This summer vacation I had the privilege of gaining knowledge in my field at The Appropriate Alternative, Kolkata under the expert guidance of Mr. Anjan Mitra and also Mr. Surajit Bhat.
I learnt about the practical utilization of engineering drawing for the drafting of plans, section and elevation of basic objects as well as buildings. Not just drafting but I also learnt how to read and understand a plan and the structural elements drawn.
I got a broader view of the structural side of civil engineering. I understood that as a structural engineer it would be my job to design a structure capable of resisting all applied loads without failure during its intended life. I learnt about the types of structural systems with major emphasis on load bearing wall system and frame structures. I have gained an insight about conditions when these systems should be applied.
I now know the difference between a floor plan, Structural layout plan, Foundation layout plan and the details that should be included in these since I have practically drawn them for one of their projects at Santipur.
I also learnt about the various types of foundation and when they should be used like strip foundation is mostly used for load bearing masonry walls, columns are supported on isolated footing, pile foundation is used where the top layer of soil has weak bearing capacity.
After learning the importance of these plans, I further learnt about the placement and details of structural elements like columns and beams.
I also learnt basic load calculation of a building with the same project of Santipur as my sample, this included the live and dead load calculation only since it was a 2 story building. This was further utilized to decide the dimension of the footing and how much load would be exerted on the footing and whether it was within the bearing capacity of the soil or not.
After this I learnt how to make an estimate for a building and how to refer to the codes for knowledge of the rates for different materials at various districts.
All in all, it was an extremely enriching and a learning experience. Through this opportunity I have understood that I would like to pursue and increase my knowledge in the field of structural engineering in the future. I shall use this experience in future for the construction of buildings and for better development in the field of civil engineering.
1.1 Company Profile:
Established in 1991, the Appropriate Alternative aims at offering effective design solutions. It is an alternative design process that addresses cultural values, intangible issues along with environmental concerns and cost-effective architectural solutions. This holistic approach caters to different segments of the society.
The Appropriate Alternative is involved in building positive relationships a
the architecture and the site
the site and its context,
the clients and their users,
the project and the environment.
Many projects have been undertaken by the company, some of them include Redevelopment of College Street Market, Restoration of Howrah Town Hall, Boys Hostel at IIT and many more.
1.2 What is Structural Design?
Structural design is the systematic examination of the stability, strength and rigidity of structures. The basic objective in structural analysis and design is to build a structure capable of resisting all applied loads without failure during its intended life. The primary purpose of a structure is to transmit or support loads. If the structure is improperly designed or fabricated, or if the actual applied loads exceed the design specifications, the device will probably fail to perform its intended function, with possible serious consequences. A well-engineered structure greatly minimizes the possibility of costly failures.
The structural design of any structure first involves establishing the loading and other design conditions, which must be supported by the structure and therefore must be considered in its design. This is followed by the analysis and computation of internal gross forces, (i.e. thrust, shear, bending moments and twisting moments), as well as stress intensities, strain, deflection and reactions produced by loads, changes in temperature, shrinkage, creep and other design conditions. Finally comes the proportioning and selection of materials for the members and connections to respond adequately to the effects produced by the design conditions.
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
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.
Once the form of the structure is selected, the structural design process starts. Structural design is an art and science of understanding the behaviour of structural members subjected to loads and designing them with economy and elegance to give a safe, serviceable and durable structure.
Structural system, in building construction, is the particular method of assembling and constructing structural elements of a building so that they support and transmit applied loads safely to the ground without exceeding the allowable stresses in the members. Basic types of systems include bearing-wall, post-and-lintel, frame, membrane, and suspension. They fall into three major categories: low-rise, high-rise, and long-span. Below is the description of the two most common construction systems in todayâ€™s time that is the load bearing wall system and concrete frame systems.
Load Bearing Walls
Load bearing masonry construction was the most widely used form of construction for large buildings from the 1700s to the mid-1900s. It is very rarely used today for large buildings, but smaller residential-scale structures are being built. It essentially consists of thick, heavy masonry walls of brick or stone that support the entire structure, including the horizontal floor slabs, which could be made of reinforced concrete, wood, or steel members. Every wall had a simple continuous strip foundation below it.
In contrast, most construction today is not load-bearing masonry but frame structures of light but strong materials, that support floor slabs and have very thin and light internal and external walls.
The key idea with this construction is that every wall acts as a load carrying element. In a load bearing structure, you cannot punch holes in a wall to connect two rooms - you would damage the structure if you did so. The immense weight of the walls actually helps to hold the building together and stabilize it against external forces such as wind and earthquake.
Concrete Frame Structures
Concrete frame structures are a very common - or perhaps the most common- type of modern building. As the name suggests, this type of building consists of a frame or skeleton of concrete. Horizontal members of this frame are called beams, and vertical members are called columns. Humans walk on flat planes of concrete called slabs. Of these, the column is the most important, as it is the primary load-carrying element of the building. If you damage a beam in a building, it will usually affect only one floor, but damage to a column could bring down the entire building.
The structure is actually a connected frame of members, each of which are firmly connected to each other. In engineering parlance, these connections are called moment connections, which means that the two members are firmly connected to each other.
