R.C.C Design & Concrete Technology

Guide to Doubly Reinforced RCC Beam Design

R.C.C Beams

R.C.C beams are cast in cement concrete reinforced with steel bars. Beams take up compressive and add rigidity to the structure.

Beams generally carry vertical gravitational forces but can also be used to carry horizontal loads (i.e., loads due to an earthquake or wind). The loads carried by a beam are transferred to columns, walls, or girders, which then transfer the force to adjacent structural compression members. In Light frame construction the joists rest on the beam.

Doubly Reinforced Beam

In this article, we are going to discuss types of beam construction and RCC design of Doubly reinforced beam…

RCC beam construction is of two types:

• Singly reinforced beam
• Doubly reinforced beam

Singly reinforced beam

A singly reinforced beam is a beam provided with longitudinal reinforcement in the tension zone only.

Doubly reinforced beam

• Beams reinforced with steel in compression and tension zones are called doubly reinforced beams. This type of beam will be found necessary when due to head room consideration or architectural consideration the depth of the beam is restricted.

• The beam with its limited depth, if reinforced on the tension side only, may not have enough moment of resistance, to resist the bending moment.

• By increasing the quantity of steel in the tension zone, the moment of resistance cannot be increased indefinitely. Usually, the moment of resistance can be increased by not more than 25% over the balanced moment of resistance, by making the beam over-reinforced on the tension side.

• Hence, inorder to further increase the moment of resistance of a beam section of unlimited dimensions, a doubly reinforced beam is provided.

Besides, this doubly reinforced beam is also used in the following circumstances:

• The external live loads may alternate i.e. may occur on either face of the member.

For example:

• A pile may be lifted in such a manner that the tension and compression zones may alternate.

• The loading may be eccentric and the eccentricity of the load may change from one side of the axis to another side.

• The member may be subjected to a shock or impact or accidental lateral thrust.

Design procedure for doubly reinforced beam

Step 1

Determine the limiting moment of resistance for the given c/s(Mulim) using the equation for singly reinforced beam

M_ulim = 0.87.f_y.A_st1.d [1 – 0.42X_umax]

Or

Balanced section

A_st1 = (0.36.f_ck.b.X_umax)/(0.87fy)

Step 2

If factored moment M_u > M_ulim, then doubly reinforced beam is required to be designed for additional moment.

M_u – M_ulim = f_sc.A_sc (d – d’)              [f_sc value from page no. 70]

Step 3

Additional area of tension steel Ast₂

Ast₂ =Asc.fsc/0.87fy

Step 4

Total tension steel Ast, Ast = Ast₁ + Ast₂

Guide to Design of Simple Beam | Design of Steel Structures

Design of Simple Beam

A member carrying loads perpendicular to its axis is defined as a beam.

For a simple floor beam, I-sections are used.

M/I = (sigma) /y

M = (I/y)(sigma)

I/y = Z (section modulus)

Therefore, M = z(sigma)

When beams are loaded, bending stresses are developed at all sections.

The bending stresses developed in beams can be determined by the equation theory of simple bending.

For laterally supported beams, the permissible bending stress in tension as well as in compression should not exceed (sigma)bc or (sigma)bt = 0.66fy

For laterally unsupported beams, the permissible stress in bending compression is calculated by using tables from the the IS code book (IS:800).

Load carrying capacity of the Beam

From structural steel tables for the given beam, the section modulus (Zxx) is obtained.

Depending upon whether the beam is laterally restrained or unrestrained; the value of permissible stress in bending compression ((sigma)bc) is calculated.

The moment of resistance of the beam is found out.

MR = Zxx .(sigma)bc

Equating the moment of resistance to the maximum bending moment equation, the total load (w) the beam can carry is calculated.

RCC Column

A column forms a very important component of a structure. Columns support beamswhich in turn support walls and slabs. It should be realized that the failure of a column results in the collapse of the structure. The design of a column should therefore receive importance.

Supporting the slabs is the main function of the columns… Such slabs are called Simply Supported Slabs. Simply supported slabs could be either one way slab or a two-way slab. It depends on the dimensions of the slab.

Reinforced Cement Concrete Column Plan and Section

A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete.

In this article, we are going to discuss in detail the basis of classification of columns and different types of reinforcement required for a certain type of column.

A column may be classified based on different criteria such as:

1. Based on shape

• Rectangle
• Square
• Circular
• Polygon

2. Based on slenderness ratio

• Short column, ? ? 12
• Long column, ? > 12

3. Based on type of loading

• Axially loaded column
• A column subjected to axial load and unaxial bending
• A column subjected to axial load and biaxial bending

4. Based on pattern of lateral reinforcement

• Tied columns
• Spiral columns

Minimum eccentricity

E_min > l/500 + D/30 >20

Where,  l = unsupported length of column in ‘mm’

D = lateral dimensions of column

Types of Reinforcements for columns and their requirements

Longitudinal Reinforcement

• Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.

• Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross cross-section area of the column.

• The bars should not be less than 12mm in diameter.

• Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.

• Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.

Transverse reinforcement

• It maybe in the form of lateral ties or spirals.

• The diameter of the lateral ties should not be less than 1/4^th of the diameter of the largest longitudinal bar and in no case less than 6mm.

The pitch of lateral ties should not exceed

• Least lateral dimension

• 16 x diameter of longitudinal bars (small)

• 300mm

Helical Reinforcement

The diameter of helical bars should not be less than 1/4^th the diameter of largest longitudinal and not less than 6mm.

The pitch should not exceed (if helical reinforcement is allowed);

• 75mm
• 1/6^th of the core diameter of the column

Pitch should not be less than,

• 25mm
• 3 x diameter of helical bar

Pitch should not exceed (if helical reinforcement is not allowed)

Least lateral dimension

• 16 x diameter of longitudinal bar (smaller)
• 300mm