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Shear Strength of Soil by Vane Shear Test

Vane shear test is used to determine the undrained shear strength of soils especially soft clays. This test can be done in laboratory or in the field directly on the ground. Vane shear test gives accurate results for soils of low shear strength (less than 0.3 kg/cm2).

Table of Contents

• Shear Strength of Soil by Vane Shear Test
  • Apparatus
• Procedure of Vane Shear Test
• Observations and Calculations of Vane Shear Test
• Result of Vane Shear Test
• Advantages of Vane Shear Test
• Drawbacks of Vane Shear Test

Shear Strength of Soil by Vane Shear Test

Apparatus

Apparatus required for vane shear test are:

1. Vane shear apparatus
2. Soil specimen container
3. Vernier callipers

Fig 1: Vane Shear Apparatus

Vane shear apparatus consists high tensile steel rod to which four steel blades (vanes) are fixed at right angles to each other at the bottom of rod.

Fig 2: Steel Rod with Vanes

Procedure of Vane Shear Test

Test procedure of vane shear test contains following steps:

1. Clean the vane shear apparatus and apply grease to the lead screw for better movement of handles.
2. Take the soil specimen in container which is generally 75 mm in height and 37.5 mm in diameter.
3. Level the soil surface on the top and mount the container on the base of vane shear test apparatus using screws provided.
4. Lower the vane gradually into the soil specimen until the top of vane is at a depth of 10 to 20 mm below the top of soil specimen.

   

   Fig 3: Lowering Vane into the Soil Specimen

5. Note down the reading of pointer on circular graduated scale which is initial reading.
6. Rotate the vane inside the soil specimen using torque applying handle at a rate of 0.1oper second.
7. When the specimen fails, the strain indicator pointer will move backwards on the circular graduated scale and at this point stop the test and note down the final reading of pointer.
8. The difference between Initial and final readings is nothing but the angle of torque.
9. Repeat the procedure on two more soil specimens and calculate the average shear strength value.
10. Measure the diameter and height of vane using Vernier callipers.
11. Sensitivity of given soil sample is determined by repeating the above test procedure on remolded soil which is nothing but soil obtained after rapid stirring of vane in the above test.

Sensitivity of soil = undisturbed shear strength/ remolded shear strength.

Observations and Calculations of Vane Shear Test

Shear strength of given soil sample is calculated from below observations.

• Diameter of vane, D = ___ cm
• Height of vane, H = ____ cm
• Torque, T = (Spring constant /180)*(initial reading-final reading)

Shear strength of soil (S) is calculated from below formula.

S.No	Initial Reading	Final Reading	(Initial Reading – final reading)	Torque (T)	Shear strength (S)
1
2
3

Result of Vane Shear Test

Shear strength of soil specimen = ________ kg/cm2.

Advantages of Vane Shear Test

Advantages of vane shear test are as follows:

• Vane shear test is easy and quick.
• This test can be performed either in laboratory or in the field directly on the ground.
• In-situ vane shear test ideal for the determination of undrained shear strength of non-fissured, fully saturated clay.
• Shear strength of soft clays at greater depths can also be found by vane shear test.
• Sensitivity of soil can also be determined using vane shear test results of undisturbed and remolded soil samples.

Drawbacks of Vane Shear Test

Drawbacks of vane shear test are as follows:

• Vane shear test is not suitable for clays which contain sand or silt laminations in it.
• It cannot be conducted on the fissured clay.
• If the failure envelope is not horizontal, vane shear test does not give accurate results.

  

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Load Bearing Structure and Components vs. Framed Structural System

A load bearing structure has the components of a building which carries and transfers the load to the ground safely. This structure guarantee stability of the building and its performance. Commonly walls, columns, beams, foundation

Understanding the structural principles of load bearing structure is the fundamentals required for the design process and bring up the design solution. This will help in coming up with suitable materials and construction techniques.

Fig.1: Load-Bearing Structure

Statics of Load Bearing Structure

The load-bearing structures is determined by the following analysis process:

1. The whole structure is analyzed initially. The function of each structural element is hence determined.
2. The forces in the individual structural elements is determined.
3. The forces that is affecting the structural elements is determined along with the forces that it transmits. These are external forces.
4. The forces within the structural elements are the internal forces or the static forces.
5. The overall structural stability of the structural element is determined.
6. Finally, determine the proof that the planned structure withstands all the forces coming over it.

