LATERAL LOAD RESISTING SYSTEMS FOR
MULTI-STOREY BUILDINGS
Yogendra Singh
Professor, Department of Earthquake Engineering, IIT Roorkee
1. INTRODUCTION
Buildings are subjected to two types of loads: (i) Vertical load due to gravity, and
(ii) Lateral load due to earthquake and wind. The structural system of the
building has to cater for both the types of load. The structural system of a building
may also be visualised as consisting of two components (i) Horizontal framing
system, consisting of slab and beams, which is primarily responsible for transfer
of vertical load to the vertical framing system and (ii) Vertical framing system,
consisting of beams and columns, which is primarily responsible for transfer of
lateral load to foundation. However, the two components work in conjunction
with each other.
Fig. 1 Lateral Load Resistance of Traditional Buildings
The old practice before 1960s had been to design buildings primarily for vertical
loading and to check the adequacy of the structure for safety against lateral loads
in a cursory manner. It has been established now that the design of a multi-storey
building is governed by lateral loads and it should be prime concern of the
designer to provide adequately safe structure against lateral loads. Further, the
old buildings were having substantial non-structural masonry walls, partitions
and connected staircase. These provided a significant safety margin against lateral
loading. The modern buildings are heaving light curtain walls, lightweight
flexible partitions along with high strength concrete and steel reinforcement. This
reduces the safety margins provided by non-structural components. Figs. 1and 2
Lateral Load Resisting Systems / 1
show the schematic plot of resistance of various components against lateral
loading in a traditional building and a modern building, respectively. It is seen
that the non-structural components in the traditional building have a large
strength reserve and a lateral load many times the design lateral load could
exhaust this strength reserve before yielding of the frame takes place. On the other
hand, in a modern building, the lateral resistance of the building is only slightly
larger than the lateral resistance of the frame alone and no reserve capacity
against lateral load is available.
Fig. 2 Lateral Load Resistance of Modern Buildings
A number of structural systems have been developed in the last century for
optimal transfer of lateral load. The ideal design is that in which no premium is
there for lateral load i.e. the stresses due to lateral loads are accommodated within
the 33% increase in the permissible stresses. This design may not be possible but
our aim is to reduce the premium as far as possible. Fig. 3 gives the economical
height ranges for various types of structural systems in steel and concrete,
respectively.
2. LATERAL LOAD RESISTING STRUCTURAL SYSTEMS
A number of structural systems to cater the varying architectural needs are
available in steel as well as concrete. Nowadays, computers are widely used for
analysis of structures, as computers and software are cheaply available. For
proper design of structure an understanding of the behaviour of the structural
system is necessary. Otherwise, the designer is bound to make mistakes in the
modelling of the structure and may have erroneous designs, whatever
sophisticated software he may be using. The understanding of the behaviour is
also necessary for the execution engineer, so that he can understand the critical
actions in the structure and can take special precautions in the construction. The
following sections present an overview of the behaviour of various structural
systems under lateral loading.
Lateral Load Resisting Systems / 2
Fig. 3 Comparison of Structural Systems in Concrete
3. FRAME STRUCTURES
The frames derive their lateral load resistance from the rigidity of connections
between beams and columns. The behaviour of frames is straightforward and
their computer modelling is simple. A number of software are available for
analysis of frame structures. The frames are infilled by masonry panels for the
purpose of partition. These partitions are considered to be non-structural and
their contribution to lateral load resistance is generally ignored. The behaviour of
these panels is complex. These act as diagonal bracing members (Fig. 4) before
failing and falling apart from the frame. In many cases, under severe shaking due
to earthquake these fail and fall apart before the frame is subjected to the ultimate
load and that is why their contribution in lateral load resistance is not considered.
