Advantages & Limitations of Prestressed Concrete

The prestressing of concrete has several advantages as compared to traditional reinforced concrete (RC) without prestressing. A fully prestressed concrete member is usually subjected to compression during service life. This rectifies several deficiencies of concrete. The following text broadly mentions the advantages of a pre-stressed concrete member with an equivalent RC member. For each effect, the benefits are listed.

A) Section remains un-cracked under service loads

1. Reduction of steel corrosion
   • Increase in durability.
2. Full section is utilized
   • Higher moment of inertia (higher stiffness)
3. Less deformations (improved serviceability).
4. Increase in shear capacity
5. Suitable for use in pressure vessels, liquid retaining structures.
• Improved performance (resilience) under dynamic and fatigue loading.

B) High span-to-depth ratios

1. Larger spans possible with prestressing (bridges, buildings with large column-free spaces)
2. Typical values of span-to-depth ratios in slabs are given below.

For the same span, less depth compared to RC member.

• Reduction in self weight
• More esthetic appeal due to slender sections
• More economical sections.

C) Suitable for precast construction The advantages of precast construction are as follows.

• Rapid construction
• Better quality control
• Reduced maintenance
• Suitable for repetitive construction
• Multiple use of formwork

1. Reduction of formwork

• Availability of standard shapes.

Limitations of Prestressing

Although prestressing has advantages, some aspects need to be carefully addressed.

• Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete.
• The use of high strength materials is costly.
• There is additional cost in auxiliary equipments.
• There is need for quality control and inspection.

Minimum Steel Reinforcement in Concrete and Clear Cover Requirements

The minimum amount of steel reinforcement is defined as that for which "peak load at first concrete cracking" and "ultimate load after steel yielding" are equal. In this way, any brittle behavior is avoided as well as any localized failure, if the member is not over-reinforced.

In other words, there is a reinforcement percentage range, depending on the size-scale, within which the plastic limit analysis may be applied with its static and kinematic theorems.

A minimum area of reinforcement is required to control the cracking, which occurs in the concrete due to temperature, shrinkage and creep. It enables cracking to be uniformly distributed and therefore minimizes individual crack width.

The following criteria have been used to determine the cross-section area of temperature or minimum reinforcement required in hydraulic structures. The percentages indicated are based on the gross cross-sectional area of the concrete to be reinforced. Where the thickness of the section exceeds fifteen (15) inches (380 mm), a thickness of fifteen (15) inches (380 mm) should be used in determining the temperature or minimum reinforcement.

The minimum ratio of required reinforcement is;

FOR SLABS:

f_min = 0.002 (for f_y = 40,000 psi)

S_min = 0.0018 (for f_y = 60,000 psi)

FOR WALLS:

For Vertical Steel

f_min = 0.0015

For Horizontal Steel

f_min = 0.0025

The temperature reinforcement shall not be less than ½ inch at nine 9 inch center to center. All concrete stilling basins, glacis and floors and all concrete aprons structures (with slab thickness > 15 inch ) shall be reinforced in the exposed (top) face with ¾ inch bars at twelve (12) inch center to center, both ways, placed three (3) inch clear from concrete face, unless otherwise designed.

Nominal reinforcement of concrete chute blocks, baffle blocks and sills for stilling basins, aprons and other portion of structures shall consist of ¾ inch bars at twelve (12) inch center to center, both.

Temperature and shrinkage reinforcement should be uniformly distributed alongside faces of structure elements to control cracking due to temperature changes, creep, and shrinkage.

Depending on the thickness of the structural element, it is preferred that the centre-to-centre spacing of the primary and secondary reinforcement be equal to or less than 300 mm; however, in no case should it exceed 450 mm. The minimum clear distance between bars should not be less than 1.4 times the bar diameter or 1.4 times the nominal maximum size of the coarse aggregate, whichever is greater. This requirement also applies to the clear distance between a contact lap splice and adjacent splices and bars.

The minimum thickness of concrete cover over the reinforcement has been determined by considerations of adequate fire resistance and durability. The cover for the reinforcement to meet specified period of fire resistance is detailed as follows:

Fire Resistance (hours)	Beams		slabs		columns
	Simply supported	continuous	Simply supported	continuous
0.5	20	20	20	20	20
1.0	20	20	20	20	20
1.5	20	20	25	20	20
2.0	40	30	35	25	25
3.0	60	40	45	35	25
4.0	70	50	55	45	25

Cover in excess of 40 mm (1.57 inch) may require additional measure to reduce risk of spalling.

