Showing posts with label notes. Show all posts
Showing posts with label notes. Show all posts

Sunday, May 19, 2019

Foundation of structure and soil stabilization : all you need to know

Welcome to Nepal Engineering Services.

Foundation is a part of structure that withstands the weight of the structure, other different types of loads transmitting into the subsurface beneath the ground level. It acts as the legs of the structure . As legs support our body, foundation supports the structure and its dynamics. The soil of the foundation should not fail in shear and the settlement should be within limit.

Foundations are generally classified as below:

1) Shallow foundation: The foundation in which depth of the foundation is less than its width is called shallow foundation. They transmit load to relatively small depth. They are used if the loads of structure are relatively less. They are of following types:

A) Strip footing:-
Strip footing is generally used for load bearing walls. So it is also called wall footing. In this type of footing length is very very greater than width of the footing.

B)Isolated/Spread footing
This type of footing supports individual columns. The slab of the isolated footing may be circular, rectangular, square in plan. The footing generally is trapezoidal but it may be cubic or cuboidal.

C) Combined footing:
This type of footing supports two or more columns in a row. This type of footing is used when thae area required is limited. This type of footing is used in providing expansion gap between building.

D) Cantilever/Strap footing:

 In this type of footing, two footings are connected by beams.There is no soil reaction in this type of footing. 

E) Mat footing:

 This type of footing supports all the columns and wall load of the structure. This type of footing is used if the allowable soil pressure is low. It reduces the soil settlement. 

2) Deep column:

 In this type of footinf, the depth is much greater than width of footing. This is used in condition of soil where the soil near the ground is not capable to support a structure.

Factors affecting the choice of foundations:
1) Function of structure
2) Subsurface soil condition
3) Service life
4) Risk level
5) Safety factor
6) Economic condtion
7) Types of superstructure

The requirements of foundation are:
1) The differential settlement of soil must be within safe limit.
2) Foundation should be safe against shear strength and soil rupture.
3) Lateral soil pressure should not affect the footings.

Improvement of foundation soil:
The improvement of soil usually indicates in increasing the shear strength of soil and decreasing the compressibility of soil reducing by reducing the airvoids of the soil. The purposes of soil improvement are :
1) Increasing the shear strenght of soil
2) Decreasing the air voids
3) Decreasing the permeability of soil
4) Decreasing compressibility

The methods of soil stabilization are:
1) Mechanical compaction
2) Dynamic Compaction
3) Preloading
4) Sand and stone columns
5) Use of admixtures
6) Injection of suitable grouts

1) Mechanical compaction: The application of mechanical energy like rollung,temping and vibration in order to increase the density of soil is called mechanical compaction. It aims at reducing air voids and decreasing permeability. For cohesive soul, the compaction should be done around the optimum moisture content so that maximum dry density is obtained. In cohesionless soil vibration is done for compaction. The purposes of mechanical compaction are:
a) to reduce permeability
b) to prolong durability
c) to control swelling and shrinkage
d) to reduce liquifaction chance

2) Dynamic compaction : Dynamic compaction are generally used for
deeper layers of soils.
The types of compaction for deeper layer of soil are:
i) Vibroflotation
ii) Dropping of heavy weight
iii) Blasting

i) Vibroflotation:
It is used for compacting granular soil . A
vibrofloat is a cylindrical tube containing water jets at top and botom and
with a rotating eccentric weight, which develops a horizontal vibratory motion. This method is very effective for increasing density of sand deposit for depths up
ii)Dropping of heavy weight:
Loose soils are compacted by repeated dropping of a heavy weight on the ground surface so as to cause compaction up to sufficient depths. This
method can be used for compacting cohesive as well as cohesion less soils. A heavy concrete or steel blook weighing 600 kN and up to a height of 45 m, from which height it is allowed to fall
freely on to the ground surface. The process is repeated as to cover the entire area.

iii) Blasting

Explosives buried under the soil are used to densify loose soils. In this
method, holes are bored and explosives are placed in them. The holes
are filled back with soil. These explosives when detonated cause shock waves in
the ground causing densification of loose surrounding soil.

In this method large mass of load is kept on the soil before construction for long period of time. The purpose of preloading is:
i) To make enough settlement so that later the settlemnet is permissible.
ii) To increase the bearing capacity of soil

Disadvantages of preloading method
i) The time of consolidation is long
ii) It requires large space.


Grouting is the process by which fluid like materials, is injected in to the void spaces of the underground soil and is allowed to solidify, therefore reducing the void
space and hence increase the load caring capacity of the soil or to reduce
permeability or both.

