Editor’s Note

This February 2011 issue contains five technical papers, three on light-frame construction (wood and cold-formed steel) and two on heavier concrete elements. Each of these papers provides new information and practical design content on their subject. Unfortunately, there is no forum section in this issue. We again solicit readership to submit forums of current and general professional design and construction interest for publication in future issues. Forums can cover news events, highlight major changes in building codes or standards, comment on professional practice issues, or provide digests of important research published in other sources.

The first paper in this issue was written by Sheikh S. Ahmed, Ivan Canino, Arindam Gan Chowdhury, Amir Mirmiran, and Nakin Suksawang from Florida International University and addresses connections in wood-frame residential construction between the roof rafters and the top plate of the wall. This type of “hurricane tie” connection is particularly critical for uplift capacity in strong wind regions and typically uses mechanical fasteners, such as metal straps or clips fastened with nails. The research presented investigates the common practice of simply scaling the connection capacity to the total number of fasteners actually used, based on a single fastener test. The key implicit assumption is that the connection’s capacity is only proportional to the number of fasteners per connection joint. This simple approach, based on tests of a single fastener per joint, disregards other different failure modes that may occur in a connection joint dependent on the details and number of fasteners per joint. This paper presents results of tests that confirm the connection type, size, and length effect. The research 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 more realistic relationship between the capacity of the connection joint and its number of fasteners. These results show that current design practices may overestimate the strength of these joints and can, therefore, be the cause of roof-to-wall connection failures in extreme wind events. The research reported is limited to the case of unidirectional loading, but future research is planned for the case of multidirectional loading.

As specified in chapter 2 of the ANSI/TPI 1-2007 wood truss standard, referenced by the 2009 International Building Codes, the registered design professional is required to provide design criteria that addresses in-service creep deflection behavior of flat roofs subjected to ponding loads and floor trusses supporting heavier stone or ceramic tile finishes. In the second paper, Donald A. Bender and Frank E. Woeste examine the potential for such long-term creep deflections, which, however, may not usually be fully considered in the design of metal-plate-connected (MPC) wood trusses. The authors review the relevant standards and research studies on creep behavior of MPC wood trusses. A rational calculation method is then suggested based on 10- and $15\text{-year}$ 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 based on the sustained load percentage relative to total design load. Some practical ways to reduce the potential for flat roof ponding are discussed and illustrated with an example that includes the application of the creep deflection test data.

O. Bedair next examines the behavior of light-weight steel channels under combinations of axial compression and major and minor axis bending. Research to date has been limited to a member’s simple one-way bending or axial compression. Analytical results are presented to cover the full design space of several North American standard cold-formed steel sections subjected to this load combination. The influence of the length and thickness of the flange and the ultimate strength and serviceability of the channel section are highlighted. Finally, design guidelines and an example that can be utilized by practicing engineers to maximize channel section performance are presented.

The fourth paper, by Subash Chandra Bose Gurram and Arghya Deb from the Indian Institute of Technology, investigates the elastic buckling of a column with an internal hinge, both with and without a rotational spring that restores some flexural capacity to the internal hinge location. This model represents the very real possibility that a locally damaged column requires repair and strengthening. The problem is solved analytically for the critical member buckling load, with an additional numerical solution obtained by use of the ABAQUS finite-element analysis software. The authors compared these results and developed design curves for the critical buckling load of columns for various values of the rotational spring stiffness and internal hinge location. The strength increase in a concrete column with the internal hinge (weakened area) wrapped with one or more layers of reinforcing material is evaluated. 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 finite-element analysis.

The final paper, by Jassim M. Hassan, Tom G. Brown, and George F. Jergeas, proposes new coefficients for design bending moments of interior reinforced concrete flat plates. These are intended to enhance their rebar distribution for serviceability. Hence, application of this method could serve to improve structural design and performance of flat slabs. The proposed design coefficients are based on Hillerborg’s advanced strip method design approach and the finite-element method. Hillerborg-Wood-Armer rules were used to convert field moments to design moments. Graphical design aids that can be used to determine the length of the top reinforcement in an interior flat plate are provided. The findings are the only available solution for designing flat plates with aspect ratios higher than 2.0.

In conclusion, we trust this issue’s collection of five papers not only provides interesting reading but also adds another set of valuable resources for structural design applications. Please remember that the Practice Periodical is always looking for submissions of forums, technical papers, and technical notes.