Quick RevisionCivil Enigneering
Kneading:
- It is the process of mixing water with blended clay (clay with additional minerals) to make it plastic.
- Kneading of the brick earth occurs after clay has been weathered, and blended and is the last step in the preparation of brick earth before moulding it into the shape.
- Kneading is done either manually or by combined kneading-tempering by adding moisture to the already blended soil and placing it in pug mills (often called as clay mixers).
- Kneading is done to obtain a homogeneous mass and to develop plasticity for the following moulding process.
Tempering of clay:
- It is the process of adding water to the clay and allowing it to remain un-disturbed for few days before mixing occurs.
Pugging:
- It is also the process of working and tempering of clay to make it plastic and consistent.
Moulding:
- It is the process of giving a required shape to the brick from the prepared brick earth. It can be hand moulded or machine moulded.
In case of lime concrete, the curing starts after 24 hours and is done for a minimum of 7 days period. Hence, the masonry works can only be done on foundation after a minimum of 7 days.
In case of cement concrete, the curing starts after 3-48 hours to ensure the initial and final setting time of concrete. The curing is done for a minimum period of 7 days or up to the period at which the strength of concrete becomes 70% of the characterized strength.
Important Point:
- Curing is done to ensure sufficient moisture in the structure, it also ensures the heat of hydration thereby helps in increasing the strength and durability.
- In case of structures formed by rapid hardening of cement or extra- rapid hardening cements, the curing is required for initial 2-4 days only as the strength achieved in early days is more in these cases.
Dip:
- It is the angle in the vertical plane aligned with magnetic north (the magnetic meridian) between the local magnetic field and the horizontal.
Declination:
- It is the angle between the magnetic and geographic meridians, or the angle in the horizontal plane between magnetic north and true north.
.
Magnetic Lines | Definition |
Agonic Line | The line joining the places of zero declination. |
Aclinic Line | The line joining the places of zero dip |
Isoclinic Line | The line joining the places of the same dip |
Isodynamic Line | The line joining the places of the same value of the horizontal component of earth's magnetic field. |
DAD curve:
- The areal characteristics of a rain storm are represented by a depth-area-duration curve. Once the sufficient rainfall records for the region are collected the basic or raw data can be analysed and processed to produce useful information in the form of curves or statistical values for use in the planning of water resources development projects.
- Many hydrologic problems require an analysis of time as well as areal distribution of storm rainfall. Depth-Area-Duration (DAD) analysis of a storm is done to determine the maximum amounts of rainfall within various durations over areas of various sizes.
Hyetograph:
- It is a graph between rainfall intensity and time. Rainfall intensity progressively increases until it reaches a maximum and then gradually decreases.
Mass curve:
- It is graphical representation between accumulated rainfall v/s time. A mass curve of inflow can be prepared from the flow hydrograph of a stream for a large number of consecutive previous years
Double mass curve:
- It is used to check consistency of a rainfall for a particular area. The current mass curve is plotted and compared with previous data to check variation over a single curve.
Rigid-jointed frames
- In a rigid jointed frame, the joints are considered to rotate as a whole.
- These are framed structures in which the members transmit applied loads by axial, shear, and bending effects.
- Rigid-joints (moment connections) are designed to transfer axial and shear forces in addition to bending moments between the connected members.
The elastic strain energy stored in a member of length s (it may be curved or straight) due to axial force, bending moment, shear force and torsion are summarized below:
:
Axial Force, P | |
Bending, M | |
Shear Force, V | |
Torsion, T |
The strain energy of a beam
Depends on the shear force in the beam
Depends on the bending moment in the beam
It is different than the potential energy
It is found that the approximate ratio of the strength of cement concrete at 3 months to that at 28 days of curing is 1.15.
The table below shows the compressive strength gained by concrete after 1, 3, 7, 14 and 28 days with respect to the grade of concrete we use
:
Age | Strength in % |
1 day | 16 |
3 days | 40 |
7 days | 65 |
14 days | 90 |
28 days | 99 |
The strength of the concrete keeps on increasing initial age (up to few years) and then starts decreasing. Based on the the graphs/relationship obtained from the practical test conducted on concrete cubes.
This tension which is caused in the tensile zone of the beam due to shear, at or near the supports is called as diagonal tension which is caused by shear, cannot be resisted by concrete alone. So shear reinforcement is provided in the R.C.C. beams to take up diagonal tension and prevent cracking of beam.
