AISC swanson2002Q3.pdf
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1、136 / ENGINEERING JOURNAL / THIRD QUARTER / 2002 ABSTRACT S everal recognized prying models are discussed and evaluated using experimental data collected during 21 component tests conducted as part of a SAC investigation at the Georgia Institute of Technology as a basis. Four exist- ing prying model
2、s are considered in addition to the model that appears in the European design specification. A modi- fication of an existing design model is proposed. INTRODUCTION In the recent past, several research projects have been con- ducted to investigate bolted connection behavior. One of those projects, SA
3、C subtask 7.03, was conducted at the Georgia Institute of Technology and focused on bolted T- stub connections (Figure 1) as an alternative to fully welded connections for light to moderate beam sizes. Several behavioral characteristics of T-stub connections were exam- ined including strength, stiff
4、ness, and deformation capacity. The experimental program was broken into two phases. During the first phase, the component testing phase, 48 Ultimate Strength Prying Models for Bolted T-stub Connections individual T-stubs were subjected to axial loads. During the second phase, the connection testing
5、 phase, complete T-stub connection assemblies were subjected to moments. The goal of testing individual T-stubs as components was to allow a larger number of parameters to be systematically varied than would have been feasible using connection tests. A discussion of the experimental program was pro-
6、 vided by Swanson and Leon (2000) and additional model- ing issues are addressed in Swanson and Leon (2001) and Swanson, Kokan, and Leon (2001). The focus of this paper will be the strength of T-stubs that fail with the formation of a prying mechanism in their flange. The determination of the ultima
7、te strength of a T-stub component subjected to an axial load is a complex process. Of the possible failure modes, the most studied case is the development of a bending mechanism in the T-stub flange followed by failure of the tension bolts (i.e. the formation of a prying mechanism). In this paper, e
8、xisting prying models will be evaluated by comparing their predictions to the results of 21 of the Georgia Tech component tests that failed as a result of tension bolt fractures. Several models for pre- dicting the ultimate strength of T-stub connections are avail- able. Most are based on work by St
9、ruik and de Back (1969), Nair, Birkemoe, and Munse (1974), Douty and McGuire (1965), or Jaspart and Maquoi (1991). Because the model proposed by Struik and de Back provided very good results, it will be discussed in the most detail. BACKGROUND In this discussion of the existing models, the notations
10、 used by the various authors will be converted to that used in this work so that a clearer comparison can be made. The nota- tion that will be used is illustrated in Figures 2 and 3. The analysis of a T-stub flange is made easier by considering a width of the T-stub that is tributary to one pair of
11、bolts. This tributary width will be called p and can be calculated as where WT-stub= the width of the T-stub at the column flange ntb= the number of tension bolts connecting the T-stub flange Other parameters that appear in the discussion of T-stubs are: JAMES A. SWANSON James A.Swanson is assistant
12、 professor, department of civil and environmental engineering, University of Cincinnati, Cincinnati, OH. 2 Tstub tb W p n =(1) (8) 1“ A490 (typ) (4) 1“ A490 W14 x 145 (A572 Gr 50) E-70 5/16“ (8) 1“ A490 (typ) W24 x 55 (A572 Gr 50) T-stubs cut from W16 x 100 1/2“ Continuity Plate (4 places) 1/2“ Doub
13、ler (one side) Fig. 1. Typical bolted T-stub moment connection. 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. ENGINEERING JOURNAL / THIRD QUARTER / 2
14、002 / 137 tips of the flange is generally accepted and is considered accurate until the length of the flange exterior to the bolt becomes large or until the flange thickness becomes small. Figure 4 shows the flange of a T-stub prior to a tension bolt fracture. STRUIK AND DE BACK MODEL The prying mod
15、el developed by Struik and de Back (Struik and de Back, 1969; Kulak, Fisher, and Struik, 1987) for ulti- mate strength prediction is the one most widely used. Vari- ations of the model are used by the LRFD Specification (AISC, 1993), the Canadian steel design code (CISC, 1997) and the EUROCODE 3 (Eu
16、rocode, 1993). Figure 3 shows T= the applied T-stub tension per tension bolt B= the force present in a tension bolt at any given time Bn= the tensile capacity of a bolt Bo= the initial pretension of a bolt Q= the prying force per bolt gt= the distance between the center lines of the ten- sion bolts
17、a= the distance measured from the bolt line to the edge of the T-stub flange b= the distance from the bolt line to the face of the T-stem Other parameters that are specific to particular models will be introduced as needed. It is crucial to understand that T is the applied T-stub load per tension bo
18、lt and is the load that is applied to one half of the tributary width of a T-stub. Therefore, the total applied load is equal to T ntb. In all of the models considered in this work, a prying force is assumed to develop as the flange deforms. This prying force is added to the conventional force prese
19、nt in the tension bolts lowering the applied load that can be safely applied to the T-stub. The basic mechanism is shown in Figure 2 and fundamental equilibrium shows that the bolt tension is the sum of the prying force and applied load, B = T + Q. The prying forces can generally be minimized by red
20、ucing the tension bolt gage or by increasing the flange thickness. The assumption that the prying forces act at the QQ BB 2T gt gt ba tf ts Fig. 2. Typical flange prying mechanism. QQ gt BB 2T b b a a B = Bolt Force Q = Prying Force M M Fig. 3. Prying model of Struik and de Back (1969). Fig. 4. T-st
21、ub flange prying prior to tension bolt fracture. 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. 138 / ENGINEERING JOURNAL / THIRD QUARTER / 2002 the n
22、otation and dimensions used. In this model, the bolt force is assumed to act at the inside edge of the bolt shank as opposed to acting at the centerline of the bolt. This assumption is based on the assumed load transfer from the bolt head to the flange that is shown in Figure 5. This pres- sure dist
23、ribution is the product of the stiffness of the bolt head and the degree of bending present in the flange and bolt. As a result, equilibrium is based on the dimensions a and b instead of a and b. Equations 2 and 3 show the def- initions of a and b. The prying forces, Q, are idealized as point loads
24、that are assumed to develop at the tips of the flange as the T-stub is loaded. These forces could more accurately be modeled as non-uniform pressure distributions acting on the flange exterior of the tension bolts. The point load idealization, though, provides reasonable results with much less compu
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