Bracing System in Building

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MULTI-STOREY BUILDINGS

INTRODUCTION

The tallness of a building is relative and can not be defined in absolute terms

either in relation to height or the number of stories. But, from a structural

engineer's point of view the tall building or multi-storeyed building can be defined

as one that, by virtue of its height, is affected by lateral forces due to wind or

earthquake or both to an extent that they play an important role in the structuraldesign. Tall structures have fascinated mankind from the beginning of civilization.

The Egyptian Pyramids, one among the seven wonders of world, constructed in

2600 B.C. are among such ancient tall structures. Such structures were constructed

for defence and to show pride of the population in their civilisation. The growth in

modern multi-storeyed building construction, which began in late nineteenth

century, is intended largely for commercial and residential purposes.

ANATOMY OF MULTI-STOREY BUILDINGS

The vertical or gravity load carrying system of a multi-storey steel-framed building comprises a system of vertical columns interconnected by horizontal

beams, which supports the floors and roofing. The resistance to lateral loads is

provided by diagonal bracing or shear walls or rigid frame action between the

beams and columns. Thus, the components of a typical steel-framed structure are:

• Beams

• Columns

• Floors

• Bracing Systems

•

Connections

Lateral load resisting systems

Lateral forces

Lateral forces due to wind or seismic loading must be considered for tall

buildings along with gravity forces. Very often the design of tall buildings is

governed by lateral load resistance requirement in conjunction with gravity load.

High wind pressures on the sides of tall buildings produce base shear and

overturning moments. These forces cause horizontal deflection in a multi-storey

building. This horizontal deflection at the top of a building is called drift. The driftis measured by drift index, D/h, where, D is the horizontal deflection at top of the

building and h is the height of the building. Lateral drift of a typical moment

resisting frame is shown in Fig. 1.



The usual practice in the design of multi-storey steel buildings is to provide a

structure with sufficient lateral stiffness to keep the drift index between

approximately 0.0015 and 0.0030 of the total height. Normally, the provision of

lateral stiffness requires about 5 to 10% of extra steel. The extra steel is used for

bracing systems as described in the next section. The IS code require drift index to

be not more than 0.002 of total height.

Lateral loading systems

A multi-storey building with no lateral bracing in which the beams and

columns are connected with simple beam connections, the frame would have

practically no resistance to the lateral forces and become geometrically unstable.

The frame would laterally deflect even under a small lateral load. One of the

following three types of structural systems is used to resist the lateral loads and

limit the drift within acceptable range mentioned above:

• Rigid Frames

• Shear walls

• Braced frames

Combinations of these systems and certain other advanced forms are also used for

very tall buildings.

Rigid Frames

Rigidly jointed frames or sway-frames are those with moment resistingconnections between beams and columns. A typical rigid frame is shown in Fig.2

(a). It may be used economically to provide lateral load resistance for low-rise

buildings. Generally, it is less stiff than other systems. However, moment resisting

connections may be necessary in locations where loads are applied eccentrically

with respect to centre line of the columns.



Shear WallsThe lateral loads are assumed to be concentrated at the floor levels. The rigid

floors spread these forces to the columns or walls in the building. Lateral forces are

particularly large in case of tall buildings or when seismic forces are considered.

Specially designed reinforced concrete walls parallel to the directions of load are

used to resist a large part of the lateral loads caused by wind or earthquakes by

acting as deep cantilever beams fixed at foundation. These elements are called as

shear walls. Frequently buildings have interior concrete core walls around the

elevator, stair and service wells. Such walls may be considered as shear walls. The

advantages of shear walls are (i) they are very rigid in their own plane and hence

are effective in limiting deflections and (ii) they act as fire compartment walls.However, for low and medium rise buildings, the construction of shear walls takes

more time and is less precise in dimensions than steelwork. Generally, reinforced

concrete walls possess sufficient strength and stiffness to resist the lateral loading.

Shear walls have lesser ductility and may not meet the energy required under

severe earthquake. A typical framed structure braced with core wall is shown in

Fig. 2(b).

