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)

What is meshing? What is role of meshing in design of concrete structures?

Meshing is the task of partitioning a spatial domain into simple geometric elements such as triangles (in 2D) or tetrahedrons (in 3D). Meshes typically have to conform to boundaries. Furthermore, for many applications there are quality criteria that need to be met, such as ensuring that angles are not too small. Meshing is a huge industry, with dozens of companies selling meshing software, hundreds of companies using software, and a diverse set of applications, including graphics, geographic information systems, computer vision, and air flow and structural simulations.

Meshing of an element

Use of Meshing

In the past decade there have been many important theoretical advances in algorithm design for meshing related problems, but only some of these have made it into meshing software.
Lateral walls and semi-rigid diaphragms are composed of shell elements. The subdivision of these elements into smaller pieces is commonly referred to as meshing.
In Ram Frame, the size of the mesh is controlled by the “Maximum distance between nodes” parameter in Ram Frame – Criteria – General. When a lateral beam has an internal node at a column or brace end, the beam is segmented in the finite element model and the beam is represented by two beam segments. These finite element segments are denoted by the use of I’ and j’ in the results. The first segment ends are I and j’ and the second segment ends are I’ and j. Therefore, I’ and j’ are always the ends at the internal node.

Meshing types

Coarse: Faster computation; not concerned about stress concentrations, singularities, or warping. Not near changes in geometry or displacement constraints or changes in material including thickness.
Fine: Best approximation but at the cost of the computation time. Look for disproportionate stress level changes from node to node or plate to plate and large adjacent node displacement differences to determine if need to refine the mesh. Nodes should be defined at locations where changes of geometry or loading occur. Changes in geometry relate to thickness, material and/or curvature. A simple check, if you can, is to decrease the mesh size by 50%, re-run analysis, and compare the change of magnitude of stresses and strains. If there is no significant change, then ok. In most companies, all of this knowledge of mesh size will be known and might
be set a FEA control file.

In case of the slab supported on beams we know in actual conditions the beam is monolithic with the slab that means that beam acts as a T-Beam with the slab having flange the portion of the slab. But during modeling in finite element analysis the slab or area or shell element is acted separately and the beam is acted separately. To make them monolithic we do the meshing of the shell element and divide the panels in to various sub-panels or parts which would then be attached to the beam from all the separated segments.

What is finite element analysis and how many types are used in analysis and design of concrete structures

Advancements in the field of computer aided engineering during the last two decades have been quite extensive and have led to considerable benefits to many engineering industries. Similarly the advantages can also be observed in the building industry where the softwares and tools which uses finite element have allowed the introduction of innovative and efficient building products along with development of accurate design methods.

Finite Element Analysis

Definition of Finite Element Analysis

According to one definition;
Finite element method (FEM) is a technique for numerical solution of complex structural problems. In this method the structural system (consisting of all the elements which make up a structure) is modeled by a set of appropriate finite elements which are interconnected at points called nodes. Elements may have physical properties such as thickness, coefficient of thermal expansion, density, compressive strength, yield strength, young’s modulus, shear modulus and Poisson’s ratio.

Use of FEM

Traditionally, engineers have used laboratory testing to investigate the structural behavior of steel building products and systems subject to the expected wind and earthquake loads and to develop appropriate design rules. Laboratory testing was also used to develop new building products and systems. However, such reliance on time consuming and expensive laboratory testing has hindered progress in this area. The product manufacturers and designers often decided on conservative designs in order to avoid expensive and time consuming laboratory testing. However, advances in the field of computer aided engineering during the last two decades have changed this situation significantly in many engineering industries.

The finite element method (FEM) is the dominant discretization technique in structural mechanics. The basic concept in the physical interpretation of the FEM is the subdivision of the mathematical model into disjoint (non-overlapping) components of simple geometry called finite elements or elements for short. The response of each element is expressed in terms of a finite number of degrees of freedom characterized as the value of an unknown function, or functions, at a set of nodal points.

