Resistance Factor Design Code Research Articles

Effects of Cross-Frame on Stability of Double I-Girder System under Erection

Double-girder systems investigated here consist of two steel I-shaped straight girders connected by intermediate cross-frames. In the erection stage, loading on the girder system, consisting mainly of girder self-weight, may not be significant. However, the concrete deck typically is not present to provide continuous lateral support to girder flanges, and there may be fewer cross-frames than in the service condition, resulting in fewer discrete lateral constraints to girder flanges and torsional constraints to girder sections. Such conditions could contribute to significant lowering of lateral torsional buckling capacity of the girder system compared with the service condition, and the girder system may become unstable during construction if the girder span is too long or if cross-frames do not provide enough discrete lateral support. Thus, it is important to investigate the effects of cross-frames on the stability of double-girder systems during construction. The AASHTO load and resistance factor design code has no specific requirements for maximum allowable spacing of cross-frames. To identify the dominant parameters and quantify the effects of cross-frames on double-girder system stability, finite element analyses were conducted to investigate the effects of the area of the cross-frame braces and cross-frame spacing on lateral torsional buckling capacity of double-girder systems under self-weight. The area of cross-frame members had negligible effects, but cross-frame spacing above a certain value affects system capacity. A method is presented to determine maximum cross-frame spacing (i.e., minimum number of cross-frames) for double-girder systems during erection. The goal is to achieve construction safety and labor reduction at the same time.

Convenient Method for On-Site Check of Single Steel I-Girder Stability during Erection

The behavior of a single steel I-girder during erection is quite different from that of a girder under service: the girder during erection usually has limited or no lateral bracing along the span. Elastic lateral torsional buckling of the steel girder may occur if the girder span becomes too large. Rules of thumb have been used to check girder stability during erection by using the ratio of unbraced length L to compression flange width b; this is based on experience. For any longitudinally unstiffened girder section, formulas were developed on the basis of the current AASHTO load and resistance factor design code for the unique maximum L/b ratio. Parametric studies were then conducted to calculate Lmax and the (L/b)max ratio for a variety of girder sections that cover the range of practical design in order to identify dominant section parameters on the (L/b)max ratio as well as girder stability during erection and to develop simplified formulas for the upper and lower bounds of the (L/b)max ratio for all girder sections within the range of practical design. On the basis of the simplified formulas, an on-site check of single steel I-girder stability during erection could be conducted by checking the girder L/b ratio against the (L/b)max ratio for that given section or, even more easily, by checking the girder L/b ratio against the upper and lower bounds of the (L/b)max ratios for all girder sections.

Comparative Study of Optimum Designs of Steel High Rise Building Structures Using Allowable Stress Design and Load and Resistance Factor Design Codes

The writers recently created a fuzzy discrete multicriteria optimization model for automated design of large steel structures subjected to the actual constraints of commonly used design codes. In this work, the model is used to perform a comparative study of optimum designs of steel high rise building structures using AISC allowable stress design and load and resistance factor design (LRFD) codes. Results are presented for two large structures, a 36-story space moment-resisting frame with cross bracings and a 144-story super high-rise building structure with over 20,000 members. LRFD code results in the cost savings in the range of 3.0-6.9% for the 36-story steel structure and 1.7-4.3% for the 144-story steel structure. When serviceability (stiffness) is more of a controlling factor, the LRFD code in general yields a much smaller advantage for weight savings.

Reliability-Based Criteria for Load and Resistance Factor Design Code for Wood Bridges

Recently AASHTO adopted a load and resistance factor design code for highway bridges. The new code provides a rational basis for the design of steel and concrete structures. However, the calibration was not done for wood bridges. Therefore, there is a need to fill this gap. The development of statistical models for wood bridge structures is discussed. Recent test results provided a considerable amount of new data for sawed wood and glulam components. Statistical methods provide a good tool for development of rational models for loads and resistance. Because of the random nature of load and resistance, reliability is a convenient measure of structural performance that also provides a rational basis for comparison of wood and other structural materials. The results of a recent project that led to development of rational design criteria for wood bridges are presented. The structural reliability of selected wooden bridges designed by the AASHTO codes are determined, and inadequacies in load distribution and material resistance in the current specifications are identified.

Comparison of BS 5950 and AISC–LRFD Codes of Practice

In this study I compare two codes of practice, the British Standard (BS 5950): Part 1 (1990) Structural Use of Steelwork in Building and the American Institute of Steel Construction Load and Resistance Factor Design (1986). Steel members such as laterally supported and unsupported beams, concentrically loaded columns, and beam-columns are considered. Only dead and imposed gravity loads are taken into account; the ends of the members are assumed simply supported and standard American sections (W shape) and standard British sections (universal beam and universal column) are used in this paper. A comparison of designs of the various members based on British Standard code 5950 with those based on the American Institute of Steel Construction Load and Resistance Factor Design code shows that the latter yields more economical design for most types of members.

