Zipper Braced Frame: A review - Juniper Publishers
Juniper Publishers - Open Access Journal of Engineering
Technology
Abstract
This paper reviews research conducted on zipper
braced frames. Various braces are used worldwide for Residential,
educational and recreational structure for seismic performance. Most of
them resist the earthquake loads effectively, but each and everybrace
are having their own advantages over another than others. Zipper braced
frames are one of the patterns for prove braces in inclined direction
for avoiding horizontal deflection, formation of hinges and flexural
forces due to earthquake forces. Tremendous research has been done on
zipper braced frames by researchers in steel by using software’s and
analytical methods. This paper represents the review on that analysis of
zipper frame.
Keywords: Zipper braced
frames; Chevron braced frames; Inverted braced frames; V-braced frames;
X-braced frames; Pushover analysis, SAP2000Introduction
Structures having the components as beam, columns,
slab to resist lateral, gravity, wind, earthquake load. To resist the
loads structures should be strong enough, so proper bracing should
provide so that structure must effective. There are basically two types
of frame structures: Rigid frame structures and Braced frame structure.
The word rigid means the ability of structure to resist the deformation
and collectively resist the moments are generated due to applied load.
This type of frame structures resists the shear, moment, torsion more
effectively. That’s why this frame is used in high rise structures.
Braced structural frames provides more efficient resistance against the
earthquake and wind forces. Bracing also used to minimise the hinges
that occur in buildings Bracings are generally provided between the
beams and columns.in frame structures. Selections of frame structures
for the high-rise building is due to their versatility and advantages
over the normal traditional load bearing structures. Tremendous braced
frames are available in construction world like Chevron braced frame,
inverted braced frames, zipper braced frames etc. And most of our
buildings were constructed as earthquake resistance, but when we talk
about earthquake resistance structures then our mind totally diverted
towards fully steel structures. Every time it is not possible that we
will construct steel structures, so using that steel frames in concrete
structures may helpful most of the time. Tremendous works done on the
steel structures using braced frames. Concrete is such a wonderful
construction material that an Italian architect one said: “Concrete has
liberated us from the rectangle”.Bracing element in structural system
plays vital role in structural behavior during earthquake. For analysis
of structures various software, methods were applied in field of
structural analysis
such as time history analysis, response spectra method, push over
analysys.one of this method is push over analysis which were using more
in structural fields for r.c.frames.
Previous studies on zipper braced frames
Evolution of braces was taken since long
ago.Ductility behaviour shown by chuang-sheng yang [1] is a pioneering
work in field of braced frames using the method of pushover analysis in
sac. He proposes a design methodology of zipper braced frames aimed at
achieving a good ductile behaviour.it is observed that nonlinear dynamic
analysis under an ensemble of 2% in 50-year pulse type near fault
ground motions. From various braces one of the frame is concentrically
braced frames which are prone to excessive inters Torey drift, so to
overcome this Lucia Tirca et al. [2] apply the innovative idea of zipper
to existing frame. A Kadid et al. [3] proposes the seismic behavior of
RC buildings strengthened with different types of steel braces,
X-braced, inverted V braced, ZX braced, and Zipper braced. Pourbaba M et
al. [4] analysis the Behaviour of zipper braced frame was investigate
and compared with concentrically braced frames. Earlier concentrically
braced frames were used, but the frame behaviour is not so effective in
terms of lateral loads, it has poor mechanism with a negligible
ductility, so they introduced new members as a structural element called
zipper to overcome the problems of concentrically braced frames
problems as a result no sudden loss of bracing forces which shows the
behaviour of ductility (Figure 1). Similarly, Nouri et.al investigated
the limitations of concentric braced frames subjected to seismic loading
and thereby proposes the another bracing to overcome the problem of
vertical unbalances forces in case of chevron braced frames [5].
Comparisons of different Braced Frames
Various researches have been done by comparing various
frames with each other with the result of showing excellent
behaviour of zipper braces frames as compared to others.
The seismic behaviour of different bracings systems in high
rise 2-D steel buildings were carried by Dhanraj M Patil [6] to
access the structural performance on different bracing systems.
According to that zipper braced frames shows higher capacity
than other buildings. Inverted v which is also known as the
chevron braced frames have been seen as being prone to soft
storey responses once the braces buckles under earthquake
loading.to overcome this chevron braced frames problems
zipper column were proposed which redistribute the inelastic
demand over the height of the building [7]. Zipper frames are
intended to improve on the behaviour of conventional inverted
-v braced frames which exhibits poor performance which arises
from the early buckling of the lower storey braces, for remedy
on this Roberto Leon et al. [8] introduces the hat truss at top
of building which prevents the formation of an overall collapse
mechanism.in order to investigate the seismic performance of
zipper frames with respect to chevron braced frames Haifeng Yu
[9] examined the four parameters: inters Torey drifts, maximum
axial deformations, vertical downward deformations, indicating
zipper frames is better than chevron braced frames.it is also found
that the design method of the zipper columns is conservative
and a more reasonable [9,10]. There has been a considerable
improvement in the field of replaceable links in the past decade.