However, presence of masonry panels alters the dynamic characteristics of frames
and the behaviour is particularly complex when the ground storey of the frame
buildings does not have masonry infills for the purpose of parking. Such
buildings behave as soft ground storey. There is a sudden change in the stiffness
of the building at the first floor level. This increases the storey drift and ductility
demand of the ground storey tremendously and may lead to failure of the ground
storey due to insufficient ductility.
In such situation a safe approach to design the buildings with open ground storey
for parking purpose is to increase the stiffness and ductility of the ground storey
by bigger sections of beams and columns and closely spaced stirrups. Further. The
Lateral Load Resisting Systems / 3
analysis of the frames with masonry infills should be made in both the conditions:
(i) considering the contribution of infills, and (ii) ignoring the contribution of
infills.
Fig. 4. Frame with Masonry Infills
Infills also result in extremely high shear force in columns due to horizontal
component of the force in equivalent diagonal strut of masonry (Fig. 5). This has
resulted in failure of columns in infilled frames in many past earthquakes (Fig. 6).
Fig. 5. Infills acting as diagonal struts result in excessive shear force in columns
Lateral Load Resisting Systems / 4
Fig. 6. Failure of columns due to high shear force from infills
In case of RC frame buildings the floor slabs are usually casted monolithically
with the frames. The floor slabs are quite rigid in their plane and are responsible
for distribution of lateral load among the various frames. This action should be
properly modelled in the space frame model. The modelling is particularly
important in buildings having large differences in lateral stiffness of various
lateral load resisting components and asymmetric buildings.
Fig. 7. Typical Shear Wall arrangements in buildings
Lateral Load Resisting Systems / 5
4. SHEAR WALL STRUCTURES
Shear wall is a slender vertical cantilever resisting the lateral load with or without
frames. Fig. 7 shows some typical arrangements of shear walls in buildings. Fig. 8
shows a building consisting of shear walls as lateral load and vertical load
resisting elements.
Fig. 8. Shear wall building
Fig. 9. Deformation Modes of Frame-Shear Wall System
Lateral Load Resisting Systems / 6
The behaviour of a shear wall is opposite to what its name suggests. A shear wall
primarily resists the lateral load in flexure with very little shear deformations. The
deformation of a shear wall is different than that of a frame (Fig. 9). Therefore,
when used in conjunction with frame, shear wall results in complex interaction
with the resultant lateral load on the shear wall and frame varying in a complex
manner along the height.
Fig. 10. Coupled Shear Wall
Some times a shear wall is used with openings for passage or windows. Such
shear walls behave as two shear walls coupled through the portion of the shear
wall between the openings (Fig. 10). Depending on the size of openings, the
behaviour of the shear wall varies. The efficiency of the system depends of the
strength of the coupling beam. The design of coupling beams in a coupled shear
wall requires attention. The beams are subjected to very high shear stresses. In
cases the shear walls are coupled only by slabs, the slabs should have provisions
for this shear.
5. BRACED FRAME SYSTEMS
A variety of braced frame systems are possible in structural steel as shown in Fig.
11. They are briefly introduced as follows:
[A] Concentrically Braced Frames
Concentrically braced frames (CBF) consist of beams columns and braces which
are connected with pinned connections. Thus, the members can be said to be
Lateral Load Resisting Systems / 7
arranged to form a vertical truss. They resist lateral force by this truss action and
columns and develop ductility by inelastic action in braces experiencing tension.
They have high elastic stiffness but low ductility as the braces in compression can
buckle which is a brittle failure.
BRACE
Moment Resisting Frame
Concentrically Braced Frame
LINK
BRACE
BUCKLING
RESTRAINED
BRACE
Eccentrically Braced Frame
Buckling Restrained Braced
Fig. 11. Different Steel Framing Systems
[B] Eccentrically Braced Frames
“An eccentrically braced frame (EBF) is a type of steel framing system including
beams, columns and braces, where these members are arranged in a manner
where at least one end of each brace is connected to isolate a segment of the beam
called a link” (Dara, 2010). They resist lateral force by combination of frame and
truss action and develop ductility by flexural and shear yielding in links.
