3D Analysis and Design of a multi storey building

3D Analysis and Design of a multi storey building

10

Fig2.Working plan

1.2Project statement:


11

Salient features:

Utility of the building: Residential complex

Shape of building : Apartments

Number of storeys: G+4

Type of construction : R.C.C Framed Structure

Type of walls : Brick wall

Geometric details:

Ground floor height : 3.5m

Floor to floor height : 3.5m

Height of plinth : 1.5m

Depth of foundation : 20cm

Materials:

Concrete grade : M25

All steel grades : Fe415

Bearing capacity of soil:20ton/m^2

1.3 STAAD. Pro:

• STAAD is the powerful design software licensed by Bentley. STAAD stands for

Structural Analysis and Design

• STAAD is comprehensive structural engineering software that addresses all aspects of

structural engineering as analysis, design, verification, and visualization.


12

• STAAD performs the analysis and design of the structure for different types of

structures, such as trusses, plane and space.

• STAAD has ability to perform design of members following more than 20

international building codes

• Alternatives for STAAD:

Struts,robot,sap,adds pro gives details very clearly regarding reinforcement and

manual calculations. But these software‘s are restricted to some designs only whereas

staad can deal with several types of structures.

• Limitations of the STAAD:

1. Huge output data

2. Even analysis of a small beam creates large output data

3. Unable to show plinth beams.

1.4 Excel:

Microsoft Excel is a spreadsheet tool capable of performing calculations, analyzing

data and integrating information from different programs. Microsoft Excel is comprised of

organizational units called workbooks. A standard workbook contains worksheets and chart

sheets. Worksheets perform calculations, store and organize data, present graphics and

controls like a web page; they are extremely versatile. A worksheet in turn is comprised of

millions of cells. The job of a cell is to store a formula that performs a calculation or

communicates with some other application (i.e. program) such as a database. They also store

and present data. A chart sheet's job is to present a chart or graph developed from data stored

on a worksheet.

IMPORTANCE OF HAND CALCULATIONS

The students have been asked to perform hand calculations. Computer analysis

and

design programs offer great benefits to the design engineer. However, the computer

programscan be easily misused without proper precautions in analysis and design procedures. If the design of any structure is based on the results obtained from erroneous computer analysis, it can lead to structural failures, costly disputes and poor performing structures. Performing the following procedures can eliminate many of the errors.

1. Model the structure as closely to the real structure as possible.

2. Recognize the important structural reactions


13

3. Check the input and understand the material behaviour and boundary conditions 4. Perform simple equilibrium and compatibility checks using hand calculations.

5. Know and understand the limitations of the software.

A series of hand design calculations were performed on a typical slab panel, a

randomly

selected set of three beams and columns, one critical footing supporting the

highest column load in the structure, and a typical combined footing. The purpose of the hand design calculation was to verify manually, the analysis from the STAAD software package, and the Indian code driven RC Design Suite programs.

2. STRUCTURAL PLANNING

2.1. Introduction:

Engineering is a professional art of applying science to the efficient conversion of natural

resources for the benefit of the mankind. Engineering, therefore, requires above all creative

imagination to innovate useful application for natural phenomenon.

The entire process of structural planning and design requires not only imagination and

conceptual thinking but also sound knowledge of science of structural engineering besides

knowledge of practical aspects, such as recent design codes and bye laws, backed up by

ample experience, intuition and judgment. It may be clarified that Code of practice, which is

commendable of good practices drawn up by good experienced engineers, should never be

allowed to replace the conscience and competence of the engineer. The purpose of standards

is to ensure and enhance the safety, keeping careful balance between economy and safety.

The process of design commences with planning of the structure, primarily to meet its

functional requirements. Initially, the requirements proposed by the client are taken into

consideration. They may be vague, ambitious or even unacceptable from engineering point of

view because he is not aware of the various implications involved in the process of planning

and design, and about the limitations and intricacies of the structural science.

It is emphasized that any structure to be constructed must satisfy the need efficiently for

which it is intended and shall be durable for its desired life span.


14

2.2. Structural Planning:

After getting an architectural plan of the buildings, the structural planning of the building

frame is done. This involves determination of the following:

(a) Positioning and orientation of columns.

(b) Positioning of beams

(c) Spanning of slabs

(d) Layout of stairs

(e) Selecting proper type of footing

The basic principle in deciding the layout of component members is that the loads should be

transferred to the foundation along the shortest path.

2.2.1. Positioning and orientation of columns :

Following are some guiding principles which help in deciding the column positions.

• Columns should preferably be located at or near the corners of the building and at the

intersections of the beams/walls.

• Since the basic function of the columns is to support beams which are normally

placed under the walls to support them, their position automatically gets fixed. The

commercial buildings have normally rectangular pattern of grid type but especially for

residential buildings.The said type of pattern for columns does not become possible.

• Select the position of column so as to reduce the bending moments in beams, when

the location of two columns are very near then one column should be provided instead

of two at such a position so as to reduce beam moment.

