Rc4 lecture 2- eng. samer akil

SEISMIC DESIGN OF
BUILDING STRUCTURES
Samer Akil
Arab International University /2017/ RC4

THE PHILOSOPHY OF SEISMIC DESIGN
Lecture 2
 THE PHILOSOPHY OF SEISMIC DESIGN
 STRUCTURAL CONFIGURATION
 LATERAL FORCE RESISTING SYSTEMS

In general, most earthquake code provisions require that structures be
able to resist:
1. Minor earthquakes without any damage.
2. Moderate earthquakes with negligible structural damage and some
nonstructural damage.
3. Major earthquakes with some structural and nonstructural damage
but without collapse.
The Philosophy of Seismic Design
SAMER
AKIL

Then, one eliminates the likelihood of inelastic action or failure
elsewhere in the structure by making the capacities of adjoining
structural members greater than that needed to reach the
maximum capacity of the inelastic zone.
In seismic design, the engineer first acknowledges that inelastic
action is unavoidable during severe earthquakes.
The designer then dictates where inelastic response should occur.
Such zones of possible inelastic action are selected to be regions
where large inelastic deformations can develop without significant
loss of strength; these regions are detailed to suppress premature
undesirable failures modes, such as member instability.
SAMER
AKIL

So
The Hysteretic damping: the
energy that is dissipated in every
loading cycle, due to the plastic
behaviour of the material.
By allowing yielding at
some fraction of the
elastic seismic demand,
the design forces are
reduced and the desired
economy is achieved
The structure has a lot of
reserve capacity beyond first
significant yield if the designer
detailed the structure to obtain
this type of Performance.
The design forces are much lower
than those that would be
required if the structure were to
remain elastic.
It would certainly be desirable
to design structures to remain
elastic during extreme events.
However, elastic seismic forces
can be several times the wind
force and design for such
forces is simply not
economically feasible.
There is a possibility that an
earthquake may not occur
during the life of the structure
WHY ?
+
+
SAMER
AKIL

Yielding of a beam section due to bending
Direct stress due to bending
Formation of a plastic hinge in a simply supported
beam
Firstly, What is Plastic Hinge ??
Stress- Strain Curve for steel
 Capacity Beyond First Significant yield
SAMER
AKIL

R.C. Beam case ;
• At low loads the section is uncracked.
• After the concrete cracks, the concrete on the tension side of the
beam is neglected, and a cracked-transformed section analysis can be
used to predict behavior. However, this method is only valid as long as
both the steel and the concrete stress-strain behaviors are linear.
Concrete can be assumed to have a linear stress-strain behavior up to
approximately 50% of maximum concrete stress (f’c )
•After the concrete stress exceeds about 50%f’c, a strain compatibility
approach can be used, using a realistic concrete stress-strain model.
• After the steel yields,
there is typically an
extended plateau in
which the displacement
increases significantly
with very little increase
in applied load.
SAMER
AKIL

Load- Deflection curve for Fixed End beam
Collapse Load Vs First Yielding load ?
SAMER
AKIL

Frame Case
• In most structures that are subjected to moderate-to-strong
earthquakes, economical earthquake resistance is achieved by
allowing yielding to take place in some structural members.
• It is generally impractical as well as uneconomical to design a
structure to respond in the elastic range to maximum expected
earthquake-induced inertia forces.
SAMER
AKIL

Structures that contain facilities critical to postearthquake
operations—such as hospitals, fire stations, power plants, and
communication centers—must not only survive without collapse,
but must also remain operational after an earthquake. Therefore,
in addition to life safety, damage control is an important design
consideration for structures deemed vital to postearthquake
functions.
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AKIL

Seismic Performance Level According to ATC
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AKIL

The test setup. A predefined cycle of
displacements is applied, and the
resisting force is monitored. The
displacement cycle is often provided
in material-specific loading protocols.
 The Hysteretic damping
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AKIL

Example of lab test
Shear wall under reverse load
SAMER
AKIL

• The area contained within a single hysteresis loop is often a better
indicator of performance than is ductility. For example, each of the
structures tested have about the same overall ductility and neither
suffers appreciable strength loss.
the energy dissipated per cycle for is greater the robust system and,
hence, this system is preferable.
The more energy
dissipated per cycle
without excessive
deformation, the
better the behavior
of the structure.
SAMER
AKIL

A typical hysteresis loop.
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AKIL

Response of elastic and inelastic SDOF system
SAMER
AKIL

Even if the motion can be predicted it is unlikely than we can
precisely predict the response. This is due to the rather long list of
things we do not know and can not do, as well as uncertainties in
the things we do know and can do.
The best we can hope for is to predict the characteristics of the
ground motion and the characteristics of the response.
It must be made very clear that the purpose of analysis is
NOT to accurately predict the response of a certain structure
to a certain ground motion. This is impossible due to the
large number of uncertainties in modeling, loading, analysis,
and interpretation of results.
What we are trying to do is to use analysis to get a handle
on the likely behavior of a structure, and to estimate
whether or not such behavior will meet pre-established
performance objectives.
The “design” ground motion cannot be predicted.
SAMER
AKIL

The Difference Between Wind-Resistant Design
and Earthquake-Resistant Design
• The intuitive philosophy of structural design uses force as the basis,
which is consistent in wind design, wherein the building is subjected
to a pressure on its exposed surface area; this is force-type loading.
However, in earthquake design, the building is subjected to random
motion of the ground at its base which induces inertia forces in the
building that in turn cause stresses; this is displacement-type loading.
Difference in the design effects on a building during natural actions of (a) Earthquake
Ground Movement at base, and (b) Wind Pressure on exposed area
SAMER
AKIL