STAGES IN STRUCTURAL DESIGN
The process of structural design involves the following stages.
1) Architectural plans.
2) Structural planning.
3) Member design.
4) Action of forces and computation of loads.
5) Detailing, Drawing and Preparation of material schedules and estimates.
Architectural plans are a must before any kind of structural planning. The drawings and specifications for a building will form an integral part of the construction contract will determine the scope of your project. In order to minimize discrepancies during construction, it is critical to have an accurate and detailed set of drawings and specifications. It also allows you to gather accurate pricing information before construction, allowing you to make any necessary changes to maintain your anticipated budget. One such important plan is the floor plan which is the most basic plan out of all.
A floor plan is a drawing to scale, showing a view from above, of the relationships between rooms, spaces and other physical features at one level of a structure.
Dimensions are usually drawn between the walls to specify room sizes and wall lengths. Floor plans may also include details of fixtures like sinks, water heaters, furnaces, etc. Floor plans may include notes for construction to specify finishes, construction methods, or symbols for electrical items.
2) 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.
a. Position 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.
b. 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).
c. 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.
d. Layout of Stairs:
The staircase, when carefully designed and built, adds dignity and charm to a home. In general, stair work is considered a special field of carpentry. The main stairway, which may have several artistic features that are difficult to make on the job, is usually made in a mill and assembled at the house. It is essential that every carpenter have the necessary information regarding the general principles involved in stair building, as well as knowledge of the layout and construction. It is very important to know the width of the stair as well as the dimensions of the tread and riser for the design of a staircase. A section of stairs gives a proper understanding of the structure of the stairs.
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. For load bearing system strip footing is normally preferred.
3) MEMBER DESIGN:
Design of Columns:
Guidelines to be followed for making a column layout
In this article, we will go through the essential thumb rules to be followed for giving a column layout. Of course columns have to be designed in accordance to the total forces acting on the structure, but apart from that, it is essential for every Civil engineer and Architect to remember a few thumb rules so that they are prevented from making mistakes.
Three thumb rules to be followed are as follows:
Size of the Columns
Distance between Columns
Alignment of columns
Minimum Size of RCC Columns
The size of the columns depends on the total load on the columns. There are axial loads and lateral loads. Large beam spans induce bending moment not only in the beams, but also in columns which are pulled by the stresses in the beams. The size of the columns increase because of two factors:
Increase in the distance between two columns (This increases the dimensions of the columns as well as the depth of the beam).
Height of the building (Increase in the number of floors is directly proportional to the dimensions).
It is important to use advanced structural design software like ETabs or Staad pro.
Distance between Columns
Try to maintain equal distance between the centers of two columns. Always plan a column layout on a grid. The distance between two columns of size 9â€x9â€ should not be more than 4m centre to centre of column. If larger barrier free distances are required then going for larger column size is to be used. The size of the columns increase because of two factors: Increase in the distance between two columns (This increases the dimensions of the columns as well the depth of the beam.) Height of the building (Increase in the number of floors is directly proportional to the dimensions of the columns.
Alignment of Columns
Placing of columns depend completely on the plan. A planner has a very important job. A grid column placement is always preferred in order to reduce point loads and unnecessary complications while construction. This reduces the cost of construction as well as time required for construction. Beams which have continuity with other simply supported beams have reduced bending moments, and thus require less steel and concrete depth to be safe.
Columns have to be connected with each other for smooth transfer of loads. An experienced planner will keep such things in mind when planning the structure.
4) ACTION OF FORCES AND COMPUTATION OF LOADS:
Structural loads or actions are forces, deformations, or accelerations applied to a structure or its components. Loads cause stresses, deformations, and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis. Excess load or overloading may cause structural failure, and hence such possibility should be either considered in the design or strictly controlled.
The loads are broadly classified as vertical loads, horizontal loads and longitudinal loads. The vertical loads consist of dead load, live load and impact load. The horizontal loads comprises of wind load and earthquake load.
1. Dead Loads- Dead loads are permanent or stationary loads which are transferred to structure throughout the life span. Dead load is mainly due to self-weight of structural members like columns, beams, slabs; permanent partition walls; fixed permanent equipments and weight of different materials.
2. Imposed or Live Loads- Live loads are either movable or moving loads without any acceleration or impact. They are assumed to be produced by the intended use or occupancy of the building including weights of movable partitions or furniture etc. The floor slabs have to be designed to carry either uniformly distributed loads or concentrated loads whichever produce greater stresses in the part under consideration. Since it is unlikely that any one particular time all floors will not be simultaneously carrying maximum loading, the code permits some reduction in imposed loads in designing columns, load bearing walls, pier supports and foundations.
3. Impact Loads- Impact load is caused by vibration or impact or acceleration. Thus, impact load is equal to imposed load incremented by some percentage called impact factor or impact allowance depending upon the intensity of impact.
4. Wind Loads- Wind load is primarily horizontal load caused by the movement of air relative to earth. Wind load is required to be considered in design especially when the heath of the building exceeds two times the dimensions transverse to the exposed wind surface. For low rise building say up to four to five storeys, the wind load is not critical because the moment of resistance provided by the continuity of floor system to column connection and walls provided between columns are sufficient to accommodate the effect of these forces.