• Under certain rare circumstances to satisfy the functional requirements, it may not be

possible to provide upper storey columns above the columns at the parking level.

Then the column at parking level is required to support the eccentric columns at the

upper storeys. In such a case the column S at parking level is splayed or provided with

a bracket to support the columns at the upper storey. However, the column at parking

level will be subjected to heavy concentrated loads transferred from the columns of

the upper storeys.

Avoid larger spans of beams.


15

• When the centre to centre distance between intersections of walls is large or where

there are no cross walls, the spacing between two columns is governed by limitations

on spans of supported beams, because spacing of columns decides the span of the

beam. As the span of the beam increases, the required depth of the beam, and its self

weight, and total load on beam increases. It is well known that the moment governing

the beam design varies with the square of the span and directly with the load. Hence

with the increase in span, there is considerable increase in the size of the beam.

• On the other hand, in the case of column, the increase in total load due to increase in

length is negligible as long as the column is short. Therefore the cost of the beam per

unit length increases rapidly with the span as compared to that of column. Columns

are, therefore in general, always cheaper compared to beams on the basis of unit cost.

Therefore, large spans of beams should preferably be avoided for economy reasons. In

general, the maximum spans of beams carrying live loads up to 4 KN/m2 may be

limited to following values.

Avoid large centre to centre distance between columns.

• Larger spacing of columns not only increases the span and the cost of beams but it

increases the load on the column at each floor posing problem of stocky columns in

lower storey of a multi-storey building. Heavy sections of column lead to offsets from

walls and obstruct the floor area.

Columns on property line

The columns on property line need special treatment. Since column footing requires certain

area beyond the column, difficulties are encountered in providing footing for such columns.

In such cases, the column may be shifted inside along a cross wall to make room for

accommodating the footing within the property line. Brackets may be taken out from the

column in continuation of cross beams to support walls along the boundary line.

Alternatively, a combined footing or a strap footing may be provided.

Orientation of columns:

• Avoid projection of column outside wall.

• According to requirements of aesthetics and utility, projections of columns outside the

wall in the room should be avoided as they not only give bad appearance but also

obstruct the use of floor space, and create problems in placing furniture flush with the

wall.


16

• Provide depth of the column in the plane of the wall to avoid offsets. The problem of

projection of column normally occurs in the internal walls sincethey are usually

thinner. Now-a-days 150 mm thick walls are provided to get more floor space. This

has posed problem for external walls too, because the width of wall is required to be

kept not less than 200 mm to be to prevent column from being slender.

• Use L shaped columns at the corners or T shaped corners at the intersection of

intermediate cross walls. Alternatively spacing of the columns should be considerably

reduced so that the load on column at each floor is less and the necessity of large

sections for columns does not arise.

• Orient the column so that the depth of the column is contained in the major plane of

bending.

Bearing in mind the guiding points given above, the principles governing orientation of

columns given below can easily understood.

• When a column is rigidly connected to beams at right angles it is subjected to

moments in addition to the axial load. In such cases column should be oriented that

the depth of the column is perpendicular to major axis of bending so as to get larger

moment of inertia and hence greater moment resisting capacity it will also reduce

Leff/D ratio resulting in increase in the load carrying capacity of the column.

• It should be born in mind that increasing depth in the plane of bending not only

increase the moment carrying capacity but also increases its stiffness, thereby more

moment is transferred to the column at the beam column junction.

However, if the difference in bending moment in two mutually perpendicular

directions is not large as the depth of the column has sufficient strength in the plane of large

moment. This will avoid offsets inside the rooms.

2.2.2. Positioning of beams :

Following are some of the guiding principles for positioning of beams.

• Beams, shall, normally be provided under the walls to avoid the loads directly coming

on slabs.

• Since beams are primarily provided to support slabs, its spacing shall be decided by

the maximum spans of slabs. Slab requires the maximum volume of concrete to carry

a given load. Therefore the thickness of the slab is required to be kept minimum. The


17

maximum practical thickness for residential/office/public buildings is 200 mm and

minimum is 100 mm.

• Avoid maximum spacing of beams from deflection and cracking criteria.

• It is well known that the deflection varies directly with the cube of the span and

inversely proportional with the cube of depth i.e. L3/D3. Hence larger span may be

avoided

• However for larger spans, normally higher L/D ratio is taken to restrict the depth from

considerations of headroom, aesthetics and psychological effect. Therefore spans of

beams which require the depth of beam greater than one meter should as far as

possible be avoided.

2.2.3. Spanning of slabs:

• When the rectangular slab is supported at its four edges, it acts as one way slab when

(ly/lx)>2 and two way slab for (ly/lx) <2. However the two way action of slab not only

depends on the aspect ratio (ly/lx) but also on ratio of reinforcement in the two

directions.

