Cem Topkaya

Development of computational software for analysis of curved girders under construction loads

The analysis of horizontally curved, trapezoidal steel girders presents a variety of computational challenges. During the erection and construction stages before a concrete deck is available to form a closed section, these girders are weak in torsion and susceptible to warping. Considering the design of an entire bridge system, current design approaches favor the use of a grid analysis methodology. While the use of a grid analysis procedure offers the advantage of computational efficiency, it is unable to capture girder stresses and brace member forces with sufficient accuracy, particularly during the critical erection and construction stages. In this paper, we present an alternative analysis approach based on the finite element method. The developed software has been designed to be computationally efficient and easy to use for bridge designers.

Ideal Location of Intermediate Ring Stiffeners on Discretely Supported Cylindrical Shells

Silos in the form of a cylindrical metal shell are commonly elevated to provide access to the space beneath, permitting the contained materials to be directly discharged. A few discrete column supports at evenly spaced intervals are commonly used. However, the structural design of discretely supported cylindrical shells presents a variety of challenges. The presence of discrete supports results in circumferential nonuniformity in the axial compressive stress as well as a progressive vertical decay above the support. Several approaches can be adopted in design depending on the severity of the nonuniformity of the stresses. Relevant research to date has focused mostly on the behavior of cylinders supported on brackets, local forces at the base, or stiff ring beams. The use of intermediate ring stiffeners to provide circumferential uniformity in the axial membrane stresses has long been recognized, but few studies have given a clear view of the practical requirements for such rings. In this paper, a combination of base and intermediate ring stiffeners is explored to develop a practical and cost-effective solution that leads to more uniformity in the axial membrane stresses above the intermediate ring stiffener. For the purposes of obtaining a simple analytical solution, the cylindrical shell is subjected to the fundamental harmonic of the column support and analyzed using membrane theory. It is shown that an ideal location exists for an intermediate ring stiffener such that the axial membrane stress above this ring is circumferentially completely uniform. The ideal location of this ring is determined analytically and is expressed in terms of the basic geometric variables. This ideal ring location is then independently verified using many linear finite-element analyses. A further study explores the effect of placing the intermediate ring stiffener below the ideal location. The results are presented in a manner that makes them suitable for direct adoption into traditional design specifications.

Ring Beam Stiffness Criterion for Column-Supported Metal Silos

Cylindrical metal silos are commonly elevated to provide access space beneath to directly discharge the contained materials into transportation systems. Evenly spaced column supports are commonly utilized. In larger silos, the discrete forces from supports are more evenly transferred and distributed into the cylindrical shell wall by using a ring beam. A fundamental assumption in the design of the silo shell is that the meridional compressive stresses are relatively uniformly distributed around the circumference. This assumption can easily be violated if the ring beam is flexible, so it is necessary to determine the ring stiffness needed to achieve a particular degree of uniformity of support. Current methods of assessing this stiffness rely on onerous finite-element analysis, which only provides information for the specific design being checked. In this paper, a criterion is developed to identify the required ring beam stiffness to achieve a particular degree of uniformity in the shell stresses. It is ba...

Evaluation of Seismic Response Factors for Eccentrically Braced Frames Using FEMA P695 Methodology

This paper reports details of a numerical study undertaken to evaluate seismic response factors for steel eccentrically braced frames (EBFs) using the FEMA P695 methodology. Six archetypes were designed by making use of the current U.S. specifications, and their behavior was assessed by making use of nonsimulated collapse models. Results indicate that the current values of response factors result in designs with higher collapse probabilities than expected. Two modifications were developed to bring the collapse probability of these archetypes to acceptable levels. The first modification is on the deflection amplification factor while the second one is on the response modification coefficient. Six archetypes were redesigned using the proposed modifications and reevaluated using the FEMA P695 methodology. The results indicate that the proposed modifications are adequate to satisfy the target collapse probability. Maximum and cumulative link rotation angles were observed to be less than the predefined limits.

Application of ring beam stiffness criterion for discretely supported shells under global shear and bending

Silos in the form of a cylindrical metal shell are commonly elevated to provide access to the space beneath. In general, a few discrete column supports at evenly spaced intervals are commonly utilized. The presence of discrete supports results in circumferential non-uniformity in the axial compressive stress above the support. Depending on the size of the structure, several different support arrangements may be chosen. A stiff ring beam is utilized in larger silos to transfer and evenly distribute the discrete forces from the supports into the cylindrical shell wall. A stiffness criterion was developed by Rotter to assess the degree of non-uniformity in axial compressive stresses around the circumference. The stiffness criterion is based on the relative stiffnesses of the ring beam and the cylindrical shell and was verified for loading conditions that produce circumferentially uniform axial stresses around the circumference. A study has been undertaken to investigate the applicability of the stiffness criterion to cylindrical shells under global shear and bending. Pursuant to this goal, extensive finite element analyses were conducted where different ring beam and cylindrical shell combinations are subjected to global shearing and bending actions. The results revealed that the stiffness criterion can be extended to shells under this loading condition. The degree of non-uniformity in axial stresses is quantified and presented as simple formulas that can be readily adopted by design standards.