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ENGINEERING STRUCTURES

Volume 175, 15 November 2018, Pages 27-40




SEISMIC PERFORMANCE OF CONCENTRICALLY BRACED FRAME WITH HEXAGONAL PATTERN OF
BRACES TO MITIGATE SOFT STORY BEHAVIOR

Author links open overlay panelNiloufar Mashhadiali, Ali Kheyroddin
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https://doi.org/10.1016/j.engstruct.2018.08.036Get rights and content




ABSTRACT

This paper aims to propose an innovative braced frame named hexa-braced frame
for the improvement of the seismic response of conventional steel-braced frames.
In the proposed system, vertical structural elements connect the V and
inverted-V bracings over three stories to form the hexagonal bracing
configuration, in which two stories are braced at the top and bottom. The
objective is to distribute deformation demands along the height of the frame in
order reduce the possibility of the soft-story mechanism, which is a concern in
conventional steel-braced frames. To achieve this objective, the braced columns
were designed to resist bending moments calculated by means of a simple analysis
procedure. A set of 4-, 10-, and 20-story building models were used to evaluate
the seismic response of the proposed bracing system through nonlinear static
(monotonic and cyclic) and dynamic analyses. The results were compared with the
responses of similar X-braced frame models as the benchmark. Analytical results
reveal that the hexa-braced frame system has the desired structural behavior for
the seismic resistant and can achieve the goal of uniform distribution of
lateral deformation in order to reduce the soft-story failure.


INTRODUCTION

In recent years, many engineers have turned to the use of innovative earthquake
resistant structural systems for the provision of stiffness and ductility and
prevention of damage concentration. Several deficiencies of the conventional
steel seismic load-resisting systems are listed, including concentrically braced
frames (CBFs) [1], [2], [3] and moment-resisting frames (MRFs) [4], [5]. For MRF
structures, brittle fracture of the beam-to-column connections was observed in
the 1994 Northridge earthquake and was thoroughly addressed in the subsequent
design provisions, such as AISC 358 in the U.S [6]. For CBF structures, a
comparison of the ductility capacity of the system with the seismic ductility
demands is necessary. Steel CBFs may be used in regions where the ductility
demand is not significant and the frame still provides the adequate ductility
capacity [7], [8], [9]. Additionally, the main reason for the undesirable
performance of CBF buildings is the soft-story mechanism, in which a single
story incurs major damage whereas the remaining stories are relatively undamaged
[10].

Over the past few decades, for simplicity and economic considerations, the
popularity of CBFs has increased significantly compared to MRFs [11], [12],
[13]. CBFs act as vertical concentric trusses which resist lateral loads through
axial forces. In past earthquake events, the major concern for CBFs was low
drift capacity [14]. Post-earthquake observations, numerical studies and
experimental investigations indicate that the damage concentration occurred in a
few weak stories of the CBFs during severe earthquakes. Several studies have
been conducted to attempt to reduce the damage concentration in CBFs. Approaches
include: (1) the use of dual structural systems for a combination of MRFs and
CBFs as a backup system in which the frame action compensates for the loss of
story shear capacity due to brace buckling [15], [16]; (2) application of
slender braces in which a large tension to compression capacity results in the
redistribution of forces from the compression brace to the tension brace [17],
[18]; (3) the use of BRBs with relatively large deformation hardening value in
which large plastic strains lead to yielding in the adjacent stories [19], [20],
[21]; (4) the use of a continuous column in which the flexural strength and
stiffness to prevent soft-story mechanisms in inelastic demand that transfers
the load capacity lost by localized brace buckling to other stories in the
system [22], [23]; (5) the use of a tie column in a zipper-braced frame to
distribute the inelastic demand over the height of the building [24], [25],
[26]; (6) the use of a strong back system in which an elastic truss system
mitigates soft-story behavior [27], [28], [29]. Other variants such as friction
dampers [30], a rocking brace core [31], [32] and negative stiffness device
[33], [34] constitute a possible solutions to reducing the damage concentration
in CBFs.

