WORKING STRESS METHOD:
Structure is designed assuming that it is in working
condition, it uses permissible stresses and factors of safety
LIMIT STATE METHOD:
Structures are designed with this method assuming that the
structure has reached its limit state.
The object of design is based on the concept that the
structure should not become unserviceable in its
lifetime for the use for which it is intended.
Concerned with functioning
of structure or structural
members under normal
use, or comfort of people or
appearance of the
Concrete should be able to perform satisfactorily in
anticipated exposure conditions during its service life.
Durability is mainly affected by:
1. Permeability of concrete
3. Cover to embedded steel
4. Type and quality of constituent material
5. Cement content and water cement ratio
6. Workmanship; for full compaction and curing
7. Shape and size of member
Low permeability is achieved by having adequate
cement content, sufficiently low water cement ratio,
complete compaction and adequate curing.
General environment to which concrete will be
subjected to is classified in 5 levels of severity i. e.
mild, moderate, severe, very severe & extreme.
3) COVER TO CONCRETE: #cls.26.4
• For longitudinal reinforcing bars in column
nominal cover should not be less than 40mm or the
dia. of such bar. For columns with dimensions less
than 200mm a cover of 25mm may be used.
• For footings minimum cover shall be 50mm.
4) CONSTITUENT MATERIAL: #cls.8.2.5
Deleterious constituents such as chlorides and
sulphates should not exceed limits
5) CEMENT CONTENT & WATER CEMENT RATIO :
OPC in excess of 450 kg/m3 should not be used unless
special consideration in design has been given to the risk
6) COMPACTION AND CURING:
• Adequate compaction without segregation should be
• Overworking the surface and addition of water/cement
should be avoided.
• Curing is essential to reduce permeability of concrete .
• #cls.13.5 provides for moist curing which should be
done for atleast 7 days for OPC and minimum 10
days(extendable to 14) for other types of blended
7) SHAPE AND SIZE OF MEMBER:
• Design of exposed structures should promote good
drainage of water.
• For structures subjected to partial hydrostatic
pressure life can be lengthened by providing extra
cover to steel, chamfering the corners or using
circular cross sections.
1) OVERTURNING: #cls.20.1
Restoring moment shall not be less than
1.2(max. overturning moment due to characteristic DL)
+1.4(max. overturning moment due to characteristic IL)
*in cases where only DL provides restoring moment,
0.9(characteristic DL) shall b considered.
The anchorages provided for overhanging members
shoulrd be such that static equilibrium is maintained.
The structure should have a factor of not less than
0.4 against sliding.
3)VARIATION IN DEAD LOAD:
probable variation in dead load during construction,
repair etc should be taken into account.
For designing framework of building, adequate
moment connections or system of bracing should
be provided to transmit horizontal forces to the
Lateral sway at the top of building should not
*H is total height of building.
Fire resistance of concrete depends on member size,
cover to reinforcement and type of aggregate.
A structural element when designed for design load
should not undergo excessive deflection.
Causes for short term deflection are:
1. Magnitude and distribution of live load
2. Span and type of end restraints
3. Cross sectional properties and % of steel.
4. Stress in reinforcement
5. Amount and extent of cracking
Causes of long term deflection are:
1. Humidity and temperature conditions at the time of
curing of concrete.
2. Age of concrete at the time of loading
3. Other factors influencing shrinkage and creep: type
and size of aggregate, water cement ratio, size of
member, presence of compression reinforcement
The total deflection due to all loads should not exceed
span/250 for slabs, beams and other horizontal members.
The deflection should not exceed span/350 or 20 mm
whichever is less for these members, after the
construction of partitions and finishes etc, in order to
avoid damage to them.
IS 456 cls.23.2.1 recommends that if span/effective depth
ratio are kept below the following( for span less than 10m)
then vertical deflection will not be excessive.
For simply supported and continuous span over 10m the
ratio must be multiplied by a factor F given as: 10/span in
For cantilevers over 10m in length, actual deflection has to
be calculated and checked with the provisions of LSM for
TYPE OF SUPPORT RECTANGULAR
SIMPLY SUPPORTED 20
* For flanged beams the values for rectangular section need to
be multiplied by reduction factor which is worked out from the
EXPOSURE INTENSITY CRACK WIDTH
MODERATE EXPOSURE Limited to 0.2mm
SEVERE EXPOSURE Limited to 0.1mm
AGGRESSIVE EXPOSURE Not more than 0.004mm
Cracks develop when tensile strength of concrete is
Cracks are an eyesore, affect strength of structure &
exposes reinforcement making it vulnerable to
As per cls.35.3.2 the crack width at the surface of
concrete should not exceed 0.3mm for the sake of
In general while designing flexural members,
compliance with spacing requirements as per cls.26.3.2
are sufficient to control cracking
The horizontal distance between 2 parallel main
reinforcing bars should not be less than:
When there are more than one row of bar:
Diameter of bar if diameter is equal
Diameter of the larger bar if diameter is equal
5mm more than the nominal size of coarse aggregate
The bar should be vertically in line
The minimum vertical distance between the bars shall be greater of:
2/3 of the nominal max size of aggregate
max size of bars
If greater spacing is required crack width has to be
checked as per formula mentioned in Annexure F of
Those beams in which breadth of compression face is
small as compared with its depth and thus the
compression side has a tendency to buckle.
TYPE OF BEAM CLEAR DISTANCE
SUPPORTS OR FROM
FREE END TO FIXED END
SIMPLY SUPPORTED OR
<60b OR <250b2/d
(whichever is less)
CANTILEVER BEAM <25b OR < 100b2/d
(whichever is less)
Vibrations may be caused by machines with rotating
parts, electric overhead cranes, wind or earthquake
loads or simply by intense repair work in a structure.
These vibrations lead to cracks in the structural
elements overtime not to mention disturbance to the
inhabitants, rendering the structure unserviceable.
Code is silent but research is needed on this topic.