Wind force on the building has a non-zero mean component
superposed with a relatively small oscillating component. Thus,
under wind forces, the building may experience small fluctuations in
the stress field, but reversal of stresses occurs only when the
direction of wind reverses, which happens only over a large
duration of time. On the other hand, the motion of the ground
during the earthquake is cyclic about the neutral position of the
structure. Thus, the stresses in the building due to seismic actions
undergo many complete reversals and that too over the small
duration of earthquake.
Nature of temporal variations of design actions: (a) Earthquake Ground Motion – zero
mean, cyclic, and (b) Wind Pressure – non-zero mean, oscillatory
SAMER
AKIL

Note that the force-displacement plot shows three points -- 10 year
wind, 50 year wind, and factored 50 year wind. The 10 year wind is
used for serviceability issues (drift) and the factored 50 year wind is
used for design (assuming strength based design). Under the
factored 50 year wind, the structure is still responding in a linear
elastic fashion. (By linear, we mean no yielding of steel or crushing of
concrete. Cracking of concrete will occur under the factored 50 year
wind (and perhaps the 10 year wind).
For most buildings, dynamic
wind response may be
neglected.
However, for very flexible
buildings and for buildings of
unusual shape, aeroelastic
interaction between the wind
load and structural response
is possible, leading to a true
dynamic response in the
structure.
SAMER
AKIL

Lateral-Force-Resisting Systems
SAMER
AKIL

The following table shows the common structural system in RC
buildings used effectively for providing resistance to seismic lateral
forces. (in Syria: shear wall system is the most common system for
buildings while frame system is used in case of special architectural
requirements, Dual system is used frequently for taller buildings (say
10 stories or more).
SAMER
AKIL

• A frame is considered rigid when its beam-to-column
connections have sufficient rigidity to hold virtually unchanged the
original angles between intersecting members. In buildings where
a space frame resists the earthquake forces, the columns and
beams act in bending.
• In this system, lateral loads are resisted primarily by the rigid
frame action; that is, by the development of shear forces and
bending moments in the frame members and joints.
• The continuity at both ends of beams also assists in resisting
gravity loads more efficiently by reducing positive moments in
beam spans.
• Moment frames have certain advantages in building applications
due to their flexibility in architectural planning.
1. Moment Rigid Frames
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AKIL

(a) Response of rigid frame to lateral loads; (b) flexural deformation of beams
and columns due to nondeformability of connections
• The strength and ductility of the connections between beams and
columns are also important considerations, particularly for frames
designed to resist seismic loads.
• The depths of frame members are often controlled by stiffness
rather than strength to limit story drift under lateral loads.
SAMER
AKIL

>>> FLAT SLAB-FRAME SYSTEM
• Perhaps one of the simplest framing techniques for a concrete
building consists of a two-way floor slab framing directly into
columns without beams. Although physically no beam exists
between the columns, for analytical purposes, a certain width of
slab may be considered as a beam framing between the columns.
• Particular concern in the design of a flat slab-frame is the
problem of shear stress concentration at the column–slab joint.
Shear reinforcement is necessary to improve joint behavior and
avoid early stiffness deterioration under lateral cyclic loading. This
is one of the primary reasons that two way slab systems are not
permitted by the ACI in regions of high seismic risk (UBC zones 3
and 4)
Cracking reduces the stiffness of the slab-beams. The magnitude of
the loss of stiffness will depend on the type of slab system and the
reinforcement details.
SAMER
AKIL

Typical floor systems for Flat slab-Frames
Flat plate/slab systems: (a) without column capitals, (b) with column capitals
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2. Shear Walls
• A shear wall building is normally more rigid than a framed
structure. With low design stress limits in shear walls, deflection
due to shear forces is relatively small.
• Notable exceptions to the
excellent performance of
shear walls occur when the
height-to-width ratio becomes
great enough to make
overturning a problem and
when there are excessive
openings in the shear walls.
• Also, if the soil beneath its
footings is relatively soft, the
entire shear wall may rotate,
causing localized damage
around the wall.
SAMER
AKIL

Some illustrative structural wall elevations
• Shear wall construction
is an economical method
of bracing buildings to
limit damage, and this
type of construction is
normally economically
feasible up to about 15
stories.
Various wall cross sections
SAMER
AKIL

>>> COUPLED SHEAR WALLS
-This system is economical for buildings
in the 40-story range.
- Since planar shear walls carry loads only
in their plane, walls in two orthogonal
directions are generally required to resist
lateral loads in two directions. resistance
to torsional loads must be considered in
determining their location
-.
Plastic Hinges occur at the end
of couplings beams then at the
base of shear walls >>>
Excellent dissipating of energy
SAMER
AKIL

• It is, however, difficult to define small openings quantitatively. As
a rough estimate, it can be assumed that small holes for windows
and doors are those with width ( lo ) less than 10 – 15% of the wall
length ( Lw ) . (i.e. lo / Lw < 0.10 – 0.15).
• For small openings, SWs behave like monolithic cantilever
columns; the influence of windows and doors is negligible.
Lateral load-resistance of single and coupled shear walls.
(See the behavior and design of coupled shear walls under cyclic loads in the lecture No.8)
SAMER
AKIL

In this system, resistance to horizontal loading is provided by a
combination of shear walls and rigid frames.
The shear walls are often placed around elevator and service cores
while the frames with relatively deep spandrels occur at the building
perimeter.
3. Shear Wall – Frame System.
Shear wall–frame interaction.
Shear walls with interior frames.
SAMER
AKIL

• Overall lateral deformations are primarily generated by shear
forces (frame) and flexural bending (bracing system or wall). Frame
lateral displacements reduce as the height increases; conversely,
lateral deflections of braced frames and structural walls increase
with the height.
• The net effect is that at lower storeys, bracing systems and walls
are stiffer than frames, while, in turn, the latter possess higher
stiffness at upper floors.
The potential advantages of a wall–frame structure depend on the
intensity of horizontal interaction, which is governed by the relative
stiffness of the walls and frames, and the height of the structure.
The taller the building and the stiffer the frames, the greater the
interaction.
SAMER
AKIL

