RCC Column A column is a very important component in a structure. It is like the legs on which a structure stands. It is designed to resist axial and lateral forces and transfer them safely to the footings in the ground. Columns support floors in a structure. Slabs and beams transfer the stresses to the columns. So, it is important to design strong columns.
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. The axial load carrying capacity of a column is deduced from the formula
Please see the link for formulas to calculate axial loads in columns. I would recommend using advanced structural design software like ETabs or Staad Pro for design of structures. Column design does not depend only on axial loads, but also on many other factors. There are bending moments and torsional forces
induced due to beam spans, wind loads, seismic loads, point loads and many other factors. 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 The ratio of the effective length of a column to the least radius of gyration of its cross section is called the slenderness ratio.
Short RCC column, =< 10
Long RCC column, > 10
Short Steel column, =<50
Intermediate Steel column >50 & <200
Long Steel column >200
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 RCC columns
Spiral RCC columns
Minimum eccentricity Emin > 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 at least 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 may be 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
1/6th 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
RCC Beams RCC beams are cast in cement concrete reinforced with steel bars. Beams resist compression and tensile forces 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 simply supported reinforced beam. Simply supported 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. Compressive forces are handled by the concrete in the beam. 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, in order 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(Mu lim) using the equation for singly reinforced beam Mulim = 0.87.fy.Ast1.d [1 – 0.42Xumax] Or Balanced section Ast1 = (0.36.fck.b.Xumax)/(0.87fy) Step 2 If factored moment Mu > Mulim, then doubly reinforced beam is required to be designed for additional moment. Mu – Mulim = fsc.Asc (d – d’) no. 70] Step 3 Additional area of tension steel Ast 2
[fsc value
from
page
Ast2 =Asc.fsc/0.87fy Step 4 Total tension steel Ast, Ast = Ast1 + Ast2
What are Simply Supported Slabs? Before we discuss the technical design rules of Simply Supported slabs, let’s just go through its definition and learn why they are named so… As the name suggests, simply supported slabs are supported on columns or stanchions.
Simply Supported Slab Simply supported slabs are classified as One way slabs and Two way slabs. One way slabs bend in one direction only and transfer their loads to the two support beams in opposite directions. Their main steel in on shorter span length. L/B ratio is generally less than 2.
Two way slabs bend in both directions, and transfer their loads and stresses on all four sides. L/B ratio is equal to or greater than 2.
Simply supported slabs don’t give adequate provision to resist torsion at corner to prevent corner from lifting. The maximum bending moment will be given if the slabs are restrained. But at least 50% of the tension reinforcement provided at the mid span should extend to the support. The remaining 50% should extend to within 0.1Lx or Ly at the support as appropriate. RCC Slab Design depends on the on the dimensions of the slab after which the slab is termed as a one-way slab or a two-way slab… In the design of RCC structures, Column Design and Beam Design are to be done before we start with RCC Slab Design…
Basic Rules followed in the design of simply supported Slab Thickness of slab l/d ratio should be less than the following:
Simply supported slab
Continuous slab, l/d = 26
Cantilever slab, l/d = 7
In any case of the above, the thickness should not be less than 100mm
Effective span
Distance between centre to centre of support
Clear span plus effective depth
Minimum main reinforcement
0.15% gross c/s of slab – for MS bars
0.12% gross c/s of slab – for HYSD bars
Spacing of main bars The spacing or c/c distance of main bars shall not exceed following:
Calculated value
3d
300mm
Distribution or Temperature reinforcement This reinforcement runs perpendicular to the main reinforcement in order to distribute the load and to resist the temperature and shrinkage stresses. It should be at least equal to;
0.15% gross c/s of slab – for MS bars
0.12% gross c/s of slab – for HYSD bars
Spacing of distribution bars The spacing or c/c distance of distribution bars shall not exceed the following
Calculated area
5d
450mm
Diameter of bars The diameter of the bars varies from 8mm to 14mm and should not exceed 1/8th of the overall depth of the slab. For distribution steel, the diameter varies from 6mm to 8mm. Cover The bottom cover for reinforcement shall not be less than 15mm or less than the diameter of such bar.
Foundation Design Foundation is the base of any structure. Without a solid foundation, the structure would not hold for long. We have to be very cautious with the design of foundations because our entire structure rests on the foundation. The job of a foundation is to transfer the loads of the building safely to the ground.
Laying of Column Footing Reinforcement | Foundation Design
The strength of the foundation determines the life of the structure. As we discussed in the earlier article, design of foundation depends on the type of soil, type of structure and its load. Higher the load bearing capacity of the soil, the larger the load it could safely carry. Foundations are Foundations.
basically
divided
into
Shallow
Foundations
and
Deep
In this article, we are going discuss the step by step guide to Column Footing Design for a shallow foundation.