• A slab with (ly/lx)>2 is designed as one way, since in that case one way action is

predominant. In one way slab, main steel is provided along the short span only and

load is transferred to two opposite supports only. The steel along the long span just act

as distribution steel and is not designed for transferring the load but to distribute the

load.

• A two way slab having aspect ratio (ly/lx) <2 is general economical compared to one

way slab because steel long both the spans acts as main steel and transfer the load to

all its four supports.

• The two way action is advantageous essentially for large spans and for live loads

greater than 3 KN/m2. For short spans and light loads, steel required for two way slab


18

does not differ appreciably as compared to steel for one way slab because of the

requirement of the minimum steel.

Fig3. Positioning of beams and columns


19

2.2.4 Layout of stairs:

Initially it is necessary to know the different parts of stairs and guide lines for fixing their

dimensions. The guidelines for fixing dimensions of the component parts of stairs are as

under:

• The rise R should not be more than 200 mm and tread T not less than 200 mm. For

residential buildings rise R lies between 150 mm to 180 mm and tread T between 220

mm to 250 mm. For public buildings rise R lies between 120 mm to 150 mm and

tread between 250 mm to 300 mm.

• The sum of tread plus twice the rise should be between 500 mm 650 mm.

The width of the stairs is dependent on its usage and shall be such as to avoid

overcrowding. For residential buildings the width of the stairs should be between 0.8

m to 1mfor public buildings width of the stairs should be between 1.8 m to 2 m

• The width of the landing should not be less than width of stairs.

• For comfortable ascend on stairs, the number of steps on each flight should not be

greater than 12.

• The pitch of the stair way should not be greater than 38 degrees.

• The head room measured vertically above any step or below mid landing shall not be

less than 2 m

• Avoid winders as far as possible.

The type stairs and its layout is governed by the available size of staircase room and the

positions of beams and columns along the boundary of the staircase.

2.2.4. Choice of footing type:

The type of footing depends upon the load carried by the column and bearing capacity of the

supporting soil.Even under small building the soil may vary from soft clay to hard morrum.

• For framed structures under study isolated column footings are normally preferred.


20

• In case of soils with very low bearing capacities or if black cotton soil exists for great

depths pile foundations can be an appropriate choice.

• If columns are very closely spaced and bearing capacity of the soil is low, raft

foundation can be an alternative solution.

• For a column on the boundary line, a combined footing or a strap footing may be

provided

Fig4.Figure showing the staad generated model.


21

3. ANALYSIS OF FRAMED STRUCTURE

3.1 Types of Loads :

The loads are broadly classified as vertical, horizontal and longitudinal loads. The vertical

loads consist of dead load, live load and impact load. The horizontal load comprises of wind

load and earthquake load. The longitudinal load (viz. Tractive and braking forces are

considered in special cases of design of bridges, design of gantry girder etc.)

3.1.1. Dead Load(DL):

Dead loads are permanent or stationary loads which are transferred structure throughout their

life span. Dead load is primarily due to self weight of structural members, permanent

partition walls, fixed permanent equipment and weights of different materials.

Dead load calculations:

o Dead load:

Slab weight: 0.120*25 = 3 KN/m2

Floor finishes: =1.50 KN/m2

Beams under external walls 0.230*3.13*20 =14.4KN/m


22

Fig5.figure showing assigning of dead loads to the structure

3.1.2. Imposed load or Live loads(LL):

Live loads are either movable or moving loads without any acceleration or impact. These are

assumed to be produced by the intended use or occupancy of the building including weights

of movable partition or furniture etc.

The floor slabs have to be designed to carry uniformly distributed loads or concentrated

loads whichever produce greater stresses in the part under considerations. Since it is unlikely

that any one particular time all floors will not be simultaneously carrying maximum loading,

the code permits some reduction in imposed loads in designing columns, load bearing walls,

piers, their supports and foundations.

o Live load:

2.00 KN/m2 on all typical floors

1.50 KN/m2 on the terrace,3.00 KN/m2 on Stair case


23

Fig6.figure showing assigning of live load to the structure

3.1.3 Wind load(WL):-

WIND LOAD is defined as the load on a structure due to windintensities. Generally wind

intensities vary from time to time. So it is suggested to take maximum probable wind

intensities calculation for a structure for which damage can be avoided.

WIND LOAD CALCULATION:

According to IS CODE (875 PART 3), Vz =Vb×K1×K2×K3

Where Vz=design wind speed at a height z meter in m/s.

Vb=basic design wind speed at 10m height. For example Vb is 50 m/s for cities like Cuttack

and

Bhubaneswar and 39 m/s for Rourkela.K1, K2, K3 can be calculated from the IS-CODE(875


24

part3).

Pz=Design wind pressure at a height z meter.

Pz=0.6V z2

3.1.4 Seismic load (SL):

Seismic load can be calculated taking the view of acceleration response of the ground to the

super structure.

According to the severity of earthquake intensity they are divided into 4 zones.

