Roof Framing Connections in Conventional Residential Construction

Roof Framing Connections in Conventional Residential Construction

February 2002

INTRODUCTION

The NAHB Research Center has been engaged in a multifaceted research program for the National Association of Home Builders (NAHB) and the U.S. Department of Housing and Urban Development (HUD) to quantify the structural performance of homes and to develop or refine engineering methods that accurately model conventional wood construction. Previous studies have focused on the system effects of the whole building performance and specific assemblies such as built-up headers and shear walls. However, little effort has been made to investigate the systemic load path with respect to roof framing connections, particularly in the context of conventional, wood-framed homes. The engineering knowledge needed to cost-effectively design homes or evaluate residential construction guidelines are lacking in this area. In addition, the practice of making connections in conventional wood-frame construction has evolved from its original use of hand-driven fasteners to the predominant use of pneumatic fasteners. The prescriptive fastening schedules developed based on historic experience with hand-driven fasteners need to be verified and updated for use with pneumatic fasteners to ensure consistency with the intended performance objectives. This problem further extends to the design practice for engineered wood-frame connections. The focus of this project was on connections used with conventional light-frame wood roof construction. A literature review was conducted and supplemented with new research on the performance of conventional roof systems and components including ceiling joist-to-rafter connections and roof framing-to-wall connections.

Individual connections and connections within full-scale roof systems were tested to quantify potential system effects. Hand-driven and pneumatic fasteners were included in the test program. Test results were compared to the provisions of the National Design Specification for Wood Construction and to predictions of the yield theory using the general dowel equations for shear connections. Finally, the results were analyzed with respect to an interest in establishing a consistent capacity basis for design of wood-frame connections.

The key objectives for this study were to:

1. Survey relevant research on conventional and engineered nailed connections.

2. Benchmark the capacity and stiffness of conventional ceiling joist-to-rafter connections (i.e. heel joints) assembled with hand-driven common and pneumatic nails in paired assembly tests.

3. Benchmark the shear capacity of roof-to-wall connections (load direction parallel to wall) assembled with hand-driven common nails, pneumatic nails, and a combination of pneumatic nails and light-gage steel roof clips using full-scale roof assembly tests and individual connection tests.

4. Better understand system effects in connection behavior in conventional wood-frame roof construction.

5. Evaluate the applicability of the yield theory methodology for predicting connection capacity.

This report is organized in seven sections and an appendix. Section 1 formulates the problem statement, summarizes the major tasks completed under the project, and presents the project objectives. In Section 2, background information is provided on the design of nailed connections in light-frame wood construction. A summary of relevant research is included with the focus on key roof framing connections. Properties of materials used in the testing program are reported in Section 3. Section 4 includes three subsections that present the corresponding tasks of the research program on the performance of various conventional roof framing connections. Each subsection is organized as a self-contained document that presents objectives, experimental methods, results and discussion, conclusions, and a design application example (Tasks 1 and 2). The research program addresses specific loading conditions and aspects of system performance not documented in the reviewed literature. Observed performance is compared to current engineering methods for nailed wood connections. Project summary and conclusions are provided in Section 5. Section 6 provides recommendations and Section 7 includes references. Calculations of lateral load resistance of nailed connections investigated in this project are summarized in Appendix A.

Prepared for

The U.S. Department of Housing and Urban Development

Office of Policy Development and Research Washington, DC

66 pages

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