Simplified Design for Footings - Simplified Design of Reinforced Concrete Buildings - page 212

Chapter 7
Simplified Design for Footings

7.1 INTRODUCTION

A simplified method for design of spread footings is presented that can be used to obtain required footing
thickness with a one-step design equation based on minimum footing reinforcement. Also included are simplified
methods for shear, footing dowels, and horizontal load transfer at the base of a column. A simplified one-step
thickness design equation for plain concrete footings is also given. The discussion will be limited to the use of
individual square footings supporting square (or circular) columns and subject to uniform soil pressure. The design
methods presented are intended to address the usual design conditions for footings of low-to-moderate height
buildings. Footings that are subjected to uplift or overturning are beyond the scope of the simplified method.

Concrete specified strength for footing design must satisfy strength design requirements and durability
requirements (ACI Chapter 4). The design examples in this chapter assume concrete specified strength of
4000 psi. In cases where the foundation is exposed to freezing and thawing, deicing chemicals or severe levels
of sulfates, the required concrete strength should be determined based on ACI Chapter 4 requirements.
The simplified design equation provided in this chapter is applicable to a wide range of concrete strength.
Also, charts are provided for concrete strength ranging from 3 to 6 ksi.

7.2 PLAIN CONCRETE VERSUS REINFORCED CONCRETE FOOTINGS

Reinforced concrete footings are often used in smaller buildings without considering plain footings. Many
factors need to be considered when comparing the two alternatives, the most important being economic
considerations. Among the other factors are soil type, job-site conditions, and building size (loads to be
transferred). The choice between using reinforcement or not involves a trade-off between the amounts of
concrete and steel. The current market prices of concrete and reinforcement are important decision-making
parameters. If plain footings can save considerable construction time, then the cost of the extra concrete may
be justified. Also, local building codes should be consulted to determine if plain concrete footings are allowed
in certain situations. For a given project, both plain and reinforced footings can be quickly proportioned by the
simplified methods in this chapters and an overall cost comparison made (including both material and
construction costs). For the same loading conditions, the thickness of a plain footing will be about twice that
of a reinforced footing with minimum reinforcement (see Section 7.8).

7.3 SOIL PRESSURE

Soil pressures are usually obtained from a geotechnical engineer or set by local building codes. In cities where
experience and tests have established the allowable (safe) bearing pressures of various soils, local building codes