How to design a Septic Tank?

A Septic Tank is a small scale sewage treatment system used in areas which have no connection to the main sewage pipes that are provided by the Local governments or private Corporations.

In this article, we will detail out all the calculations that are to be performed to get the required volume of a Septic Tank with the help of examples.

Schematic Diagram of a Septic Tank

We would begin with a small scale example so that your concept of Septic tank design is clear.

Here are the thumb rules to be kept in mind which will help you in calculating the volume of a Septic Tank.

For a three bedroom house:

The quantity of sewage considered for a three bedroom house should not be less 900 gallons.

For a two bedroom House:

The quantity of sewage to be considered for the design of Septic Tank should not be less than 700 gallons.

For one bedroom unit:

The quantity of sewage to be considered for the design of a Septic Tank should not be less than 550 gallons.

Let’s begin with an example of a three bedroom residence.

Septic Tank calculations for a three bedroom Residence

As per the thumb rule;

Quantity of sewage to be considered for the design of a Septic Tank = 900 gallons

1 gallon of liquid = 3.78 litres

Quantity of sewage in litres = 900×3.78 = 3402litres

Volume of Sewage in Cubic metres

3402/1000 = 3.402 cubic metres

Volume of Sewage in Cubic feet

3402/28.52 = 119.28 cubic feet

Dimensions of a Septic Tank in metres:

Width of the Septic Tank = 1.70m

Depth of the Septic Tank = 1.00m

Length of the Septic Tank = 2.00m

Dimensions of a Septic Tank in feet:

Width of the Septic Tank = 5.61’

Depth of the Septic Tank = 3.3’

Length of the Septic Tank = 6.6’

In this article, we will move forward with our discussion on the Septic Tank calculations for an Office Complex.

First question that is likely to occur to your mind would be, “How are Septic Tank Design calculations for an Office Complex different from that of a Residence?”

Let us first answer this question before we move forward with the calculations.

Residences are structures which are occupied with families living, eating and sleeping there. Let’s take an example of a person who regularly goes to an office for his job.Let’s have a look at his Daily schedule:

He wakes up in the morning.

Takes a bath, uses a toilet.

Has his breakfast after which he washes his hands. Apart from him using the services in the house, his family members also use the same.

He leaves for his office. After he reaches the office, he wouldn’t take bath which consumes the maximum amount of water which turns into sewage water after use.

His usage of toilet and bathroom would be minimum. Hence, it is important to understand the logic behind the Septic tank calculations.

Calculations for an Office Complex with 8000 employees

As per thumb rule;

Quantity of sewage to be considered for an Office Complex is to be calculated on the basis of the capacity of the Office.

Now, we are calculating the Volume for the Septic Tank required for an Office Complex with 8000 employees.

Hence, quantity of sewage to be considered per person should not be less than 20 gallons.

Office Complex = 8000 persons

Total quantity of sewage = 8000*20 = 160,000 gallons

1 gallon = 3.78 litres

Quantity of sewage in litres = 160,000 x 3.78 = 604,800 litresVolume of Sewage in cubic metres:

604,800/1000 = 604.8 cubic metresVolume of Sewage in cubic feet:

604800/28.52 = 21206.17 cubic feetDimensions of a Septic Tank for an Office Complex in Metres:

Length of the Septic Tank = 15m

Width of the Septic Tank = 13.44m

Depth of the Septic Tank = 3mDimensions for a Septic Tank in Feet:

Length of the Septic Tank = 49.5’

Width of the Septic Tank = 44.352’

Depth of the Septic Tank = 9.9’

Alamillo Bridge, Seville, Spain | Suspended Structures

In my previous article, we discussed the development of Suspended Structures and the technical aspects involved in the construction of Suspended Structures.

Alamillo Bridge, Seville, Spain, 1987-1992, designed by Calatrava Santiago, total span of the bridge is 200m and height of the tower is 142m. Materials used are steel for tower with concrete infill where needed, steel bridge deck structure and concrete abutments. The extraordinary weight of the concrete filled steel mast, which angles away from the roadbed at 58 degrees is enough to support the deck without the need for counter-stay cables or support piers.