• This difference in lateral stiffness along the height between the
structural components significantly affects the distribution of
seismic actions as shown:
SAMER
AKIL

In Shear Wall–Frame System:
It is observed that the total shear carried by the MRF at top storeys
can exceed the applied seismic action ‘negative storey shear share ’.
Effects of negative storey shear share are exacerbated if rotation of
the wall anchors is allowed.
Interaction between frame and structural wall
SAMER
AKIL

4. BRACED FRAMES
• Generally speaking, rigid frame systems are not efficient for
buildings taller than about 20 stories because deflection due to
bending of columns and girders causes the drift to be too large.
• A braced frame improves upon the efficiency of a rigid frame by
virtually eliminating the bending of columns and girders. This is
because by adding web members such as diagonals or chevron
braces, the horizontal shear is resisted by the web.
SAMER
AKIL

• Braced frames may be grouped into two categories as either
Concentric Braced Frames (CBFs) or Eccentric Braced Frames (EBFs).
In CBFs, the axes of all members, that is, columns, beams, and
braces intersect at a common point such that the member forces
are axial without significant moments. On the other hand, EBFs,
utilize axis offsets to intentionally introduce flexure and shear in
preselected beam segments to increase ductility.
• CBFs are of questionable value in seismic regions because of their
poor inelastic behavior. Although moment-resistant frames exhibit
considerable energy dissipation characteristics, they are relatively
flexible when sized from strength considerations alone.
Eccentric bracing is a unique structural system that
attempts to combine the strength and stiffness of a braced
frame with the inelastic energy dissipation characteristics of
a moment frame.
SAMER
AKIL

Eccentric bracing system
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AKIL

5. TRUSS MOMENT FRAMES
• Truss moment frames consist of horizontal trusses rigidly
connected to columns.
• The resistance to lateral displacement is by traditional frame
action.
• In these buildings, a
special segment of the truss
is designed to provide a
yield mechanism, while the
truss elements outside of
the special segment are
designed and detailed to
remain nominally elastic.
SAMER
AKIL

6. STAGGERED TRUSS SYSTEM
• In this system, story-high trusses span in the transverse direction
between the columns at the exterior of the building.
• The floor system acts as a diaphragm transferring lateral loads in
the short direction to the trusses.
Lateral loads are thereby resisted by truss diagonals and are
transferred into direct loads in the columns. The columns therefore
receive no bending moments.
• In regions of low
seismicity, the system has
been used for buildings in
the 35- to 40-story range.
SAMER
AKIL

Load path in staggered truss system
SAMER
AKIL

7. FRAME TUBE SYSTEM
The term tube, in usual building terminology, suggests a system of
closely spaced columns say, (2.5–4.5 m) on center, tied together with
a relatively deep spandrel.
This system works efficiently as a hollow vertical cantilever.
SAMER
AKIL

What is ‘SHEAR LAG’
in tube system ??
Axial stress distribution in a square hollow
tube with and without shear lag.
• The stresses in the corner column
will be greater than those from a
pure tubular action, and those in
the inner columns will be less. The
stresses in the inner columns lag
behind those in the corner
columns, hence the term shear lag.
• Because the column stresses are
distributed less effectively than in
an ideal tube, the moment
resistance and the flexural rigidity
of a tubular building are much
less. Thus, although a framed tube
is highly efficient, it does not fully
utilize the potential stiffness and
strength of the structure because
of the effects of shear lag.
SAMER
AKIL

>>> TUBE - IN - TUBE SYSTEM
• In this system floor slabs, acting as horizontal rigid diaphragms, tie
exterior and interior tubes together so that they interact under
horizontal loads.
SAMER
AKIL

• Generally, The exterior tube resists most lateral loads in the
upper floors, while the interior tube (shear walls or braced frames)
carries most lateral loads at lower storeys.
Typical tube - in - tube system: layout ( left ) and action distribution ( right )
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AKIL

Torre Agbar, Barcelona, Spain, 2004
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>>> EXTERIOR DIAGONAL TUBE
-Trussed Tube System improves
the efficiency of the framed
tube by increasing its potential
for use in taller buildings and
allowing greater spacing
between the columns.
-This is achieved by adding
diagonal bracing at the faces of
the tube to virtually eliminate
the shear lag in both the
flange and web frames.
SAMER
AKIL

John Hancock Center,
Chicago, USA, 1969
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8. BUNDLED TUBE
-The underlying principle to achieve a
bundled tube response is to connect two or
more individual tubes into a single bundle.
-A bundled tube
typically consists of a
number of individual
tubes interconnected
to form a multicell
tube, in which the
frames in the lateral
load direction resist the
shears, while the flange
frames carry most of
the overturning
moments.
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AKIL

Willis Tower, Chicago, USA, 1974
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9. THE DIAGRID FRAMED TUBE
• The diagrid-framed-tube system can be formed by using closely
spaced diagonal braces instead of vertical columns.
• This system is more effective against lateral loads than the
conventional framed-tube system. Placing the elements in a closely
spaced diagrid pattern provides sufficient resistance against vertical
and lateral loads.
• While the shear forces caused by lateral loads are met by the
bending strength of the columns and beams in the framed-tube
system, in the diagrid-framed-tube system they are met by the axial
compressive and tensile strength of the diagonal braces which
significantly increases the efficiency of the structural system.
SAMER
AKIL

