By
Prof. V.D.Gundakalle, Professor ,Department of Civil Engineering, K.L.E. College of Engineering and Technology, Belgaum, Karnataka, (India)

Prof.Abhishek.S.Pathade, Professor, Department of Civil Engineering, K.L.E. College of Engineering and Technology, Belgaum, Karnataka, (India).

Mubashar Munshi Post Graduation Student, K.L.E. College of Engineering and Technology, Belgaum, Karnataka, (India).

Abstract
The actual earthquake force in considerably higher than what the structures are designed for. We cannot design the structures for the actual value earthquake intensity because the cost of construction will be too high. The actual intensity of earthquake is reduced by a factor called response reduction factor ‘R’. The value of ‘R’ depends on how we design the frame members. From previous study it is noted that the ‘R’ factor depends on ductility factor (Rµ), strength factor (Rs), structural redundancy (RR) and damping associated with structure. The objective of this work is to evaluate the response modification factor (R) for RC elevated water tanks supported on framing system are considered having staging height of 15m and 21m with varying capacities and staging configuration. These tanks are designed for gravity as well as seismic loads. A non-linear pushover analysis is used to calculate the base shear capacity and ductility of tanks. Two different cases of collapse criterion are used for defining ultimate stage on the capacity curve. It is observed that the Rµ are increasing with time period but the variation is not consistent. RS is higher for lower staging height.

Key words
Response reduction factor, Seismic design, static nonlinear pushover analysis
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By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

Concrete Strength
Cement like water, aggregates and some times admixtures is one of the ingredient of concrete. The mixing of these materials in specified proportions produces concrete. Accordingly cement alone is not a building material, it is the concrete which is a building material. For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregates, and water (under same mixing, curing and testing conditions) is influenced by the :

a) Ratio of cement to mixing water
b) Ratio of cement to aggregates, the strength of the mortar, the bond between the mortar and the coarse aggregate.
c) Grading, surface texture, shape, strength, and stiffness of aggregate particles.
d) Maximum size of aggregate.

Strength of concrete is directly related to the structure of the hydrated cement paste. Air in concrete produces voids. Excess of water in concrete evaporate leave the voids in the concrete. Consequently, as the W/C ratio increases, the porosity of the cement paste in the concrete also increases. As the porosity increases, the compressive strength of the concrete decreases.

STRENGTH OF CEMENT V/S STRENGTH OF CONCRETE
It is not possible to design a concrete mix of high strength with cement of low strength. The variation in strength of cement is due largely to the lack of uniformity in the raw materials used in its manufacture, not only between different source of supply, but also with in a quarry. Further, differences in details of the process of manufacture and above all, the variation in the ash content of coal used to fire the kilin, contribute to the variation in the properties of commercial cements. This is not to deny that the modern manufacturing of cement is a highly sophisticated process.

Upto 1975, the mass production of cement in India was only OPC-33 Grade. It was found difficulty in obtaining high strength concrete with this cement. The consumer has been normally finding it difficult to get consistent and ensured supply of high strength cement for prestressed concrete and certain items of precast concrete. For these special requirements BIS published IS:8112, Specification for OPC-43 Grade cement. Now, the varieties of cement manufactured in India are:

1. Ordinary Portland Cement (Grade OPC-33, OPC-43 and OPC-53. OPC-33 Grade almost vanished from Indian market)
2. Portland Pozzolana Cement (PPC)
3. Sulphate Resistance Cement (SRC)

Test results of different brand of cement minimum to maximum compressive strength are given in the table-1.

Due to variation of cement strength, the concrete made from these cement will also have variable strength. For a correct approach in the Concrete Mix Design, if the facilities at site are available, with the given set of materials, requirements and site conditions own W/C ratio v/s compressive strength of concrete curve should be developed at site itself.

It is often observed that cement bags marked as OPC-43 Grade may really be containing cement of much higher grade. PPC cement as per IS Code is only of 33 Grade. Where as on bags it is marked as 43 MPa or 53 MPa. Site cement samples should be tested for its actual strength and other properties. There are instances where higher grade cement is being used even for low strength concrete, as mortar or even for plastering. This can lead to unnecessary cracking of concrete/surfaces.

In low grade OPC, the gain in strength will continue beyond 28th day. Due to early strength gain of higher grade of OPC the concrete strength do not increase much beyond 28th day. The heat of hydration of higher grade OPC being higher, the chances of micro-cracking of concrete is much greater. Thus during initial setting period of concrete, the higher head of hydration can lead to damaging micro-cracking with in the concrete which may not be visible at surface. The situation can be worse when we tend to increase the quantity of the cement in concrete with a belief that such increase are better for both strength and durability of concrete.

Table-1 : Compressive Strength of different Grade of Cement:

The variation in cement strength is given below : (in N/mm2)
OPC-43 Grade : 7-days from 24.7 to 57.7 and 28-days from 40.0 to 74.5
OPC-53 Grade : 7-days from 37.8 to 57.8 and 28-days from 52.5 to 73.9
PPC : 7-days from 25.5 to 52.6 and 28-days from 48.0 to 67.0
Note: Compressive Strength (CS) of cement given are average values.

