Difference Between Shallow and Deep Foundation

• Foundations are defined as a substructure element which is used to transfer the heavy superstructure loads to the soil in such a way that neither the surrounding soil is stressed in excess of allowable stress of soil nor it undergoes deflection more than permissible deflection.
• Foundations are broadly classified of two types:-
• Shallow Foundation 
• Deep Foundation

Difference between Shallow and Deep Foundation

• For a Shallow foundation, the depth of foundation is equal to or less than its width. Whereas for Deep foundation, the depth of footing is equal to or greater than its depth.
• A shallow foundation is located at very low depth, whereas a deep foundation is constructed at more depth.
• In case of Shallow foundation, loads are primarily supported by the action of end bearing. Whereas for Deep foundation, loads are supported partly by frictional resistance around the surface and partly by bearing at the base of foundation.
• The construction of shallow foundation is done in open excavation. Whereas the construction of deep foundation is generally carried by boring or drilling well beneath the ground. This will lead to a better visual inspection in case of a shallow foundation over deep foundation.
• Shallow foundation are in general cheaper than deep foundation in construction. Deep foundation requires more machinery and more technicality which increases the costs.
• Allowable load in case of shallow foundation is way less in comparison of deep foundation. So, for heavy superstructure construction, Deep foundation is always preferred over shallow foundation.
• Lateral effect of soil is better resisted by the deep foundation in comparison to that of shallow foundation.
• Shallow foundation requires less labour in comparison to that of deep foundation. Deep foundation even requires skilled labour.
• Shallow foundation is not suitable for soils which are weak in upper stratum. In this scenario Deep foundation is preferred over shallow foundation.

Difference between Shallow and Deep foundation

AASHTO Soil Classification System

• Soil classification is generally done to organize the different types of soil in different groups on the basis of their engineering properties. 
• There are so many popular system of  soil classification system. AASHTO soil classification system is one of them.
• AASHTO soil classification system is given by the American Association of State Highway and Transportation officials.
• The AASHTO soil classification system was first developed by Terzaghi and Hogentogler in 1929 and has been revised many times. The AASHTO soil classification system was initiated by Highway Research Board in 1943.
• The AASHTO soil classification system is generally used for soil classification for highway construction projects and is very much used worldwide.
• AASHTO soil classification system is mainly based on sieve analysis and consistency limits.
• In AASHTO soil classification system, soils are classified in group and subgroups. There are 7 major groups present in AASHTO soil classification system which are further divided into subgroups.

A1 ➡ A-1-a , A-1-b

A2 ➡  A-2-4 ,  A-2-5 ,  A-2-6 ,  A-2-7

A3

A4

A5

A6

A7 ➡ A-7-5 , A-7-6

• In  AASHTO soil classification system, Group index is also included along with soil groups and subgroups. Group index is a number used to refer the quality of soil used as subgrade material in highway. Its value generally ranges from 0 to 20. For two soils falling under same group and subgroup , the soil having lower GI value is better highway subgrade material in comparison to that of soil having lower GI value.

 

Step by Step Procedure for AASHTO soil classification system 

1. Distinguishing coarse grained / fine grained

First of all, find the percentage of soil passing US # 200 sieve (0.075 mm opening)

• If % passing ≤  35%  ➡ coarse grained soil
•  If % passing＞35%  ➡ fine grained soil

2. Determination of  group and subgroups

• For coarse grained soil, % of soil passing through US sieve # 10, #40 and #200 is also required in addition to liquid limit and plasticity index.

Now, after finding all of the mentioned values, compare the data from the AASHTO table of coarse grained. To compare go through the top to bottom until a line is found matching all of the properties calculated here.

• For fine grained soil, only liquid limit and plasticity index is required.

 Now, just compare the data from the AASHTO table of fine grained. To compare go through the top to bottom until a line is found matching all of the properties calculated here.

3.  Determination of Group Index

• For coarse grained

for A-1-a, A-1-b, A-3, A-2-4, A-2-5,   GI = 0

for A-2-6 and A-2-7, GI = 0.01 (F₂₀₀ -15) (PI -10)

• For fine grained

GI = (F₂₀₀ - 35) [ 0.2+0.005(LL-40) ] + 0.01 (F₂₀₀ -15)(PI - 10)

if GI ＜ 0 then take GI =0 and if GI ＞0 , the round off to nearest whole number.

4. Naming of soil

Finally as per AASHTO soil classification system, soil➡ soil group/subgroup followed by GI in parenthesis. e.g. A-7-5(10) , A-5(9).

AASHTO Table for coarse/fine grained soil 

 

AASHTO table for coarse grained

AASHTO table for fine grained

AASHTO soil classification example

Q. The result of sieve analysis of three soil is given next. It is required to classify these soils according to the AASHTO Classification system. Use the attached AASHTO Classification of Highway Subgrade Materials Table.

