Reinforcement of Concrete Slabs

The determination of a formwork framework ought to be made based on the chosen floor framework that satisfies the basic stacking conditions. Floor slabs in concrete structures are classified into two essential sorts, in view of the heap appropriation applied on the Reinforced Concrete slab:

Two-way slab, in which the rectangularity proportion (slab length/width) is somewhere in the range of 1 and 2, and the slab load is moved to the supporting pillars in two directions.Two-way development incorporates flat plate, flat slab, waffle slab, and two-way slabs bolstered by drop shafts.

Single direction slab, in which the rectangularity proportion (slab length/width) is more than 2, and the slab load is moved to the supporting bars a single way. Single direction development as a rule remembers strong slabs for shafts or dividers, single direction joist (ribbed) slabs upheld on bars or bearing dividers.

Two-Way Flat Plate: Such slabs might be cantilevered at the outside of the structure to allow the utilization of outside balconies.The supporting segments for flat plates are normally similarly dispersed to encourage the plan and development of such slabs.

This framework is prudent for ranges of up to 23 ft (7.0 m) with mellow reinforcing.Flat plates can be built in least time since they use the easiest conceivable formwork. Level plates have been utilized effectively in multi storey inn, lodging, medical clinic, and high rises.

Two-Way Flat Slab: A flat slab basic framework comprises a steady thickness of Reinforced Concrete slab with drop boards at the sections areas. Note that the framework is normally appropriate for square or about square boards.

In prior years, section capitals were utilized alongside drop boards, but since of the higher formwork cost, segment capitals are less preferred in the present development practice. Level slabs are utilized to oppose heavier burdens and longer ranges than flat plates. Generally, the framework is generally appropriate for square or about square boards.


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Rajib Dey
www.constructioncost.co
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Some useful terms of construction materials

In civil engineering sectors, there are different types of terms associated with various construction materials. Given below, the details about them.

Aggregate: It belongs to all particles of sand, broken stone or gravel etc. which are applied to form Concrete.

Bulking: It means the surge in volume of sand or aggregate resulting from the absorption of Water.

Concrete: Concrete belongs to a building material that comprises of specified quantities of Cement, Sand, Aggregates and water. At the time of mixing, transmission and placing, the concrete sets and solidifies through the method of hydration as the water reacts with cement and binds other elements finally results in producing a material similar to stone.

Curing: The process of retaining the concrete damp once it is arranged in its position to finish the chemical combination of cement and water.

Final Setting: It happens when the concrete has been properly set but has not been solidified yet.

There should be adequate time for the framework to be detached. Final set happens in about 3 to 4 hours with ordinary cement and should not require in excess of 10 hours.

Hardening: It specifies the increase in strength of a mortar or concrete and occurs at the end of the initial set.

Initial Setting: The period passed between the time when water is added initially to time of cement to develop a paste and the time when that paste is stopped to be fluid and plastic to a certain degree under the specified conditions of test.

Lean mix: It belongs to a concrete mix with a low cement content.

Screeding: It means acquiring a level surface as the exact height with the use of a piece of wood or metal containing a straight edge.

Setting: It stands for the chemical action that starts when water is added to cement and causes the plastic nature of cement to go away gradually. There are initial set and final set of cement.

Segregation: It means the detachment of the particles with various sizes in a concrete mix. The bigger aggregates settle at the bottom. Segregation impacts the strength of the concrete significantly.

Striking: It means dismantling and elimination of form work or centering.

Workability: It is the property of the freshly mixed concrete(or mortar) that ascertains the ease or difficulty with which it can be treated in order to make full compaction.

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Benefits of I-beams in construction

Hot-rolled steel beam having I-shaped cross section, and tapered flanges closer than wide flanged beam is known as I-beam.

I Beam contains one vertical plane and two horizontal planes or flanges which develop 'I' or 'H' structure. The vertical plane withstands the shear stress , whereas horizontal planes withstand the bending movement. I Beam is majorly utilized in construction industry like construction of manufacturing plants, multi-story buildings etc.

I-beam is frequently applied as important support trusses, or the primary framework, in buildings. Steel I beams retain structure’s integrity with persistent strength and support. The extreme power of I beams minimizes the requirement for several support structures and as a result huge time and money is saved. The stability of the structure is improved considerably.

Some vital jargons of I-Beam:

1. Flange thickness: Top and bottom horizontal plate-like segments of an I-beam are known as flange. The density of the flanges is defined as the flange thickness.

