AISC Seismic Provisions Research Articles

Seismic Performance Assessment of Multitiered Steel Buckling-Restrained Braced Frames Designed to 2010 and 2022 AISC Seismic Provisions

Seismic Performance Assessment of Multitiered Steel Buckling-Restrained Braced Frames Designed to 2010 and 2022 AISC Seismic Provisions

Experimental and numerical investigation of through-diaphragm in H-shaped steel beam to CFST column connections

This research proposes a cruciform through-diaphragm (CTD) for an H-shaped steel beam to concrete-filled steel tubular (CFST) column and investigates its seismic performance experimentally and numerically. The proposed connection consists of through-plates passing through the aligned slots in the panel zone and end plates directly welded to an H-shaped steel beam. This connection eliminates welding inside the steel tube for installation of the diaphragm, while providing a reliable load path from the steel beam to the CFST column. The experimental program examined the connection hysteretic behaviors, including the moment-rotation response, ductility, initial stiffness, and energy dissipation capacity. The proposed connection shows stable hysteric behavior and good energy dissipation up to a story drift up to 4 % and satisfies the AISC seismic provisions criteria for special moment connection. A finite element (FE) model was established and verified against the experimental results. The effects of concrete infill, steel tube column thickness, axial load ratio, and through-plate thickness on the hysteretic behavior of the proposed connection were investigated through parametric analysis of 24 FE models. This study provides the information on the optimized design parameters that ensures the stable seismic performance of the proposed connection that could be used in structural engineering practice.

Frictional mid‐spliced shear links for eccentrically braced frames

AbstractAccording to the AISC Seismic Provisions for Structural Steel Buildings (AISC341‐16) and EC8, the inelastic rotation demand at the design story drift is limited to 0.08 rad for I‐shape shear links in eccentrically braced frames (EBFs). Numerical studies on EBF archetypes show that the single‐sided inelastic rotation demands can be much higher than the limiting value. In addition, these links can fail due to low‐cycle fatigue (LCF) which depends on the loading history. A mid‐spliced end‐plated detachable replaceable link has recently been developed to promote easy replacement of end‐plated links. In this paper, a frictional mid‐spliced shear link is developed to increase the inelastic link rotation capacity and LCF life of shear links. The proposed link utilizes a splice connection at the mid‐length, where frictional faying surfaces are introduced to dissipate energy. Slip at the mid‐splice connection causes a relative vertical displacement between the link ends which eventually reduces the rotation demands on the I‐shape members. Experimental and numerical studies were conducted to study the proposed link concept. Three conventional and eight frictional mid‐spliced links were tested using a nearly full‐scale test setup. The results showed that the proposed links have a pinched link shear versus link rotation response. The links were able to sustain a link rotation demand of 0.23 rad together with a significant increase in their LCF life. Numerical studies were conducted to investigate the link rotation, interstory drift, and residual interstory drift of EBF archetypes equipped with the proposed frictional link.

Seismic performance of bolted T-stub Beam-to-Box column connection

Using internal continuity plates in box columns due to accessibility problems and complicated detailing of connection was perceived as fraught with shortcomings and limitations. To overcome this deficiency, the effectiveness of external continuity plates in conjugation with T-stub connectors in I-beam to box-column connections under cyclic action is investigated in this study. Three damage control strategies based on the fuse action of T-stubs (T-F), strengthening the connections (T-S), and using reduced beam section (RBS) connections (T-R) are pursued and are compared to a corresponding end-plate connection (E). Nonlinear finite element analyses are conducted using the ABAQUS/STANDARD software on the connections under the qualification cyclic loading conditions to elucidate all the main seismic behavioral aspects, including the strength, initial stiffness, energy dissipation, ductility, von-Mises stress, and equivalent plastic strain distributions of models. First, the accuracy of numerical models is validated by a series of available experimental results. Then, a parametric study is performed on the effects of the moment ratio of the beam to column, the number of continuity plates, stiffeners embedded on the beam, and using high strength materials on the seismic performance of the connections. From the results, it is found that the T-S model was the most efficient among the different connections studied, supplying the stiffness, strength, energy absorption, and ductility increases by 2%, 21%, −17%, and 22%, respectively with respect to the E model, satisfying the requirements of AISC Seismic Provisions for SMF. Increasing the beam-to-column moment ratio and the number of continuity plates, had limited effects by less than 10% on the seismic performance of the connections. In the absence of continuity plates, the connections exhibited worse inelastic seismic performance with decreases in stiffness, strength, and energy dissipation up to 67%, 44%, and 74%, respectively. The T-F connection gained a 13% strength increase and satisfied the AISC requirements when high-strength T-stubs are used. Based on the numerical observations, design strength and rotational stiffness of the proposed connections on the component-based joint design strategies, are finally outlined.

