Limit State Method
IS 456 Standards for Beams and Slabs and Columns
Effective span
A. Simply supported beams and slabs (leff)
Here, l0 = clear span
w = width of support
d = depth of beam or slab
B. For continuous beam
(i) If width of support < 1/12 of clear span
(ii) If width of support > 1/12 of clear span
(a) When one end fixed other end continuous or both end continuous.
leff = l0
(b) When one end continuous and other end simply supported
C. Cantilever
(i)
(ii)
D. Frames
leff = Centre to centre distance
Control of deflection
(i) This is one of the most important check for limit state of serviceability.
(a) The final deflection due to all loads including the effect of temperature, creep and shrinkage and measured from as cast level of the support of floors, roofs and other horizontal members should not normally exceed span/250
(b) The deflection including the effect of temperature, creeps and shrinkage occurring after erection of partition and application of finishes should not normally exceed span/350 or 20 mm which ever is less.
(ii) The vertical deflection limit may generally be satisfied if
(a) Basic span to effective depth ratio for span upto 10m is
Types of Beams:
For cantilever → 7
For simply supported → 20
For continuous → 26
(b) For span > 10 m effective depth
Where 'A' is span to effective depth ratio for span upto 10m.
(c) Depending upon the tension reinforcement the value 'A' can be modify by multiplying a factor called modification factor (MF1)
effective depth
where,
(d) Depending upon area of compression reinforcement, value (A) can be further modified using a modification factor (MF2)
effective depth =
(e) For flanged beam : A reduction factor is used
(f) Deflection check for two way slab:
Slenderness limit
- For simply supported or continuous beams
where, l0 = Clear span
b = Width of the section
and, d = Effective depth - For cantilever beam
- Minimum tension reinforcement
- Maximum tension reinforcement = 0.04 bD
- Maximum compression reinforcement = 0.04 bD
where, D = overall depth of the section - Where, D > 750 mm, side face reinforcement is provided and it is equal to 0.1% of gross cross-section area (b × D). It is provided equally on both face.
- Maximum spacing of side face reinforcement is 300 mm.
- Maximum size of reinforcement for slab/beam is 1/8 of total thickness of the member
- Nominal cover for different members
Beams → 25 mm
Slab → 20 to 30 mm
Column → 40 mm
Foundations → 50 mm - Moment and shear coefficient for beams/slabs
- Minimum tension reinforcement
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Comments
write a commentAmit Singh RanaSep 14, 2019
Tarun BhatiaSep 22, 2019
Ramkrishan SoniSep 22, 2019
Aham Khan ArkSep 23, 2019
Aham Khan ArkSep 23, 2019
TOP 10 LISTOct 9, 2019
And for two way slab the ratio is less than 2.
Nikhil KumarFeb 23, 2020
ThakurJul 2, 2020
slab ke L/D ratio me L kon sa span hai
longer yaa shorter
Arjun HodshilSep 9, 2020
SirichandanaSep 14, 2020