Lateral Load Resisting Systems / 8
[C] Buckling Restrained Braced Frames
Buckling restrained braced frames (BRBF) are similar to CBF in construction. The
only difference being a special type of buckling braced brace is provided which
prohibits brace to buckle under compression. Thus ductility is developed by
yielding in both tension and compression. This system combines high elastic
stiffness with high ductility.
The moment resisting frames consist of rigidly connected beams and columns
which develop ductility by formation of flexural hinges. The flexural stiffness of
beams and columns is the major source of lateral stiffness in MRF. The rigid body
deformed shape of a MRF showing development of plastic hinges after
application of lateral load is shown in Fig. 12. Experimentally obtained lateral
load versus displacement plot for a MRF is given in Fig. 13 (Wakabayashi M. et al.
1974, as quoted from Popov and Engelhardt, 1988). The figure shows a fully
developed hysteresis loop thus demonstrating their ability to show high ductile
behaviour. In the curve, H represents the applied lateral load measured in tonnes.
LOAD
MOMENT HINGE
Fig. 12. Deformed Shape for MRF (AISC, 2007)
Fig. 13. Lateral load versus displacement for MRF (Wakabayashi M. et al. 1974, as quoted from
Popov and Engelhardt, 1988)
Lateral Load Resisting Systems / 9
5.1 Concentrically Braced Frames
Concentrically braced frames consist of diagonal brace members pinned to beam
column junction, such that only axial force is developed in them. The braces
impart high elastic stiffness thus allowing designing the structure with smaller
drift. The primary mechanism for development of ductility in CBF is axial
yielding under tensile force in the bracing members. Since axial ductility is lower
than flexural ductility, the ductility level of CBF is lower than that of MRF.
However, earthquake induced vibrations are reversible in nature. Thus, when a
brace is subjected to tensile forces, at the same time, an oppositely placed brace
will be subjected to compressive forces. These braces under compression buckle
prematurely thus producing a brittle failure in frame.
LOAD
BUCKLING OF
COMPRESSIVE
BRACE
AXIAL HINGE IN
TENSILE BRACE
Fig. 14. Deformed Shape for CBF (AISC, 2007)
Fig. 15. Lateral load versus displacement for CBF (Wakabayashi M. et al. 1974, as quoted from
Popov and Engelhardt, 1988)
Lateral Load Resisting Systems / 10
The formation of axial hinge in tension and buckling of brace under compression
is can be seen from the rigid body deformed shape of a CBF as given in Fig. 14.
Experimentally obtained lateral load versus displacement plot for a CBF
(Wakabayashi M. et al. 1974, as quoted from Popov and Engelhardt, 1988) is given
in Fig. 15. As evident from figure, the hysteresis loop is pinched due to
compressive buckling of frames. Also, the plot deteriorates as the cyclic load
progresses thus demonstrating poor energy dissipation capacity of CBF.
5.2 Eccentrically Braced Frames
“Eccentrically Braced Frames (EBFs) are a lateral load resisting system for steel
building that can be considered as a hybrid between conventional Moment
Resisting Frames (MRFs) and Concentrically Braced Frames (CBFs)” (Popov &
Engelhardt, 1988). Properly designed EBF display high levels of ductility as by
MRF and at the same time possesses high elastic stiffness similar to CBF. Also EBF
display better architectural versatility than CBF to provide space for openings.
When an EBF is subject to lateral load, the axial force induced in the braces is
transferred in the form of high levels of shear and bending moment in the link.
However, the link is usually subjected to typically low levels of axial force.
