Yield Strength Reduction Factor Research Articles

Web crippling design of cold-formed high strength steel SHS and RHS at elevated temperatures

This study investigates the web crippling capacities of cold-formed high strength steel (CFHSS) hollow structural section members at elevated temperatures. A numerical investigation on square and rectangular hollow sections (SHS and RHS) undergoing web crippling was performed at various temperatures from 21 °C to 1000 °C. Finite-element (FE) models were firstly developed to replicate the available web crippling tests on CFHSS SHS and RHS members. After validation, extensive parametric studies with 784 specimens covering all four codified web crippling load cases in the current cold-formed steel design specifications were performed. The reductions of web crippling capacities at elevated temperatures were compared against the corresponding yield strength and elastic modulus reduction factors of the materials. The suitability of available web crippling design provisions in current codes of practice and literature to CFHSS sections at elevated temperatures was evaluated. Comparison results reveal that the direct strength method (DSM) previously proposed by the authors at ambient temperature could be extended to elevated temperature conditions and would provide rather consistent and reliable predictions than other existing provisions. A modified DSM is put forward in this paper and it is found that the modified DSM is suitable for web crippling design of the CFHSS sections at both elevated and ambient temperatures, and moreover, can also be used for cold-formed stainless steel SHS and RHS of various grades. Therefore, the modified DSM is recommended to be used for web crippling design of CFHSS and stainless steel SHS and RHS at elevated and ambient temperatures.

Determination of structural reliability of a reinforced concrete slab under fire Load

Four reinforced concrete rooms are used in this study, with varying floor areas, openings, and fuel loads. These rooms are office, Classroom, computer, and chemical laboratories of institutional buildings. The time versus Eurocode (EC) parametric fire relationship is determined and compared with ISO 834 and ASTM 119. The performance of the slab is evaluated based on the EC target reliability index of 3.8 for 50 years design period using the First Order Reliability Method (FORM). The probability of failure and structural reliability of each fire compartment at different fire duration is determined. Variables used were concrete compressive strength, a reduction factor of concrete compressive strength, steel yield strength, concrete cover, slab depth, unit width, and area of steel reinforcement provided. The reliability is determined at each 30 minutes fire duration for all fire compartments. The starting point is ambient temperature, then after every thirty minutes until the time when the maximum gas temperature is reached. The fire compartments were able to show fire resistance for two hours and then started to drop over time. Yield strength reduction factor, concrete cover, and area of reinforcement respectively were the most significant variables to determine the reliability index. The structure was able to withhold fire until two hours with a reliability of more than 3.8after that, the slab reliability was less than the target reliability and will need a repair.

Local buckling of corroded cold-formed steel stub columns

Cold-formed steel (CFS) is widely used as the load-carrying members in infrastructure engineering. However, CFS structure, which has been in the corrosive environment for a long time, is often difficult to avoid corrosion. Five corroded CFS stub columns with different corrosion degrees were tested under axial compression and the material properties of the coupon specimens were obtained by monotonic tensile tests. The thickness and initial geometric imperfection were measured using a dial indicator and an ultrasonic thickness gauge, respectively. The failure modes, the relationship between the axial load and lateral displacement, strain and mechanical parameters of the corroded CFS stub columns were discussed. A numerical study was undertaken using validated finite element models. Finally, the design rules for the corroded CFS columns were analyzed. The results indicated that the yield strength and ultimate strength reduction factor for corrosion decreased linearly with respect to the thickness loss ratio and the elastic modulus reduction factor for corrosion decreased nonlinearly with the thickness loss ratio. The axial displacement corresponding to the axial strength and the axial strength decreased with the increase of the thickness loss ratio. The axial strength predicted by the North American Specification (NAS) were more accurate than that predicted by the Chinese Technical Specification (CTS).

