PART 1- MODELING AND ANALYSIS
SAFE FOUNDATION AND SLAB DESIGN
CHAPTER 1: Model & Design of Concrete Floor & Foundation Systems CSI SAFE: SAFE offers a single user interface to perform: Modeling, Analysis, Design, Detailing and Reporting SAFE give a facility for quickly starting a new model including flat slabs, two-way slabs, base mats, waffle slabs, ribbed slabs, and single or combined footings. Easily model soil supports and zero tension soil models with uplift analysis. The area assignment of soil supports is based on the sub grade modulus, and they automatically adjust whenever the mesh changes. Base mat foundation models can include pedestals, walls, columns, beams, and piles in addition to the foundation area. Safe offers 2 methods to model Foundation or Slab Design 1) Direct Design Method or 2) Export from ETABS and import to SAFE. A very useful and powerful way to start a model in SAFE is to import the model from ETABS. Floor slabs or base mats that have been model in ETABS can be exported from ETABS.
Foundation Design: Procedure for exporting ETABS model to SAFE: Steps: First of all take Base plan in ETABS and make footings as per the loads come from support reactions after analysis.
Steps to draw footings in ETABS: After taking base plan. Go to DRAW > draw joint objects> then make plan offset in x and y direction. After getting offset drag slab on total corners and mark as footing.
Note: what offset u has to take? Whatever total load you are getting on column just take that load and divide with soil bearing capacity u will get the required area from that u will get the footing size. After getting the joints we can directly drag the slab from joint to joint or otherwise we can draw a null beam from joint to joint which we have got from plan offset in above figure. After getting the slab i.e. footing we can delete that null beams. See below figure how we will get the footings in ETABS using joint object.
Footings in ETABS
There are 3 options to Export from ETABS to SAFE: 1) Floor loads: This is a common option and should be used for most gravity floor systems, when there is no transfer of loads from floors above, or the floors from above has small influence on floor being exported 2) Floor loads and loads from above: This option is recommended when loads from above have an important effect of floor being exported (i.e. transfer floor), for foundation design and also this is the recommended option to export gravity and lateral loads. In this case loads in Safe will be separated in two load patterns floor loads and loads from above (i.e. Dead Above, Dead), these load patterns will be combined in one load case (i.e. DEAD) 3) Floor loads plus columns and wall distortions: This option should be used when exporting temperature loads, in this case walls and columns will be replaced by fixed supports in SAFE, and deformation will be assigned to these nodes
For foundation design take 2 option (floor loads and loads from above) we have to take. 1. When exporting from ETABS for foundation design (as opposed to elevated slab), use the export floor loads and loads from above option with the BASE Story to export selected.
Notes: Model must be analyzed and locked to export. From the above figure appropriate floor load option can be selected along with the desired load cases. After the model has been exported as an .f2k text file the same file can be imported into safe Using the file menu > import command. Using the import and export steps will complete the transfer of the foundation and slabs geometry, section properties, and loadings for the selected load cases. 1 Import .f2k file to SAFE.
2 Check material property whether it has import correctly from ETABS or not.
3 Define footing property and assign to all footings.
Note: when defining footing property there will be one option STIFF better to give same property in STIFF option as given in footing property same material same footing thickness etc.
4 Define soil sub grade modulus to foundation as per soil condition and bearing capacity of the soil and assign to all the columns as supports.
Formula for finding soil sub grade modulus: 40 *3* bearing capacity 5 Check load cases and load combinations as there are correctly import from ETABS or not if not then define the load patterns and load combinations which are useful for foundation design. Define > load patterns & load combinations > ok
6 Check design preferences. 7 Then Run the building analysis using the Analyze menu > Run Analysis Command. 8 After analysis checking required
Soil pressure: Display menu > show reaction forces. Check soil pressure for (service) load combination and the condition for checking soil pressure. Soil pressure must be > zero Soil pressure must be < bearing capacity. Note: If soil pressure is showing more than bearing capacity then we have to increase the footing size and it has to be increase in 2 directions also ( Go to Edit > edit areas > expand or shrink area) with this option footing area will increase in 2 directions.
Punching shear: for footings punching shear must be less than 1. If greater than 1 then increase the footing size. Note: after analysis and design if N/C is showing it means then is no problem of punching shear there. This will be shown when strap beam is provided from footing to footing.
10. Footing Design (Reinforcement Details): After analysis. Go to run analysis and Design. Go to Display > Show Slab Design > design basis > finite element method. Contour range: Minimum - 4* 89 = 356 mm2 / m
Maximum - 4* 89 = 356 mm2 / m Check
Direction 1 – Top rebar Direction 2 – bottom rebar.
Note: for knowing how the footing is safe or not if pink color appears on footings then all the footings are safe in reinforcement ( reference see below figure ). If you get blue color on footings then u have to change reinforcement in contour range. Ex (change contour to 6*89 = 539) like this.
Floor slab Design: Procedure for exporting ETABS model to SAFE for floor design: Steps: First of all take floor plan in ETABS and Go to file > export > story as SAFE V12.f2k file. Next open SAFE software Go to file > import > SAFE .F2K file. After importing the floor slab from ETABS first of all u has to check the loads on slabs what assign
in ETABS are it correctly import or not? Go to display > show loads> live or dead etc. And check the load combinations what has given in ETABS. Note: Especially for long term deflection checking only we have to add more load combinations in SAFE otherwise for normal deflection we have to check for Service load combination it’s enough.
Fig: Floor Slab Design
Fig: Floor Loads Check material property whether it has import correctly from ETABS or not. Go to Define >Materials> ex (M30, FE 415) etc.
Check slab property whether it has import correctly from ETABS or not. Go to Define> slab properties > ex (S200) etc.
Note: when defining SLAB property there will be one option STIFF better to give same property in STIFF option as given in slab property same material same slab thickness etc. Checking required in floor slab design: o Deflection check o Punching shear. Then Go to > Analysis > Run analysis After analysis > go to Display > show deformed shape > check for load combination > ultimate. See the min displacement at the bottom of the screen as shown in below figure. It has to be less than 25mm which is allowable deflection as per code. If it is more than 25mm then u have to increase the slab thickness for deflection control. Note: for long term deflection also same procedure but there will be some extra load cases and load combinations. And especially we will check for load combination (long term deflection).
PUNCHING SHEAR CHECK: After deflection check Go to > Analysis > Run analysis and design.
Go to > Display > show punching shear > if less than one near columns then ok otherwise select the columns were the pinching is more than 1 then Go to design > punching shear overwrites > use the option reinforcement allowed ( rebar ties) and we can use studs ( but studs are so costly) instead of studs we can use rebar ties around the columns. For slabs punching shear must be less than 1. If greater than 1 then . Note: after analysis and design if N/C is showing it means then is no problem of punching shear there. This will be shown when beam is provided from column to column. After all checking slab design reinforcements required: Go to display > slab design > design basis > finite element method > Contour range: Minimum - 4* 89 = 356 mm2 / m Maximum - 4* 89 = 356 mm2 / m Check
Direction 1 – Top rebar Direction 2 – bottom rebar.
Note: for knowing how the slab is safe or not if pink color appears on footings then all the footings are safe in reinforcement ( reference see below figure ). If you get blue color on footings then u have to change reinforcement in contour range. Ex (change contour to 6*113 = 678) like this. Mostly for flat slab is there then u have to put total 12mm for total slab and then check mostly u will find the extra steel near columns then u have to provide extra reinforcement near column only. See below fig: steel reinforcement.
