IS CODES FOR RCC STRUCTURAL DESIGN

This article describes the basic codes for RCC structural design as per Indian standard codes. The structural design of reinforced concrete structures should be carried so as to conform to the Indian codes for reinforced concrete design, published by Bureau of Indian standards, New Delhi.

Purpose of design codes:

National building codes have been formulated in different countries to lay down guidelines for the design and construction of structures. The codes have been evolved from the collective wisdom of expert structural engineers, gained over the years. These codes are periodically revised to bring them in line with current research, and often current trends.

Following are the functions of design codes:

Firstly, the design codes ensure adequate structural safety, by specifying certain essential minimum reinforcement for design.

Secondly, they render the task of the designer relatively simple, often the result of sophisticate analysis is made in the form of a simple formula or chart.

Thirdly, the codes ensure a measure of consistency among different designers.

Finally, they have some legal validity in that they protect the structural designer from any liability due to structural failures that are caused by inadequate supervision and/or faulty material and construction.

Following are the design codes in India:

(i) IS456: 2000 – plain and reinforced concrete – code of practice (fourth revision)

(ii) Loading standard codes

The loads to be considered for structural design are specified in the following loading standards:

IS 875 (Part 1 to 5) : 1987 – code of practice for design loads (other than earthquake) for buildings and structures (second revision).

Part – 1: Dead loads

Part – 2: Imposed (Live) loads

Part – 3: Wind loads

Part – 4: Snow loads

Part – 5: Special loads and load combinations

IS 1893: 2002 – criteria for earthquake resistant design of structure (fourth revision).

IS 13920: 1993 – ductile detailing of reinforced concrete structures subject to seismic forces.

Design Handbooks:

The bureau of Indian Standards have also published the following handbooks which serve as useful supplement to the 1978 version of the codes. Although the handbooks need to be updated to bring them in line with the recently revised (2000 version) of the code, many of the provisions continue to be valid (especially with regard to structural design provisions).

SP 16 – 1980 – Design Aids (for Reinforced Concrete) to IS456: 1978

SP 24: 1983 – Explanatory handbook on IS 456: 1978

SP34: 1987 – Handbook on Concrete Reinforced and Detailing.

LIST OF IS CODES FOR REINFORCEMENT

1.  IS:432 – Mild steel & medium tensile steel bars and hard drawn steel wires for concrete reinforcement : Part-II -Hard drawn steel wire.

2. IS:1786 -  Specification for High strength deformed steel bars and wires for concrete reinforcement.

3.  IS:2502 -  Code of practice for bending & fixing of bars for concrete reinforcement.

4.  IS:2751 -  Recommended practice for welding of mild steel plain  & deformed bars for reinforced construction.

5.  IS:5525 -  Recommendation for detailing of reinforcement in reinforced concrete works.

6.  IS:9077 -  Code of practice for corrosion protection of steel reinforcement in RB & RCC construction.

7.  SP:34 – Handbook on concrete reinforcement detailing.

Construction Error during concreting

We had show below the common error happens while doing concreting at site. These errors not only occur during new construction, but may also happen during repair or rehabilitation works.

(1) Adding water to concrete: Water is usually added to concrete in one or both of the following circumstances:

First, water is added to the concrete in a delivery truck to increase slump and decrease pouring or placement effort. This will lead to concrete with lowered strength and reduced durability. As the water/cement ratio of the concrete increases, the strength and durability will decrease.

In the second case, water is commonly added during finishing of structural member. This leads to scaling, crazing, and dusting of the concrete.

(2) Improper alignment of formwork: Improper alignment of the formwork will lead to discontinuities on the surface of the concrete. While these discontinuities are unsightly in all circumstances, their occurrence may be more critical in areas that are subjected to high velocity flow of water, where cavitation-erosion may be induced, or in lock chambers where the “rubbing” surfaces must be straight.

(3) Improper consolidation or compaction of concrete: Improper compaction of concrete may result in a variety of defects, the most common being bugholes, honeycombing, and cold joints.

Bugholes are formed when small pockets of air or water are trapped against the forms. A change in the mixture to make it less “sticky” or the use of small vibrators worked near the form has been used to help eliminate bugholes.

Honeycombing can be reduced by inserting the vibrator more frequently, inserting the vibrator as close as possible to the form face without touching the form, and slower withdrawal of the vibrator. Obviously, any or all of these defects make it much easier for any damage-causing mechanism to initiate deterioration of the concrete.

