Design of RCC structure.... Storey building

1. MultistoreyMultistorey
BuildingBuilding
Introduction

2. Multistorey BuildingMultistorey Building

3. Multistorey BuildingMultistorey Building
• Reinforced concrete buildings consist of floor slabs,
beams, girders and columns continuously placed to
form a rigid monolithic system as shown in Fig. 19.1.
Such a continuous system leads to greater
redundancy, reduced moments and distributes the
load more evenly. The floor slab may rest on a
system of interconnected beams. Interior beams B4,
B5, B6 are supported on the girders, whereas, the
exterior beams BI, B2, B3 are directly supported by
the columns. Girders such as GI, G2, G3 are also
supported directly by the columns. Beams BI, B2, B3
and girders GI, G2 and 03 are not only continuous
but are also monolithic with upper and lower
columns.

4. Multistorey BuildingMultistorey Building
• Thus, a building frame is a three-dimensional
structure or a space structure. It is idealized as a
system of interconnected two-dimensional vertical
frames along the two mutually perpendicular
horizontal axes for analysis. These frames are
analyzed independently of each other. In frames
where the columns are arranged on a rectangular
grid, loading patterns giving biaxial bending need
not be considered except for corner columns.

5. Multistorey BuildingMultistorey Building
• The degree of sophistication to which a structural
analysis is carried out depends on the importance
of the structure. A wide range of approaches have
been used for buildings of varying heights and
importance, from simple. approximate methods
which can be carried out manually, or with the aid
of a pocket calculator, to more refined techniques
involving computer solutions. Till a few years ago
most of the Multistorey buildings were analyzed by
approximate methods such as substitute frame,
moment distribution, portal and cantilever methods.

6. Multistorey BuildingMultistorey Building
• The recent advent of personal computers (PC's)
and the abundance of ready-made computer
package programs has reduced the use of
approximate methods which at present are useful
for preliminary analysis and verification. The
application of computers is not restricted merely to
analysis; they are used in almost every phase of
concrete work from analysis to design, to plotting,
to detailing, to specification writing, to cost
estimating etc.

7. Sections of multistorySections of multistory
BuildingBuilding

8. Structural AnalysisStructural Analysis
• A building is subjected to various load such as dead
load, live load, lateral load such as wind load or
earthquake load.
• A structural systems may be classified as follows
i. Load bearing wall systems
ii. Building with flexural systems
iii. Moment resisting frame system
iv.Dual frame system
v. Tube systems

9. i.i. Load bearing wall systemLoad bearing wall system
• Walls provide support for all gravity load s as well as
resistance to lateral loads
• There are no columns
• The walls and partition wall supply in-plane lateral
stiffness and stability to resist end and earthquake
loading.
• This systems lacks in providing redundancy for the
vertical and lateral load supports, that is, if the walls
fail, the vertical loads as well as lateral loads
carrying capacity is eliminated to instability.

10. ii.ii. Building with flexural wallBuilding with flexural wall
systemsystem
• The gravity load is carried primarily by a frame
supported on columns rather than bearing walls.
• Some minor portion of the gravity load can be
carried on bearing walls but the amount so carried
should not should not represent more than a few %
of the building area.
• The resistance to lateral loads is provided by
nominal moment resistance be incorporated in the
vertical load frame design.

11. iii.iii. Moment resisting frameMoment resisting frame
systemssystems
• If the system in which members and joints are
capable of resisting vertical and lateral loads
primarily by flexural.
• To qualify for a response reduction factor R=5. the
frame should be detailed conforming to IS
13290:1993 to provide ductility excepts in seismic
zone2.
• In moment resistant frame, relative stiffness of
girders and columns is very important.
• A frame may be designed using weak column-
strong girder proportions or strong column-weak
girder proportions.

12. iv.iv. Flexural shear (wall)Flexural shear (wall)
systemssystems
• It is reinforced concrete wall designed to resist
lateral forces parallel to the plane of the wall and
detailed to proved ductility conforming to IS 13290-
1993.
• The international building code of America IBC 2000
permits the use of flexural shear wall systems up to
height of about 45m.
• However, it can be used up to height of 70m, if and
only if; flexural walls in any plane do not resist more
than 33% of the earthquake design force including
torsional effects.

13. v.v. Dual frame systemsDual frame systems
• It is structural system with the following features:
i. Moment resisting frame providing support for gravity
loads.
ii. Resistance resisting loads is provided by :
a) Specially detailed moment resisting frame which is capable of resisting at
least 25% of the base shear including torsion effects
b) Flexural walls must resist total required lateral force in accordance with
relative stiffness considering interaction of walls and frames as single
systems.

