Seismic Analysis Of Steel Concrete Engineering Essay

 

Now a day’s earthquakes are very frequently occurred and maximum loss of life and loss of property occurred due sudden failure of the structure, therefore special attentions are required to evaluate and to improve the seismic performance of multistoried buildings. Hence in this paper the seismic analysis of G+4 story office building is carried out using composite structure in which composite beams (RCC slab rest over steel beams) and composite columns (encased composite columns) are used. The 3-D model static analysis is carried with the help of advanced analysis software (SAP software) according to codal provision by considering different load combination. The results obtained from this type of structure are compared with results of same R.C.C. structure to describe earthquake resistant behavior and performance of the structure.

Such type of constructions has many advantages like high strength, high ductility and stiffness, ease in erection of high rise buildings, fire resistance, and corrosion resistance and helps to achieve modern trend in architectural requirement.

KEY WORDS

Composite structure, problem, composite beams, encased composite column, earthquake analysis, codal provision, different load combination, comparison with RCC building.

INTRODUCTION

In India, earthquakes occurrence have been increased during last few years and it has been studied that maximum loss of life and property occurred due to sudden failure of structure. In composite construction economy of the construction and proper utilization of material is achieved. The numbers of structures are constructed using composite structure in most of the advanced countries like Britain, Japan and America but this technology is largely ignored in India despite its obvious benefits (1).

In composite structure the advantage of bonding property of steel and concrete is taken in to consideration so that they will act as a single unit under loading. In this structure steel is provided at the point where tension is predominant and concrete is provided at the point where compression is predominant. In conventional composite construction, concrete rests over steel beam (2), under load these two component acts independently and a relative slip occurs at the interface of concrete slab and steel beam, which can be eliminated by providing deliberate and appropriate connection between them. So that steel beam and slab act as composite beam and gives behavior same as that of Tee beam. In steel concrete composite columns both steel and concrete resists external loads and helps to limit sway of the building frame and such column occupies less floor area as compared to reinforced concrete columns. The number of studies related to economy of the composite construction shows that the composite construction are economical, light weighted, fire and corrosion resistant and due to fast track construction building can be utilize or occupied earlier as compared to reinforced concrete structure(3).

In this paper an office building considered and seismic analysis is carried using composite beam (RCC slab rest over steel beam), encased composite column (concrete around Hot Rolled steel I section) and the results obtained from this type of structure are compared with the results of same RCC structure.

EXAMPLE OF BUILDING

The building considered is the office building having G+4 stories. Height of each storey is 3.5m. The building has plan dimensions 24 m x 24 m, which is on land area of about 1200 sqm and is symmetric in both orthogonal directions as shown in the figure 1. Separate provisions are made for car parking, security room, pump house and other utilities. However they are excluded from scope of work. The building provision is made for 180 employees and considered to be located in seismic zone III built on hard soil. In composite structure the size of encased composite column is 450mm x 450mm (Indian standard column section SC 250+ 100mm concrete cover), size of primary composite beam is ISMB 450 @72.4 Kg/m and size of secondary composite beam is ISMB 400 @61.6 Kg/m. Here channel shear connector ISMC 75 @ 7.14 Kg/m are used. Concrete slab rest over steel beam having thickness of about 125mm. The unit weights of concrete and masonry are taken as 25 kN/m3 and 20 kN/m3 respectively. Live load intensity is taken as 5 kN/m 2 at each floor level and 2 kN/m2 on roof. Weight of floor finish is considered as 1.875 kN/m2 (4). In RCC structure the size of column is decided by taking equivalent area of encased composite column that is 400mmx 700mm; size of primary beams is 300mm x 600mm and secondary beams is 300mm x 450mm with slab thickness is about 125mm. The unit weights of concrete and masonry are taken as 25 kN/m3 and 20 kN/m3 respectively. Live load intensity is taken as 5 kN/m 2 at each floor level and 2 kN/m2 on roof. Weight of floor finish is considered as 1.875 kN/m2. In the analysis special RC moment-resisting frame (SMRF) is considered.

MODELLING OF BUILDING

The building is modeled using the software SAP 2000. Beams and columns are modeled as two noded beam element with six DOF at each node. Slab is modeled as four noded shell element with six DOF at each node. Walls are modeled by equivalent strut approach (5). The diagonal length of the strut is same as the brick wall diagonal length with the same thickness of strut as brick wall, only width of strut is derived. The strut is assumed to be pinned at both the ends to the confining frame. In the modeling material is considered as an isotropic material.