2. Flange width: The width of the flanges is known as flange width.
3. Beam depth: The height among the top and bottom surface of the steel I beam is termed as beam depth.
4. Web thickness: The vertical segment of steel I beam is known as web, and the thickness of the web is termed as web thickness.
5. Fillet radius: The curved section, where the changeover among the web and flange occurs is known as a fillet. The radius of the fillet is defined as the fillet radius.

A properly sized I Beam can be chosen on the basis of the following criterion :-

The entire method of choosing the proper size of the I beam is dependent on the basic mechanical design calculations as given below:

1. The first input necessary belongs to the steel I beam load specifications or loading details on the steel I beam.
2. Draw bending moment diagram for the specified loads and get the value of maximum bending moments (suppose M) that the steel I beam is likely to experience.
3. Select an exact size of steel I beam from a standard I beam table.
4. Determine the area moment of inertia (suppose I) of the selected steel I beam.
5. Obtain the beam depth (suppose d) of the selected steel I beam.
6. The stress developed (f) in the beam can be measured with the formula given below :

f/(d/2)=M/ I

f denotes the bending stress.
M denotes the moment at the neutral axis.
y denotes the perpendicular distance to the neutral axis.
I denotes the area moment of inertia about the neutral axis x.

7. Compare the calculated value of the bending stress with the yield stress of the steel with the purpose of verifying the safety factor of your design.

The structural design will be perfect when the size of the I-beam is accurate. The method described above is dependent on static I beam load specifications. In case where dynamic loads are concerned, it is necessary to apply FEA tools like ANSYS, Pro Mechanica, etc.

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Types of reinforcement or mesh in several footings (foundations)

Different types of Reinforcement in footings or types of mesh used in foundation:-

Several types of reinforcement exist in footings. The reinforcement should be provided in footings for tension requirements. Normally, the percentage of reinforcement in footings should remain among 0.5% to 0.8%. Based on the load analysis, the structural engineer design the type of Mesh in footings. Given below, the details about he types of mesh (reinforcement) implemented at several types of footings or foundations.

Usually, four different types of reinforcement in footings or foundations are found :–

1. Plain Mesh: This type of Mesh is normally implemented at plain or isolated or combined footings. It is specifically useful for low-rise buildings. Prior to use plain mesh to high rise buildings, the load should be analyzed in accordance with this mesh and determine either the type of mesh is balanced with the load or not.

Under this type, bars are arranged as a grid. It may contain bars with various diameter and spacing in either direction. The spacing may or may not vary in both directions.

2. Mesh with hooks (Hook Mesh): It is suitable for both low rise and high rise buildings. The footing is reinforced as grid and the bars are arranged with hook at the ends of the mesh. The perfect anchorage of the reinforcement can be obtained by bending the bars ends. Normally, the standard length of hook is 10D where D stands for the diameter of the bar.

3. Footing Mesh up to the depth of Footing: It has similarity with Plain footing. Under this type of footing, the bars are bent at ends up to a height of footing. The concrete cover of 1″ to 4″ should be arranged in all the sides of footing.

4. Raft Mesh: This type of Mesh is ideal for raft footing. Raft footing is suitable when the bearing strength of soil is very low. Under this type, mesh is segregated into two parts like top mesh and bottom Mesh.

Initially, the bottom mesh is arranged on covering blocks, ends of a bottom mesh are bent at an angle of 90 degree up to a height of 50D where D stands for Dia of Bar. After that top mesh is attached with the bottom mesh in opposite direction. Besides, the top mesh equivalent to bottom mesh is bent with 90 degrees but an additional bar of 50D is not arranged since it is already equipped on bottom mesh.

The 50D extra bar is arranged either on bottom or top mesh.

Single ring or double rings are attached with top mesh and bottom mesh to retain the proper framework. The rings allow the steel reinforcement not to distort in any direction. Least diameter of bars used for rings should be 6 mm.

In single ring raft mesh, rings are arranged in only one direction either horizontal or vertical, while in double ring system, the rings are arranged in both the direction.

The following points should be taken into consideration :-

1. Concrete cover differs from 1" to 4" depending on the size of the footing.
2. Hook length in Hook mesh is always 9D, where D stands for Dia of bar.
3. Additional bar is arranged either on top or bottom mesh and additional bar length is 50D.



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Reason for damaging & collapsing of concrete buildings

A reinforced concrete building gets damaged and collapses due to several reasons such as sliding of roofs, falling of walls, crushing of columns, short column effects, diagonal cracking, foundation sinking and tilting etc.

