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CONSTRUCTION KNOWLEDGE  >> CONCRETE >>

CONCRETE FOOTINGS
 


 

1. What are the Basics of Concrete Footings?
2. How do I Stage and Plan the Job for Footings?
3. How do I Place Concrete Footings?
4. Tricks of the Trade and Rules of Thumb for Concrete Footings:


What are the Basics of Concrete Footings?


Concrete footings exist to transfer the structural loads from the building (snow load, live load, dead load, wind load, earthquake load, etc.) to the Earth. Obviously, the foundations on a building are critical from a structural standpoint. However, since they are usually unseen after backfilling, too many times the foundations are not built without enough attention to detail.

It’s often said that any building project that doesn’t come out of the ground well will be difficult to ever get back on track. It’s important for the Construction Supervisor to understand both the function and the acceptable tolerances on foundations. In order to understand the function, some terms need defined:

  1. Concrete spread footing under a single column: this footing typically has the column in the middle of the footing. As the force acts downward from the column, the spread footing distributes that force to the soil. The bottom of the spread footing is in tension from the bending moment created by the column force and the earth resistance. Many typical spread footings with a single column are designed using Construction Reinforcing Steel Institute (CRSI) Tables for Footings. Engineers tend to use these tables, or computer design software, rather than design footings manually each time. While this information is not in the public domain (so I can't just provide those tables here) you could pick up a used CRSI Design Handbook at a low cost.
  2. Concrete wall footing: this footing has continuous wall above. The forces acting downward through the wall are distributed by the wall footing to the Earth.
  3. Concrete spread footing under multiple columns: depending on the column locations and loads, the layout of this footing can vary greatly. Footing rebar is often required in the top and bottom of spread footings with multiple columns because of changes in the bending moments.

How do I Stage and Plan the Job for Footings?


While some site work needs to proceed the footing installation, the planning for the footings typically begins the building construction process. It’s difficult to overstate the importance of getting that beginning done right. All other building work follows the footings, literally sitting upon them. Take the time to get the footing planning and execution done well.
 
One of the first steps in planning the footing work concerns the location of the surrounding sub grade. In some instances, it’s better to keep the existing grade low, install the footings with minimal excavation, then place the fill in the building after footing and foundation wall excavation. In most instances, it’s easier to bring the building pad to final sub-grade prior to starting footing excavation. This is a decision that should be evaluated on each project.
 
The layout of the footings and what to do with the footing excavation excess soil material should be considered together. Some construction supervisor’s use batter boards at each corner to give the line and the grade of the building. Others use offset hubs to determine line and an elevation benchmark for grade. With either method, you need to consider how much offset is required from the actual wall lines in order to accomplish the excavation. These items should be thought through in advance of the actual layout and footing excavation in order to proceed in the most effective manner.
 
Also consider water related problems at his stage. What will a hard rain do to the site in the middle of footing excavation? Will footings needed pumped? Can they be gravity drained? Will mud be created and how will it be dealt with? Since this is such an important project stage, it’s prudent to have some contingency plans ready for a few predictable scenarios.
 
The construction supervisor need not be paralyzed by planning, spending all his time considering worst case scenarios. But the construction supervisor should have put some thought into a few scenarios that may happen and at least have an idea of how he will proceed. A bit of extra planning at this stage just makes sense.


How do I Place Concrete Footings?


The actual installation of concrete footings varies greatly from job to job. Sometimes footings need to be formed, many times earth forming works fine (using the Backhoe bucket to neatly excavate the footing size in the soil). The need for formwork in footings sometimes depends on the building pad elevation, as discussed above.
 
The rebar placement in the concrete footing stage generally will be required to be inspected by the local building official. It’s important the proper clearances are available (3” from earth, 1 ” from forms, etc.) in order to provide protection for the rebar from future corrosion. It’s also important to tie the rebar securely, due to all the jostling and pushing that happens during footing concrete placement. The rebar needs to be in the specified locations, not only before the concrete is poured, but after.
 
The concrete placement of footing again varies greatly depending on the project circumstances. Sometimes the Ready Mix Concrete truck can run along the footing trench and chute concrete directly into the footings. Other times concrete buggies or wheel barrows need to carry the concrete from the truck to the footing. Of course, some projects use concrete pumps for transporting the concrete to the foundations.
 
Before the concrete placement starts, the construction supervisor should discuss some contingencies with the concrete foreman. For example, what happens if the there is a plant shut down and the concrete trucks stop coming? What route through the site may the concrete trucks use? Will this change in foul weather? Where does any extra concrete from a truck get placed? Where may Ready Mix concrete trucks wash out their mixers and chutes?
 
The construction supervisor should be especially vigilant about the concrete footings. The method of layout should be approved and verified. The concrete foreman should describe how he will get the top of footings at the correct elevation. When earth forming is used, there are too many cases when concrete footing elevations are too high or too low.
 
There are several ways to make sure the footings are installed correctly (temporary rebar grade stakes that get pounded down into the wet concrete after grade is set, wet benching with a laser level and establishing acceptable grade benches, or several other options). The construction supervisor doesn’t need to specify the method used, but does need to understand it and agree that the proposed method will achieve acceptable results. It’s also valuable to wander by during the first few concrete footing placements and verify that the proposed method is actually being appropriately followed.


Tricks of the Trade & Rules of Thumb for Concrete Footings:


  1. Think about building pad fill placement and concrete footing excavation carefully, weigh the several options that may be available.
  2. Be prepared for heavy rains during footing installation.