2.1 Shell Element

Slab modeled as shell element of 125mm thickness having mesh of 1mx1m of this shell element. Material used for shell element is M25 grade cement concrete in both composite and RCC structure

2.2 Beams

In composite structure beams are steel I section from IS code and steel table. The length of each beam is divided into small parts of 1m intervals and connected with concrete slab so as to get composite action. In RCC the length of each concrete beam is divided into small parts of 1m intervals and connected with concrete slab so as to get behavior same as that of Tee beam action.

2.3 Columns

In composite structure column is modeled by giving section properties of both steel and concrete to the software. Also in RCC structure column is modeled by giving sectional properties to the software

ANALYSIS OF BUILDING

Equivalent static analysis is performed on the above 3D model. The lateral loads are calculated and is distributed along the height of the building as per the empirical equations given in the code (IS 1893:2002). The building modeling is done then analyzed by the software SAP 2000. The bending moment and shear force of each beam and column are calculated at each floor and tabulated below.

RESULTS AND DISCUSSION

4.1 Results of Composite Structure:

Floor Level

Max. Shear Force

(kN)

Max. Bending Moment (kN-m)

+ve B M

-ve B M

Plinth Level

73.32

19.64

168.6908

1

177.925

134.31

306.1174

2

175.075

132.34

299.477

3

165.571

132.34

274.038

4

153.64

132.39

236.546

Roof Level

65.59

82.15

125.52

Table 1: Bending Moment and Shear Force of Beam

4.2 Results of RCC Structure:

Floor Level

Max. Shear Force

(kN)

Max. Bending Moment (kN-m)

+ve B M

-ve B M

Plinth Level

115.00

62.45

230.42

1

244.772

177.96

449.82

2

236.744

183.89

418.69

3

223.675

175.28

380.04

4

207.023

174.63

324.58

Roof Level

119.83

115.1004

181.00

Table 2: Bending Moment and Shear Force of Beam

4.3 Results of Composite Structure:

Column No.

Max. Axial Force (kN)

Max. Shear Force (kN)

Max. Bending Moment

(kN-m)

Column-1

1462.307

83.868

251.1801

Column-2

2865.903

101.64

271.4602

Column-3

2828.667

100.091

269.33

Column-4

2865.903

101.64

271.46

Column-5

1462.307

83.87

251.18

Table 3: Axial Force, Shear Force and Bending Moment of Column

4.4 Results of RCC Structure:

Column No.

Max. Axial Force (kN)

Max. Shear Force (kN)

Max. Bending Moment

(kN-m)

Column-1

2453.516

148.942

495.89

Column-2

3526.32

161.64

510.50

Column-3

3538.64

160.995

509.61

Column-4

3519.463

161.83

511.142

Column-5

2455.27

149.047

496.432

Table 4: Axial Force, Shear Force and Bending Moment of Column

From above results of bending moment and shear force of composite structure and RCC structure it is found that bending moment and shear force for composite structure

are less than RCC structure. Hence the cross section area of section and amount of steel for structural element reduced in composite structure than RCC structure so that large space meets for utilization.

CONCLUSIONS

In this paper a three dimensional model is analyzed using SAP 2000 software in terms of the structural characteristics of encased composite column and composite beam. It is concluded that:

The dead weight of composite structure is found to be 15% to 20% less than RCC structure and hence the seismic forces are reduced by 15% to 20%. As the weight of the structure reduces it attracts comparatively less earthquake forces than the RCC structure.

The axial force in composite columns is found to be 20% to 30% less than RCC columns in linear static analysis.

The shear force in composite column is reduced by 28% to 44% and 24% to 40% in transverse and longitudinal directions respectively than the RCC structure in linear static analysis.

The bending moment in composite column in linear static analysis reduces by 22% to 45%.

In composite beams the shear force is reduced by 8% to 28% in linear static analysis.

It also provides fire, corrosion resistance, sufficient strength, ductility and stiffness.

Hence Composite structure is one of the best options for construction of multistory building as well as for earthquake resistant structure.

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Study Of Compressive Strength Of Concrete Engineering Essay

This research study comprises of concrete cubes made with Ordinary Portland Cement and with different configurations of fly ash by replacing cement and fine aggregate. To achieve the aim of this study, total 81 concrete cubes were cast. Among 81 cubes, 9 cubes were made with normal concrete, 36 cubes were made by replacing 25%, 50%, 75% and 100% of fine aggregate with fly ash and 36 cubes were made by replacing 10%, 25%, 50%, and 75% of cement with fly ash. The cubes were 6" x 6" in cross-section, and the mix design was aimed for 5000 psi. After proper curing of all 81 cubes, they were tested at 3, 7 and 28 days curing age. The cubes were tested in Forney Universal Testing Machine in the Concrete Laboratory of Civil Engineering Department, Mehran University Jamshoro. By analyzing the test results of all the concrete cubes, the following main findings have been drawn.