Axial-flexural strength of steel-concrete composite shear walls

A predictive equation is proposed for the axial-flexural strength of rectangular and flanged Composite Plate Shear Walls-Concrete Filled (C-PSW/CF), with and without endplates and boundary elements. The proposed equation is based on plastic stress distribution, adjusted by coefficients to account for influential design parameters. The adjustment coefficients are established by a regression analysis performed on the data developed using an extensive numerical parametric study. The continuum-based finite element model used for the simulations is first validated using the available test data. The effects of various design variables, including the aspect ratio, slenderness ratio, reinforcement ratio, axial load ratio, infill concrete compressive strength, and yielding strength of the steel faceplates on axial-moment are then numerically examined. Four cross-sections, including double skin plate, double skin plate with endplates, double skin plate with boundary elements, and double skin plate with flanges, are considered. The results are used to develop a new predictive equation to obtain the flexural strength of C-PSW/CFs in presence of axial loading. The proposed equation is finally validated using the experimental test data and its accuracy is evaluated by comparing it to available predictive equations and the method proposed by the 2016 AISC Seismic Provisions. The results show that the proposed design equation can well predict the flexural moment capacity of C-PSW/CFs with various cross-sections in presence of axial loading.

Cyclic behavior of 3D moment connections subjected to bidirectional load: Experimental approach

This paper presents an experimental research to assess the cyclic behavior of a bolted moment connection with the use of optimized end-plate connected to built-up box column subassemblies subjected to bidirectional and unidirectional loading. Seven real scale specimens were tested: three specimens with four beams connected to column as interior joint, two specimens with two beams connected as corner joint configuration and two specimens with two beams connected to column as interior joint, according the protocol established in AISC Seismic provisions. The seismic performance was evaluated in terms of hysteretic behavior, failure mechanism, stiffness and dissipated energy. The joints studied were manufactured from of hot-rolled I-beams and square built-up box columns. The elements of connection such as bolts, welding, outer stiffeners and end-plates were designed to remain in elastic range from the flexural expected capacity of beams. The results showed that the required minimum moment of 0.8Mp at 0.04 rad of drift angle was achieved for all specimens tested. However, a higher stiffness and resistance was reached in configuration with unidirectional load (interior joint) in comparison to joint subjected to bidirectional load (corner joint). The damage was concentrated uniquely in beams, while an elastic behavior in columns and connection components was reached for 0.04 rad of drift angle. Finally, this moment connection configuration can be used as an alternative to design buildings with special moment frames under bidirectional loading considering the cyclic behavior of joints evaluated.

Overstrength of I-Shaped Shear Links for EBF Design

Past experimental research on EBF indicated that the link overstrength, particularly that for short (i.e., shear) links, could be much higher than that specified in the AISC Seismic Provisions, thus potentially leading to an unsafe design of beams, columns, and gusset connections per the capacity design requirements. This study aims to identify key factors contributing to the high overstrength and to derive an expression to predict the overstrength of short links. Available experimental data were first collected, and main parameters affecting the overstrength were identified from the database and used for a multi-variate regression analysis. It was found that the following two parameters affect the link overstrength the most: (1) the Fu/Fy ratio between the actual tensile strength and yield stress and (2) Kw, a factor that represents the contribution of localized bending of link flanges. The link length, to a lesser extent, also affects the overstrength. A predictive overstrength equation based on these three parameters was proposed for capacity design of EBF with short links.

