PARAMETRIC STUDY OF EFFECT OF COLUMN SHAPES ON EARTHQUAKE RESISTANCE OF BUILDINGS

BY: KARAN SHAH KATHA DESAI

GUIDE :Dr. A.S SANTHI

INTRODUCTION

  It is estimated that by the year 2025 more than 5500 million people will

live in cities. Astoundingly, a significant fraction of the largest of these megacities is

located close to regions of known geological hazard, the earthquake. The statistics show that the risk of earthquake damage to lives and property

has more than quadrupled in the past decade, because of rise in megacities.

global level debate to reduce the carbon emissions in which cement ranks 2nd most hazardous material on the list.

need to build structures which are optimum in performance under seismic loads and have a reasonable construction cost and have carbon free credentials.

We experimented with the size and shape of column one of the most important links in load transfer for the structure keeping the volume of concrete same. The structure is to be tested with lateral loads and the percentage of reinforcements shall be calculated.

Our small attempt towards making globe a better and a safer place to live.

ABSTRACT The present study was carried out to study the effect of column

shape on the seismic resistance of a RCC framed structures for Ground + 10 upper stories which consisted of rectangular shaped columns as against equivalent square and circular shaped columns.

Static equivalent seismic analysis was carried out for each of the two categories of buildings

i) having three, 4m x 4m panels in both lateral directions forming a plan area of 12m x 12m

ii) having four 4m x 4m panels in one direction and three 4m x 4m panels forming plan area of 16m x 12m.

All the beams were considered to be of 230mm x 450 mm in size and the columns were of 3m height in each floor. The columns extended upto 2m below the ground level. Cross section of columns was considered as 450mm x 300mm for rectangular 370mm x 370mm for equivalent square columns and 415mm dia for circular.

Analysis was carried out using STAAD Pro and results were compared.

It was concluded that there is significant decrease in steel reinforcement requirement for square and circular cross section of columns for both buildings when tested under seismic loads. The quantity of steel required and concrete volume consumed is evaluated and the structural cost is compared.

It is concluded that the square cross section of columns give a saving of about 7.7% and circular shape of columns show a saving of about 7.8% as that against a rectangular column shape for a square plan building. The same values works out to be 7.6% for square and 7.3% for circular for the rectangular shaped building

•MODEL-1•MODEL-2•DIMENSIONS OF STRUCTURAL ELEMENTS

GEOMETRIC DEFINATION

MODEL-1•plan area of 12m x 12m with a three panel grid of 4m x 4m in both lateral directions.

•The space frame thus formed consists of nine columns placed at each panel points.

•The columns were 3m in height for each storey and are considered to extend for 2m below plinth level upto foundation which was considered to be fixed.

MODEL-2• Plan area of 16m x 12m with three panel grid of 4m x 4m in one direction and four panel grid of same dimension in other direction.

•The space frame thus formed consists of twelve columns placed at each panel points.

•The columns were 3m in height for each storey and were considered to extend for 2m below plinth level upto foundation which was considered to be fixed.

DIMENSIONS OF STRUCTURAL ELEMENTS

Both the models had been analysed with the following dimensions of structural elements.

Beam size for all models was fixed as 230mmx450mm. All external wall of thickness 230mm All floor slabs of thickness 130mm Rectangular column size was taken as 300mmx450mm. Equivalent square columns were taken as

370mmx370mm. Equivalent circular columns were taken as 415mm in dia.

MODELLING

MODELLING All the supports and end of columns considered fixed in

both types of models Seismic load in the two lateral directions was considered

as per the equivalent static load method according to Indian standard IS:1893- 2002.

The structure was analyzed for Zone IV with zone factor 0.24.

Response reduction factor(IR) of 5 Importance factor (I) of1. Rock soil type was taken as medium soil with factor 2. Damping ratio was taken as 5% .

Typical Floor load 1. dead load 4.3 kN/sq.m2. live load 2 kN/sq.m 3. wall load 11.75 kN/m Terrace Floor load 1. dead load 6.05 kN/sq.m (to account for water

proofing).2. live load 1.5 kN/sq.m 3. wall load 4.15 kN/m (parapet wall.) Analysis was carried out for 13 load combinations that

were defined based on recommendations as given in IS 456, IS 1893 and IS 13920.

DESIGN

DESIGN The static analysis was carried out for the given dead and live loads. The design was carried out by the most critical load combination of

1.5 x D.L + 0.9 x EQ in +X,-X,+Z,-Z directions respectively for both, rectangular and square model.

