What is Wobble Coefficient in post tensioning system?

Lets us answer this question from scratch, the word wobble means unsteady jerking movement from one end to other or move unevenly while coefficient is a symbol or number that defines the relation between two or more variables.

While post tensioning we use strand wire that is being joined by twisting around one and other to make a tendon, this tendon is to be inserted within a profile pipe called sheathing or duct after the concrete will achieve a specified compressive strength.

Sometimes the jacking force is applied from one end or both ends. The duct or profile pipe is to be inserted as per the designed curvature and location. The strand wire or tendon can’t be completely set as per the designed curvature and profile due to field restrictions and faulty workmanship.

Due to these reasons the curvature is bit shifted from the designed curvature and thus causes some loss in tensioning force. When jacking force is applied at one end, it decreases as we moves to the farther end due to friction between the tendon and the profile pipe or duct, this friction losses is of two types,

1)      Curvature friction loss

2)      Wobble friction loss

Curvature friction loss is basically the loss in tensioning or jacking force due to design curvature while wobble friction or wobble coefficient is a function of unintended deviation due to faulty workmanship or field restrictions.

Wobble coefficient is usually denoted by K and is measured in units of per length. It is found by multiplying the jacking force at any distance from jacking end with the average of the intended angular deviation from the design profile.

Usually having value of 0.0088. 

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Characteristics of a Good Formwork

Concrete being plastic material in fresh state, has to kept within an enclosure (moulds) till it gain reasonable strength, this temporary structure which has to be built for any concrete member is called “Formwork”.

Click me to read more about the Form work defintion and other defintions

Characteristics of a Good Formwork

Basic Requirements of a Good Formwork:

"Form Work must be strong, smooth, true, water tight and economical"

Strength

i) Formwork must be strong enough.  This is very essential as it has to withstand the weight of concrete, which weighs about 145 lbs/ft3 and exerts a lot of pressure on the members surrounding it while it is in wet state. However, after drying, it may not exert any pressure. Extra allowance for use of vibrators must also be made. 

Smoothness

ii) Formwork should be smooth get a smooth surface of the required member: It is always desirable to have smooth surfaces on all concrete structures particularly on the exposed concrete work. IF the surface is not smooth over soofits of beams and slabs, we can apply 3/8” thick plates to remove undulations and irregularities. But this plastering is not generally recommended as the contractor can hide it’s not properly compacted concrete and further plastering increases the cost unnecessarily. All the impressions which are visible on the surrounding formwork will get transformed onto the concrete surface. If rough texture of the surface is required, formwork can be made rough. 

Followed Dimensions

iii) Formwork should be true: It should strictly follow dimensions of the structure. 

Easy to remove

iv) Formwork should be such that it can easily be removed without damaging the surface, so that, a suitable mould oil (release agent) is recommended for easy striking of forms. 

Avoid Deflection

v) Deflection is the most important consideration for design of formwork, the limit for deflection varies according to class of the work. For simply supported spans, this limit is not more than span/360. The loads for design of formwork include the following.

a. Weight of wet concrete and self weight of forms (dead load),

b. Live load due to the men working as well as the impact of machine such as vibrators. The maximum live load is normally taken as 75 lbs per sq-ft. 

Water Tight

vi) Formwork must be water tight: so that there is not loss of fine material (cement paste etc.)

Skilled Labor

vii) Formwork determines the shape and position of finished concrete so accuracy of finished structure depends upon the skill of formworks, designers, builders and erectors. Properly skilled persons should be employed for formwork construction.

Economical

ix) Fulfilling all of the above requirements, formwork should be as economical as possible. 

Fatigue (Alligator) Cracking

The word Fatigue means repetitive action of loads, thus fatigue cracking by names means cracking due to the repetition of loads. As the pavement is subjected to number of vehicles of different sizes and loads the stress keeps on pilling up and thus the cracks appears which are interconnected in nature. 