The response of the mathematical model is then considered to be approximated by that of the discrete model obtained by connecting or assembling the collection of all elements. The disconnection-assembly concept occurs naturally when examining many artificial and natural systems. For example, it is easy to visualize an engine, bridge, building, airplane, or skeleton as fabricated from simpler components. Unlike finite difference models, finite elements do not overlap in space.

A typical finite element analysis on a software system requires the following information:
1. Nodal point spatial locations (geometry)
2. Elements connecting the nodal points
3. Mass properties
4. Boundary conditions or restraints
5. Loading or forcing function details
6. Analysis options

Because FEM is a discretization method, the number of degrees of freedom of a FEM model is necessarily finite. They are collected in a column vector called u. This vector is generally called the DOF vector or state vector. The term nodal displacement vector for u is reserved to mechanical applications.
FEM Solution Process

Procedure

1. Divide structure into pieces (elements with nodes) (discretization/meshing)
2. Connect (assemble) the elements at the nodes to form an approximate system of equations for the whole structure (forming element matrices)
3. Solve the system of equations involving unknown quantities at the nodes (e.g., displacements)
4. Calculate desired quantities (e.g., strains and stresses) at selected elements

Basic Theory

The way finite element analysis obtains the temperatures, stresses, flows, or other desired unknown parameters in the finite element model are by minimizing energy functional. Energy functional consists of all the energies associated with the particular finite element model. Based on the law of conservation of energy, the finite element energy functional must equal zero. The finite element method obtains the correct solution for any finite element model by minimizing the energy functional. The minimum of the functional is found by setting the derivative of the functional with respect to the unknown grid point potential for zero. This is based on the principle of virtual work, which states that if a particle is under equilibrium, under a set of a system of forces, then for any displacement, the virtual work is zero. Each finite element will have its own unique energy functional.

Retaining Walls, the support soil needs to stand up

We know that soil or any other loose granular material has a safe angle of repose at which the soil or material can withstand without any requirement of confinement or support. But due to practical and property restrictions we in some cases need to cut or fill the soil at some unstable angle which if unsupported might cause failure of the slope.

Concrete Retaining Wall

If we provide stable slope rather than steep slope; we might need more space there and sometimes more economy is required; to cater this problem we use the retaining wall to support the material laterally after justifying its usage from economic considerations.
Thus we can define retaining wall;

What is Retaining Wall?

As a structure or a part of structure that holds back and retained the loose material behind within a limited space to provide safety and economy one at the same end.

 Or
McCormac and Nelson defines it as;

Retaining wall is a structure built for the purpose of holding back or retaining or providing one-sided lateral confinement of soil or other loose material.

Aurther H. Nilson and others define the retaining wall as;

Retaining walls are used to hold back masses of earth or other loose material where conditions make it impossible to let those masses assume their natural slopes.

Example Of Retaining Wall

Typical example of the retaining wall might be that of highways; in highways we have restricted property space called the right-of-way and within that limited space we have to provide the required width of the highway by using the cut and fill soil, and there those steep slopes come which needs retaining wall to get that support;

Types of Retaining Walls

Most commonly five types of retaining walls are used named as under;
(a)    Gravity Retaining wall
(b)    Cantilever Retaining Wall
(c)    Counter-fort Retaining wall
(d)    Buttress Retaining wall
(e)    Bridge-abutment type retaining wall

Gravity Retaining Wall;

The Gravity Retaining wall as the name suggests provide support or restraint to the loose material laterally because of its self-weight, it generally doesn’t contain reinforcement and mostly is used for 10 ft height and no more than that because more height will increase the need of more thicker cross-section and ultimately it will not be economical.

