STAAD Foundation Advanced Tutorials: Series 2 – Isolated Footing Design

Design an Isolated (spread/pad) Footing with STAAD Foundation Advanced

 

This is the second post to introduce and explain a series of tutorials created to demonstrate different features of STAAD Foundation Advanced. In a former blog post, the STAAD Foundation Advanced Tutorial: Series 1 – The Basics taught users how to set up proposed foundations and about the importance of global data. The following is the precursor of the video related to isolated footing design.

Isolated footing is probably the most used foundation type in the program. It is also probably the simplest of most foundation types—rectangular in shape and placed under one column or support point. Engineers often use traditional equations as shown below to calculate soil pressures at footing corners.

But the above mentioned equation may not work well if the eccentricity falls beyond the “kern.” Due to biaxial moments, stress at all four corners can be different and if the moments are large enough, there could be uplift or a part of the slab may not be in contact with the soil as shown in the figure below. It can be tedious to capture all these scenarios by manual calculations and often assumptions and approximations are made. A program like STAAD Foundation Advanced uses innovative techniques to accurately calculate soil pressures at all four corners including foundation uplift.

Please note, the above image is for illustration purposes only. Finite element analysis (FEA) is used here to analyze an isolated footing and base pressure contour shows how soil pressure is acting on slab. One corner is not in contact with the soil and a pink line shows a large portion of slab is in uplift. It also shows one corner is subjected to excessive pressure. The same support was designed as a rigid foundation for maximum allowable bearing capacity of 4 kip/ft2 and the program automatically picked up a proper foundation dimension. The table shows the soil pressure at four corners and evidently it matches the base pressure contour shown above. Also note, pressure at corner 2 shows zero value as that corner is out of contact with the soil. Similarly, corner 4 shows maximum base pressure and is very close to allowable bearing capacity.

Load Case

Pressure at
corner 1 (q1)
(kip/ft2)

Pressure at
corner 2 (q2)
(kip/ft2)

Pressure at
corner 3 (q3)
(kip/ft2)

Pressure at
corner 4 (q4)
(kip/ft2)

1

0.5056

0.0000

0.5056

3.7741

1

0.5056

0.0000

0.5056

3.7741

1

0.5056

0.0000

0.5056

3.7741

1

0.5056

0.0000

0.5056

3.7741

 

The other important benefit of STAAD Foundation Advanced (SFA) is to calculate the most optimized foundation dimensions by iterating through hundreds of load combinations as needed. Depending on project requirements one or more dimensions could be fixed. The software offers many other design utilities like input for cohesion to resist excessive uplift or the depth of water table to calculate the effect of buoyancy.

“Rigid Foundation Settings” options under “Global Settings” should be carefully reviewed before designing an isolated footing. Some of the most important options are:

  1. Type of Bearing Capacity (Gross capacity or Net Capacity)
  2. Concrete Check self weight settings
  3. Reinforcement Placement Option 

Gross and net bearing capacity

The difference between gross and net bearing capacity is the soil overburden pressure. It can be written:

 Gross Bearing Capacity = Net Bearing Capacity + Soil Overburden

 

 

 

 

 

 

 

 

 

 

Concrete check self weight settings

Self weight and surcharge are normally not added to design load cases as the shear due to these loads are nullified by the resulting base pressure. But in some cases eccentricity might fall beyond the footing length and the solution may never converge. In that scenario, self weight is added to obtain a solution for shear check and moment design. STAAD Foundation Advanced provides three options to handle these situations. It is recommended to use “net Pressure” option which should handle most scenarios automatically.

Reinforcement placement option

Moment lever arm to calculate reinforcement requirement is dependent on this option. The following figure is self explanatory. In previous versions of SFA used to assume Mz (moment about global Z axis) is the major axis moment and the bottom most layer of reinforcement will always be parallel to global X direction. Now the user has the option to place reinforcement based on the structural requirements.

 

Soil depth for top or bottom fixed option

This option greatly influences the calculation of dead load for service design. It is part of the design parameters under “Soil & Cover” section. The user can either fix the depth of the base of the footing or top of the footing. These options are called “bottom fixed” and “top fixed” respectively (refer to the image below).

 

 

 

 

 

If “top fixed” option is chosen, program will increase footing depth downwards; whereas if “bottom fixed” option is chosen, program will increase footing depth upwards. “Bottom fixed” option is normally chosen when engineers/contractors want to keep soil excavation depth same for all foundations of a building or similar structure.

If initial user-provided minimum footing depth increases during foundation sizing and “bottom fixed” option is used, the program will automatically change actual soil depth on top of the footing and pedestal height. As footing depth increases during concrete checks, the program will re-iterate through all service load cases and calculate actual bearing pressure for new soil and pedestal height.

Conclusion:

Isolated footing is the simplest form of foundations to design, but to take full advantage of a software program, it is important to go through the tutorials and understand various options and utilities. STAAD Foundation Advanced is the leading foundation design software program which automatically offers a detailed step-by-step calculation sheet along with code clauses, equations, and references. But different options influence those calculations, some options like cohesion may not be used in all cases, but in other special cases those could be very useful. So, to optimize your foundation design it is important to know the assumptions made and options offered by STAAD Foundation Advanced.

In addition, these videos explain more specific about Isolated Footing job:

1)      STAAD Foundation Adv 07: Isolated footing jobs

2)      STAAD Foundation Adv 05: Creating Multiple Jobs on a Single Foundation