Frequently, a fear of overconsolidation is used to justify a lack of effort in consolidating concrete.

Overconsolidation is usually defined as a situation in which the consolidation effort causes all of the coarse aggregate to settle to the bottom while the paste rises to the surface. If this situation occurs, it is reasonable to conclude that there is a problem of a poorly proportioned concrete rather than too much consolidation.

(4) Improper curing: Curing is probably the most abused aspect of the concrete construction process. Unless concrete is given adequate time to cure at a proper humidity and temperature, it will not develop the characteristics that are expected and that are necessary to provide durability. Symptoms of improperly cured concrete can include various types of cracking and surface disintegration.

In extreme cases where poor curing leads to failure to achieve anticipated concrete strengths, structural cracking may occur.

(5) Improper location of reinforcing steel: This section refers to reinforcing steel that is improperly located or is not adequately secured in the proper location.

Either of these faults may lead to two general types of problems. First, the steel may not function structurally as intended, resulting in structural cracking or failure. A particularly prevalent example is the placement of welded wire mesh in floor slabs. In many cases, the mesh ends up on the bottom of the slab which will subsequently crack because the steel is not in the proper location. The second type of problem stemming from improperly located or tied reinforcing steel is one of durability. The tendency seems to be for the steel to end up near the surface of the concrete. As the concrete cover over the steel is reduced, it is much easier for corrosion to begin.

(6) Movement of formwork: Movement of formwork during the period while the concrete is going from a fluid to a rigid material may induce cracking and separation within the concrete. A crack open to the surface will allow access of water to the interior of the concrete. An internal void may give rise to freezing or corrosion problems if the void becomes saturated.

(7) Premature removal of shores or reshores: If shores or reshores are removed too soon, the concrete affected may become overstressed and cracked. In extreme cases there may be major failures.

(8) Settling of the concrete: During the period between placing and initial setting of the concrete, the heavier components of the concrete will settle under the influence of gravity. This situation may be aggravated by the use of highly fluid concretes. If any restraint tends to prevent this settling, cracking or separations may result. These cracks or separations may also develop problems of corrosion or freezing if saturated.

(9) Settling of the subgrade: If there is any settling of the subgrade during the period after the concrete begins to become rigid but before it gains enough strength to support its own weight, cracking may also occur.

(10) Vibration of freshly placed concrete: Most construction sites are subjected to vibration from various sources, such as blasting, pile driving, and from the operation of construction equipment. Freshly placed concrete is vulnerable to weakening of its properties if subjected to forces which disrupt the concrete matrix during setting.

(11) Improper finishing of flat concrete surface:The most common improper finishing procedures which are detrimental to the durability of flat concrete surface are discussed below:

Adding water to the surface: Evidence that water is being added to the surface is the presence of a large paint brush, along with other finishing tools. The brush is dipped in water and water is “slung” onto the surface being finished.

Timing of finishing: Final finishing operations must be done after the concrete has taken its initial set and bleeding has stopped. The waiting period depends on the amounts of water, cement, and admixtures in the mixture but primarily on the temperature of the concrete surface. On a partially shaded slab, the part in the sun will usually be ready to finish before the part in the shade.

Adding cement to the surface: This practice is often done to dry up bleed water to allow finishing to proceed and will result in a thin cement-rich coating which will craze or flake off easily.

Use of tamper: A tamper or “jitterbug” is unnecessarily used on many jobs. This tool forces the coarse aggregate away from the surface and can make finishing easier. This practice, however, creates a cement-rich mortar surface layer which can scale or craze. A jitterbug should not be allowed with a well designed mixture. If a harsh mixture must be finished, the judicious use of a jitterbug could be useful.

Jointing: The most frequent cause of cracking in flatwork is the incorrect spacing and location of joints.

Soil Consideration for Building Design

Subsurface investigations:

Subsoil conditions are examined using test borings, provided by soil engineer (geotechnical).Number of borings and location of borings depends on building type and site conditions.Typically for uniform soil conditions borings are spaced 100-150′ apart, for more detailed work, where soil footings are closely spaced and soil conditions are not even borings are spaced 50′ apart.Larger open warehouse type spaces, where fewer columns are present (long span) required less boring samples.Borings must extend to firm Strata (go through unsuitable foundation soil) and then extend at least 20 feet more into bearable soil.Location of borings samples are indicated on engineer plan.Borings are not taken directly under proposed columns.Borings indicate: depth, soil classification (according to the unified soil system), and moisture content and sometimes ground water level is shown as well. (Physical properties: particle size, moisture content, density).Soil report recommendation should be based on testing of materials obtained from on site borings and to include:Bearing capacity of soil.Foundation design recommendations.Paving design recommendations.Compaction of soil.Lateral strength (active, passive, and coefficient of friction).Permeability.Frost depth.