This paper proposes an innovative braced-frame configuration in relation to the
hexa-braced frame (Hexa-BR) system. It contains both V- and inverted V-braces in
different stories, forming the hexagonal bracing configuration over three
stories. It is a combined system comprising continuous columns and braces for
seismic resistance. In the proposed system, all the beams are connected to the
columns with simple pins. Fig. 1 presents two possible bracing configurations of
hexa-braced frame. Based on the length of braced bay, two types of chevron and
diagonal braces are used to form hexa-bracing configuration. In hexa-braced
frame column, bending moment, shear and axial forces are the internal actions
due to seismic loads. Compared with the conventional CBF, the hexa-braced frame
columns are subjected to substantial flexural demand which is led to be
stronger.

In a typical research, the design of CBF columns for seismic resistance is based
on truss action and is governed by the column axial forces, while neglecting the
flexural demand on columns in the design of braced frames. Braced frame with no
column flexural strength and stiffness allows soft-story mechanism in inelastic
demand.

MacRae et al. [22], in a study on the seismic behavior of steel frames, reported
that the pattern of column moments required against to a soft-story mechanism
should be considered in the design of braced frames in order to reduce damage
concentration. Some researchers concluded that the continuous columns in the
real frame significantly decreased the possibility of drift concentration due to
the flexural demand [35], [36]. Wang et al. [37] compared the influence of a
continuous column in a X-bracing system using V-braced and inverted V-braced
frames in which the columns were considered to be continuous over two stories
and over the height of the building, respectively. They concluded that providing
continuous columns over the height of the building is more beneficial. However,
it is known that the prevention of a soft story can be assured only if a
rigorous design procedure is considered, even when experimental evidence shows
that such a mechanism can be delayed by the use of continuous columns [38],
[39]. A proposed technology to create more uniform displacement involves the use
of rocking cores as a continuous column, which is pinned to the building of CBF
through the pin-pin link [40] (Fig. 2). This technique applies a strong column
to distribute lateral demand, thereby preventing soft-story mechanism.

The proposed hexa-braced frame configuration combines the following two
concepts: continuous strong column and flexural demand to achieve uniform drift
distribution along the height of a building and to prevent the formation of
soft-story mechanism.

The hexa-braced frame structural system is a new approach for hexagrid
structures. The hexagrid system is located at the perimeter of the building; it
includes multiple hexagonal grids limited by the mega columns. Mashhadiali and
Kheyroddin [41], for the first time in a study of the structural behavior of
hexagrid system, investigated its lateral load and progressive collapse
resistance for tall buildings [42], [43]. Recently, the use of hexagrid patterns
for structural effectiveness and aesthetics has attracted the attention of
engineers [44]. The Sinosteel International Plaza [45] is a tall building in
which hexagonal pattern was utilized for the first time for the structural
system, and other engineering plans of hexagrid structures were also proposed
(Fig. 3) [46], [47]. However, the design intention of the hexagrids and
hexa-braced frame are different. The hexagrid system is a tube type lateral
resistance system, but the hexa-braced frame is a new bracing system.

This study presents a description of the proposed hexa-braced frame as a new
seismic-resistant bracing system. A simple static analysis methodology is
presented to determine the column bending moment. Different structural models
are designed to evaluate the seismic behavior of the hexa-braced frame compared
to similar X-braced frame models as the benchmark. In order to indicate that the
proposed bracing system are extendable to a wide range of buildings, the model
buildings are conducted with different height representing low-, mid- and
high-rise structures. Nonlinear static and dynamic analyses are performed to
examine seismic responses of model structures.


SECTION SNIPPETS


DESCRIPTION

The hexa-braced frame consists of a hexagonal bracing system in which vertical
structural elements (strong column or tie column) in a story connect the V and
inverted-V bracings at the stories below and above that story, respectively. The
tie columns behave like the zipper columns. Chevron braces provide limited
ductility from the brace-intersected beam, such that the beam stiffness deeply
affect the damage distribution [48], [49]. Based on this, the ties are designed
to have enough strength to


ANALYTICAL MODELS

Three types of model buildings 4-, 10-, and 20-story were selected to examine
the structural response of hexa-braced frame for low-, mid-, and high-rise
buildings respectively (Fig. 8). In this research, different types of
hexa-braced frame models were considered based on the different lengths of the
braced bays. Chevron braces and single diagonal braces were used for beam spans
of 9.1 and 6.1 m, respectively. The dead and live loads were considered for
gravity loads as 5 and 2.5 kPa,


MONOTONIC PUSHOVER

The monotonic pushover is applied to estimate the seismic capacity and
progressive failure of the models. In this procedure, the models are subjected
to a monotonic increase of the specified lateral force pattern until the
predetermined story drift is reached. The capacity of the models in terms of
base shear with the response corresponding to the roof drift ratio is reflected
in the pushover curve. The pushover curves obtained for the model structures are
illustrated in Fig. 10.