14. vi.vi. Space frameSpace frame
• It is three dimensional structural systems without
shear or bearing walls composed of interconnected
members laterally supported so as to function as
complete self contained unit with or without aid of
horizontal diaphragm of floor systems.

15. vii.vii. Tube systemTube system
• If consisting of closely spaced exterior columns tied
at each floor level with relatively deep spandrel
beams.
• Thus it creates the effect of hollow concrete tube
perforated by opening for the windows.
• The exterior columns are generally spaced between
1.2m to 3m.
• The spandrel beams interconnecting the closely
spaced columns have a depth varying from 60cm
to 1.25m and width from 25cm to 1m.
• Such building has very high moment of inertia
about the two orthogonal axes in controlling lateral
displacements in very tall building.

16. Stiffness ElementsStiffness Elements
• In tall buildings stiffness elements are required so as
to control the lateral drift from serviceability
considerations.
• Stiffness may be provided through walls, wall panels
or diagonal bracing members.

17. RegularityRegularity
• Regularity of a building can significantly affect its
performance during a strong earthquake.
• Past earthquakes have repeatedly shown that buildings
having irregular configurations suffer greater damage than
buildings having regular configurations.
• Regular structures have no significant physical discontinuities
in plan or vertical configuration or in their lateral force
systems.
• Whereas irregular structures have significant physical
discontinuities in configuration or in their lateral force resisting
systems.
• They may have either vertical irregularity or plan irregularity or
both.

18. Vertical StructuralVertical Structural
IrregularityIrregularity
• Stiffness soft- storey- a soft storey is one in which the lateral
stiffness is less than 70% of that in the storey above or less than
80% of average stiffness of the three storeys above.
• Strength weak storey- a weak storey is one in which the storey
strength is less than 80% if that in the storey above. The storey
strength is the total strength of all seismic force resisting
elements sharing the storey shear for the direction under
consideration.
• Vertical geometry
• In plane discontinuity and
• Weight or mass - A mass irregularity is considered to exist
where the effective mass of any storey is more than 150% of
the effective mass of an adjacent storey. A roof which is
lighter than the floor below need not be considered.

19. Plan Structural IrregularityPlan Structural Irregularity
It may be caused on account of the following
aspects:
(1) Torsional irregularity,
(2) Re-entrant corners,
(3) Diaphragm discontinuity,
(4) Out-of-plane offsets, and
(5) Non-parallel systems.

20. NEED FOR REDUNDANCYNEED FOR REDUNDANCY
• It is strongly 'recommended that the lateral force resisting system be made as
redundant as possible within the functional parameters of the building because of
many unknowns and uncertainties in the magnitude and characteristics of the
earthquake loading, in the materials & systems of construction, and in the method
of analysis.
• Redundancy plays an important role in determining the ability of the building to
resist earthquake forces. In a statically determinate system every component must
remain operative to preserve the integrity of the structure.
• On the contrary, in a highly indeterminate system, one or more redundant
components may fail and still leave a structural system which retains its integrity
and continue to resist the earthquake forces although with reduced effectiveness.
• It is, therefore, preferable to provide multiple lines of bracing to perimeter bracing,
and multiple bents or bays of bracing in each bracing line than a single braced bay.
Good torsional stiffness is also essential. The objective is to create a system that
will have its inelastic behavior distributed nearly uniformly throughout the plan and
elevation of the system. The back up system can prevent progressive or
catastrophic collapse if distress occurs in the primary system.

21. PARTITION WALLS ORPARTITION WALLS OR
INFILL WALLSINFILL WALLS
• Brick masonry is a highly non-homogeneous and orthotropic material and it is
difficult to model its behavior and properties. The behavior of a framed
building with masonry infill's is quite complex. There are several problems
associated with the masonry panels during earthquakes. Soft storey effect is
the most serious. The presence of windows and absence of any rigid contact
between the masonry panel and the beam above and below further
complicates the problem. The infill's are brittle and weak compared to the
concrete members. The infill's contribute to the stiffness of the building during
the initial stage of loading but fail much earlier before the ultimate capacity
of the frame is reached. Similarly, they do not contribute to the strength and
ductility of the frame. The usual practice is to ignore the strength and stiffness
of the infill's but to consider its mass and design the bare frame for earthquake
load.
• It is desirable to provide a 10-20 mm clear gap between the masonry panel
and the adjoining beam and columns. The gap may be filled with weak
mortar. The intention is to let the infill panel separate from the moment
resisting frame during an earthquake and let the moment resisting frame resist
the earthquake force through ductile behavior. It is recommended that for
such ductile moment resisting frames with infill wall panels a R value equal to 4
should be taken instead of as implied in the. IS: 1893-2001.

