3. STRUCTURAL DESIGN?
and
for
• Process of determining
, selection of
determination of
the structure to be built
• Aim: ensure that the structure will perform
satisfactorily during its design life
Structural Design
4. STRUCTURAL DESIGN PURPOSES?
• Fitness for purpose
• Safety and reliability
• Economy
• Maintability
Structural Design Purpose?
5. Fitness for purpose
• Arrangement of spaces, spans, ceiling height,
access and traffic flow must complement the
intended use.
• The structure should fit its environment and
be aesthetically pleasing
Fitness for Purpose
6. • Structure must be strong to safely support all
anticipated loadings
• Structure must not deflect, overturn, tilt,
vibrate or crack in a manner that impairs its
usefulness
Safety and Reliability
7.
8.
9.
10. Economy
• Overall cost of structure should not exceed
the client’s budget
• Designer should take into account: cost of
materials, buildability, construction time, cost
of temporary structures and maintenance
costs
Economy
16. Structural Elements
• Beams: horizontal members carrying lateral loads
• Slabs: horizontal plate elements carrying lateral loads
• Columns: vertical members carrying primarily axial
loads but generally subjected to axial load and moment
• Walls: vertical plate elements resisting vertical, lateral
or in-plane loads
• Foundations: pads or strips supported directly on the
ground that spread loads from columns or walls to the
ground
• Stairs: plate elements consists of a flight of steps,
usually with one or more landings provided between
the floor levels
Structural Elements
18. Code of Practice
• Document that gives recommendations for
the design and construction of structures
• Contains detailed requirements regarding
loads, stresses, strengths, design formulas and
methods of achieving the required
performance of complete structure
Codes and Specifications
19. Codes and Specifications
A code represents the consensus of opinion of experienced engineers
and professionals.
These codes provide the guidelines for the design and construction of
structures. They are revised at regular intervals to reflect new
developments based on the experience gained from past design practice,
behaviour of existing structures, and failure of structures.
Codes contain the recommended loads for a given locality and the
recommended fire and corrosion protection. They also contain rules
governing the ways in which loads are to be applied and design rules for
steel, concrete, and other materials.
20. Codes and Specifications
The codes serve at least the following four distinct functions:
1. They ensure adequate structural safety, by specifying certain
essential minimum requirements for the design.
2. They aid the designer in the design process. Often, the results of
sophisticated analysis are made available in the form of simple formulae
or charts.
3. They ensure consistency among different engineers.
4. They protect the structural engineer from disputes, though codes
in many cases provide legal protection.
21. Project specifications along with the design drawings are given to the
builder by the architect or project manager. These specifications and the
way in which the drawings are prepared and presented vary from
organization to organization.
They include the following items:
1. Materials that must be used in the structure
2. Sizes of structural members
3. Joint details
4. Expected quality and tolerance
5. Instructions on how the construction work is to be done
Codes and Specifications
23. Introduction
Determination of various loads is a very important phase in a structural
design process.
Before designing any structure or the different elements such as beams
and columns, one has to first determine the various natural and man-
made loads acting on them.
Dead loads, imposed loads, snow, ice, and earth loads, and hydraulic
pressure are caused due to mass and gravitational effect.
Indirect loads are caused due to environmental effects such as
temperature difference, settlement, and shrinkage.
25. Characteristic Actions (Loads)
The determination of the loads for which a given structure has to be
proportioned is one of the most difficult problems in design.
Decisions are to be made on the type of loads the structure may
experience during its lifetime, combinations of loads, and so forth.
The probability that a specific load will be exceeded during the life of a
structure usually depends on the period of exposure (or life) of the
structure and the magnitude of design load.
Loads applied to a structure during its life should be considered
statistically and a characteristic load determined.
27. Dead Loads
The load that is fixed in magnitude and position is called the dead load.
Determination of the dead load of a structure requires the estimation
of the weight of the structure together with its associated ‘non-
structural’ components.