1. Zone I and II are combined as zone II.

2. Zone III.

3. Zone IV.

4. Zone V.

SEISMIC LOAD CALCULATION:

According to the IS-CODE 1893(part 1) the horizontal Seismic coefficient Ah

for a

structure can be formulated by the following expression

Ah=ZISa/2RG

WHERE

Z=Zone factor depending upon the zone the structure belongs to.

For Zone II (z=0.1)

For Zone III (Z=0.16)

For Zone IV (Z=0.24)

For Zone V (Z=.36)

I=Importance factor.

For important building like hospital it is taken as 1.5 and other for other

building it is

taken as 1.


25

R=Response reduction factor.

Sa/g=Average Response Acceleration coefficient.

However it should be notice that the ratio of I and R should not be greater

than 1.

Combination load case:1.5(DL+LL) As the design is for the limit state a

safety factor of 1.5 is considered.

3.2 Design parameters taken in STAAD:

• Horizontal members are designed as beams.

• Vertical members are designed as columns.

• fck = 25 N/mm2

Table1:

Parameter Main reinforcement Secondary reinforcement

fy 415 N/mm2 415 N/mm2

Maximum bar diameter 25 mm 12 mm

Minimum bar diameter 12 mm 8 mm

• Longitudinal reinforcement in column is taken along 4 faces.

• Maximum pt is taken as 6 % in columns.


26

Fig7.Figure showing assigning of wind load(+ve X direction) to the structure


27

Fig8.figure showing assigning of wind load (-ve X direction)to the structure


28

Fig9.figure showing assigning of wind load (+veZ direction)to the structure


29

Fig10.figure showing assigning of wind load (-veZ direction)to the structure

3.3 Analysis of frame:

A multi- storied frame consists of beams and columns whose degree of indeterminacy is

very high. The design of various members such as beams, columns trusses requires

bending moment, shear force and direct thrust at all sections. The bending moment at all

sections of beams or columns are obtained by analysis of frames.


30

Frames are analyzed for vertical and horizontal loads separately final moments are

obtained by super imposing.

Fig11.Deflection diagram


31

Fig12.Bending Moment diagram


32

Fig13.ShearForce diagram

BEAMS:

Beams are the simplest structural elements that are used extensively to support loads.

They may be straight or curved ones. For example, the one shown in Fig. 1.2 (a) is

hinged at the left support and is supported on roller at the right end. Usually, the loads

are assumed to act on the beam in a plane containing the axis of symmetry of the cross

section and the beam axis. The beams may be supported on two or more supports as

shown in Fig. 1.2(b). The beams may be curved in plan as shown in Fig. 1.2(c). Beams

carry loads by deflecting in the same plane and it does not twist. It is possible for the

beam to have no axis of symmetry. In such cases, one needs to consider unsymmetrical

bending of beams. In general, the internal stresses at any cross section of the beam are:

bending moment, shear force and axial force.


33

Fig14. Types of beams

4 DESIGN OF STRUCTURAL MEMBERS

4.1 DESIGN OF SLABS:

Slabs are plate elements forming floors and roofs of building and carrying distributed loads

primarily by flexure. A slab may be supported by beams or walls and may be used as the

flange of a T or L-beam. The common shapes of slabs are square, rectangular, triangular and

circular.

� Slabs are classified mainly into two types:

1. One-way slabs 2. Two-way slabs

4.1.1 ONE-WAY SLABS:

One-way slabs are those supported continuously on the two opposite sides so that the

loads are carried along one direction only, in general when the aspect ratio ly/lx is greater

than 2. The direction in which the load is carried in one-way slabs is called the span. It may


34

be in the long or short direction. One-way slabs are usually made to span in the shorter

direction since the corresponding bending moments and shear forces are the least. The main

reinforcements are provided in the span direction. Steel is also provided in the transverse

direction to distribute any unevenness that may occur in loading and for temperature and

shrinkage effects in that direction. The steel is called distribution steel or secondary

reinforcement. The main steel is calculated from the bending moment consideration and

under no circumstances should it be less than the minimum specified by the code. The

secondary reinforcement provided that, is usually the minimum specified by the code for

such reinforcement.

4.1.2 TWO-WAY SLABS :

Two-way slabs are those slabs that are supported continuously on all four sides and of

such dimensions that the loads are carried to the supports along both directions. In two-way

slabs, the slab is stiffened along both the directions by providing main steel reinforcement

along both the directions. In general slabs are designed as two-way slabs when the aspect

ratio (ly/lx)is less than 2. Generally two-way slabs are economical than one-way slabs.

� The maximum permissible span length of slabs are considered as follows:

IS-

CODE PROVISIONS FOR DESIGN OF SLABS:

As per IS: 456-2000, Code of practice for design of R.C.C structures recommends the

following:

• For frames the effective spans taken as per Clause No.21.2 © of IS:456-2000

• Effective depth is the distance between the centroid of the area of the tension

reinforcement to the top of compression fiber excluding the finishing.