Braced frames

To resist the lateral deflections, the simplest method from a theoretical standpoint

is the intersection of full diagonal bracing or X-bracing as shown in Fig. 2(c). The

X-bracing system works well for 20 to 60 storey height, but it does not give room

for openings such as doors and windows. To provide more flexibility for the

placing of windows and doors, the K-bracing system shown in Fig. 3(a) is

preferred instead of X- bracing system. If, we need to provide larger openings, it is

not possible with K-bracing system; we can use the full-storey knee bracing system

shown in Fig. 3(b). Knee bracing is an eccentric bracing that is found to be

efficient in energy dissipation during earthquake loads by forming plastic hinge in

beam at the point of their intersection of the bracings with the beam.



ADVANCED STRUCTURAL FORMS

The bracing systems discussed so far are not efficient for buildings taller

than 60 stories. This section introduces more advanced types of structural forms

that are adopted in steel framed multi-storeyed buildings larger than 60 storeys

high. Common types of advanced structural forms are:

Framed -Tube Structures

The framed tube is one of the most significant modern developments in

high-rise structural form. The frames consist of closely spaced columns, 2 - 4 m

between centers, joined by deep girders. The idea is to create a tube that will actlike a continuous perforated chimney or stack. The lateral resistance of framed tube

structures is provided by very stiff moment resisting frames that form a tube

around the perimeter of the building. The gravity loading is shared between the

tube and interior columns. This structural form offers an efficient, easily

constructed structure appropriate for buildings having 40 to100 storeys. When

lateral loads act, the perimeter frames aligned in the direction of loads act as the

webs of the massive tube cantilever and those normal to the direction of the

loading act as the flanges. Even though framed tube is a structurally efficient form,

flange frames tend to suffer from shear lag. This results in the mid face flangecolumns being less stressed than the corner columns and therefore not contributing

to their full potential lateral strength. Aesthetically, the tube looks like the grid-like

façade as small windowed and is repetitious and hence use of prefabrication in

steel makes the construction faster. A typical framed tube is shown in Fig. 4(a).

3.2 Braced tube structures

Further improvements of the tubular system can be made by cross bracing the

frame with X-bracing over many stories, as illustrated in Fig. 4(b). This

arrangement was first used in a steel structure, in Chicago's John Hancock

Building, in 1969. As the diagonals of a braced tube are connected to the columns

at each intersection, they virtually eliminate the effects of shear lag in both theflange and web frames. As a result the structure behaves under lateral loads more

like a braced frame reducing bending in the members of the frames. Hence, the

spacing of the columns can be increased and the depth of the girders will be less,

thereby allowing large size windows than in the conventional framed tube

structures. In the braced tube structure, the braces transfer axial load from the more

highly stressed columns to the less highly stressed columns and eliminates

differences between load stresses in the columns.



Tube-in-Tube Structures

This is a type of framed tube consisting of an outer-framed tube together

with an internal elevator and service core. The inner tube may consist of braced

frames. The outer and inner tubes act jointly in resisting both gravity and lateral

loading in steel-framed buildings. However, the outer tube usually plays a

dominant role because of its much greater structural depth. This type of structures

is also called as Hull (Outer tube) and Core (Inner tube) structures. A typical Tube-in-Tube structure is shown in Fig. 5.



Bundled Tube

The bundled tube system can be visualised as an assemblage of individual

tubes resulting in multiple cell tube. The increase in stiffness is apparent. The

system allows for the greatest height and the most floor area. This structural form

was used in the Sears Tower in Chicago. In this system, introduction of the internal

webs greatly reduces the shear lag in the flanges. Hence, their columns are more

evenly stressed than in the single tube structure and their contribution to the lateralstiffness is greater.