 

Alamillo Bridge, Seville, Spain | Suspended Structures

The mass and the bed of the tower exerts a backwards downward force while the cable stays and roadbed mass exert a forward downward force. The mass of the tower was so calculated so that its backward and downward force wouldn’t lift the bridge off the ground.

For this reason, an additional mass was needed to counter the reaction of vertical forces. In order to increase the mass, the steel box girder of the tower and the steel deck structure were filled with concrete. This added to the stability of the structure.

 

Alamillo Bridge, Spain | Designed by Santiago Calatrava

The joint formed by the meeting of the tower, deck and abutment near the ground was then cast out of concrete.

Horizontal and Vertical forces acting on the structure have to be studied carefully. This structure is designed in such a way that the horizontal force component in the bent tower and the horizontal force of the roadway counteract each other so that the abutment only needs to resist vertical forces.

What is a Truss?

In Architecture and Structural Engineering, a truss is a structure comprising one or more triangular units constructed with straight slender members whose ends are connected at joints referred to as nodes.

External forces and reactions to those forces are considered to act only at the nodes and result in forces in the members which are either tensile or compressive forces.

Moments (torsional forces) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes.

In this article, we are going to discuss the various types of roof trusses in wood and steel and their uses in various kinds of construction.

Different types of Wooden and Steel Roof Trusses:

1. King Post Truss2. Queen Post Truss3. Howe Truss4. Pratt Truss5. Fan Truss6. North Light Roof Truss7. Quadrangular Roof Truss

Trusses for large span constructions

Tubular Steel Roof Truss Tubular Monitor Steel Roof Truss

King Post Truss

King Post roof truss (spans upto8M)

King Post Truss is a wooden truss. It can also be built of combination of wood and steel. It can be used for spans upto 8m.

Queen Post Truss

Queen Post Truss (spans upto 10M)

Queen Post Truss is also a wooden truss. It can be used for spans upto 10m.

Howe Truss

Howe Truss (spans upto 6M to 30M)

It is made of combination of wood and steel. The vertical members or tension members are made of steel. It can be used for spans from 6-30m.

Pratt Truss

Pratt truss (spans upto 6M to 10M)

Pratt Truss is made of steel. These are less economical than the Fink Trusses. Vertical members are tension and diagonal members are compression. Fink Trusses are very economical form of roof trusses. It can be used for spans from 6-10m.

Fan Truss

Fan Truss (spans upto 10M to 15M)

It is made of steel. Fan trusses are form of Fink roof truss. In Fan Trusses, top chords are divided into small lengths in order to provide supports for purlins

which would not come at joints in Fink trusses. It can be used for spans from 10-15m.

North Light Roof Truss

North light roof truss (spans upto 20M to 30M)

When the floor span exceeds 15m, it is generally more economical to change from a simple truss arrangement to one employing wide span lattice girders which support trusses at right angles.

In order to light up the space satisfactorily, roof lighting has to replace or supplement, side lighting provision must also be made for ventilation form the roof.

One of the oldest and economical methods of covering large areas is the North Light and Lattice girder.

This roof consists of a series of trusses fixed to girders. The short vertical side of the truss is glazed so that when the roof is used in the Northern Hemisphere, the glazed portion faces North for the best light.

It can be used for spans from 20-30m. Used for industrial buildings, drawing rooms etc.

Quadrangular roof Trusses

Quadrangular Roof Truss (for large spans)

These trusses are used for large spans such as railway sheds and Auditoriums.

Large Span Trusses

Large span trusses

Related posts:

1. Tubular Steel Monitor Roof truss | Large Span Constructions Steel Trusses The trusses are structural members and comprise of one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. The..

RCC StructuresRCC Structures are nothing but reinforced concrete structures. RCC structure is composed of building components such as Footings, Columns, Beams, Slabs, Staircase etc.

These components are reinforced with steel that give stability to the structure. Staircase is one such important component in a RCC structure.

Dog Legged Stair

In this article, we will discuss different types of staircases and study the RCC design of a dog-legged staircase…

Stairs

Stairs consist of steps arranged in a series for purpose of giving access to different floors of a building. Since a stair is often the only means of communication between the various floors of a building, the location of the stair requires good and careful consideration.

In a residential house, the staircase may be provided near the main entrance.

In a public building, the stairs must be from the main entrance itself and located centrally, to provide quick accessibility to the principal apartments.