Static resistance of axially loaded multiplanar gap KK-joints of Circular Hollow sections at elevated temperatures

This paper presents the results of a numerical investigation of the behaviour, and proposes a method to calculate the static resistance of axially loaded Circular Hollow Section Double K-joints (DKK) at elevated temperatures. This type of joints is widely used in building construction, but currently, there is no method to calculate their static resistance at elevated temperatures. The numerical investigations of this paper cover a wide range of design parameters, including brace axial load combination, joint temperature, angles between brace members, longitudinal and transverse gap sizes, chord diameter to thickness ratio and brace diameter to chord diameter ratio.The results of this paper indicate that the ratio of joint elevated temperature resistance to ambient temperature resistance may be lower than the reduction factor for the yield strength of steel at elevated temperatures.There are two joint failure modes, one for small and one for large transverse gap. These failure modes and the demarcation between small and large gaps are independent of joint temperature. For joints with large gap, the ratio of joint resistance at elevated temperature to that at ambient temperature is close to the average of steel yield strength and Young’s modulus reduction factors at elevated temperature. For joints with small gap, the joint resistance ratio follows the steel Young’s modulus factor variation at elevated temperature if all four braces are under compression, but the trend changes to the average ratio of steel Young’s modulus and yield strength reduction factors if the axial brace loads are balanced (compression in two brace members equal to tension force in the other two brace members). Linear interpolation can be applied between these two different loading cases.

High-temperature material degradation of Q345 cold-formed steel during full-range compartment fires

High-temperature material properties are the most important input parameters for the study of the fire resistance of steel structures. Meanwhile, the structural failure of steel structures might occur during compartment fires. This paper presents steady-state experiments on Q345 cold-formed steel (CFS) during the rising and falling temperature periods. The results show that whether in the rising or falling temperature periods, the material properties of Q345 CFS decrease as the load testing temperature increases. Meanwhile, the difference between the test results of the rising and falling temperature periods can not be ignored. Under the same load testing temperatures, the differences in the elastic modulus reduction factors of Q345 CFS between the rising and falling temperature periods are within −0.16 to 0.057; meanwhile, the differences in the yield strength reduction factors between the rising and falling temperature periods are within −0.009 to 0.142. For the fire resistance design, if the material properties during the rising temperature period are used to replace those during the falling temperature period, the calculation results of the fire-induced deformation for Q345 CFS members may be conservative, but the corresponding high temperature bearing capacity will be over-predicted. In addition, the differences in the reduction factors of material properties for Q345 CFS caused by the maximum temperature are significantly lower than those for G550 CFS. Finally, a maximum temperature influence coefficient is introduced and unified expressions are put forward, which can be appropriately used to predict the material properties and stress-strain curves of Q345 CFS during full-range compartment fires.

High-temperature steady-state experiments on G550 cold-formed steel during heating and cooling stages

Steel structures usually experience heating and cooling stages during a compartment fire. Most previous investigations on the high-temperature material properties of cold-formed steel (CFS) focused on the heating stage of steel, corresponding to the growth and fully developed phases of a compartment fire. The influence of the cooling stage of steel, which corresponds to the decay phase of a compartment fire, has not been properly considered. This study conducted 88 steady-state tests on G550 CFS at elevated temperatures and investigated the material properties during the heating and cooling stages. The results show that the prediction of the time-dependent load-bearing capacity of CFS structures under compartment fires might become non-conservative if the material properties of the G550 CFS during the cooling stage are replaced with those during the heating stage. For instance, when the tensile temperature is less than 500 °C and the peak temperature is greater than or equal to 600 °C, the yield strength reduction factors of G550 CFS during the cooling stage are significantly lower than those during the heating stage under the same tensile temperature. In addition, the distribution of the yield strength (ultimate strength) reduction factors of G550 CFS during the cooling stage is affected by the peak and tensile temperatures and displays two branches. The difference in the reduction factors between the two branches is significant under the same tensile temperature and different peak temperatures. Finally, unified equations for the material property reduction factors and stress–strain curves are proposed and can provide a reasonable prediction of the material properties of G550 CFS under full-range compartment fires. • Steady-state tests of CFS were conducted during heating and cooling stages. • The difference in the test results during heating and cooling stages is significant. • The distribution of yield strength curves during the cooling stage displays two branches. • The prediction of the bearing capacity of CFS structures in fires might become non-conservative. • Expressions are provided for the material properties of G550 CFS under full-range fire.

