Non-Linear Finite Element Analysis of Steel Base Plates on Leveling Nuts

Abstract

Base plates are used to connect structural members to their foundations. They are routinely used in cantilevered structures supporting traffic utilities like overhead cantilevered direction signboards, traffic signals, speed cameras and high mast roadway light poles as well as in industrial warehouses and garages. These poles are usually supported on concrete footings by means of steel base plates on leveling nuts. The purpose of the leveling nuts is to adjust alignment of the supported member. Currently, there are no simple design methods in the relevant structural codes. Therefore, the objective of the proposed study is to develop a rational procedure for sizing a base plate on leveling nuts subjected to gravity and eccentric loads. To accomplish the stated objective, three full-scale models were tested in laboratory under the effect of concentric axial load, biaxial bending and uniaxial bending. The results obtained from lab testing were used to rectify a finite element model in the ANSYS software similar to pile caps on piles. A parametric study was conducted under concentric and eccentric loads with various base plate thicknesses, column sizes, number of bolts and bolt eccentricities in order to check the sensitivity of the flexuralbehavior of the base plate to the design variables. The results from ANSYS showed that the failure occurs at the face of column for the concentricaxial and uniaxial bending loadcases. For biaxial loading, the failure occurs in most cases along a line tangent to the column corner under load, unless more bolts are located close to column face. The most load critical case was due to biaxial bending where the failure occurred at a relatively small load, compared to the other two considered load cases. The location of anchor bolts and the width of column are the most important factors that influence the flexural stress distribution and intensity in the base plate. Based on the results of the finite element analysis, equations of influence angles are proposed to calculate the effective width of the base plate in resisting flexure as a function of the plate thickness and bolt eccentricity. The results obtained from these equations are intended to help Structural Engineers size base plates following the load and resistance factored design approach. The location of anchor bolts, width of column, and plate thickness are the most important factors that influence the flexural stress distribution and intensity in the base plate.