AISC hewitt2004Q1.pdf
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1、ENGINEERING JOURNAL / FIRST QUARTER / 2004 / 3 I n 1990, AISC and ASCE jointly commissioned a task group to develop a design philosophy for predicting the strength of bracing connections. The design methods in place prior to that time were only applicable to connections made to the webs of columns,
2、as a proper methodology for brace-to-column flange connections had not been agreed upon. The overall goal was to create a complete design phi- losophy that could be used to accurately predict the strength of bracing connections and ensure ductile structural response. The task group devised a study o
3、f five bracing connection design methods. Among the five design methods evaluated were the Uniform Force Method, developed by Thornton (1991), and a modified version of a design method devel- oped by Richard (1986). The study included a comparison of the predicted response of each model to the actua
4、l response found by testing in both isolated, Bjorhovde and Chakrabarti (1985), and complete frame action cases, Gross (1990), for each bracing connection. After extensive study and comparison the committee recommended the use of the Uniform Force Method as the most accurate predictor of the true st
5、ructural response of bracing connections. Although it was found that the Uniform Force Method was the most accurate predictor of true connection per- formance, the virtues of each design method were consid- ered for application to an overall design philosophy. Richards design method was based on emp
6、irical results derived from 54 finite element models. Richards studies showed that frame action significantly affects the gusset-to- frame fastener force distributions and that the gusset con- nection force distributions primarily depend on the plate aspect ratio and the brace angle. Richards data s
7、uggested that uneven stress distributions are produced across a gusset edge in a bracing connection, the peak stress of such a dis- tribution being approximately 1.4 times the average pre- dicted stress value at ultimate load (see Figure 1). As explained by Thornton (1984), the Uniform Force Method
8、captures the effect of frame action, but obviously the method does not capture the uneven stress distribution. In response to Richards findings, the ability of a gusset connection to redistribute forces and achieve a uniform dis- tribution was considered. In the case of a gusset welded to the web of
9、 a W-shape member, the flexibility of the web was assumed to be sufficient to redistribute an uneven force distribution. However, in the case of a bracing connection welded to the flange of a member, which has significantly more rigidity than a connection to the web, the potential inability of the s
10、ystem to accommodate force redistribution was considered. The development of a peak stress induced at some point across the welded connection might cause the weld to fail at the point where the stress is concentrated, causing an unzipping of the weld and a progressive failure of the welded connectio
11、n. In this case, the uniform force distribution used in the Uniform Force Method might be violated, resulting in failure below the theoretically pre- dicted strength of the connection. An example of this scenario can be seen in the 3rd Edi- tion Load and Resistance Factor Design Manual of Steel Rati
12、onale Behind and Proper Application of the Ductility Factor for Bracing Connections Subjected to Shear and Transverse Loading CHRISTOPHER M. HEWITT and WILLIAM A. THORNTON Christopher M. Hewitt is staff engineer, American Institute of Steel Construction, Inc., Chicago, IL. William A. Thornton is pre
13、sident, Cives Engineering Corpo- ration, Roswell, GA. Fig. 1. Summary of finite element model results from Richards work displaying the ratio of peak vs. average yield stress. 4 / ENGINEERING JOURNAL / FIRST QUARTER / 2004 Construction (LRFD Manual), Chapter 13, Example 13.1 (AISC, 2001). Because of
14、 the proximity of the brace claw angle to the gusset to beam connection, a peak stress can be expected at point A in Figure 2. To maintain ductility in the connection for the case of a welded joint subjected to both shear and transverse loading, such as the attachment of a gusset to the flange of a
15、mem- ber, the LRFD Manual states, on page 13-28: From Richard (1986) it is recommended that the design factored force be increased by 40 percent to ensure ade- quate force redistribution in the weld group and the valid- ity of the Uniform Force Method. Thus, (Note that if a moment existed on this in
16、terface the con- nection would be designed for the larger of the peak stress and 1.4 times the average stress.) This 40 percent increase in the design force for the welded joint was adopted to account for uneven distribu- tions in a directly welded gusset edge connection to a mem- ber flange, predic
17、ted by a ratio of peak versus average stress in the joint, and has been the basis for the recommendations of the Manual. By providing over strength in the connec- tion, this factor maintains the validity of the design assump- tions in the Uniform Force Method. Looking back at Richards test results,
18、the upper bound value of a statistical 90 percent confidence interval of the graphed data points suggests a value of 1.25 as an appropriate design value, assuming a normal distribution. The 90 percent confidence interval upper bound of 1.25 should replace the value of 1.4 currently recommended in th
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