Effect of Depth on Shear Capacity of Transfer Beam in High Rise Building Ppt

8/12/2019 Effect of Depth on Shear Capacity of Transfer Beam in High Rise Building Ppt

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Effect of Depth on Shear capacityof Transfer Beam in High rise

building

By- Kalpesh Patel

SD0308


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Index

Introduction to transfer beam

Study on shear capacity of RC beam

Experimental Programme

Material properties Test Procedure

Test Result And Discussion

Conclusions


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What is Transfer beam?

To provide for functional requirement of large columnfree space in high rise building ,the RC column are

placed at the periphery of the built-up plan area.

With a view to developing high flexural and torsionalstiffness ,these column are very closely spaced and

connected through very stiff beam ,called as Spandrel

beam.

These closely spaced columns at the periphery,however, pose hindrance to the movement of people and

the goods at the ground floor and basement levels.


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To fulfill this requirement , the columns at these floorlevels have to be placed at larger spacing.

As a result an interface has to be provided between the

closely spaced columns of the upper floor and the widelyspaced columns at the ground or basement level. This

interface has to be a horizontal RC element and , hence,

is referred as Beam.


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Conventionally , a beam is taken to be a flexural member

of the structural system. The above mentioned interfacebeam, dose not behave as a flexural member ,since itgets sandwiched between closely spaced upper columnsand a little widely spaced supporting columns below it.

Also to transfer the high magnitude of loads collectedfrom all the upper floor of a high-rise building , the depthof the interface beam has to kept much higher than theconventional beams , ranging from 1 m to 4.5 m.


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As the result of this , the load transfer mechanism

through this beam becomes altogether different from theconventional mechanism of flexure , and failure of such

beam is in brittle mode.

Such a beam is referred to as TRASFER BEAM


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It is therefore , required to generate the shear capacityof transfer beams.

Figure shows a 25 storey building having Transfer beamof 1.7 m depth provided at ground floor level to create aspace for free movement of people and for parkingpurpose.

In spite of its wide structural application , only a fewnational codes include their design.

For example , the British standard BS 8110 for structuraluse of concrete explicitly state that, for the design ofdeep beams , reference to be made to specialistliterature


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Similarly,Euro-2 for design of concrete structure statethat itis not apply to the deep beam.

Consequently, there is no specific , unified , and rational

design procedure available in the codes of thesecountries.

I describe the experimental studies conducted on beam

of different depth to investigate the effect of depth onshear capacity of transfer beam in high rise buildings.


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Study on shear capacity of RC beam

What is shear span?

it is the horizontal distance between load on top of

beam and support.


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The study on beam related to varying shearspan todepth ratio (a/d) has been carried out by varying the depth

(d) and not by varying the shear span (a).

This has been done to achieve flow of applied loadthrough the entire depth of body of concrete.

This allows the concrete to develop stress over the fulldepth of beam which varies non-linearly across the depth.

This aspect is very distinct, important and relevant from

view points of the structural response of the beams.


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It is to be particularly noted that the same value of a/dcan be attained by varying the shear span (a) which iseasy to implement since it simply demands shifting of thetop loading points toward the supports.

This way of varying the shear span-to-depth ratio dosenot result in the true structural response as in a deepbeam to be used as a transfer beam.

The failure patterns are significantly different in the twosets of beams , one obtained by varying the depth (d) ,and second by varying the shear-span (a) having thesame a/d ratio.

The primary objective of the research is to build-up shearresisting capacity in RC beams by increasing the depth.


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Experimental Programme

Shear strength of concrete is evaluated experimentally.

This is so primarily due to interlocking of the coarseaggregates among themselves.

As a result of particle interlocking , it is not feasible toapply a shearing action ( direct shearing force) in a plane ,as is customarily done in the case of metals.

Experiments have to be , therefore , devised to indirectlyasses the shear strength of concrete.


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In one of the popularly adopted devices, a beam of

appropriate length is subjected to shearing and

bending action under 4 points loading system (2-

active and 2-passive forces) as shown in fig.


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Beam subjected to be constant shear is refereed to asshear span , which offers itself to be studied forperformance under shear , bending being negligible forshort shear span.

In the present study, such a device has been adopted tostudy the performance of concrete under direct shearing

action.

Once steel bars are introduction generally along adirection perpendicular to shearing force , these bars startcoming in to action to resist shearing force.


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Thus , steel bars becomes essential linked to resisting

shearing force along with the inherent concrete resistance.

The shear resistance due to these longitudinal steel barsis commonly referred as dowel effect.

The primary design variable was depth in term of shearspan-to-depth ratio.

The study aims to investigation the effect of depth ofbeams in terms of shear span-to-depth ratio on shearstrength of concrete.


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Material properties

The test specimens were cast using cement , fine

aggregate, coarse aggregate , water and

susperplasticizer. The materials, in general, conformed to

the specification laid down in the relevant Indian Standard

Codes. For grading of fine and coarse aggregate , sieve

analysis was carried out.


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Cement:

ordinary portland cement of 43-grade conforming toIS:8112:1989 was used throughout the experimentalwork.

All test were carried out as per IS:4031-1988

The specific gravity and finess respective were 3.14 and275 m2/kg


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Coarse aggregate:

Crushed granite obtained from a local source was usedas coarse aggregate and maximum size used was 20

mm along with 10 mm size.

Fine aggregate:

Locally available sand was used as fine aggregate. The

specific gravity was 2.6 and fineness modulus was 2.29.


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Superplasticizer:

A modified melamine based highly effective high range

water reducing concrete admixture was used throughout

the investigation. It was dark brown in colour having 1.22

specific gravity.