WIND LOAD

The wind loading is the most important factor that determines the design of

tall buildings over 10 storeys, where storey height approximately lies between 2.7

– 3.0 m. Buildings of up to 10 storeys, designed for gravity loading can

accommodate wind loading without any additional steel for lateral system. Usually,

buildings taller than 10 storeys would generally require additional steel for lateralsystem. This is due to the fact that wind loading on a tall building acts over a very

large building surface, with greater intensity at the greater heights and with a larger

moment arm about the base. So, the additional steel required for wind resistance

increases non-linearly with height as shown in Fig. 6. As shown in Fig. 6 the lateral

stiffness of the building is a more important consideration than its strength for

multi-storeyed structures. Wind has become a major load for the designer of multi-

storeyed buildings. Prediction of wind loading in precise scientific terms may not

be possible, as it is influenced by many factors such as the form of terrain, the

shape, slenderness, and the solidarity ratio of building and the arrangement of

adjacent buildings. The appropriate design wind loads are estimated based on twoapproaches. Static approach is one, which assumes the building to be a fixed rigid

body in the wind.

This method is suitable for buildings of normal height, slenderness, or susceptible

to vibration in the wind. The other approach is the dynamic approach. This is

adopted for exceptionally tall, or vibration prone buildings. Sometimes wind

sensitive tall buildings will have to be designed for interference effects caused by

the environment in which the building stands. The loading due to these interference

effects is best ascertained using wind tunnel modelled structures in the laboratory.

However, in the Indian context, where the tallest multi-storeyed building is only 35storeys high, multi-storeyed buildings do not suffer wind-induced oscillation and

generally do not require to be examined for the dynamic effects. For detailed

information on evaluating wind load, the reader is referred to IS: 875 - 1987 (Part -

III).



EARTHQUAKE LOAD

Seismic motion consists of horizontal and vertical ground motions, with the

vertical motion usually having a much smaller magnitude. Further, factor of safety

provided against gravity loads usually can accommodate additional forces due to

vertical acceleration due to earthquakes. So, the horizontal motion of the ground

causes the most significant effect on the structure by shaking the foundation back

and forth. The mass of building resists this motion by setting up inertia forces

throughout the structure. The magnitude of the horizontal shear force F shown inFig. 7 depends on the mass of the building M, the acceleration of the ground a, and

the nature of the structure. If a building and the foundation were rigid, it would

have the same acceleration as the ground as given by Newton’s second law of

motion, i.e. F = Ma. However, in practice all buildings are flexible to some degree.

For a structure that deforms slightly, thereby absorbing some energy, the force will

be less than the product of mass and acceleration [Fig. 7(b)]. But, a very flexible

structure will be subject to a much larger force under repetitive ground motion

[Fig. 7(c)]. This shows the magnitude of the lateral force on a building is not only

dependent on acceleration of the ground but it will also depend on the type of the

structure. As an inertia problem, the dynamic response of the building plays a large part in influencing and in estimating the effective loading on the structure. The

earthquake load is estimated by Seismic co-efficient method or Response spectrum

method. The later takes account of dynamic characteristics of structure along with

ground motion. For detailed information on evaluating earthquake load, reader is

referred to IS: 1893 - 1984 and the chapter on Earth quake resistant design.



Fig. 7 Force developed by earthquake

MEMBER FORCE ANALYSIS

Analysis of the forces in a statically determinate triangulated braced frame

can be made by the method of sections. For instance, consider a typical single-

diagonal braced pinpointed panel as shown in Fig. 8. When this bent is subjected to

an external shear Qi in ith storey and external moments Mi and Mi-1 at floors i and

i-1, respectively, the force in the brace can be found by considering the horizontal

equilibrium of the free body above section XX, thus,

FBC Cos θ = Qi

Hence,FBC = Qi / Cos θ

The axial force FBD in the column BD is found by considering moment

equilibrium of the upper free body about C, thus

FBD* ℓ = Mi-1

Hence,

FBD = Mi-1 / ℓ

Similarly the force FAC in column AC is obtained from the moment equilibrium of

the upper free body about B. It is given by

FAC = Mi / ℓ

This procedure can be repeated for the members in each storey of the frame. Themember forces in more complex braced frames such as knee-braced, X-braced and

K-braced frames can also be obtained by taking horizontal sections.