5. Earthquake Loads- Earthquake loads are horizontal loads caused by the earthquake and shall be computed in accordance with IS 1893.
Actual loadings in a building are typically either concentrated or uniformly distributed over an area. The former need no further consideration other than to characterize them as a force vector. In the latter, however, some modeling is needed when the area considered is actually made up of an assembly of one-way line and surface elements. These elements would pick up different portions of the total load acting over the surface, depending on their arrangement.
Consider the simple structural assembly shown in Figure 1 (a). Eight pre-cast concrete elements are supported by three beams Both external beams have to carry the weight of a half concrete element The middle beam carries the weight of one element (Â½ of the left and right element as illustrated in Figure 1 (b)). The reactions from all the elements supported by a beam then become loads acting on the beam. Note that these loads form a continuous line load on the beam. Loads of this type are expressed in terms of a load or force per unit length (i.e. N/m) and are commonly encountered in the structural analysis process.
The loading considered should, of course, include both live- and dead-load components. The exact value of the latter can be found by calculating the volume of the contributory area and multiplying it by the unit weights for that material of which the element is made of. Determining these values can be tedious. An alternative is to use a unit weight, e.g. the weight for one square metre, typically expressed as a force per unit area, to represent the weight expressed as N/m2. Since live loads are also expressed in terms of a force per unit area, the calculation process is facilitated, since both loads can be considered simultaneously.
5) DETAILING, DRAWING AND PREPARATION OF MATERIAL SCHEDULES AND ESTIMATES:
Blueprint or drawing that is subject to clarifications but is complete with enough plan and section views (with dimensions, details, and notes) to enable the construction or replication without additional information.
Production information may include:
ï‚§ Drawings, such as working drawings.
ï‚§ Bills of quantities or schedules of work
Working drawings provide dimensioned, graphical information that can be used; by a contractor to construct the works, or by suppliers to fabricate components of the works or to assemble or install components. They may include architectural drawings, structural drawings, civil drawings, mechanical drawings, electrical drawings, and so on.
Traditionally, working drawings consist of 2 dimensional orthogonal projections of the building or component they are describing, such as plans, sections and elevations. These may be drawn to scale by hand, or prepared using Computer Aided Design (CAD) software. However, increasingly, building information modelling (BIM) is being used to create 3 dimensional representations of buildings and their components for construction. This may be described as a virtual construction model (VCM) and can comprise a number of different models prepared by different members of the project team.
Working drawings may include title blocks, dimensions, notation and symbols. It is important that these are consistent with industry standards so that their precise meaning is clear and can be understood. Specification information can be included on working drawings or in a separate specification, but information should not be duplicated as this can become contradictory and may cause confusion.
The scale at which drawings are prepared should reflect the level of detail of the information they are required to convey. Different line thicknesses can be used to provide greater clarity for certain elements.
It is important that the purpose of the drawings and the people that will use them are considered. Working drawings might be prepared for; statutory approvals, for contractors to plan the construction works, to provide instructions on site, for the procurement of components, for the preparation of shop drawings, for the appointment of subcontractors and so on.
The preparations of detailed estimate consist of working out quantities of various items of work and then determine the cost of each item. This is prepared in two stages.
1) Details of measurements and calculation of quantities:
The complete work is divided into various items of work such as earth work concreting, brick work, R.C.C. Plastering etc., The details of measurements are taken from drawings and entered in respective columns. The quantities are calculated by multiplying the values that are in numbers column to Depth column as shown below:
S.No Description of Item No. Length(L)
(m) Quantity Explanatory notes
2) Abstract of Estimated Cost:
The cost of each item of work is worked out from the quantities that already computed in the details measurement form at workable rate. But the total cost worked out in the prescribed form is known as abstract of estimated cost. 3-4% of estimated cost is allowed for Petty Supervision, contingencies and unforeseen items.
Item no. Description Quantity Unit Rate Amount
The detailed estimated should be accompanied with:
iii) Drawings( Plans, Sections, Elevations)
iv) Design charts and calculations
v) Standard schedule of rates.
Data for detailed estimate:
The process of working out the cost or rate per unit of each item is called as Data. In preparation of Data, the rates of materials and labour are obtained from current standard scheduled of rates and while the quantities of materials and labour required for one unit of item are taken from Standard Data Book (S.D.B).
Fixing of Rate per Unit of an Item:
The rate per unit of an item includes the following:
1) Quantity of materials & cost: The requirement of materials is taken strictly in accordance with standard data book (S.D.B). The cost of these includes first cost, freight, insurance and transportation charges.
ii) Cost of labour: The exact number of labourers required for unit of work and the multiplied by the wages/ day to get of labour for unit item work.
iii) Cost of equipment (T&P): Some works need special type of equipment, tools and plant. In such case, an amount of 1 to 2% of estimated cost is provided.
iv) Overhead charges: To meet expenses of office rent, depreciation of equipment salaries of staff postage, lighting an amount of 4% of estimate cost is allocated.
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