SOFTWARE FOR INTEGRATED ANALYSIS DESIGN OF RCC BUILDINGS (IADB)

1.1     SCOPE OF THE SOFTWARE

This integrated software comprises of the following three components:

1.1.1   Pre-processor Module:

This module accepts the data for all the modules as a single file. The data file broadly comprises of :

·         geometry of building frames

·         member sizes

·         geometry of panels

·         floor and line loads

·         earthquake parameters

It also carries out the load calculations and generates the data for Analysis Module. The load calculation includes:

Transfer of floor loads acting on panel/sub-panel and end reactions of     secondary  beams to the main beams and Determination of fixed end forces at the end points of the main beams

The load calculations can only be performed for rectangular panel supported on 4 or 5 columns and having layout of the secondary beams similar to one of the pattern as given in the user manual.

 In addition, it also generates data files comprising the cross-sectional details of the structural members and grade of concrete and steel for the Post Processor Module.

1.1.2   Analysis Module

This module carries out the three dimensional analysis of a building block assuming that it is composed of frames which are connected at floor levels. The frames are assumed to be rectangular in elevation. Non-rectangular frames i.e. stepped in elevation, can be generated by assigning zero properties to particular beams/columns. In plans the frames can have inclined orientation with respect to global axes. The floors are assumed to be rigid in their own plane. The analysis is carried out for five different load cases i.e. three load cases for different distributions of vertical loads and two load cases for lateral loads applied along the two Global axes. Using these five load cases, user can define load combinations as required.

1.1.3   Post Processor Module

This module sorts the output of analysis module and carries out the design and detailing of ;

·         Beams

·         Columns

·         Slabs and

·         Isolated footings

according to relevant codal provisions given in IS:456 and IS:13920. It also prepares the tables of details of reinforcement for beams and columns in the format currently being used in the structural drawings. It also estimates the quantities of steel for each diameter of bars and volume of concrete used in beams and columns. In addition, it also carries out analysis of raft foundation using the finite difference method.

1.2      SCOPE OF INPUT

Only one data file is to be prepared by the user. This data file comprises of the following details. In addition, the user is prompted to key in few data during execution of various programs.

A)        Frame geometry (required for analysis)

The number of bays and storeys define the geometry of a frame. The identification number of main beams defines the bays. The main beams are defined by the identification number of left and right end columns and length of beam measured centre to centre of the columns The storey heights are defined once for the whole building and are identical for all the frames. The co-ordinates of first and the last column points define the locations of frames.

B)                Panel geometry (required for load calculation)

The identification numbers of surrounding main beams defines the panels. These panels are used only for load calculation purpose. The structural stiffness of these panels is not considered in structural analysis.

The layout of secondary beams within a panel and floor loading on sub-panels and line loads on secondary beams are defined in accordance with details given in user manual.

C)       Member properties

The various beams and columns are grouped into typical beam types and typical column types.

The properties of typical beams and columns are to be defined only once for the whole building. The cross-sectional details of beam/column and grade of concrete and steel define the beam/Column types.

Each beam is defined by the beam identification number and beam type number and similarly, each column is defined by the column identification number and column type number.

D)       Loading

The vertical loads are prescribed as uniformly distributed floor loads on the panels/ sub-panels and uniformly distributed line loads on main beams/secondary beams.

The floor/ line loads can only be given on entire area/length of the beam. The line load on part length of the beam is not accepted. Similarly floor load on the part area of a panel/sub panel is not accepted.

Additional fixed end forces can also be prescribed on main beams. This option is particularly used for the panels for which load calculations cannot be carried out directly by this software. For such panels fixed end forces can be determined separately using the graphical user interface

The software calculates the lateral loads due to earthquake based on the procedure given in IS: 1893. The earthquake parameters are to be prescribed by the user.

The lateral storey forces due to wind are to be calculated and supplied by the user.

 

1.3     SCOPE OF THE OUTPUT

The following output are generated by the software, which can be viewed and printed by the user.