MULTI-STOREY BRACED STRUCTURES

Lateral stability in two directions approximately at right-angles to each other

should be provided by a system of vertical and horizontal bracing within the

structure so that the columns will not be subject to sway moments. Bracing can

generally be provided in the walls enclosing the stairs, lifts, service ducts, etc.Additional stiffness can also be provided by bracing within other external

or internal walls. The bracing should preferably be distributed throughout the

structure so that the combined shear centre is located approximately on the line of

the resultant on plan of the applied overturning forces. Where this is not possible,

torsional moments may result, which must be considered when calculating the load

carried by each braced bay. Braced bays should be effective throughout the full

height of the building. If it is essential for bracing to be discontinuous at one level,

provision must be made to transfer the forces to other braced bays.

Forms of bracing

Bracing may consist of any of the following:

• Horizontal bracing

–– triangulated steel members

–– concrete floors or roofs

–– adequately designed and fixed profiled steel decking

• Vertical bracing

–– triangulated steel members

–– cantilever columns from moment resistant base –– reinforced concrete walls preferably not less than 180mm in thickness

–– masonry walls preferably not less than 150mm in thickness adequately pinned

and tied to the steel frames. Precautions should be taken to prevent such walls

being removed at a later stage, and temporary bracing provided during erection

before such masonry walls are constructed

–– uplift forces generated from the vertical bracing system shall be adequately

(safely) held down by the foundations.



BRACING SYSTEMS

INTRODUCTION

In common design practice and in design guides and manuals, bracing

systems are very often identified with triangulated trusses or with concrete cores or

shear walls which are present in buildings to accommodate shafts and staircases. It

is very common to find bracing systems represented as shown in Figure 1.

The simplification of representing a bracing system by a triangulated truss also

arises because in steel structures, in contrast to concrete structures where all the

joints are naturally continuous, the most immediate way of making connections between members is to hinge one member to the other. As a result structures are

created which need bracing systems in order to prevent failure mechanisms

forming. Based on this simplifying consideration, all the joints of Figure 9 can be

assumed to be hinged. Therefore bracing can only be obtained by use of

triangulated trusses or concrete cores or, exceptionally, by a very strong frame.

In conclusion, a topological definition of a bracing system equates a bracing

system to a triangulated truss or to a shear wall. This definition covers the majority



of actual cases but is not sufficient to clarify the function of a bracing system. For

this purpose a definition based on the requirements of a bracing system is provided

below.

Engineering Definition

A bracing system can be defined as a structural system capable of resisting

horizontal actions and limiting horizontal deformations. On the basis of this

definition, all the systems shown in Figure 10 can be considered bracing systems.

Within one building more than one of these systems can be present. In that case

some systems are more effective than others in resisting horizontal loads, the

others are neglected.

The definition allows a simple frame, and even a column, to be considered as a

bracing system. The column or the frame may not have enough strength or stiffnessto resist the horizontal actions with reasonable sizing of its members (columns and

girders) and then to satisfy the strength and serviceability checks which require

limited interstorey and global drifts. In this case it is necessary to add other bracing

systems to the frame itself.

BRACED AND UNBRACED FRAMES

Introduction

Sometimes the term "bracing system" is inappropriately identified with theterm "braced frame". It is clear that the definitions of the two terms are different.

The word "braced" in the second case is used as an adjective to the word "frame"

and therefore at least two structures have to be identified: a bracing and a frame.

A braced frame is commonly intended as a frame to which a triangulated truss is

attached.




The main function of a bracing system is: to resist horizontal actions, and is

derived from the separation of the resisting systems: vertical and horizontal. In

some cases the vertical system also has some capability to resist horizontal actions.

It is necessary therefore, from an engineering point of view, to identify the two

sources of resistance and to compare their behavior with respect to the horizontal

actions. Sometimes this identification is not obvious since the bracing is integralwithin the frame and therefore there is only one structure. However, even in this

case, it is possible to make some assumptions in order to define the two structures

to be compared. The examples given below clarify these concepts.