A numerical investigation of static resistance of welded planar steel tubular joints under in-plane and out-of-plane bending at elevated temperatures

This study presents the results of a numerical investigation on static resistance of welded planar steel tubular joints under in-plane and out-of-plane bending moments at elevated temperatures. The numerical simulations were carried out using the commercial finite Element software ABAQUS v6.14-1 and the simulation model was validated by comparing against test results for CHS joints under brace in-plane bending at elevated temperatures.Extensive numerical parametric simulations were performed for CHS/SHS/EHS joints under in-plane and out-of-plane bending at elevated temperatures, to establish a database of results covering a wide range of geometrical parameters, including brace to chord diameter ratio, the angle between brace to chord members, and chord diameter to twice chord thickness ratio.The ratios of joint elevated temperature resistance to ambient temperature resistance were compared to the steel yield strength reduction factor, the steel Young’s modulus reduction factor, and the average of the above these reduction factors at different elevated temperatures. Based on the comparisons, it has been found that using the reduction factor for the yield strength of steel to modify the ambient temperature equations will overestimate the resistance of joints at elevated temperatures. As a simplified and safe recommendation, the ambient temperature joint resistance equations can be modified by the average reduction factors for the Young’s modulus and yield strength at elevated temperatures. This modification factor can be increased to the yield strength reduction factor when the brace and chord widths/diameters are the same.

Higher modes contribution for estimating the inelastic deformation ratios and seismic demands of structures

In order to estimate the seismic demand by using the nonlinear static procedure, different approximate methods have been developed. One of the most useful methods is called displacement coefficient method (DCM), which is based on some modification factors. One of these coefficients denoted C1, concerns the inelastic deformation ratio and usually depends on either the yield-strength reduction factor or the ductility factor. In general the evaluation of the inelastic deformation ratio is based on the response of single degree of freedom (SDOF) systems, where the response of the structure is mainly controlled by the fundamental mode, knowing that the inelastic deformation ratio will not capture the contribution of higher modes in the overall structural response. A developed theoretical approach with the aim of estimating the inelastic deformation ratio for structures, considering contribution of higher modes of vibration, is introduced. In this assessment, the normalized yield strength coefficient (η) and the post-to-preyield stiffness ratio (α) are key factors. The results are compared to the uncoupled modal response history analysis (UMRHA) procedure and some existing formulations for a nine story building subjected to the El Centro 1940 ground motion. It appears that the new theoretical approach leads to enough accurate estimation of the inelastic deformation ratio compared to the UMRHA one.

Constant yield displacement procedure for seismic evaluation of existing structures

The starting point of the proposed procedure for seismic evaluation of existing structures is that the yield displacement of a structure in flexure is constant and that it depends only on the yield strain of the yielding material and the geometrical characteristics of the structure, not on the yield strength of that structure. The fundamental vibration period of the structure is, thus the dependent variable derived from the estimated yield strength and yield displacement of the structure. To facilitate an evaluation of the maximum inelastic deformation of an existing structure using a corresponding single-degree-of-freedom system approach, a new relation between the yield strength (defined using a new yield strength reduction factor) and the displacement ductility demand of a corresponding single-degree-of-freedom system is proposed. This relation is consistent with the constant yield displacement assumption and characterizes the relevant properties of the structure using the yield strain of its yield material, its aspect ratio and its size. The proposed Constant-Yield-Displacement-Evaluation (CYDE) procedure for seismic evaluation of existing structures has four steps. Given an existing structure, its seismic hazard environment, and an estimate of its strength, the CYDE procedure estimates the displacement ductility demand, i.e. the maximum inelastic displacement, the structure may experience at the examined seismic hazard levels. The proposed CYDE evaluation procedure is similar to the current constant-period procedures, but provides a more realistic estimate of the displacement ductility demand for stiff structures, enabling a more accurate seismic assessment of numerous existing structures.

Response of high-strength steel beam and single-storey frame in fire: Numerical simulation

The design principles for high strength steel (HSS) structures exposed to fire are under development. In this paper, the response of HSS structures in fire is studied through numerical simulation of a beam and a two-bay frame. Geometrical imperfections and residual stresses are introduced into the structural models. Deformation limit criteria are used to compare the critical temperatures of the structures made of HSS and mild steel. The comparisons show that HSS structures have higher strength reserve than mild steel structures. Using the mechanical properties at elevated temperatures from literature sources, it is observed that the deflection behaviour of the studied structures depends on the ratio of strength to elastic modulus. The deflection of the studied beam is sensitive to yield strength reduction factors as the beam fails by plastic hinge mechanism. Whereas, the deflection of the HSS frame is sensitive to the reduction factors of the elastic modulus as the frame fails by inelastic instability. The above-mentioned observations on the studied structures are made using a three-stage mechanism which is developed for interpreting the deformation response.