The 41-storey,
30 St Mary Axe,
London, UK, 2004
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Hearst Magazine Tower, New York, USA, 2006
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O-14, Dubai, U.A.E, 2010
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10. Megacolumn (mega frame, space truss) Systems
• Mega column systems consist of reinforced concrete or
composite columns and/ or shear walls with much larger cross-
sections than normal, running continuously throughout the height
of the building. In this system, mega columns and/or mega shear
walls can resist all the vertical and lateral loads.
Taipei 101, Taipei, Taiwan, 2004
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Commerzbank Tower, Frankfurt, Germany, 1997
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11. Mega Core Systems
• Mega core systems consist of reinforced concrete or composite
core shear walls with much larger cross-sections than normal,
running continuously throughout the height of the building.
• Since the mega core can resist all vertical and lateral loads in this
system, there is no need for columns or shear walls on the
perimeter of the building. In mega core systems, floor slabs are
cantilevered from the core shear wall.
• Mega core systems can also be used with strengthened cantilever
slabs . In this case, floor slabs are supported by the core shear
walls and discontinuous perimeter columns. Perimeter columns
are supported by strengthened cantilever slabs repeated on some
storeys. Strengthened cantilever slabs protrude from the core, and
are strengthened in order to support the load coming from the
storeys above.
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Slabs in the mega core system: (a) cantilever slab, (b) supported cantilever slab
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Mega core systems efficiently and economically provide sufficient
stiffness to resist wind and earthquake induced lateral loads in
buildings of more than 40 storeys.
8 Shenton Way, Singapore, 1986
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Aspire Tower, Doha, Qatar, 2006
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12. OUTRIGGER AND BELT WALL SYSTEM
-The structural arrangement for
this system consists of a main
concrete core connected to
exterior columns by relatively
stiff horizontal members such
as a one or two-story deep
walls commonly referred to as
outriggers.
-The core may be centrally
located with outriggers extending
on both sides or it may be located
on one side of the building with
outriggers extending to the
building columns on one side.
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-The basic structural response of the system is quite simple. When
subjected to lateral loads, the column-restrained outriggers resist
the rotation of the core, causing the lateral deflections and moments
in the core to be smaller than if the freestanding core alone resisted
the loading.
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Shanghai Tower, Shanghai, China, under construction
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The 88-storey, 452 m high
The PETRONAS Twin Towers
Kuala Lumpur, Malaysia
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Burj Khalifa, Dubai, U.A.E, 2010
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STRUCTURAL CONFIGURATION
•To achieve adequate performance, basic principles recommended
to follow for ‘ conceptual design ’ are provided here, which can be
summarized below:
(i) Simplicity: consists of clear and direct paths for vertical and
horizontal forces due to the combination of gravity and earthquake
loading.
(ii) Uniformity: uniform distribution of structural elements in plan
and elevation, allowing for smooth and direct transmission of the
inertial forces generated by the masses of structural and non -
structural components. Concentrations of stresses or large ductility
demands cause premature collapse. It may be necessary to
subdivide the entire building into independent units by using
seismic joints.
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AKIL

(iii) Symmetry: symmetrical or quasi - symmetrical structural layouts,
well distributed in - plan, are a viable solution for the achievement of
uniformity. Structural symmetry means that the centre of mass and
centre of resistance are located at, or close to, the same point.
Eccentricity produces torsion and stress concentrations.
(iv) Redundancy: this is a measure of the degree of indeterminacy
and reliability of structural systems. Redundancy primarily arises
from the capacity of structures to provide an alternative loading
path after any component failure.
(v) Bidirectional resistance and stiffness: lateral resisting elements
and systems arranged in an orthogonal in - plan pattern provide
similar resistance and stiffness characteristics in the principal
directions of the structure. High horizontal stiffness is effective in
limiting excessive displacements that may lead to instabilities (e.g.
due to P - Δ effects) or to extensive structural and non - structural
damage.
SAMER
AKIL

(vi) Torsional resistance and stiffness: adequate torsional stiffness
and resistance is necessary to reduce torsional motions which tend
to stress the structural elements non - uniformly. In this respect,
arrangements in which the main elements resisting the seismic
actions are distributed close to the periphery of the building
present clear advantages.
(vii) Diaphragm behaviour at storey level: floor and roof systems act
as horizontal diaphragms in building structures. These collect and
transmit inertia forces to the vertical elements of lateral resistant
systems, i.e. columns and structural walls. They also ensure that
vertical components act together under gravity and seismic loads.
(viii) Adequate foundation: stiff and resistant foundations and their
connections with the superstructure ensure that the whole structure
is subjected to uniform seismic excitation. Buildings with isolated
foundation elements should utilize a foundation slab or tie beams
between these elements in both main directions.
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• Typical building configuration deficiencies include an irregular
geometry, a weakness in a story, a concentration of mass, or a
discontinuity in the lateral-force-resisting system. Vertical irregularities
are defined in terms of strength, stiffness, geometry, and mass.
• When irregular features are unavoidable, special design
considerations are required to account for the unusual dynamic
characteristics and the load transfer and stress concentrations that
occur at abrupt changes in structural resistance.
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AKIL

 TORSION
• if the Centre of Mass (CoM) of a building is not coincident with
the Centre of Resistance (CoR) a torsional moment acts in the
horizontal plane causing floor diaphragms to twist about the CoR.
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• Engineers prevent building
damage arising from torsion by
using several approaches. Firstly,
they minimize the distance in
plan between the CoM and CoR.
Remember that even with a
perfectly symmetrical structural
configuration some degree of
torsion still occurs due to
torsional motions within the
ground shaking.
• Secondly, designers provide a
minimum of two lines of vertical
structure parallel to each of the
main orthogonal axes of a
building horizontally offset from
each other.
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AKIL

Also due to the diaphragm rotation, the x direction shear walls
deflect horizontally Δx in opposite directions. Like the y direction
shear walls, they react against the movement that deflects them.
They apply equal and opposite reaction forces upon the diaphragm
creating another moment couple. Even though no x direction seismic
forces act on the building, because these two shear walls orientated
parallel to the x axis are strongly connected to the diaphragm, they
nonetheless participate in resisting torsion.
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AKIL

