AISC shipp1989Q2.pdf
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1、Design of Headed Anchor Bolts JOHN G. SHIPP AND EDWARD R. HANINGER In current practice the design of base plates is controlled by bearing restrictions on the concrete (see Fig. 1); shear is transmitted to the concrete largely through anchor bolts, shear lugs or bars attached to the base plate and th
2、e tensile anchorage steel is generally proportioned only for direct stress. The embedment requirements for anchorage steel are not clearly defined by most codes and are left largely to the discretion of the design engineer. Also, there are no provisions to prevent a brittle failure in the concrete a
3、s opposed to a ductile failure in the anchor bolt, as provided for with a probability-based limit states design or Load and Resistance Factor Design (LRFD) for steel.8 Larger design forces now mandated in many areas due to the revised seismic and wind loads require design capacities for anchor bolts
4、 beyond any existing code values.6,11 Therefore, there is a need for a complete design procedure for anchor bolts that will accommodate these larger loads and incorporate the proposed design philosophy, i.e., probability-based limit states design (PBLSD).8 THE HEADED BOLT AS AN ANCHORAGE The headed
5、bolt, as designed herein, is recommended as the most efficient type of anchorage to use for both tension and shear loads. Other anchorages which have been used are L- bolts, J-bolts, rods with a bolted bearing plate and shear lugs. L-bolts have been shown to be less effective in resisting slip at se
6、rvice load levels than headed bolts.13 The authors are not aware of any published data that addresses the performance of J-bolts. For a threaded rod with a bolted washer or bearing plate embedded in concrete, tests have shown that unless the plate is properly sized it may actually decrease the ancho
7、r capacity by causing a weakened failure plane in the concrete.7,17 Shear lugs can fail in a brittle mode if not properly confined, and do not lend themselves to a shear friction analysis.7,17 The headed bolt, when properly embedded and confined, will develop the full tensile capacity of even A490 h
8、igh John G. Shipp is Supervising Structural Engineer, Fluor Engineers and Constructors, Inc., Irvine, California. Edward R. Haninger is Senior Structural Engineer, Fluor Engineers and Constructors, Inc., Irvine, California. strength bolts.3 When the tension capacity of the bolt is developed, a ducti
9、le failure can be ensured by the shear friction mechanism.3 In this paper, anchor bolt design ductility is assured by causing a failure mechanism that is controlled by yielding of the anchor bolt steel, rather than brittle tensile failure of concrete. This is accomplished by designing the pullout st
10、rength of the “concrete failure cone” (Up) such that it equals the minimum specified tensile strength (FuAt) or “full anchorage value” of the anchor bolt. See Figs. 2 and 10 for illustrations of the concrete failure cone concept. See Appendix A for the derivation of Ld to satisfy this criteria. The
11、design approach presented herein is compatible with the proposed AISC Specification for Nuclear Facilities,5 ACI 318-77,2 and the proposed revisions to ACI 318-77.7 The governing design approach is that presented in ACI 349, Supplement 1979.3 DESIGN PARAMETERS The design approach presented is genera
12、lly applicable to any of a number of bolt or concrete strengths. However, the following representative materials are used in developing the design values. Anchor bolt materials used are ASTM A36, A307 (Grade B), A325, A449 and A687. Concrete is assumed to have a minimum compressive strength (fc) of
13、3,000 psi. Anchor bolts are heavy hex bolts or threaded steel bars with one heavy hex nut placed in concrete. Bolt threads at the embedded end of each threaded steel bar are “staked” at two places below the heavy hex nut. All bolts are brought to a “snug tight” condition as defined by AISC4 to ensur
14、e good contact between attachments. The concrete is at least 14 days old prior to tightening the anchor bolts in order to prevent bolt rotation. Anchor bolts are designed for combined shear and tension loads; the area of steel required for tension and shear is considered additive. Criteria will be p
15、resented such that either Working Stress Design (WSD) or Ultimate Strength Design (USD) may be used. COMBINED TENSION AND SHEAR Many authors have presented data and interaction equations to account for the combined effects of tension and shear 58 ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CON
16、STRUCTION Fig. 1. Example of base plate loading (see Refs. 1, 3, 12, 14, 15 and 17). In this paper, the total required area of anchor bolt steel to resist tension and shear loads is considered to be additive (see Appendix B, and Figs. 1 and 9). Fig. 2. Effective stress area for limited depth (Ae) Ta
17、ble 1A. Standard Anchor Bolt Basic Types TypeDescription Bolt Spacing r Edge Distance mComments A Isolatedr rmm mvmv rm/2, mv mt B Shear reinforcement only r rmrm/2 mt C Shear reinforcement plus overlapping failure cones r rm/2; mv mt The bolt embedment depth is greater than or equal to Ld as specif
18、ied in Table 1B. The size of Type A anchor bolts is selected such that the design load (T) does not exceed the basic Nominal Design Resistance (AtFy) values tabulated in Table 2A. Fig. 3. Shear reinforcement Type B Anchor BoltsAnchor bolts are classified as “Type B,” or shear reinforcement only, whe
19、n all of the following apply: The closest bolt spacing (r) is greater than or equal to rm. The closest edge distance (m) is greater than or equal to rm/2 but less than mv. Note: rm/2 mt The bolt embedment depth is greater than or equal to Ld. The size of Type B anchor bolts is selected as per Type A
20、 anchor bolts. In addition, shear reinforcement (Asv) is provided on both sides of any critical plane of potential failure (see Fig. 3). The total area of horizontal shear reinforcing steel (Asv) is determined as follows: A F A CF sv utt y = cos45 where Fy is the specified minimum yield strength of
21、the reinforcing steel. Type C Anchor BoltsAnchor bolts are classified as Type C, or shear reinforcement plus overlapping failure cone considerations, when all the following apply: The closest bolt spacing (r) is less than rm. The closest edge distance (m) is greater than or equal to mt and less than
22、 mv. Note: mt Ld as tabulated in Table 1B. Under no condition will the closest bolt edge distance be less than mt or 4 in. The size of Type C anchor bolts is selected as per Type A anchor bolts. Shear reinforcement is provided as per Type B anchor bolts. Also, the bolt embedment depth is calculated
23、as follows: First, calculate the effective concrete tensile stress area Ae (see Fig. 2) based on r, m and an assumed embedment depth greater than Ld. The effective concrete tensile stress area (Ae) is the projected area bounded by the intersection between 45 degree lines radiating from the edge of t
24、he bolt head and the concrete surface at which the loads are applied, minus the area of the bolt heads (refer to Fig. 2). Then, calculate the pullout strength (Up), where 4 f c is the allowable uniform concrete tensile stress applied over the effective stress area Ae: UfAF A pceut =4 Note that Up mu
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