Support

condition Cantilevers

Simply

supported Fixed/continuous

Slab type One way

Two way

One way

Two way

One way

Two way

Max span(m) 1.50 2.00 3.50 4.50 4.50 6.00


35

• When (Ly/Lx) is less than 2, the slab is designed as Two-way Slab, when (Ly/Lx) is

greater than 2, the slab is designed as One-way Slab, as per the coefficients given in

table 22 of IS:456-2000 torsion reinforcement need not be provided at any corner

contained by edges over both of which the slab is continuous.

• Maximum diameter of reinforcing bar shall not exceed the 1/8th of the total thickness

of slab (clause 25.2.2).

• Cover to reinforcement, at each end of reinforcing bar not less than 25mm or less than

twice the diameter of such bar (clause 25.4.1).

• Cover to reinforcement, for tensile, compressive shear or other reinforcement in slab,

not less than 20mm nor less than diameter of such bar.

• Maximum permissible spacing of distribution reinforcement shall not be more than 3

times of the effective depth of a slab or 30cm, whichever is smaller.

• Max permissible spacing of distribution reinforcement shall not be more the 5 times

effective depth of a slab or 45cms whichever is smaller.

• No shear reinforcement should be provided for slabs less than 200mm thick. However

the increased value of shear resistance in slabs can be taken into account in design.

• Minimum reinforcement in either direction in slab shall not be less than 0.15% of

total cross-sectional area. However the value can be reduced 0.12% when HYSD bars

are used (clause25.5.2.1).

• Over the continuous edge of a middle strip the tension of the slab at a distance of

0.15L from the support and at least 50% extended to a distance of 0.3L.

• Steel is calculated from the following :

•

bdbdf

M

fy

fA

ck

uck

st ×

−−=

2

6.411

5.0

Basic Rules followed in the design of simply supported Slab:

Thickness of slab

L/d ratio should be less than the following:


36

• Simply supported slab = 20

• Continuous slab = 26

• Cantilever slab = 7

In any case of the above, the thickness should not be less than 100mm

Effective span

• Distance between centre to centre of support

• Clear span plus effective depth

Minimum main reinforcement

• 0.15% gross c/s of slab – for MS bars

• 0.12% gross c/s of slab – for HYSD bars

Spacing of main bars

The spacing or c/c distance of main bars shall not exceed following:

• Calculated value

• 3d

• 300mm

Distribution or Temperature reinforcement

This reinforcement runs perpendicular to the main reinforcement in order to distribute the

load and to resist the temperature and shrinkage stresses.

It should be at least equal to;

• 0.15% gross c/s of slab – for MS bars

• 0.12% gross c/s of slab – for HYSD bars

Spacing of distribution bars

The spacing or c/c distance of distribution bars shall not exceed the following

• Calculated area

• 5d

• 450mm


37

Diameter of bars

The diameter of the bars varies from 8mm to 14mm and should not exceed 1/8th of the overall

depth of the slab.

For distribution steel, the diameter varies from 6mm to 8mm.

Cover

The bottom cover for reinforcement shall not be less than 15mm or less than the diameter of

such bar.

Fig!5.Figure showing the slabs


38

4.1.1. Results: Design of Slabs (Using Excel Sheets):


65

LOAD COMB 11 COMBINATION LOAD CASE 11

5 1.5 6 1.5

PERFORM ANALYSIS

DEFINE ENVELOP

11 ENVELOP 1

END DEFINE ENVELOP

START CONCRETE DESIGN

CODE INDIAN

FC 25000 MEMB 1 TO 20 22 TO 35 38 TO 42 44 TO 233 235 TO 466

FYSEC 415000 ALL

DESIGN BEAM 1 TO 20 22 TO 35 38 TO 42 44 TO 212 313 TO 364 390 TO 441

DESIGN COLUMN 213 TO 312 365 TO 389 442 TO 466

CONCRETE TAKE

FYMAIN 415000 ALL

END CONCRETE DESIGN

FINISH

4.2.1(a)DESIGN of BEAMS

A concrete beam should be able to resist tensile, compressive and shear stresses

induced in it by the loads on the beam. Concrete is fairly strong in compression but very

weak in tension. Plain concrete beams are thus limited in carrying capacity by the low tensile

strength. Steel is very strong in tension. Thus, the tensile weakness of concrete is overcome

by the provision of reinforcing steel in the tension zone around the concrete to make a

reinforced concrete beam.

There are three types of beams.

a) Singly reinforced beams.


66

b) Double reinforced beams &

c) Flanged beams.

a) SINGLY REINFORCED BEAMS:

In singly reinforced simply supported beams reinforcing steel bars are placed near the

bottom of the beam where they are most effective in resisting the tensile bending stresses. In

singly reinforced cantilever beams reinforcing bars are placed near the top of beam.

b) DOUBLY REINFORCED BEAMS:

A doubly reinforced beam is reinforced both in compression and tension regions. The

section of the beam may be a rectangular, T or L section. The necessity of using steel in the

compression zone arises due to two main reasons as follows:

• When the depth of the beam is restricted the strength available from a singly

reinforced beam is inadequate.

