Cristopher D. Moen

Direct Strength Method for Design of Cold-Formed Steel Columns with Holes

In this paper, design expressions are derived that extend the American Iron and Steel Institute (AISI) direct strength method (DSM) to cold-formed steel columns with holes. For elastic buckling-controlled failures, column capacity is accurately predicted by using existing DSM design equations and the cross-section and global elastic buckling properties calculated including the influence of holes. For column failures in the inelastic regime, in which strength approaches the squash load, limits are imposed to restrict column capacity to that of the net cross section at a hole. The proposed design expressions are validated with a database of existing experiments on cold-formed steel columns with holes, and more than 200 nonlinear finite-element simulations that evaluate the strength prediction equations across a wide range of hole sizes, hole spacings, hole shapes, and column dimensions. The recommended DSM approach is demonstrated to provide a broad improvement in prediction accuracy and generality when com...

Reinforced Concrete Force Visualization and Design Using Bilinear Truss-Continuum Topology Optimization

AbstractA new force visualization and design tool employing hybrid topology optimization is introduced for RC and prestressed concrete structural members. The optimization scheme couples a minimum compliance (maximum stiffness) objective function with a hybrid truss-continuum ground structure that can generate a strut-and-tie model for any general concrete member, loading, and set of boundary conditions. The truss ground structure represents discrete steel reinforcing bars (tensile load paths) that can be sized based on axial forces output directly by the optimization routine, whereas the continuum elements simulate concrete compression struts. This separation of compressive and tensile load-carrying elements is achieved through bilinear elastic models with an orthotropic constitutive relationship for the continuum. Examples are provided demonstrating the potential value of the optimization tool to RC design. Reinforcing layouts that can minimize cracking and reduce steel quantities when compared with tra...

Experiments on Cold-Formed Steel C-Section Joists with Unstiffened Web Holes

AbstractExperiments were conducted on cold-formed steel C-section joists with rectangular unstiffened web holes. The presence of holes decreased joist capacity and amplified distortional buckling deformation. Distortional buckling was accompanied by unstiffened strip buckling of the compressed web. When hole depth approached the web depth, sudden buckling of the compressed flange and web above the hole was observed. Critical elastic buckling moments for local, distortional, and global buckling were calculated for each specimen including the influence of holes with recently introduced engineering expressions and finite strip analysis. Forthcoming direct strength method equations for cold-formed steel joists with holes were demonstrated to be viable predictors of flexural capacity for the specimens considered in the experimental program.

Three-Dimensional Force Flow Paths and Reinforcement Design in Concrete via Stress-Dependent Truss-Continuum Topology Optimization

AbstractStrut-and-tie models (STMs) are widely used by RC designers. However, selection of a viable model is a challenging task, especially in complex three-dimensional (3D) design domains with irregular cutouts, which are common in building cores and shear walls. Therefore, topology optimization has been promoted as a means of automating the development of minimum strain energy STMs, which can lead to improved structural behavior. Current drawbacks of such methods are that solutions may be difficult to construct and may fail to properly account for tensile stresses resulting from force spreading. A two-dimensional hybrid truss-continuum topology optimization scheme was recently developed to overcome these challenges with the goal of reconfiguring traditional reinforcement layouts to automatically follow principal tensile stresses, reducing cracking at service loads and increasing strength and ductility at an ultimate limit state. That work is generalized and extended herein to 3D domains and mechanics mo...

INVESTIGATION OF FIBER-REINFORCED CONCRETE FOR USE IN TRANSPORTATION STRUCTURES

Results are presented of a laboratory investigation to determine the properties of fiber-reinforced concretes (FRCs) with steel (hooked-end), polypropylene (monofilament and fibrillated), and the recently introduced polyolefin fibers (monofilament) for application in pavements and bridge deck overlays. Concrete properties in the unhardened and hardened states were evaluated and compared. Although the ultimate splitting tensile strength, compressive strength, and first-crack strength were higher in most of the FRCs, when strength values were adjusted for changes in air content, only a few batches had higher strengths. The addition of fibers resulted in great improvements in flexural toughness and impact resistance. Three FRC pavement overlays were applied in Virginia in 1995. The FRCs used in the projects were similar to those used in the laboratory investigation, with similar fiber volumes, types, and sizes. To implement the findings of the study successfully, the performance of the FRC pavement overlays is being monitored.

