Creep Deflection in Design of Metal Plate–Connected Wood Trusses

Abstract

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.