Points to be remembered while generating STAAD Model

Modeling of the structure in STAAD can be split in various stages. At the completion of

each stage the concerned Engineer must convince himself, about proper layout of model

up to that stage, before proceeding to the next stage. If they have any kind of confusion

or doubt at any stage of modeling, then they should cross check the model themselves or

take help of any other Engineer, because a small negligence can lead to disaster which

will take much more time to be caught and rectified at later stage of modeling. Since we

all are working on time bound projects we need to be 100% accurate at initial stage only.

Various Stages of Modelling

Stage 1

Load transfer mechanism should be understood and in case of any doubt the same

should be discussed with your nominated colleagues. Do not start modeling if

structural system is not clear.

Possibility of the future expansion in vertical as well in horizontal direction to be

discussed with client/architects.

Framing plan of each floor should be prepared based on the available architectural

plans and levels should be taken from sectional details..

The location of the Expansion Joint should be discussed with your colleagues and got

approved from the architect before doing the modelling

Loading sheets for all areas to be prepared, mentioning all kinds of load coming on

the structure like thickness of slab, fire tender load on the specified area, service

equipment load in basements and terrace, filling load over extended basement area,

water tank load, lift machine load, etc. A sample of loading sheet is attached in

Annexure 1.

Care should be taken to provide correct direction of the members (orientation) while

generating the geometry. The columns should be joined from bottom to top, for

beams join nodes left to right or right to left and for plate join nodes in clockwise or

anticlockwise direction.

Take extra height of 600mm below lower most finished basement level for foundation

and for non basement areas we should take as per the actual taking 300 mm from the

foundation top.

While modeling retaining wall or shear wall, at intermediate level always join the

nodes with beam member. This beam shall be given high stiffness (like size of

230mm X 1500mm), but the density of concrete shall be very low (of the range of

100 Kg/m3).

Care to be taken that in case of free standing columns the actual length to be provided

in form of even though the intermediate nodes at floor level are there. Provide Ely/Elz

member length factors for length about local Y and Z axis for columns.

Before proceeding to next stage check for following

A. Duplicate node with tolerance allowance of 0.1 meter.

B. Orphan nodes

C. Overlapping of members

D. Multiple structures

E. Beam Plate connectivity.

F. Warped plates

G. Zero Length Members

Discuss the model with your colleague/mentor as the time spent at this stage will help

in designing the structure correctly. NEVER HESITATE TO TAKE HELP OF

YOUR COLLEAGUES. GO WITH SOLUTION AND NOT ONLY WITH

PROBLEMS.

Stage 2

Provide support conditions correctly.

Always use same units throughout the model. If anywhere in the input file units are

changed, then it should be in knowledge of concerned Engineer. The mistake in units

is very common and serious happened in the STAAD file. Thus after completing the

input file the units must be checked.

Feed the sizes of all the members.

Check member sizes i.e. incase of beams check the width and depth, at various

locations. For columns, check the section outline label for correct orientation.

The beams should be entered as T or L beams with the actual area of the beam being

fed else the self weight command would calculate the load inaccurately. OR For the

conventional beam slab systems, provide Iyy = 100 m4 in beam member property.

Flat slabs shall be modeled precisely.

For irregular shaped columns like L or T shape, equivalent sections shall be provided

as per the stiffness calculations. All the irregular columns with equivalent section

shall be noted on a sheet of a paper with their actual sizes.

After completing stage I and points mentioned in stage 2, run the program for self

weight only. Observe the results carefully, especially the deflections. Normally

deflection will be in the range of 0.1mm, if at any location in the structure, excessive

deflection is observed, then check its feasibility. This check with self-weight is

extremely important, because any kind of flaws in the model can be sorted out in the

preliminary stage.

Once the concerned Engineer is convinced with the results for self-weight, then can

proceed to the next stage of modeling.

Stage 3

In stage 3, various kinds of loading happening on the structure are applied. Before

applying loading the concerned Engineer must list out the type of loading that will

come on the structure throughout the life of the structure.

The gravity loads are applied to the structure as per the loading sheet attached in

Annexure 1. Simultaneously apply filling load and the fire tender load on the

specified area.

The load basis for the gravity loads shall be written in the STAAD file so that the

same can be reviewed by any one at any stage.

Once the gravity load is applied, check each panel of the model, for proper load

distribution. The load distribution shall be triangular or trapezoidal.