The compressive strength of concrete cubes made by replacing 100 % fine aggregate by fly ash was higher than the concrete cubes made with Ordinary Portland Cement at all 3, 7 and 28 days curing ages. On the other hand, the compressive strength of concrete cubes made by replacing 10 % and 25 % cement by fly ash were slightly lower than the concrete cubes made with Ordinary Portland Cement at all curing ages, whereas the compressive strength of concrete cubes made by replacing 50 % and 75 % of cement by fly ash were quite lower than the concrete cubes made with Ordinary Portland Cement at all curing ages.

Key Words: Ordinary Portland Cement, Fine Aggregate, Fly Ash, Compressive strength of Concrete.

Concrete is a composite material which is being used in variety of structures. More commonly cement, steel bars as well as coarse and fine aggregates are to be transported from distant places to the site which is quite expensive. Therefore the aggregates are preferably to be used from whatever is available locally.

Fly ash (also known as a Coal Combustion By-Product) is the finely divided mineral residue resulting from the combustion of powdered coal in electric generating plants. Large quantities of industrial by-products are produced every year. These waste by-products must be effectively disposed to eliminate air, soil, and surface, as well as ground water pollution at added cost to the industry and thus to the society [1- 3].

Fly ash is generally used as partial replacement of Portland Cement and/or fine aggregate, an expensive and energy intensive material. Therefore use of fly ash leads to considerable saving in cost and energy consumption. Utilization of increased volumes of fly ash in concrete will lead to conservation of energy and natural resources. Bulk quantities of some industrial by-products such as fly ash, bottom ash and slag have been used as aggregates for concrete, road embankment as well as sub base construction, but such bulk uses represents low value applications. On the other hand, their use as mineral admixtures in cement and concrete due to their pozzolanic and cementitious properties represents high value applications. [4]

Addition of finely divided pozzolanic and cementitious materials like fly ash, can affect the properties of cement mortar/concrete both in fresh and hardened state. In fresh or plastic state, mix proportions, water requirements for specified consistency, setting characteristics, workability, and heat of hydration are some of the properties influenced by mineral admixtures. In the hardened state, the rate of strength development and ultimate strength, permeability, durability against frost attack, sulfate attack, alkali-silica reaction, carbonation, and resistance to thermal cracking are significantly affected with the incorporation of mineral admixtures in cement concrete. Over the years, extensive research has been conducted all over the world to investigate the influence of fly ash on the strength of plain cement concrete. In this study the fly ash produced at Lakhra Coal Power Plant is used as a replacement of cement / fine aggregate, in order to investigate its effects on the strength of concrete.

With the boom in population and industrial growth, the need for power has increased manifold. It has been observed that the power generation plants running through coal fuel are producing huge amount of ashes, which is being treated as waste. If this waste is left unutilized, it can pollute various phases of human environment like air, food, land, shelter and water [5]. However, if this waste is disposed of properly, it can be a new source of useful material.

Researchers have been attempting to convert this waste into the wealth by exploring viable avenues for use of fly ash. It has been reported that this waste stuff is being used as fine aggregate in concrete construction and higher strengths are being achieved [5]. This will inevitably reduce the cement content, which is one of the expensive item in concrete construction. Hence the use of fly ash as a construction material in those areas where it is cheaply available would be a feasible step in construction industry rather than transporting standard hill sand from a far distant source.

It is reported that fly ash has cementitious properties; hence fly ash is an inexpensive replacement for various contents of concrete construction. When fly ash is employed with portland cement, then hydrated lime combines with the fly ash forming stable cementitious compound which contributes strength. [2, 10].

Fly ash refers to the finely divided material which is added to obtain specific engineering properties of cement mortar and concrete. The other, equally important, objective of using fly ash in cement concrete include economic benefits and environmentally safe recycling of waste by products. Fly ash is generally finer than Portland Cement. Because of its fineness, pozzolanic properties and self- cementitious nature, it is widely accepted as mineral admixture in mortar and concrete [4].

Fly ash in concrete is used to enhance the performance of concrete. The various advantages of fly ash in concrete largely depend on mix proportions, mixing procedure and field conditions. Although fly ash creates environmental problems, never the less it improves the quality of concrete. It also lowers the heat of hydration. Fly ash increases strength of concrete, reduces the permeability and corrosion of reinforcing steel, increases sulphate attack resistance and reduces alkali-aggregate reaction [10].

