Practice Periodical on Structural Design and Construction

One of the most critical connections in wood frame construction is that of the roof rafter and the top plate of the wall. This type of connection typically uses mechanical fasteners, such as metal straps or clips fastened with nails. Manufacturers base the allowable capacity of connections with one fastener on results of tests performed on such connections. However, it is assumed in current design practice that the capacity of a connection with two mechanical fasteners is twice the capacity of a connection with a single fastener. Implicit in this practice is the assumption that the connection’s capacity is proportional to the number of fasteners per connection joint. This approach, based as it is on testing a single fastener per joint, disregards the fact that the failure modes of a connection joint may depend on the number of fasteners per joint. This paper presents results of tests that establish this fact. The results, based on testing with three types of wood (spruce pine fir, southern yellow pine, and douglas fir), are used to modify the current design approach and propose a realistic relationship between the capacity of the connection joint and the number of fasteners in the joint. The results show that current design practices may overestimate the capacity of these joints and can therefore be the cause of roof-to-wall connection failures in extreme wind events. The research reported herein is limited to the case of unidirectional loading. Future research is planned on the case of multidirectional loading.

Creep deflection can be an important consideration in the design of metal plate–connected (MPC) wood trusses. Chapter 2 of the ANSI/TPI 1-2007 truss standard, referenced by the 2009 International Building Code (IBC), defines the responsibility of the registered design professional when specifying MPC wood trusses. Based on Section 2.3.2.4 (h) of the standard, the registered design professional shall provide design criteria that addresses in-service creep deflection behavior of flat roofs subject to ponding loads and floor trusses supporting stone or ceramic tile finishes. In this paper, we review relevant standards and research studies on creep behavior of MPC wood trusses and offer a rational calculation method based on 10- and 15-year test data for estimating creep deflection for parallel chord MPC wood trusses. Examples are included to illustrate the calculation method for determining a design creep factor that is related to the sustained load percentage relative to total design load. Some practical ways to reduce the potential for flat roof ponding are discussed and illustrated by an example that includes the application of the creep deflection test data.

The paper examines the behavior of lightweight channels under combinations of axial compression, major and minor axis bending. Design guidelines are proposed that can be used by practicing engineers to maximize the channel section performance. Results are presented showing the design space of several North American standard cold-formed steel sections subjected to this load combination. A three-dimensional multistory building model is used to demonstrate cost-effective design strategies that cope with challenges encountered in practice. The influence of the flange, web size, and section thickness on the ultimate strength and serviceability of the channel section are also highlighted. The study leads to significant guidelines that can be used by practicing engineers and steel fabricators in order to optimize the section sizes under this load combination.

This paper examines the buckling of a column with an internal hinge that is retrofitted with a rotational spring to restore some capacity to carry bending moment at the internal hinge location. The problem is solved analytically to obtain the critical buckling load. In addition a numerical solution is obtained using finite elements with the help of the ABAQUS finite-element analysis software. The results are compared, and design curves are obtained that yield the critical buckling load for the column for various values of the rotational spring stiffness and internal hinge location. The increase in strength of the column with the internal hinge wrapped with one or more layers of reinforcing material is then studied. An analytical expression is found to characterize the reinforcing wrap in terms of the stiffness of an equivalent linear elastic rotational spring. The procedure is validated by a finite-element analysis.

This paper proposes new coefficients for design bending moments for interior reinforced concrete flat plates. These design coefficients are safe for strength and serviceability and are calculated using Hillerborg’s advanced strip method design approach and finite-element method. The Hillerborg-Wood-Armer rules were used to convert field moments to design moments. The paper also introduced graphs. These can be used to determine the length of the top reinforcement in an interior flat plate. The findings are the only available solution for designing flat plates with aspect ratios higher than 2.0.