After applying gravity loads, run the program for only dead load and live load

combination. Checks the load coming over some of the columns with tributary area

calculations. At least perform this check at 5 to 6 locations for corner and central

columns. The STAAD output and manual calculation must be in variation of 15 to

20%. If concerned Engineer is satisfied with the above check then proceed further.

Always calculate the loading intensity for the dead load and live load combinations.

In case of residential building the loading intensity comes out to be 1.5 t/m2 and for

commercial building it is about 1.8 t/m2.

If the length of the building in more than 60/70m, then temperature and shrinkage

loading shall be applied. The temperature load calculation sheet shall be made and

attached to project file. A sample calculation for temperature stress in given in

Annexure 2.

Earthquake loading shall be applied to all kind of structures and static/response

spectrum analysis shall be carried out. Proper calculation shall be done and attached

into the project file. A sample calculation for earthquake loading is given in

Annexure 3.

A separate STAAD file shall be made to find out the nodal forces. The load

combination shall be as per IS 1893 or as given in Annexure 3. For calculation of

lumped masses reduction in the live load at floor level should be done.

To calculate time period, the height of the structure shall be taken above ground floor

only when there is no basement or basement is enclosed by retaining walls at all four

sides. When structure is not enclosed by retaining wall with three sides, then for time

period calculations the height of the building shall be taken from the foundation level.

Time period calculations, two different formulas are given in code. For commercial

structures use the bare frame formula. In case of residential structure the average

time period for bare frame and in filled frame is used by few people but it I

recommended to use the bare frame formulae only..

Initially, for earthquake analysis, static method shall be used for three reasons.

Firstly, the deflections required for torsional irregularity can be best taken from static

method of analysis. Secondly, for satisfying torsional irregularity a number of

iterations has to be performed, which is a time consuming process, but with static

method it can be done in few minutes. Thirdly, the stresses for shear walls should be

extracted from the static analysis.

It should be noted that the ratio of height of building (same as that taken from for the

time period calculations) to 750 should be less than maximum deflection observed.

This check for deflection shall be done for un-factored value of static analysis.

Check for the torsional irregularity conforming to IS 1893 in both X & Y directions.

A sample calculation for torsional irregularity is attached in Annexure 4. It’s an

iterative process to satisfy the structure in permissible limits of torsional irregularity.

This check shall be made for the un-factored deflections from static analysis.

Torsional irregularity shall be matched even if the deflections are small.

Once the torsion irregularity is satisfied with static analysis, then apply the

earthquake loading as per the response spectrum. Final results for the earthquake

analysis should be taken from response spectrum method (except shear wall design).

Response spectrum analysis shall be done as the last step of analysis.

Apply wind load on the structure. Calculations for wind loading shall be written

separately.

Stage 4

Load combinations shall be applied as per the list given in Annexure 5. Live load

reduction shall be as per IS 875.

Design the columns with specified grade of concrete. If the reinforcement in columns

is above 2.5% then increase the size of columns if possible. If the column

reinforcement is very low then review the column sizes again. The code calls for

columns to be designed for reduced live load.

Compare the reinforcement coming at similar locations if there is any substantial

difference then the model to be checked for forces manually also.

Increase the column size by nominal amount if we are getting very high

reinforcement and then the reinforcement. May come down.

Stage 5

Once the concerned Engineer is convinced with the STAAD model and its output,

then get the model checked with other Engineer.

A checklist for the STAAD model is attached in Annexure 6. The concerned

Engineer and his/her colleague should sign the check list stating that the model has

been checked to best of their knowledge and no errors are detected in the model.

Annexure 1 : Loading Sheet

Annexure 2 : Temperature Loading

Annexure 3 : Earthquake Loading

Annexure 4 : Torsional irregularity

Annexure 5 : Load Combinations

Annexure 6 : Checklist for STAAD Model

Annexure "1"

Loading sheet

A sample-loading sheet is given below, similar loading sheet shall be prepared for

each project/ floor.

1. Wall Load per Running metre of height

1.1 230mm thick brick wall

Self load = 0.23 x 2.0 = 0.46 T/m

Plaster (12+ 15mm) = 0.027 x 2.0 = 0.054 T/m

Total = 0.514 T/m

= 0.52 T/m

1.2 115mm thick brick wall per Running metre of height

Self load = 0.115 x 2.0 = 0.23 T/m

Plaster (12+15mm) = 0.027 x 2.0 =0.054 T/m

Total = 0.28 T/m

1.3 Apply parapet load (height and thickness as per architectural drawings)-

Never apply load less than one metre height of 115mm wall

For peripheral walls the load of the stone cladding should be taken. Reduction due to

openings in brickwalls to be considered in application of the loads on the beams.