The four extra shear walls added in plan (a) enhance torsional
resistance slightly. Even if the new walls are identical to the
perimeter walls because they are closer to the CoR they are subject
to 50 per cent smaller displacements when the diaphragm twists
and the leverarms between them are less. With a lesser resisting
force (proportional to horizontal displacement) and half the lever
arm their torsion-resisting contribution is only 25 per cent of that
provided by the perimeter walls.
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AKIL

• If the perimeter walls are
removed, and horizontal forces
and torsion are now resisted by
the inner walls alone, the two
torsion-resisting couples must
offer the same resistance as
before since the value of the
torsion moment is unchanged.
• Although the previous figures illustrate shear walls resisting seismic
forces, moment and braced frames can also provide adequate
torsion resistance. Replace the shear walls with one- or multi-bay
moment frames and the principles outlined above still apply. The
building will be less torsionally stiff due to the lesser stiffness of the
frames but still perform adequately, especially if the frames are
located on the perimeter of the building.
SAMER
AKIL

In the examples considered so far, a recommended torsion-resistant
structure comprises a minimum of four vertical elements, like shear
walls or moment frames, with two in each direction. However, in
some situations the number of elements can be reduced to three
Any y direction forces are resisted by one
shear wall, and x direction forces resisted
by two walls. When torsion induces
diaphragm rotation, the two x direction
walls, in this case with a long lever-arm
between them, form a moment couple.
They provide torsional stability or
equilibrium irrespective of the direction of
loading – but only so long as they remain
elastic. Most shear walls and frames are
designed for relatively low seismic forces if
they incorporate ductile detailing >>>
SAMER
AKIL

>>> So when one x direction wall yields as a result of inertia forces
in the x direction as well as torsion it temporarily loses its stiffness
and the CoR migrates towards the stiffer end, increasing torsional
eccentricity. The system becomes torsionally unstable.
This configuration consisting of three vertical structural
elements is described by structural engineers as a torsionally
unbalanced system. It is not recommended unless the x
direction walls or frames are much stronger than minimum
requirements. They must be capable of resisting horizontal
forces with little or no ductility demand and therefore possibly
possess far more strength than normal.
• Until research findings update guidelines, architects should
avoid torsionally unbalanced systems unless satisfying the
criterion above.
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AKIL

 RE-ENTRANT CORNERS
Many buildings have
suffered seismic damage
due to re-entrant corners
in past earthquake.
Although re-entrant
geometries can take many
shapes, what they share
in common from a seismic
design perspective, is
their potential for
damage resulting from
the different dynamic
properties of each wing.
Typical re-entrant corner forms.
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For example, when the building in
the right Fig is shaken in the
y direction, the left-hand area of the
building, and the wing to the right,
react quite differently. The left-hand
area deflects horizontally a relatively
small amount due to its greater
depth and inherently greater
horizontal stiffness.
The more flexible wing moves
further and at a different period of
vibration. It swings about the stiffer
area, possibly damaging floor
diaphragms at the junction of the
two wings.
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• The attitude of most codes towards re-entrant corners is to require
structural engineers to undertake a 3–D dynamic analysis where
the length of a projecting area of building causing a re-entrant corner
exceeds approximately 15 percent of the building plan dimension.
• An engineer will design the re-
entrant structure to avoid either
diaphragm tearing or excessive
horizontal deflections.
• However, if they are long or
their diaphragms weakened by
penetrations for vertical
circulation or other reasons in
the critical region where they
join, that approach may not be
structurally sound. The building
might best be separated into
two independent structures. Irregular plan configurations
improved by seismic separation gaps.
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Two methods of supporting flooring
at a seismic separation gap.
Possible detail of a seismic separation gap
between two buildings at roof level
A section through a generic
floor level seismic gap.
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 DIAPHRAGM DISCONTINUITIES
• In most buildings quite large penetrations are required for vertical
circulation such as stairways and elevators. Building services,
including air ducts and pipes also need to pass through floor slabs
and in the process introduce potential weaknesses into diaphragms.
• It likens diaphragms to horizontal beams resisting and transferring
horizontal inertia forces to their supports which, in this case, consist
of vertical structural systems such as shear walls or moment frames.
A slot in the diaphragm destroys its ability to span between shear
walls for y direction forces. (X direction structure not shown.)
SAMER
AKIL

• The web of a diaphragm resists shear forces, while perimeter
diaphragm chords acting in tension or compression, resist bending
moments.
Inertia forces within a multi-storey building
shown in plan and section
SAMER
AKIL

A continuous two-span diaphragm.
A diaphragm as a simply-supported beam.
SAMER
AKIL

Where the length of a bracing
element – such as a shear
wall – is short in plan with
respect to the width of the
diaphragm transferring forces
into it, the interface between
the horizontal and vertical
element may be too weak to
transfer the forces between
them. In this case, a
‘collector’ or tie member is
required. It collects forces
from the diaphragm acting in
either tension or
compression, depending
upon the direction of force at
that instant of time and
transfers them into the wall.
Collector or tie members transfer
diaphragm shear forces into vertical structure
SAMER
AKIL

• The size of a penetration can be large enough to ruin the structural
integrity of a diaphragm altogether.
• Consider the case of a simple rectangular diaphragm spanning
between two shear walls that act in the y direction, What are the
structural options if a full-width slot is required?
The slot destroys the ability of the diaphragm to span to the right-
hand wall. If the purpose of the slot is to introduce light or services
through the diaphragm one option is to bridge the slot by
introducing a section of steel bracing.
SAMER
AKIL

