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Comparitive study of different bracing systems on High rise steel

structures
Sushant Basnet a , Rajan Suwal b ,
a Department of Civil Engineering, IOE, Thapathali Campus, Tribhuvan University, Nepal
b ADepartment of Civil Engineering, Pulchowk Campus, IOE, Thapathali Campus, Tribhuvan University, Nepal
a sushant.basnet04@gmail.com , b rajan suwal@ioe.edu.np ,

Abstract
In the context of Nepal, steel structures are constructing for residential as well as commercial purpose rapidly.
With the rapid development of steel structure, the safer design methodology is required for safer and economical
design and construction in a country located at earthquake prone region. The most common performance
improvement methods for buildings to resist lateral load is to strengthen and stiffen the system. Bracing
system are commonly used as the effective structural system. In the present study, the seismic performance of
moment resisting steel frame structure based on different parameters such as roof displacement, storey drift
and base shear are evaluated. The project shows the bare frame structure and the braced frame structure with
different types of bracings and the bracing configuration for three types of structure i.e 5 story, 10 story and
15 story steel structure modeled in E-tabs software. Equivalent Static Method (ESM) and Modal Response
spectrum Method (MRSM) are performed to determine the seismic response of the structure and hence the
parameters are compared and the results are concluded as with the use of bracings the roof displacement
and storey drifts can be reduced. X-bracing and Chevron bracing is most suitable bracing configuration as it
shows the minimum roof displacement and storey drifts in compare to diagonal and V-bracing.
Keywords
Bracing, Steel structures, Displacement, Base shear

1. Introduction seismic design of structures is essential for the safety

of structure in their life period. Most of the buildings
Nepal is considered as the earthquake prone country are damaged and collapsed during these earthquakes
since the geography of Nepal is delineated by due to improper design and construction and
Himalayan Frontal Thrust (HFT) in the south and non-engineering. Conventional buildings are designed
Main Boundary Thrust (MBT) in the north and lies in according to the codal provision but the high rise
active seismic zone. Research shows the Indian and building needs to resist the large forces due to
Tibetan tectonic plates are converging towards each earthquake and wind load. In case of high-rise
other causing the big earthquakes in Nepal in the near building there is a need to resist lots of forces both by
future. Nepal lies just above the junction of two wind as well as seismic load which may be
tectonics, Nepal is vulnerable in accordance to sufficiently fulfilled by the conventional building
seismic activity. There are number of active faults configurations. Hence to resist the lateral forces, there
throughout the country and hence there may be large needs to incorporate bracing systems. The world
number of greater earthquakes. The history shows the population is growing so rapidly in recent years that
big earthquakes in Nepal causing the death of number there has been resurgence of high-rise constructions in
of lives and the economic loss. Some of the historic the major cities. High rise buildings have become a
earthquakes are 1833 Nepal/Bihar Earthquake of 8 trend and moreover, they have paved the way to world
magnitude, 1934 Nepal/Bihar earthquake of 8 competition in constructing tall buildings to exhibit
magnitude, 1988 Kathmandu/Bihar earthquake of 6.9 the symbol of power and technology owned by its
magnitude and 2015 Gorkha earthquake of magnitude population. With the lesser area for construction in
7.8. in accordance to fatality, damage and economic Kathmandu valley and the rapidly growing population,
loss. Since Nepal lies in earthquake prone region,

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Comparitive study of different bracing systems on High rise steel structures