Load Bearing Components of a Building

The main load-bearing structural elements are:

1. Beam
2. Columns
3. Walls
4. Braces
5. Trusses

1. Load Bearing Walls

A load bearing wall transfers the loads form slabs above it to the foundation. These walls can be made of concrete, masonry or block materials. Most of the exterior walls of a building structure are considered as load bearing.

Removal of load bearing wall as a part of renovation must be conducted only after providing alternative support for the above-supported structures.

2. Beams

Beam forms one of the primary load-bearing component of structure which can be made from wood, concrete or metal. It is a primary member utilized to take the load on the building. The capacity of load bearing depends on the depth and width of the beam element.

Beam is subjected to higher amount of shear and compressive force as they have high amount of internal and external forces.

3. Columns

The structural columns are one of the important elements in a structure which have effective role in the transmission of dead and live loads to the foundation that the building structure is subjected to.

4. Braces

Braces are structural elements used in the framework structural system. This load bearing elements helps in stiffening the framework effectively.

5. Trusses

Trusses are load-bearing elements that supports the roof elements in building structure. The roof loads are uniformly transmitted to the truss. The truss is subjected to tension and compression forces. The trusses are not subjected to any kind of moments.

Framed Structural System and Load Bearing Structural Systems

The framed and load-bearing structural systems vary based on the load-bearing structural components taking part in the load transmission.

In the load bearing structural system, the loads gets transferred from slabs to foundations through walls, while in framed structural system, loads from slabs gets transferred to beams, beams to columns and finally from columns to the foundation.

The structural elements involved in a framed structural element are:

Slabs >> Beams >> Columns >> Foundation

In case, of Load bearing, structural system the path followed is

Slabs >> Walls >> Foundation

Fig.2: Load Bearing Structure vs Framed Structure

The load-bearing structural system is an old method of construction. Framed structural system has framed structure of columns and beams which have high resistance to lateral forces. The framed structural system is more flexible compared with load-bearing structural system.

  

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Size and Quantity of Reinforcement for Building Works

Different size and quantity of reinforcement is required for different concrete members in buildings. Quantity of reinforcement per cubic meter and its suitable sizes for different reinforced concrete construction is discussed.

Reinforcement is required for reinforced concrete members such as footings, beams, columns, slabs, lintels etc. Estimation of reinforcement quantity is required prior to tendering stage to calculate approximate cost of project or construction work.

Size and Quantity of Reinforcement for Building Works

Following table gives the estimated quantities of reinforcement and its size generally used for various building works:

Sl. No	RCC Member	Quantity in kg/m3	Size of reinforcement required
1	Column footings	75	10mm or 12mm
2	Grade beams	100	12mm, 16mm – 85% Stirrups – 6mm or 8mm– 15%
3	Plinth beams	125	8mm diameter – 85%, Stirrups 6mm – 15%
4	Columns	225	16mm, 20mm and 25mm – 90% Ties – 6mm or 8mm – 10%
5	Lintel beam	125	12mm, 16mm dia – 85% Stirrups – 6 mm or 8mm – 15%
6	Sunshades	60	8mm dia – 75% Distributer – 6mm – 25%
7	Canopy slab upto 2.0 m span	125	10mm dia – 80% Distributor bars – 6mm or 8mm – 20%
8	Staircase waist slab	150	12 or 16mm dia – 80% Distributor 8mm dia – 15%
9	Roof slab
	(a) One way slab	80	8mm dia – 70% Distributor – 6mm – 30%
	(b) Two way slab	100	8mm dia – 100%
	(c) Square slab – 4m to 6m size	150	10 – 12mm dia – 100%
10	Main beams above 6m	250	20mm, 16mm, 12mm – 80 – 85% Stirrups – 8mm – 15 – 20%

All above mentioned steel are round tapered steel. This data is just for estimation of quantity of steel for various RCC works. This does not provide actual steel required for all the members. Actual quantity of steel required can be calculated from the drawings prepared after structural design.