Gravity Type Retaining Wall

Cantilever Retaining Wall

Is a reinforced retaining wall that acts like a cantilever beam, it has four parts, the portion of the base slab or footing below the retained material is heel, and the other portion is toe, the vertical wall is called stem and to increase the slide friction key is provided below the base slab or footing.
The weight of the retained soil on the top of heel provides additional stability to the structure. It is mostly used for relatively greater heights upto 10 to 20 ft. 

Cantilever Type Retaining Wall

Counter-fort Retaining Wall

In Counter-fort retaining wall the stem and heel (the portion of the base slab below the retained slow) are joined together by inclined transverse walls at intervals called counterforts, Counterforts are in tension and mostly called counterfort ties. Counterfort retaining wall is mostly used for higher heights up to 25ft or more.

Counter Fort Retaining Wall

Buttress Retaining Wall

Buttress retaining wall is similar to counterfort retaining wall except that the inclined transverse walls are provided at the front of the stem called buttresses and acts like a compression member. Because of this property they are more stable and more economical, but because the buttresses are visible and occupies the space un-necessarily that might be useful if buttresses were not there; thus they are not generally used;

Buttress Retaining Wall

 Bridge Abutment Retaining Wall

It is a wall types bridge abutment, abutment and piers are the column-type structures used to support the deck of the bridge, the first and the last support of the bridge are called abutments because they retains the soil where as other supports are called piers. This type of retaining wall is just a simple cantilever retaining wall except that the bridge deck provides an additional horizontal restraint at the top of the stem. This wall is thus designed as a beam with fixed support at the bottom and simply supported or partially restrained at the top. 

Bridge Abutment Retaining Wall

From Editor’s Desk

Retaining Walls are very sensitive type of structures that protects our infrastructure like highways and roads from land-slides etc and helps us to let the roads be clear for us. In the retaining wall drain pipes are also provided along with pot-holes for reducing the amount of lateral load and to allow the wall to remain stable against overturning and sliding.
Hope you like this post, don’t forget to share it with your friends and colleagues.
Happy Civil Engineering!
 

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Things you should know about rutting

A very common premature pavement failure in most countries is seen as a surface depression in the wheel path. The depression which is formed is called a rut and along the rut uplift occurs due to shearing. Ruts are particularly evident after a rain when they are filled with water. Rutting may occur only in the pavement surface and the sub grade may be intact and rutting may also be exhibited by the sub grade as well.

What is rutting?

Problems due to rutting?

The ruts which are formed due to rutting may be filled with water and may cause further damage to the roadway. During turning the tyres of the vehicle may stuck in the rut, similarly during movement the tyres may get attracted towards the rut due to depression formed.

Causes of Rutting

Rutting is mostly caused due to consolidation of the subgrade i.e. the downward lateral movement of the soil followed by escape of moisture, or due to lateral movement of the materials i.e. bitumen, aggregate etc.

Most causes found can be;

Insufficient compaction during construction, inadequate pavement structure, improper mix design

Difference between free way (motorway) and a simple highway?

In pavement engineering we mostly hear the terms highway, motorway and freeway. These terms sometimes causes confusion about what they mean in real and what is the basic difference in them. In this post we will try to find the basic difference between these terms.

Difference between motorway and simple highway

Firstly all these terms specifies a paved surface which is used for transportation of high speed vehicles that may include trucks, cars, bikes etc. Motor way and Free way is similar in this sense that both of these have limited access which means to enter in a motorway or freeway there is specified path followed by a collector road, in a similar manner the exit is also controlled. Where as in highway the access is not controlled there are a lot of paths provided where the access is very easily provided. But even then the motorway and freeway are different in the sense that, in freeway there are no toll plazas and no toll tax is collected which means it is free and thus it is known as freeway but that is not the case with a motorway. In Motorways the toll tax is applied, in various countries mostly developing countries the project of a motorways is awarded to some foreign firm or contractor which doesn’t take the cost from the client but for some specified time the toll is being collected by the contractors to complete its cost plus profit after which the project will be handed over to the client which may apply the toll tax for maintenance and up gradation of the project.