A)       Load Output

In the load output, the details of data given by the user are reproduced in tabular form and in addition, the following details are given for each floor starting from terrace level.

·         For each beam, the values of the fixed end moments and shears and mid span moments due to dead and live loads.

·         For each column, the values of axial loads due to dead and live loads and self-weight of the column. The axial loads are calculated based on fixed end shear transferred from beams connected to it.

·         For each column, the values of the axial load per unit of contributory area supported by the column.

The contributory area for a column is determined considering the applicable part of the areas of the surrounding panels on which loads are prescribed. In case the user has carried out the load calculation manually for one or more surrounding panels of a column, the axial load per unit area for such column will be calculated by ignoring the contributed area of such panels. However, the load transferred from such panels to the column will be considered in determining the load on the column if the user has prescribed additional fixed end forces on surrounding beams of such panels.  Therefore, the value of axial load per unit area for such columns will be higher than the actual axial load per unit area, which considers the contribution of area from all the surrounding, panels.

·         For each floor, the value of floor loads per unit area due to dead and live loads.

These values are obtained by dividing the sum of axial column loads of a particular floor by the sum of the areas of the panels on which loads are prescribed. As explained above these values will be higher than actual values, in case, manual load calculations are carried by the user for one or more panels.

·         For each floor, the values of the lateral storey forces due to earthquake

B)        Analysis output

In first part of analysis output, the details of data prepared by load module for analysis module are reproduced. This part of the analysis output is not useful for the users.

The second part of the analysis output i.e. including and beyond structural displacements is useful for the user. This part of analysis output comprises of following details.

·         For each floor, the values of structural displacement along the global X and Y-axis and rotation about centre of rigidity for the basic of five load cases.

The first 3 load cases: that is (I)(II)(III) corresponds to the three different distributions of vertical loads. Load case (I) corresponds to dead loads on all the spans of the frame and load case (II) and (III) corresponds to live loads on odd and even spans, respectively. Load case (A) corresponds to lateral storey force applied along X- direction and load case (B) corresponds to lateral storey forces applied along Y- direction.

·         The details of the load combinations.

·         For each floor, the values of lateral displacement of each frame for all the load combinations.

·         For each floor, the value of design forces for each column line of the frame for all the load combinations.

 For column design forces, the user may only refer to major axis moment’s (top and bottom) axial force and major shear. The other details like torsion moment, minor axis moments and minor shear may be ignored as the torsional stiffness and minor axis stiffness of the columns is neglected in structural modelling.

·         For each floor, the value of the design forces for each beam for all the load combinations.

C)        Column Design Output

The column design output comprises of following details:

·                     The grade of concrete mix and height of column for each floor.

·                     The dimensions of column along X and Y-axes and effective cover for each            column.

·                     The load factor for each load case

·         The axial loads, bending moments about both the axes, percentage of area of longitudinal reinforcement for all the load combinations of each column. The quantity of steel required for all the column of a particular floor.

These details are arranged storey wise for each column.

          D)        Beam Design Output

The beam design output comprises of following details:

·         The design percentages and areas of  longitudinal steel at the   top and bottom of three sections of each beam i.e. at left end, right end and mid span

If the design percentage of steel is less than the minimum prescribed in the code, the minimum percentage of steel is adopted.

·         The area of stirrups at both ends of the beam.

E)        Column Detailing Output

The output of column detailing comprises of the following details for each column

·         Dimensions of columns along X and Y-axes.

·         Grade of concrete mix for each floor.

·         Longitudinal bars at corners and both the faces in each floor.

·         Spacing of stirrups and reference number of stirrup type in each floor.

F)        Beam Detailing Output

This output comprises of following details for each frame and for each floor:

·         Grade of concrete and steel.

·         The number and dia. of bars at top and bottom the various sections of the frame.

Beam Drawing Table

It comprises of the following details for each beam in a standard tabular format currently being used in CPWD

·         Size and span of  the beam

·         Longitudinal reinforcement bars and

·         Stirrups

These details are arranged frame wise and level wise.

