#82 Theory and Step Wise Design guide for Built up Beams - Useful for students

Built-up Section- Theory & Details

• A built-up beam is also known as compound beam.
• The built-up beams are used when the span, load and corresponding bending moment are of such magnitudes that rolled steel beam section becomes inadequate to provide required section modulus.
• Built-up beams are also used when rolled steel beams are inadequate for limited depth.
• In building construction, the depth of beam is limited by a space provided by the architect.
• Drawing beam of small depth do not provide required section modulus. Therefore, plates are attached to the beams.
• The strength of rolled steel beams is increased by adding plates to its flange which is one of the method forming built-in section.
• The other method is to compound a number of rolled steel sections themselves.

The built-up sections shown in figure’ A’ and ‘B’ are used for heavy loads and small spans.

The built-up section ‘C’ is also used for heavy loads and small span.

The built-up section ‘D’ is used for light loads and large spans.

The built-up sections ‘E’ & ‘F’ are provided for Gantry girders.

Design Procedure of the Built-up Beams design

Here are the simple steps that are to be followed for the design of Built-up Beams.

Step one

The effective span and load required to be carried by the built-up beam are known.

Maximum bending moment and shear force in built-up beams are calculated.

Step two

Value of yield stress (fy) for structural steel is to be assumed.

The permissible bending stress (sigma bc) is calculated.

Step three

The required section modulus (Z) for the given beam section is calculated.

Step four

From the steel table, a trial section for a beam is adopted having the modulus of section (Z) about 25% to 50% in excess of that requirement. The geometrical proportion of beam sections are noted.

Step five

When the depth of the beam is noted, then the usually the practice is to select from ISI handbook No.1 (Steel tables).

The strongest Rolled steel beam that will allow for necessary thickness of power plates at top and bottom.

Step six

The area of power plate required is found out by trial and error method are by;
I = Ibeam + 2(Ap)(h/2)(h/2)

For finding the area, divide the whole equation by h/2.

Step seven

Width of cover plate and thickness are decided with the restriction of outstands.

Step Eight

Check for bending stress/Actual bending compressive stress

(sigma bc)calculated = (maximum bending moment/Gross MI) x Distance of extreme fibre in compression from Neutral Axis

= (M/Ixx)x y1

Actual bending tension stress
sigma bt, cal = (sigma bc)cal x (gross area of tension flange/Net area of tension flange)
= Value should not exceed the permissible bending stress (sigma bc) or (sigma bt) = 0/66fy

Step nine

Check for shear and Deflection

These are the basic steps involving in the design of built up sections, so that you can tell any person in the interview and not to do numericals, for more examples and theories search for more articles on this website

Regards

RCC Construction Services

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