Diagonal tension occurs in the tensile zone of the beam, generally tensile zone In a beam lies below the neutral axis and compression zone is above the neutral axis. So the diagonal tension increase below the neutral axis where shear force is more and it decrease above the neutral axis. The shear stress distribution along the depth of a beam is shown below.
Prestressing is the process by which a concrete element is compressed, generally by steel wires or strands.
The purpose of reinforcement in prestressed concrete is to impart initial compressive stress in concrete
Precast elements may be prestressed during the construction process (pre-tensioning) or structures may be stressed once completed (post-tensioning).
Prestressing compensates for the tensile stresses introduced when the element is loaded. Hence the concrete generally remains in compression.
Prestressing serves two main purpose:
1. To improve the resistance of the member to the dead and live loads.
2. To modify the behaviour of the member or structure in such a way so as to make it more suitable for its intended purpose.
The primary purpose of prestressing steel is to apply a force to a concrete, either by bond or by means of special anchoring devices.
When a composite system is subjected to temperature rise initially the component having a higher thermal coefficient value (brass in the diagram) will try to elongate more than the component having a lower thermal coefficient value (steel in the diagram).
But due to end constraints, both components have to settle for the same amount of elongation.
Note:
That’s why the component which has a lower thermal coefficient value and which has elongated less initially will be subjected to tensile stress now to achieve the average elongation.
Similarly, the component which has a higher thermal coefficient value and which has elongated more initially will be subjected to compressive stress now to achieve the average elongation.
Truss
Bracings:
- When a strong wind flows parallel to ridge, the sloping of pitched industrial roof truss deforms and instability arises parallel to ridge. To arrest this deformation and instability the top and bottom chords of the roof truss are joined by cross diagonal members which are called bracing. This bracings helps to resist the wind load parallel to ridge.
Purlins:
- These are horizontal beam members runs parallel to ridge and connects the trusses along the length of ridge.
Truss:
- These are combined arrangement of several structural members to transfer loads from top to bottom, and usually occurs at regular interval.
Columns:
- Columns are the vertical beam members which take load from the trusses and transfer it to the foundation.
Rivet and bolt
- According to clause no. 11.1.4 of IS 800: 2007, when the effect of wind or earthquake load is taken into account, the permissible stress as specified in rivets (or in anchor bolts) may be increased by 25%.
Confusion/Mistake Point:
- According to clause no. 11.1.4 of IS 800: 2007, when the effect of wind or earthquake load is taken into account, the permissible stress as specified in structural steel member may be increased by 33.33%.
The moment distribution method is best suited for the rigid 2D frame as for rigid 2D frame because only one kind of moment (Mz) is acting at every joint of the structure the moment developed at the joint is distributed to all connected members at that particular joint.
The moment distribution method only takes into account the moment effect not the axial force effect.
As trusses are designed to carry axial force, so it is not desired to do moment distribution for pin-jointed truss or trussed beam.
For space frame, there are three kinds of the moment (Mx, My & Mz) acting at every joint. So, it is very difficult and inconvenient to do moment distribution for spaced frame.
The strength of the fillet weld is about 80 to 95 % of the main menber.
Joint strength are generally kept less than the strength of the main member, else the main member may fail before the joint.
bridge span increase and impact factor decrease
Impact factor is taken to accommodate impact of live load on structures.
As per IRC 112, different types of vehicles have different impact factor.
For Reinforced concrete structures, it is given by:
I.F=4.5/ (6+L)
Where,
L = span of the bridge
As the span of the bridge increases, the impact factor decreases.
A slab is always designed for a shorter span.
As per clause 24.1 of IS 456:2000,
The shorter of the two spans should be used to determine the span to effective depth ratios (A-value).
For spans up to 3.5 m and with mild steel reinforcement, the span to overall depth ratios satisfying the limits of vertical deflection for loads up to 3 kN/m2 is as follows:
S.No | Type | A-value |
1. | Simply supported slab | 35 |
2. | Continuous slab | 40 |
for fixed beam
The point of contraflexure lies at a distance of L/(2√3) from either end of the beam.