Lakhra Power Plant is very near to Jamshoro and Hyderabad and is the only coal fuel powered plant in Pakistan. It is about 35 km form Jamshoro and 55 km from Hyderabad. Lakhra coal field encompasses an area of 250 square kilometers. Fly ash produced through this power plant is very fine powder recovered from gases created by coal fired electric power generation. This power plant produces about 2 million tons of fly ash annually, which is being dumped like a land fill. It has been reported that dumped fly ash has occupied huge considerable space of land in the vicinity of power plant which has created environmental problem to the inhabitants who are living in this area. This alarms researchers to consume this land fill fly ash which is producing great environmental impact in the surrounding society.

There may be differences in the fly ash from one plant to another, day-to-day variations in the fly ash from a given power plant are usually quite predictable, provided plant operation and coal source remain constant. The effective utilization of fly ash in concrete requires adequate knowledge of characteristics of fly ash defined by its physical, chemical and mineralogical properties.

The various materials used in concrete mix are given in Table 1.

Cement

Dada Bhai Cement Factory

Fine Aggregatge

Bolhari sand

Coarse Aggregatge

Petaro crushing plant

Fly ash

Lakhra Power Plant

Water

Concrete laboratory, Civil Engineering Department

3.2 Properties of Materials used in Concrete mix

Standard test procedure as prescribed by ASTM C128-93 was used for this test. The specific gravity of fine aggregate used in this research study was found to be 2.61.

Standard test procedure as prescribed by BS: 812 Part 107: (Draft) and ASTM C 127- 93 was used for this test. The specific gravity of coarse aggregate used in this research study was found to be 2.66.

Standard test procedure as prescribed by ASTM C128-93 was used for this test. The specific gravity of Fly ash was found to be 2.54.

Standard test procedure as described in BS 812: Part 107: (Draft) was used for this test. The water absorption of fine aggregate was found to be 1.69 %.

Standard test procedure as described in BS 812: Part 107: (Draft) was used for this test. The water absorption of coarse aggregate was found to be 1.38 %.

Standard test procedure as described in BS 812: Part 107: (Draft) was used for this test. The water absorption of Fly ash was found to be 16.92 %.

Standard test procedure as described in BS 812: Part 2: 1975 and ASTM C 29-91a was used for this test. The unit weight of fine aggregate was found to be 103.47 lb/ft3.

Standard test procedure as described in BS 812: Part 2: 1975 and ASTM C 29-91a was used for this test. The unit weight of coarse aggregate was found to be 98.48 lb/ft3.

Standard test procedure as described in BS 812: Part 2: 1975 and ASTM C 29-91a was used for this test. The unit weight of Fly ash was found to be 44.52 lb/ft3.

The British method of concrete mix design, popularly referred to as the "DoE method", was used for design purpose. After having few trials to check the mix design for the required strength of 5000 psi, the ratio was used as: 1 : 1.25 : 2.50 @ 0.39 w/c ratio.

In this research study total 81 concrete cubes were cast. Among 81 cubes, 9 cubes were made with normal concrete, 36 cubes were made by replacing 25%, 50%, 75% and 100% of fine aggregate by fly ash and 36 cubes were made by replacing 10%, 25%, 50%, and 75% of cement by fly ash. The cubes were 6" x 6" in cross-section, and the mix design was aimed for 5000 psi. After proper curing of all 81 cubes, they were tested at 3, 7 and 28 days curing ages. The cubes were tested in Forney Universal Testing Machine in the Concrete Laboratory of Civil Engineering Department Mehran University Jamshoro.

4. TEST RESULTS AND DISCUSSION

After proper curing of all 81 cubes, these were tested at 3, 7 and 28 days curing ages. The cubes were taken out from the water tank and left for surface saturated drying condition. The cubes were then tested in Forney Universal Testing Machine in the Concrete Laboratory of Civil Engineering Department Mehran University Jamshoro. The test results of all the concrete cubes are summarized in Table 2, whereas their graphical presentation is shown in figure 1.

01.

Cubes made with Normal cement concrete

3241

4594

5129

02.

Cubes made by replacing 25% of Fine Aggregate by Fly ash

3033

4287

4896

03.

Cubes made by replacing 50% of Fine Aggregate Fly ash

3128

4381

4975

04.

Cubes made by replacing 75% of Fine Aggregate by Fly ash

3377

4561

5041

05.

Cubes made by replacing 100% of Fine Aggregate by Fly ash

3608

4614

5197

06.

Cubes made by replacing 10% of Cement by Fly ash

3146

4409

4989

07.

Cubes made by replacing 25% of Cement by Fly ash

2925

4428

5076

08.

Cubes made by replacing 50% of Cement by Fly ash

1872

1931

2283

09.