Consequently, links will normally experience shear and/or flexural yielding
during an earthquake and undergo formation of shear or flexure plastic hinges
(Dara, 2010). Thus in EBF links, by undergoing plastic deformations, allow
dissipating seismic energy and act as a fuse to prevent damage in other parts of
the frame. Other members of an EBF, including the braces, the columns and the
beams segments outside of the links are intended to remain essentially elastic
during an earthquake. The link undergoing shear yielding when lateral load is
applied is can be seen from the rigid body deformed shape for an EBF as
presented in Fig. 16. Experimentally obtained lateral load versus displacement
plot for an EBF (Wakabayashi M. et al. 1974, as quoted from Popov and
Engelhardt, 1988) is given in Fig. 17. The figure shows a full and stable hysteresis
loop for EBF with good energy dissipation capacity similar to MRF.
LOAD
SHEAR
YIELDING
IN LINK
Lateral Load Resisting Systems / 11
Fig. 16. Deformed Shape for EBF (AISC, 2007)
Fig. 17. Lateral load versus displacement for EBF (Wakabayashi M. et al. 1974, as quoted from
Popov and Engelhardt, 1988)
LINK
LINK
LINK
LINK
LINK
LINK
Fig. 18. Various Possible Configurations of Eccentrically Braced Frames (AISC, 2007)
Lateral Load Resisting Systems / 12
Different configurations of EBF are possible. The links can be provided either
horizontally or vertically. Also, the position of horizontal link can be varied so
that link is provided on end of beam and has a column on one face and a brace on
the other face or link is provided in centre of beam span and has braces on both
the faces. The different configurations are shown in Fig. 18.
Extensive research on behaviour of eccentrically braced frames and their design
was started in latter half of 1970s. Their widespread adoption in construction
industry is not very much old. In the past 2-3 decades only, the use of EBF
buildings in construction has gained momentum. However, EBF buildings are yet
to experience a significant exposure to any seismic event across the globe.
It was only in Christchurch Earthquake of 2010-2011 that an EBF building was
subjected to seismic loading. The Christchurch Earthquake consisted of M7.1
Darfield Earthquake on 04 September 2010, followed by M6.3 Lyttelton
Earthquake on 22 February, 2011 and finally strong aftershocks in June and
December, 2011. The 22 February earthquake was the most intense to impact
eccentrically braced framed systems recorded to date worldwide (Gardiner et al.,
2013). The damage observed in EBF is classified in following sections:
Preliminary field observations conducted post February, 2011 earthquake on some
selected steel structures (Bruneau et al., 2011) reveal minor damage to links of
eccentrically braced frames. Reconnaissance was done on the following two multistorey steel buildings:
Name of
building
Club Tower
Building
Pacific
Residential
Tower
No. of storeys
12
22
SeismicResisting
System
Floor System
Year of
completion
EBF and MRF
Composite
Deck and Steel
Beams
2009
EBF and MRF
Composite
Deck and Steel
Beams
2010
The Club Tower Building has eccentrically braced frames located on three sides of
an elevator core eccentrically located closer to the west side of the building, and a
ductile moment resisting frame (DMRF) on the east façade (Bruneau et al., 2011).
The steel frame is supported on a concrete pedestal from the basement to the first
storey, and foundations consist of 1.6m thick raft slab. The EBFs were designed in
compliance with the NZS 3404: Part1-1997 (Standards New Zealand, 2007)
provisions using a ductility factor of upto 4, a level of link deformations that
would correspond to significant shear deformations in the link. Only minimal
flaking of paint was observed in the links that were architecturally accessible.
The Pacific Residential Tower consists of perimeter EBF upto six floors (one on
each building face), shifting to EBFs around the elevator core from that level, with
a transfer slab designed to horizontally distribute the seismic loads at that
Lateral Load Resisting Systems / 13
transition point (Bruneau et al., 2011). In addition there are MRFs at the two
uppermost levels. The building is founded on large reinforced concrete
foundation beams and a mixture of 1500 diameter concrete caisson and 900
diameter helix steel screw piles, typically 12 metres in length. The building was
designed in 2007 using NZS 1170.5 (Standards New Zealand, 2004) (utilizing a
hazard factor, Z = 0.22 for Christchurch and design structural ductility factor, µ =
3) and NZS 3404: Part1-1997 (Standards New Zealand, 2007) (Gardiner et al.,
2013).