The following parameters were considered for design 1. Grade of concrete M25 2. Grade of steel Fe415 3. Clear cover 0.02 m 4. Maximum main reinforcement for column 25 mm 5. Maximum main reinforcement for beams 20 mm 6. Diameter of ties for beams and columns 8 mm. The Ast provided was checked with the Ast required for all corner and

centre columns. The columns were designed considering equal reinforcement on all four sides for the square and rectangular shape of columns. Only one bar diameter was used for providing the vertical steel in columns from practical point of view.

RESULTS

TABLE-1 COLUMN SHAPES

STEEL(TONNES) CONCRETE(M3)RECTANGUAR 20.54 194.83 1232400SQUARE 18.959 195.9 7.70 1137540 94860CIRCULAR 18.929 232.64 7.84 1135740 96660

COST SAVINGS COMPARED TO RECTANGULAR

COLUMN(Rs.)

SQUARE BAY%REDUCTION OF STEEL

COMPARED TO RECTANGULAR COLUMN

TOTAL COST OF STEEL (Rs.)

TABLE-2

COLUMN SHAPESSTEEL(TONNES) CONCRETE(M3)

RECTANGUAR 25.905 248.51 1554300SQUARE 23.941 249.84 7.58 1436460 117840CIRCULAR 24.013 274.57 7.30 1440780 113520

%REDUCTION OF STEEL COMPARED TO

RECTANGULAR COLUMN

TOTAL COST OF STEEL (Rs.)

COST SAVINGS COMPARED TO RECTANGULAR

COLUMN(Rs.)RECTANGULAR BAY

OBSERVATIONS

OBSERVATIONS From Table 1, it can be observed that square shaped columns have a definite

advantage over rectangular columns in terms of steel consumption for a regular RC building having 3 panels of 4mx4m in each lateral direction making an overall plan size of 12m x 12m.

The Table 1 also shows that the circular shape of columns is equally good as the square shape and performs almost the same as a square shaped column.

The advantage of a square shaped column is slightly reduced from 7.7% to 7.58% when the overall size of the building is changed to a rectangular one of 12m x 16m as seen in Table 2.

Table 2 also indicates that a circular shape of column is performing almost the same as a square column with a marginal difference of 0.28% for a rectangular building.

It is also observed from the tables that the structural cost of both the G+10 storey buildings can be reduced by nearly Rs. 1,00,000 by simply adopting a square or circular shape of column instead of rectangular shape which is the usual practice.

CONCLUSIONS It can be concluded that the seismic

performance of RCC framed structures can be improved by providing square or circular shaped columns (equal stiffness members in the principle lateral directions) instead of the rectangular columns.

LIMITATIONS The above conclusion is drawn for only symmetric plan shaped

buildings without any reduction in the column size.

The building is subjected to only equivalent static method of lateral force generation which may not be the case under actual seismic forces.

The inclusion of infill walls may cause a deviation in the behavior of the structure which is not modeled in the current analysis.

Geometric nonlinearity like the P-delta effect and large deformations are not taken into account in the present study.

REFERENCES Bimal Shah and Jignesh Shah (2004), Performance Based

Seismic Evaluaton of RCC Framed Structures for Symmetric Plan Buildings, Proceedings of Asia Conference on Earthquake Engineering ACEE 2004, Manila, Philippines. Vol-1, pp 165-175.

Diptesh Das and C.V.R. Murty (2004), Brick Masonary infills in seismic design of RC frame buildings: Part 2 – Behaviour, The Indian Concrete Journal ICJ, August 2004.

Naeim, F. (2001) , The Seismic Design Handbook second edition , Kluwer Acedemic Publishers

Indian Standard Code of Practice IS-1893(Part 1) (2002).Criteria for Earthquake Resistant Design Of Structures Part 1 General Provisions and Buildings, Fifth revision.

Chopra A. (2002) Dynamics of Structures : theory and applications to earthquake engineering, second edition, Prentice-Hall of India Private Limited, New Delhi.

Indian Standard Code of Practice IS-456 (2000). Plain and reinforced Concrete – Code of Practice (fourth Revision).

ACKNOWLEDGEMENTS OUR SINCERE THANKS TO A.S SANTHI

MADAM FOR HER VALUABLE TIME AND HER PRECIOUS INPUTS IN THE PROJECT.

THANKYOU