How Traffic engineers plays with speed? Definition, Types, Relations

Traffic speed – traffic speed characteristics – how we can define speed, distance covered by an object in unit time

Speed and its role in traffic engineering

U = s/t
In transportation engineering – speed is defined as the distance(s) that is covered by a vehicle in specific time (t). Speed is the most critical aspect of the geometric design – the roads are designed according to the speed and the cost of any highway project is dependent on the speed. The greater the speed you are providing for the riders the greater will be the cost of that project.

Important Information
The speed limit on the motarway in South Asia is usually in 120 kmph – kilo meters per hour

Speed is not consistent. The speed of any vehicle depends on many factors like the location of the vehicle, the design of the roadway, the purpose for which the person is driving the vehicle, the time in which the person is riding, the congestion condition in that locality, the weather condition and the visibility on the road etc. Because all these factors are varying and are very complex to tackle, the speed is always changing even in very small amount of time.

Types of Speed

1)    Running Speed

It is that average speed of the vehicle in which the vehicle hasn’t stopped i.e. the speed in one continuous trip. The vehicle may have slowed down but haven’t stopped. The speed which is marked on the boards present on the road is called posted speed. The speed for which the road is designed is called design speed. The speed on which mostly the cars are running on the road is called operating speed.

 Important Information
The distance between Rawalpindi and Lahore if we go from Motarway is 400 km, but if we go from G.T Road it is approx. 300 km. But more time is needed in case of G.T. Road.

2)    Journey Speed or Travelling Speed

The total average speed of the vehicle calculated by dividing the total distance between two stations with the total time taken by the vehicle plus the time spent during any stoppage. Thus it includes the speed of continuous trips plus the time of any number of stoppage.
Speed is very important aspect as far as the supply chain management is concerned, like if you have to supply a particular fresh fruit which will expire after certain hours you have to consider all the critical aspects which effects the speed, similarly in traffic engineering all the speeds are considered and studied.

3)     Instantaneous – Time mean or spot speed

It is the speed determined or recorded at some specific time. It can be calculated for example by radar checking system. It is used to study the speed patterns of the traffic. It is also used to check the percentile speed. Percentile speed is used to calculate the operating speed of the roadway.

Important Information
What is the meaning of percentile? Percentile basically compares a result with certain group of people or objects rather than comparing with absolute value. For example in statics class if your score is like only 6 % students have got better marks than you and 94% students have marks less than you, then your result is 94 percentile.

How to calculate the percentile speed?

First the speed of 100 specimen vehicles is recorded and is listed in ascending order. 85th percentile of the governing speed is then calculated and the resultant will be taken as the operating speed of the highway.
85th percentile here would mean that only 15 cars out of 100 cars have speed better then the specific speed, and 85 cars out of 100 cars have speed less than that specific speed.
In 85 percentile you have to note such a speed such that only 15 % cars have speed more than this and 85% cars have speed less than this specific speed. Thus this specific speed is taken or considered as the operating speed of the roadway.

4)    Space Mean Speed

It is the average speed of the vehicles determined at specific length of the highway section. It can be determined by first marking two sections on the roadway and then when a particular vehicle crosses that section stop watch is set on and when the vehicle left the section the stop watch is set to off and then speed which is calculated will be the space mean speed.

5)     Design speed

The speed for which the highway is designed considering all the design conditions; like vertical and horizontal alignment, lane width, median, shoulders, weather condition, visibility, volume of traffic, type of traffic, no of lanes etc.

Important Information

Standard lane width in case of Motorway and most highways is 3.65 m except the local access roads in the rural areas or the dual carriage way in case of the rural areas.