Figures 11 and 12 represent structures in which it is easy to define, within one

system, two sub-assemblies which identify the bracing system and the system to be

braced. In particular, a structure is shown in Figure 11 where there is a clear

separation of functions: the horizontal loads are carried by the first hinged sub-

assembly (A) and the vertical loads are carried out by the second one (B). In Figure12, in contrast, since the second sub-assembly (B) is able to resist horizontal

actions as well as vertical actions, it is necessary to assume that practically all the

horizontal actions are carried by the first sub-assembly (A) in order to define this

system as braced. In this case the first sub-assembly is defined as a bracing system

if its lateral stiffness expressed by the spring constant Ka is considerably higher

than the one of the second sub-assembly Kb (in this case a braced frame or

system):

Fig. 11: Pinned connection structure split into two sub-structure

Fig. 12: Partly framed structure split into two sub-assemblies



Euro code Definition

"The frame can be classified as braced if the bracing system reduces its

horizontal displacement by at least 80%" which means that the stiffness of the two

systems has to be compared and the following relationship satisfied:

K a > 0.8 (K a + K b)

or K a > 4 K b

Where, K a and K b stiffnesses of both sub-assemblies

SWAY AND NON-SWAY FRAMES

Introduction

Before defining sway frames and non-sway frames, it is useful to note the

common design practice for evaluating safety of structures against stability. It is

often convenient to isolate the columns from the frame and treat the stability of columns and the stability of frames as independent problems. For this purpose it is

assumed the columns are restricted at their ends from horizontal displacements and

therefore are only subjected to end moments and axial loads as transferred from the

frame. It is then assumed that the frame, possibly by means of a bracing system,

satisfies global stability checks and that the global stability of the frame does not

affect the column behaviour. This gives the commonly assumed non-sway frame.

This approach has led to years of research spent in the field of behaviour of

columns and beam-columns.

To explain the concept of sway, as opposed to non-sway, figures such as Figures 13

and 14 are used. The frame of Figure 13 is considered to be the non-sway type and

the one of Figure 14 is considered to be the sway type. This form of representation,

which is based on common practice and common engineering sense, leads to the

erroneous assumption that non-sway frames and braced frames are perfectly

equivalent and therefore that one definition can be used instead of the other

without causing any misunderstanding.



Fig. 13: Braced Frame (but may be sway if bracing is very flexible)

Fig. 14: Unbraced Frame (but may be non-sway if it is sufficiently rigid i.e.

insensitive to horizontal loading)


The equivalence between "braced" and "non-sway" frames cannot be

established in general since the two terms refer to different aspects of the behavior

of the structure. The fact that the definitions of “sway” and “non-sway” appear

when the problem of stability of columns and frames is evaluated suggests that

these definitions are part of a simpler treatment of this problem.

The concept of braced and enbraced frames can be defined in engineering terms by

means of comparison of the stiffness of the systems, as in previous sections, and

has no straightforward implications for stability. The concept of sway frames is not

intrinsic to the structure: it is based only on its mechanical properties.



In fact the seismic meaning of the term "non-sway frame" has no real significance.

It is only valid in an "engineering" sense. There is no structure, braced or unbraced,

in which there are no sway displacements. The displacements can only be small

enough, for particular design purposes, to be considered equal to zero in an

engineering sense.

Another reason for defining "sway" and "non-sway frames" is the need to adopt

conventional analysis in which all the internal actions are computed on the basis of the undeformed shape of the structure. To make this assumption it is necessary that

second order effects are negligible, i.e. no significant moments arise due to the

action of vertical loads on the deformed shape of the structure. This definition can

be shown to be equivalent to the previous one since the vertical design loads cause

no significant moments if their value is not close to the elastic critical load of the

structure.