• At support of continuous beam where bending moment changes sign.

c) FLANGED BEAMS:

In most reinforced concrete structures, concrete slabs and beams are cast monolithic.

Thus beam form part of the floor system together with the slab. In bending the slab forming

the top part of the beam at mid span would be in compression for a definable width greater

than the width of the rid (or the beam) thus increasing the moment of resistance for given rib

width. At continuous supports the position is reversed. The slab in tension and part of it have

cracked in tension, this beam is equivalent to rectangular section at the supports

IS CODE PROVISIONS:

• The loading on the beam is taken as per clause 24.5 of IS: 456-2000

• For continuous beam with equal/unequal spans and equal/unequal loaded, the bending

moment is obtained by using matrix displacement method.

• Effective span and effective depth of beam is same as explained in slab provisions.

• The beams at mid span are designed as T – beams and the same steel reinforcement is

provided for all beams and the reinforcement provided is minimum.

• At supports when the moment of resistance exceeds the balancing moment, the section is

designed as double reinforced section.

• Minimum reinforcement in the tension shall not be less than yfbd

Ast 85.0=


67

⇒Clause26.5.1.1 (a)

• Maximum reinforcement in tension shall not be exceeded by 0.04bD ⇒Clause

26.5.1.1(b).

• Maximum area of compression reinforcement shall not exceed 0.04bD and

reinforcement is enclosed by strength vide ⇒ Clause 26.5.1.2.

• Nominal shear stress for uniform depth shall be calculated from the equation

bd

Vuv =τ ⇒ Clause 40.1

• Minimum shear reinforcement will be provided when τv<τc given in table 19.

• Maximum spacing of shear reinforcement shall not exceed the least of 0.75d or 300 mm

for vertical stirrups vide ⇒ clause 26.5.1.5

• Shear reinforcement shall be provided to carry a shear equal to Vu - bd. The strength of

the shear reinforcement Vs shall be calculated for vertical stirrups.

v

svy

S

dAfVs

87.0= ⇒Clause 40.4(a)

• At least 1/3rd positive moment reinforcement in simple beam and 1/4th positive moment

reinforcement in continuous beam shall extend along the same face of the member in to

the support to a length equal to Ld/3⇒clause 26.2.3.3

• For curtailment, reinforcements shall extend beyond the point at which it is no

longer required to resist flexure for a distance equal to the effective depth of the

member or 12 times the diameter of the bar whichever is greater⇒Clause 26.2.3.1

• The minimum shear reinforcement in the form of stirrups shall be provided such that

vbS

Asv ≥≥≥≥

yf87.0

4.0⇒Clause 26.5.1.6

Where, Asv =Total cross-sectional area of stirrup legs effective in shear

Sv = Stirrup spacing along the length of the member

b = Breadth of the beam or breadth of flanged beam

fy =Characteristic strength of the stirrup reinforcement in

N/mm2whichshall not be taken greater than 415N/mm2.


68

• Clear cover for longitudinal reinforcement in a beam, neither less than 25 mm nor less

than diameter of such bar and 15 mm to stirrups

• At each end of reinforcing bar neither less than 25 mm nor less than twice diameter of

such bar.

• At least two bars should be used as tension steel, and not more than 6 bars

should be used in one layer of beam.

• The diameter of hanger bars shall not be less than 10 mm, and of main tension bars 12

mm. The usual diameter of bar chosen for beams are 10, 12, 16, 20, 22, 25, and 32 mm.

When using different sized bars in one layer place the largest diameter bars near the

faces, there as of steel should be symmetrical about center line of column as far

as possible.

• The minimum distance between bars has the diameter of bar or maximum size of

aggregate plus 5 mm. Size of aggregate normally used in India is 20 mm. So that clear

maximum distance between bars should be 25 mm.

• The depth of the beam should satisfy the deflection requirements with respect to L /D

ratios. In addition, for economy, the ratio of overall depth to which should be between 1.5

to 2.0

Specifications Regarding Spacing Of Stirrups In Doubly Reinforced Beams:

• Compression steel placed in doubly reinforced beams also has to be restrained against

local buckling during its action like the compression steel. The same rules regarding

restraining of column reinforcements by lateral ties apply to compression reinforcements

in beams also. Accordingly, the diameter of stirrups (ties) should be 6mm and the pitch

should not be more than the least of the following:

o Least lateral dimension

o 16 times the diameter of longitudinal bar

o 300mm

• Minimum steel is necessary to

o Guard against any sudden failure of a beam if concrete cover burst and the bond to the

tension steel is lost.

o Prevent brittle failure, which can occur without shear steel.


69

o Prevent failure that can be caused by tension due to shrinkage and thermal stresses

and internal cracking in the beams.

o d) Holds the reinforcements in place while pouring concrete and act as the

necessary ties for the compression steel and make them effective.