Seismic structural demands and inelastic deformation ratios: Sensitivity analysis and simplified models

Modern seismic codes rely on performance-based seismic design methodology which requires that the structures withstand inelastic deformation. Many studies have focused on the inelastic deformation ratio evaluation (ratio between the inelastic and elastic maximum lateral displacement demands) for various inelastic spectra. This paper investigates the inelastic response spectra through the ductility demand µ, the yield strength reduction factor Ry, and the inelastic deformation ratio. They depend on the vibration period T, the post-to-preyield stiffness ratio α, the peak ground acceleration (PGA), and the normalized yield strength coefficient η (ratio of yield strength coefficient divided by the PGA). A new inelastic deformation ratio Cη is defined; it is related to the capacity curve (pushover curve) through the coefficient (η) and the ratio (α) that are used as control parameters. A set of 140 real ground motions is selected. The structures are bilinear inelastic single degree of freedom systems (SDOF). The sensitivity of the resulting inelastic deformation ratio mean values is discussed for different levels of normalized yield strength coefficient. The influence of vibration period T, post-to-preyield stiffness ratio α, normalized yield strength coefficient η, earthquake magnitude, ruptures distance (i.e., to fault rupture) and site conditions is also investigated. A regression analysis leads to simplified expressions of this inelastic deformation ratio. These simplified equations estimate the inelastic deformation ratio for structures, which is a key parameter for design or evaluation. The results show that, for a given level of normalized yield strength coefficient, these inelastic displacement ratios become non sensitive to none of the rupture distance, the earthquake magnitude or the site class. Furthermore, they show that the post-to-preyield stiffness has a negligible effect on the inelastic deformation ratio if the normalized yield strength coefficient is greater than unity.

Inelastic deformation ratio for seismic demands assessment of structures

Modern seismic codes rely on performance-based seismic design methodology which requires from the structures to withstand inelastic deformation. Many studies have focused on the inelastic deformation ratio evaluation for various inelastic spectra. The purpose is to investigate the ratio between the inelastic and elastic maximum lateral displacement demands. Three options are considered in the present work to express the inelastic response spectra in terms of the ductility demand, yield strength reduction factor and inelastic deformation ratio. This later, a new non dimensional parameter called inelastic deformation ratio (Cη), depends on the natural period (T), the post-to-preyield stiffness ratio (a), the normalized yield strength coefficient (t) and the peak ground acceleration (PGA). The inelastic deformation ratio (Cη), is not evaluated for a fixed ductility ratio or for a fixed strength reduction factor. Though the ductility demand and yield strength reduction factor differ greatly for given ranges of T and r“ they take close values for systems for any T value when η > 1. It confirms also previous studies stating that they take unity value for periods T ≥ 1sec.

An approach for synthesizing tri-component ground motions compatible with hazard-consistent target spectrum - Italian aseismic code application

A versatile approach for synthesizing non-stationary multicomponent ground motions compatible with a given target response spectrum is presented. In conjunction with performance-based structural design, the proposed approach combines a scenario-based procedure to select the target spectra of the three components of the motion, and a stochastic scheme, with an iterative wavelet-based method to generate the artificial time histories. In this context, the selection of an appropriate target spectrum is mandatory to synthesizing accelerograms with proper frequency content, input-energy, effective duration, and inelastic demand. In the scenario-based assessments, the target spectrum is derived directly from an earthquake scenario defined by seismic hazard disaggregation and strong ground-motion attenuation relationships. Firstly, the design spectrum specified by aseismic code provisions is replaced by a set of conditional mean spectra (CMS) for each of the ground motion components. Subsequently, an inverse stochastic dynamics problem, defined by a set of parametric evolutionary power spectra (EPS), is formulated and solved in a point-wise format for each CMS and for the three components of the motion. To improve the matching between the response spectrum of the simulated records and the CMS, an iterative procedure involving the family of harmonic wavelets is then employed. Post-processing for a proper baseline correction of the records is performed to synthesize ground-motions yielding realistic velocity and displacement traces. To illustrate the effectiveness of the approach, extensive numerical results pertaining to the Italian aseismic code provisions are included in this paper. In this regard, Monte Carlo simulations are undertaken to relate the CMSs to the evolutionary power spectrum. In particular, non-constant peak factor expressions via regression analysis are derived for various ground-motion durations. Finally, the nonlinear responses of elasto-plastic SDOF oscillators with different values of the yield strength reduction factor are considered. The oscillators subjected to artificially generated records and a set of natural unscaled and spectrum-compatible accelerograms, are examined in terms of ductility demand and equivalent number of yield cycles.