Fixed and jack-up platforms: Basis for reliability assessment

Abstract The introduction of the fixed jacket Load and Resistance Factor Design code (API RP 2A-LRFD), and the Jack-up Site Assessment Recommended Practice (SNAME T&R Bulletin 5-5A), raises debates regarding differences in the reliability levels associated with the use of the two documents. In an effort to investigate whether substantial differences exist, and to understand the underlying causes of the differences, a major study has been initiated by the UK Health and Safety Executive to compare the reliabilities of these two structures when operating in similar weather conditions and in similar water depths, to the extreme weather hazard. To ensure the comparison is appropriate, both units will be assumed to be operating close to the limits of their relevant code. Both will be subject to the same analysis packages in respect to wave-current force determination, structural analysis, pushover analysis, and reliability analysis. Initially, foundation failure will be suppressed in order that structural reliabilities under extreme weather conditions can be directly compared. Foundation failure will then be introduced to quantify the effect this has on the structural safety levels. Reliabilities will be determined at both component and system level using the response surface reliability methodology. Component strength modelling will exploit the formulations currently being harmonised for the new ISO offshore code, whilst global strength will be evaluated using CAP/Seastar for both installation types, thus reducing the potential for modelling based differences occurring during the system analysis of the study.

Statistics of High-Strength Concrete Cylinders

The results of a statistical analysis on steam-cured, plant-produced high-strength concrete in compression based on about 2,200 cylinders obtained from several concrete producers in the United States is summarized. The statistical analysis considers the actual-to-specified concrete strength ratio, coefficient of variation, and probability distribution function. The seasonal effect on the statistical properties of the strength is also included. The results of the analysis showed that both the mean-to-nominal and the coefficient of variation are higher for normal strength concrete than for high-strength concrete. The mean-to-nominal ratio varies between 1.078 and 1.255, whereas the coefficient of variation is in the range of 0.06528-0.1353, depending on the specified 28-day concrete strength. Also, the analysis showed that the mean-to-nominal ratio and the coefficient of variation for concrete produced in the cold season are both lower than the corresponding statistics for concrete produced in the warm season. The study will help code developers in generating probabilistic resistance models for concrete structures that can be used in the development of load resistance factor design codes.

Behaviour of headed studs in composite beams: push-out tests

This paper briefly summarizes the results of tests conducted on 18 full-size push-out specimens and 4 pull-out specimens. The objective of the project was to study the behaviour of headed studs in composite beams with ribbed metal decks perpendicular and parallel to the steel beam. The longitudinal spacing of the headed studs and the rib geometry of the metal decks were the principal experimental parameters.Five of the push-out specimens had solid concrete slabs, in five specimens the ribbed metal deck was placed parallel to the steel beam, and in the remaining eight specimens the metal deck was perpendicular to the steel beam. In most cases 38 mm deck and 16 × 76 studs were used.Test results obtained by the authors appear to indicate that with some exceptions the current Canadian Standards Association and Load and Resistance Factor Design codes are able to predict stud strength correctly for specimens with solid slabs and with parallel ribbed slabs only when failure occurs owing to stud shearing, i.e., when the studs are spaced sufficiently apart. It is recommended that these codes include a provision to check the possibility of concrete-related failures when the longitudinal stud spacing approaches or falls below six times the stud diameter for solid and parallel ribbed slabs.Stud pull-out was the principal mode of failure in the specimens with perpendicular ribbed metal decks. An equation recently proposed by Hawkins and Mitchell appears to underestimate the capacity for specimens with 38 mm deck. For specimens with 76 mm deck, their equation overestimates the stud capacity. The authors have proposed two separate but similar empirical equations for specimens with 38 and 76 mm deck. Key words: composite beam, headed stud, stud shear capacity, stud pull-out failure, minimum stud spacing, perpendicular deck, parallel deck.

Reliability of Bolted Steel Tension Members

The reliability of bolted steel tension members designed according to the American Institute of Steel Construction Load and Resistance Factor Design Code is evaluated. The failure modes considered are yielding of the gross section and fracture of the net section. Statistics on the correlation coefficient between the yield strength and ultimate strength and the model error in calculating the effective net area are presented. A critical review is made of the code in light of the fact that the reliability index of the member is always less than the reliability index of either mode.