staad modeling
TRANSCRIPT
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