All staircases should be adequately lighted and properly ventilated.

Various types of Staircases

Straight stairs

Dog-legged stairs

Open newel stair

Geometrical stair

RCC design of a Dog-legged staircase

In this type of staircase, the succeeding flights rise in opposite directions. The two flights in plan are not separated by a well. A landing is provided corresponding to the level at which the direction of the flight changes.

Design of Dog-legged Stairs

Based on the direction along which a stair slab span, the stairs maybe classified into the following two types.

1. Stairs spanning horizontally2. Stairs spanning vertically

Stairs spanning horizontally

These stairs are supported at each side by walls. Stringer beams or at one side by wall or at the other side by a beam.

Loads

Dead load of a step        = ½ x T x R x 25 Dead load of waist slab = b x t x 25 Live load                       = LL (KN/m2) Floor finish                    = assume 0.5 KN/m

Stairs spanning Longitudinally

In this, stairs spanning longitudinally, the beam is supported ay top and at the bottom of flights.

Loads

Self weight of a step          = 1 x R/2 x 25 Self weight of waist slab  = 1 x t x 25 Self weight of plan             = 1 x t x 25[(R2 + T2)/T] Live load                               = LL (KN/m2) Floor finish                           = assume 0.5 KN/m

For the efficient design of an RCC stair, we have to first analyse the various loads that are going to be imposed on the stair.

The load calculations will help us determine, how much strength is required to carry the load. The strength bearing capacity of a staircase is determined on the amount of steel and concrete used.

The ratio of steel to concrete has to be as per standards. Steel in the staircase will take the tension imposed on it and the concrete takes up the compression.

These are the essential steps that are to be followed for the RCC Stair Design.

Archive for category Green Building

Autoclaved Aerated Concrete (AAC) – A New Green Building System

Posted by BenzuJK in Green Building on January 12, 2011

Autoclaved Aerated Concrete (AAC) | Efficient Building Systems | Green Material

Autoclaved Aerated Concrete (AAC) was developed in Sweden in the late 1920s and has been used successfully in a variety of applications in commercial, industrial and residential construction. AAC is a lightweight, high strength building material and is produced in a variety of forms from blocks, to structural floors and wall panels.

AAC is credited by LEED (Leadership in Energy and Environmental Design) and USGBC (US Green Building Council) as an “green” alternative to traditional construction materials. Indian Green Building Council (IGBC) recommends its use in India.

AAC consists of 80% of air. It is manufactured by combining silica in the form of sand or recycled flyash, cement, lime, water and an expansion agent – aluminium powder and paving it into a mold.

Autoclaved Aerated Concrete (AAC) – A New Green Building Material

Structurally reinforced AAC products like lintels or roof panels, steel rebar or mesh are also placed in the mould.

When added to concrete, the aluminium powder reacts with the silica, resulting in the formation of millions of microscopic hydrogen bubbles. The hydrogen bubbles cause the concrete to expand to roughly five times its original volume.

The hydrogen evaporates and leaving a highly closed cell aerated concrete. It is then cut into blocks or panels which are then steam and pressure cured in an autoclaved. Using AAC is very advantageous because it is environmentally friendly qualifies as a ‘green’ building material from manufacturing to recycling.

All the waste material like trimmings, rejected units are all recycled and hence there is zero wastage. All the waste on the sites is crushed and safely used as a fill or buried shallow underground. Spread AAC dust on the lawn serves as a great lime supplement.

AAC is so lightweight, it weighs 1/5th of the weight of the standard concrete, which results in lower transportation costs, faster work-flow lower material handling costs etc. AAC is in ‘ready to build’ material, requiring no onsite curing time. It has unparalleled workability because it can be sown, drilled, nailed, screwed and milled with common hand tools. AAC openings are easily and alternately cut. Retrofitting the Foundations of Old Structures

The level of deterioration of the foundation determines whether the building can be improved or needs to be demolished. Repairing and retrofitting of the building elements should be done at the initial stages of the deterioration. This would save time as well as costs of construction.

In case the deterioration is not paid attention to, it can lead to excessive damage of building components which may result into excessive retrofitting costs.