there is demand of high rise structures. Steel frame operational efficiency of a rigid frame by virtually
building are increased in construction due to its light eliminating column and beam bending factors. This is
weight and fast construction in the cities area and can achieved by adding truss elements such as diagonals
be the best alternative for RC framed structures. Thus, between the floor system. The diagonals now absorb
the use of the braces in the steel frame building may the shear force, mostly in the axial action, allowing
lead to possibility of reliable taller buildings and may almost pure cantilever behavior. All parts are
help the country for the high-rise building subjected to axial loads, creating an efficient structural
construction and cope with the insecurity of the multi system. There are different types of robust frame
storey steel buildings from the greater earthquakes systems (Designing Buildings Ltd., 2020):
that are occurring and are expected to occur to make
the building safer for the future. Now a days, the
interest for steel building has been seen increasing so, 1.2.1 Single diagonal Bracing
the steel structure analysis is needed in a Trusses are formed by placing diagonal structural
comparatively extensive way. It is not possible to members on rectangular areas of the structural frame,
prevent earthquake from occurring, but safety which help stabilize the frame. If a single clamp is
measures intended to mitigate the severity of its used, it must withstand sufficient tension and pressure.
effects can save lives and properties. This research
aims to incorporate the members in the buildings
using different bracing members to analyze the 1.2.2 Cross Bracing
improvement in the seismic performance of MRF.
Cross braces (or X braces) use two diagonal members
that cross each other. They must resist tension only,
1.1 Rigid or moment resisting frame system one support at a time, resisting lateral forces dependent
on the direction of the load. Thanks to this, steel cables
Rigid or moment-resisting frame systems to withstand
can also be used for cross supports. However, cross
lateral and vertical loads have been in practice for
bracing on the exterior of the building can interfere
many years. Rigid or moment-resisting frames are
with the placement and operation of window openings.
structures with a traditional beam-column frame. The
This also leads to greater bending of the floor beams.
joints in these frames are considered rigid because
the connection between the beam and the column is
supposed to be rigid enough to retain the initial angles 1.2.3 K-Bracing
of the intersecting components almost constant. Due to
the monolithic behavior and thus the inherent stiffness K-braces are connected to the columns at mid-height.
of the joint, the rigid frame is ideal for RC buildings. This frame has greater flexibility in the creation of
Steel buildings also use a rigid frame, although it is facade openings and results in the least bending of the
difficult, but in certain cases. Rigid frames carry the floor beams. The use of K-plugs in seismic areas is
gravity load of the floor system. The floors also act as generally not recommended because the column can
horizontal diaphragms that transmit lateral forces to be damaged if the compression brace buckles.
the beams and columns. In addition, beams or girders
withstand large moments and shears at the ends of their
1.2.4 V-Bracing and Inverted V-Bracing or Chevron
lengths, which in turn are transferred to the column
Bracing
system. In a rigid frame, strength and stiffness are
directly proportional to beam and column sizes and Two V-shaped diagonal pieces extend down from the
inversely proportional to column spacing. top two corners of the horizontal piece and meet in the
middle of the bottom horizontal piece. An inverted
V-bracket, also known as a shaft brace, consists of two
1.2 Braced Frame System
members that meet in the middle of the upper
[1] Because the rigid frames of the structure alone are horizontal section. Both systems can significantly
not sufficient to withstand lateral forces such as reduce the flexural capacity of a compression support
earthquakes and strong wind forces, because the shear so that it is less than the stress capacity of a tension
component of the deflection caused by the bending of support. This may mean that when the supports reach
the columns and beams causes the building to drift their resistance, the load must be resisted by the
excessively. A stiffened frame attempts to improve the bending of the horizontal beam instead.

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1.2.5 Eccentric Bracing Modeling To achieve the main objectives of the

study, first, the manual design of the building
Eccentric bracing is often used in seismic areas. It is
structural members (Beams and Columns) were done
similar to a V-clamp, but the support members do not
to estimate the initial member size. Then linear static
meet in the center. This means that there is space
and dynamic analysis were performed to check for the
between them in the connection above. The
sufficiency of the strength using the Etabs 20 software
supporting parts connect to separate points in the
(Computers and Structures Inc.) after which the
horizontal beams. This is because the ”bond” between
dimensions of the members of the building was
the stiffeners absorbs the energy of the seismic action
changed as required till the model was found safe for
through plastic deformation. Eccentric single
the design in the analysis. ESM and RSA were
diagonals can also be used to support the frame. This
performed to find the maximum roof displacement,
type of verification is not used as a limitation in the
base shear and story drift. The performed tasks to
study.
complete the research are explained in the following
sections of this chapter. For the seismic performance
assessment of building, the model was selected having
regular plan used as horizontally regular structure. As
the building taken for the study was a hypothetical
building, preliminary design of the building
components was carried out to define the sections and
Figure 1: Bracing Types later on for study the linear response of the structures,
the model of structure was developed. The responses
of the structures were studied under the load cases
2. Methodology dead, live and lateral loads with separate load cases.
To assess the performance of the selected structures,
the buildings were modelled in finite element software
Etabs 20.

Figure 3: 5 storey Bare Frame Building Model

Figure 2: Flowchart for Methodology

Figure 2 above shows the complete methodology to

accomplish the research.

Selection of Model In context of Nepal, high rise

steel structures are not constructed in many numbers
hence a hypothetical building of 15 story, 10 storey
and 5 storey with steel moment resisting frame was
taken and modeled. The building is assumed to be
of commercial purpose and the further details of the
structure are tabulated in further sections. Figure 4: 10 storey Bare Frame Building Model

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Comparitive study of different bracing systems on High rise steel structures

