Allowable Strength Design Research Articles

ASD and LRFD: Reliability comparison for designs subjected to wind loads

Current design practice in the United States allows engineers to use either Allowable Strength Design (ASD) or Load and Resistance Factor Design (LRFD) method, under the assumption that both methods provide similar levels of economy (available design strengths) and safety (probabilities of failure and reliability indices) regardless of the design conditions. Recent studies have demonstrated that significant differences in economy can arise between ASD- and LRFD-based designs subjected to combinations of gravity and wind loads, suggesting inconsistencies between the safety levels. This study investigates the safety levels provided by the ASD and LRFD methods for such design cases. The safety levels were estimated using a formal reliability analysis with consideration of geometrically non-linear second-order effects. The reliability analysis was performed using Monte Carlo simulations with appropriate probability distributions for the random variables associated with both resistance and loads. The findings of this study indicate that generally accepted uniformity of reliabilities achieved by the ASD and LRFD methods does not hold for design cases governed by wind loads. The safety levels achieved by using the ASD method are less consistent than those achieved by using the LRFD method. The use of the ASD method generally entails the acceptance of failure risks many times greater than for those of the LRFD method, and target safety levels lower than those stipulated by the ASCE 7 Standard. These results highlight the need for a reevaluation of the current design practices to ensure that the required target levels of structural safety are consistently met.

Experimental study of novel wrapped CFRP-strengthened CFS-to-sheathing screw connections

Cracking damage of the sheathing wallboards in screw connections is the typical failure mode of cold formed steel (CFS) structures under seismic effects, which limits the shear capacity and weakens the hysteretic performance of CFS structures. To improve the shear performance of CFS-sheathing screw connections, a novel screw connection with carbon fiber reinforced polymer (CFRP) reinforcement is proposed in this paper. Cyclic tests were conducted on a total of 26 sets of screw connections with different configurations and wall sheathings, and the effects on the failure modes and shear performance of the connections with and without CFRP reinforcement were investigated. The strengthening effects of the sheathing material, panel thickness and end distance on the shear properties of the connections were discussed. The results indicated that (1) except for specimens with 15 mm end distance, CFRP reinforcement can effectively avoid brittle cracking failure of the connections, and the failure mode tended to ductile failure of the bearing and pull-through damage. (2) in addition to the stiffness, the shear strength, deformation capacity, ductility and energy dissipation capacity of CFRP-strengthened connections are significantly enhanced. The effect of CFRP on the shear performance improvement of connections sheathed with gypsum board (GWB) and magnesium glass board (MgO) is greater than that of oriented strand board (OSB), and the strengthening effect is not significantly influenced by the board thickness. A toughness evaluation method for CFRP-strengthened connections was proposed, and the design formulae for the shear strength of the CFRP-strengthened screw connections were recommended based on the load resistance factor design (LRFD) method and allowable strength design (ASD) method.

Improved Design Shear Method for the Bolted Cold-Formed Steel Clip-Angle Connector

In this paper, the ultimate shear capacity of the 3-bolt cold-formed steel clip-angle between the cold-formed steel (CFS) beam and column is evaluated through 54 laboratory tests. A series of experiments were conducted by varying (1) thickness, and (2) aspect ratio (L/D) of clip-angles for different depths (D) and widths (A). The experimental program consists of three phases of tests: (1) Phase-I: direct shear load tests on clip-angle attached to a CFS column through 4.6-grade bolts; (2) Phase-II: CFS column replaced with a hot-rolled steel (HRS) column, since the CFS column experienced bearing failure in Phase-I; and (3) Phase-III: 10.9-grade bolts used instead of 4.6, as the 4.6-grade bolts subjected to bolt shear failure in Phase-II. Failure modes observed in the test specimens are (1) shear local buckling of clip-angle; (2) column bearing failure; (3) bolt shear failure; and (4) tear failure in clip-angle. Design shear equations from the literature, for the bolted clip-angle, were found to be inefficient for the high-grade steel (fy=375 MPa to 550 MPa), and conservative for the commonly available low-grade steel (fy=275 MPa). Hence a new shear strength equation is suggested for the clip-angle from the collated data of the present study and past research work. A comparative study between 2-bolt and 3-bolt clip-angle configurations was conducted to evaluate the increase in shear strength. Reliability studies were conducted, and corresponding resistance and safety factors were suggested for the design shear strength calculation corresponding to load and resistance factor design (LRFD), limit state design (LSD), and allowable strength design (ASD) methods.