Development of Fatigue Cracking

The fatigue failure of a pavement consists of 4 states as;

Initiation of minor hairline cracks

Development of micro cracks into wider macroscopic cracks

Propagation of these cracks across the thickness of pavement

Development of Pothole and ultimate premature failure of pavement

Fatigue Failure leading to Development of Pothole

 Fatigue cracking is sometimes also called alligator cracking, as they resembles the shape of the skin of an alligator due to the interconnectedness of the micro cracks in asphaltic pavements. Factors influencing the fatigue cracks 

Factors affecting Fatigue cracking 

There are numerous factors that influence the development of alligator cracks, some of them can be number and magnitude of applied loads, the structural design of pavement including the type of layer materials and thicknesses of various layers, the quality and uniformity of foundation support, How much workable or consistent is the asphalt cement used, the quantity and quality of the asphalt, the presense of air voids and characteristics of the asphalt concrete mix and the last one can be the temperature or the climate condition of the site under consideration.

Problems due to Fatigue Cracking

Due to fatigue cracking there are numerous problems to be faced by the pavement if not proper maintenance is ensure, the structure failure is at the top having, moisture infiltration, roughness and development of pothole are further worse situation inter-related to fatigue cracking.

Causes of Fatigue Cracking

Despite of the fact that fatigue stresses are directly caused because of traffic loading but the factors that can make the situation more worse are the environment factors including weather. 

Similarly due to moisture or any other reason when the subbase or base of the pavement is weakened the pavement will not be able to handle more loads and thus failure would occur.

Sometimes when the drainage characteristics of pavement is not properly optimized the bed of the road degraded causing base or sub-base weakness. 

Sometimes due to poor adhesion between the asphalt and the aggregates, the aggregates started to dislodge from the surface this process is called stripping or raveling due to this exercise same result of fatigue cracking can be observed. 

Fatigue Cracking and Edge Cracking

Alligator Cracking (Fatigue cracking)

Sometimes there is confusion between the edge cracking and fatigue cracking. Both of these are not similar, the basic difference between edge cracking and fatigue cracking is that edge cracking starts from surface and propagate downwards where as fatigue cracking starts from bottom and propagate to the surface. 

Presentation of Fatigue Cracking

As we now, know the reasons of fatigue cracking we can easily prevent them, by making sure the pavement have sufficient resistance against the causing factors, these my include, 

Lowering or reducing the load on asphalt pavement by increasing the thickness of the pavement. 

Provide adequate drainage at the ends and shoulders to avoid collection of water there as well as its percolation into the body of pavement which may worsen the situation. 

How to repair Fatigue Cracking

Similarly like presentation, in repair work one must know the reason of fatigue cracks, but sometimes the specific cause is fairly difficult to determine, and prevention is there correspondingly difficult. Any investigation should involve digging a pit which means coring of the pavement body to determine the structural makeup of the pavement as well as determining whether or not subsurface moisture is a contributing factor. 

While the fatigue crack is in its early stage, crack sealant may heal the bad effects and avoid further damage of the road, but ultimately the crack will propagate and failure will be unavoidable at the end only one solution exists and that is to do overlay of the cracked pavement by a hot Mix asphalt (HMA) of sufficient strength to resist the loading effects. 

According to the conclusion from some of the laboratory tests one can say; for thin asphalt concrete layers which are less than 5 inches, asphalt cement of low stiffness or low viscosity should be used to avoid premature failure due to fatigue, and thus for the asphalt concrete layer of 5 and more than 5 inch thick asphalt cement of high stiffness should be used. 

What is Formwork, Shuttering, Scaffolding, Shouring

During construction of reinforced concrete works as well as their maintenance, we need to support the structure as long as it is in its premature state or plastic state as it could not even bear its own load. After the suitable duration of time, i.e. when concrete sets enough to take its own load, these temporary supports are removed systematically.

Here are some of the definitions that must be known to a construction engineer as well as civil engineer.