The pushover


CONCLUSION

This study proposes a new seismic bracing system hexa-braced frame with the
hexagonal configuration of the braced frame. This system develops a conventional
braced framed for the consideration of column moment demand to reduce soft-story
mechanism and damage concentration. The proposed system combines strong
continuous columns with the braces forming hexagonal configuration. Although
beam to column connections are simple, columns can inherently carry moment due
to its configuration. This makes




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CITED BY (12)


 * PURE-BENDING YIELDING DISSIPATER FOR THE SEISMIC RETROFITTING OF REINFORCED
   CONCRETE BUILDINGS WITH SOFT-STORY IRREGULARITY
   
   2023, Structures
   Show abstract
   
   Removing masonry infill walls from the ground or lower stories of buildings
   to make more open spaces, may lead to forming soft-story structural
   irregularities. Since considering the masonry infill walls as non-structural
   components is a common assumption in the evaluation of the seismic demand of
   buildings, there is a lack of knowledge dealing with the influence of the
   soft-story effect on the seismic response. In this study, the performance of
   4-, 8-, and 12-story moment-resisting reinforced concrete frames with and
   without a ground soft story is compared first. Then, a proposal including the
   use of a yielding dissipater device to retrofit soft-story buildings is
   presented. The low cost, the simple manufacturing technology, and the same
   behavior in tension and compression, which makes the new axial force not
   imposed on the columns, are some of the advantages of this device. In order
   to assess the seismic performance of the proposed system, incremental dynamic
   analysis was conducted on the frames. Five states were considered for each
   frame: (a) bare frame, (b) fully-infilled frame, (c) ground soft-story frame,
   (d) and (e) retrofitted frames with two different configurations of yielding
   dissipater devices in the ground soft story. The responses of the structures
   are compared in terms of capacity curves, progressive damage curves,
   inter-story drifts, and story damage values. The results demonstrate that the
   frames equipped with the yielding dissipater devices experience higher
   lateral load capacity, lower overall damage, lower drift, and a lower damage
   value in the ground story than the soft-story ones.


 * PERFORMANCE ASSESSMENT AND CALCULATION OF ROBUSTNESS OF HIGH-RISE DIAGRID
   TUBE STRUCTURES WITH VARIOUS CONFIGURATIONS
   
   2023, Structures
   Show abstract
   
   Resilience is an appropriate criterion for evaluating the ability of a
   structural system to reduce the probability of damage caused by an event. To
   date, the seismic resilience of novel structural systems such as diagrid
   structures has not been studied. In the present research, the effect of
   various perimeter geometric patterns on the robustness index of diagrid
   structures is evaluated. For this purpose, six diagrid structures of 24 and
   48 stories with perimeter configuration angles of 49°, 67°, and 74° are
   designed. The seismic capacity of the structures is calculated using the
   incremental dynamic analysis (IDA) based on the maximum story drift parameter
   and under near-field earthquakes. Next, the fragility function of these
   structures in limit states are determined based on lognormal statistical
   distribution. Seismic performance levels for diagrid structures are also
   developed. The proposed formulation in the MCEER report is used to calculate
   the loss function of structures. In addition to the structural damage,
   non-structural damage is also considered in estimating the seismic resilience
   of the structures, according to the HAZUS guideline. Furthermore, an
   additional factor is defined in the loss function as an assurance of the
   quality of construction. Finally, the resilience of high-rise buildings with
   diagrid structures is determined for a specific hazard level. The results
   show that diagrid structures have relatively high robustness and using
   perimeter triangular elements can reduce the degradation of structures after
   the earthquake. As observed in the results, considering damage to
   non-structural elements causes a decrease in the quality function of the
   studied structures. It is demonstrated that increasing the perimeter
   configuration angles decreases the robustness index of diagrid structures.
   Based on this analysis results, insufficient knowledge of the quality of
   construction can decline the seismic performance of diagrid structures by up
   to 6%.


