Details of stress-strain curve in steel bars

The objective of stress-strain curve is to define the behavior of steel bars under loads. To develop the curve, the steel specimens are examined. A steel specimen is progressively drawn through a testing machine unless it breaks, and then stress and respective strains are recorded. The stresses are then outlined along the vertical axis, and because of these stresses, respective strains are outlined along the horizontal axis.

Several mark points exist on the stress-strain curve which demonstrate different stages for steel specimen to move through before occurrence of fracture. It is very important to recognize the stress-strain curve for getting the ability to understand the response of steel bars when exposed to loads.

When the steel specimen is exposed to load, it functions like an elastic material to indicate that the stresses and strains are proportional. When the load is raised, the specimen begins to lose its proportionality and ultimately collapses or yields. When the load is raised beyond the yield point, the steel bar undergoes stress hardening and gets the strength to resist larger stress, after it attains the fracture point.

Stress-strain Curve of Steel Bars: When the steel specimen is exposed to load, it passes through different stages ranging from elastic stage, yield point, and fracture.

Limit of Proportionality: This stage is demonstrated on the stress-strain curve from the initial point until point “A”. In this region, the stress remains low and fails to produce permanent strain. The stress and strain are relative to each other, as a result when the stress is eliminated, the steel bar would recapture its original shape.

Elastic Limit: It is situated among point “A” and “B” on the curve. When the stress on steel specimen is again raised, it will produce elastic strain. The stress and strain are not relative to each other.

Yield Point: It is the most vital point on the stress-strain curve from the design perspective. This point is indicated by letter B on the curve and treated as the failure point in the design of reinforced concrete structure. Therefore, when the steel bar attains the yield point in the reinforced concrete element, it will be treated as a failed member.

The yield point is the origin of steel plastic deformation. The stress and strain are not relative. The point B is defined as the upper yield point while the point C is the lower yield point.

Ultimate Strength: When the stress is again raised beyond yield point, strain hardening occurs that is demonstrated from point C to D, beyond which necking commences. Throughout strain hardening, the material is subjected to changes in its atomic and crystalline structure that leads to greater resistance of the material to further deformation. The maximum ordinate in the stress-strain diagram i.e. point D belongs to the ultimate strength or tensile strength.

Rupture Strength: Rupture strength stands for the strength of the material at rupture. It is also called the breaking strength. It refers to the point “E” on the stress-strain diagram.

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Some vital tips to develop a column independently

A column footing generally stands for a block of concrete that is poured in the bottom of a hole with the purpose of allocating the weight provided on the column to a greater area. As a result, the columns can’t go down into the ground in due course. Given below, some useful guidelines to pour the footings independently.

Step 1 - Measure the Footing: Label the spot to locate the correct center point of the column.

After that, work out and mark out a square that is 12 inches larger regarding the size of the column, and excavate the hole. Standard depth should be 12 inches along with one extra inch for each three inches of column. As for instance, the minimum depth should be 14 inches for a six-inch column. More is always recommended.

Step 2 - Level the Bottom: The bottom of the hole should be leveled. If it is not possible to work with a shovel because of shortage of space, utilize a concrete trowel.

Then, take the measurement from the bottom 12 inches along with the depth added for column size, and fix a nail into the side of the hole at that depth on all four sides. Instead, provide a stake into the bottom of the hole, leaving that amount of the stake uncovered.

Step 3 - Measure the Concrete required: Cut a piece of wire mesh as per size of the hole, and put aside. It is assumed that minimum one full bag of concrete should be needed for the column footing or possibly more, but the formula for making perfect measurement is L x W x H / 12 to obtain the result for the number of bags required, rounded up. The height should be the distance from the bottom of the hole to the nails (or the top of your grade stake).

While arranging a column permanently, work out the concrete to fill the hole to within three inches of the top. Apply a mix containing small rocks in it since these rocks will arrange reinforcement and avoid cracking.

Step 4 - Pour the Footing: Mix the concrete based on your calculations. It is suggested not to mix excessively than required amount to fill the hole up to the nails at this time. It will be the actual footing and it should be poured discretely first, prior to adding the permanent concrete setting. Shovel or pour the concrete into the hole, but take precautions not to cave in the sides. Set the wire mesh down into the concrete, roughly halfway to the bottom.

The finishing of the surface should be smooth, but not slippery. Employ the nails as a guide for the finish depth along with them hardly over the top of the footing.

As these nails are only a guide, therefore, apply a torpedo level for leveling the footing. Allow a footing to be cured for minimum 48 hours prior to setting the column.

Step 5 - Set the Columns: While setting permanent columns, arrange one end on the footing after it is cured. Brace the column to retain it perfectly in vertical direction while checking on all sides. After that, mix the remaining concrete from your calculation and fill the hole upto three inches of the top.

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Some vital characteristics of concrete in fresh & hardened state

1. STRENGTH: Concrete has good strength against compression but it is moderately weak in tension and bending. Significant amount of force is required to crush concrete, but marginal force is required to pull it apart or produce bending cracks.

