STRUCTURAL DESIGN PROCESS

The entire process of structural planning and design requires not only imagination and conceptual thinking but also sound knowledge of practical aspects, such as recent design codes and bye-laws, backed up by ample experience, institution and judgment.

It is emphasized that any structure to be constructed must satisfy the need efficiency for which it is intended and shall be durable for its desired life span. Thus, the design of any structure is categorizes into following two main types:-

1. Functional design

2. Structural design

FUNCTIONAL DESIGN:

The structure to be constructed should primarily serve the basic purpose for which it is to be used and must have a pleasing look.

The building should provide happy environment inside as well as outside. Therefore, the functional planning of a building must take into account the proper arrangements of room/halls to satisfy the need of the client, good ventilation, lighting, acoustics, unobstructed view in the case of community halls, cinema theatres, etc.

STRUCTURAL DESIGN:

Once the form of the structure is selected, the structural design process starts. Structural design is an art and science of understanding the behavior of structural members subjected to loads and designing them with economy and elegance to give a safe, serviceable and durable structure.

STAGES IN STRUCTURAL DESIGN:

The process of structural design involves the following stages.

1) Structural planning.

2) Action of forces and computation of loads.

3) Methods of analysis.

4) Member design.

5) Detailing, Drawing and Preparation of schedules. 

1. STRUCTURAL PLANNING:

After getting an architectural plan of the buildings, the structural planning of the building frame is done. This involves determination of the following.

a. Position and orientation of columns.

b. Positioning of beams.

c. Spanning of slabs.

d. Layouts of stairs.

e. Selecting proper type of footing.

1. Positioning and orientation of columns:Following are some of the building principles, which help in deciding the columns positions.

1. Columns should preferably be located at (or) near the corners of a building, and at the intersection of beams/walls.

2. Select the position of columns so as to reduce bending moments in beams.

3. Avoid larger spans of beams.

4. Avoid larger centre-to-centre distance between columns.

5. Columns on property line.

Orientation of columns:

1. Avoid projection of columns:

The projection of columns outside the wall in the room should be avoided as they not only give bad appearance but also obstruct the use of floor space, creating problems in placing furniture flush with the wall. The width of the column is required to be kept not less than 200mm to prevent the column from being slender. The spacing of the column should be considerably reduced so that the load on column on each floor is less and the necessity of large sections for columns does not arise.

2. Orient the column so that the depth of the column is contained in the major plane of bending or is perpendicular to the major axis of bending.

This is provided to increase moment of inertia and hence greater moment resisting capacity. It will also reduce Leff/d ratio resulting in increase in the load carrying capacity of the column.

2. POSITIONING OF BEAMS:

1. Beams shall normally be provided under the walls or below a heavy concentrated load to avoid these loads directly coming on slabs.

2. Avoid larger spacing of beams from deflection and cracking criteria. (The deflection varies directly with the cube of the span and inversely with the cube of the depth i.e. L3/D3. Consequently, increase in span L which results in greater deflection for larger span).

3. SPANNING OF SLABS:

This is decided by supporting arrangements. When the supports are only on opposite edges or only in one direction, then the slab acts as a one way supported slab. When the rectangular slab is supported along its four edges it acts as a one way slab when Ly/Lx < 2.

The two way action of slab not only depends on the aspect ratio but also on the ratio of reinforcement on the directions. In one way slab, main steel is provided along with short span only and the load is transferred to two opposite supports. The steel along the long span just acts as the distribution steel and is not designed for transferring the load but to distribute the load and to resist shrinkage and temperature stresses.

A slab is made to act as a one way slab spanning across the short span by providing main steel along the short span and only distribution steel along the long span. The provision of more steel in one direction increases the stiffness of the slab in that direction.

According to elastic theory, the distribution of load being proportional to stiffness in two orthogonal directions, major load is transferred along the stiffer short span and the slab behaves as one way. Since, the slab is also supported over the short edge there is a tendency of the load on the slab by the side of support to get transferred to the nearer support causing tension at top across this short supporting edge. Since, there does not exist any steel at top across this short edge in a one way slab interconnecting the slab and the side beam, cracks develop at the top along that edge. The cracks may run through the depth of the slab due to differential deflection between the slab and the supporting short edge beam/wall. Therefore, care should be taken to provide minimum steel at top across the short edge support to avoid this cracking.

A two way slab is generally economical compare to one way slab because steel along both the spans acts as main steel and transfers the load to all its four supports. The two way action is advantageous essentially for large spans (>3m) and for live loads (>3kN/m2). For short spans and light loads, steel required for two way slabs does not differ appreciably as compared to steel for two way slab because of the requirements of minimum steel.

FOOTING:

The type of footing depends upon the load carried by the column and the bearing capacity of the supporting soil. The soil under the foundation is more susceptible to large variations. Even under one small building the soil may vary from soft clay to a hard murum. The nature and properties of soil may change with season and weather, like swelling in wet weather. Increase in moisture content results in substantial loss of bearing capacity in case of certain soils which may lead to differential settlements. It is necessary to conduct the survey in the areas for soil properties. For framed structure, isolated column footings are normally preferred except in case of exists for great depths, pile foundations can be an appropriate choice. If columns are very closely spaced and bearing capacity of the soil is low, raft foundation can be an alternative solution. For a column on the boundary line, a combined footing or a raft footing may be provided.

ASSUMPTIONS

The following are the assumptions made in the earthquake resistant design of structures:

• Earthquake causes impulsive ground motions, which are complex and irregular in character, changing in period and amplitude each lasting for small duration. Therefore resonance of the type as visualized under steady-state sinusoidal excitations, will not occur as it would need time to build up such amplitudes.

• Earthquake is not likely to occur simultaneously with wind or max. Flood or max. sea waves.

• The value of elastic modulus of materials, wherever required, maybe taken as per static analysis.

Types of Concrete Beams and their Reinforcement Details

Reinforced concrete beams are structural members that support the transverse load which usually rest on supports at its end. Girder is a type of beam that supports one or more smaller beam.

Types of Concrete Beams

Beams are classified as

1. Simple Beam
2. Continuous Beam
3. Semi-Continuous Beam
4. cantilever beam
5. T- beam

1. Simple Concrete Beams

Simple concrete beam refers to the beam having a single span supported at its end without a restraint at the support. Simple beam is sometimes called as simply supported beam. Restraint means a rigid connection or anchorage at the support.

Fig: Simply supported beam

2. Continuous Beam

It is a beam that rest on more than two supports. It can be a single beam provided for long span between columns or walls with intermediate supports of smallar beams or a single continuous beam for entire length of the structure with intermediate column or wall supports.

Fig: Continuous beam with reinforcement details

3. Semi-Continuous Beam

Refers to a beam with two spans with or without restraint at the two extreme ends.

Fig: Semi-continuous beam

4. Cantilever Beam

Cantilever beams are supported on one end and the other end projecting beyond the support or wall.

Fig: RCC Cantilever beam reinforcement details

5. T – Beam

When floor slabs and beams. are poured simultaneously  producing a monolithic structure where the portion of the slab at both sides of the beam serves as flanges of the T-Beam. The beam below the slab serves as the web member and is sometime called stem.

Fig: RCC T-beam

Vastu Shastra For Building Construction – Beliefs, Benefits,Tips and Limitations

Vastu Shastra is an ancient Indian science which capitalizes on the concept of prosperity. It links person’s health and happiness directly with layouts and construction of a building.

Promoters of Vastu Shastra propagate that it is most authentic and proven advice science. They say that nature consists of 5 basic elements such as energy of planets, air, water, fire and earth. Each carries its own set of dynamics and with vastu one’s home and business can be synchronized.

As per promoters of shastra it creates a tremendous impact over all facts of life, i.e., health, life, education, thinking, prosperity, marriage and peace of mind.

In fact “Vastu Shastra” is the science of the ancient “Vedic Ages” of India under the guidance of which one can construct building of structures on a particular plot of land. For some people these principles are still relevant today also. As per their belief the science of “Vastu Shastra” controls forces of gravity and magnetic power of the earth.