Seismic retrofitting of beam to double-I built-up column moment connections by lateral T-stiffeners: Experimental and numerical study

In the present paper, the retrofitting of welded flange plate moment connections joined to double-I built-up columns is discussed by presenting the design method and details of construction retrofitting. Some of the usual construction problems of this connection include the defective complete joint penetration groove weld at the connection of the flange plate and the column, the large out-of-plane deformations of the column cover plate at the level of the beam-to-column connection, and as a result, its inappropriate seismic performance and the connection's partially restrained behavior. Thus, an attempt is made to evaluate the advantages of strengthening this defective connection with the help of lateral T-stiffeners using analytical and experimental studies. For this purpose, the experimental specimen and the high-fidelity finite element model are considered for both retrofitted and defective connections by evaluating the proposed design method, cyclic behavior of connection, collapse potential of weld fasteners, and failure modes. The results indicate that by using the proposed retrofitting method, the connection is classified in the group of fully restrained connections, while also having the necessary strength and ductility to accept the conditions of the special moment-resisting frames according to AISC seismic provisions. By performing a sensitivity analysis on the T-stiffener, changing its dimensions and examining the damage indices, the accuracy of the proposed design method is proven. The examinations indicate that the aforementioned retrofitting method is a proper method for the introduced defective connection.

Development of a loading protocol for long links in eccentrically braced frames

Loading protocols are generally used for qualification testing of structural members. In eccentrically braced frames (EBFs), the connections of short links attached to columns are evaluated based on the loading protocol given in the AISC Seismic Provisions for Structural Steel Buildings (AISC 341–16). Behavior of many short links without column attachments was also investigated by making use of this loading protocol. The AISC341 loading protocol is suitable for only short links and does not take into account the one-sided nature of link rotation angle (LRA) demands due to mean effects. Long link behavior differs from short link behavior in a sense that long links are subjected to strength and stiffness degradation due to local buckling effects. A nonsymmetrical loading protocol for long links was developed as a part of this study. Twenty-four long link EBF archetypes with variable floor plans, bay widths, number of stories and link length to bay width (e/L) ratios were designed according to the US standards. The responses of EBFs under maximum considered earthquake (MCE) and collapse level earthquake (CLE) were obtained by making use of numerical analysis employing degrading link models. The link rotation angle time histories were reduced and converted into a series of cycles and the peaks of the LRA response were identified by using the rainflow counting algorithm. The nonsymmetrical loading protocol was represented as a function of maximum rotation range, which depends on the seismic hazard and e/L ratio.

Development of new types of bolted joints for cold-formed steel moment frame buildings

It has been a growing trend to use cold-formed steel (CFS) sections as primary structural members in low to multi-story buildings due to their high strength-to-weight ratios, ease of construction and flexibility in manufacturing to a variety of cross-sectional shapes. However, CFS frames are not widely used in seismic regions; one important reason is that the less redundant CFS joints generally have low ductility and energy dissipation capacity due to their inherently thin-walled properties. To address this issue, this paper presents an experimental programme and the corresponding results on two types of beam-to-column CFS joints, which are assembled to the two adjacent surfaces of the built-up CFS column in this paper. Different from the previous research, the two types of joints proposed are potentially suitable to connect CFS members in T-, L- and cruciform joints for multi-story framed-structure buildings. It was observed from the tests that the beams buckled before the occurrence of obvious damage to the tested joints. The two types of joints satisfied the seismic design philosophy of ‘strong connection-weak beam’. Joint Type 2 (JT2) exhibits overall higher performance in terms of strength, ductility, and energy dissipation capacity than the Joint Type 1 (JT1). JT1 satisfies the rotational requirement for Ordinary Moment Frames (OMFs) while JT2 satisfies the rotational requirement for Special Moment Frames (SMFs), as defined in AISC Seismic Provisions. The connection zones within both joints can be classified as semi-rigid according to the approach recommended by Eurocode 3: Part 1–8, with JT1 exhibiting higher stiffness than JT2. Overall, the proposed two joints connected with very slender beams and less bolts, have so far shown moderate performance to provide resistance and ductility to be applied in CFS framed buildings in seismic regions, which can be improved in further work.