Analysis of LRFD factors for a linear limit-state equation with correlated and time-independent random variables: Closed-Form formulation and numerical algorithm

The calibration of load and resistance factors (LRFs) of multiple loads can be a demanding process, where multiple loads must be considered in a potentially large set of combination scenarios to maintain consistent reliability in structural design. In this study, a set of closed-form formulas of the LRFs are developed with the aid of the coordinates of the design point, under the assumption that the limit-state equation is linear, and the load and resistance variables are mutually correlated and time-independent. These formulas can be used to efficiently calculate LRFs for a combination of multiple loads. Each load ratio is defined in a way that the derived equations can explicitly show the relationship between the LRFs and the conventional safety factor used in the allowable strength design or allowable stress design (ASD). All parameters used in the LRF formulas are dimensionless, making them more general and capable of facilitating comparative observations in a direct and quantitative manner. The closed-form formulas are kept simple by limiting the scope to linear limit-state equations with normally distributed load and resistance variables. For cases where one or more variables are nonnormal, an efficient single-loop iteration algorithm based on the developed formulas and the Rackwitz-Fiessler method is proposed. Four numerical examples are provided to demonstrate the feasibility of the presented formulas and algorithm used for LRFD calibration, followed by discussions of several interesting observations. For example, the fundamental advantage of the calibrated LRFD over the conventional, but calibrated ASD as load ratios change, and the effect of correlation between resistance and loads to the LRFs are illustrated.

Studi Struktur Portal Baja Gedung Workshop Alat Berat Di Balai Wilayah Sungai Sumatera VI Tahap II Dengan Metode DFBK dan DKI

Analysis of calculations and planning in steel structures there are 2 methods that are usually used, the methods are the method load factor and resistance design (DFBK) and the method allowable strength design (DKI). The method DFBK is a method that takes into account the working load multiplied by several safety factors so as to produce a factored load which is then used to calculate or plan a building, while the method DKI is a method that does not depend on the combined load factor in the calculation and in its planning, but uses stresses. permission which basically this method emphasizes the duration factor of the load that occurs on the structure. In this case study, the author aims to compare the method DFBK and so what distinguishes the two methods in a gable frame steel portal building. Both methods use the H-Beam 200.200.7.11 steel profile for the column, the I-WF 200.100.5.5.5.8 steel profile for the beam, and the CNP steel profile 100.50.5.7.5 with internal force analysis on the steel portal carried out with the program SAP 2000 with loading in accordance with SNI 1727-2020 and the results of the program SAP 2000 taken the largest moment, shear and axial for the calculation of the steel structure portal. Based on the results of the calculation of the deflection of the method DFBK of 2.4 cm, the bolts used were 10 bolts and the base plate thickness was 15 mm. While the method DKI of deflection is 2.1 cm, the bolts used are 6 bolts and the base plate thickness is 14 mm. Based on the results of the study and the conclusion that the deflection of the method DKI is smaller than the method DFBK, the bolt requirements and base plate thickness of the method DFBK are closer in terms of construction, so the method DFBK is safer and stronger.

Experimental study on ultimate strength of bolted L shaped sleeve joints between CFS lipped channel sections

This study aims to understand the behavior of bolted L shape sleeve joint connecting two Cold-Formed Steel (CFS) channel sections. A total of thirty-two specimens were experimentally tested under three-point bending. Three different series namely, (A) control specimen with two different span lengths, (B) sleeved specimen with the flange of sleeve connector capping the compression zone, and (C) sleeved specimen with the flange of sleeve connector capping the tension zone were tested. The parameters considered in this study include (i) the span length (Lt), (ii) sleeve length to section depth ratio (β), and (iii) thickness of sleeve connector (LS). The experimental results revealed that the ultimate moment resistance of specimens with a 2.5 mm thick sleeve connector was 19 to 73% higher than 2.0 mm thick sleeve specimens. The moment capacity of series C specimens was about 90–103% of that of the series B specimens. Based on the experimental results, design predictor equations were suggested to determine the moment resistance of sleeve specimens with respect to β. In addition, reliability studies were done for test series B to determine the safety and resistance factors for allowable strength design (ASD) method and load and resistance factor design (LRFD) method respectively. The equivalent and design uniformly distributed load (UDL) was determined based on the combined effect of shear and bending.

Structural performance on bolted sleeved connections between two CFS channel sections subjected to combined bending and shear

An experimental investigation was carried out to study the behavior of the sleeve connection between two cold-formed steel (CFS) channel sections subjected to combined bending and shear. A total of thirty-nine specimens including three control specimens were tested under three-point bending for different parameters such as three types of the span to section depth ratio, three different sleeve lengths, two sleeve thicknesses and two bolt configurations. Each tested specimen consisted of a pair of channel sections braced together using hot rolled angle sections along the length to ensure lateral stability. The test results indicate two different failure modes; local buckling due to combined bending and shear in the purlin section adjacent to sleeve end, and the other is shearing of the self-drilling screw at the tension zone. The moment resistance of the sleeve connection varied directly with the sleeve length to section depth ratio. The interaction study carried out between the proposed critical sleeve moment and shear capacity of the sleeve specimens is found to be conservative with respect to the AISI interaction equation. The resistance factor for load and resistance factor design (LRFD) method and safety factor for allowable strength design (ASD) method is proposed for the design of sleeve connection by considering it as a flexural member. The design uniformly distributed load (wD) of sleeve specimens having an average sleeve length to section depth ratio of 2.17, 3.51, and 4.85 are found to be 80, 103, and 125% the capacity corresponding to control specimen respectively. The proposed design method is also applicable to the design of continuous channel purlin/girt sections with 150 mm depth for any number of spans subjected to UDL.