For steel eccentrically, braced frames Sina Kazemzadeh review
the research for system level response [11]. Due to buckling’s
of compressive braces in inverted-chevron braced frames, an
unbalanced vertical forces has been applied also shear capacity
of the frame decreases.to overcome this shortcomings, S Naeimi
[12] apply zipper to counteract the unbalanced forces.as a result
by applying zipper base shear lateral displacement behaviour
is enhanced. Simultaneously it increases the energy absorption
capacity of the structures. Vertical displacement of the mid span
point of the braced bay beam is considerably reduced. Zahrai
[13] has conducted a thorough research on shear panel system
proposing the method for modifications of V-braced framesas a
result v-braced frames offers high levels of shear ductility and
a great deal of energy dissipation. For improvements over it
zipper was applied which connect the midpoint of shear links
in all stories.by imposing acceleration on the structures and
conducting time history analysis it was observed that zipper
show stronger tendency to form shear links. Chevron braced
frames have high hardness and weak buckling behaviour for
preventing this condition zipper placed between the beams
[14] as a result hysteresis absorbed energy in zipper frames are
uniform on other side chevron has un-uniform hysteresis curves.
Suspended zipper braced frames are a modified configuration
of chevron braced frames in which zipper columns are added
between storey beams and a hat truss is attached between top
surfaces in order to redistribute the unbalanced vertical forces
[15].
Zipper braced frames
Most of the braces have problems such as weakness, soft
storey mechanism. There are two important dimensions to the
behaviour of bracings energy absorption/dissipation capacity:
ductility and failure behavior [16] According to the previous
studies ductility is the ability of the constituents parts, to bear
non-elastic deformations, so to counteract such problems
wonderful work done by A. Mirza Gol Tabar et al. [16] for analysis
they consider a chevron braced frames and turn it into zipper
frames.as a result amount of energy absorption and dissipation
of zipper braced frames is more than chevron bracing braces.
Zipper braced frames subjected to seismic excitations [17].
Buckling of compressive braces results in a localization of the
failures and loss of the lateral resistances.one of the ways to
overcome this problem is to use a vertical structural element
at the beam mid-span from the second to stories above called
zipper strut [18]. To solve the traditional problems associated
with conventional braced frames, a new class of bracing systems
known as zipper frames were developed and tested. The
research developed analytical tools and methodologies to allow
practicing engineers to determine potential benefits of a variety
of applications of zipper frames [19].
Seismic effects of zipper brace frame
Seismic performances of braced frames are governed by the
buckling of brace members in compression. Structures should
be strong enough to resist the seismic effect. For seismic effect
of zipper braced frame some researchers had done research on
the frames with respect to the seismic effect. Jay Shen et al. [20]
studied on the seismic demands of two-storey x braced frames
as a result required seismic strength on the brace intersected
beams in two-storey x-braced frames was lower than the
inverted v-braced frames. The structural responses of frames
are studied in terms of capacity curve, drift ratio, global damage
index, base shear, storey displacements, roof displacements
[20] choosing an appropriate lateral force resisting systems
has a significant effect on a performance of the structures. The
results showed a good improvement in the seismic resistance
of frames with the incorporation of bracings. For zipper braced
frames ductility is more compared to others frame, but Rahimi
R [21] recommended the suspended zipper braced frames to be used in high rise building, however for the low and midrise
building it was recommended to use zipper frames due to
economic efficiency. Zipper columns bracing system are more
ductile system and efficient than any other [22]. Razavi M et al.
[23] investigates seismic behaviour of zipper braced frames with
different pre-stressed ratio. Moreover, it was observed that the
use of prestressed cables enhances the seismic performances of
zipper braced frames.
Analysis of frames by applying push over method in sap2000
Simplified method for nonlinear static analysis of buildings
subjected to monotonically increasing horizontal loading {push
over analysis}. Vojovo et al. [24] applied the pushover analysis of
a seven-storied reinforced concrete frame wall building.
Push over technique based on the conventional displacement
method of elastic analysis [25]. Mostly hinges formed in beam
than in column [26], push over analysis has been developed
over past 20 years ago and has become simple in analysis using
sap2000,which also considers post elastic behavior [26-28].
Traditional pushover analysis is widely used for the seismic
performance evaluation process. But improved pushover
overcome the limitations of traditional pushover procedures
[29]. By conducting the push over analysis weak zones in the
structures can identified. For achieving the objective of push over
analysis Abhijeet A Maske et al. [30] work on 5 and 12 stories
buildings. According to them push over analysis were relatively
simple ways to explore the non-linear behaviour of buildings.
Various structures were analysed using push over analysis [31-
40].
Conclusion
Results of experimental researches shows that zipper bracing
systems performed better results as compared to normal frame
structure. Performance results is depending on the type and size
of the cross sections and also enhances the global capacity of
building in terms of strength, deformation, ductility [3,8]. The
design methods of zipper braced frames are conservative and a
more reasonable design [7,9]. Distribution of inters Torey drift
over the structure height is uniform using zipper braced frames
[22,29]. Furthermore, the shear capacity of the frames which was
obtained through hysteric analysis shows that the ultimate shear
capacity of zipper strut frames 2% higher than that of regular
eccentrically braced frames [23].
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