A Procedure for Assessing Low-Cycle Fatigue Life of Buckling-Restrained Braces

AbstractBuckling-restrained brace frames (BRBFs) have been a popular seismic force-resisting system in the US since it was first introduced to the AISC Seismic Provisions and ASCE 7 in 2005. The st...

Gene expression model to estimate the overstrength ratio of short links

Short links are used as fuse members in Eccentric Braced Frames (EBFs), hybrid coupled-wall systems, and self-anchored suspension steel bridges. Compared to the intermediate and long links, short links have superior performance such as high strength and plastic deformation capacity. However, several researchers observed a considerably high overstrength ratio for short links. This study found that the AISC Seismic Provisions underestimate the plastic shear strength of short links, the experimental results of 103 specimens showed that only 8% of the specimens comply with the recommended AISC Seismic Provisions overstrength value of 1.25. The average of all data is 1.67 with COV 17.6% and RMSE 0.732. Therefore, a genetic expression-based model (GEP) was proposed to estimate the plastic shear strength of short links. A 96% of the overstrength ratio calculated by GEP of the 103 specimens comply with the recommended AISC overstrength value of 1.25. The average of the GEP overstrength values is 1.00 with COV 13.6% and RMSE 0.136. The GEP model considers the contribution of six variables such as the flange slenderness ratio (bf/tf), the web slenderness ratio (d/tw), the flange to web area ratio (Af /Aw), the flange force (Af fyflange), the web force (Awfyweb), and the link length ratio e/(M/V). Unlike the AISC overstrength ratio estimates, the GEP predictions are not sensitive to the variables. This is evidence that the GEP model is an inclusive and efficient equation to estimate the overstrength ratio of short links.

Stability of laterally unsupported shear links in eccentrically braced frames

AbstractI‐shaped links in eccentrically braced frames (EBFs) are susceptible to lateral or lateral torsional buckling when subjected to cyclic link rotations. Lateral bracing should be provided at the ends of the I‐shaped links in order to prevent these failures. Requirements for such braces are available in widely used design specifications such as the AISC Seismic Provisions for Structural Steel Buildings (AISC341‐16) and EC8. These requirements limit the use of I‐shaped links in bridge piers and elevator shafts. A combined experimental and numerical study was undertaken to investigate the behavior of laterally unsupported I‐shaped links under cyclic loading. The experimental study consisted of testing of six nearly full‐scale EBFs where the link length, link length ratio, and presence of lateral supports were considered as the prime variables. The test results demonstrated that short links with link length ratios less than 1.22 can experience inelastic link rotations greater than the codified limit of 0.08 rad even without lateral bracing. A numerical parametric study was conducted to develop more generalized design recommendations for laterally unsupported I‐shaped shear links. Stability of one‐story one‐bay EBFs was studied using geometrically and materially nonlinear finite element analysis including imperfections. The numerical results showed that I‐shaped links without lateral bracing can provide a stable response when the link length ratio is less than 1.15. In addition to this limit, the ratio of the elastic critical buckling capacity to the plastic shear capacity should be greater than 3.5 and 2.5 for links with and without axial force, respectively.