Wednesday, May 15, 2019

Timber Structures (Theory)

Welcome to Nepal Engineering Services.

                                         Fig. Freshly cut timber 

Timber is the earliest building material known to human kind. Timber is used to refer to any stage of trees after they are cut off. Such cut off can be both raw and processed one. Timber perhaps was the only material used to make their building and living structures in the past. Today timber can be used as structural elements like beam, column, piles, roofing, trusses, doors,etc. 

Benefits of timber structures

  1.  Easy to use.
  2.  Environmental friendly 
  3.  Cheap
  4.  Easy to move, mend, and work.
  5. Has high axial compressive strength
  6.  Can be framed in low temperture
  7.  Has high thermal function 
  8.  Support to environmental conservation for the timber benefit by plantation

Disadvantages Of Timber Structure   

  1.  May require additional time for design and fabrication
  2.  Low sound insulation
  3.  More prone to rottening, infection, woodworm,etc.
  4. High risk to fire
  5.  Weak in tension and flexure

Structural Timber:  

The timber used for various structural elements is called structural timber. Timber beams, columns, truss,etc. are examples of structural timber.

Characteristics of Good Timber

The characteristics of good timber are:

                                                   Fig. Cross-section of a timber
  •   structurally strong to sustain structural loads.
  •   has close annular rings.
  •   strong enough to resist shocks and vibrations.
  •   able to resist fungi, woodworms,etc.
  •  gives sweet smell in the freshly cut stage.
  •  well seasoned.                                                 
  •  dense.
  •  has dark uniform color.
  •  hard enough to resist deformation 


Defects in timber

Depending upon type of soil, climatic conditions, natural calamities, human activities,etc. irregularity in grains of timber might have occured which is known as the defects in timber which in result decreases the mechanical asthetic property of timber. These defects decrease the quality of timber. The defects in timber are generally of following types: 

1. Knots : Knots are caused by natural forces. Knots are caused by the base of dead branches which remains in the tree during growth of it. They are of two types:
   a) Dead knots
   b) Live knots

2. Shakes: The cracks and splits in wood are generally called shakes. They are separations and defects in the annular rings. Shakes are of following types:
 a) Star shakes
 b) Cup shakes
 c) Heart shakes

3. Rind galls: The abnormal growth in the bark timber is called rind gall. These are caused due to improper cutting of branches.

4. Defects due to attack of insects   Beetles, marine boars, termites,etc. are the insects that eat wood weakening the strength of wood. 

5. Defects due to fungi: Fungi while feeding on the sap wood of timber leaves stain causing colour degradation but not structural hinderance. Some type of fungi breaks down the cell structure causing in decay of timber.

Seasoning of timber:

1. Natural Seasoning:

  • A concrete base is provided at level higher than ground level and timbers are stacked on it,
  • Timber are prevented from sun and storm winds,
  • It takes minimum of six months for completition 
  • Well seasoned timber contains moisture less than 15 percentage.

2. Artificial Seasoning:

  • Its a quick method,
  • Timber are heated in masonary channel with regulated  heat intensity,controlled humidity and adequate air circulation,
  • Timber are stacked in the masonary units,
  •  Carried out for 4-5 days.

Thursday, May 9, 2019

Relation of an Engineer with Client, Contractor and Fellow Engineers

Welcome to Nepal Engineering Services.

The relation between engineer, client, contractor must be a mutual one with full understanding and ethics and moral values.

Engineer's Relation with client:
The relation between client and engineer should strictly be professional, even though the client is closely familiar. There should be no discrimination among  clients based on their color, sex, religion,etc. An engineer must work on the best interest of client with loyalty within legal limit. Engineer should deliver the result on time with quality. Engineer should warn client about the potential risk of certain decisions. Engineer should supervise work and prevent misuse of client's property. Engineer should keep information confidential unless required by law.

Engineer's Relation with Contractor:
There should be strict professional relationship among engineer and contractor even though they are familiar. Engineer should provide due respect to contractor. There should be no discrimination among contractors on the basis of race, religion, region, sex,etc. Engineer should check and approve running bills on time as per specification. Engineer should assist contractor when variations are technically needed. Engineer should warn potential risks of decisions.

Engineer's relation with fellow Engineer:
 There should be professional relationship with all engineers despite of their status and relationship. They should criticize cautiously and objectively with respect to the person's professional status. Their should be professional relationship. They should encourage fellow engineers to follow the code of conduct. They should help, guide and orient graduate engineers.