Initial observations revealed paint flaking and residual shear link deformations in
links of levels upto six (Bruneau et al., 2011). Few doors of hotel rooms along the
corridor at level six could not close suggesting greater residual deformations at
that level. Most of the links were hidden by architectural finishes. Absence of
damage to these finishes suggested no inelastic damage to the links. However, in
August 2011 a fractured link was detected at the north-western corner of the
building at the underside of Level 6 as shown in Figs. 19 and 20.
After detection of above fracture, detailed inspections for all 126 active links were
undertaken, but it did not reveal any further fracture in links. No damage or sign
of any permanent displacement was observed to steel MRFs, columns, braces or
welded or bolted connections. Material properties of active links with permanent
rotation of 1.5% from horizontal were tested using Magnetic Particle (MP)
inspection. Hardness and Charpy Impact Test were also conducted on fractured
and highly deformed active links.
Fig. 19. Fractured link at level six of Pacific Residential Tower, Christchurch (NZ)
(Gardiner et al., 2013)
Lateral Load Resisting Systems / 14
Fig. 20. Close-up of the fractured link (Gardiner et al., 2013)
After detailed investigations on fractured link, following possible reasons of
failure were identified (Gardiner et al., 2013):


Low fracture toughness (low Charpy Impact Test)
The location of a shear stud(s) above the brace web stiffeners forming the
active link was a potential fracture initiation site.
Shorter link length imposing higher hinge rotations in this frame compared
to the others in the building.
Field observations were also done on a hospital parking garage having
eccentrically braced frame as seismic load resisting system (Bruneau et al., 2011).
It employs at least six EBF frames at each level in each of the building principle
direction. Paint flaking in links was observed in some bays at first storey as shown
in Fig. 21, while 2 bays showed link fractures. Evidence of soil liquefaction was
also observed over parts of the slab on grade.
The fracture in parking garage link indicated a ductile failure of link due to
overloading rather than a brittle failure. The probable reason of fracture can be
offset of link end vertical stiffener from the point flange of brace meets the
link/beam. When the brace was axially loaded, the axial force instead of going
directly into stiffener fed into the brace panel zone of beam thus overloading it
and eventually fracturing the link as shown in Fig. 22. This emphasizes the
importance of end link stiffener as well as good workmanship while fabricating
the link.
Lateral Load Resisting Systems / 15
Fig. 21. Paint flaking in EBF link (Bruneau et al., 2011)
Fig. 22. Fracture in EBF beam due to offset of link and vertical stiffener
(Bruneau et al., 2011)
6. CORE AND OUTRIGGER SYSTEM
The lateral stiffness of a multi-storey building can be greatly enhanced by tying
the centrally located shear wall core with the peripheral columns through deep
(usually storey high) girders at top of the building and preferably at some
intermediate level also. In steel buildings the core is made of vertical truss and the
outrigger is a horizontal truss. These outriggers mobilise the axial stiffness of the
columns in resisting the lateral load and at the same time reduce the bending
moment in columns and beams (Fig. 23). This increases the efficiency of the
structural system as the axial mode is a more efficient mode as compared to the
Lateral Load Resisting Systems / 16
bending mode. It has been reported that even in frame buildings without core, if a
stiff girder is provided at the top or at some intermediate level, the efficiency of
the structure increases significantly and a considerable saving of material may be
achieved.
Fig. 23 Core and Outrigger System
7. TUBULAR SYSTEMS
An efficient way to increase the lateral stiffness of tall buildings is to put the most
of the lateral load resisting material along the perimeter of the building. The
resulting system is called a Tubular building. Most of the tall buildings of world,
both in steel as well as in concrete have been constructed based on this concept.