 * NON-LINEAR STUDY OF THE METHOD OF TRANSITION IN MIXED CONCRETE/STEEL
   STRUCTURES
   
   2023, Soil Dynamics and Earthquake Engineering
   Show abstract
   
   A critical zone is formed at the junction of the lower concrete and upper
   steel sections in mixed concrete/steel structures in terms of gravity and
   lateral loads caused by the support reactions of the steel frame. There are
   two common approaches to transition from concrete section to steel one in
   this critical zone: the first is the direct pin connection of the steel frame
   to the concrete frame, and the second is the application of a concrete-steel
   composite story called the transition story. Until now, various studies have
   been conducted on mixed concrete/steel structures, but no study has
   investigated and compared the two mentioned approaches. Considering the
   importance of this issue, 18 models of mixed concrete/steel structures were
   formed in the present study according to the two approaches and the different
   places of connection of the concrete and steel sections along the height of
   the structure. Fragility curves were generated for all models using
   Incremental Dynamic Analysis (IDA) under 44 far-field ground motion records
   and probability function at the complete damage state based on HAZUS
   technical report. A comparison was made from the strength point of view
   considering the two approaches to transition. Next, to investigate the
   non-linear response of the structure at the complete damage state, the Peak
   Inter-Story Drift ratio (PISD) was obtained in the different levels of
   collapse intensity from IDA for different stories. The results showed that
   the application of transition story improves the seismic behavior of mixed
   concrete/steel structures and increases Median Collapse Capacity (MCC) in 7,
   13, and 19-story structures up to 24.63%, 19.54%, and 17.99%, respectively.
   In addition, it makes the displacement of the structure more uniform and
   reduces the significant difference of drift in the critical zone.


 * EVALUATION OF SEISMIC PERFORMANCE FACTORS FOR DUAL STEEL SMF-SCBF SYSTEMS
   USING FEMA P695 METHODOLOGY
   
   2022, Soil Dynamics and Earthquake Engineering
   Citation Excerpt :
   
   Nevertheless, failure of braces in high-amplitude cyclic loading due to their
   global and local slenderness and the occurrence of low-cycle fatigue is a
   major drawback of SCBFs, which results in a significant reduction in
   stiffness, damage concentration, increased P-Delta effect, increased residual
   drift, and even structural collapse [3–6]. In order to improve the seismic
   performance of conventional SCBF systems, other bracing systems have been
   proposed by researchers, such as buckling-restrained braced frames [7],
   zipper braced frames [5,8], hexa-braced frames [9], and many more [10–12].
   These improved bracing systems prevent buckling, damage concentration, and
   soft story formation through appropriate distribution of the seismic demand
   among all stories of a building.
   
   Show abstract
   
   The present study evaluates the seismic performance factors of typical
   buildings with dual lateral force resisting system (LFRS) consisting of
   special moment frames (SMFs) combined with special concentrically braced
   frames (SCBFs) using FEMA P695 methodology. For this purpose, six building
   archetypes with 2-, 4-, 8-, 12-, 16- and 20-stories, in two short- and
   long-period performance groups were analyzed by the response spectrum method
   and designed based on the requirements of ASCE 7–16 and AISC 341–16.
   Validated nonlinear models were developed for each building in the OpenSees
   software platform. Then, the static and dynamic behavior of each building was
   investigated up to the collapse stage by nonlinear static analyses and
   nonlinear time history analyses under 44 far-field ground motions of FEMA
   P695. In this investigation, the peak and residual drift ratios, hysteresis
   behavior of the stories during the earthquake, collapse fragility curves, and
   seismic performance parameters (R, Ω, and Cd) were obtained and discussed.
   The results of nonlinear analyses demonstrated that code-compliant long
   period dual SMF-SCBFs (16 stories or higher) performed poorly and can be
   severely damaged in strong earthquakes.