What is the classification of soil A,B and C?

sol:-

• soil A

% passing # 200 sieve = 38 ➡ fine grained soil

plasticity index= (LL- PL) = 42-23 = 19

on comparing from AASHTO table for fine grained

soil group = either A-7-5 or A-7-6

as PI= 19 > (LL-30)  ⇒ soil group = A-7-6

Group Index, GI = (F₂₀₀ - 35) [ 0.2+0.005(LL-40) ] + 0.01 (F₂₀₀ -15)(PI - 10)

= (38-35) ( 0.2+0.005(42-40)) + 0.01 (38-15)(19-10)

= 2.7 ≈ 3 

so, soil A = A-7-6(3) 

• soil B

% passing # 200 sieve = 33 ➡ coarse grained soil

% passing # 10 = 77 , #40 = 50 

and LL = 46 , PI= 46-29= 17

on comparing from AASHTO table for coarse grained

soil group = A-2-7

Group Index, GI = 0.01 (F₂₀₀ -15)(PI - 10)

 = 0.01 (33-15) (17-10) = 1.26 ≈ 1

 so, soil B = A-2-7(1)

• soil C

% passing # 200 sieve = 63 ➡ fine grained soil

plasticity index= (LL- PL) = 47-24 = 23

on comparing from AASHTO table for fine grained

soil group = either A-7-5 or A-7-6

as PI= 23 > (LL-30)  ⇒ soil group = A-7-6

Group Index, GI = (F₂₀₀ - 35) [ 0.2+0.005(LL-40) ] + 0.01 (F₂₀₀ -15)(PI - 10)

= (63-35) ( 0.2+0.005(47-40)) + 0.01 (63-15)(23-10)

= 12.82 ≈ 13  

so, soil C = A-7-6(13)  

 

 

 

SUPERPOSITION METHOD/ METHOD OF TABLES OF FINDING DEFLECTION

• Table method of finding deflection of beam is one of the best method of finding deflection of beam.
• This method of finding deflection is very much beneficial in case of finding slope and deflection as per superposition method. 
• As per superposition method , the effect of each and every load is considered separately and finally the effect of each loading is summed up.
• This method is even very much helpful in case of analysis of indeterminate structure with the force method of analysis. By choosing the redundant , the effect of various loading is calculated at the desired position with the help of the table.
• The table consists of slope and deflection equation of some standard type of beam under various standard loadings.

1. CANTILEVER BEAM WITH POINT LOAD

• For 0 ≤ x ≤ a 

Ө = (P/2EI) (X² - 2aX)

y = (P/6EI) (X³ - 3aX²)

• For a ≤ x ≤ L

Ө = - Pa²/2EI

y = (Pa² / 6EI) (a - 3x)

2. CANTILEVER BEAM WITH CONCENTRATED COUPLE

• For 0 ≤ x ≤ a 

Ө = - Mx/EI

 y = - Mx²/ 2EI

For a ≤ x ≤ L

Ө = -Ma/EI

y = (Ma/2EI) (a - 2x) 

3. CANTILEVER BEAM WITH UDL

• For 0 ≤ x ≤ a 

Ө =(w/6EI)(3ax²-3a²x-x³)

y = (w/24EI)(4ax³-6a²x²-x⁴)

•  For a ≤ x ≤ L

Ө = - wa³/6EI

y = (wa³/24EI) (a - 4x)

4. CANTILEVER BEAM WITH TRIANGULAR UVL

• For 0 ≤ x ≤ a

Ө = (w/24EIa)(x⁴-4ax³+6a²x²-4a³x)

y = (w/120EIa)(x⁵-5ax⁴+10a²x³-10a³x²)

•  For a ≤ x ≤ L

Ө = (- wa³/24EI)

y = (wa³ /120EI)( -5x+a)

5. SIMPLY SUPPORTED BEAM WITH POINT LOAD

• For 0 ≤ x ≤ a

Ө = (Pb/6EIL)(3x²+b²-L²)

y = (Pb/6EIL)(x³+b²x-L²x)

 

•  For a ≤ x ≤ L

Ө = (Pa/6EIL)(L²-a²-3(L-x)²)

y = (Pa(L-x)/6EIL)(x²+a²-2Lx) 

6. SIMPLY SUPPORTED WITH CONCENTRATED COUPLE

• For 0 ≤ x ≤ a

Ө = (M/6EIL)(-3x²+6aL-3a²-2L²)

y = (M/6EIL)(-x³+6aLx-3a²x-2L²x) 

