Structural load paths in low-rise, wood-framed structures.

摘要

Light-framed wood residential buildings continue to suffer damage from extreme wind events, even at wind speeds below design levels. One of the major findings in post-hurricane damage investigations is that a continuous load path to transfer the wind uplifts loads acting on the roof into the foundations was missing. The roof-to-wall connections are major components in this load path that have inadequate capacity during extreme wind events causing catastrophic failure. The objective of this dissertation is the evaluation of the vertical structural load paths due to wind loading on the roof of a low-rise, light-framed wood building. Through the use of static influence coefficients developed on a 1/3rd-scale wood building instrumented at twenty (20) roof-to-wall and wall-to-foundation connections, the vertical structural load paths were determined. Using a database-assisted design (DAD) methodology combining the structural influence coefficients with wind tunnel-derived pressure coefficients, estimated peak structural connection loads were determined. These peak loads were compared with loads estimated from two other previously established methods, namely the covariance integration (CI) and load-response-correlation (LRC) methods. Generally, the CI and LRC methods estimated the mean connection loads well but underestimated the peak loads.;The hypothesis of the research is that a wood-framed roof is sufficiently flexible to be excited by dynamic wind loads. An analytical finite element model of a 21-truss roof was developed to investigate the effects of dynamic loads on the roof. A fundamental frequency analysis was performed on the roof and various roof components to determine the first vertical mode of vibration. The analysis showed that the roof is sufficiently stiff with fundamental frequencies well above the low frequencies that dominate natural wind flows. This finding discredits the hypothesis and shows that the static influence coefficients are valid for light-frame wood structures.;The DAD-derived peak reactions were also compared with design loads determined using the ASCE 7-05 wind load provisions and the prescriptive loads provided by the Wood Frame Construction Manual (WFCM). The roof-to-wall connection design loads determined using the ASCE 7-05 main wind force resisting system (MWFRS) procedures (both the low-rise and all heights provisions) underestimate significantly the peak loads developed using the DAD methodology. However, the design loads determined using the ASCE 7-05 components and cladding (C&C) provisions provide a better estimation of the peak loads, but still underestimate the DAD-derived peak loads at some roof-to-wall connections. This significant and unanticipated finding is due to the fact that the influence coefficients used to develop the MWFRS were for steel portal frames, which are not applicable to light-frame wood construction.