When there is interaction between global and column behavior, it is not possible to

isolate the column. The column or the frame then has to be assumed to be the"sway" type. Unfortunately, research has been limited in this field and therefore

extrapolation of the same procedures already used for non-sway frames to sway

frames has been used. As a result inaccuracies occur also due to the fact that the

actual behavior is inelastic and is therefore affected by all types of imperfections,

i.e. cross-section, column and frame imperfections. In addition the inelasticity in

the columns prevents the use of the familiar concept of "effective length". The

design of sway frames has to consider the structure as a whole.

On the basis of those considerations, the following definitions can be established

for sway and non-sway frames:

A non-sway frame is a structure which, from the points of view of stability and the

definition of the internal action, can be considered to have small interstorey

displacements. Therefore column buckling is independent by frame buckling, i.e.

the problems can be uncoupled. This definition will be true if the safety factor

against overall buckling is sufficiently large that global buckling can be neglected

when carrying out the check against column buckling. On the basis of this

definition, it is clearly that to be a non-sway frame is not a characteristic intrinsic

of the frame since the safety factor against critical load depends on the magnitude

of the design vertical loads acting on the structure.

Whilst it is possible to define whether a frame is braced or not by evaluating the

stiffness of its members, in order to evaluate whether a frame is the non-sway type,

i.e. second order effects can be neglected, the design vertical loads have to be

known. This is understandable since even a very flexible structure has no second

order effects if the vertical loads are practically equal to zero.



Euro code Definition

"A frame can be classified as non-sway if its response to in-plane horizontal forces

is sufficiently stiff for it to be acceptably accurate to neglect any additional internal

forces or moments arising from horizontal displacements of its nodes".

"A frame may be classified as non-sway for a given load case if the elastic critical

load ratio Vsd/Vcr for that load case satisfies the criterion:

VsdVcr≤0,1

where Vsd is the design value of the total vertical load

and Vcr is its elastic critical value for failure in a sway mode."

This application rule confirms that the definition of a frame as non-sway dependson the vertical loads. Furthermore it establishes that a safety factor against overall

buckling equal to 10 is enough for considering the problem uncoupled from

column buckling.

FRAME CLASSIFICATION

[i] Braced or unbraced.

[ii] Non-sway or sway.

Braced Frames

A frame may be classified as braced if its sway resistance is supplied by a bracing

system with a response to in-plane horizontal loads which is sufficiently stiff for it

to be acceptably accurate to assume that all horizontal loads are resisted by the

bracing system.

This may be further quantified as:

A steel frame may be classified as braced if the bracing system reduces itshorizontal displacements by at least 80%.

When the above conditions are not satisfied the frame must be considered as

unbraced.

The initial sway imperfections plus any horizontal loads applied to a braced frame

may be treated as affecting only the bracing system.



Unbraced Frames

An unbraced frame may be classified as a non-sway frame according to Eurocode

providing:

Its response to in-plane horizontal forces is sufficiently stiff for it to be acceptably

accurate to neglect any additional internal forces or moments arising from

horizontal displacements of its nodes.

This classification may be further quantified as:

A frame may be classified as non-sway for a given load case if the elastic critical

load ratio VSd /Vcr for that load case satisfies the criterion:

VsdVcr≤0,1

where Vsd is the design value of the total vertical load.

Vcr is its elastic critical value for failure in a sway mode.

Beam-and-column type plane frames in building structures with beams connecting

each column at each storey level may be classified as non-sway for a given load

case if the following criterion is satisfied. When first-order theory is used, the

horizontal displacements in each storey due to the design loads (both horizontal

and vertical), plus the initial sway imperfection applied in the form of equivalent

horizontal forces, should satisfy the criterion:

where d is the horizontal displacement at the top of the storey, relative to the

bottom of the storey.

h is the storey height.

H is the total horizontal reaction at the bottom of the storey.

V is the total vertical reaction at the bottom of the storey.