22. MEMBER STIFFNESSMEMBER STIFFNESS
• Stiffness of a member in elastic analysis is defined as EI/L were
E is modulus of elasticity of concrete, I is moment of inertia
and L is center to center length of a member.
• The codes generally allow the use of any reasonable
assumption when computing the stiffness for use in a frame
analysis provided the assumptions made are consistent
throughout the analysis.
• Ideally, the member stiffness El should reflect the degree of
cracking and inelastic action which has occurred along each
member immediately prior to the onset of yielding.
• The value of El varies along the length of a member and is
also a function of stress level.
• The exact determination of El is quite complex, hence simple
assumptions are required to define the flexural stiffness for
practical analysis. The results of an analysis obviously depend
on the values of El.

23. Modulus of elasticityModulus of elasticity
• A suitable value of the modulus of elasticity of
concrete is required if a building frame is to be
analyzed by the stiffness method using a computer.
The modulus of elasticity of concrete is considerably
more variant than its compressive strength.

24. Moment of inertiaMoment of inertia
• The moment of inertia of a section can be determined on the basis
of any one of the following cross-sections throughout the building:
(a) Gross concrete section -'the cross-section of the member
ignoring reinforcement,
(b) Gross equivalent section - the concrete cross-section plus the
area of reinforcement transformed on the basis of modular ratio.
(c) Cracked section - the area of concrete in compression plus
the area of reinforcement transformed on the basis of modular
ratio.
• For the purpose of computing the moment of inertia, the value of
modular ratio may be taken as 15 irrespective of the grade of
concrete in the absence of better information. A consistent
approach should be used for all elements of the structure.

25. Moment of inertiaMoment of inertia
• The moment of inertia of beams/girders and columns is
generally calculated on the basis of gross-section with no
allowance made for reinforcing steel. There is a difficulty
involved in the determination of moment of inertia to be used
in continuous T-beams. The moment of inertia is much greater
where there is sagging moment with the flanges in
compression than where there is hogging moment with the
flanges cracked due to tension. Thus, there is a need to use
an equivalent value which is constant throughout its length. A
general practice is to assume equivalent moment of inertia
equal to twice the moment of inertia of the web. The depth
of web is taken as the overall depth of the beam.

26. LOADSLOADS
• The dead load on a frame is calculated floor-wise and consists of
weight of floors, girders, partition walls, false ceiling, parapets,
balconies, fixed or permanent equipment and half the columns
above and below a floor. The load acting on a column is calculated
from all the beams framing into it.
• Live loads the magnitude of live load depends upon the type of
occupancy of the building. IS 875-1987 (part 2) has specified certain
minimum values of live loads (or imposed loads) for specific purpose
as given in Appendix C. l. The live load distribution varies with time.
Hence, each member is designed for the worst combination of dead
and live loads. A reduction in live load is allowed for a beam if it
carries load from an area greater than 50 m2. The reduction is 5 % for
each 50 m2 area subject to a maximum reduction of 25 %.

27. LOADSLOADS

28. Wind LoadsWind Loads
Wind is essentially a random phenomenon. In the past
it was considered sufficient to the highest wind speed
that had been recorded at the meteorological
stations nearest concerned place. The corresponding
wind pressure was applied statically. This was
erroneous practice since wind loading varies with
time. Moreover, the wind speed )depends on several
factors such as : density of obstructions in the terrain,
size of gust, return period, and probable life of
structure etc. Thus no deterministic method can do
Lice with wind loading.

29. • The wind loads in IS: 875-1987. (Part 3) are based on
two considerations:
(1)The statistical and probabilistic approach to the
evaluation of wind loads, and
(2)Due recognition to the dynamic component of
wind loading and its interaction with the dynamic
characteristics of the structure.

30. • The design wind speed Vz at any given height and
at a given site is expressed as a product of four
parameters
• Vz = Vbk1k2k3
Where
Vb = basic wind speed in meter/sec at 10 m height
K1= probability or risk factor
k2 = terrain, height, and structure size factor
k3 =local topography factor

31. Effect of Sequence ofEffect of Sequence of
ConstructionConstruction
• Most computer softwares for the analysis of building
frames are based on the stiffness matrix method.
They require input of the building geometry and
loading before beginning the analysis. The quantum
of data depends whether the analysis is 2-D or 3-D.
In actual practice, a building is built up gradually,
hence dead load is also built up gradually. In a 15-
storey building, at the time 6th floor is being raised,
there is only a 6- storey frame and not a 15- storey
frame. Hence, the dead load of 6- storey frame is
resisted by a 6- storey frame and not a 15- storey
frame. The procedure of simultaneous analysis of a
complete frame for dead and live loads may lead
to erroneous results.