7. SIMPLY SUPPORTED WITH UDL

• For 0 ≤ x ≤ a

Ө = (-w/24EIL)[4Lx³-6a(2L-a)x²+a²(2L-a)²]

y = (-w/24EIL)[Lx⁴-2a(2L-a)x³+a²(2L-a)²x] 

•  For a ≤ x ≤ L

Ө = (-wa²/24EIL)(6x²-12Lx+a²+4L²)

y = (-wa²/24EIL)(L-x)(-2x²+4Lx-a²) 

8. SIMPLY SUPPORTED WITH TRIANGULAR UVL

Ө = (-w/360EIL)(15x⁴-30L²x²+7L⁴)

y = (-w/360EIL)(3x⁵-10L²x³+7L⁴x)

 

 

 

CONJUGATE BEAM METHOD

 Conjugate beam method is one of the best method of deflection which is used to find deflection of beams.

Important facts about conjugate beam method

• Conjugate beam method is only used to find deflection due to bending forces applied on structure. This means that conjugate beam method cannot be used to find deflection due to shear, torsion, axial force, temperature change, etc.
• For beams having non-prismatic cross-section, the slope and deflection can be easily obtained by conjugate beam method.
• Conjugate beam method is very much helpful in case of structure having some kind of discontinuity like internal hinge or slider.

Comparison of Conjugate beam method with moment area method

• Conjugate beam method is often compared with moment area method as in both of the cases, the M/EI diagram is calculated.
• In moment area method of deflection, it is usually required to have some prior understanding of geometry of deflected shape. While in conjugate beam method of deflection no such prior understanding of geometry of deflected shape is required. In Conjugate beam method of deflection simple use of principle of statics is required to find deflection and slope.
• The process of finding slope and deflection by moment area method in case of discontinuity like internal hinge or slider is very much complex, while in conjugate beam method this can be calculated easily.

What is CONJUGATE BEAM

Conjugate beam is an imaginary beam having length as same as that of real beam and the loading on conjugate beam is equal to M/EI diagram of real beam.

Theorem of CONJUGATE BEAM METHOD

As per conjugate beam method:-

1. Slope at any point in real beam= Shear force at that point in conjugate beam
2. Deflection at any point in real beam= Bending moment at that point in conjugate beam

Support modification in CONJUGATE BEAM METHOD

As per theorem of conjugate beam method, slope and deflection at any point in real beam is equal to the shear force and bending moment at that point in conjugate beam respectively. So, the supports of a conjugate beam must be modified in such a way that they supports the theorem of conjugate beam method.

SIGN CONVENTION OF CONJUGATE BEAM METHOD OF DEFLECTION

1. If M/EI diagram of real beam is (+) , then the loading on conjugate beam is upward/(+) and if M/EI diagram of real beam is (-), then the loading on conjugate beam is downward/(-).
2. If shear force on conjugate beam is (+) ,  then the slope in real beam is anticlockwise and vice-versa.
3. If bending moment in conjugate beam is (+) , then the deflection in real beam is upward and vice-versa.

Which DEFLECTION METHOD to use in which case in structures?

INTRODUCTION

•  Whenever a structure got affected from any external sources, then there may be a chance of having deflection in the structures.
• The external sources mentioned above may be anything such as loading applied (axial force, shear force, bending moment, torsion), self weight, change in temperature (increase/decrease),fabrication or misfits errors, support settlements, etc.
• The calculations of  these deflections in the structures are very important to know. 
• The deflections of structures must be within permissible limit as per design to prevent cracking or even failure of structure. So, it is very much necessary to calculate deflection at all critical points of the structure so that the structure should be designed as such deflection in structure remains within the permissible limit as per the design.
• The calculation of deflection is also very much necessary in case of analysis of indeterminate structures, where deflection is required to write compatibility equation.

 TYPES OF DEFLECTION METHOD

There are so many different types of method which are used to find deflection in any structure. 

1. Double integration method
2. Method of superposition
3. Moment area method
4. Conjugate beam method
5. Castigliano's method
6. Method of virtual work/ unit load method

DOUBLE INTEGRATION METHOD

• This method is also sometimes known as Macaulay's method/Singularity function method/ Discontinuity function method.
• This method of finding deflection is very much used in case of beams.
• This method is only used for finding deflection for bending.
• This method gives the equation of deflected elastic curve of whole beam at once.
• This method can't be applied directly for the structures having some kind of discontinuity in the elastic deflected curve. By discontinuity we mean to say presence of any hinge or slider.
• This method can also be not applied directly for the non-prismatic beams.
• This deflection method is best for the case when deflection or slope is required to be found at too many points.  