Table 2: Material Properties

Desciption Data
Steel Fe250
Concrete M20
Modulus of Elasticity 22360 N/mm2

Table 3: Structural Data

Desciption Data
Roof Live Load 1.5 kN/m2
Live Load 4 kN/m2
Figure 5: 15 storey Bare Frame Building Model
wall Load 4kN/m
parapet wall load 2 kN/m
Description of Building The building taken for the Seismic Load Calculation NBC 105:2020 [4]
research is a simple steel Moment Resisting Frame Location of Building Kathmandu
structure. The structures taken into consideration are Soil Type D type
fifteen-story, ten story and five story with story height Importance Factor 1.25
of 3m. The floor consists of composite deck. The total Seismic Zoning Factor 0.35
height of the building is 45m,30m and 15m for
15-storey, 10 story, 5 story respectively with similar
floor characteristics. The building is considered 3. Results and Discussion
having walls with glass panel at interior and exterior
parts. The beams, columns and deck were taken with Different 12 number of models were prepared with
the elements that was present in the software itself. bare frame, X-bracing, Diagonal bracing, Chevron
Primary and secondary beams were taken by bracing and V-bracing for each 5 story, 10 story, and
importing the ISWB and ISMB sections from the 15 story steel structures. The sample configurations for
Indian standard code.[2] The columns were designed 5 storey structure are as shown in figure below. Same
in section designer taking I-section and plates [3] configurations were made for 10 storey and 15 storey
interconnected with web similar to I section crossing storey too which is not shown in paper.
each other as the single section was not able to resist
the load on the building. The deck material was used
75mm thick deck considering the load to transfer from
slab to the secondary beam then to the primary beam
and finally to the column transferring to the
foundation. The details of the building are given
below:

Table 1: Description of Building

Description Data
Building Type Steel MRF Building Frame
Length 25m (5 bays @ 5m)
Width 15m (3 bays @ 5m)
Storey 5,10,15
Height 15,30,45 Figure 6: Different Configurations of Bracing
Storey Height 3m Compared for Displacement
Column I-section and plates
Beam ISWB600,500,400 Response spectrum method is used for the analysis
Secondary Beam ISMB225 and hence the displacement and drift are calculated
Deck Slab Depth 87.5mm for 5 storey, 10 storey and 15 storey structures. For
Deck Rib Depth 75 mm simplicity only the Y-direction responses are

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evaluated and compared for all different • 10 storey Building

configurations for 5 storey buildings whereas the
optimal bracing configuration for each bracing types Table 5: Maximum Roof Displacement and Base
are only studied in case of 10 storey and 15 storey Shear Comparison for 5 storey Building
structures.
Model Displacement(mm) percent change
BF 118.03 -
• 5 storey Buildings
CB-2 63.67 46.05
DB-1 77.21 34.58
Table 4: Maximum Roof Displacement and Base
VB-2 72.469 38.6
Shear Comparison for 5 storey Building
XB-2 63.696 46.03
Model Displacement(mm) percent change
BF 36.88 -
CB-2 13.66 62.45
DB-1 18.63 48.79
VB-2 16.37 55
XB-2 13.38 63

Figure 10: Displacement graph for Seismic

Coefficient and Response Spectrum Methods at Y
direction

Figure 7: Displacement graph for Seismic Coefficient

and Response Spectrum Methods at Y direction

Figure 11: Storey drift graph for Seismic Coefficient

and Response Spectrum Methods at Y direction

Figure 8: Storey drift graph for Seismic Coefficient

and Response Spectrum Methods at Y direction

Figure 12: Base Shear

Figure 9: Base Shear • 15 storey Building

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Comparitive study of different bracing systems on High rise steel structures

Table 6: Maximum Roof Displacement and Base comparison for different bracing system for 5 storey
Shear Comparison for 5 storey Building buildings and for 10 storey and 15 storey structure
Model Displacement(mm) percent change similar pattern of curve can be seen.
BF 157.35 -
CB-2 125.66 20.13
DB-1 141.32 10.18
VB-2 135.37 13.97
XB-2 125.5 20.24

Figure 16: Drift Ratio comparison

The curve above shows the reduction in storey drift

using the bracing system and the x-brace and chevron
braces shows the best result among the bracing system
in terms of story drift ratio.

Figure 13: Displacement graph for Seismic

4. Conclusion
Coefficient and Response Spectrum Methods at Y
direction Different configuration of bracing system for 5 storey,
10 storey, and 15 storey steel structures with and
without bracing systems were analysed and following
conclusions are obtained.

• X-bracing system and Chevron bracing system

are best in terms of reduction in maximum roof
displacement and maximum storey drift ratio.

• With the increase in storey height the efficiency

Figure 14: Storey drift graph for Seismic Coefficient of bracing system gets decreased and hence for
and Response Spectrum Methods at Y direction high rise structure bracing with energy
dissipating devices can be installed for better
performance.

• The base shear increment is less for chevron

bracing than x-bracing while the maximum roof
dispalcement is also nearly equal, chevron
bracing system can be used as best option as
bracing system.

Figure 15: Base Shear 5. Future Enhancement

The study can be further extended using eccentric
For the concentric braces, the ductility factor is bracing system and may be compared with concentric
reduced from 4 to 3 and over strength factor is bracing systems. Moreover, now a days the dampers
reduced from 1.5 to 1.3 according to NBC code which are being installed for energy dissipation and hence
makes the base shear coefficient to increase. the dampers may be used. The comparison of
The figure below shows the storey drift ratio bracings system and damping system can be studied.

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References [3] SK Duggal. Limit State Design of Steel Structures.

McGraw Hill, 3rd edition, 2010.
[1] Gayatri Thakre and A.R. Kambekar. Effect of steel
bracings in steel structure. 2017.
[2] Indian. IS code part I,II,III. [4] Nepal. Nepal Building Code 105:2020.