Reinforced Concrete Footings Footing comprises of the lower end of a column, pillar or wall which i enlarged with projecting courses so as to distribute load. Footings shall be designed to sustain the applied loads, moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as uniform as possible and the safe bearing capacity of soil is not exceeded. In sloped or stepped footings, the effective cross-section in compression shall be limited by the area above the neutral plane, and the angle of slope or depth and location of steps should be such that the design requirements are satisfied at every section.
Design Procedure of Column Footings | Foundation Design Here is a step-by-step guide to Column Footing Design:
Column Footing Plan and Section | Foundation Design
Step 1 Area required for footing Square = B = (w+w1)/P0 Where, Po = safe bearing capacity of soil w1 = self weight of footing
w = self weight of footing For Rectangle = b/d = B/D A=bxd Net upward pressure on the footing q/p = W/A
Step 2 Bending Moment Critical section for maximum bending moment is taken at the face of the column For a square footing, Mxx = q x B/8 (L – a)2 Mxx = q x L/8 (B – b)2 Myy = q x B/8 (L – a)2
Step 3 To fix the depth of the footing shall be greater of the following: Depth from bending moment consideration d = √(M/Qb) where, Q = moment of required factor
Depth from shear consideration Check for one-way shear Check for two-way shear or punching shear Critical shear for one-way shear is considered at a distance ‘d’ from face of the column. Shear force, V = qB [ ½(B – b) d] Nominal shear stress, Tv = k . Tc Tc
= 0.16√fck
Step 4 Check for two-way shear Critical section for two-way shear is considered at a distance at a distance d/2 from all the faces of the column.
SF, V = q [ B2 – (b + d)2] SF, V = q [L x B – (a + d) (b + d)] Nominal shear stress, Tv = V/2((a+d) (b+d) d)
——- {for a rectangle}
Tv = V/4((b+d) d)
——- {for a square}
Tv = k. Tc k = 0.5 + β > 1 of the column]
——- [Beta β = ratio of sides
Tc = 0.16√fck Area of steel, Ast = M/((σ)stjd)
RCC Staircase Design RCC Structures are nothing but reinforced concrete structures. RCC structure is composed of building components such as Footings, Columns, Beams, Slabs, Staircase etc. These components are reinforced with steel that give stability to the structure. Staircase is one such important component in a RCC structure.
Dog Legged Stair | Staircase design
In this article, we will discuss different types of staircases and study the doglegged reinforced cement concrete staircase design.
Stairs Stairs consist of steps arranged in a series for purpose of giving access to different floors of a building. Since a stair is often the only means of communication between the various floors of a building, the location of the stair requires good and careful consideration. In a residential house, the staircase may be provided near the main entrance.
In a public building, the stairs must be from the main entrance itself and located centrally, to provide quick accessibility to the principal apartments. All staircases should be adequately lighted and properly ventilated.
Various types of Staircases
Straight stairs
Dog-legged stairs
Open newel stair
Geometrical stair
RCC Dog-legged Staircase design In this type of staircase, the succeeding flights rise in opposite directions. The two flights in plan are not separated by a well. A landing is provided corresponding to the level at which the direction of the flight changes.
Procedure for Dog-legged Staircase design Based on the direction along which a stair slab span, the stairs maybe classified into the following two types. 1. Stairs spanning horizontally 2. Stairs spanning vertically
Stairs spanning horizontally These stairs are supported at each side by walls. Stringer beams or at one side by wall or at the other side by a beam.
Loads
Dead load of a step
= ½ x T x R x 25
Dead load of waist slab = b x t x 25
Live load
= LL (KN/m2)
Floor finish
= assume 0.5 KN/m
Stairs spanning Longitudinally
In this, stairs spanning longitudinally, the beam is supported at top and at the bottom of flights.
Loads
Self weight of a step
= 1 x R/2 x 25
Self weight of waist slab = 1 x t x 25
Self weight of plan
Live load
= LL (KN/m2)
Floor finish
= assume 0.5 KN/m
= 1 x t x 25[(R2 + T2)/T]
For the efficient design of an RCC stair, we have to first analyse the various loads that are going to be imposed on the stair. The load calculations will help us determine, how much strength is required to carry the load. The strength bearing capacity of a staircase is determined on the amount of steel and concrete used. The ratio of steel to concrete has to be as per standards. Steel in the staircase will take the tension imposed on it and the concrete takes up the compression. These are the essential steps that are to be followed for the RCC Stair Design.