Various assumptions made while analysing the structure subjected to torsion are:
a) Circular sections remain circular.
b) Plane sections remain plane before the application of twisting moment continue to remain plane after the application twisting moment.
c) The projection upon a transverse section of straight radial lines in the section remains straight.
d) The shaft is loaded by twisting couples in planes that are perpendicular to the axis of the shaft.
e) Stresses do not exceed the proportional limit.
f) The material of the bar is homogeneous and perfectly elastic, and obeys Hooke’s Law.
When a cylindrical shaft is subjected to equal and opposite couples at the ends, either it will be in equilibrium or it will rotate at a uniform rate.
In either case, it is subjected to torsion and the stresses set up by every cross-section are shear stresses.
The shear strain developed in the cross-section (either hollow or solid) will linearly vary from zero at centre and maximum at boundaries.
The first plastic hinge is formed at the center and two plastic hinges is formed at supports, hence even if the first plastic hinge is formed at the center, the degree of static indeterminacy of the structure remains the same.
Number of Plastic hinge formed in a structure is given by:
N = Ds + 1
Where,
Ds = Degree of static indeterminacy
N = Number of plastic hinge formed
Let us take an example of fixed beam subjected to uniformly distributed load throughout the span, Therefore the Number of plastic hinge formed = Ds + 1
Where, Ds = 2
Hence number of plastic hinge is formed is three i.e. one at centre and two at support.
Centrifugal Pump
The centrifugal pump converts the mechanical energy into fluid energy. Also, it increases manometric head by converting the pressure head into manometric head. Hence, during its operation there is a chance that the pressure at the eye of the impeller may reaches to zero or negative pressure which may lead to the cavitation effect (if it reaches below vapour pressure). Hence to reduce this effect, it must be placed at certain height from the sump level called as suction height.
This minimum manometric head is required to make possible the suction which is represented by NPSH (net positive suction head).
Hence, the centrifugal pump height is such that the negative pressure does not reaches below vapour pressure.
Important points:
NPSH is defined by the Thomas’s cavitation number times the manometric head.
Cavitation is a low-pressure phenomenon which causes wear and tear of the equipment.
In step methods, the computations must be done upstream of subcritical flow. This is done, as in subcritical flow the depth on the upstream side is high and on the downstream side it is low, hence stability is more on the upstream side.
Similarly, for a supercritical flow, the step method computations are done on the downstream side as the depth is more hence stability is more on the downstream side.
The step method analysis requires a certain amount of time hence high velocity and low depth flows cannot be analyzed through it.
We know that the centre of pressure is the point at which resultant pressure force due to fluid acts and it is given by-
h= x¯ + (I/Ax¯)(sinθ)square
h=x¯+IGAx¯(sinθ)2
Where, h represents the centre of pressure, IG = second moment of area, A = area, and x̅ = centre of gravity.
Now, as the depth of immersion increases, x̅ increases which results into increase in h but the rate of increment would be decreased because the formula has x̅ in the denominator.
Hence h will come closer to x̅.
For θ = 90°, means vertical, the centre of pressure is farthest to the centre of gravity.
For θ = 0°, means horizontal, the centre of pressure is coinciding with the centre of gravity.
True area = measured area × (R.F.)
Representativefactor(R.F.)=(truelengthstandardlength)2
Normally earthwork is estimated for 30 m lead and 1.5 m lift.
Lead: It is the average horizontal distance between the centre of excavation to the centre of deposition.
Lift: It is the average height through which the earth has to be lifted from the source to the place of spreading or heaping.
Normally earthwork is estimated for 30 m lead and 1.5 m lift.
Dummy activity: It is an activity which has zero duration and is created for the sole purpose of establishing the relationship between two activities. It is represented by dashed lines.
Excavate the foundations: This will be an internal activity (a part of project which requires some duration and cost as well to reach the tail event.
Waiting for the arrival concrete materials: It won’t require any cost nor any time hence it can be called as a dummy activity.
Lay the foundation concrete: This is the second stage after the excavation which will require money and time hence not a dummy activity
Cure the foundation concrete: This is the third stage after the lay of concrete which requires minimum 7 days of curing and manpower & resource cost.
Cost slope
Cs=CrashCost−NormalCostNormalTime−CrashTime
Cost Slope: The direct cost curve is a curve that can be approximated by a straight line, depending upon the flatness of the curve. The slope of this straight is the cost slope. It is very helpful in the project cost analysis.
