient of friction used should be that of steel-to-steel.) The main reason anchor bolts should not be used to resist shear forces is that they have a
48、 relatively low bending resistance. If a base plate were to ease sideways into bearing against an anchor bolt, the load (discounting the grout) would be delivered an inch or so above the concrete top. The anchor bolt would act like a vertical cantilivered member. A in. diameter A36 anchor bolt resis
49、ts less than a 5 kip lateral load delivered 2 in. above the top of concrete. Also the random location of anchor bolts as they are located in the oversized base plate holes results in a situation where seldom does more than one bolt of a multi-bolt group go into bearing at any one time. Large uplift forces are resisted primarily by the anchor bolts going into tension. Significant uplift forces must be resisted by a greater weight anchored to the lower end of the anchor bolts or by drilling direct into basic rock. For large uplift forces, it may not be adequate to rely mere