Column Drawing Table

It consists of the following details for each column in a standard tabular format currently being used in CPWD:

·         Size of the column.

·         Longitudinal reinforcement bars and

·         Reference number to the pattern of lateral ties. (Refer Appendix D for different pattern of lateral ties)

These details are arranged level wise in-groups of columns having identical details.

G)        Slab Design Output

Following outputs are generated.

a)     Complete design details

b)     Detailing of bars in tabular form

The design details comprises:

·         Input details

·         Detailing notes

·         Factored moments

·         Required and provided steel area

·         Detailing of bars

·         Check for shear stress and effective span to effective depth ratio

·         Deflection details

The detailing of bars in tabular form comprises:

·         Identification of slab panel

·         Plan dimensions and thickness of slab

·         Detailing of bars at bottom and top along shorter and longer spans.

H)        Isolated footing Design Output

The output comprises the following details for the footing of each column:

·         Size of footings and pedestal.

·         Depth of footing.

·         Reinforcement along the two directions and detailing.

·         Quantities of concrete, steel and shuttering.

·         Cost of concrete, steel and shuttering.

I)         Estimate of Steel and Concrete in Beams

The output gives the following details for each floor

·         The length and weight of longitudinal steel for each diameter.

·         The length and weight of stirrups for each diameter

·         Volume of concrete in main beams for each floor.

J)         Estimate of Steel and Concrete in Columns

The output gives the following details:

·         The length and weight of longitudinal steel for each diameter.

·         The length and weight of stirrups for each diameter

·         Volume of concrete in column for each grade of concrete.

1.4      ADVANTAGES

1.4.1   Analysis Model

General structural analysis programs e.g. Space Frame Program and Finite Element Program etc. have flexibility to model any complicated shapes of structures. But the input and output of these programs do not take the advantage of regular characteristic of a building i.e. the continuity of column lines all through the floors, beams at same floor levels and rigid floor diaphragm at each storey. Thus, in such programs a building can not be modelled as assemblage of columns, beams and floors. That is why the input and output of such programs are not user friendly and it is difficult to visualize the structural behaviour of a typical building in terms of displacement of floors and forces in columns and beams.

The analysis program considered here i.e. Three-dimensional Analysis of Building “(ETABS – Extended Version of TABS) takes the advantages of the characteristics of buildings stated above. Thus, the input and output of this analysis program are easy to comprehend.

1.4.2   Results

The results of analysis are given column/beam wise for each floor. Thus, it is easy to understand the outputs by the user. In addition, the outputs are easy for post processing for design and detailing of beams and columns.

All the results of analysis, design and detailing of various structural members are tabulated in very user-friendly format.

1.4.3   Detailing

The detailing of longitudinal reinforcement in beams is carried out considering the continuity of reinforcement at supports. Similarly, the continuity of longitudinal reinforcement is maintained among various floors in the columns. Beside, the number of bars in a column is optimised to provide minimum quantity of steel for longitudinal bars and lateral ties.

The pattern of lateral ties in rectangular columns is automatically selected by the program based on the numbers of bars on two adjacent faces. While doing so all the provisions given in I.S 456 are followed. The length of ties for a particular pattern is determined based on actual size of the column. This information is used in optimisation of steel in columns and in determination of quantity of steel of lateral ties.

1.4.4   Draft Drawing Tables

The details of reinforcement in beams, column, and isolated footings are given in tabular form that can readily be incorporated in structural drawings.

1.5      LIMITATIONS

·         The program is  applicable for regular frame buildings.

·         Member inclined in plane other than horizontal cannot be considered.

·         Compatibility is not enforced with regard to joint displacement  at joints that are common to more than one frame. Thus axial deformations in columns common to many frames will not be same; however for design purposes, these column axial forces may be added  directly to give reasonable results.  Similarly, the joint rotations of the frames with common members orthogonal in plan view are mutually uncoupled.