Surface investigations:

High Water Table.Presence of trouble soils: Peat, soft clay, loose silt, or fine water bearing sands.Rock close to the surface (require blasting for excavations).Dumps or Fills.Evidence of slides or subsidence.

Above ground indicators of soil conditions:

Near Buildings – require shoring or earth and existing foundations.Rock Outcropping – indicate bedrock, good for bearing and frost resistance, bad for excavations.Water (lake) – indicate high water table, some waterproofing of foundations is required.Level Terrain – easy site work, fair bearing, but poor drainage.Gentle Slopes – easy site work, and excellent drainage.Convex Terrain (Ridge) – dry solid place to build.Concave Terrain (Valley) – wet soft place to build.Steep Terrain – costly excavations, potential erosion, and sliding soils.Foliage – some trees indicate moist soil. Large trees indicate solid ground.

Soil Classifications:

Engineers dealing with soil mechanics devised a simple classification system that will tell the engineer the properties of a given soil.The unified soil classification system is based on identifying soils according to their textural and plasticity qualities and on their grouping with respect to behavior.Soils are usually found in nature as mixtures with varying proportion of particles of different sizes, each of these components contribute to the soil mixture.

Soil is classified on the basis of:

Percentage of gravel, sand, and fines.Shape of grain.

Plasticity and compressibility characteristics.

In the unified soil classification system (uscs) the soil is given a descriptive name and a letter symbol indicating its principal characteristics.Placement of solid into its respective group is accomplished by visual examination and laboratory tests.In the unified soil classification, the terms cobbles, gravel, sand, and fines (silt or clay) are used to designate the size ranges of soil particles.

Soil particle size ranges from largest to smallest:

CobblesGravel (Coarse + Fine)Sand (Coarse + Medium + Fine)Fines consisting of Clay or SiltSoil shear strength is made up of cohesion (water content, how sticky it is) and internal friction (based on size of grains). This is determined by triaxial compression testing

Soil Groups:

Soils are then grouped into three groups consisting of:Coarse Grained – divided into gravely soils (G) and sands and sandy soils (S)Fine Grained – divided based on their plasticity properties. (L,H)Highly Organic – are not subdivided. (Pt)

Coarse Gained – are soils which composed of gravel and or sands and which contain a wide variety of particles. These are most suitable for foundations when well drained and well confined. They are soils with good bearing value. Particularly the G series (GW, GP, GM, GC).

Identified on the basis of the percentage amount of gravel and sand.

Fine Grained – are soils that are Silts and Clays (L,H). Contain smaller particles of silt and clay.These are suitable for foundations but require compactions. The most suitable of this series (L) is the CL.

Based on their cohesive properties and permeability.

Highly Organic – are soils that are usually very compressible and are not suitable for construction. They contain particles of leaves, grass, and branches. Peat, Humus, and swamp soil with highly organic texture are typical of this group (Pt).These are identified readily on the basis of color, texture, and odor. Moisture content is also very high in this type of soil.Soil names shown on the unified soil classification system are associated with certain grain size and textural properties. This is the case for the coarse grained soils. For silts and clay the names are based on the plasticity basis of the soil.Relevant information of samples taken by borings which can aid the geotechnical engineer in determination of foundations includes:1. For coarse grain soil – the size of the particles, mineralogical composition, shape of grains, and character of the binder.For fine grained soils – strength, moisture, and plasticity.In the preliminary stages, a visual inspection can determine the behavior of the soil when used as component in the construction of a proposed building. Soil can be classified according to the classification categories of the unified soil classification system. (Later on laboratory testing can be performed).Strength and consolidation which make up the compaction characteristics of the soil determines its suitability forbuilding foundations.

Soil Problems:

The problem of uplift pressures in soil can be reduced by having well drained and free draining gravels (GW, GP). Uplift pressures can occur in fine grained soils consisting of silts and clays; such soils can cause heaving of foundations and formation of boils.

Due to potential frost action

Regardless of the frost susceptibility of the various soil groups, two conditions must be present simultaneously before frost action will be a consideration – a source of water during the freezing period and a sufficient period of the freezing temperature to penetrate the ground.In general silts and clays (ML, CL, OL) are more susceptible to freezing (as they contain moisture). Well drained granular soils are less susceptible to freezing and creating foundation problems.