After the design process, the initially assumed dead load of the
structure (based on experience) has to be compared with the actual dead
load.
If the difference between the two loads is significant, the assumed
dead load should be revised and the structure redesigned.
28. Weights of Some Building Materials
as per IS 875 (Part 1)
S. No. Material Unit Weight
1. Brick masonry in CM 1:4 20 kN/m3
2. Plain concrete 24 kN/m3
3. Reinforced cement concrete 25 kN/m3
4. Stone masonry 20.4–26.5 kN/m3
5. Cement mortar 20.4 kN/m3
6. Steel 78.5 kN/m3
7. 20 mm cement plaster 450 N/m2
8. 5 mm glass 125 N/m2
9. Floor finishes 600–1200 N/m2
10. Water 10 kN/m3
Table 3.1 Weight of some building materials as per IS 875 (Part 1)
29. Imposed Loads
Imposed loads (also referred to as live loads) are gravity loads other
than dead loads and include items such as occupancy by people, movable
equipment and furniture within the buildings, stored materials such as
books or machinery.
The code provides uniformly distributed loads (UDLs) as well as
concentrated loads for various occupational categories. The distributed
and concentrated imposed loads shall be considered separately and the
design carried out for the most adverse conditions.
30. Live Loads on Floors as per IS 875
(Part 2 )
S. No. Type of Floor Usage Imposed Load (kN/m2)
1. Residential 2.0
2. Office
(a) with separate storage
(b) without separate storage
2.5
4.0
3. Shops, classrooms, restaurants, theatres, etc.
(a) with fixed seating
(b) without fixed seating
4.0
5.0
4. Factories and warehouses 5.0-10.0
5. Book stores and stack rooms in libraries 10.0
6. Garages with light vehicles 4.0
7. Stairs, landings, and balconies
(a) not liable to overcrowding
(b) liable to overcrowding
4.0
5.0
Table 3.2 Live loads on floors as per IS 875 (Part 2 )
31. Imposed Loads
Imposed load may change from room to room.
To account for the most adverse load cases, analysis should be carried
out for the following:
1. Factored live load on all spans
2. Factored live load on two adjacent spans resulting in high bending
moment (BMs) over the support between the two loaded spans
3. Factored live load on alternate spans resulting in high BMs at mid-
span at the loaded beams
32. Imposed Loads
When large areas are considered, the code allows for a reduction in the
imposed load, unless earthquake loads are considered.
Code IS 875 (Part 2) also provides the values of horizontal loads acting
on parapets and balustrades. These loads should be assumed to act at
handrail or coping level.
Roofs are considered non-accessible except for normal maintenance
and minor repairs. If roofs are frequently accessible and used for floor-
type activities, they should be treated as floors and the corresponding
loads should be considered.
33. Consideration of Slab Loads on
Beams
One-way Slab
When a slab is supported on four
sides and the length to width ratio
is greater than two, the slab acts as
a one-way slab and the beams
along the long spans are assumed
to carry the load from the slab.
Fig. 3.3 Load distribution - One-
way slabs
34. Two-way Slab
In two-way slabs (i.e., when a slab is supported on four sides and the
length to width ratio is lesser than or equal to two), the load distribution
is based on yield-line analysis.
In a square slab, the yield lines running at 45° from each corner will
meet at a single point in the centre.
In a rectangular slab, the same four yield lines will not meet at one
point, and hence, there will be a fifth yield line running between the
intersections of these yield lines. Thus, the long beams will be subjected
to a trapezoidal load and the short beams to a triangular load.
Consideration of Slab Loads on
Beams
36. Other possible loadings on beams and equivalent uniform
loads:
1. Two triangular loads in span
2. Triangular load in the middle of span
3. Triangularly varying load
Consideration of Slab Loads on Beams
37. Fig. 3.5 Other possible loadings on beams and equivalent uniform loads (a) Two triangular loads in
span (b) Triangular load in the middle of span (c) Triangularly varying load
Consideration of Slab Loads on
Beams
38. Consideration of Wall Loads on
Beams
When the height of the masonry over a beam is
greater than about 0.6 times the span of the
beam, it can be assumed that there will be an
arch action in the masonry and hence some part
of the load will be transferred to the columns on
either side. In such situations, only wall loads
bounded by a° lines from the columns cause
bending moment and shear force in the beam, as
shown in Fig. 3.6(b).
