Effect of clamping pressure and joint geometry on corrosion induced bowing and distortion of bolted joints in weathering steel

Abstract

Field experience with bare weathering steel indicates that bolted joints of sufficient stiffness will resist the distortion caused by the pressure of corrosion products forming within the joint. As a result of these observations, guidelines were previously written for the design of joints in transmission towers and similar structures. To gain more information on the pressure caused by corrosion-product formation, specimens were exposed to the atmosphere for test periods of up to seven years. The test specimens include stacks of Cor-Ten A steel (A242 Type 1) and carbon steel compressed together with spring washers and exposed at Monroeville, Pennsylvania, and the 800-foot lot, Kure Beach, North Carolina. Also, stacks of Cor-Ten A steel and carbon steel compressed together at lower pressures were exposed in Monroeville. Various joints of Cor-Ten B steel (A588 Grade A), including geometries that did and did not meet the previously developed guidelines, were exposed at both sites, along with test joints similar to those used on a transmission tower and comparison joints modified to meet the guidelines. The results of the stack tests indicate that at sufficiently high initial force no expansion of the stacks occurs, but at very low pressures there will be continuous expansion of stacks. Cor-Ten A steel was found to produce more expansion than carbon steel at similar initial pressures. Bolting, as specified, with A325 bolts minimized distortion caused by corrosion product pressure for a given joint design. At both sites, stiffer joints produced less distortion at free edges; however, joints that met the guidelines of free edge distance did not eliminate the distortion. Joint designs that met the guidelines developed very little bowing between bolts during exposure at either site. Also, tests of comparison specimens showed that the bowing between bolts in a transmission-tower joint with A325 bolts can be reduced significantly by simply reversing one set of details so that the spacing meets the guidelines. All observations confirm that the use of the previously published guidelines for the design of bolted joints in bare Cor-Ten steel transmission towers and similar structures will prevent excessive bowing and joint distortion.