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Rectangular and continuous footing design is one of the most important aspects of any building project. The footing supports the foundation and bears the entire weight of the structure. Make a mistake in the design or construction of the footing, and the results could be disastrous.
The purpose of the footing
The footing provides a stable flat supportive base that distributes the weight of the load of the structure into the surrounding soil. The weight distribution is spread out through the soil as the distance increases from the footing. For this reason, the texture of the soil is crucial to the success of any rectangular or continuous footing design.
The soil immediately surrounding the footing is the most crucial area and is also generally the most damaged area as well. When the soil is disturbed during the digging of the footing, the texture of the soil is changed and air is introduced. This reduces the bearing capacity of the soil. For this reason, the soil must be tightly compacted before pouring a concrete footing. This can be done with a vibrating plate compactor, a jumping jack compactor, or by adding soil, sand, or gravel to create the proper consistency.
Uncompacted soil leads to settling in the foundation of the structure. This can lead to
- crack plaster
- even structural failure if the displacement is severe
1/2 inch or larger gravel is often used as a base for backfill during rectangular and continuous footing design. The size of the stone ensures that concrete maintains contact with rock, and creates a self-compacting base that is capable of supporting the footing.
Different types of soils such as sedimentary rock, sand, gravel, silt, and clay all have different load bearing pressures. Soil testing is often required before the footing design process can proceed. A hand penetrometer can be used on-site to test the pressure capacity of the soil.
Soil that is incapable of supporting larger load can be compensated for by installing a wider footing. If the footing extends beyond the structure wall for distance that is greater than the thickness of the footing, the concrete can crack. Proper concrete dimensions are crucial to providing a stable footing. Rectangular and continuous footing design software is often used to calculate precise floating dimensions for varying soil types, structural sizes, and steel reinforcement requirements. The precise calculations provided by the software ensure proper low distribution and structural integrity.
StruCalc’s footing design module allows for the design of square, rectangular and/or continuous footings. The footing module will provide size and reinforcement requirements for any concrete footing loaded with pure vertical load. After a footing type has been specified StruCalc will then want to know what is providing the load; this will mean specifying the column type for the square or rectangular footing, or the stemwall type for the continuous footing.
Here you can see the load selection criteria for square and rectangular footings:
And here you can see selecting a continuous footing allows for selecting the stemwall type and weight in pcf:
After the footing type has been entered it will then be appropriate to enter the loads on the footing. The loads can come from two sources: calculations elsewhere in the structure that might result in reactions from beams, columns, walls, etc., or they might come from the use of the load calculator. The load calculator will generate loads on the footing based on user set floor, roof, and wall loads followed by specifying tributary widths or areas.
You can see the standard load inputs of Live Load and Dead Load above. Selecting the Load Calculator option gives you this calculator:
Simply filling in the load options in the calculator will automatically calculate the Live Load and Dead Load.
After the loads have been entered the there remains some additional footing/environment information that will need to be entered. Steel yield strength, concrete compressive strength, soil bearing pressure, reinforcement cover, and reinforcement bar size will all need to be entered. StruCalc preloads some common values for the above mention properties, but the user is free to change any that might not be accurate. More information about the column would also need to be entered now when doing a square or rectangular footing design.
Now, when designing a square or rectangular footing StruCalc will require a depth and a trial footing width (and length for rectangular footing) to be entered.
After, they have been entered StruCalc will verify their adequacy as well as give the reinforcement requirements. If the dimensions of the footing are inadequate a red bar will appear in the lower right hand corner of the screen and new dimensions will need to be entered. It will be at this point though, that StruCalc will have calculated a required area for the footing. This will allow the user to specify dimensions to satisfy the area required.
If the user is designing a continuous footing then the stemwall thickness and height, as well as the footing depth will all need to be entered. After they have been entered StruCalc will generate a footing width required as well as the continuous reinforcement that is required.
For either of the three types of footings you can also add a soil weight load and it will compensate in the footing’s total load for the additional required load. You can select this option using the Calculate Soil Weight Above Footing as shown below: