Designers and detailers should detail connections to ensure that welders have ample space for positioning and manipulating electrodes and for observing the operation with a protective hood in place. Electrodes may be up to 18 in long and 3⁄8 in in diameter.
In addition, adequate space must be provided for deposition of the required size of fillet weld. For example, to provide an adequate landing c (in) for the fillet weld of size D (in) in Fig. 5.21, c should be at least D + 5⁄16. In building-column splices, however, c = D + 3⁄16 often is used for welding splice plates
DESIGN OF CONNECTIONS
Overview. Connection design is both an art and a science. The science involves equilibrium, limit states, load paths, and the lower bound theorem of limit analysis. The art involves determination of the most efficient load paths which is necessary because most connections are statically indeterminate. The lower bound theorem of limit analysis can be stated as follows. If a distribution of forces within a structure (or connection, which is localized structure) can be found which is in equilibrium with the external load and which satisfies the limit states, then the externally applied load is less than or at most equal to the load which would cause connection failure. In other words, any solution for a connection which satisfies equilibrium and the limit states yields a safe connection, and this represents the science of connection design. Finding the internal force distribution (or load path) that maximizes the external load at which a connection fails represents the art of connection design.
This maximized external load is also the true failure load when the internal force distribution results in satisfaction of compatibility (no gaps and tears) within the connection in addition to satisfying equilibrium and the limit states. Strictly speaking, the lower bound theorem applies only to yield limit states in ductile structures. Therefore, limit states involving stability and fracture must be considered to preclude these modes of failure.
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. 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, but for indeterminate connections there are many possibilities.
Use judgment, experience, and published information to arrive at the best load path. 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, so that it is adequate to carry the given loads. Complete the preliminary layout, check specification required spacings, and finally, check to ensure that the connection can be fabricated and erected economically. The examples of this chapter will demonstrate this procedure.
Economic Considerations. For any given connection situation, it is usually possible to arrive at several satisfactory solutions. Where there is a possibility of using 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 principles of connection design are followed to achieve a safe connection, local preferences should be accepted. Some additional considerations which will result in more economical connections are as follows:
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, with the gravity moment and lateral moment due to wind or seismic loads listed separately. This is needed to do a proper check for column web doubler plates. If stiffeners are required, allow the use of fillet welds in place of complete joint penetration welds. To avoid the use of stiffeners, 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, sometimes called ‘‘drag’’ or ‘‘drag through’’ 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 obtained from a computer analysis of the structure. A misunderstanding of transfer forces can lead to both uneconomic and unsafe connections.