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, stage, before proceeding proceeding to the next stage. stage. If they have any kind kind of confusion 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 much more time to be caught caught and rectifie rectified d at later later stage of modeling. modeling. Since 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 and levels levels should be be taken from from sectional sectional details.. 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 generating the geometry. geometry. The columns should 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 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
1.2
1.3
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
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
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
2.2
False ceiling including electrical fixtures
=
0.025 T/Sqm
HVAC Ducts
=
0.025 T/Sqm
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
Step
Finishing
Total
4.
=
[0.15 x 2.5 x I]/[Cos 33.40°]
=
0.449 T/Sqm
=
(0.5 x 0.184 x 0.25 x 2.5)/0.25
=
0.23 T/Sqm
=
[(0.184 + 0.250) x 2.5 x 0.04]/0.25
=
0.174 T/Sqm
=
0.853 T/Sqm
=
0.2 T/Sqm (residential building)
=
0.4 T/Sqm (commercial /institutional building)
=
0.5 T/Sqm (for parking in basements)
=
0.3 T/Sqm (residential building)
=
0.5 T/Sqm (commercial/institutional building)
≅
0.9 T/Sqm
Live Load
a)
b)
All Floor
Balconies
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
c) Base Shear VB = WAh
W=
Z
= Zone factor
I
= Important factor
R
= Response Reduction factor
Sa/g
= Average response accel. Coefficient
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. For wind forces also Torsional Irregularity shall be checked in the same manner.
Annexure '5' 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
1
Height of structure from architectural drawings and for
2 3 4
time period calculations taken from foundation or ground Framing has been correlated with architectural drawings Shape/Size and orientation of retaining walls/columns For non orthogonal column orientation 30% Earthquake
4 5
has been taken in both sides as per code Members sizes have been checked. IYY for beams has been provided OR T-L Beams prop.
6
Along with area of rectangular section been provided. Orphan nodes, duplicate nodes and members, member
7 8
connectivity, member plate connectivity, etc. Loading sheet calculations Loading pattern on whole model and check at few
9 10
locations Typical calculations for column load as per tributary areas Wall load on beam member, 230/115mm thk. Wall
11
as/Arch. Extra loading due fire tender and filling on the specified
12 13 14
part as per the Arch. Drawing. Extra loading due to services in basement or terrace Loads for lift machine room and water tanks Sunken load in toilets, kitchen, extra load in staircase, no
15 16
load in lifts area and cutouts Temperature load & shrinkage load Wind loading, check calculation for few nodes, and
17
confirm the direction of application for all face of building Check earthquake calculations, application of nodal
18
forces, time period for bare frame or average Torsional irregularity in structure due to gravity loads,
19 20 21
earthquake and wind loads Whether the torsional irregularity in limits and how much Match the theoretical base shear with the STAAD output Load combinations
Engineer
Mentor
22 23
Grade of concrete for the design of columns and beams Torsion due accidental eccentricity (also check torsional
24
irregularity after applying it) 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