Alternatively, if the geometry of diagonal members isn’t acceptable
aesthetically a horizontal vierendeel frame, with its far larger
member sizes, can be inserted to restore structural function
If the intention of the
penetration is to provide a
staircase. The only option is
to no longer consider that
wall as a shear wall but to
provide a new shear wall to
the left of the penetration.
SAMER
AKIL

A serious diaphragm
discontinuity occurs where a
potential floor diaphragm
consists of more than one
level. If a relatively small area
is raised or lowered it can be
treated, as far as seismic
behaviour is concerned, as if
it were a penetration.
A stepped diaphragm.
A kinked beam showing internal
compression and tension forces that can
not be achieved.
SAMER
AKIL

• Two ways to overcome these problems are; firstly, to fully
separate the building into two structures as discussed previously;
or secondly, to introduce a shear wall or frame along the line of the
step and provide x direction shear walls at each end of the building.
If the step is higher than several hundred millimetres, one
diaphragm will apply y direction forces directly to the columns of
the centre frame. This could lead to their premature failure and so
the best approach would be to separate the diaphragms and their
supporting members into two independent structures.
SAMER
AKIL

Plan irregularities:
(b) irregularity due
to mass-resistance
eccentricity;
(c) irregularity due
to discontinuity in
diaphragm stiffness.
Unfavourable core
arrangement;
diaphragm at risk
due to shear failure
at the connections
to the cores.
SAMER
AKIL

 NON-PARALLEL SYSTEMS
The following figure illustrates two
non-parallel systems. In each case
the directions of strength of the
vertical structures are angled with
respect to any sets of orthogonal
axes.
Two examples of non-parallel
systems. Gravity-only structure not shown
The ability of each configuration
to resist horizontal forces and
torsion is understood by
considering the length of each
vertical system as a strength
vector. A vector can be resolved
into components parallel to, and
normal to, a set of axes.
SAMER
AKIL

In the symmetrically configured
building, as the shear walls resist
y direction forces, the
diaphragms must provide tension
and compression forces to keep
the system stable. When the
configuration of non-parallel
systems is asymmetrical the
distribution of these internal
forces becomes far more
complex. For this reason codes
insist that structural engineers
model non-parallel systems in
3–D in order to capture these
effects and design for them. A non-parallel system showing the
orthogonal force components of each wall
and secondary diaphragm stresses for a y
direction force.
SAMER
AKIL

 SOFT STOREYS
Soft storey configuration describes structure where one storey of a
building is more flexible and/or weaker than the one above it from
the perspective of seismic forces.
Rather than earthquake energy
absorbed by ductile yielding of
steel reinforcing bars, or
structural steel sections in plastic
hinge zones, or structural fuses
throughout the whole structure
as shown in Fig. (b), in a soft
storey configuration earthquake
energy concentrates on the soft
storey. Serious damage is caused
especially to the columns of that
soft storey.
SAMER
AKIL

A collapsed building with weak
columns and strong beams. 1985
Mexico earthquake.
A soft storey building is doomed,
since columns in the soft storey
usually lack the resilience to
absorb seismic damage and still
continue to support the weight of
the building above.
Of all vertical configuration
problems, the soft storey is the
most serious and is by far the
most prevalent reason for multi-
storey building collapses. So many
buildings, located in seismically
active regions throughout the
world possess relatively open
ground floors and are at risk of a
soft storey mechanism forming.
SAMER
AKIL

Soft storeys are also caused by other configuration irregularities as
illustrated in the next figure.
Examples of soft storey configurations.
if the soft storey irregularity is reasonably minor, a seismic code may
permit the system to resist horizontal forces. However, the structural
engineer must undertake special analyses and provide members
within that storey with additional strength and ductile detailing.
SAMER
AKIL

So the questions arise:
‘ Is it possible for a building to exhibit the visual characteristics of a
soft storey for architectural reasons and still perform satisfactorily
in a quake; and if so, how? ’
Let’s take some real examples :
In more severe soft storey cases even the most advanced structural
design cannot prevent poor performance in a design-level
earthquake.
SAMER
AKIL

 Imagine that you are designing a building whose façade is
modulated by slender columns and deep beams (see next Fig.).
How can you achieve this presumably architecturally desirable
layout without creating a hazardous weak column–strong beam
configuration?
A weak column–strong beam structure
develops a soft storey at ground level once
columns are damaged.
SAMER
AKIL

One of two strategies is employed:
>> either separation or differentiation.
• Separation involves isolating from the
force path those stiff and strong
elements – like infill walls and deep
beams – which cause adjacent
elements – like columns – to be
relatively more flexible and weaker.
• If, for any reason the strategy of
separation is unacceptable, then
consider differentiation as a solution. In
this approach the seismically flawed
frame configuration remains on the
façade but is relieved of any
expectation of withstanding horizontal
forces by provision of an alternative
and stiffer system elsewhere in plan.
SAMER
AKIL

The internal moment frames
and shear walls of the next Fig.
resist all horizontal forces
because they are stiffer and
stronger. They are designed so
as the perimeter frames need
only carry gravity forces.
Whether or not that
intentional softening is
undertaken the perimeter
frame must be flexible and
possibly possess some
ductility. It has to undergo the
same horizontal drifts as the
stronger alternative seismic
resisting structure without
distress while, at the same
time, resisting gravity forces.
SAMER
AKIL

 What if, you require a double-height floor at ground floor level, or
anywhere else up the height of a building for that matter?
Begin by accepting that the frames with such a flexible and soft
storey must be excluded from the primary seismic resisting system.
So keep their irregular configuration and design them to resist gravity
forces only. Once again provide an alternative stiffer structure to
resist all seismic forces. At least two other approaches are possible.
First, introduce beams without floor
slabs . This may achieve the intended
spaciousness of the double-height
storey yet avoid a soft storey by
restoring the regularity of the
moment frame. Now that the weight
of the level without a floor is far less
than that of the floor above, a special
engineering analysis and design is
required.
(a) Provided beams
without slabs
SAMER
AKIL

