Beams

UNIT – III

Chapter - 6(B)

BEAMS, COLUMNS and LINTELS

BEAMS

R.C.C. Structural Members

Now-a-days, Plain and R.C.C. structural members like Foundation, Beams, Columns, Lintels, Roofs and Roof Slabs, etc., are widely used in the construction of all types of buildings.

Advantages: R.C.C. structural members are highly durable, strong, economical, quickness in construction with improved appearance. Also, different types of finishes can be given to concrete.

1. R.C.C. BEAMS

Beam: Beam is a horizontal member of a structure, carrying transverse loads. Beam is rectangular in cross-section. Beams carry the Floor Slab or the Roof Slab. R.C.C., Pre-stressed Concrete and Steel I-Sections are used as beams to support the slabs.

The beams transfer all the loads (Dead and Live Loads) including its Self-weight to the Columns or Walls. The loads from the columns or walls are transmitted to the foundation. From the foundation, the loads are safely transmitted to the soil.

R.C.C. Beam: R.C.C. beam is subjected to bending moments and shear. Due to the vertical external load, bending compresses the top fibers of the beam and elongates the bottom fibers. The strength of a R.C.C. beam depends on the composite action of concrete and steel. Concrete is strong in compression and weak in tension.

Main Reinforcement Bars: Reinforcing Main Steel Bars are embedded in the tension zone to give the tensile strength. Concrete takes all the compressive stresses and the main steel bars take all the tensile stresses.

Stirrups: Vertical Shear Bars called Stirrups are provided to withstand shearing forces.

2. DEFINITIONS

Loads Acting on a Structure: (i) Dead load and (ii) Live load.

Dead Load: Dead load is the self-weight of the various components of a building.

Live Load: Live load is the external superimposed load on a structure. Live loads may be (i) Uniformly distributed load, (ii) Uniformly varying load, (iii) Concentrated load.

Horizontal or Sloping Beams: Beams may be mostly horizontal or sloping like roof beams.

Span: The distance between the two supports of a beam is called the Span of the beam.

Clear Span: It is the clear horizontal distance between the two supports of a beam.

Effective Span of a Beam: It is the horizontal distance between the center of one support to the center of the next support. For design purposes, the effective span is always considered.

3. TYPES OF BEAMS

1. Based on Support

(i) Simply Supported Beam

(ii) Fixed Beam

(iii) Cantilever Beam

(iv) Continuous Beam

(v) Overhanging Beam

1. Simply Supported Beam (Fig. 1)

It is a beam supported freely at the two ends on walls or columns. In actual practice, no beam rests freely on the supports (walls or columns) without fixing on the supports. Moment is not induced at the supports, since it allows rotation. Hence, no beam is practically simpl supported.

2. Fixed Beam (Fig. 2)

In this beam, both the ends of the beam are rigidly fixed into the supports (walls or columns). Also, Main Reinforcement Bars and Stirrups (in the form of rectangular rings) are provided. Here, Bending is constrained at the supports. A Moment is induced at the supports, since it will not allow rotation.

3. Cantilever Beam (Fig. 3)

When a beam is fixed in a wall or column at one end and the other end is free, it is called a Cantilever Beam. It has tension zone in the top side and compression zone in the bottom side.

Uses: It is used to support slabs projecting outside the wall/column.

Examples: Portico, Balcony, etc.

4. Continuous Beam (Fig. 4)

A Continuous Beam is supported on more than two supports. This beam is more economical for any span lengths.

5. Overhanging Beam (Fig. 5)

In Overhanging Beam, its end extends beyond the wall or column support. Overhanging of the beam is the unsupported portion of the beam. It may be one side or both the sides of the support.

2. Based on Reinforcement

a) Singly Reinforced Beam (Fig. 6)

If the main reinforcement steel bars are provided only on one side of the beam, it is End Cover known as Singly Reinforced Beam.

b) Doubly Reinforced Beam (Fig. 7)

If the main reinforcement steel bars are provided both at top and bottom (tension and compression zones) of the beam, it is known as Doubly Reinforced Beam.

Concrete Covers: Main bars are placed uniformly along the perimeter of the column leaving required Covers. Concrete cover is the distance between the outer surface of the member and the nearest point of reinforcement. It may be End Cover or Bottom Cover.

4. TYPES OF LOADING ON BEAMS

See Figs. 8(i), (ii), (iii) and (iv)

(i) Concentrated Load,

(ii) Uniformly Distributed Loads (U.D.L.),

(iii) Uniformly Varying Loads

(iv) Arbitrary Loading.

5.  STEEL BEAMS

Steel Beams are generally Rolled Steel I-Sections.

I-Section may be split and rejoined by welding and used for beams with large spans and light loads.