Pier Foundation Design Process:

Shallow pier foundations are a relatively simple design process. Piers are defined by the International Building Code, IBC, as having a ratio of diameter to depth of 12 or less. For flexural reinforcement piers are analyzed as short columns. However due to their relatively short length in comparison to their width, and the lateral support offered by the soil, in many cases by analysis piers do not require much if any flexural reinforcement. However the IBC requires certain minimum amounts of reinforcement. If the designer wants to be conservative in their design, they may disregard the lateral support offered by the soil when designing the pier. Sizing the cross section area of a pier is simply a matter of using the allowable soil pressure and soil frictional resistance to resist the vertical load placed on the pier. The depth of the pier must be calculated to resist any lateral load or overturning moment. If however there is no lateral load or overturning moment, the depth should be set deep enough to go below the local frost depth, to bear on suitable soil and to comply with the pier diameter or width to depth ratio, (typically 2 to 12), chosen for the trial pier design section. The step by step process of pier design is as follows:
   
1. Choose a trial diameter to depth ratio for the trial design pier and then determine the required pier diameter based on the allowable soil bearing capacity or soil bearing pressure and the friction resistance of the soil if applicable. Repeat this step as desired until the suitable depth to width ratio is achieved and the pier depth is below the local frost depth and is bearing on the desired subgrade soil layer.  For convenience typical soil values for different soil types are given in the table below (source of table IBC).
2. Next determine required depth to resist any overturning moment if applicable.  Also check uplift resistance for round or uplift resistance for square pier, if there is a significant uplift force that must be resisted.
 
 
3. Validate the pier design, accounting for any adjacent floor loading etc.
 
4. Design flexural steel required. Analyze the pier as a short column, and if it is desired to be ultra conservative, disregard any lateral support offered by the adjacent subgrade soils. If the pier is very short in relation to the diameter, ratios of 2 to 3, there is adequate lateral support offered by adjacent subgrade, and there is no lateral load or overturning moment placed on the pier, the IBC minimum reinforcement may be all the reinforcement required.
Start Using The Calculations >>> Become A Member Now!