The main disadvantage of the
mega-frame solution is that
the frame member sizes are
considerably larger than
normal in order to control the
increased drift and bending
and shear stresses due to the
increased storey heights. The
columns must also be
designed to resist mid-height
inertia forces acting at
alternate storeys.
(b) Create a two-storey mega-frame by pinning
the ends of beams on alternate storeys
If the idea of inserting beams to create regularity is unattractive,
consider a mega-frame solution. The moment frame storey height is
extended to two storeys. At alternate storeys floor beams are pinned
at their ends to prevent them participating as moment frame
elements.
SAMER
AKIL

 SHORT COLUMNS
• Short columns are to be avoided just
as assiduously as flexible columns.
• There are two types of short
column problems;
firstly, where some columns are
shorter than others in a moment
frame, and
secondly, where columns are so
short they are inherently brittle.
• The stiffness of a column against a
horizontal force is extremely sensitive
to its length; in fact, inversely
proportional to the column length
cubed (L3). Examples of short columns among
longer columns of moment frames
SAMER
AKIL

In following Fig. Two
columns together, one
that is half the height of
the other, resist a
horizontal force. The
shorter column is
therefore eight times
stiffer than the other, so
it tries to resist almost
eight times as much
force as the longer
column. It is unlikely to
be strong enough to
resist such a large
proportion of the
horizontal force and
may fail.
Two unequal height columns resisting seismic force.
SAMER
AKIL

 This type of short column problem can be overcome in several ways:
• If the beams that frame into the
columns forming short columns (in the
next Fig.) are pinned at both ends, that
effectively doubles the column lengths
and makes them all of equal length as far
as seismic resistance is concerned. Of
course, that creates a soft storey scenario
that then needs to be addressed.
• An alternative approach to structuring is to neglect the seismic
strength of the long columns altogether and to resist all seismic
forces by four one-bay frames; two acting in each direction to
achieve a symmetrical structural configuration.
SAMER
AKIL

• On a sloping site, short columns can be lengthened by integrating
them with the piles . If the piles are monolithic with columns and
protected from contact with the ground by sleeves or casings that
allow unrestrained horizontal drift, then a short column is avoided.
Finally, check that a soft storey does not result from this foundation
modification.
A method of avoiding a short column on a sloping site
SAMER
AKIL

Comparison between a regular and a short
column subject to horizontal drift.
Now we return to short
columns which have a
very short distance over
which they can flex
horizontally.
short column has an
unrestrained or free-
length less that twice its
depth. The problem is
that the free-length is too
short to allow for the
development of a ductile
plastic hinges.
In the event of seismic
overload the column fails
in shear.
SAMER
AKIL

• One solution is to avoid
them. by confined
masonry or structural
masonry walls to function
as shear walls and the
masonry is partial height,
• Guevara and Garc´ia
suggest continuing a short
length of masonry up the
sides of columns so that
diagonal compression
struts can act at the beam-
column joint and thereby
avoid short column failure
Reduction in the width of an opening
above a partial-height masonry.
SAMER
AKIL

• Another solution is to
separate that masonry
infills to prevent short
column configuration.
Methods to avoid a short column configuration
with reinforced concrete infills.
• The same approach
applies if infill walls are of
reinforced concrete
construction.
• Alternatively, designers
can infill one or more
windows to form shear
walls which are strong
and stiff enough to resist
seismic forces without
short columns being
damaged.
SAMER
AKIL

Short column failure. 2007, Peru earthquake.
Typical short column damage. 1994 Northridge
Note that even if strong infills are
separated from the moment frame
as shown, the ductility of the frame
is reduced due to the stiffening
and strengthening effect the infills
have on the beams. The beams
cannot bend when the building
sways.
SAMER
AKIL

Members reinforced conventionally,
have substantially different
behaviour under cyclic loading
characterised by a high vulnerability
to brittle failure in a mode of x-
shaped diagonal splitting of concrete
due to a diagonal compressive field
leading to a premature explosive
cleavage shear fracture.
alternative arrangements of
reinforcement were proposed
aiming at the improvement of the
behaviour of short columns under
seismic action and especially
aiming at avoiding premature
explosive cleavage fracture.
See Lecture No (7) later for
more details
SAMER
AKIL

 DISCONTINUOUS AND OFF-SET WALLS
Consider the building in next Fig.
At its upper levels y direction
forces are resisted by shear walls
at each end, but at ground floor
level the left-hand wall, Wall 1, is
discontinuous.
SAMER
AKIL

• Two perimeter moment frames resist x direction forces. When
struck by a quake in the y direction, the ground pulses will distort
the ground floor columns under Wall 1. Their ‘softness’ prevents
Wall 1 from providing the seismic resistance perhaps expected of it
and exemplifies the worst possible case of a soft storey.
• At the other end of the building the base of Wall 2, which is
continuous, moves with the ground motion. Due to the more
substantial overall strength and stiffness of Wall 2, as compared to
Wall 1, Wall 2 tends to resist the inertia forces from the whole
building.
Since Wall 1 resists almost no inertia force due to its discontinuity,
yet Wall 2 is fully functional the building experiences serious
torsion. To some degree, but limited by the modest lever-arm
between them and their inherent flexibility, the two x direction
moment frames try to resist the torsion. As the building twists
about its CoR located close to Wall 2, the columns furthest away
from the CoR are subject to large drifts and severe damage.
SAMER
AKIL

Two strong columns, one at each end of Wall 1 must withstand vertical
tension and compression forces.
See Lecture No (8) later
for more details about
design of shear walls.
SAMER
AKIL