Factors Influencing the Speeds

There are two basic characteristics of traffic stream which influences the speed, by traffic stream we mean combination of different types of traffic present on the highway at specific time. Like the traffic stream we see when a traffic light signal is just turned to green, we saw motorbikes, cycles, trucks, cars etc. If we are in China we would see bicycles mostly if we are in some industrial area we would see trucks mostly, in Pakistan in cities we would rather see small cars mostly.
(a)    Macroscopic characteristics of traffic
(b)    Microscopic characteristics of traffic

Macroscopic Characteristics of Traffic

By macroscopic we mean the accumulated effect of vehicular characteristics and not considering the individual effect of traffic streams i.e. the overall effect. There are 3 basic parameters
1)    Speed – U
2)    Volume or flow – q
3)    Density or concentration - K 

Speed, we have already discussed it, what is volume, it is not the multiplication of length width and depth,
Volume or flow: no. of vehicles passing through specific section of the highway at the specific time.
Flow – if time is in hours then flow is no. of vehicles per hour. Thus its unit is veh. / hr
q = veh / hr
Density: no. of vehicles occupying specific length of a highway at specific time is known as density.
The basic difference between volume and density is that volume is in terms of time (hours) while density is in terms of length of the highway section (miles)
K = veh. / mile

Units:

-    Speed (U) = ft/sec.
-    Volume (q) = veh. / hr.
-    Density (k) = veh. / mile

Macroscopic Characteristics of Traffic

 Those characteristics which takes the individual effect of the vehicles they are
(a)    Spacing (s) and headway (h)
(b)    Clearance (c) and gap (g)

(a)    Spacing (s)

It is the front to front distance or the distance from front bumper of first vehicle to front bumper of the other vehicle. Units is in ft/vehicle
S=ft/vehicles
1/S = vehicle / ft
K = vehicles / mile = 5280 / s
K = 5280 / s
Where k is concentration or density – veh. / mile
And s is spacing which is in ft/ veh.

Important Information
1 mile = 5280 ft.

You can measure spacing between vehicles, by taking any two vehicles travelling on the road by satellite image and then calculating the spacing from which you can calculate the density or concentration (k).

(a)    Headway (h)

It is the time a car is lacking behind another car a head of it.
Important Information
In formula f1 racing cars the score of all the vehicles is calculated by headway the time It is behind or a head of any other vehicle.
h = sec / veh.
1/h = veh. / sec.
q=veh./hour
= 3600 / h
Now;
Speed (U) – ft/ sec ; density (k) – veh / mile ; flow or vol (q)  – veh / hr.
                                    Spacing (s) – ft / veh; headway(h) – sec / veh.
q/k =u
q = u.k
-    Where u is speed in miles / hour
-    q is flow or volume in terms of veh. / hour
-     k is concentration or density in terms of veh. / mile 

  Speed Density Relation

 Flow-density relation

 

Speed Flow Curve 

 

Example

Problem: Find the speed of a vehicle if its headway is 2.5 sec / veh. And spacing is 200 ft/ veh.

Solution:

 h=2.5 sec. / veh.

s=200 ft/veh.

q=uk => u = q/k

q=3600/h = 3600 / 2.5 = 1440 veh. / hr

k = 5280 /s = 5280 / 200 = 26.4 veh. / mile

u = q/k = 1440 / 26.4 = 54.4 miles per hour - answer

Speed is the first criterion in the feasibility report – speed directly effects the cost of a highway project. Cost vs benefit ration which is calculated for any project should be in positive if that project is to be initiated – the social and economical benefits of any project is estimated.
There are 3 components of cost of any project
(i)    Construction cost
(ii)    Vehicle maintenance cost
(iii)    Time delay cost
The last two costs comprises of 70% of the total cost, while the first one comprises of 30 % of the total cost. These costs are estimated considering 40 years.
Then Benefits of that project on the plain area, rolling terrain, or hilly or mountainous terrain is considered.
Freeway: A type of highway in which minimum posted speed is 120 Kmph and level of service is A1
Expressway: a type of highway in which speed may vary a bit but level of service is A1
Then we have collectors, etc.
A policy on Geometric Design of Highway – AASHTO Green Book