METHOD OF SUPERPOSITION

• In this method of finding deflection, deflection at any point on the structure can be calculated by summing up the effects of each load acting on the structure one by one.
• This method of superposition is only helpful in the case if deflection/slope at any point is already known for the given applied loading. And so, the limitation of this method is that this method can only be used for standard types of loading or supports present in the beams.

MOMENT AREA METHOD

• This method of finding deflection is only used to find bending deflections.
• This method is generally used to find deflection for beams subjected to a series of concentrated loads and for non-prismatic beams.
• This method can't be applied directly for the structures having some kind of discontinuity as hinge or slider.

CONJUGATE BEAM METHOD

• This method of finding deflection is only used to find bending deflections.
• This method is generally used to find deflection for beams subjected to a series of concentrated loads and for non-prismatic beams.
• This method of finding deflection can be even applied directly for the structures having any discontinuity like hinge or slider.

 CASTIGLIANO'S METHOD

• This method of finding deflection can be used for all types of external loading.
• This method of finding deflection can also be applied for ant discontinuity case or even in non-prismatic case.
• The limitation of the castigliano method is that it can't be applied for support settlement case(can be applied for support settlement in case of self  restraining structure) and in temperature change case.

METHOD OF VIRTUAL WORK/ UNIT LOAD METHOD

• This method of finding deflection can be applied for all types of external loadings.
• This method of finding deflection can also be applied for ant discontinuity case or even in non-prismatic case.
• This method can also be applied for support settlement case or in case of temperature change also.

You can also check for some extra details Types of loading a structure resist in civil engineering/ Different types of structure

TYPES OF CONTRACT IN CIVIL ENGINEERING

 What is CONTRACT

The most broad and simple definition of contract is that " A contract is an agreement enforceable by a law".

Now as per Civil Engineering, a contract is an agreement between two or more firms/individuals based on which, one agrees to do some work for other under the given terms and conditions. Now this work mentioned above can be anything like construction or maintenance or repairs of any structures, supply of labour or materials, transportation of materials, storage of materials, etc.

Types of CONTRACT IN CIVIL ENGINEERING   

By the term types of contract in Civil engineering , we mean to say different methods of payments which are used to reimburse contractors for the construction or other services they provide.

1. ITEM RATE CONTRACT

• This types of contract in Civil engineering is also known as Unit price contract or Schedule contract.

TYPES OF STRUCTURES USED IN CIVIL ENGINEERING

In civil engineering, a structure is defined as anything which is made up of various components which will resists external loading applied on them without  affecting the functions for which  the particular structure is designed.

 It is a very important task  for a structural engineers to select the type of structure which will be used in supporting or transmitting loads. On the basis of types of primary stresses which can be developed in a structure on the application of load, structures are classified in to five types:-

1. Tension carrying structures

2. Compression carrying structures

3. Trusses

4. Shear structures

5. Bending structures

The above mentioned structures can be considered as a basic types of structures. A single structure may be composed of combination of above mentioned structures.

TENSION STRUCTURES

These types of structures supports loading applied on them by generating tensile forces.

Some of the common types of tension structures we usually see are Cables, Vertical rods, etc.

Cables are usually used in supporting bridges. Vertical rods are used as hangers.

COMPRESSION STRUCTURES

These types of structures supports loading applied on them by generating compressive forces.

Some of the most common types of compressive structures we usually see are Columns and Arches.

Column is a straight member subjected to axially compressive loads while Arch is a curved structure. Column and Arches develops mainly compressive stresses on application of loads.

TRUSSES

These types of structures supports loading applied on them by developing axial forces. The axial forces generated can either be tensile or compressive in nature.

Trusses are generally composed of straight members with a pin connections at the ends. In real life , the trusses are generally constructed by connecting members to gusset plates by bolted or welded connections. This makes connections at the ends somewhat fixed and so induces some bending in the members of trusses. But as these secondary stresses are so small in magnitude, so that they can be neglected in design, and so the assumptions of pinned connections at the ends holds good.

Loading on trusses are only subjected to joints.

SHEAR STRUCTURES

These types of structures supports loading applied on them by developing mainly in-plane shear.

e.g. Reinforced concrete shear wall in multistorey buildings. This shear wall helps in reducing lateral movements of building due to wind loads and earthquake loads.

BENDING STRUCTURES

These types of structures supports external loadings applied on them by developing bending stresses.

Some of the most common types of  bending structures which we usually see are Beams, Rigid frames, Slabs. Beam is a straight member which is usually loaded perpendicular to its longitudional axes. Frames are composed of straight members connected together to support external loads applied on them. Unlike trusses , the loading on frames may be applied on joints as well as on members. In general a rigid frame develops bending moment, shear force and axial force also. But the design of horizontal members or beams of frames  is generally governed by bending and shear stresses only.