32. • The simultaneous analysis of a complete frame is
correct only for live loads. It is correct for dead
loads if all columns have identical stress level or
axial deformations. If the adjoining columns have
differential elastic shortening, the analysis may show
significant positive bending moments over the
highly stressed column. In fact, by the time 7-th
storey is being raised, the elastic axial shortening in
the 6- storey frame due to dead loads has already
taken place and, there won't be any positive
moment over the highly stressed column. A similar
situation may arise in a shear wall-frame structure
near top region of the shear wall.

33. ANALYSIS FOR LATERALANALYSIS FOR LATERAL
LOADSLOADS
• A building should be carefully designed for lateral
forces because not only must buildings have
sufficient lateral resistance to prevent overturning,
hence failure, but they also must have sufficient
lateral resistance to deflections so as to satisfy the
limit state of serviceability. Approximate analysis, of
building frames can be carried out either.by portal
method or by cantilever method. The portal
method is supposed to be satisfactory for most
buildings upto about 25 storeys, whereas, the
cantilever method is good enough for about 35
storeys.

34. Portal methodPortal method
• In this method, the following assumptions are made:
• (1) There is a point of inflection at the centre of
each girder.
• (2) There is a point of inflection at the centre of
each column.
• (3) The total horizontal shear on each storey is
divided between the columns of that storey so that
each interior column carries twice as much shear as
each exterior column.

35. • These assumptions reduce a highly statically
indeterminate structure to a statically determinate
one. The method neglects the effect of axial
deformations in the columns. he assumptions
associated with the portal method results in errors in
the vicinity of the base and top of the frame, and
at set backs or locations where significant changes
in ember stiffness occur.

36. Cantilever methodCantilever method
In this method, the following assumptions are made:
•1) There is a point of inflection at the centre of each
girder.
•2) There is a point of inflection at the centre of each
column.
•3) The intensity of axial stress in each column of a
storey is proportional to horizontal distance of that
column from the centre of gravity of all the columns of
the storey under consideration.
•It is suggested that if height of the building is more
than five times its least lateral dimension, a more
precise method of analysis should be used.

37. TORSION IN BUILDINGSTORSION IN BUILDINGS
• There are series of frames in orthogonal directions x
and y to resist gravity loads and lateral loads. A floor
is generally quite rigid in its own plane. Each frame
may have a different stiffness distribution and mass
distribution. At each floor, it is possible to centre of
rigidity due to lateral stiffness and centre of mass. If
the building is symmetric with respect to lateral
stiffness and mass, the two entre would coincide.
Otherwise, there will be an eccentricity in the two
directions.

38. TORSION IN BUILDINGSTORSION IN BUILDINGS

39. Steps in torsional analysisSteps in torsional analysis
• Step I : Arrange all the frames in the building along
the y-direction interconnected through axially rigid
links at the floor levels, apply the lateral loads and
carry out a plane frame static analysis. The frame
shears computed by analyzing this hypothetical
building are taken as the relative stiffnesses of the
lateral load resisting elements.

40. • Step 2 Arrange all the y-direction frames in the
building along the y-direction interconnected
through axially rigid links at the floor levels, apply the
lateral loads and carry Out a plane frame static
analysis. The frame shears computed by this analysis
are used to compute the x-coordinates of the
reference centers (shear centre by the storey
eccentricity approach and centre of rigidity by the
floor eccentricity approach Another such analysis in
the x-direction gives the y-coordinates of the
reference centers.
• Step 3 Compute the, torsional stiffness K9 of the
building with reference to the reference centres
computed in Step 2 using the relative stiffnesses of
the frames computed in the Step 1. The values of K9
are different for the two sets of reference centres.

41. Steps in torsional analysisSteps in torsional analysis
• Step 4 Compute the location of the reference
centres of mass. In the case of storey eccentricity
approach these are the cumulative centres of mass
while in the case of floor eccentricity approach
these are the nominal centres of mass. Then
compute the eccentricity at each floor.
• Step 5 : Compute the design torsional moments
corresponding to eda for y-direction loading and
compute the frame shears.

42. Monolithic Beam To Column JointsMonolithic Beam To Column Joints
• A beam-column joint is a very critical element in
reinforced concrete construction where the
elements intersect In all the three directions.
• Floor slab has been removed for convenience.
Quite often in design the details of joint are simply
ignored. Joints are most critical because they insure
continuity of a structure and transfer forces that are
present at the ends of members into and though
the joint.
• Frequently joints are points of weakness due to lack
of adequate anchorage for bars entering the joint
from the columns and beams.

43. The shear in jointThe shear in joint