Types and Causes for Damage and Collapse of Concrete Buildings.

Given below, the details about the most common types of damages in reinforced concrete buildings:

1. Sliding of Roofs off the Supports: Where the beams are just supported on walls or columns, they are susceptible to slide if the severity of earthquake surpasses the frictional resistance and several times come out of the support and collapse, specifically when the bearing length is minor.

2. Collapsing of Infill Walls: The infill panel walls amid reinforced concrete columns overturn outer the framework when they are not firmly retained or secured with the frames.

3. Crushing of Column Ends and Virtual Hinging: When extreme shaking occurs, the column ends are susceptible to serious eccentric compressive stresses which compel the concrete to get crushed and broke down from the exterior surfaces. In frequent cycles, the damage proceeds interiors, consequently the effective section is shortened significantly. Both the column ends substantially function as pins and the entire framework falls down like a mechanism.

4. Short Column Effect: If infill walls having wide openings are joined to the columns, the sections of the columns to be deformed against lateral seismic loads turn out to be very short with reference to their normal height.

Such short columns develop into much harder as compared to other columns and pull greater shear forces under which they experience extreme diagonal tension which result in collapsing of the column.

5. Diagonal Cracking in the Columns: Columns are exposed to diagonal cracking resulting from large seismic shears occurred under extreme ground shaking. When the building also sustains the twisting action, the crack may change to a spiral form that decreases load bearing strength of the columns significantly.

6. Diagonal Cracking of Column Beam Joint: Several times, diagonal cracking happens through the intersection of the columns with the beams that considerably damages the strength of the frame.

7. Drawing Out of the Reinforcing Bars: Where the anchor length of the column bars or overlaps among the longitudinal bars are insufficient for producing full tensile strength of the bar, they are frequently drawn out because of tensions occurred in the column against reversal of stresses.

8. Collapse of Gable Frames: Reinforced concrete gable frames, frequently applied for school workshops, gymnasia and assembly halls, and cinema halls, may be expanded devoid of secondary resistance obtainable as soon as a joint fails. These are frequently found to fail and collapse if not properly designed and detailed.

9. Foundation Sinking and Tilting: Sinking or tilting of foundations of columns because of seismic shaking happens in loose soft soils and can result in extreme cracking of the superstructure and even fall down.



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Different Sorts Of Scaffolding

Definition of scaffolding

The impermanent framework containing platforms at various stages which permits masons to sit and continue their construction work at different height of building is termed as scaffolding. Scaffolding is required for masons to sit and put construction materials when the height of wall, column or any other structural members of a building exceeds 1.5m. It offers a temporary and a safe working platform for different types of work such as construction, maintenance, repair, access, inspection, and so forth.

Divisions of Scaffolding:

There are the following various divisions available:

Standards:

Standards indicate to the vertical member of the frame work that is used to support on the ground.

Ledgers:

Ledgers are the horizontal members which are running equivalent to the wall.

Braces:

Braces are diagonal members running or fixed on the standard to provide stiffness to the scaffolding.

Transoms:

While both the ends of put logs are sustained on ledgers, then they are called transoms.

Boarding:

Boarding is a platform which is horizontal to support workmen and elements which are supported on the put log.

Guard Rail:

Guard rails are supplied at the working level similar to a ledger.

Classification of Scaffolding

Scaffolding can be categorized into various kinds which are described below in a nutshell:

Single Scaffolding

For brick masonry works single scaffolding is widely used. This type of scaffolding contains a single framework of standards, putlogs, ledgers, etc. Standards are placed at a distance of about 1.2m from the wall at 2-2.5 m interval and are attached by ledgers at a vertical interval of 1.2 to 1.5m. One end of putlog is on the ledgers and the other one in the hole left in the wall at an intermission of 1.2 to 1.5m.

Double Scaffolding

Double scaffolding is usually used for stone masonry works. Stipulation of holes to withstand putlogs is tough enough, so double scaffolding is preferred containing two rows of standards segregated by ledgers. The first row of the scaffolding is located at 20-30 cm away from the wall and the next row is kept 1m away from the first row. Transoms are situated on the ledgers and the cross braces in the scaffolding assists to build them more powerful and durable.

Steel Scaffolding

Steel scaffolding is similar to mason scaffolding. It consists of steel tubes instead of wooden members. In such scaffolding, standards are placed at a space of 3m and are connected with the help of steel tube ledgers at a vertical interval of 1.8m.

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