Compressive strength is mainly defined by the amount of cement applied, but is also influenced by the ratio of water to cement and proper mixing and placing as well as the suitability and range of hydration and curing.

Tensile strength generally varies from 7 or 8 % of compressive strength in high-strength mixes to 11 or 12% in low-strength mixes.

Both tensile strength and flexural bending strength are raised with the addition of steel or fiber reinforcement.

2. DURABILITY: Durability means the capacity to retain standard performance over an expanded service life.

Concrete upon which the commuters will walk on should have the resistance capacity against abrasion in order that it doesn’t erode easily.

Concrete uncovered on the outside of a building should have good resistance capacity against weather in order that it doesn’t lose it’s strength from recurring freezing and thawing.

The longevity of concrete uncovered to recurring freeze-thaw cycles can be raised greatly with air entrainment.

3. VOLUME STABILITY: All materials are enlarged and contracted with variations in temperature, and as the concrete is a porous material, it also enlarged and contracted with variations in moisture content. Cement-based products like concrete, concrete masonry, and stucco undergo initial shrinkage since the cement moisturizes and excess mixing water vaporizes.

This initial shrinkage is persistent, and it is other than reversible expansion and contraction resulting from later temperature or moisture variations.

Extreme shrinkage can lead to crack and as a result the moisture enters into concrete and a harmful cycle of deterioration is started.

4. WORKABILITY: Workability belongs to the relative ease with which a fresh concrete mix is treated, arranged, compacted, and finished devoid of segregation or separation of the materials.

Proper workability is necessary to form inexpensive and superior quality concrete.

Fresh concrete contains strong workability if it can be developed, compacted, and finished to its final shape and texture with least effort and devoid of segregation of the materials.

Concrete having inferior workability fails to flow smoothly into forms and perfectly enclose reinforcing steel and embedded items, and it becomes complicated to compact and finish.

5. CONSISTENCY: Consistency stands for the aspect of workability associated with the flow characteristics of fresh concrete.

It signifies the fluidity or wetness of a mix and is computed with the slump test. Fresh concrete is arranged in a metal cone. When the cone is detached, the concrete slumps a specific amount on the basis of how fluid it is. A wet, soft mix slumps more as compared to a drier, stiffer one.

A high-slump concrete is very fluid whereas a low-slump concrete is drier and more hard.

A high-slump mix leads to unnecessary bleeding, shrinkage, cracking, and dusting of the hardened concrete surface.

6. COHESIVENESS: Cohesiveness means the element of workability which specifies whether a mix is harsh, sticky, or plastic.

Plasticity is a suitable property in concrete stating that a mix can be moulded and retained a shape at the time of being formed.

A harsh mix does not have plasticity and the materials may be easily detached.

Harshness is resulting from either excessive or shortage of mixing water (high- or low-slump mixes), a shortage of cement (lean mixes), or a shortage of fine aggregate particles.

Harshness may also occur by an excess of rough, angular, flat, or elongated aggregate particles. Harsh mixes can sometimes be enhanced with air entrainment or by raising the fine aggregate or cement content, but modifications should be done to the overall mix to sustain the exact ratios of all materials.

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Details about high Rise Buildings Structural Systems

Requirement for high-rise buildings: The High rise buildings are gaining popularity in recent times for the following purposes -

• Shortage of land.
• High demand for business and residential space.
• Economic Growth.
• Technological Progression.

• Inventions in Structural Systems.
• Aspiration for aesthetics in urban settings.
• Cultural significance and prestige.
• Human Desire to build higher.

Now-a-days, the high rise buildings are constructed with steel-framed construction, reinforced concrete construction and steel- framed reinforced concrete construction. Besides, hybrid systems are developed during this time. These systems apply in excess of one type structural system in a building.

Structural Concern

Fighting gravity

• The weight of the building is supported with a wide array of vertical columns.
• Each floor is supported with horizontal steel girders driving among vertical columns.
• Curtain wall with steel and concrete are affixed to the outside.

Wind Resistance

• Building longer than 10 stories would normally need extra steel for lateral system.

• The elementary process for managing horizontal sway is to just brighten the structure. At the position, where the horizontal girders are connected to the vertical column, the construction crew bolt, welds them on the top and bottom as well as the side. It will facilitate the complete steel super structure to move more as one unit, like a pole, instead of a flexible skeleton.

• The impact of wind is also reduced by aerodynamic shaping of the building. Wind tunnel testing provides perfect loading for overall lateral system design and cladding design, and indicates motion perception and pedestrian level effects.



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Advantages of cross wall construction

Cross wall construction stands for a building construction method. Under this system, the precast load-bearing walls are arranged perpendicular to the lateral axis of the building.

In cross wall construction, the load-bearing walls provide support to the floors, roofs and the beams. The structures built up with the cross wall method are structurally strong and cost-effective.