Performance of tubular T-joints at elevated temperature by considering effect of chord compressive stress

This paper aims to study the performance of welded circular tubular T-joints subjected to axial compressive loads at the ends of both the brace and the chord at elevated temperature. Different from previous studies on circular hollow section (CHS) T-joints under brace axial load at elevated temperature, the effect of the chord stress is also taken into consideration. Overall six specimens are designed and tested in an environment in accordance with ISO 834 heating process. Different levels of compressive loads are applied in the axial direction of the chord to investigate their effect on the fire resistance of the specimens. Temperature development at some critical positions around the brace/chord intersection is measured during experimental test. The developments of defined global and local deformations during heating process are also obtained. The failure modes of all specimens are found to be characterized by chord plastification around the brace/chord intersection. By introducing an elastic modulus reduction factor and a yield strength reduction factor, the feasibility of some equations provided by Eurocode EN 1993-1-2, CIDECT and API RP 2A WSD for calculating the static strength of the specimens at elevated temperature are evaluated based on the experimental results.

Mechanical properties of cold-formed steels at elevated temperatures

Mechanical properties have an important role in the fire safety design of cold-formed steel structures due to the rapid reduction in mechanical properties such as yield strength and elastic modulus under fire conditions and associated reduction to the load carrying capacities. Hence there is a need to fully understand the deterioration characteristics of yield strength and elastic modulus of cold-formed steels at elevated temperatures. Although past research has produced useful experimental data on the mechanical properties of cold-formed steels at elevated temperatures, such data do not yet cover different cold-formed steel grades and thicknesses. Therefore, an experimental study was undertaken to investigate the elevated temperature mechanical properties of two low and high strength steels with two thicknesses that are commonly used in Australia. Tensile coupon tests were undertaken using a steady state test method for temperatures in the range 20–700 °C. Test results were compared with the currently available reduction factors for yield strength and elastic modulus, and stress–strain curves, based on which further improvements were made. For this purpose, test results of many other cold-formed steels were also used based on other similar studies undertaken at the Queensland University of Technology. Improved equations were developed to predict the yield strength and elastic modulus reduction factors and stress–strain curves of a range of cold-formed steel grades and thicknesses used in Australia. This paper presents the results of this experimental study, comparisons with the results of past research and steel design standards, and the new predictive equations.

강도한계 이선형 단자유도 시스템의 동적 불안정

강도한계 이선형 단자유도 시스템의 지진 하중 하에서의 동적 불안정에 대해 연구하였다. 강도한계 이선형 이력 모델은 철골 모멘트 골조의 이력거동을 가장 잘 모사한다. 단자유도 시스템의 동적 불안정을 판단하기 위해 붕괴 강도비를 사용하였고, 이것은 붕괴가 일어날 때의 항복강도 저감계수이다. 단단한 지반에서 측정된 240개의 지진을 이용하고 고유주기, 강성 경화 기울기, 음강성 기울기, 연성 그리고 <TEX>$2{\sim}20%$</TEX>의 감쇠비를 변수로 하여 강도한계 이선형 단자유도 시스템의 붕괴 강도비의 평균과 편차 값들을 구할 수 있도록 통계 분석을 하였다. 비선형 회귀분석을 통해 강도한계 이선형 단자유도 시스템의 붕괴 강도비의 평균과 표준편차를 계산할 수 있는 식을 구하였다. 제안한 식을 이용하여 붕괴 강도비의 확률적 분포를 구하였고, 실제 값과 비교하여 제안한 식의 정확성을 입증하였다. This study investigates the dynamic instability of strength-limited bilinear single degree of freedom (SDF) systems under seismic excitation. The strength-limited bilinear hysteretic model best replicates the hysteretic behavior of the steel moment resisting frames. To estimate the dynamic instability of SDF systems, the collapse strength ratio is used, which is the yield-strength reduction factor when collapse occurs. Statistical studies are carried out to estimate median collapse strength ratios and those dispersions of strength-limited bilinear SDF systems with given natural periods, hardening stiffness ratios, post-capping stiffness ratios, ductility and damping ratios ranging from 2 to 20% subjected to 240 earthquake ground motions recorded on stiff soil sites. Equations to calculate median and standard deviation of collapse strength ratios in strength-limited bilinear SDF systems are obtained through nonlinear regression analysis. By using the proposed equations, this study estimated the probabilistic distribution of collapse strength ratios, and compared this with the exact values from which the accuracy of the proposed equations was verified.

