Here we will learn with some simple examples how to calculate Effective Span of Beam & Slab of various type As per IS 456 2000 Clause 22.2.
Effective span: It is that length of Beam or Slab
using which we can calculate the moment acting on it & it is further
help us to design & analyze the members. (Show
in Fig:1) [IS 456 2000 Clause 22.2]
Clear span: It is the clear distance between
two supports over which the member (Beam or Slab) resting. (Shown in Fig
:1)
Effective depth: It is the distance between top most compression fiber of the concrete member (Beam or Slab) to the center of the tension reinforcement. (Shown in Fig:3) [All Beam & Slab here have same dimension profile shown below]
Effective span is an
important parameter for design & analysis of a structure parts like Beams
& Slabs also its necessary for calculating “MOR” (Moment
of resistance:the moment which can be safely withstand by a structure
or parts of structure’s without showing any considerable deformation), but the effective span depends on lots of other factors, because of that we divided effective span into Four main categories:-
a) Simply supported Beam & Slab
b) Continuous Beam & Slab
c) Cantilever Beam & Slab
d) Frames
a) Simply supported Beam & Slab: The effective span of a member that is not built integrally (monolithic-ally) with its supports shall be taken as lesser than of these two: -
Clause (i): Clear Span
(L) + effective depth (d) (shown in fig:2 &2.1)
Or
Clause (ii): Clear Span (L) + support/2 +support/2 (Center to Center distance b/w support)
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Example 1: (as shown in fig 1; where
span length= 3000 mm & width of support= 300 mm)
Clause i)
Effective span of the Beam = Clear Span (L) + effective depth (d)
= 3000 mm + 252 mm
= 3252 mm
Cause
ii)
Effective span of the Beam = Clear Span (L) + support/2 +support/2 (Center to Center distance b/w support)
=3000 mm+300 mm/2+300 mm/2
=3300 mm
Example 2: (as shown in fig 2; where
span length= 3000 mm & width of support= 200 mm)
Clause
i)
Effective span of the Beam = Clear Span (L) + effective depth (d)
= 3000 mm + 252 mm
= 3252
mm
Cause
ii)
Effective span of the Beam = Clear Span (L) + support/2 +support/2 (Center to Center distance b/w support)
=3000 mm+300 mm/2+200 mm/2
= 3200 mm
if the width of the support is less
than 1/12 of the clear span, the effective span of a continuous Beam or Slab is
calculated as per clause (a) explain above. (As shown in Fig:4)
width of support < 1/12 of clear span
or
12 x width of support < clear span (we can write this
expression mathematically)
“The clear span length should be greater than twelve times of width”
If the supports are wider than 1/12
of the clear span or 600 mm whichever is less, the effective span shall be
taken by following below mentioned criteria’s
Width of support > 1/12 of clear
span
Or
12 x width of support > clear span (we can write this expression
mathematically)
“The clear span should be less than twelve times of support width”
Effective span = Clear span
For Span 2: effective spa = clear span = 3400 mm (This is the intermediate span)
[All span is less than twelve times of the width which is 3600 mm]
2) For end span with one end free & other continuous the effective span shall be equal to lesser of these TWO
Or
Clause (ii) Clear span + width of support/2
Example 1: (as shown in fig 5; where
span 1; clear span= 3500 mm; support width= 300 mm; effective depth=252 mm)
Clause (i) Clear
span + effective depth (d)/2 = 3500 + 252/2 = 3626
mm
or
Clause (ii) Clear
span + width of support /2 = 3500 + 300/2 = 3650 mm
Here clause (i) is the deciding
clause for the effective length =3626 mm
Example 2(as shown in fig 5; where span
3; clear span= 3850 mm; support width= 200 mm; effective depth=252 mm)
Clause (i) Clear
span + effective depth (d)/2 = 2300 + 252/2 = 2426 mm
or
Clause (ii) Clear span + width of support /2 = 2300 + 200/2 = 2400 mm
Here clause (ii) is the deciding
clause for the effective length = 2400 mm
[All span is less than twelve times of the width which is 3600 mm & 2400 mm]
3) For Roller or Rocket bearing
= clear span +
roller dia/2 + roller dia/2
= clear span
+roller radius +roller radius
= clear span + dia of roller
Example: as show in fig 6; clear span
=3400 mm; roller dia=250 mm
Effective span = clear span + dia of
roller
= 3400 + 250
= 3650 mm
[All span is less
than twelve times of the width which is 3600 mm]
c) Cantilever Beam & Slab: The cantilever beam or slab are categorized
into two types for calculating their effective length there are:
- Fixed at the end (not continuous over
support): For these
cantilever bear or slab effective length consider as the clear length from the
face of the support plus effective depth. (as shown in fig:7)
Effective length = length from face of the support + effective depth(d)
Example: as show in fig 7; clear length
=1350 mm; effective depth (d) = 252 mm
Effective length= 1350 mm + 252 mm
= 1602 mm
- Continuous over support: The cantilever which is continuous
over support to form a cantilever, the effective length of that cantilever
should be taken as length from face of the support plus half the width of the support.
Effective length = length from face of the support + support width/2
Fig:8 Cantilever Beam or Slab continuous over support
Example: as show in fig 8; clear
length =1410 mm; support width = 300 mm;
Effective length= 1410 mm + 300/2
= 1410 mm + 150 mm
= 1560 mm
d) Frame: For analysis of continuous frame center to center distance shall be
consider for effective length.
Effective length= Center to center
= Clear span + width of support/2
Example: as show in fig 9; Frame 1;
clear span =3600 mm; support width = 300 mm;
Effective length= 3600 mm + 300/2
= 3750 mm
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