70

4.2.1(b)Results of beam design:


75

STAAD DESIGN OUTPUT FOR BEAM:


76

4.2.2(a)DESIGN OF COLUMN:

A column is defined as a compression member, the effective length of which exceeds

three times the least lateral dimension

A column forms a very important component of a structure. Columns support

beams which in turn support walls and slabs. It should be realized that the failure of a

column results in the collapse of the structure. Supporting the slabs is the main

function of the column.

Reinforced Cement Concrete Column Plan and Section

In this article, we are going to discuss in detail the basis of classification of columns and

different types of reinforcement required for a certain type of column.


77

A column may be classified based on different criteria such as:

• Based on shape

o Rectangle

o Square

o Circular

o Polygon

• Based on slenderness ratio

o Short column Leff/d <12

o Long columnLeff/d> 12

• Based on type of loading

o Axially loaded column

o A column subjected to axial load and uni-axial bending

o A column subjected to axial load and biaxial bending

• Based on pattern of lateral reinforcement

o Tied columns

o Spiral columns

• Minimum eccentricity

o Emin >(� 500� ) + (� 30� ) > 20

o Where, l = unsupported length of column in ‘mm’

o D = lateral dimensions of column

Types of Reinforcements for columns and their requirements

• Longitudinal Reinforcement:

o Minimum area of cross-section of longitudinal bars must be at least 0.8% of

gross section area of the column.

o Maximum area of cross-section of longitudinal bars must not exceed 6% of the

gross cross-section area of the column.

o The bars should not be less than 12mm in diameter.

o Minimum number of longitudinal bars must be four in rectangular column and

six in circular column.

o Spacing of longitudinal bars measures along the periphery of a column should

not exceed 300mm.


78

o Transverse reinforcementmay be in the form of lateral ties or spirals.

o The diameter of the lateral ties should not be less than 1/4th of the diameter of

the largest longitudinal bar and in no case less than 6mm.

• The pitch of lateral ties should not exceed

o Least lateral dimension

o 16 times the diameter of longitudinal bars (small)

o 300mm

• Helical Reinforcement

o The diameter of helical bars should not be less than 1/4th the diameter of

largest longitudinal and not less than 6mm.

o The pitch should not exceed (if helical reinforcement is allowed).

o 75mm

o 1/6th of the core diameter of the column

• Pitch should not be less than,

o 25mm

o 3 times the diameter of helical bar

o Pitch should not exceed (if helical reinforcement is not allowed)

• Least lateral dimension

o 16 times the diameter of longitudinal bar (smaller)

o 300mm


79

4.2.2(b) Results of column design:


84

STAAD DESIGN OUTPUT FOR COLUMN:


85

4.3. DESIGN OF FOOTINGS USING EXCEL SHEETS:

Footing or foundation is defined as the part of substructure, which transmits the load from

superstructure to surrounding soil stratum safely.

Footings may be Isolated, Combined:

Isolated or independent footings are the footings that support the individual columns.

They distribute and spread the load over a sufficiently large area of the soil stratum to

minimize the bearing pressure. Isolated footings may be square, rectangular or circular.

In general, it is assumed that the soil behaves elastically i.e. the strain in the soil is

proportional to applied stress i.e., stress and strain distribution in the soil immediately under

the base of the footing is linear. Stress distribution is different for different soils.

For analysis purpose, a footing can be compared with a rigid body in equilibrium

subjected to loads. Like other structural members, a footing is designed to resist shear forces

and bending moments. In design, for any soil the pressure distribution is assumed to uniform.

In design, the critical section for one way shear (beam shear) is at a distance equal to

the effective depth, d from the face of column footing. The critical section for two way shear

or slab type shear shall be at a distance d/2 from the periphery of column, perpendicular to

the plane of the slab. The critical section for bending moment is at the face of the column.

Generally the footing is sensitive to punching shear.


86

Design Procedure of Column Footings

Column Footing Plan and Section

Step 1: Area required for footing

Square = B = (w+w1)/P0

Where, Po = safe bearing capacity of soil

w1 = self weight of footing

w = self weight of footing

For Rectangle = b/d = B/D

A = b x d

Net upward pressure on the footing


87

Q/p = W/A

Step 2: Bending Moment

Critical section for maximum bending moment is taken at the face of the column

For a square footing,

Mxx = q x (B/8) (L – a)2

Mxx = q x (L/8) (B – b)2

Myy = q x (B/8) (L – a)2

Step 3: To fix the depth of the footing shall be greater of the following:

Depth from bending moment consideration

d =�(M/Qb)

where, Q = moment of required factor

Depth from shear consideration.

Check for one way shear

Check for two way shear or punching shear

Critical shear for one way shear is considered at a distance‘d’ from face of the column.

Shear force, V = qB[((B – b)/2) d]

Nominal shear stress, τv = k .τc

τc = 0.16 √Fck

Step 4: Check for two way shear

Critical section for two way shear is considered at a distance at a distance d/2 from all the

faces of the column.