Approximate Method for Predicting Inelastic Displacement of SDF Systems

The objective of this study is to develop inelastic displacement ratio for calculating peak inelastic displacement of SDF systems. The inelastic displacement ratio is defined as the ratio of peak displacement of inelastic SDF system to the peak displacement of corresponding elastic SDF system. Statistical studies are carried out to estimate median inelastic displacement ratios and those dispersion of bilinear SDF systems with given yield-strength reduction factors, natural periods, post-yield stiffness ratios and damping ratios ranging from 1 to 20% when subjected to 60 earthquake ground motions recorded on stiff soil sites. This study shows that equal displacement rule overestimates the peak inelastic displacement of the system which has a fundamental period ranging from the intermediate to long period. It is also observed that no significant difference was observed in the value of median inelastic displacement ratio across site classes.

이선형 단자유도 감쇠시스템의 비탄성변위비

NEHRP 지반조건 B,C,D에서 이선형 단자유도 감쇠시스템의 지반조건, 후탄성기울기, 감쇠비, 항복강도 감소계수, 고유 주기 등의 변화가 비탄성변위비에 미치는 영향을 조사하였다. 기존의 제안식은 변위일정 법칙을 따라 일정주기 이상에서 비탄성 변위비를 과대평가하게 된다. 또한 기존식은 5%이상의 감쇠비에 대하여만 제안되었다. 본 연구는 후탄성기울기, 감쇠비 20% 이하의 이선형 시스템의 비탄성 변위비의 평균과 편차를 제안하였고 범용적으로 사용할 수 있음을 보였다. 제안식을 사용하여 비탄성 변위비의 확률적 분포를 계산하여 구조물의 성능기반설계에 이용할 수 있다. This study investigates the effect of site class, post-yield stiffness ratio, damping ratio, yield-strength reduction factor, and natural period on inelastic displacement ratio of bilinear SDF systems located at the sites classified as NEHRP site class B,C,D. The previous studies developed inelastic displacement ratio using equal displacement rule in the intermediate and long period range. But, this approximation overestimates the inelastic displacement ratio. Furthermore, inelastic displacement ratio has not been developed for the systems having a damping ratio less than 5%. This study conducts nonlinear regression analysis for proposing equations for calculating median and deviation of the inelastic displacement ratio of the bilinear SDOF system having damping ratios ranging from 0 to 20%. Using median and deviation of the inelastic displacement ratio, probabilistic inelastic displacement ratio is estimated, which can be used for performance-based seismic evaluation.

Inelastic Deformation Ratios for Design and Evaluation of Structures: Single-Degree-of-Freedom Bilinear Systems

The relationship between the peak deformations of inelastic and corresponding linear single-degree-of-freedom (SDF) systems is investigated. Presented are the median of the inelastic deformation ratio for 214 ground motions organized into 11 ensembles of ground motions, representing large or small earthquake magnitude and distance, and National Earthquake Hazards Reduction Program (NEHRP) site classes B, C, and D; near-fault ground motions are also included. Two sets of results are presented for bilinear nondegrading systems over the complete range of elastic vibration period, Tn: Cμ for systems with known ductility factor, μ, and CR for systems with known yield-strength reduction factor, Ry. The influence of postyield stiffness on the inelastic deformation ratios Cμ and CR is investigated comprehensively. All data are interpreted in the context of acceleration-sensitive, velocity-sensitive, and displacement-sensitive regions of the spectrum for broad applications. The median Cμ versus Tn and CR versus Tn...

リングスチフナで補強される送電用鋼管トラス鉄塔節点の設計方法に関する研究 : その2 リングスチフナ・ガセットプレート一体型補強モデルの鋼管局部変形耐力

Strength and deformation criteria for a joint of the steel tubular trussed tower reinforced by ring-stiffeners and gusset plates are quantitatively derived from experiments and FEM nonlinear analyses to predict its local behavior under bending or horizontal loading. Yield strength of the concerned joint under bending loading is evaluated more safely than that of the model joint reinforced by single ring-stiffener. In case of a joint having multiple number of ring-stiffeners under horizontal loading, its yield strength shows a tendency to be small, because sheared force of the each ring-stiffener is not even. Under the unified condition, yield strength reduction factor correlates a ratio of the ring width outside the tubular steel to the tubular steel diameter.