Deflections of Wood Domes: Deficiencies in American Codes

摘要

The experimental work clearly shows that the localized crushing of wood fibers in joints of wood domes using steel connectors, cause deflections of the dome that are far in excess of the theoretically predicted elastic deflections of the dome. This crushing is not easily visible because it is hidden behind the steel plates in the body of the joint. Though the actual total deflections are still small, because of the doubly-curved shape of the dome structure, care must be taken in those situations in which deflection-sensitive, nonstructural finish materials are applied to the dome. While the tests were carried out only under vertical static loading, the application of cyclic wind loading with uplift conditions could cause localized wood fiber crushing on both sides of the metal connector, making the problem even more acute. In order to take this localized crushing of the wood fibers into account, the author suggests that the NDS (or other codes) allow for the increased deformations resulting from such crushing by the provision of a "connection factor" of a value less than one. This connection factor would reduce the allowable E-value for the wood in question. The connection factor itself will depend on a number of variables such as the density of the wood, diameter and numbers of steel bolts, and the nature of the connection whether in tension, compression, bending or shear. While such a connection factor at each joint could modify the E-value of the wood, for purposes of theoretical calculation, at an individual connection, there would still need to be some overall modification that would be necessary depending on the type of structure itself. For instance, in the case of the model domes considered in this paper, there are a total of 65 joints. For load at the crown of the dome, a connection factor of 0.9 applied at each joint would cause an increase in deformation of 1/0.9 or 11% at each joint. When multiplied by 65 joints, the theoretical deflection of the dome at the apex would be increased by 7.15. However, this would apply only to domes, where the deformations at the individual joints are cumulative leading to the largest deflection at the crown of the dome.