SF, V = q [ B2 – (b + d)2]

SF, V = q [L x B – (a + d)(b + d)]

Nominal shear stress, Tv = V/2((a+d)(b+d)d)

τv = V/4((b+d)d)


88

τv = k .Tc

k = 0.5 + β > 1

τc = 0.16 *Fck1/2

IS CODE RECOMMENDATIONS FOR DESIGN OF FOOTINGS (IS: 456-2000):

Footings shall be designed to sustain the applied loads, moment and forces and safe bearing

capacity is not exceeded.

• In R.C.C. Footing, the thickness at the edge shall not be less than 15 cm for footing on

soil.

• The greatest bending moment to be used in the design of an isolated concrete footing,

which supported on column, shall be computed at the face of the column.

• The critical section for diagonal cracking is taken at a distance equal to the Effective

depth from the face of the column in hard soils and shall not exceed nominal shear

stress.

•

Fig15.figure showing the footings of the structure


89

4.3.1 Results of footing design:


96

4.4 DESIGN OF STAIR CASE USING EXCEL SHEETS:

The purpose of staircase is to provide pedestrian access to different levels within a building.

The geometrical forms of staircases may be quite different depending on the individual

circumstances involved.

• They are two main components of a staircase

o Stairs

o Landing slab.

The stairs and landing slab can be arranged in different forms to get different types of

staircases. In the shape and structural arrangement of a staircase would generally depend on

two main factors.

a. Type of construction of the structure around the stair case i.e. load bearing

brick structure or reinforced concrete structure

b. Availability of space

• Classification of Staircases:

o Straight stair

o Quarter turn stair

o Half turn stair (open newel type or open well stair)

o Dog-legged stair

o Open newel stair with quarter space landing

o Geometrical stairs such a circular stair, spiral stair etc.

• General Principles:

o Between consecutive floors there should be an equal rise per every parallel

step. Similarly there should be equal going

o There should be at least 2m head room measured vertically above any step.

o The sum of going of a single step plus twice the rise should be between

550mm and 700mm.

o The rise of step should not be more than 200mm.

o The slope or pitch if stair way should not be more than about 380.

Effective Span in Stairs:

The effective span of stairs without stringer beams shall be taken as the following

horizontal distances:


97

• Where supported at top and bottom risers by beams spanning parallel with the risers,

the distance center to center of beams,

• Where spanning on to the edge of a landing slab, which spans parallel, with the risers,

a distance equal to the going of the stairs plus at each end either half the width of the

landing or one meter, whichever is smaller, and

• Where the landing slab spans in the same direction as the stairs, they shall be

considered as acting together to form a single slab and the span determined as the

distances center to center of the supporting beams or walls, the going being measured

horizontally.

DISTRIBUTION OF LOADING:

In the case of stairs with open wells, where spans partly crossing at right angles occur,

the load on the areas common to any two such spans may be taken as one half in each

direction. Where flights or landings are embedded into walls for a length of not less than

110mm and are designed to span in the direction of the flight, a 150mm strip may be

deducted from the loaded area and the effective breadth of the section increased by 75mm for

purpose of design.

In this, stairs spanning longitudinally, the beam is supported at top and at the bottom of

flights.

Loads

• Self weight of a step = 1 x R/2 x 25

• Self weight of waist slab = 1 x t x 25

• Self weight of plan = 1 x t x 25[(R2 + T2)/T]

• Live load = LL (KN/m2)

• Floor finish = assume 0.5 KN/m

For the efficient design of an RCC stair, we have to first analyze the various loads that are

going to be imposed on the stair.

The load calculations will help us determine, how much strength is required to carry the load.

The strength bearing capacity of a staircase is determined on the amount of steel and concrete

used.

The ratio of steel to concrete has to be as per standards. Steel in the staircase will take the

tension imposed on it and the concrete takes up the compression.

These are the essential steps that are to be followed for the RCC Stair Design.


98

4.4.1 Results of staircase design:

DistributionSteel

Ast=185mm^2

Diameter ofbar=8mm

Spacing=270mm

Development Length Ld=(Øxσs)/(4xTbd)

Therefore,Ld=564mm

Provide,Ld=570mm


99

5. SUMMARY

• The design of slab, beam, column, rectangle footing and staircase are done in limit

state method which is safe at control of deflection and in all aspects.

• For the construction of residential building, the safe bearing capacity is taken as

200 KN/m² in order to bear all the loads coming on to the building.

• Using Staad.pro software, the design consideration has been taken as per the

IS codes. The design is safe in all conditions.

• On comparison with the drawings, manual design and the geometrical model

developed using staad.pro; the area of the steel requirement for the beam,

Column, footing and slab are comparatively similar to that of the requirement.

• The percentage of variation in area of steel for the beams, column, footing and slab

are less than 20 percent of the required steel.

Documents

3D Analysis and Design of a multi storey building