For commercial buildings normally the glazing is provided at the periphery, instead of

glass we should take load of 115mm walls on the beams.

2. Slabs load

2.1 125mm thick floor slab

Dead Load: Self weight = 0.125 x 2.5 = 0.3125 T/Sqm

Plaster = 0.006 x 2.0 = 0.112 T/Sqm

Floor Finish = 0.05 x 2.4 = 0.12 T/Sqm

Total = 0.445 T/Sqm

Additional loads for the following should also be taken

False ceiling including electrical fixtures = 0.025 T/Sqm

HVAC Ducts = 0.025 T/Sqm

2.2 Terrace (Say 150mm thick)

150mm thick floor slab

Dead Load: Self weight = 0.15 x 2.5 = 0.375 T/Sqm

Plaster = 0.006 x 2.0 = 0.112 T/Sqm

Water Proofing = 0.15 x 2.0 = 0.300 T/Sqm

Total = 0.800 T/Sqm

Additional loads for the following should also be taken

False ceiling including electrical fixtures = 0.025 T/Sqm

HVAC Ducts/Pipe Rack/Cable Tray = 0.025 T/Sqm

Note:

In commercial Buildings, extra load of 400Kg/Sqm shall be applied on terrace for

service equipment.

Extra loading shall be applied in the basement for service load like AC plant room,

D.G. room, transformer room, etc.

Water tank load shall be applied at the terrace level.

Lift machine room load shall be applied.

Fire Tender load shall be applied in the extended basement area.

Filling load shall be applied in the basement area or any part of the structure. It

should be mentioned with which material filling shall be done (along with density).

Sunken load for the toilets and kitchen shall applied. Material for sunken load shall be

specified (along with density).

In case of cantilever balconies, the dead load shall be transferred to the adjoining

beams..

2.3 Staircase Loading (Dead Load)

Loading Per Meter Width of Flight

Waist Slab = [0.15 x 2.5 x I]/[Cos 33.40°]

= 0.449 T/Sqm

Step = (0.5 x 0.184 x 0.25 x 2.5)/0.25

= 0.23 T/Sqm

Finishing = [(0.184 + 0.250) x 2.5 x 0.04]/0.25

= 0.174 T/Sqm

Total = 0.853 T/Sqm 0.9 T/Sqm

4. Live Load

a) All Floor = 0.2 T/Sqm (residential building)

= 0.4 T/Sqm (commercial /institutional building)

= 0.5 T/Sqm (for parking in basements)

b) Balconies = 0.3 T/Sqm (residential building)

= 0.5 T/Sqm (commercial/institutional building)

c) Terrace = 0.15 T/Sqm

d) Staircase = O.15 T/Sqm

= 0.3 T/Sqm (residential building)

= 0.5 T/Sqm (commercial building)

Annexure "2"

Temperature Loading

When the span of the building is more than 60m, then the temperature stresses may

governs the design. Thus after 60m, it becomes mandatory to apply temperature loads in

the structure. Temperature load is applied in followings ways:

Shrinkage Load

Shrinkage Load shall be applied at all the floor levels. Stresses due to shrinkage are

compressive in nature. Thus this load is always applied with negative sign.

Maximum shrinkage strain (є) in concrete = 0.00003 (IS 456)

It is assumed that the 40% of the total shrinkage strain act as long term shrinkage.

Thus long term shrinkage strain (є) = 0.4 X 0.0003 =1.2 X 10-4

As we know, δ = Lαt

Also є = δ/L Thus, δ/L = αt ..................... (a)

Where L = length of building (meters) in the desired direction

α = Coefficient of Thermal Expansion of Concrete = 1.2 X 10-5/0C

t = Temperature Variation

Thus from Equation (a), t = [δ/L]/ α = [1.2 X 10-4]/[1.2 X 10-5] = lO0C

Thus, shrinkage in structure in converted into equivalent temperature of 100C

Temperature load due to seasonal variation

Lowest temperature in summer = 25°C

Highest temperature in summer = 50°C

Thus temperature variation in summer = 25°C

Similarly, temperature variation in winter = 25°C

Temperature load due to diurnal variation

The temperature variation in day & night temperature for both summer & winter = 25°C

Note:

When the concrete area (like terrace area and extended basement area) is directly

exposed to the sunlight, then the temperature variation of 25°C is taken into the

account.