39. Consideration of Wall Loads on
Beams
Fig. 3.6 Consideration of wall loads (a) Typical frame with walls (b) Loads transferred to
columns and beams (c) Load on beam due to walls (d) Short span walls
40. Impact Loads
Impact due to vertical crane, moving machinery, and so on is
converted empirically into equivalent static loads through an impact
factor, which is normally a percentage (20% to 100%) of the
machinery load (as specified in IS 875—Part 2).
The loads due to cranes and other machineries are often
obtained from the manufacturers or suppliers.
The impact load is an important criterion in industrial buildings
where machinery will be mounted on floors and also in bridges.
41. Snow and Ice Loads
Snow and ice loads are to be considered in the mountainous
(Himalayan) regions in the northern parts of India. Thus, the roofs
in these regions should be designed for the actual load due to snow
or for the imposed loads specified in IS 875 (Part 2), whichever is
more severe.
Although maximum snow and maximum wind loads are not
considered to act simultaneously, it is important to consider drift
formation due to wind, since the majority of snow-related roof
damage is due to drifted snow.
42. Wind Loads
Winds are produced by the differences in atmospheric pressures,
which are primarily due to the differences in temperature.
Horizontal wind flow exerts lateral pressure on the building
envelope and hence has to be considered in the design.
Code IS 875:1987 (Part 3) provides the basic wind speeds,
averaged over a short interval of 3 seconds and having a 50-year
return period at 10 m height above ground level in different parts of
the country.
43. Wind Loads
The wind pressure or load acting on the structural system and the
structural or non-structural component being considered depends
on the following:
1. Velocity and density of air
2. Height above ground level
3. Shape and aspect ratio of the building
4. Topography of the surrounding ground surface
5. Angle of wind attack
6. Solidity ratio or openings in the structure
7. Susceptibility of the structural system under consideration to steady
and time-dependent (dynamic) effects induced by the wind load
44. Wind Loads
The wind load on a building can be calculated for the following:
1. The building as a whole
2. Individual structural elements such as roofs and walls
3. Individual cladding units including glazing and their fixings
45. Wind Loads
The code provides the pressure coefficients (derived on the basis
of models tested in wind tunnels) for a variety of buildings. Force
coefficients are also given for clad buildings, unclad structures, and
structural elements.
Wind causes pressure or suction normal to the surface of a
structure. Pressures are caused both on the exterior as well as the
interior surfaces, the latter being dependent on the openings (or
permeability) in the structure, mostly in the walls.
46. Factors Affecting Wind Pressure
Coefficients
1. Shape of the building or roof
2. Slope of the roof
3. Direction of wind with respect to building
4. Zone of the building
47. Wind Loads
Code IS 875 (Part 3) provides the external
coefficient for mono-slope and hipped roofs,
canopy roofs, curved roofs, pitched and saw-
tooth roofs of multi-span buildings,
overhangs from roofs, cylindrical structures,
roof and bottom of cylindrical structures,
combined roofs, and roofs with skylight,
grand stands, and spheres.
48. Wind Loads
Fig. 3.7 Typical industrial building elevation along with the wind pressure coefficients: (a) Typical elevation
with wind pressure coefficients Cpe and Cpi
(b) Half plan
49. Earthquake Loads
In IS 1893 (Part 1) code, the following seismic design philosophy has
been adopted:
1. Minor and frequent earthquakes should not cause any damage to the
structure.
2. Moderate earthquakes should not cause significant structural damage
but could have some non-structural damage (structure will become
operational once the repair and strengthening of the damaged main
members are completed).