We at engineeringcivil.com are thankful to Sir Kaushal Kishore for submitting this research paper and helping all civil engineers understand the concept of Concrete Strength and factors affecting it.

By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

In about 80% of our construction sites, the water in the concrete mixer is added in a very crude manner either direct from a hoze pipe or by some container without any proper measured quantity. Thus no consideration is given to maintain free Water/Cement ration to its correct specified value resulting production of poor quality of concrete. The addition of mixing water in the concrete mixer with these crude methods always add more water then actually required. This excess water in due course evaporated leaving voids and increasing the porosity of the concrete. Such concrete will have lower strength and also will be not durable.

Therefore it is very important to maintain free W/C ration to its correct value in all the batches of concrete. Free W/C ratio means mixing water added to saturated and surface dry aggregates ie, if the site aggregates are dry extra water is to be added in the mixing water as per the absorption of aggregate, and if the site aggregates contains surface water, this surface water is to be deducted from the mixing water. The weight of aggregates should also be adjusted accordingly. A Concrete Mix Design is reported in standard moisture condition of aggregates and this is saturated and surface dry aggregates. If aggregates are being taken by volume bulking of sand should be taken into consideration.

To solve the construction sites mixing water problems, a simple graduated transparent plastic jar of least count 0.5 ltr, as per drawing should be supplied along with the mixer or may be fabricated at site. This Jar be installed at site near concrete mixer as shown in the drawing. The water may be filled in the jar to the quantity of required gauging water. While mixer is running the measured water in the jar slowly drain in the mixer drum through rubber hoze by opening the valve. If ADMIXTURES are to be used and required to be mixed with the gauging water, this may be mixed with water of the jar.

We at engineeringcivil.com thankful to Sir Kaushal Kishore for publishing his paper on “Water Measuring Jar for Concrete Mixer”.

By
Preetpal Kaur Ragbir Singh, Assoc. Prof. Dr, Nasir Shafiq
University Technology Petronas, Bandar Seri Iskandar.31750,Tronoh, Perak, Malaysia

Abstract
Design, construction and maintenance requirements of tall buildings and industrial complexes are very different from those applicable for normal building design and construction. For example, for conveying the services and other facilities such as water supply, electricity, air-conditioning and sewerage discharge; a complex network of system routing is provided, which usually align vertically and horizontally and spread throughout the floor area. This complex network is often obstructed by the structural components such as beams, columns and floors and requires to penetrate through such obstruction, which is called the structural penetrations. The size, location and configuration of structural penetration are derived from the type of services, magnitude and speed of facility to be provided. The most prevalent location, size and configuration of structural penetration are always an issue between structural engineers and service or facilities design engineers. This research focuses on the effects of static loading on reinforced concrete beams with openings. This research also studies the prospect of strengthening the beams by using external bonded CFRP in different combinations or arrangement to regain bending capacity that was lost due to the openings. The openings are circular, rectangular, square and elliptical shaped.

Keywords
CFRP Sheets, Large Opening, Static, RC Beam.
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By
Prof. Pravin B. Waghmare
Acharya Shrimannarayan Polytechnic Pipri (M)- Wardha-Maharashtra

Abstract
The objective of this study is to identify an efficient retrofitting method for existing open ground story reinforced concrete frame buildings. Failure of several soft-stored buildings in the past earthquakes underscores the need to retrofit existing soft-story buildings. A common cause for the collapse of multi-storied buildings is the occurrence of soft story in the ground floor due to the presence of infill walls in the upper story. During the Bhuj (Gujarat) earthquake of 6thJanuary 2001 several soft storied building failed there by confirming the vulnerability of such buildings to earthquake loading. This underscores the need to retrofit existing soft story buildings to prevent their total collapse. The existing building structures, which were designed and constructed according to early codal provisions, do not satisfy requirements of current seismic code and design practices. A two dimensional R.C. frame designed with linear elastic dynamic analysis using response spectrum method. The computer software package STAAD Pro–2005 is used for dynamics analysis technique is used to assess the performance of a (G + 4) reinforced concrete buildings, of which the ground storey is a parking facility the ground storey is 3.5m high while the upper stories giving a total height of 15.5 m. the building is located in Seismic Zone IV.

The RC frame is retrofitted by three methods namely,
1) Brick masonry infill in the ground story.

2) Steel braces in the ground story.

3) R.C. Structural wall in the ground story.

The study concludes that the building designed as per provisions of IS: 456:2000 using limit state method of design, and analyzed as per existing seismic code IS: 1893-2000 of all these three methods studied the use of structural wall in the ground story panel gave the maximum strength and ductility.

Keywords: Open ground storey, brick infill, RC wall Infill and Steel Bracing.
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