For all the intermediate floors the temperature variation of 10°C is taken into the

account.

ANNEXURE"3"

Earthquake Calculations

a) Time Period Calculations

i) Tal = 0.075 (h)O.75 --- For Bare frame (without infills)

ii) Ta2 = 0.09/vd --- With brick infills panels

h = Height of building -- From foundation or Ground in meters

d = Base dimension at the plinth level in considered lateral for direction in meters

NOTE: - (i) For commercial buildings Ta1 shall be used

(ii) For residential building Ta = (Tal + Ta2)/2 [Average time period shall be taken]

b) Ah = [ZI{Sa/g}]/2R Z = Zone factor

I = Important factor

R = Response Reduction factor

Sa/g = Average response accel. Coefficient

c) Base Shear VB = WAh W = Seismic weight of building calculated for

[DL + 0.25LL or (DL + 0.5LL]

* Where VB is the theoritical base shear from IS 1893

d) Scaling Factor = [VB/Vx] in X Direction

= [VB/Vz] in Z Direction

Note:

Scaling factor shall not be less than 1

Final Base shear shall be matched with the STAAD output.

Seismic weight shall be calculated without considering Fire Tendor load

ANNEXURE"4"

Torsional Irregularity in Structure

Torsional Irregularity in a structure shall be checked for unsealed values of deflection

with Static analysis in Earthquake.

Sample Calculation in X - Direction

a & b are deflections at two corners.

Thus,

[(a+b)/2]/1.2 < a & b < 1.2(a+b)/2]

[(a+b )/2]/1.2 - Lower bound limit of deflection

1.2 [a+b)/2] - Upper bound limit of deflections. .

NOTE:

Similar Calculation shall be made for Z direction also.

All load cases considered as unfactored.

Governing load case is the one in which maximum deflection occurs.

Annexure '5'

For wind forces also Torsional Irregularity shall

be checked in the same manner.

LOAD CASES

I. EQX

2. EQZ

3. DL (self weight, slab weight, floor finish, partition load, sunken load, water tank load, filIing load, etc.)

4. WDL (Wall load)

5. VTL (Vehicular including Fire Tender Load)

6. LL (100%)

7. LL (90%)

8. LL (80%)

9. LL (70%)

10. LL (60%)

11. LL (50%)

I2. +WLX

13. -WLX

14. +WLZ

15. -WLZ

16. TL1 [Shrinkage Load (-10°C)]

17. TL2 [Temperature Load in Exposed Area/Intermediate level (25°C/l0°C)]

Load Combinations

19. Nodal Forces (DL + 0.25/0.5 LL)

20. Reactions DL + 0.5LL - Reaction & Foundation Design

Earthquake Combinations For Orthogonal columns/shear walls orientation

21. DL+EQX

22. DL- EQX

23. DL+EQZ

24. DL-EQZ

25. 1.5(DL + EQX)

26. 1.5(DL - EQX)

27. 1.5(DL + EQZ)

28. 1.5(DL - EQZ)

29. 1.2(DL + LL + EQX)

30. 1.2(DL + LL - EQX)

31. 1.2(DL + LL + EQZ)

32. 1.2(DL + LL - EQZ)

33. O.9DL + I.5EQX

34. O.9DL - 1.5EQX

35. O.9DL + 1.5EQZ

36. O.9DL - 1.5EQZ

Earthquake Combinations for non orthogonal columns/shear wall orientation

21. DL + EQX + O.3EQZ

22. DL + EQX - O.3EQZ

23. DL - EQX + O.3EQZ

24. DL - EQX - O.3EQZ

25. DL + EQZ + O.3EQX

26. DL + EQZ - O.3EQX

27. DL - EQZ + 0.3EQX

28. DL - EQZ - O.3EQX

29. 1.5(DL + EQX + O.3EQZ)

30. 1.5(DL + EQX - O.3EQZ)

31. I.5(DL - EQX + O.3EQZ)

32. 1.5(DL - EQX - O.3EQZ)

33. 1.5(DL + EQZ + O.3EQX)

34. 1.5(DL + EQZ - 0.3EQX)

35. 1.5(DL - EQZ + 0.3EQX)

36. I.5(DL - EQZ - 0.3EQX)

Annexure '5' Page2/4

37. 1.2(DL + LL + EQX + 0.3EQZ)

38. 1.2(DL + LL + EQX - O.3EQZ)

39. 1.2(DL + LL - EQX + O.3EQZ)