3. Major and infrequent earthquakes should not cause collapse (the
structure will become dysfunctional for further use, but will stand so
that people can be evacuated and property recovered).
51. Earthquake Loads
For the purpose of determining seismic forces, India is classified into
four seismic zones (zones II to V) by IS 1893 (Part 1) code.
The code requires that the designer either use a dynamic analysis of the
structure or, for the usual generally rectangular medium height buildings
(regular buildings), use an empirical lateral base shear force.
There are various calculations such as determination of natural
frequencies using the design horizontal seismic coefficient of a structure,
approximate fundamental natural period of vibration, etc. calculated for
a building depending on its height.
52. Thermal and Shrinkage Effects
If the lateral dimension of the building exceeds 45 m, temperature
effects must be considered in the design, or suitable expansion or
contraction joints should be provided.
Structures that have abrupt changes in plan should be provided with
expansion joints at places where such changes occur. These expansion
joints facilitate the necessary movements to occur with minimum
resistance at the joint.
53. Thermal and Shrinkage Effects
The spacing of expansion joints is affected by many factors such as
building shape, material type and associated properties, restraints to
movement such as walls and bracing and their relative location in the
structure, etc.
Temperature stresses are important in the design of chimneys, cooling
towers, and structures designed to resist loads due to fires.
54. Slabs and other elements exposed to the sun’s radiation also develop
temperature stresses. In such occasions, nominal reinforcements are
often provided, close to the surface that is being affected, to take care of
temperature and shrinkage.
Concrete shrinks as it dries out. Usually, slabs and other members are
joined rigidly to other parts of the structure and cannot contract freely.
This will result in tensile stresses, known as shrinkage stresses.
Thermal and Shrinkage Effects
55. Thermal and Shrinkage Effects
Fig. 3.13 Length between expansion joints versus design temperature change ΔT, as given
in ACI 224.3/R (1995) (a) Graph as per Martin and Acosta method (1970) (b) Graph as
per National Academy of Sciences (1974)
56. A shrinkage strip or pour strip is a temporary joint in the structure that
is left open for a certain time during construction to allow a significant
part of shrinkage to take place.
A better alternative is shrinkage compensating concrete which is made
with expansive cement, which will expand by an amount equal to or
slightly greater than the anticipated drying shrinkage.
Shrinkage Strip and Shrinkage
Compensating Concrete
58. Working Stress Method
This was the traditional method of design not only for RC but also for
structural steel and timber design. The conceptual basis of the WSM
assumes that the structural material behaves in a linear elastic manner
and that adequate safety can be ensured by suitably restricting the
stresses in the material due to the expected working loads (service loads)
on the structure.
WSM also assumes that both the steel reinforcement and concrete act
together and are perfectly elastic at all stages, and hence the modular
ratio can be used to determine the stresses in steel and concrete.
59. Working Stress Method
The stresses under the working loads are obtained by applying the
methods of ‘strength of materials’ like the simple bending theory. The
limitations due to non-linearity and buckling are neglected.
The stresses caused by the ‘characteristic’ or service loads are checked
against the permissible (allowable) stress, which is a fraction of the
ultimate or yield stress. The permissible stress may be defined in terms of
a factor of safety, which takes care of the overload or other unknown
factors.
Permissible (allowable) stress = Ultimate or yield stress/Factor of safety
Thus, in WSM,
Working stress ≤ Permissible stress
60. Limitations of Working Stress
Method
1. The main assumption of a linear elastic behaviour and the implied
assumption that the stresses under working loads can be kept within
the ‘permissible stresses’ are found to be unrealistic. Many factors are
responsible for this, such as the long-term effects of creep and
shrinkage and other secondary effects.
2. The use of the imaginary concept of modular ratio results in larger
percentage of compression steel and generally larger member sizes
than the members designed using ultimate load or limit states design.
However, as a result of the larger member sizes, they result in better
performance during service.