• if those columns under
Wall 1 are omitted, the
overturning-induced axial
force can also be resisted
by two deep transfer
trusses or beams. They
must remain elastic
during the design level
quake to prevent
permanent downwards
movement of the wall.
Alternatives to a discontinuous wall.
• Another possibility is to
introduce an off-set
single-storey wall back
from Wall 1 ( Fig. (b) ).
SAMER
AKIL

Since the trusses or deep beams create a strong beam–weak
column configuration, ground floor shear walls in the x direction will
be required as well as the whole of the first floor slab being
designed as a transfer diaphragm. Another reason the offset
solution is not ideal is that torsion is introduced due to eccentricity
between the CoM and CoR at ground floor level for y direction
forces.
The danger posed by off-set walls supported on cantilever beams
has been tragically and repeatedly observed during five Turkish
earthquakes in the 1990. After categorizing building damage a
report concludes:
‘Buildings having architecturally based irregular structural systems
were heavily damaged or collapsed during the earthquake.
Cantilevers of irregular buildings have again proven to be the
primary source/cause of seismic damage. Many buildings have
regular structural systems but [even if] roughly designed
performed well with minor damage ’ .
SAMER
AKIL

The next figure shows a less
extreme wall discontinuity.
A large penetration
weakens the most highly
stressed region of the wall
creating an undesirable soft
storey.
Traditional engineering
wisdom would advise
designing the wall to be
nonstructural, but given the
sophisticated 3-D analysis
and design tools available
to contemporary structural
engineers, a careful design
might achieve satisfactory
seismic performance. A partially discontinuous wall and options for the
location of its structural fuse ormplastic hinge region.
SAMER
AKIL

When approaching the design of an element with a discontinuity
such as this, it is crucial that designers first identify the ductile
overload mechanism ( Fig. (b) or (c) ), and then using the Capacity
Design approach, ensure dependable ductile behaviour.
One approach is to design for plastic hinging at ground floor level
and detail the wall and unattached column accordingly with the
wall above strengthened to avoid premature damage.
Another approach is for the first floor section of wall to be
designated the fuse region. This means the ground floor section
and the wall above first floor will be stronger than the fuse
so damage occurs only in that specially detailed area.
SAMER
AKIL

 SETBACKS
• A setback is where a plan
dimension of a storey above
a certain level in a multi-
storey building reduces.
• Sophisticated structural
analyses quantify the ‘notch
effect ’ of a setback, but even
though structural engineers
avoid notches wherever
possible because of stress
concentrations, setbacks can
be designed satisfactorily.
Typical setback configurations.
SAMER
AKIL

The podium and tower form
represents a rather severe setback
configuration. Designers are faced with
several choices. They can treat the
building as one structure. In this case,
the podium roof is probably designed
as a transfer diaphragm to force the
podium framing to contribute to the
horizontal force resistance at the
bottom storey of the building .
Alternatively, designers can provide
the podium with little if any horizontal
resistance and tie it strongly to the
primary structure of the main tower,
which then resists the seismic force of
the entire building.
Finally, the podium can be seismically
separated from the tower.
SAMER
AKIL

 INFILL WALLS
• Infill walls are non-structural walls constructed between columns.
• Although they are not designed to resist either gravity or
horizontal forces no one has informed them! By virtue of their
inherent in-plane strength and stiffness they cannot avoid resisting
forces even if they wanted to.
• Infill walls can helpfully resist seismic forces in buildings, but only
in certain situations. These include where there is no other seismic
resisting system provided; the building is low-rise; the masonry
panels are continuous from foundation to roof; there are enough
panels in each plan orthogonal direction to adequately brace the
building; the infills are not heavily penetrated; and finally, where
infill walls are placed reasonably symmetrically in plan. Most infill
walls do not satisfy these criteria and may introduce configuration
deficiencies
SAMER
AKIL

So what are the difficulties with infill walls given that they are
commonly used in so many countries? Why do they require special
attention in seismically active regions?
• Firstly, infill walls stiffen a building against horizontal forces
additional stiffness reduces the natural period of vibration, which
in turn leads to increased accelerations and inertia forces
• Secondly, an infill wall prevents a structural frame from freely
deflecting sideways. In the process the infill suffers damage and
may damage the surrounding frame. The in-plane stiffness of a
masonry infill wall is usually far greater than that of its surrounding
moment frame – by up to five to ten times!
• The final problem associated with the seismic performance and
influence of infill walls is that of torsion. Unless infill walls are
symmetrically placed in plan their high stiffness against seismic
force changes the location of the Centre of Resistance (CoR).
SAMER
AKIL

Solutions to problems caused by infill walls : three solutions are
available: the first is often not feasible and the other two, while
simple in theory, are difficult to achieve in practice.
The first solution is for infill walls to be transformed into
confined masonry construction that is fully integrated with the
structural frame.
Another alternative is to provide very stiff primary structure
(RC shear walls). In this situation the less stiff infill walls do not
attract horizontal forces.
 The final option is to separate infill walls from their frames by
gaps of sufficient width as calculated by the structural engineer.
SAMER
AKIL

Some typical architectural details of separation gaps between an infill wall and frame.
SAMER
AKIL

Two possible structural
details
that resist out-of-plane
forces yet allow
relative movement
between an infill wall
and structure above.
Separated unreinforced
masonry infill wall with
‘practical columns ’ providing
out-of-plane strength.
SAMER
AKIL

Unfavourable and favourable geometric configuration in plan.
UNFAVOURABLE FAVOURABLE
SAMER
AKIL

Distribution of mass and stiffness in plan.
SAMER
AKIL

Unfavourable and favourable configuration in elevation.
SAMER
AKIL

Distribution of mass and stiffness in elevation.
SAMER
AKIL

Unfavourable and favourable configuration of the foundation.
SAMER
AKIL

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