Experiments on a Hybrid-Composite Beam for Bridge Applications

This paper details a study of the structural behavior of hybrid-composite beams (HCBs), which consist of a fiber-reinforced polymer (FRP) shell with a tied concrete arch. The HCB offers advantages in life-cycle costs through reduced transportation weight and increased corrosion resistance. Through a better understanding of system behavior, the proportion of load in each component can be determined, and each component can be designed for the appropriate forces. A long-term outcome of this research will be a general, structural analytical framework, which can be used by transportation departments to design HCBs as rapidly constructible bridge components. This study focused on the identification of the load paths and load sharing between the arch and FRP shell in an HCB and on the test of an HCB with a composite bridge deck. Tests were performed through the application of point loads on simple span beams (before the bridge deck was placed) and with a three-beam, skewed composite bridge system, which resulted in strain data for the arch and FRP shell. The test results showed that strain behavior was linear elastic at service loads, and the FRP shell had a linear strain profile. Curvature from strain data was used to find internal bending forces, and the proportion of load within the arch was found. A stress integration method was used to confirm the internal force contributions. The arch carried about 80% of the total load for the noncomposite case without a bridge deck. When composite with a bridge deck, the arch made a minimal contribution to the HCB stiffness and strength, because most of the arch was below the neutral axis and cracked under the maximum live load expected for the bridge. For this composite case, the FRP shell and prestressing strands resisted about 80% of the applied load, while the bridge deck carried the remaining 20% to the end diaphragms and bearings.

Extending the Direct Strength Method for Cold-Formed Steel Design to Through-Fastened Simple Span Girts and Purlins with Laterally Unbraced Compression Flanges

AbstractA direct-strength method (DSM) prediction approach is introduced and validated for metal building wall and roof systems that are constructed with steel panels through-fastened with screws to girts or purlins. The focus is capacity prediction for simple spans under wind uplift or suction; however, the DSM framework is generally formulated to accommodate gravity loads, continuous spans, standing-seam roofs, and insulated roof and wall systems in the future. System flexural capacity is calculated with the usual DSM approach; global buckling, local-global buckling interaction, and distortional buckling strengths are determined with a finite-strip Eigen-buckling analysis, including a rotational spring that simulates restraint provided by the through-fastened steel panel. The DSM flexural capacity is then reduced with a code-friendly equation consistent with existing standard provisions to account for the additional stress at the intersection of the web and free flange that occurs as the girt or purlin ...

Analysis of Elastic, Doubly Symmetric, Horizontally Curved Beams during Lifting

The lifting of horizontally curved beams (or almost-straight beams with an imperfection in shape) is considered, with application in the construction of bridges. A circularly curved beam that is suspended at two symmetric locations by vertical or inclined cables is analyzed. The cross section of the beam is assumed to be doubly symmetric, the material is assumed to be linearly elastic, the cross-sectional dimensions are assumed to be small relative to the radius of curvature, and the deformations are assumed to be small. Both uniform (St. Venant) torsion and inclusion of nonuniform (warping) torsion are treated. Analytical equations are derived for the overall roll angle of the beam, the internal forces and moments, the weak-axis and strong-axis deflections, and the cross-sectional angle of twist. The behavior depends crucially on the locations of the lift points.

Reinforced Concrete Design with Topology Optimization

Topology optimization techniques are employed to automate the design of reinforced concrete members. Truss models are derived with maximum stiffness (minimum total strain energy) from an initial ground structure defined over a general concrete member. The optimization routine, implemented with a freely available computer program, produces strut and tie geometries consistent with elastic tensile and compressive stress trajectories, resulting in steel reinforcement layouts with the potential to minimize crack widths and improve member performance over traditional strut and tie models. Ongoing work in continuum topology optimization of reinforced concrete members is summarized, including consideration of constructability in the optimized solution and the development of solutions with curved compressive struts which are more consistent with elastic stress trajectories than traditional strut-and-tie models derived by hand.

Field Verification of Simplified Analysis Procedures for Segmental Concrete Bridges

Load tests on segmental bridges are uncommon in the literature given their relatively short history and comparatively smaller presence in the national bridge inventory. This paper presents results from two segmental concrete bridge field tests and compares them with common simplified longitudinal and transverse analysis procedures. These single-cell structures, built with balanced cantilever construction, represent two significantly different segmental concrete bridges. Designers frequently use a beamline model for longitudinal analysis. When compared with the load test results, this simple method produces conservative predictions of longitudinal behavior within 20%, which is also reflected in the literature. Conversely, little information exists in the literature on transverse bending analysis. When analyzing the localized transverse bending from concentrated wheel loads, designers commonly use an equivalent frame model. Most frequently, designers use influence surfaces to estimate the scaled loads to apply to these two-dimensional frame models. This simplified approach is shown to be conservative overall but cannot always predict bending sense and frequently overpredicts demand in excess of 100%.