61. Limitations of Working Stress
Method
3. The stress–strain curve for concrete is non-linear and is time
dependent. Thus, the elastic modulus is a function of the stress level (it
may also change with age) and hence the modular ratio is not really
constant. This method does not consider the consequences of this
material non-linearity.
4. WSM does not discriminate between the different types of loads that
act simultaneously, but have different degrees of uncertainty. This
may result in unconservative designs, particularly when two different
loads (say, dead loads and wind loads) have counteracting effects.
62. Ultimate Load Design
This method is also referred to as the load factor method or the
ultimate strength method. The shortcomings of WSM led to the
development of the ultimate load design.
In this method, the non-linear stress–strain curves of concrete and steel
and the stress condition just before collapse are considered. Sufficient
safety is achieved by the use of a load factor, which is defined as the ratio
of the ultimate load (design load) to the working load.
63. Ultimate Load Design
In this method of design, different types of loads are assigned with
different load factors under combined loading conditions, thereby
overcoming the related shortcomings of WSM.
It should be noted that even though non-linear stress–strain behaviour
is considered in the design, the analysis is still based on linear elastic
theory.
One of the disadvantages of this method is that the performance at the
normal service loads is not considered.
64. Principles of Limit States Design
•Limit States Design is a design approach that
combines the best features of the ultimate strength
design and working stress design.
• Will result in better structural performance in
strength and serviceability.
•Principles behind this method are explained in the
following slides.
65. Uncertainties in Design
1. To safeguard against the risk of failure (collapse or unserviceability), safety margins
are normally provided in the design. These are based on reliability-based design
methods.
2. The variables such as loads, material strength, and member dimensions are subject
to varying degrees of uncertainty and randomness. Hence, the design should take
into account the possibility of overload or under strength.
3. There are also several unforeseen factors that influence the prediction of strength
and serviceability. They include construction methods, workmanship and quality
control, intended service life of the structure, human errors, possible future change
of use, and frequency of loading. In order to provide reliable safety margins, the
design must be based on the probabilistic methods of design.
67. •In the limit states design, the term ‘limit states’
is preferably used instead of the term ‘failure’.
•Thus, a limit state is a state of impeding failure,
beyond which a structure ceases to perform its
intended function satisfactorily.
Limit States
68. 1. Ultimate (safety) limit states, which correspond to the maximum load
carrying capacity and are concerned with the following:
(a)Loss of equilibrium (collapse) of a part or the whole structure when
considered as a rigid body
(b)Progressive collapse
(c)Transformation of the structure into a plastic mechanism collapse
(d)Rupture of critical sections due to the stress exceeding material
strength or by deformations
(e)Loss of stability (buckling, overturning, or sliding)
(f) Fracture due to fatigue
Types of Limit States
69. 2. Serviceability limit states, deal with the discomfort to occupancy
and/or malfunction, caused by excessive deflection, excessive crack
width, undesirable vibration, etc.
3. Special limit states deal with the abnormal conditions or abnormal
loading such as damage or collapse in extreme earthquakes, damage
due to fire, explosions, or vehicle collisions, damage due to corrosion
or deterioration, elastic, plastic, or creep deformation, or cracking.
Types of Limit States
70. The design for the ultimate limit state may be conveniently explained
with the help of Fig. 4.13 (in the following slide). This figure shows the
hypothetical frequency distribution curves for the effect of loads on the
structural element and the resistance (strength) of the structural
element.
When the two curves overlap, shown by the shaded area, the effect of
the loads is greater than the resistance of the element, and the element
will fail. Thus, the structure and its elements should be proportioned in
such a way that the overlap of the two curves are small, which means
that the probability of failure is within the acceptable range.
Ultimate Limit State
72. A full-scale probabilistic analysis is generally described as a level III
reliability method, which uses the probability of failure to evaluate the
risk involved.
