Foundation Design Foun Found dation tion is the the base ase of any stru struct ctur ure e. Witho ithout ut a fir firm foun found dati ation, on, the the str structu ucture re cannot nnot stand. That is the reas on on wh y we have t o be very cautious wit h the des ig ign of foun da dations because our entire structure rests on the foundation.
Laying of Column Footing Reinforcement The stre streng ngth th of the the foun found dati ation dete etermin rmine es the the life life of the the str structu ucture re.. As we disc iscusse ussed d in the the earl earlie ierr arti artic cle, desig esign n of foun found dati ation dep depend ends on the the typ type of soil soil,, typ type of stru struc cture ture and its its load. On that that basi basis, s, the the foun founda dati tion ons s are are basi basic call ally divid ivided ed into into Shal Shalllow Foun Found dati ations ons and Deep Deep Foundations.
In this article, we are going discuss the step by step guide to Column Footing Design….
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 reac reacti tion ons s and and to ensu ensure re that that any any sett settle leme ment nt whic which h may may occu occurr shal shalll be as unif unifor orm m as poss possib ible le and the safe bearing capacity of soil is not exceeded.
In slop sloped ed or step steppe ped d foot footin ings gs,, the the effe effect ctiv ive e cros crosss-se sect ctio ion n in comp compre ress ssio ion n shal shalll be limi limite ted d 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 Here is a step-by-step guide to Column Footing Design:
Column Footing Plan and Section
Step 1 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
q/p = W/A
Step 2
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 2
Mxx = q x L/8 (B – b) 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 =square root(M/Qb) where, Q = moment of required factor
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, T v = k . Tc Tc =
0.16square rootfck
Step 4 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 [ B – (b + d) ] SF, V = q [L x B – (a + d)(b + d)] Nominal shear stress, T v = V/2((a+d)(b+d)d) ——- {for a rectangle Tv =
V/4((b+d)d)
Tv =
k . Tc
——- {for a square
k = 0.5 + Beta > 1 ; [Beta = ratio of sides of the column Tc =
0.16square rootfck
Area of steel, Ast = M/((sigma)stjd)
RCC Column A column forms a very important component of a structure. Columns support beams which 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
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 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 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. th
The diameter of the lateral ties should not be less than 1/4 of the diameter of the largest longitudinal bar and in no case less than 6mm.
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 t
1/6 of the core diameter of the column
25mm 3 x diameter of helical bar Pitch should not exceed (if helical reinforcement is not allowed)
16 x diameter of longitudinal bar (smaller) 300mm
Types of Column bases 1. Slab base 2. Gusseted base
Slab Base
are used where the columns have independent concrete pedestals.A thick steel base plate and two cleat angles connecting the flanges of the column to the base plate.In addition to thes e, web cleats are provided to conn ect the web of the column to the base plate. These web cleats guard against the pos sible dislocation of the column durin g erection.The ends of the column and also the base plate should be mechanized so that the column load is wholly transferred to the base plate.
Area of base plate= (load of column)/(permissible bearing stress in concrete) Gusseted base
are provided for columns carrying heavier loads requiring large base plates.A gusseted base consists of a base of reduced thickness and two gusseted plates are attached one to each flange of the column.
Gusseted Column Base The gusseted plates, cleat angles and fastenings (bolts, rivets) in combination with bearing area of shaft shall be sufficient to take all loads.