Reinforcing steel rebar:

Thermo mechanically treated (TMT) rebar of Fe 415grade was used.


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Concrete Mix Design

The concrete mix were designed in accordance with the

Indian standard recommended method of concrete mix

design (IS 102621982).

The concrete mix table was prepared for 400kg/m3

cement content.


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Table-1: Concrete Mix properties

Sr. No. Description Value

1 Cube compressive strenth (MPa) 43.00

2 Cement (kg) 24.50

3 Fine Aggregate (Send) (kg) 44.50

4 Coarse Aggregate (kg) 81.00

5 Water - Cement Ratio 0.32

6 Water (Litres) 7.80

7 Plasticizer as % of wt. of cement 0.85

8 Mix Proportion 1:1.82:3.31


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Test specimen


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Twelve specimens were designed and fabricated. The

span of beam kept constant at 1 m with 0.1 moverhanging on either side of the supports.

The spacing between the top two pointsloads has been

kept at 200 mm as shown in fig . The depth of the beamvaried at 150 , 200, 250, 300, 350, and 400 mm to

achieve desired six shear span to depth ratio (a/d ratios).


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All beam were rectangular in cross section, 100 mm

wide .

Standard cubes (150 mm X 150 mm X 150 mm), cylinder

(150 mm X 300 mm), were cast with each mix to know

the various mechanical properties of concrete.


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Test Procedure

After 28 days curing period, the beam specimens were

removed from the curing tank and both sides of the beam

were white-washed to aid observation of crack

development during testing.

Load was applied gradually with the help of jack and

deflection of proving ring was recorded to find the failure

load.


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All the beams were tested to failure under four-point

loading test set-up (2-active, 2-passive) as shown in fig-


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The cube specimens were tested for compressive

strength, the cylinder specimens for split tensile strength.

Crack in beam


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Test Result And Discussion

The result obtained from the experimental investigation

are shown in table-2.


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All beam specimens failed in shear i.e. a sudden failure

without warning , loud noise at failure with the

appearance of single shear crack in the shear span andfine flexural cracks in the middle portion of the beam.

The shear crack crosses the compression zone of the

beam. Figure shows typical crack pattern for RC beamspecimens of different a/d ratio.


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1.for depth D=150 mm




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From, the result obtained , the effect of depth of beam in

term of shear spantodepth ratio on shear strength of

concrete are analyzed and discussed as follows.


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Effect of depth of beams in terms of

shear span-to-depth (a/d) ratio

The shear strength of concrete beams for different

depths at 28 days curing age and the variation of shear

strength with different shear span-to-depth ratio is shownin table-2.

It is evident from the plots that high shear strength is

developed at lower value of span-to-depth ratio and theshear strength decreases at higher value of span-to-

depth ratio.


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0.75

1.25

1.75

2.25

2.75

0.25

1.1 1.23 1.45 1.78 2.29 3.2

Shear spantodepth ratio

Shearstress(MPa)


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Figure shows the effect of shear span-to-depth ratio onnominal shear stress at diagonal cracking , which isobtained by dividing measured failure load to the nominalcross sectional area (bXd).

As the shear span-to-depth (a/d) ratio decreases, theshear strength increases .

The increase in shear strength is significant in RC beamspecimens with a/d ratio less than about 1.78 (AI-0.8/1.10, AII-0.8/1.23 , AIII-0.8/1.45) , because a

significant portion of the shear is transmitted directly tothe support by an inclined strut.


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This mechanism is frequently referred to as arch action

and the magnitude of direct load transfer increases with

decreasing a/d ratio.

The shear strength of RC beam with a/d ratio less than

1.78 is higher than those of RC beam with a/d ratio more

than 1.78.

This result is due to the beneficial effect of direct load

transfer to the support by arch action or so called strut-

and-tie load transfer mechanism.


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The transition point between the arch action and beam

action ( or transfer beam and normal beams) lies

between a/d ratio 1.45 to 1.78.

Either side of this a/d ratio , behaviors of RC beams in

term of load resisting mechanism , failure pattern and the

noise at failure , were entirely different.


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Conclusions

The primary objective of the research was to built-upshear resisting capacity in beam by increasing the depthof the experimental result and the nature of variation of

shear strength against independent parameters , thefollowing broad conclusions are arrived at:

1.shear resisting capacity of beam significantly dependsupon the shear span-to depth ratio (a/d)


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Beam with higher value of a/d (a/d > 1.8) exhibitincreasing influence of moment and hence developflexuralshear crack in the tension zone.

Such beam may be categorized as normal beams(flexural beams) and have a/d ratio in excess of 1.8

2.shear resisting capacity of beam in the lower range of a/dratio (a/d


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The beams fail in sudden splitting mode with increasingloud sound. Sudden splitting with loud sound is a

measure of brittleness.

This may be taken as a transition zone between a normalbeam and a deep beam.

3.beams with a/d


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This is of particular interest since transfer beam fall inthis rang of a/d ratio.

While the beam shows very high shear resistance , theytend to fail in highly brittle mode.

This mode is characterized by sudden failure in shear

with very loud sound .

This mode of failure is considered undesirable and isbranded as treacherous.


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References:

Building code requirement for structural concrete and commentary

(ACI-318)

Structure use of concrete-part 1:code of practice for design and

construction (BS-8110)

Civil engineering and construction review magazine.

IS : 456-2000

Reinforced concrete design : by H.J.Shah


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Effect of Depth on Shear Capacity of Transfer Beam in High Rise Building Ppt

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