Effective span of beam for simply supported, Cantilever, Continous- IS

The beam is a structural member that takes loads from the slab and transfers them to the column. The effective span of beam is different for different types of beams. It depends upon the clear span, width of support, and depth of the beam as well.

The effective span of the beam is required to analyze the structure for safety and economics. The beam of the structure should be strong and durable enough to carry the load and remain stable for a long period of time.

The spans of different beams are given below. But, first of all, you should know about some technical terms given below.

What is Clear Span?

The span of the beam without measuring the length of support is known as the clear span of the beam. The clear span can be calculated by subtracting the width of the support, i.e., column, along the direction of the beam span from the total span of the structure.

What is effective Span?

It is measured according to IS code 456-2000.

What is effective cover?

It is the distance between the outer surface of the beam and the middle of the main steel rod provided in the beam. The effective clear cover depends on the load on the beam. However, the thumb rule to provide effective cover in the beam should not be less than 25 mm.

What is effective depth?

It is the (Total depth of beam – effective cover of beam). You can see in picture below.

Calculation for Effective span of beam

Here I have described the effective span of different types of beams given below,

1. Effective span for Simply supported beam
2. For Continuous beam
3. Cantilever beam

a) Effective span for Simply supported beam

The effective span of a simply supported beam is taken as the least of the following:

a) Clear span + the effective depth of beam.

b) Center to center ( c/c ) distance between supports.

b) For Continuous beam

In case of continuous beam, if the width of the support is less than 1/12 of the clear span, the effective span is taken as in (a).

If the width of support is more than 1/12 of the clear span or 600 mm whichever is less, the effective span is taken as:

For end span with one end fixed and the other continuous or for intermediate spans, the effective span is taken as the clear span between supports;

For end span with one end free and the other continuous, the effective span is equal to the clear span + half the effective depth of the beam or the clear span + half the width of the discontinuous supports, whichever is less.

In case of span with roller or rocket bearings, the effective span always the distance between the centre of bearing.

c) Cantilever beam

The effective length of a cantilever beam is taken as its length to the face of the supports + half the effective depth except where it forms the end of a continuous beam.

How to Calculate Effective span of beam?

Q) Calculate the effective span of beam having a depth of 500 mm, a Clear span of 3 meters, and a supported on column having a width of 400×400 mm.

Solution:-

Given, Depth (D) = 500 mm

Clear Span (S) = 3 m = 3000 mm

Dimension of support = 400×400 mm

Assume, effective cover = 45 mm

Now,

We have two method to calculate effective span of beam,

A) Effective span (l)

= Clear span + Effective depth of Beam

= 3000 + (500-45) (Taking effective cover 45 mm)

= 3455 mm

B) Effective span (l) (center-to-center distance between supports)

= Clear span + Width of support

= 3000 + (400/2+400/2)

= 3400 mm

Hence, from the above calculation, B has a minimum value of effective span (l). So the effective span of beam will be 3400 mm.

I hope you got something new.

Happy Learning – Civil Concept

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