I-shaped Members Research Articles

ELASTIC DISTORTIONAL BUCKLING OF SINGLY SYMMETRIC I-SHAPED FLEXURAL MEMBERS WITH SLENDER WEBS

The effectiveness of the measures provided in the 2005 American Institute of Steel Construction (AISC) Specification for elastic distortional buckling of doubly symmetric I-shaped flexural members with slender webs was evaluated in a previous study. It was demonstrated that the code equations generally provide conservative strength estimates for the slender-web I-beams, and the amount of the conservatism was found to be rather dramatic for some cases. As a continuation of this effort, the effectiveness and accuracy of the 2005 AISC code provisions as well as predictions for elastic distortional buckling of slender-web singly symmetric I-shaped members is investigated in this paper. Comparisons are made with the finite strip analysis results for distortional buckling and the two design equations for elastic distortional buckling proposed by other researchers. It is demonstrated that the code predictions are by and large conservative, and even overly conservative in some cases, which does not seem to be justifiable economically.

A comparative study of AISC-360 and EC3 strength limit states

A study has been undertaken to evaluate the similarities and differences between the steel building design specifications used in the United States and Europe. Expressions for nominal strength presented in the AISC-360 Specification and the Eurocode 3 Specification were compared for fundamental limit states. In particular, rules for cross-section classification, tension members, compression members, I-shaped members subjected to flexure, I-shaped members subjected to shear, and fasteners were studied. Results of the investigation revealed that, in general, both specifications provide nominal capacities that are close to each other. Significant differences were reported for some limit states such as flexure in I-shaped members with non-compact flanges, shear and lateral torsional buckling in I-shaped members, and bearing strength at bolt holes. In this paper, the details of the comparative study are presented along with observations that are useful for practicing engineers.

Elastic distortional buckling of doubly symmetric I-shaped flexural members with slender webs

Intermediate length I-section members, particularly those with slender webs and stocky flanges, have been shown to buckle in a distortional mode, in which the web distorts and the flanges displace sideways with less twist than for the lateral–torsional buckling mode. This, in turn, reduces the torsional rigidity of the member, and hence results in a lower buckling strength. The web distortional flexibility is not addressed explicitly in standards for flexural design of steel I–section members. However, in the 2005 AISC specification it has been tried to account for the influence of web distortional flexibility on the lateral–torsional buckling resistance for slender-web I-section members through implicit use of J=0 in the lateral–torsional buckling equations. This paper evaluates the amount of effectiveness of this consideration within the elastic limit. By comparing the AISC code predictions with the accurate finite strip analysis (FSA) distortional buckling solutions as well as the theoretical predictions of two elastic distortional buckling formulae developed by other researchers, it is found that the assumption of J=0 for slender-web members results in overconservative estimate of the buckling strength in some cases which may cause economic losses.

Elastic buckling of I-beam flanges subjected to a linearly varying stress distribution

Lateral forces or torsion combined with major axis bending result in a stress gradient across the compression flanges of I-shaped flexural members. Sources that cause significant stress gradient include situations where unequal reactions come to opposite sides of a girder or beams that frame into one side of a girder, skewed bridge girder supports, wind loading on facia girders, curvature induced warping, etc. In this study, three sets of support conditions are considered for evaluating the elastic buckling capacity of flanges of I-shaped beams subjected to a stress gradient. This includes a Galerkin series solution that considers the full width of the flange plate with a variable rotational stiffness along the center of the plate. For a stress gradient leading to tension over part of the flange, equations for predicting the buckling capacity are developed using finite element analyses. Simplified equations are developed for I-shaped cross sections, and numerical examples are used to demonstrate the accuracy of the solution. A definition of the noncompact limit for flanges of I-shaped beams and girders subjected to stress gradient is presented.

Minor Axis Moment-Thrust Response Behavior in Steel I-Shaped Members

Most current design specifications employed throughout the world prescribe the use of so-called interaction equations for the case of beam–columns. Most often these interaction equations are optimized for use with the case of I-shaped cross sections bent about the major principal centroidal axis. The current study focuses on the case wherein an I-shaped member is loaded in compression and bending about the minor principal centroidal axis. It is shown that the current AISC interaction equations can be improved on in terms of their ability to predict failure in these types of members. Alterations to the existing AISC interaction equations are suggested for improving on strength predictions relative to this case.