Cover Against Spalling

Concrete Element	Minimum Concrete Cover
	(in)	(mm)
Face in contact with earth	3	75

Concept of Demand & Capacity in Structures

Definition of Demand (Loads on structures) Demand refers to all external actions (self weight) gravity, wind, earthquake, snow all are external actions. If differential settlement occurs stresses are induced in structure. Internal stresses are also called load effects. Definition of Capacity (Load capacity of structures) Overall ability of a structure to carry an imposed demand is called capacity. Analysis of Capacity and Demand To find out whether capacity is enough to carry demands or not. To enhance capacity, either change the material or its geometry. Failure occurs when capacity is less than demand. To avoid failure, capacity to demand ratio should be kept greater than one or at least equal to one. This can be done by increasing the area or reducing the loads or increase the strength of the material by replacing the material Steel Reinforcement area should be opposite to the direction of demand e.g. in slabs the demand direction is in the direction of compression and tension i.e. along the length of bending. So steel is placed along the length and along the width.

Reinforced Concrete Building Elements

Beam Beam is a flexure member of the structure. It is subjected to transverse loading such as vertical loads, and gravity loads. These loads create shear and bending within the beam. Columns A long vertical member mostly subjected to compressive loads is called column Strut A compressive member of a structure is called strut. Beam-Column A structural member subjected to compression as well as flexure is called beam column Grid A network of beam intersecting each other at right angles and subjected to vertical loads is called grid. Cables and Arches Cables are usually suspended at their ends and are allowed to sag. The forces are then pure tension and are directed along the axis of the cable. Arches are similar to cables except hath they are inverted. They carry compressive loads that are directed along the axis of the arch. Plates and Slabs Plates are three dimensional flat structural components usually made of metal that are often found in floors and roofs of structures. Slabs are similar to plates except that they are usually made of concrete. 1st Floor It is the floor that has 1 storey height above ground. Basement Floor The floor of the basement of the building. It is also called cellar. The basement floor is either completely or partially below the ground floor. A basement can be used in almost exactly the same manner as an additional above-ground floor of a house or other building. However, the use of basements depends largely on factors specific to a particular geographical area such as climate, soil, seismic activity, building technology, and real estate economics. The concrete floor in most basements is structurally not part of the foundation; only the basement walls are. Since warm air rises, basements are typically cooler than the rest of the house. In summer, this makes basements damp, due to the higher relative humidity. Basement Wall The wall surrounding the basement floor is called the basement wall. The basement walls can be regarded structurally as part of the foundation. The basement walls are shear walls which can resist lateral laods as well. Moreover, these walls are meant to be higly non-porous and water resistant. Column Bracket Column Bracket is protrusion from the column also used for hanging or attaching lamps, bulbs or other accessories to it like road signs. Column Capital Column capital is an architectural element used for aesthetic purposeswhich forms the topmost member of a column. Drop Panels Drop panels are used to thicken the slab around the column in flat slabs to avoid punching shear. Since flat slabs have no stirrups shear is resisted by thickening the slab around the column to increase the concrete in shear. Beams can also be used, but generally drop panels are preferred to avoid conflicts with the electromechanical works of the structure. Exterior Columns The columns supporting the main structure of the building. Usually in frame structures the exterior columns are of extreme importance and bear the load of the building as well as resist environmental factors like wind, rain, and other physical factors. Flat Plate Slabs connecting to columns directly. Flat plate system is widely adopted by engineers as it provides many advantages . The system can reduce the height of the building, provide more flexible spatial planning due to no beams present, and further reduce the material cost. However, the main problem in practice is the brittle failure of flat plate under punching shear. Due to the relatively small floor loading and the close column spacing, flat plate construction is preferred. For heavier loading and larger column spacing, column capitals are required, and for even larger spans to reduce the self-weight, waffle slabs are used. Flat Slab The flat plate is a two-way reinforced concrete framing system utilizing a slab of uniform thickness, the simplest of structural shapes. The flat slab is a two-way reinforced structural system that includes either drop panels or column capitals at columns to resist heavier loads and thus permit longer spans. Construction of flat slabs is one of the quickest methods available. Lead times are very short as this is one of the most common forms of construction. Interior Columns Interior columns in a frame structure suppor the slab and beams internally. They are not as susceptible to buckling and environmental effects as the external ones but still are extremely important considering the safety and stability of the building. Interior columns can also serve aesthetic and architectural purposes. Pedestal An architectural support or base, as for a column or statue. Roof A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather and the invasion of animals. Structures that require roofs range from a letter box to a cathedral or stadium, house buildings being the most numerous. The elements in the design of a roof are: the material the construction the durability & Serviceability Spread Footing To distribute the load of the foundation on the soil, spread footings are installed below the building's foundation. This type of footing is continous below the perimeter of the house walls and may be thickened or widened at the points where concentrated loads are applied e.g. columns. These components are constructed from concrete and are often reinforced with rebar or steel to add additional support. Depending on the size and configuration of the building, the footers can be buried just below ground level or several feet below the surface. In cold climates, they are always placed below the frost line to minimize problems with concrete heaving that occurs during freeze/thaw cycles. This type of footer design is highly beneficial to builders and homeowners. Since they transfer the weight of the building over a large area, they have little risk of failure Upturned Beam Through the use of upturned concrete T-beams, designers created a naturally ventilated workspace that employs the thermal mass of an exposed concrete ceiling. This concrete absorbs heat during the day and is purged at night by cool breezes. The term is usually used in concrete construction, in parking structures, but here is how it works: The beam is above the floor it supports, or a combination. Take a parking structure, there is the required barrier wall, so if you turn the beam up it acts as support and the barrier. Think of your simple beam diagram with a uniform load on it. The beam supports this load, so it doesnt matter if the load is applied at the bottom (simply, other than there are the compression/tension face) This also works well in buildings, rather than have a large beam under the floor, the beam is cast above and below the floors, acts as bearing and shear tension and compression face reinforcement will be some what different, but beam cross section area will stay the same.

