Practice Periodical on Structural Design and Construction

Connections in wood structures are important when designing for ductility. The 1997 Uniform Building Code has taken this into consideration when designating wind and earthquake load duration factors for connections. Factors of 1.6 or 1.33 may be applied to the connection strength, depending on the type of yield mode exhibited by the connection, which may be determined from the yield limit equations supplied in the National Design Specification for Wood Construction (NDS). The NDS provides the designer with multiple tables containing capacities for various common connections. Unfortunately, yield modes are not published along with tabulated capacities. Therefore, the designer must carry out potentially cumbersome calculations using the NDS yield limit equations simply to determine the governing yield mode before an appropriate Uniform Building Code load duration factor can be applied. In this paper, several NDS tables are extended to include capacity and yield mode, smaller side member thickness configurations are added to the existing nail/spike tables, and a useful toe-nail table is provided. The overall purpose of these tables is to accelerate the design process by eliminating time-consuming calculations.

This paper presents a direct solution method for computing prestress loss in pretensioned concrete bridge girders caused by elastic shortening that occurs at transfer. This method yields the same results as those obtained using an iterative solution based on the method for computing elastic loss, Δf_pES, given in Section 5.9.5.2.3a of the AASHTO LRFD Bridge Design Specification. The method is simple enough that it can be implemented on a programmable calculator.

Formulas are given for locating the girder-to-girder connections in cantilever and suspended span systems so that the maximum bending moments in the cantilevered girders are minimized, which facilitates least weight design. That optimum is achieved if the maximum positive moment on the backspan of each cantilevered girder is equal to the maximum magnitude of the negative moment over the support. An example derivation implements that concept and demonstrates that the formulas are based on principles of statics. The formulas cover an unlimited number of equally wide bays and include uniformly distributed loads as well as concentrated loads spaced at equal intervals. The formulas are valid for the general situation in which maximum loads occur at different times on different spans. A well-known design guide does not account for that situation and is shown to give results that may lead to designs that are not conservative with respect to strength.

The responsibility for determining and installing temporary bracing for metal-plate-connected (MPC) wood trusses is reviewed. According to industry standards accepted by the model building codes, the contractor or truss installer is responsible for the determination and installation of temporary truss bracing and for installing the permanent bracing as designed and specified by the building designer. A conceptual explanation is given as to how temporary truss bracing works to stabilize truss compression chords when a series of lateral and diagonal braces are used to resist the buckling action of members. Four documents to aid bracing designers and contractors involved with MPC wood truss installation are available from the Truss Plate Institute and the scope of each document is summarized. Temporary bracing ideas and suggestions not covered in the current Truss Plate Institute temporary bracing literature were presented. A temporary bracing video is available from the Wood Truss Council of America (WTCA).