What is Formwork:

Concrete construction in the modern building works has achieved considerable importance, roughly speaking 80% of civil engineering works utilize concrete for their constructions. Concrete being plastic material in fresh state, has to kept within an enclosure (moulds) till it gain reasonable strength, this temporary structure which has to be built for any concrete member is called “Formwork”.

shuttering, formwork, scaffolding, shoring

Formwork is thus a sort of mould used for pouring fresh or plastic concrete. It is closed from all sides and can carry the hydrostatic load of fluid concrete, additional load due to vibration, and load of men and machinery. Formwork should be such that it can be easily removed after hardening of concrete.

Timber, plywood panels or steel sheets are used for formwork. Timber is the most common material for formwork construction, it is easy to work with and it is also economical. However the use of steel forms thus gained lot of popularity in the recent years. They can be obtained for all types of structures. The initial cost of steel forms may be high but they can be repeatedly used for a number of times.

What is Shuttering?

Shuttering is a temporary plateform constructed with the help of wooden planks, wooden logs, steel rods or bamboos over which formwork is supported and ultimately pouring of concrete is done.

What is Scaffolding?

Scaffolding is a grid of bamboo, wooden planks or circular steel pipes by which labour can have access to any point of the structure to be constructed and further formwork and shuttering can rest over it. Steel scaffolding is most commonly used as it is easy to dismantle and reassemble.

What is Shoring?

Shoring is the temporary support given to the existing structure for repair purposes. When dismantling any central building, the surrounding building is also temporarily supported.

What is Timber, Merits, Demerits Growth and Structure

Variety of Construction Materials are being used now a days depending upon the availability and transportation cost, lesser the material available expensive will be the structure and as a result structural engineer might have to think to change the material accordingly. In mountains where brick is not easily available due to less availability of plain surface timber is preferred, similarly in areas where there is great danger of earthquake and light structure is needed Timber is preferred.  

In this post I will be sharing with you some key terms related to Timber engineering, merits demerits, of timber, growth of trees and structure of trees. It is a series of post related to timber construction so stay tuned for more updates. 

Timber Construction

"In developed nations like the United States, timber is used as lumber in the construction industry and to produce furniture, pulp and paper, and wood-based composites."

Definition of Timber:

“Wood which is suitable for building or any other civil engineering purpose is called Timber”

When it is a port of living tree it is called standing Timber.When that tree has been felled it is called Rough Timber. When it is sawn or converted into various forms it is called “Converted Timber”

Uses of timber

Despite of the aesthetic use of timber for decoration and finishing work, Timber is used as a structural material in houses as well as structural components like beams, columns, slabs etc. Because Timber is easily available in hilly and cold areas it is used for partial and full insulation. Some times Timber is used to construct Small Span bridges as well, we can see many of the bold bridges being that of timber along with the cables. Similarly in past Railway engineers used Timber as sleepers under the railway track to bear the thrust of the steel rail gauges and to provide them stability as well. 

Merits and De-merits of Timber

Timber is cheap and economical as it is easily available, in comparison to concrete where which need specified gradation of aggregates and sand as well which is not readily available in some hilly backward areas. Timber can be cut easily to any shape as required in the shape of column, beam or even you can construct truss out a timber log. Timber is durable and long lasting as far as it is dynamite proof. It is harder and light weight which enhances its ability to be constructed on sites of low bearing capacity soils. 

"Early citizens in the Massachusetts Bay Colony started to refer to sawed planks as lumber"

But Timber has some demerits as well, It can easily be attacked by insects which can convert whole of the structure into powder just in few days. Moisture and water logging problems in few areas are destructive for it. It is less strong as compared to steel and concrete. In some plain areas it is very costly and environmental hazards and new environmental laws and regulations do not appreciate its usage. 

Classification of Timber (Trees)

There are two types of trees depending upon their mode of growth;

Endogenous 

Exogenous

Endogenous

They grow inward in longitudinal fibrous mass. Examples are banana, bamboo, palm and cane.Their stem is rough and light yet it is flexible enough to be useful for engineers.