The problem may be simplified by limiting the probability information
of the basic variables to their ‘second moment statistics’ (i.e., mean and
variance). Such a method is called a level II reliability method where the
structural failure (the achievement of a limit state) is examined by
comparing the resistance with the load effect in a logarithmic form.
Level I reliability method or first-order second moment reliability
method is used in the code to obtain a probability-based assessment of
structural safety.
Levels of Reliability Methods
74. In normal design calculations, a single value is usually used for each
load and for each material property, with a margin to take care of all
uncertainties. Such a value is termed the characteristic strength (or
resistance) or characteristic load.
The characteristic strength of a material is defined as the value of
strength below which more than a prescribed percentage of test results
will fall. This prescribed percentage is normally taken as 95.
The characteristic load, Qc, is load that is not expected to be exceeded
with more than five per cent probability during the lifespan of a
structure.
Characteristic Load and Characteristic
Strength
75. Fig. 4.15 Normal frequency distribution of concrete strengths
Characteristic Load and Characteristic
Strength
76. Sampling and Acceptance Criteria
IS 456 accepts a size of 30 samples for normal distribution. When
insufficient test results (less than 30 samples) are available, an assumed
value of standard deviation has to be used.
The standard deviation of the results from the mean value is regarded
as an index of the scatter and hence may reveal the site control. A small
value of standard deviation will result in a curve with a dominant peak,
while a larger value will result in a flatter curve as shown in Fig. 4.16 (in
the following slide).
Three test specimens form a sample. The minimum frequency of
sampling depends on the quantity of concrete, as shown in Table 4.3.
77. Sampling and Acceptance Criteria
Fig. 4.16 Typical normal frequency curves for different levels of
control
78. Sampling and Acceptance Criteria
Quantity of Concrete Involved,
m3
Required Number of Samples
1-5 1
6-15 2
16-30 3
31-50 4
>51 4 + 1 additional for each
additional 50 m3
Table 4.3 Sampling frequency
79. Limit States Method
Limit states method aims for a comprehensive and rational solution to
the design problem, by considering safety at ultimate loads and
serviceability at working loads.
The limit states method philosophy uses a multiple safety factor format
that attempts to provide adequate safety at the ultimate loads as well as
adequate serviceability at the service loads, by considering all possible
limit states.
The selection of the various multiple safety factors is supposed to have
a sound probabilistic basis, involving the separate consideration of the
different kinds of failure, types of materials, and types of loads.
80. Multiple Safety Factor Format
The limit states design has to ensure that the probability of any limit
state being reached is acceptably low. This is accomplished by specifying
appropriate multiple safety factors for each limit state (level I reliability).
The values of multiple safety factors are chosen by a careful reliability
study in order to achieve the target reliability.
Limit States of Strength
81. As per LRFD, the expression for structural safety is given as follows:
Design resistance (j Rn) ≥ Design load effect (ΣgiQi)
where the left-hand side of the equation represents the strength (or
resistance) of the system or component and the right-hand side
represents the load expected to be carried by the system or component.
The nominal strength Rn is multiplied by a strength reduction (or
resistance reduction factor) j to obtain the design strength.
Similarly, the various load effects Qi (such as dead, live, and wind loads)
are multiplied by their respective overload factors gi to obtain the sum
ΣgiQi.
Load and Resistance Factor Design
Format
82. Resistance factor accounts for the ‘under strength’ and is less than
unity.
The load factors which accounts for ‘overloading’ and the uncertainties
associated with various load effects, are generally greater than unity.