Testing and design of cold-formed steel clip angles in tension: Pull-over and serviceability

A cold-formed steel (CFS) clip angle is a common connector used in CFS building structures. This research project was conducted to investigate the behavior of the CFS clip angles in tension and to develop the corresponding design methods. The test program included two phases of testing: Phase I of program focused on the pull-over strength of screw connections on the anchored leg of the clip angles, and Phase II of program concentrated on the tensile strength of the anchored leg of the clip angles within the service deflection limit. Design methods for determining the nominal pull-over strength as well as the nominal tension strength of the clip angles with consideration of the serviceability were developed based on the test results. The Allowable Strength Design safety factors and the Load and Resistance Factor Design, Limit State Design resistance factors were also produced to support the proposed design methods.

Allowable Strength Design of Mining Structures

AbstractThis paper reviews the basic differences between allowable strength design (ASD) and LRFD and the suitability of each methodology for the design of structures in the mining industry. The reasons the LRFD approach is of limited use for this industry are discussed. The LRFD method, as formulated in current codes, would need extensive and complex reformulation to be of general use for mining structures. The ASD method offers a rational straightforward way to design structures. Safety and design criteria for the ASD methodology are presented for various types of mining structures.

Bond Behavior of Concrete-Filled Steel Tube (CFT) Strucutres

To achieve internal force transfer while avoiding the use of steel stud anchors or a bearing mechanism within concrete-filled steel tubes (CFTs), an accurate assessment of the bond strength of CFTs is required. However, calculation of the bond within CFTs remains a challenging problem due to lack of a general procedure that can account for the range of connection configurations seen within composite construction. A new approach for assessing the nominal bond strength for both rectangular and circular CFTs is proposed. Based on the results of push-out experiments of CFTs, the nominal bond stress is shown to vary with tube shape and dimensions, and formulas are proposed to capture this behavior. The longitudinal bond transfer length is derived by examining the distribution of bond stress along the height of the column as well as experimental data from CFT connection tests. The circumferential bond transfer width is identified as the entire perimeter of the interface, accounting for the contribution to the bond strength from the interface on the sides that do not have girders or braces framing in. The resulting nominal bond strength is then shown to have a resistance factor of 0.45 for load and resistance factor design (LRFD) and safety factor of 3.33 for allowable strength design (ASD).

Design methods of integral‐lift tubular steel scaffolds for high‐rise building construction

AbstractIn this paper, we discuss the design and calculation methods of integral‐lift scaffolds under condition of operating, climbing up/down and falling. Dead loads, construction live loads and wind loads for integral‐lift scaffolds are given in this paper. The key factors such as additional load factor κ are introduced and defined for recommendation as well. Load and resistance factor design method based on probability theory is introduced for the design of the main load‐bearing structures. Herein, the additional partial safety factor of material strength γ′m is introduced specifically for scaffolding frames which are often frequently reused under poor outdoor conditions. Some important checking methods proposed specifically for integral‐lift scaffolds such as checking of anti‐slide strength of couplers and anti‐overturning analyses are also presented. Meanwhile, allowable strength design method is suggested in the design of the lifting equipments, suspending rods and slings. The proposed methods in this paper will be useful guidelines for safe design of integral‐lift scaffolds. Copyright © 2010 John Wiley & Sons, Ltd.

The new 2005 AISC specification

In late 2005, the American Institute of Steel Construction issued its most recent Specification for Structural Steel Buildings (ANSI/AISC 360-05). This specification includes updated design provisions in both allowable strength design (ASD) and load and resistance factor design methods (LRFD), and incorporates the design provisions for hollow structural sections and single angles. Amongst the major changes are a complete revamping of the methodologies for assessing stability of framed structures, new provisions for composite columns and updated material requirements. This paper will describe the changes and highlight those of practical significance.

Technical Note: Determination of Allowable Strength Design Safety Factors in the 2005 AISC Specification

The 2005 AISC Specification for Structural Steel Buildings includes both allowable stress design (ASD) and load and resistance factor design (LRFD) methods. The determination of safety factors for use in ASD are directly proportional to the resistance factors used for LRFD. The relationship between the resistance and phi factors is derived based on a live load-to-dead load ratio of 3 and the development of an equivalent load factor for LRFD. Both the resistance and phi factors are tabulated in this paper for the various limit states and structural elements that appear throughout the 2005 AISC Specification.