·         Floor levels must be same for all the frames.

·         Torsional stiffness is neglected for all members.

 

1.6      GRAPHICAL USER INTERFACE

Graphical user interface of IADB has been developed with the following objectives to perform following activities in user friendly manner:

 

·         To provide common platform for executing the various back-end programs.

·         To prepare the input data file and simultaneous validation of data.

·         To facilitate viewing the geometry of the building and properties of various structural members. 

·         To adjust sizes of overlapping isolated footings.

·         To determine the fixed and forces for any arbitrary loading on beams.

·         To view various results of analysis, design and detailing including the draft drawings.

 

OTHER UTILITY PROGRAMMES OF STRUCTURAL ANALYSIS AND DESIGNS

 

3.1      ANALYSIS OF PLANE FRAME AND TRUSSES

 

This program is based on stiffness matrix analysis. It analyses the plane frame and trusses. The programmer accepts concentrated load, U.D.L, triangular load or trapezoidal load applied on the members and joints load.

 

The output consists of displacement and rotations of joints, moments at mid span and the ends of beams, shear force at the ends of beams, axial force and end shears and moments in columns.

 

3.2      DESIGN OF COLUMNS WITH LIMIT STATE

 

Two separate programmes for design of columns are available as given below:-

 

a)       Equal steel on all four sides

b)       Unequal steel on adjacent sides

 

The program has been developed for designing square/rectangular column sections for axial load and biaxial moments. It is based on limit state method. The codal provisions, as given in IS:456:2000, for minimum reinforcement requirement and moments due to, minimum eccentricity in both the directions are taken into account. In case of long columns, additional moments due to slenderness can be added to the biaxial moments at the input stage, if required. The grade of steel considered in this program is Fe 415 only. The concrete of any grade can be specified for the design.

 

The program has been developed for designing square/rectangular column sections for axial load and biaxial moments and is based on limit state method. The codal provisions, as given in IS:456, for minimum reinforcement requirement and moments due to, minimum eccentricity in both the direction are taken into account. In case of long column, additional moments due to slenderness can be added to the biaxial moments at the input stage.

The input data consists of the sectional dimensions of the column, grade of steel and concrete, axial load and biaxial moments.

 

The program works out the percentage and area of reinforcement, check factor and alpha factor of interaction formula as given in IS:456. In program with unequal steel on adjacent sides, it gives reinforcement area along both the axes.

 

3.3      ANALYSIS OF STRIP FOOTINGS BY RIGID METHOD

 

In this package, the combined and raft foundations are analysed using rigid method. The planer distribution of pressure underneath the foundation is assumed. The analysis is carried out by simple statics without taking into account soil-structure interaction. The strip footings can be sub-divided into several elements as per the requirements of the user in accordance to points at which moment/shear is to be determined. The nodal points are defined at the two ends of the elements. This program gives soil pressure, moments and shear at every node.

 

3.4      ANALYSIS OF STRIP FOOTING BY FLEXIBLE METHOD

Strip and raft foundations are analysed using flexible method wherein soil characteristic and rigidity of the structure is taken into account. This software analyses the strip or raft foundations by flexible method using the analogy of beam on elastic foundation. The foundation is modelled as beam supported over a bed of elastic springs representing the supporting soil.

 

The input consists of modulus of subgrade reactions (ks), modulus of elasticity of concrete , loading details at different nodal points and geometry of the raft.

 

The program generates deflection, rotations and soil pressure at different nodes along with moments and shears in the members.

 

3.5      ANALYSIS OF RAFT BY FINITE DIFFERENCE METHOD.

This software is based on flexible method of analysis. The slab type rectangular type raft foundations is analysed as a plate supported on elastic foundation using finite difference method.

 

Input consists of number of grid liens and spacing between grid lines along X and Y direction, raft thickness, modulus of subgrade reaction and loading details.

 

Output consists of deflections at each grid points, bending moments per unit width in both X and Y directions at each grid points as well as nodal reactions and soil pressures at each grid point.