Strength and ductility of HPS flexural members

An experimental and analytical research program was recently completed that examined in detail the parameters affecting the strength and ductility of high-performance steel (HPS) flexural members. HPS is a term used to describe a new class of steels being produced under strictly controlled conditions that have high strength, usually greater than 448 MPa (65 ksi) and exceptional toughness and weldability. The mechanical characteristics of these steels are different from conventional steels, leading to concerns over their use in some structural applications. Under earthquake loading, flexural members are expected to deform inelastically, so members fabricated with HPS steels must possess adequate ductility. This paper discusses the inelastic behavior of welded, I-shaped flexural members fabricated from an HPS steel, HSLA-80, having a nominal yield stress of 550 MPa (80 ksi) and an ultimate strength between 610–690 MPa (90–100 ksi) and compares the results to similar flexural members fabricated from conventional A36 steel. The effects of material properties: yield stress, strain-hardening modulus, yield stress-to-ultimate strength ratio, and strain at ultimate stress; cross-section geometry: flange slenderness, web slenderness, and lateral slenderness; and loading condition: monotonic moment gradient, monotonic uniform moment, and cyclic moment gradient are described from the results of experimental testing and analytical modeling. The results are evaluated against the existing design criteria established in the AISC-LRFD specifications and recommendations are made for revising the specifications.

Straight, Single‐Tapered Composite I‐Beams of Orthotropic Materials

Design methods for tapered members of multiple materials with nonrectangular cross sections are not available. This finite-element study of straight, single-tapered I-beams was conducted to aid in developing a rational design method for nonprismatic, I-shaped structural members of layered orthotropic materials. Particular emphasis is placed on wood composites. Taper geometry (O≤\Nθ\N≤\N6.3°) is considered in concert with the elastic constants of the web substrate. Comparisons of the stresses and deflections in a prismatic beam and a tapered beam of the same average depth and having the same elastic properties for the web are used to identify potential design considerations. The bending, vertical, and shear stress maximums are greater in the tapered beam than in the prismatic beam. In addition, the stresses are distributed differently if the beam is tapered rather than prismatic. For example, the maximum shear stress is not necessarily in the web. Deflection of the tapered member is 19% greater and occurs at 0.43\IL\N rather than at midspan.

Rapid design of built-up I-shaped compression members

A procedure is presented based on Canadian Standard CSA S16.1 for a rapid proportioning of a symmetrical built-up I-shaped compression member. The method considers the general case having different effective lengths in the two principal axes. The failure axis is not known initially. The method is design oriented and eliminates time-consuming trials to establish an economical section.

Plastic Bending of A514 Steel Beams

To determine how variations in the geometric ratios of b/t (the flange width is 2 b ) and L/ry affect the ability of A514-steel members to fulfill the requirements for plastic design, seven A514 steel I-shaped beams were tested in pure bending and two others were tested as centrally loaded beams in which the moment varied linearly from zero to maximum between bracing points. The moment-gradient and pure-bending tests simulated, respectively, first-forming and last-forming plastic hinges of frames. Both the pure-bending and moment-gradient tests indicated that A514 steel has sufficient rotation capacity for its use in plastically designed structures loaded in this way. The tests also indicated that premature plastic buckling of A514 steel I-shaped members will not occur if the value of b/t is limited to 5 and if the value of L/ry is limited to 21 for uniform-moment regions and 36 when the moment varies linearly from zero to maximum over the unbraced length.

The use of unstressed wires in pre-tensioned concrete members

Summary: In the serial fabrication of pre-tensioned concrete members the odd lengths of wire left over are often considered as waste. At the very slight cost of straightening them, these odd lengths of wire could be put to very efficient use. Used as a tensile reinforcement, unstressed wires make it possible to reduce or prevent the formation of cracks and so to increase the ultimate safety factor or to reduce the section or the prestress of a member designed for a given load and span. Used as a shear reinforcement in I-shaped members, unstressed wires are very efficacious in preventing failure by differential shear and so make possible the use of a thinner web; quite short lengths of wire may be used in this fashion.