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Design and Construction Practices that Affect Diaphragm Strength and Stiffness of Postframe Buildings
Abstract
Including diaphragm action in sizing building posts (columns) and postembedment depths (foundation) is more consistent with the actual building performance. A large portion of in-plane loading, such as wind, is transferred to the end walls through in-plane shear via metal roofing-a phenomenon called "diaphragm action." Because of load sharing and redundancies that are not accounted for, postframe buildings are stronger and stiffer than what is normally assumed. Various construction and design practices that increase diaphragm strength and stiffness of postframe buildings are discussed. The objective of this paper is to discuss design and construction practices that contribute to the diaphragm strength and stiffness of metal-clad postframe rectangular buildings. The paper is based on data and information obtained from the full-scale metal-clad timber-framed postframe building tested over a period of 9 years, and the literature. Understanding the functions, contributions, interactions, and load sharing characteristics of the different framing, cladding, and redundant elements is critical in understanding the diaphragm action and optimizing designs of postframe buildings. The study benefits the engineers, architects, builders, and code officials of postframe buildings.
In Australian domestic structures, steel framing and sheathing panels are usually connected using screws. The ceiling diaphragm plays an important role in distributing lateral loads to the bracing walls. Shear connection tests could be used to observe the ceiling diaphragm performance without the complexity of testing full-scale isolated diaphragms. In this research, numerous inexpensive tests have been conducted to replicate representative tests to determine the parameters outlining the load-displacement behaviour for the screw connections between cold-formed steel framing members and plasterboard sheathing. All investigations of shear connection tests have focused on monotonic loading testing. These tests are aimed to determine the lower and upper bounds of the load-displacement curves for typical cold-formed steel framing-to-plasterboard screw connections in ceiling diaphragms. Finally, finite element (FE) models were developed by utilising these test results and predicted the performance of full-sized plasterboard-clad cold-formed steel framed ceiling diaphragms under lateral loading.
A computer program for calculating the distribution of horizontal loads among the individual post-frames and roof diaphragm sections of a building is presented. The program, entitled DAFI, can be used to analyze buildings in which bay spacings vary, the stiffness of individual post-frames differ, endwalls are not assumed infinitely rigid, and/or the stiffness of individual diaphragms are not the same.
Two different definitions of diaphragm shear stiffness as defined by L.D. Luttrell and R.C. Hoagland and D.S. Bundy are presented, and their applications with different diaphragm-frame interaction methods are discussed. The use of Luttrell's length adjustment equation for each diaphragm shear stiffness and the validity of its use is discussed. The treatment of pitched roofs as one or two diaphragms is discussed.
A rational procedure for the design of end walls and corner post foundations is proposed. When diaphragm action is considered in the design of post-frame buildings, there is evidence that a solid stitched end wall is at least as strong and stiff as a solid stiched roof. ASAE EP484.1 acknowledges that supplemental reinforcement may be required to replace strength and stiffness lost, due to placement of large doors and openings in the end walls. In the design of end walls, flat steel strapping is used to return the deflection of an end wall with a centered door to a target deflection corresponding to that of a solid end wall. The design of the post foundations include a wood-collar connection to resist net uplift forces.
End-wall diaphragm stiffness values were predicted from horizontal eave deflection measurements of a full-scale post-frame building. The effectiveness of steel strapping, plywood sheathing, and stitching of panel laps as supplemental reinforcement alternatives for an end wall with a large (up to 50% of the area of the end wall) centered door opening was tested and analyzed. A relationship between end-wall stiffness and roof diaphragm stiffness is derived when similar materials and construction are used in the end wall and roof.
A three-dimensional stiffness model was developed to provide procedures to incorporate the frame-diaphragm interaction into design of metal-clad post-frame buildings. The model determines cave displacements of post-frame buildings based on a stiffness of a scaled-down test panel. The predicted cave displacements were compared with measured displacements from three full-scale building tests. A procedure for estimating the upper bound strength of a full-scale building is suggested.
This study presents an evaluation of the 'stressed-skin' design of pole-frame agricultural buildings based on the results of full-scale load testing of a building. The effect of different types of roofing systems as well as the need for knee bracing in pole buildings are discussed.
A scaled-down (test panel) and full-scale post-frame building were extensively instrumented to determine the interaction between the wood roof framing system and the metal-cladding membrane. The behavior of the system was analyzed more from a global rather than localized viewpoint. The measurements provided these insights: (a) contributions of interior purlins in resisting diaphragm forces, (b) interaction between the two roof halves, (c) interaction between the roof purlins and the metal cladding, and (d) load sharing between frames including end-wall frames.
A full-scale post-frame building was constructed and tested in various stages of construction to determine the contributions of the metal cladding to the stiffness of the building. Horizontal loads were applied to interior post-frames to simulate wind loading. Measured horizontal eave deflections were used to predict frame stiffness, roof diaphragm stiffness, and endwall stiffness.
Post-truss frames were analyzed with different assumptions regarding diaphragm action and knee bracing. Slips of the knee brace end joints were modeled as short fictitious members. Diaphragm action significantly increase the lateral stiffness of the building. The maximum combined stress interaction for the critical posts was much lower when diaphragm action was included. Smaller post bending moments and axial force were predicted when knee brace end joints were molded to include slip.
Diaphragm test results of a full-scale post-frame building are presented. The post-frame building was tested at various stages of construction to determine the contributions of the different components of the building system as metal cladding was added. Subsequently, the effectiveness of different bracing systems (steel bracing, plywood sheathing, and stitching of panel overlap seams) was evaluated for supplemental reinforcement of an end wall that contains an opening equal to 25% or 50% of the area of the end wall. Wind loading was simulated by applying horizontal loads at the eave. Measured eave deflections and applied loads were used to predict stiffness values of interior frames, roof diaphragm, and end walls. Strain gauges were installed on roof purlins in end, middle, and intermediate bays across the building width to determine the interaction between the roof framing system and the metal-cladding membrane. The full-scale tests provided several insights, including (a) in-plane deflection of end walls, (b) continuity between the two roof halves, (c) load sharing between frames, (d) interaction between roof purlins and metal cladding, and (e) contribution of interior purlins in resisting diaphragm forces.