Seismic response of dual concentrically braced steel frames with various bracing configurations

Steel dual structures incorporating special concentrically braced frames (SCBFs) and special moment frames (SMFs) are, in general, effective for their high lateral stiffness and ductility. As such, ductile dual CBFs are often treated as a remedy for mitigating permanent drift demands along with the potential drift concentration in isolated stories. However, despite the benefits and interest, the effect of the bracing configuration on the seismic response of the dual concentrically braced systems, designed following the current capacity design approach stipulated in AISC Seismic Provisions, in particular, has not been thoroughly examined to date. The present study aims to shed light on the impact of the bracing scheme on the seismic behavior of dual SCBFs coupled with SMFs as backup frames. For this purpose, 4- and 10-story dual SCBFs with the commonly used chevron, split X, V, and cross X type bracing configurations are designed according to ASCE 7 and AISC 341. Then, the frames are subjected to a suite of ground motions. The nonlinear time-history analysis results are contrasted and discussed in terms of the peak and permanent drift demands, horizontal floor acceleration demands along with brace ductility demands.

Elimination of intermediate stiffeners in box link beam using low yield point steel

The use of intermediate stiffeners in the link beam, in addition to increasing the cost of construction and reducing the speed of execution due to the need to welding operations, can lead to residual stresses. In this study, in order to remove the intermediate stiffeners in box link beam, the idea of using two types of low yield point steel (made in Japan and China) in the web of box link beam has been investigated using the finite element modeling in Abaqus software. According to this study, using low yield point steel in link beam without intermediate stiffeners, the maximum permissible compactness ratio of link web can be considered up to 56% greater than the maximum compactness ratio provided for common structural steels. The overstrength factor for the two types of low yield point steel made in Japan and China is calculated as 4.14 and 2.58 on average, respectively, while the value of this parameter proposed in accordance with AISC seismic provisions for box link beams made of common structural steels is 1.4. Using low yield point steel in the link beam and eliminating the intermediate stiffeners, the energy dissipation of the link beam can be increased to about 30% and its elastic stiffness can be increased to about 45%. In addition, by using low yield point steel and elimination of the intermediate stiffeners, undesirable fracture mode in the vicinity of the stiffeners is removed, and the rotation limit corresponding to fracture of the link beam increases to about 85%.

Experimental Validation of Detachable Links for Eccentrically Braced Frames

AbstractEccentrically braced frames (EBFs) subjected to seismic actions dissipate energy through yielding of link members. Other members of the framing are on the other hand designed to remain mainly elastic during a seismic event. The framing can be reused after an earthquake if the link members can be replaced. In line with this, the last decade has witnessed development of replaceable links. However, most of the proposed details are not well suited for replacement under residual drift conditions. Consequently, this paper presents details of an experimental validation of a detachable link concept where a splice connection is introduced at the mid‐length of the link to facilitate replacement under residual frame drift. The concept was studied by conducting six nearly full scale EBF tests under quasi‐static cyclic loading. Different splice details employing bolted connections with vertical slotted or site drilled holes and welded connections were developed. The link length ratio, type of mid‐splice connection and the amount of residual drift were considered as the variables of the testing program. The specimens were subjected to the loading protocol recommended in AISC Seismic Provisions for Structural Steel Buildings (AISC‐341). The experimental results suggested that the detachable links with the proposed connection details can satisfy the link rotation capacity limits of AISC‐341. The detachable links performed similar to conventional links while their replaceability under residual drift was demonstrated.

Technical Note: Unbraced Length Requirements for Steel Special Cantilever Column Systems

AISC Seismic Provisions Section E6.4b for steel special cantilever column systems (SCCS) requires clarification based on inquiries to the AISC Steel Solution Center. In the 2016 edition, it is unclear if bracing is required for all special cantilever columns or for columns with unbraced lengths that exceed the maximum beam brace spacing of Lb per Section D1.2a for moderately ductile members. Instead of using Equation D1-2, which is applicable to I-shaped beams only, equations for SCCS columns have been derived for both I-shaped members and rectangular HSS or box-shaped members. The proposed revision provides specific situations when bracing is required.