 

3.6      ANALYSIS OF GRID FLOORS

This programme analyses the grid floors consisting of intersecting secondary beams supported on main beams or on load bearing walls. Mathematically, the grid floors are modelled as a structure that lies in a horizontal plane with loads acting perpendicular to the plane of the structure. This programme can take into account the nonorthogonally intersecting secondary beams also The connection between intersecting beams are considered as rigid joints and each joint is assigned a maximum of three components of displacement i.e. one vertical and two rotational displacements. The programme calculates flexural moments, torsional moments and shear force for each member.

3.7      PROJECT MANAGEMENT THROUGH CPM/PERT

This software essential carries out the analysis of the project network, comprising of activities and their duration. It identifies the critical path and critical activities along with the dates of early start, early finish, late start and late finish. This program also gives the total and free floats of non-critical activities.

 

The programme also projects the Resource allocation for maximum three resources based on early and late finish dates of activities for each month. The programme also works out the completion dates of milestones.

 

3.8      ANALYSIS OF SPACE FRAME AND TRUSSES

 

This program analyses three dimensional space trusses / frames of any configuration .This program has been developed based on stiffness matrix approach. All joints are assumed to be pinned in space truss and restrained in space frame problem. The program accepts the data that describe the geometry and loading. It calculates the joints displacement and end forces for each member.

 

3.9      DESIGNING AND DETAILING OF ISOLATED FOOTING

 

The design of isolated footing is carried out for various combinations of axial loads and biaxial moments.  If the user has not opted for fixing one or both the dimension of footing, the software designs a square footing.  The user is given the flexibility of fixing one or both the dimensions. The software considers only the constant depth footing. Sloped or stepped footings have not been considered

 

The software finds out the size of footing, size and  height of pedestal, depth of footing and the details of  reinforcement along both the axes. The software performs the detailing according to IS:456.  The diameter of the bar is also checked for the requirements of development length. It also calculates the quantity of concrete, steel and shuttering and cost based on DSR-97.

 

User has been given the flexibility of fixing the depth of footing and the pedestal size. In cases where the parameters fixed by the user lead to unsafe design, appropriate message is given, accordingly.

         

3.10    DESIGN AND DETAILING OF SLABS

 

The software designs and details the rectangular slab panels in accordance with IS:456-2000. The depth is optimised satisfying bending, shear and deflection criteria.  Two ways slabs supported on four sides and one way slabs supported on two opposite sides are considered.  The moments are computed as per the coefficients given in the code.  Detailing requirements of IS:456 are satisfied. Though span to effective depth ratio is controlled to satisfy deflection requirements, deflection is also calculated using expression of moment of inertia as specified in the code.  Sound engineering practice has been followed for detailing, in addition to the provisions given in IS:456-2000. The quantity of concrete and cost of concrete on DSR 97 have also been computed.    

3.11    DESIGN OF COLUMNS OF ANY SHAPE

It designs the columns, with axial load and bi-axial bending moments, of any shape. The regular shapes of columns considered are

          i)        Rectangular

          ii)       'T' (symmetrical about Y axis)

          iii)       '+' shape (symmetrical about both the axis)

          iv)      'L'

          v)       'O'  ( Solid )

For regular shape the programme defines the co-ordinates of the boundary points of the column section and reinforcement bars. The user may select the spacing between the main bars, layer of main bars and skin reinforcement bars.

For any odd shape (other than regular shapes) the user is to define the co-ordinates of boundary points of the column and Rbar

For rectangular sections, it assumes equal amount of steel on opposite sides and unequal amount of steel on adjacent sides. If user desires, equal amount of steel can be assumed on all the faces.

For column sections of shape 'T', '+' and 'L', the program places main bars on the outer faces of the column and skin reinforcement on the inner faces at the rate of 8 cm2 per meter length. This way, the program optimises the amount of reinforcement required to the carry design loads applied on the columns.

The program can take different grades of concrete but only one grade of steel, at present.