Both requirements follow from the idea that, if satisfied, the load-carrying

resistance determined by neglecting sway effects will be only a ten per cent less

than that calculated by including such effects. This approach is, in turn, based upon

the Merchant-Rankine concept for estimating the true ultimate load of a frame that

fails by some from of inelastic instability from knowledge of its elastic critical load

and its first-order, rigid-plastic collapse load. Both loads are relatively

straightforward to calculate.



The original Merchant-Rankine formulae for the failure load Vsd is:

where:

Vcr is the elastic critical load.

Vpl is the first-order, rigid-plastic collapse load.

From this it is clear that when Vcr >> Vpl, then Vsd ~ Vpl.

Frames that do not meet the above requirements must be designed as sway frames.

Sway Frames

Sway frames shall be analysed under those arrangements of the variable

loads which are critical for failure in a sway mode. In addition, sway frames shall

also be analysed for the non-sway mode.

The initial sway imperfections and member imperfections where necessary, shall

be included in the global analysis of all frames.

The allowance for imperfections in the analysis of sway frames is intended to

cover effects such as lack of verticality, lack of straightness, residual stresses, etc.

It is expressed in Euro code 3 by means of a set of equivalent geometrical

imperfections [1]. These imperfections are not actual construction tolerances but,

because they are intended to represent the effect of a number of factors, are likelyto be larger than such tolerances.

FRAME CLASSIFICATION

When analysing a principle structural frame, it is important to classify it as

being either sway or non-sway, braced or unbraced. The classification of the frame

determines the method of global analysis which may be employed and the

influence of secondary effects on frame actions.

Classification as Braced or Unbraced

The frame is classified as braced or unbraced depending on the relative stiffness of

the bracing system providing the resistance to lateral forces.

The criterion for a braced frame is that the bracing system is at least five times

stiffer than the lateral stiffness of the frame. It should be noted that this

requirement will automatically be satisfied for simple frames incorporating bracing



systems. In the absence of a bracing system a simple frame has zero lateral

stiffness.

For a frame which is deemed to be braced, the bracing system must be designed to

resist all lateral loads applied to the structure including those arising due to frame

imperfections.

A frame which is classified as braced is automatically designed as non-sway. A

frame which is deemed to be unbraced must be further classified as either sway or

non-sway. Classification as Sway or Non-Sway

A frame is classified as non-sway for a given load case if the following criterion is

satisfied:

VsdVcr≤0,1

Where, Vsd is the design value of the total vertical load

Vcr is the elastic critical load for failure of the frame in a sway mode.

For multi-storey beam-and-column plane frames, with beams connecting each

column at each storey level, classification as non-sway can be made for a given

load combination, simply by determining whether the following condition is

satisfied:

where

δ is the deflection over a storey height (interstorey drift)

h is the storey height

V is the vertical reaction at the bottom of a storey

H is the horizontal reaction at the bottom of a storey.

REASONS FOR FRAME TO SWAY

(i) Eccentric or unsymmetrical loading on the portal frame.

(ii) Unsymmetrical outline of portal frame.

(iii) Different end conditions of the columns of the portal frame.

(iv) Non-uniform section of the members of the frame.



(v) Horizontal loading on the columns of the frame.

(vi) Settlement of the supports of the frame.

(vii) A combination of the above.

What is a Shear Wall Building?

Reinforced concrete (PC) buildings often have vertical plate-like RC walls

called Shear Walls (Figure 15) in addition to slabs, beams and columns. These

walls generally start at foundation level and are continuous throughout the building

height. Their thickness can be as low as 150mm, or as high as 400mm in high rise

buildings. Shear walls are usually provided along both length and width of

buildings (Figure 15). Shear walls are like vertically oriented wide beams that

carry earthquake loads downwards to the foundation. Advantages of Shear Walls in

RC Buildings Properly designed and detailed buildings with shear walls have

shown very good performance in past earthquakes. The overwhelming success of

buildings with shear walls in resisting strong earthquakes is summarized in the

quote:

Figure 15. Reinforced concrete shear walls in buildings-an excellent structural

system for earthquake resistance.