Evaluation of AISC Seismic Design Methods for Steel Multi-Tiered Special Concentrically Braced Frames

Steel multi-tiered concentrically braced frames (MT-CBFs) are commonly used in North America as a lateral load resisting system of tall single-story buildings. Past studies show that MT-CBF columns designed in accordance with the 2010 AISC Seismic Provisions are prone to buckling due to a high axial compression force combined with in-plane bending moments caused by the nonuniform distribution of inelastic brace deformations along the frame height. Special design provisions have been introduced in the 2016 AISC Seismic Provisions to address flexural demands imposed on MT-CBF columns and prevent column instability. In this paper, the seismic design methods for multi-tiered special concentrically braced frames are evaluated using the nonlinear finite element analysis method. A two-tiered special concentrically braced frame was then created, and nonlinear static and dynamic analyses were performed to evaluate the seismic performance of both frames. Analysis results confirmed that the inelastic deformations in the frame designed using the 2010 requirements are not uniformly distributed but rather concentrated in one of the tiers and cause column instability under large story drifts, whereas, the 2016 design method significantly improves the distribution of inelastic deformation along the height of the frame and prevents column instability. Furthermore, it was found that the 2016 AISC Seismic Provisions accurately estimate the axial load but overestimate the in-plane flexural demands and underestimates the out-of-plane flexural demand. Nonetheless, the overestimation of in-plane flexure demands results in acceptable strength capacity even though out-of-plane flexural demands is underestimated.

Cyclic behavior of steel moment connections with built-up columns in weak direction

In many countries, due to fabrication restrictions in producing large hollow tubular rolled sections, use of built-up sections is a common practice. One of those built-up sections being a suitable choice as column in two directional steel moment frames is the double-I that attain high moment of inertia about their two main axes. In this study, using a through-plate concept, a new detailing and corresponding design procedure proposed for moment connections with double-I built-up columns in weak axis direction. A series of finite element analyses performed in order to envisage the connection behavior and its load transfer mechanism. To ensure the accuracy of the results, a computer model similar to the experimental prototype assembled and analyzed via finite element method. The numerical results illustrate reasonable agreement with the experimental results. In addition, the influence of the pertinent parameters on the proposed connection such as thickness and height of the column reinforcing plate, and thickness and height of the trough plate examined. The outcome of this research reveals that the proposed connection satisfies the rigidity, strength and ductility criteria according to AISC seismic provisions for special moment frame connections

Numerical Study on Cyclic Response of End-Plate Biaxial Moment Connection in Box Columns

The 2008 Wenchuan-China earthquake showed the importance of considering the bidirectional seismic action as a cause of failure in column hinge mechanisms. Subsequently, the large 2011 Tohoku-Japan earthquake revealed that Special Moment Frames buildings, made of tubular columns (Hollow Structural Section or Built-up Box Section) and rigid connections with I-beams, did not suffer serious damage. However, only the ConXtech® ConXL™ moment connection has been prequalified according to the (American Institute of Construction) AISC Seismic Provisions for use with tubular columns and the rest of connections do not consider biaxial resistance. The research reported herein investigated the cyclic response of box-columns joints, connected to I beams using the four-bolt extended endplate connection, subjected to bidirectional bending and axial load on the column. To conduct the study, complex nonlinear finite element models (FEMs) of several I beam to box column joint configurations were constructed and analyzed under cyclic loading using the ANSYS software. The results reveal that the failure is concentrated in the beams of all joint configurations except for the columns with axial load equal to 75% of the column capacity, where a combined failure mechanism is achieved. The energy dissipation capacity of joints with a greater number of beams is lower than joints with fewer beams. The bidirectional effect of the seismic action and the level of axial load must be considered to avoid the formation of a column-hinge fragile failure mechanism also the behavior exhibited by 3D joints is more realistic than 2D joints according to real structures.