PLIEGO DE CONDICIONES DE NATURALEZA FACULTATIVA

2.5.3 Extended tab with axial 165

2.6 References 169

2.1 Introduction

Connection design is an interesting subject because it requires a great deal of rational analysis in arriving at a solution. There are literally an infinite number of possible connection configurations, and only a very small number of these have been subjected to physical testing. Even within the small group that has been tested, changes in load directions, geometry, material types, fastener type, and arrangement very quickly result in configurations that have not been tested and therefore require judgment and rational analysis on the part of the designer. This chap- ter provides design approaches to connections based on test data, when available, supplemented by rational design or art and science in the form of equilibrium (admissible force states), limit states, and ductility considerations. The limit states are those of the AISC 13th Edition Manual (2005).

38 Chapter Two

(Courtesy of The Steel Institute of New York.)

2.1.1 Philosophy

Connection design is both an art and a science. The science involves equi- librium, limit states, load paths, and the lower bound theorem of limit analysis. The art involves the determination of the most efficient load paths for the connection, and this is necessary because most connections are statically indeterminate.

The lower bound theorem of limit analysis states: If a distribution of forces within a structure (or connection, which is a localized structure) can be found, which is in equilibrium with the external load and which sat- isfies the limit states, then the externally applied load is less than or at most equal to the load that would cause connection failure. In other words, any solution for a connection that satisfies equilibrium and the limit states yields a safe connection. This is the science of connection design. The art involves finding the internal force distribution (or load paths) that maximizes the external load at which a connection fails. This maximized external load is also the true failure load when the internal force distri- bution results in satisfaction of compatibility (no gaps and tears) within the connection in addition to satisfying equilibrium and the limit states. It should be noted that, strictly speaking, the lower bound theorem applies only to yield limit states in structures that are ductile. Therefore, in applying it to connections, limit states involving stability and fracture (lack of ductility) must be considered to preclude these modes of failure. 2.1.2 General procedure

Determine the external (applied) loads, also called required strengths, and their lines of action. Make a preliminary layout, preferably to scale. The connection should be as compact as possible to conserve material and to minimize interferences with utilities, equipment, and access, and to facil- itate shipping and handling. Decide on where bolts and welds will be used and select bolt type and size. Decide on a load path through the connection. For a statically determinate connection, there is only one possibility, but for indeterminate connections, there are many possibilities. Use judgment, experience, and published information to arrive at the best load path. Now provide sufficient strength, stiffness, and ductility, using the limit states identified for each part of the load path, to give the connection sufficient design strength, that is, to make the connection adequate to carry the given loads. Complete the preliminary layout, check specification-required spac- ings, and finally check to ensure that the connection can be fabricated and erected. The examples of this chapter will demonstrate this procedure. 2.1.3 Economic considerations

For any given connection situation, it is usually possible to arrive at more than one satisfactory solution. Where there is a possibility of using Design of Connections for Axial, Moment, and Shear Forces 39

bolts or welds, let the economics of fabrication and erection play a role in the choice. Different fabricators and erectors in different parts of the country have their preferred ways of working, and as long as the prin- ciples of connection design are followed to achieve a safe connection, local preferences should be accepted. Some additional considerations that will result in more economical connections (Thornton, 1995B) are: 1. For shear connections, provide the actual loads and allow the use of

single plate and single angle shear connections. Do not specify full- depth connections or rely on the AISC uniform load tables.

2. For moment connections, provide the actual moments and the actual shears. Also, provide a “breakdown” of the total moment, that is, give the gravity moment and lateral moment due to wind or seismic loads separately. This is needed to do a proper check for column web dou- bler plates. If stiffeners are required, allow the use of fillet welds in place of complete joint penetration welds. To avoid the use of stiff- eners, consider redesigning with a heavier column to eliminate them. 3. For bracing connections, in addition to providing the brace force, also provide the beam shear and axial transfer force. The transfer force is the axial force that must be transferred to the opposite side of the column. The transfer force is not necessarily the beam axial force that is obtained from a computer analysis of the structure. See Thornton (1995B) and Thornton and Muir (2008) for a discussion of this. A misunderstanding of transfer forces can lead to both uneconomic and unsafe connections.