 

Failure of Structure due to ignoring the need for retrofitting

Small cracks

In case there are cracks having occurred on the building are small,  the building can be declared to be in a sound condition. It involves nominal repairs and lower repair costs which would save your time as well.

Large scale damage

For older buildings where the foundations of the building have been severely affected, all large retrofit work is then subject to professional assistance. This would require the evaluation of geotechnical and structural conditions of the structure.

Surveys to be carries out:

Majorly two important surveys are to be carried out:

1. Technical Survey2. Historical Survey

The extent of analysis of the building would depend on the following factors:

Size of the building

Age of the building

Conditions of the foundation

Type of usage of the building (Purpose for which the building was constructed)

Change in the type of usage of the structure

The analysis of the structure subject to retrofitting would involve the following:

Soil conditions have to be checked

Foundations and foundation walls have to be surveyed

Excavation of the ground and visual survey

Reporting the amount of damage

Surveys of the cracks having occurred on the foundation and the structure

When the foundation survey is conducted, drilling and testing should also be included as a part of the survey

Load on the foundation has to be calculated

Ground water level and pore water pressure has to be measured

Foundations and should be straightened and levelled properly

Material quality of the built structure is to be checked

Problems occurring with Foundation retrofitting of a building

When retrofitting is carried out, the ground is excavated in order to reach the foundation of the building undergone deterioration. Changes and improvements in the foundation can result in great problems to the buildings adjacent to the retrofitted structure. This might result into legal

conflicts. Therefore care must be taken to consider the conservation of the surrounding structures.

We will discuss the possible causes of the deterioration of the foundations in my succeeding articles…

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Foundations

Foundation of a structure is like the roots of a tree without which the tree cannot stand. The construction of any structure, be it a residence or a skyscraper; starts with the laying of foundations.

Before designing the foundation, the type of soil is determined. Depending on whether the soil is hard soil or soft soil, a specific type of foundation is adopted.

Shallow Foundations versus Deep Foundations

Foundations are made in various materials… They could be reinforced cement concrete foundations or brick foundations or stone rubble masonry foundations etc. The choice of material to be used in the construction of foundations also depends on the weight of the structure on the ground.

The bearing capacity of soil plays a major role in deciding the type of foundation. The safe bearing capacity of soil should be 180N/mm2 to 200N/mm2.

Foundations are broadly classified into shallow foundations and deep foundations. The depth of the foundation means  the difference of level  between the ground surface and the base of the foundation. If the depth of the foundation is greater than its width the foundation is classified as a deep foundation.

Shallow foundations are commonly used in smaller structures such as residences and small buildings whose floor height is limited to 10m whereas Deep Foundations are used in Skyscrapers…. Piles are the most commonly used Deep Foundations used in skyscrapers…

Types of Shallow foundations

Footings

Footings are structural members used to support columns and walls and to transmit their load to the underlying soils.

Mats or rafts

Combined footings, strap and strip footings

Column Footing

In this type of foundation the base of the column is sufficiently enlarged to act as the individual support. The widened base not only provides stability but is useful in distributing the load on sufficient area of the soil.

Column footings are usually used in the foundations of residences and buildings where the soil is hard enough has has sufficient bearing capacity.

Pressure distribution Under a Foundation

The law of distribution of pressure under a foundation depends on the homogeneity of the soil and flexibility of the base. If really the soil is homogeneous and the base of the foundation is flexible, the pressure distribution under the foundation will be uniform. On the contrary if the foundation base is absolutely rigid, the pressure distribution will not  be uniform but may follow such pattern.

In our designs it is usual to assume a flexible base and hence to regard the pressure distribution to be uniform. This can be achieved by gradually decreasing the thickness of the base towards the edges so that the base is only as much thick as it is regarded to resist the induced moments and shears.

General rules of Foundation Design

While designing a foundation the following points must be borne in mind.

When a soil is yielding soil, a certain amount of settlement must be reduced as much as possible by bringing down the pressure intensities.

It is necessary that a foundation shall be designed so that if at all a settlement should occur, it will be uniform. In other words, the settlement of all the footings must be more or less the same.

This is a very important point in reinforced concrete structures due to the rigid connection between the different components of the structure.

In our next article, we will discuss the procedure of designing an isolated foundation and also justify the foundation design rules mentioned above.