Cubes made by replacing 75% of Cement by Fly ash

1038

1090

1323

The compressive strength of concrete cubes made by replacing 100 % Fine aggregate by Fly ash was higher than the concrete cubes made by Ordinary Portland Cement at all 3, 7 and 28 days curing ages as shown in Figure 1. The compressive strength of concrete cubes made by replacing 75 % of Fine aggregate by Fly ash was higher at 3 days but it was slightly lower than the O.P.C made normal cubes at 7 and 28 days as presented in Figure 2.

The compressive strengths of concrete cubes made by replacing 10 % and 25 % cement by Fly ash were slightly lower than the concrete cubes made by Ordinary Portland Cement at all curing ages. The compressive strengths of concrete cubes made by replacing 50 % and 75 % of cement by Fly ash were quite lower than the concrete cubes made by Ordinary Portland Cement at all curing ages as shown in Figure 3.

5. CONCLUSIONS AND RECOMMENDATIONS

By analyzing the test results of all the concrete cubes with Ordinary Portland Cement concrete and with different configurations of fly ash made concrete by replacing fine aggregate and Ordinary Portland Cement, the following conclusions have been drawn.

As the compressive strength of concrete cubes, made by replacing 100 % fine aggregate with fly ash is more higher than the concrete cubes, made by using Ordinary Portland Cement and common fine aggregate, therefore the use of fly ash is recommended in plain cement concrete as a replacement of fine aggregate.

As the compressive strength of concrete made by replacing 10% and 25% cement with fly ash is relatively same as made with Ordinary Portland Cement at 7 & 28 days, therefore the use of fly ash as a replacement of Ordinary Portland Cement in plain cement concrete is recommended up to 25 %.

This paper emphasizes on the suitability of fly ash to replace the fine aggregate and cement in plain concrete. Further extensive research is required to use fly ash for design and construction of R.C.C members which may be economical for our construction industry.

The first author is extremely thankful to honorable project supervisor Professor Dr. Ghous Bux Khaskheli, Chairman, Department of Civil Engineering, and Director Post Graduate Studies, Mehran University of Engineering & Technology, Jamshoro, Pakistan, for his encouragement and necessary help at each and every stage of this research work.

ASTM C 618-01, “Standard Specifications for Coal Fly ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete”, Annual Book of ASTM Standards, 2001.

ACI Committee 116, “Cement and Concrete Terminology”, ACI 116R-90”, American Concrete Institute , Farmington Hills, MI, pp. 46, 1990

ASTM C618-92a., "Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as Mineral Admixture in Portland Cement Concrete”, American Society for Testing and Materials, Annual Book of ASTM Standards, Vol. 04.No. 02, West Conshohocken, Pennsylvania. 1994

V.S. Rama Chandram (1996), Concrete admixture Hand book Properties, science, and Technology, IInd Edition, pp. 657-680

Memon Amanullah, ”Experimental Study For Utility of Fly Ash of Lakhra Coal Plant as a Structural Concrete Construction Material”, ME Thesis, Department of Civil Engineering, Mehran University of Engineering & Technology, Jamshoro, Pakistan. 2004.

Sott, Allan N; Thomas, Micheal DA, “Evaluation of Fly ash from Co-Combustion of Coal and Petroleum Coke for use in Concrete”’, ACI Materials Journal Vol. 104, No. 1, pp. 62-70, Jan-Feb 2007,.

A.Oner, S.Akyuz and R.Yildiz, “An experimental study on strength development of concrete containing fly ash and optimum usage of fly ash in concrete”, Science Direct Cement and Concrete Research 35, pp. 1165-1171, 2005.

Kejin Wang, Alexander Mishulovich and Surendra P.Shah , “Activation and Properies of Cementitios Materials Made with cement-kiln dust and class F fly ash” Journal of Materials in Civil Engineering, ASCE, pp. 112-119, January 2007.

Akhtar Naeem Khan, Attaullah Shah and Qaiser Ali , “Use of Fly Ash as cementitious material in Concrete”, Research Journal of Engineering and Applied Science, N.W.F.P University of Engineering & Technology Peshawar, Pakistan, Vol.20, No.1, pp.37-45 Jan-June 2001.

Pathan Amjad “Study of Fly ash made Mortar Concrete”, ME Thesis, Department of Civil Engineering, Mehran University of Engineering & Technology, Jamshoro, Pakistan, 2007

Memon, F.A., “Experimental Study of Fly ash of Lakhra Coal Power Plant in RCC Beams”, M.E Thesis, Department of Civil Engineering, Mehran University of Engineering & Technology, Jamshoro, Pakistan, 2007.

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