Due to drainage Characteristics

The drainage characteristics of soils are a direct reflection of their permeability. The presence of moisture in base, sub-base and sub-grade materials may cause the development of pore water pressure and loss of strength.The gravelly and sandy soils with little or no fines (GW, GP, SW, SP) have excellent drainage characteristics.Fine grained soils and highly organic soils have poor drainage characteristics.

Compaction:

The sheepsfoot and rubber tired rollers are common pieces of equipment used to compact soils. Some advantage is claimed for the sheepsfoot roller in that it leaves a rough surface that affords better bond between layers.Granular soils consisting of well graded materials (GW, SW) furnish better compaction results than the poorly graded soils (GP, SP).

Fine grained soils can also be compacted.

For most construction projects of any magnitude, it is highly desirable to investigate the compaction characteristics of the soil be means of a field test section.Suitability of soils for foundations depends primarily on the strength, cohesion and consolidation characteristic of the soils. The type of structure, load and its use will largely govern the adaptability of a soil as a satisfactory foundation material.A soil might be entirely satisfactory for one type of construction but might require special treatment for other building.In general, gravel and gravely soils (GW, GP, GM, GC) have good bearing capacity and undergo little consolidation under load.Well graded sands (SW) usually also have good bearing capacity.Poorly graded sands and silty sands (SP, SM) have variable capacity based on their density.Some soils containing silts and clays (ML, CL, OL) are subject to liquefaction and may have poor bearing capacity and large settlements when subject to loads. Of the fine grained soil group CL is probably the better for foundations.Organic soils (OL and OH ) and highly organic soils (Pt) have poor bearing capacity and usually exhibit large settlement under load.

Foundations:

For most of the fine grained soils (containing silt and clays) it might be sufficient to use simple spread footings, it is largely depending on the magnitude of the load. The location of the foundations in relation to the soil (need to be aware of foundation walls and hydrostatic pressure as moisture is present in the soil).If the soil is poor and structure loads are relatively heavy, then alternate methods are required.Pile foundations might be required in some cases where fine cohesive silt and clay soil is present. (CH, OH).Sometimes it might be desirable and economically feasible to over excavate remove such soils that are not of bearing capacity; can remove compact and fill back or import other engineered soil.The geotechnical engineer based on borings will recommend suitable foundations systems or alternative solutions, also beating capacity, minimum depths, and special design or construction procedures might be established.Safe bearing capacity of soil equals to the ultimate bearing capacity divided by a safety factor (usually 2-4). ultimate bearing capacity is defined as the maximum unit pressure a soil can sustain without permitting large amounts of settlements.Bedrock has the highest safe bearing capacity.Well graded gravel and sand that are confined and drained have a safe bearing capacity of 3,000 – 12,000 PSF.Silts and clays have lower safe bearing capacity of 1,000 – 4,000 PSF.

Role of Foundations:

1. Transfer the building load to the ground.

2. Anchor building against wind and seismic load.

3. Isolate building from frost heaving.

4. Isolate building from expansive soils.

5. Holds building up from moisture.

6. Provide living spaces (basement, storage).

7. Houses mechanical systems.

Foundation configurations are: Slab on Grade, Crawl Space, and Basement.

Pre Stressed Concrete

What is Prestressed Concrete?

Prestressed Concrete is an architectural and structural material possessing great strength. The unique characteristics of prestressed concrete allow predetermined, engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads. This is accomplished by combining the the best properties of two quality materials: high strength concrete for compression and high tensile strength steel strands for tension.

Actually, prestressing is quite simple. High tensile strands are stretched between abutments at each end of long casting beds. Concrete is then poured into the forms encasing the strands. As the concrete sets, it bonds to the tensioned steel. When the concrete reaches a specific strength, the strands are released from the abutments. This compresses the concrete, arches the member, and creates a built in resistance to service loads.

Prestressed Concrete Beam
Prestressed or pretensioned before it leaves the plant, a slight arch or camber is noticeable. Energy is stored in the unit by the action of the highly tensioned steel which places a high compression in the lower portion of the member. An upward force is thereby created which in effect relieves the beam of having to carry its own weight.

The upward force along the length of the beam counteracts the service loads applied to the member.

Ordinary Concrete Beam
Even without a load, the ordinary concrete beam must carry its own considerable weight - this leaves only a portion of its strength available to resist added loads.Under service loads, the bottom of the beam will develop hairline cracks.