Given below, the construction details and the features of cross wall construction :-

Features of Cross Wall Construction: Under cross wall construction, the prefabricated walls are arranged in series. These load-bearing walls transmit the structural loads securely to the foundation devoid of any concrete or steel frame. They also serve as shear walls and safeguard the building against lateral movement.

Cross wall construction is ideal for the buildings which contain maximum of 5 stories where all the floors come with similar plans. The intermediate floor span among the load-bearing walls offers adequate lateral control.

If both the load-bearing walls and the floors belong to precast members, then a series of boxes should be created. This type of construction is normally defined as box frame construction.

Cross Wall Construction Systems:

1. Precast materials can be sent at the right moment.
2. The precast wall components are arranged both horizontally and vertically. They are grouted and attached according to the plan through hidden joints and ties.
3. Prior to finish the complete structure, mechanical & electrical installation works, finishing works and other multifaceted works on the walls should be finished.

The cross walls are attached with any of the following key connections:

1. Wall-to-wall by Horizontal Joints
2. Wall-to-wall by vertical joints
3. Wall to the base foundation

When the cross walls are placed along the floor, the front and the rear walls can be non-load bearing claddings. The connections among the non-load bearing walls and the cross walls should be designed and built up with a proper care to avoid moisture infiltration and other issues.

Advantage of Cross Wall Construction:

1. Cross wall construction forms a long-lasting structure.
2. The initial construction cost is fewer.
3. With regard to conventional construction method, cross wall construction method save huge time and cost.
4. The building built up is structurally strong that contains greater fire and acoustic separation among the nearby rooms.
5. The construction achieves good thermal mass.
6. Site co-ordination is simple at the time of providing cross wall construction.
7. The structure ensures superior security.
8. The maintenance cost of the structure developed with cross wall construction method is fewer.
9. No scaffoldings are required for cross wall construction.
10. The structure remains monolithic with good air tightness.

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How to generate a floor plan to scale for any space

Given below, some vital steps and items for creating a solid diagram for floor plan:

List requirements: Create a brief list of the requirements of the people who will capture the space.

Measure - Ensure to obtain the exact measurements for the following items :

Exterior walls or footprint of your space, any doorways or entries, and windows. The walls from side to side and from the floor to the ceiling. Positions of the electrical outlets, switches, and other controls.

Permanent fixtures: Any and all architectural features along with fireplaces, brackets, shelves, benches, and any other in-built features.

Adjoining space and outside or overall dimensions of these items, and then trace each on your plan.

Objects inside the space

Scale: Floor plans should be drawn up to scale, it signifies that the size of your drawing should be minimized in order that the total space can be accommodated properly on a piece of paper or screen. A common scale is 1/4 inch = 1 foot. This similar scale should be employed to objects like furniture or exhibit booths which will be contained in the floor plan.

Consider sightlines: Standing, sitting, or working anything you prefer to see. Consider about windows, doorways, walls and what makes the most sense for the purposes you plan for the space.

Render it: Based on the all elements, arrange your floor plan.

Evaluate: Evaluate the requirements lists and the spatial arrangement along with circulation. Other factors comprise of utilizable space, privacy, and accessibility.

Share: Collaborate and take help from someone else to examine your floor plan to obtain helpful feedback to double-check your thinking and design.



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Some useful tips to eliminate the deficiencies of the reinforced concrete shear walls

Reinforced concrete shear walls are considered as perfect structural systems for a long time which offer both lateral resistance and drift control in RC buildings.

However these aging shear walls were normally designed for combined actions against gravity loads and wind loading. Seismic loading and design were not taken into consideration for day to day mid-height building structures.

There are lots of shortcomings in design and detailing of these shear walls and because of these, the shear walls become susceptible to seismic hazard.

General shortcomings of Thin Reinforced Concrete Shear Walls :-

The following shortcomings are mostly found in several existing buildings:

• Deficient wall thickness containing only one curtain of distributed horizontal and vertical reinforcement;
• Deficient lap splice lengths of the longitudinal reinforcement;
• lap splices are situated in the zones of potential plastic hinging;
• Insufficient confinement of the end regions of the walls;
• Deficiency in controlling the buckling of the flexural reinforcement;
• Inadequate amounts and defectively detailed transverse (shear) reinforcement.

Points to be followed for improvements:

To improve the condition of these defective walls, various repair and retrofit schemes should be undertaken.

The details of these schemes are given below:

Carbon-Fiber Reinforced Polymer (CFRP) wraps:

Under this method, the shear wall is covered with a layer of CFRP sheet. This layer makes the confinement better around the boundary elements to some extent. As this confinement effect is nominal, it enhances the ductility of the section, and prevents brittle failure of lap splices.

Fibre-Reinforced Self Consolidating Concrete Jacketing

A combination of steel fibers and reinforcement are applied to enhance the flexural strength of concrete section, and the simultaneously, increases the ductility of the wall to the new plastic hinge location.



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