Load and Resistance Factor Design
Format
83. Resistance factors will take care of the following aspects:
1. The possibility of unfavourable deviation of material strength
from the characteristic value
2. The possibility of unfavourable reduction in member strength due
to fabrication and tolerances
3. The possibility of unfavourable variation of member sizes
4. The uncertainty in the theoretical assumptions
5. The uncertainty in the calculation of strength of members
Load and Resistance Factor Design
Format
84. The loads factors will account for the following:
1. The possibility of unfavourable deviation of the load from the
characteristic value
2. The possibility of inaccurate assessment of the load
3. The uncertainty in the assessment of the effects of the load
4. The uncertainty in the assessment of the limit state being
considered
Load and Resistance Factor Design
Format
85. The multiple safety factor format adopted by the Indian code was
initially recommended by CEB-FIP 1970. It is called the partial safety
factor format and is expressed as follows:
Here, Rd is the design strength (or resistance) computed using the
reduced material strengths Ru/gm where Ru is the characteristic material
strength and gm is the partial safety factors for the material that allows
for uncertainties of element behaviour and possible strength reduction.
Qid is the design action (load effect) computed for the enhanced loads,
involving separate partial load factors such as for dead load, imposed
load, wind or earthquake load.
Partial Safety Factor Format
Rd ≥ ΣgifQid
86. The partial safety factors for loads, make allowances for possible
deviation of loads and the reduced possibility of all loads acting together.
When more than one imposed load acts simultaneously, the leading load
is that causing the larger action effect.
All the load factors are generally greater than unity, because
overestimation usually results in improved safety.
The load factors are reduced when different types of loads (DL, LL, and
WL or EL) are acting simultaneously at their peak values. (This is
sometimes referred to as the load combination effect.)
Partial Safety Factor for Loads
87. The material partial safety factors gm for concrete and reinforcing
steel is taken as 1.5 and 1.15, respectively.
The partial safety factor of 1.15 as applied to steel reinforcement
accounts for the following:
1. The reduction in strength of any member as a result of inaccurate
positioning of steel
2. The reduction in strength of steel reinforcement due to any
manufacturing defect
A higher value of gm is prescribed by the code for concrete as the
strength of concrete may deviate much from the assumed strength due
to deviation in properties of aggregates and defects or variations that
may occur during the production of concrete.
Partial Load Factors for Materials
88. In this limit state, the variable to be considered is a serviceability
parameter Δ (representing deflection, vibration, etc.). Limit state or
failure is considered to occur when a specified maximum limit of
serviceability, Δall is exceeded.
The limiting failure is deterministic and not probabilistic.
Serviceability limit states relate to satisfactory performance and
correspond to excessive deflection, vibration, local deformation,
durability, and fire resistance.
Serviceability Limit States
89. The maximum deflection affecting the strength and stability of the
structure is controlled by the strength limit state. However, excessive
deflection should not produce sagging appearance, plaster cracking, or
failure to align plant and machinery.
Excessive deflection of beams causes damage to supported non-
structural elements or impair the usefulness of the structure. On roofs, a
major deflection-related concern is ponding of water.
Excessive deflections are often indicative of excessive vibration and
noise transmission, both of which are serviceability problems.
Deflections and Crack Widths
90. Deflections are to be calculated for all combinations of loads specified
in the code, by using an elastic analysis and checked for the maximum
values specified in the code.
Serviceability, instead of strength, may often control the design of
beams. The code suggests an empirical method (by limiting the span to
effective depth ratios) and a theoretical method to calculate the
deflection (by considering the effective moment of inertia) and compare
it with the limiting deflection.
For controlling crack widths too, the code recommends an empirical
method (detailing by spacing of bars, minimum steel ratios, etc.) and a
theoretical method to calculate the actual width of cracks and compare it
with the limiting crack width.
Deflections and Crack Widths
91. With the development of lighter construction using high-strength
concrete and use of longer spans, there is a higher risk of vibrations
becoming critical in a number of situations.
Vibration will have to be checked when vibrating loads such as due to
machinery, washing machines, and cranes applied to slabs. In these
cases, care must be taken to ensure that the structural response will not
amplify the disturbing motion.
Vibrations
92. It is recommended that flexible structures (with height to effective
width ratio exceeding 5:1) should be investigated for lateral vibration
under dynamic loads.
When vibration becomes a problem, one may have to change the
natural frequency of the structure by some means. Increasing the load
factor will not help in reducing this problem.
Vibrations