Post Tensioning in Concrete Slab

Bonded Slab Post Tensioning

This can be defined as the method of applying compression force on concrete after it has been poured and cured.

Introduction to Post tensioning and benefits of post tensioning in concrete slab.

Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands rebars, typically referred to as tendons. It has the following benefits as compared to unbonded post tensioning.

Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element.

A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete.

The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-tensioning is also used in the construction of various bridges; both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.

The advantages of this system over unbonded post-tensioning are:

1. Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
2. Tendons can be easily 'weaved' allowing a more efficient design approach.
3. Higher ultimate strength due to bond generated between the strand and concrete.
4. No long term issues with maintaining the integrity of the anchor/dead end.

Advantages of Post-tensioning

1. Allows longer clear spans, thinner slabs
2. Lower overall building height for the same floor-to-floor height.
3. Allows a high degree of flexibility in the column layout, span lengths and ramp configurations
4. The use of traditional reinforcement requirement can be reduced as tendons cannot de-stress in accidents.
5. Increases the ultimate strength due to strong bond between the strand and concrete.
6. Significant savings in costs can be achieved by post tensioning in concrete slabs because of:
1. Reduced cracking and deflections
2. Reduced storey height
3. Better Water tightness

Placement of Tendons

• Positioning and fixing of casting and block-outs to the edge formwork or construction joint form work
• The support bars shall be prepared in advance.
• Lay tendons according to tendon layout in accordance with the drawings.
• Fix tendons to correct profiles with support bars and chairs and the tendons are made with provisions for grouting using grout using grout vents and grout hoses
• Prepare installation report for every installation as per the enclosed format.
• Tolerance of tendon profiles is recommended as follows:
  • vertical: + 5 mm (at lowest and highest points)
  • Horizontal: + 100 mm

Structure - Types of Structures - Definition & Classification

Definition:

A combination of members connected together in such a way to serve a useful purpose is called structure.

Types of Structure Rigid Frame Its is that type of structure in which the members are joined together by rigid joints e.g. welded joints.

Truss (Pin connected joints) A type of structure formed by members in triangular form, the resulting figure is called a truss. In truss joints are pin connected and loads are applied at joints. No shear force & bending moment are produced. Only axial compression and axial tension is to be determined while analyzing a truss Structural Members Those members that are interconnected in such a way so as to constitute a structure are called structural members. Beam Beam is a flexure member of the structure. It is subjected to transverse loading such as vertical loads, and gravity loads. These loads create shear and bending within the beam. Columns A long vertical member mostly subjected to compressive loads is called column Strut A compressive member of a structure is called strut. Beam-Column A structural member subjected to compression as well as flexure is called beam column Grid A network of beam intersecting each other at right angles and subjected to vertical loads is called grid. Cables and Arches Cables are usually suspended at their ends and are allowed to sag. The forces are then pure tension and are directed along the axis of the cable. Arches are similar to cables except hath they are inverted. They carry compressive loads that are directed along the axis of the arch. Plates and Slabs Plates are three dimensional flat structural components usually made of metal that are often found in floors and roofs of structures. Slabs are similar to plates except that they are usually made of concrete.