 * DYNAMIC PERFORMANCE OF SOFT-STOREY STRUCTURES WITH GAP ELEMENTS AT
   BEAM-COLUMN JOINTS
   
   2022, Materials Today: Proceedings
   Citation Excerpt :
   
   Rodrigues, H. et al. [4] conducted experiments on RC jacketing for the soft
   storey and found it useful in reducing soft storey effects, but if not
   executed properly, it leads to monolithic behaviour of the element under
   lateral loading. Mashhadiali and Kheyroddin [3] proposed an innovative
   Hexa-braced frame system for the improvement of seismic response. They found
   that its structural behaviour achieved the goal of uniform distribution of
   lateral deformation.
   
   Show abstract
   
   Soft-storey structures are more susceptible to failure for seismic
   vibrations, which is well-known to the world. But, in multi-storey
   apartments, it is a practice to provide a parking place at the ground storey,
   inducing soft-storey effects. In the present study, an effort is made to
   reduce the soft-storey effect on the structure's dynamic performance by
   introducing an arrangement of gap elements at beam-column joints. To that
   context, a numerical assessment is carried out to check the performance of
   the RC structure. The gap element is modelled as a spring either in a beam or
   in a column or both.
   
   The research has been performed in two parts; the first part of the analysis
   is focused on optimizing the gap element pattern. Diagonal compressive struts
   replace the infill walls, and the ground storey is modelled as an open ground
   storey without infill walls, which is considered a soft-storey. From the
   first part, an optimal gap element pattern from five different arbitrary
   patterns is obtained from the results of linear dynamic response spectrum
   analysis, depending upon the comparison of various seismic parameters. The
   second part of the analysis involves checking the effect of the gap element's
   optimized pattern for four levels of structures with vertically varied aspect
   ratios. Response spectrum analysis is performed, and a study is conducted to
   compare the seismic parameters for the four levels. The models with gap
   elements have shown an apparent reduction in storey displacements,
   inter-storey drift ratios, over-turning moments, and increment in storey
   stiffness when compared to the conventional models, thus exhibiting
   constructive outcomes.


 * SEISMIC PERFORMANCE ANALYSIS OF TOWER CRANE MAST TYPES WITH DIFFERENT BRACING
   CONFIGURATIONS
   
   2021, Structures
   Citation Excerpt :
   
   Goel and Tang [33] studied the seismic behavior of a steel building that had
   inverted V-braced frames in order to question the relation between design
   forces and frame ductility. Mashhadiali and Kheyroddin [34] used several
   building models to study on developing a new braced frame type for
   improvement of seismic performance. Nayak et al. [35] investigated the
   seismic performance of X-braced frames and diamond braced frames for 3-story
   building by doing pushover analysis.
   
   Show abstract
   
   Tower cranes, which are composed of a lattice structure, are mostly used to
   transport heavy materials and equipment in construction sites. The delicate
   structure of tower cranes and operating conditions make them very sensitive
   to dynamic excitation such as instant overload for any reason caused by
   transported material or wind and earthquake effect. This study was carried
   out for investigating the seismic performance of tower crane mast types with
   different kinds of bracing configurations. A program with respect to finite
   element method was written in MATLAB which is capable of meshing, modal
   analysis, and transient analysis. While nodal coordinates, material
   information, cross-section data, and the orientation of cross-section for
   each element in the structure and earthquake motion in the time domain are
   used for input data in the program, natural frequencies, mode shapes, mass
   participation factors of the structure and displacement, velocity and
   acceleration response of each node are obtained as output parameters. In this
   research, different types of mast frames were designed in terms of the most
   commonly used bracing configuration and were used in modal and transient
   analysis using two different earthquake motion inputs. The seismic
   performance of each mast type was determined in the time domain in terms of
   dynamic response characteristics.

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RECOMMENDED ARTICLES (6)

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 * Research article
   
   
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 * Research article
   
   
   STRENGTH AND DUCTILITY TYPE RETROFIT OF SOFT-FIRST-STORY RC FRAMES THROUGH
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   Engineering Structures, Volume 186, 2019, pp. 255-269
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 * Research article
   
   
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 * Research article
   
   
   SEISMIC ASSESSMENT FOR TYPICAL SOFT-STOREY REINFORCED CONCRETE STRUCTURES IN
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   International Journal of Disaster Risk Reduction, Volume 41, 2019, Article
   101332
   Show abstract
   
   In this study, a seismic assessment is performed for a low-code high-rise
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