The major beliefs of Vastu Shastra are as follows:

• Existence of roads on the northern or eastern side of the house improves the health and progress of the members of the house. If a house has road on the 3 sides, its inhabitants suffer tension and tension related problems.
• When the digging is happening to lay the foundation, make sure that you start from the east, go to the north, then to the west, and lastly, southwest.
• The plot at the end of a road is also not good for its inhabitants.
• It is good to have main door on east, northern and north eastern corner of the house. A house having main door at the southwest end provides less peace and perpetually tense atmosphere.
• A house having massive main door gives a lot of unsavory events and members residing suffer health problems.
• There should be no doors in the southwest corners or the southwest portion of the house. It is best if the doors face north or east. Doors should also open inside and not outside. When the door opens inside, it stands for inviting popular powers and positive energy. However, the door opening outside would be bad for the positive energy- it would actually drive the good energy away.
• Wall of house should have soothing and soft colors on walls as this gives a tension free atmosphere. Deep yellow, red or orange colored walls give irritation to the occupants.
• All water related appliances or resources of the house should be placed on north east area. This provides prosperity.
• A well or a tube well should never be provide on the southeast end of the house. A well or tube well at center of the house brings good luck to its head.
• Presence of plants of Tulsi, Banana, Champa, Ashoka, Aawla outside the house keeps inhabitants healthy, happy and peaceful.

Tips of Vastu Shastra for Building Construction

In the urban areas, due to the scarcity of land the construction of flats has become a common practice. Flats are economical and have some added benefit such as security, common recreation centers, shopping area etc.

Therefore, in urban areas purchase of flats are more common than constructing a house on a plot which becomes quite expensive.

In fact, construction of flats as per vastu is difficult still satisfactory results could be achieved if one follows principles of vastu meticulously in selection of plot and construction of buildings.

As per vastu experts, maximum benefits to the flat owners can be achieved keeping in mind the following principles of “Vastu”.

• The site should be a square or a rectangle. If possible southwest corner should have 90 degree.
• The main door should be on north, east or northeast side of the plot. 2 gates are preferable, one on east and other in the north. In addition to this northeast, southwest or northwest blocks are also good.
• Ground level should slope towards northeast and levels in southwest should be higher than all other sides.
• A bore well should be provided in north east before the construction takes place.
• More space should be provided in the east and north compared to west and south of the complex.
• Balcony towards north, east and northeast are preferable.
• Kitchen is advisable in south east or northwest but never in northeast.
• Staircase should be provided in the south, west or southwest and should be avoided in northeast because head room of the staircase higher than southwest is not acceptable.
• Underground room or space (cellar) should be kept under the northeastern or eastern portion of the complex.
• Parking for cars, scooter and bicycles are preferred in northeast cellar.
• Open area in north and south should be used for lawns and sumps.
• AC equipment should be kept in southeast of the complex and should never be installed in northeast under any circumstances.
• Washbasins should be provided in north or east or northeast of the hall.

Benefits of Vastu Shastra

One often thinks about how Vastu Shastra works and not enough about how they can benefit from the same. Here are some of the biggest benefits derived from Vastu Shastra to become successful:

1. Comfort
2. Inner realization
3. Strength
4. Easy to use
5. Best use of space
6. Good structuring
7. Can enhance one’s personality
8. Enhance relationship with other people
9. Use this for spiritual knowledge
10. Greater mental peace and skill

Limitations of Vastu Shastra

A general recommendation from vastu shastra has been provided for information of readers. While going through these recommendations they will conclude that most of the recommendations are governing the principles of good orientation and ventilation which are national building code also has recommended.

Since the principles of vastu are coming from Vedic period, to encourage people to get more benefits from nature, the principles have been associated mythological so the people will follow them.

Many examples could be cited to show that if principles of vastu followed in constructing one’s house one could be quite happy and prosperous.

The figure above describes the best and worst position of rooms in house according to Vaastu.