40. 1.2(DL + LL - EQX - O.3EQZ)

41. 1.2(DL + LL + EQZ +O.3EQX)

42. 1.2(DL + LL + EQZ - 0.3EQX)

43. 1.2(DL + LL - EQZ + Q.3EQX)

44. 1.2(DL + LL - EQZ - 0.3EQX)

45. 0.9DL + 1.5(EQX + 0.3 EQZ)

46. 0.9DL + 1.5(EQX - 0.3 EQZ)

47. 0.9DL - 1.5(EQX + 0.3 EQZ)

48. 0.9DL - 1.5(EQX - 0.3 EQZ)

49. 0.9DL + 1.5(EQZ + 0.3 EQX)

50. 0.9DL + 1.5(EQZ - 0.3 EQX)

51. 0.9DL - 1.5(EQZ + 0.3 EQX)

52. 0.9DL - 1.5(EQZ - 0.3 EQX)

Wind Load Combinations

53. DL+WLX

54. DL- WLX

55. DL+ WLZ

56. DL- WLZ

57. 1.5(DL + WLX)

58. 1.5(DL - WLX)

59. 1.5(DL + WLZ)

60. 1.5(DL - WLZ)

61. I.2(D L + LL + WLX)

62. 1.2(DL + LL - WLX)

63. 1.2(DL + LL + WLZ)

64. 1.2(DL + LL - WLZ)

65. 0.9DL + 1.5WLX

66. 0.9DL - 1.5WLX

67. 0.9DL + 1.5WLZ

68. 0.9DL - 1.5WLZ

69. 1.5[DL + (100%) RLL]

70. 1.5[DL + (90%) RLL]

71. 1.5[DL + (80%) RLL]

72. 1.5[DL + (70%) RLL]

73. 1.5[DL + (60%) RLL]

74. 1.5[DL + (50%) RLL]

75. 1.5(DL +LL) [Beam Design]

76. DL + TLl + TL2

77. DL + TLl - TL2

78. DL + LL + TLl + TL2

79. DL + LL + TLl - TL2

80. 1.5(DL + TLl + TL2)

81. 1.5(DL + TLl - TL2)

82. I.2(DL + LL + TLl + TL2)

83. 1.2(DL + LL + TLl - TL2)

LEGEND

DL: Dead Load

LL: Live Load

EQX: Earthquake in X Direction

EQZ: Earthquake in Z Direction

RLL: Reduced Live Load

WLX: Wind Load in X Direction

WLZ: Wind Load in Z Direction

TL1: Shrinkage Load (10°C)

TL2: Temperature Load (25°C/l0°C)

WDL: Wall Load

Annexure ‘6”

Check List for STAAD Model

S.No. Description Engineer Mentor

1 Height of structure from architectural drawings and for

time period calculations taken from foundation or ground

2 Framing has been correlated with architectural drawings

3 Shape/Size and orientation of retaining walls/columns

4 For non orthogonal column orientation 30% Earthquake

has been taken in both sides as per code

4 Members sizes have been checked.

5 IYY for beams has been provided OR T-L Beams prop.

Along with area of rectangular section been provided.

6 Orphan nodes, duplicate nodes and members, member

connectivity, member plate connectivity, etc.

7 Loading sheet calculations

8 Loading pattern on whole model and check at few

locations

9 Typical calculations for column load as per tributary

areas

10 Wall load on beam member, 230/115mm thk. Wall

as/Arch.

11 Extra loading due fire tender and filling on the specified

part as per the Arch. Drawing.

12 Extra loading due to services in basement or terrace

13 Loads for lift machine room and water tanks

14 Sunken load in toilets, kitchen, extra load in staircase, no

load in lifts area and cutouts

15 Temperature load & shrinkage load

16 Wind loading, check calculation for few nodes, and

confirm the direction of application for all face of

building

17 Check earthquake calculations, application of nodal

forces, time period for bare frame or average

18 Torsional irregularity in structure due to gravity loads,

earthquake and wind loads

19 Whether the torsional irregularity in limits and how

much

20 Match the theoretical base shear with the STAAD output

21 Load combinations

22 Grade of concrete for the design of columns and beams

23 Torsion due accidental eccentricity (also check torsional

irregularity after applying it)

24 Typical check for wind calculations

We with our best knowledge have checked this STAAD model. All the points

mentioned in the checklist are thoroughly checked and no error has been observed.

For any kind of modeling error or comments by proof consultants, then we are

responsible for that.

Structural Engineer