Foundation Design using SAFE
Question 1-
Aim- Model the isolated footing provided in week 11 challenge on SAFE. Analyze and design it to verify the size, depth and steel provided as per calculations.
Answer-
Data required for SAFE analysis and design
Unfactored load= 2000/1.5= 1333.33 kN
Dead load= 1333.3/2= 666.7 kN
Live load= 1333.3/2= 666.7 kN
Sub-grade modulus for soil= Safe bearing capacity X Factor of ultimate/ Maximum settlement
k = 150 x 3/0.05= 9000kN/m3
Rebar Yield strength (Fy)= 500 M/mm2
Rebar Ultimate strength (Fu)= 575 N/mm2
Elastic modulus of M25 grade concrete= 25000 N/mm2
Modeling In SAFE
Step 1- Open SAFE and open a new model with Indian Standards
Step 2- Choose the Single Footing option from the footing templates and provide the footing details
Step 3- Go to Define> Material Properties> Add New Property
1) Add M25 grade concrete
2) Add Fe415 grade Steel Rebar
Step 4- Go to Define> Section Properties> Slab Section> Add New Property
Add Footing Slab and Stiff with the following properties
Step 5- Go to Define> spring properties> Area spring> Add New Property
Add Soil150 with the following properties
Step 6-Check the assigned loads
Go to Display> Show Load Assigned> Joints
Dead Load
Live Load
Step 7- Go to Define> Load Combination> Add New Combo
Add SLS and ULS as shown below
Step 7- Go to Design> Concrete Slab/ Beam Design> View/ Revise Preferences
Check design preference
Step 8- Go to Design> Concrete Slab/ Beam Design> Select design combination
Step 9- Go to Edit> Add/ Edit Design Strips> Add Design Strips Using Grids
Step 10- Run the analysis
Step 11- Perform Checks
1) Check for Settlement
Go to Display> Show Deformed Shape
Check SLS combo
For isolated footings, maximum displacement OF 50mm is allowed as per IS 1904-1986.
The settlement, in this case, is way less than the said limit. Hence OK.
2) Check for Soil Pressure
Go to Display> Show Force/ Stress Diagram> Soil Pressure
Check SLS combo
The SBC of soil is 150kN/m/m2 > 149. Hence OK.
3) Punching Shear Check
Go to Display> Show Punching Shear Design
Here it is less than 1. Hence OK.
4) Longitudinal Reinforcement Requirement
For reinforcement requirements Go to Display> Show Slab Design
5) Ultimate Bending Moment
Go to Display strip forces
Question 2-
A column of size 600x600 carrying an unfactored gravity load (DL+LL) of 1175 KN is to be supported on pile cap, which ultimately transfers load to the piles underneath. Piles of 16 metres length and 500 mm diameter are to be used. Each pile has a load carrying capacity of 200 KN. Model, analyse and design the pile cap on SAFE to evaluate the following:
- Pile group to be used
- Depth of pile cap
- Longitudinal steel on top and bottom face in both directions
Answer-
Data required for SAFE analysis and design
Unfactored load= 1175 kN
Dead load= 1175/2= 587.5 kN
Live load= 1175/2= 587.5 kN
Point sprng of pile= A X E/L = 196349 x 25000/ 16000 = 306796 kN/m
k = 306.8 kN/m3
Rebar Yield strength (Fy)= 500 M/mm2
Rebar Ultimate strength (Fu)= 575 N/mm2
Elastic modulus of M25 grade concrete= 25000 N/mm2
Number of Piles = 1.1 x Total load / Bearing Capacity of single pile
= 1175 x 1.1 / 200 = 6.46
Consider 8 piles for symmetric placement
Minimum distance between two piles is 3 x diameter of pile = 3 x 500 = 1500 mm
As the diameter of the pile is given as 500 mm
Now the distance from the edge of the footing to the edge of the pile is 1 x pile diameter= 500mm
Modeling In SAFE
Step 1- Open SAFE and create a new model with IS standard data
Step 2- Enter the grids as per the pile and raft distances.
Step 3- Go to Define> Material Properties> Add New Property
Add M25 grade concrete and Fe500 grade Steel Rebar
Step 4- Go to Define> Section Properties> Slab Section> Add New Property
Add Pile Cap, Column and Pile500 with the following properties
Step 5- Go to Define> spring properties> Point spring> Add New Property
Add PSpr1 with the following properties
Step 6- Draw a Rectangular floor with the property as “Pile cap”
Select “Quick Draw Area around point” > Draw Piles with the property as “Pile500” at each Gridpoint
Select “Quick Draw Area around point” > Draw column with the property as “Column” at the centre
Step 7- Select all joints of Pile500 and assign point spring
Go to Assign> Joint> Springs
Step 8- Select the column joint
Go to Assign> Joint Loads> Force
Assign Dead load and Live load
Step 9 See the loads assigned by display joint loads option
Step 10- Go to Define> Load Combination> Add New Combo
Add SLS and ULS as shown below
Step 11- Go to Design> Concrete Slab/ Beam Design> View/ Revise Preferences
Check design preference
Go to Design> Concrete Slab/ Beam Design> Select design combination
Step 11- Go to Edit> Add/ Edit Design Strips> Add Design Strips Using Grids
Step 12- Run the analysis
Perform Checks
1) Check for Settlement
Go to Display> Show Deformed Shape
Check SLS combo
2) Punching Shear Check
Go to Display> Show Punching Shear Design
Here it is less than 1. Hence OK.
3) Base Reactions at Piles
Go to Display> Show Force/ Stress Diagrams> Support/ Spring Reactions
The pile capacity is 200kN. Hence OK.
4) Longitudinal Reinforcement Requirement
For reinforcement requirements Go to Display> Show Slab Design
5) Ultimate Bending Moment
Go to Display strip forces
