Strength analysis of multiple-bolt joints in orthotropic materials

Multiple-bolt joint design is largely based on an allowable strength format derived more than 30 years ago and is applicable only below the proportional limit strength. A more sophisticated and reliable limit-states design approach will require new analytical methods to better understand the ultimate strength behavior of whole connections.;A two-dimensional finite element analysis model was developed to predict the ultimate strength, failure modes, load-slip curves, and load distribution information among bolts in a row for multiple-bolt joints with steel plates and small ratios of main member thickness to bolt diameter in orthotropic materials. A total of twenty-two full-sized Douglas-fir single- and three-bolt joints with slenderness ratio l/d equal to 1.6 were tested to failure under tension parallel to grain loading to verify the numerical model.;The study showed that ultimate strength of multiple-bolt joints will decrease when the main member's end distance and bolt spacing are less than four times the bolt diameter. The current end distance design requirement in the U.S. of seven times the bolt diameter for tension loading is conservative. The main member's thickness and bolt diameter also have a significant effect on the ultimate strength when the ratios of main member thickness to bolt diameter are small. Parallel to grain compressive strength and shear strength are the primary material properties that had significant effects on joint strength. A misfabrication offset along the longitudinal loading direction does not significantly impact the ultimate strength, but strongly influences the proportional limit strength.;This study has established load modification factors for multiple-bolt joints in a row for current design practice are valid only under the service loading condition. Unequal loading distribution among bolts is nearly balanced at ultimate load when joints have adequate spacing and end distance. The applied ultimate loading tends to be redistributed among bolts until all bolts in a row are failed.;Multiple-bolt connections with adequate end distance will have a crushing parallel to the grain bearing failure. A perpendicular to the grain crack opening failure will occur at free end location when the end distance is less than four times the bolt diameter.