The simplest form of this type is called Framed-Tube (Fig. 24), which consists of
closely spaced columns along the perimeter, interconnected by deep spandrel
beams. The overall system is similar to a hollow vertical cantilever tube having
perforations for windows and horizontal diaphragms for floor slabs. The system
offers large open floor areas with only a few interior columns designed for only
vertical loads. The exterior columns are designed to carry the total lateral load.
A framed-tube consists of four frame panels. The panels parallel to loading are
called web panels, while those perpendicular to loading are called flange panels.
This distribution of column axial forces in the web and flange panels of a framedtube subjected to lateral load is shown in Fig. 25. The ideal tube behaviour
predicts uniform column axial force in the flange panel and linearly varying
column axial force in the web panel. In the actual framed-tube the column axial
force is highest in the corner column and reduces towards the centre of the flange
panel. This is because of finite shearing stiffness of the spandrel beams. Due to
lack of shearing rigidity of the interconnecting beams the axial force in the interior
columns of a panel lags behind the axial force in the corner columns. The
phenomenon is known as Shear Lag. It has been observed that in the top portion
of the framed-tube building the distribution of the column axial force is opposite
to that shown in Fig. 25. The Phenomenon is termed as Negative Shear Lag.
Lateral Load Resisting Systems / 17
Fig. 24 Framed-Tube Building
Fig. 25 Shear Lag in a Framed-Tube Building, (a) Column axial force distribution in web,
(b) Column axial force distribution in flange
Two approaches had been used for the analysis of Framed-Tubes: (i) Continuum
approach, and (ii) discrete approach. Using discrete approach, framed-tube can be
analysed using any software available for frames. The framed tube can be
modelled either as a space frame or as an equivalent plane frame. The results of
the equivalent plane frame models have been shown to yield quite accurate
results.
Tall multi-storey buildings have a large service core, which is usually made of
shear walls. Such a core has high lateral resistance. When a core is acting in
conjunction with the outer frame-tube the resulting system is called Tube-in-Tube
(Fig. 26).
To further increase lateral resistance of frame-tubes an exterior truss type
structural system is obtained by adding diagonal members in the steel frametubes (Fig. 27(a)). In concrete buildings the same effect is obtained by filling the
frame panels in a diagonal pattern (Fig. 27(b)). Such buildings have very high
lateral resistance and very large heights can be achieved with this type of
structural system. Mega trussed-tube systems with heavy corner columns,
Lateral Load Resisting Systems / 18
diagonal bracing, stiff shear wall/ vertical truss core and outrigger are being
considered for the future super high multifunctional towers.
Another way to increase the efficiency of a framed tube is to provide cellular
planform. A number of tube may be bundled together to have more than two web
panels and flange panels. The increased number of panels result in reduced shear
lag effect and enhanced efficiency of the structural system compared to a singlecell framed tube. The system is called multi-cell tube or bundled tube (Fig. 28).
The system provides flexibility in architectural plan-forms by considering
different combination of cells up to different heights of building. The system has
been successfully used in the famous Sears Towers, one of the world’s tallest
buildings.
Fig. 26 Tube-in-Tube Building
Fig. 27 (a) Trussed-Tube
Building in Steel
Fig. 27 (b) Infilled-Tube
Building in Concrete
Computer modelling of framed-tube, tube-in-tube, trussed-tube and multi-cell
tube can be done using a software for frame analysis. A space frame model can be
used to get the exact results. As mentioned earlier, the buildings have deep
spandrel beams with relatively short spans. Therefore, the shear deformations in
the members and the finite size of joints should be considered in analysis.
Ignoring the finite joint size may predict flexible structure and conservative
member forces. As the size of the structure and the number of degrees of freedom
is very large, equivalent plane-frame models are available, which are economical
from the computational time and memory point of view.
Lateral Load Resisting Systems / 19
Fig. 28. Multi-cell Tube Building
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