Exogenous 

They grow outward by the addition of one concentric ring every year. These concentric rings are called annual rings. The no of concentric rings indicates no of years or age of that tree. Exogenous trees are mostly used for engineering purpose.

Exogenous trees are further classified into two types;

a) Trees yielding soft wood, 

        a. Conifers or evergreen

        b. Trees with pointed leaves

        c. Deodar, pine, chir, kail belong to this group

b) Trees yielding hard wood,

        a. Deciduous with broad leaves

        b. Teak, sal, shisham belong to this group

Growth and Structure of Timber Trees

The growth is done in summer and autumn season;

In spring, roots suck a solution of salt from the soil, this solution is than transmitted to the branches and leaves. The salt solution losses moisture because of evaporation, then it absorbs carbon dioxide from the atmosphere, then in the presence of sun light this solution is transformed into a viscous solution called a sap.

In autumn this sap is descend below the bark where it gets thick and its transformed into wood forming a cambium layer. Cambium layer is again with the passage of time gets thicker and thicker and is then form an annual ring. The modullary rays carry the sap from inside the bark to the interior of tree getting itself nourished. 

Structure of Timber Construction

The cross-section of the an exogenous trees shows the following structure

i) Pith or Medalla

ii) Annual Rings

iii) Heart wood

iv) Sap wood

v) Cambium layer

vi) Medullary rays

vii) Bark

Pith

The first formed round dark portion of the tree is called pith, around pith annual rings are formed the pith, when the plant is young contains large amount of fluid and it nourishes the plant. It dies up and decays when the plant becomes old. 

ANNUAL RINGS

Woody fibre formed and arranged in concentric rings around the pith are called annual rings, they are called annual because each year one ring is developed, with the help of annual rings we can find the age of tree by counting dark or bright rings, each annual ring consists of two parts outer part being darker and inner one being lighter. The outer portion is dark, solid as compared to inner portion. Wood formed during spring season is called spring wood and the wood formed during summer season is called summer wood. 

Bark or Cortex

It is the outermost protective covering of the tree which is exposed to air. 

Heart Wood or Duramen

The central rings surrounding the pith and is near to the pith is generally darker and is called heartwood. It is non-active part of the tree and generally gives rigidity or strength to the trees.

Sap Wood

Ring near to the bark or away from pith is generally lighter in color and is weaker as well, its main task is to provide sap from root to branches. Heart wood  is generally harder than sap wood thus it must be used for all types of engineering works and sap wood must be avoided as is not rigid is more exposed or liable for decomposing. 

Cambium Layer

Rings between bark and sap wood constitutes cambium layer, it is that part which is not yet converted into wood. If the cambium layer is exposed to atmosphere by removing the bark it will result in ultimate death of the tree.

Medullary Rays

These are thin horizontal veins radiating from the pith towards the bark. The main purpose of modularly rays is to carry sap from inner to bark, and they also keep the inner or outer rings bound together. 

What is Brick Masonary and some important terms civil engineers must know.

When bricks are laid in mortar in a proper systematic manner, they form a homogeneous mass, which can withstand forces without disintegration. This mass of the structure, so made by the use of bricks is called "Brick Masonry" or simply "Brick work".

Bricks are of uniform size and shape, light in weight, durable, fire resistant, have high resale value, low maintenance cost and are easily available in plain areas.

Brick Masonry is commonly used for construction of ordinary as well as important buildings in plain areas now-a-days.

Some Important Terms Used in Brick Masonry

What is Brick Masonry?

(1) BRICK

An artificial structural element in the form of a rectangular block of clay is called a “Brick ".

Bricks can be manufactured of any required shape and size. The sizes of some standard bricks are given as follows:

These sizes are called "Nominal, designated or format sizes" and are used while estimating the number of bricks in a given volume of structure.
The actual sizes in which bricks are manufactured, are slightly smaller to allow for the layer of mortar present all around the brick, usually taken as 3/8 in thick. The Actual or Work size of English standard brick, which is mostly used in Pakistan, is usually taken as 8 5/8 in 4 1/8 in x  2 5/8 in.