Shear wails in high seismic regions require special detailing. However, in

past earthquakes, even buildings with sufficient amount of wails that were not

specially detailed for seismic performance (but had enough well-distributed

reinforcement) were saved from collapse. Shear wall buildings are a popular

choice in many earthquake prone countries, like Chile, New Zealand and USA.

Shear walls are easy to construct, because reinforcement detailing of walls is

relatively straight-forward and therefore easily implemented at site. Shear wails are



efficient, both in terms of construction cost and effectiveness in minimizing

earthquake damage in structural and non-structural elements (like glass windows

and building contents).

Architectural Aspects of Shear Walls

Most RC buildings with shear walls also have columns; these columns

primarily carry gravity loads (i.e., those due to self weight and contents of

building). Shear walls provide large strength and stiffness to buildings in the

direction of their orientation, which significantly reduces lateral sway of the building and thereby reduces damage to structure and its contents. Since shear

wails carry large horizontal earthquake forces, the overturning effects on them are

large. Thus, design of their foundations requires special attention. Shear walls

should be provided along preferably both length and width. However, if they are

provided along only one direction, a proper grid of beams and columns in the

vertical plane (called a moment resistant frame) must be provided along the other

direction to resist strong earthquake effects. Door or window openings can be

provided in shear walls, but their size must be small to ensure least interruption to

force flow through walls. Moreover, openings should be symmetrically located.Special design checks are required to ensure that the net cross-sectional area of a

wall at an opening is sufficient to carry the horizontal earthquake force.

Shear walls in buildings must be symmetrically located in plan to reduce ill-effects

of twist in buildings (Figure 16).They could be placed symmetrically along one or

both directions in plan. Shear walls are more effective when located along exterior

perimeter of the building - such a layout increases resistance of the building to

twisting.

Figure16. Shear walls must be symmetric in plan layout- twist in buildings can

be avoided.



Ductile Design of Shear Walls

Just like reinforced concrete (RC) beams and columns, RC shear walls also

perform much better if designed to be ductile. Overall geometric proportions of the

wall, types and amount of reinforcement, and connection with remaining elements

in the building help in improving the ductility of walls. The Indian Standard

Ductile Detailing Code for RC members (IS: 13920- 1993) provides special design

guidelines for ductile detailing of shear walls.

Overall Geometry of Walls: Shear walls are oblong in cross-section, i.e., onedimension of the cross-section is much larger than the other. While rectangular

cross-section is common, L- and U-shaped sections are also used (Figure 17).

Thin-walled hollow RC shafts around the elevator core of buildings also act as

shear walls, and should be taken advantage of to resist earthquake forces.

Figure 17. Shear walls in RC Buildings- different geometries are possible.

Reinforcement Bars in RC Walls:

Steel reinforcing bars are to be provided in walls in regularly spaced vertical

and horizontal grids (Figure 18 a). The vertical and horizontal reinforcement in the

wall can be placed in one or two parallel layers called curtains. Horizontal

reinforcement needs to be anchored at the ends of walls. The minimum area of reinforcing steel to be provided is 0.0025 times the cross-sectional area, along each

of the horizontal and vertical directions. This vertical reinforcement should be

distributed uniformly across the wall cross-section.

Boundary Elements: Under the large overturning effects caused by horizontal

earthquake forces, edges of shear walls experience high compressive and tensile

stresses. To ensure that shear walls behave in a ductile way, concrete in the wall

end regions must be reinforced in a special manner to sustain these load reversals

without loosing strength (Figure 18 b). End regions of a wall with increased



confinement are called boundary elements. This special confining transverse

reinforcement in boundary elements is similar to that provided in columns of RC

frames. Sometimes, the thickness of the shear wall in these boundary elements is

also increased. RC walls with boundary elements have substantially higher bending

strength and horizontal shear force carrying capacity, and are therefore less

susceptible to earthquake damage than walls without boundary elements

Figure18. Layout of main reinforcement in shear walls as per IS: 13920-1993

Bracing System in Building

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