(2) FROG

The depression provided in the face of a brick is called a "Frog". 

It is provided in the brick to achieve the following purposes:

(a) To form a key of mortar in between any two adjacent courses of brick work, so as to increase the lateral strength of the structure.

(b) To reduce the weight of the bricks, so that the bricks can be laid with convenience.

(c) To provide a place for putting the impression of trade-mark or the year of manufacturing of the bricks.

(3) POSITION OF BRICKS

(a) The position of brick, when laid with its Frog upward in the horizontal plane, is termed as "Brick on bed".
(b) The position of the brick when laid on its side "9 in  x  3 in", with frog in the vertical plane is called "Brick on edge".
(c) The position of brick when laid on its side "4 1/2 in  x  3 in", with frog in the vertical plane is called
"Brick on end".

(4) COURSE

Each horizontal layer of bricks laid in mortar in a brick work is called a "course".

(5) STRETCHER

Brick, laid with its length horizontal and parallel with the face of the wall or other masonry member is called a "Stretcher" 

and a course, in which, all the bricks are laid as Stretchers is called a “Stretching course" or "Stretcher course".

(6) HEADER

A brick laid, so that only its end shows on the face of a wall is called a "Header" and a course, in which all the bricks are laid as headers, is known as "Heading Course" or "Header course".

(7) QUOIN

The external corner of the wall is called a "Quoin".

(8) QUOIN BRICK

The brick, which forms the external corner of a wall is known as "Quoin brick".

(9) QUOIN HEADER

A corner header, in the face of wall, which is a stretcher in the side wall is known as "Quoin header".

(10) QUOIN STRETCHER

A corner stretcher in the face of a wall, which is header in the side wall is known as "Quoin stretcher".

(11) BRICK BATS

The pieces of bricks, cut long their length and having width equivalent to that of a full or half brick are called "Brick bats".

Some common Brick Bats are shown below:

(12) QUEEN CLOSER

Queen closer is a brick, which is half as wide as full brick and is made by cutting a whole brick lengthwise into two portions.
These are generally used next to the Quoin header for creating bonds in brickwork.

(13) KING CLOSER

A brick, whose one diagonal piece is cut off one corner by a vertical plane passing through the center of one end to the center of one side.

It is actually 7/8 of a full brick but is usually called a 3/4 brick

(14) BEVELED CLOSER

A brick cut longitudinally along a vertical plane, starting at the middle of one end to the far corner.
One quarter of the brick is cut off in this way.
           

(15) BULL NOSE BRICK

 
A  brick with rounded corners is called a “Bull Nose Brick”
                               

(16) SQUINT BRICKS

These bricks are used to construct acute (>90 degree) or obtuse (< 90 degree) corners in brick masonry.
These are special forms of bricks.

(17) JAMB

The vertical sides of door or window openings provided in a wall are known as "Jambs".

(18) REVEALS

The part of the Jamb opening , which is exposed between a door or window frame and the face or back of a wall is known as "Reveal".

(19) SILL

The horizontal part (either of timber, concrete, stone, metal, etc) at the bottom of a door or window, supporting the vertical members of the frame is known as " Sill " and its height window base from the floor level is known as " Sill level ".

(20) MORTAR

The paste obtained by mixing a binding material and a fine aggregate in suitable proportions in addition to water is known as "Mortar".

Cement and Lime are used as binding materials and Sand, Surkhi, Cinder, etc. are used as fine aggregates. The mortars are named according to the type of binding material used in their preparation such as, cement mortar, lime mortar, etc. The mortar prepared from simple earth is known as "Mud Mortar". The mortar not only acts as a cementing bed between any two courses of bricks but also, gives strength to the structure by holding the individual bricks together to act as a homogeneous mass.

Credits

These notes have been developed by Prof. Dr. Liaqat Ali Qureshi (UET, Taxila Pakistan)