ACI 318 and monolithic footings

[christopher.banbury]

Posted: Mon Nov 17, 2008 5:38 pm
Post subject: Re: ACI 318 and monolithic footings

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Note that by ACI 318 15.1.1, Chapter 15 applies to the design of isolated footings, combined footings, and mats. The typical monolithic footing is none of these.

When you try to apply the structural concrete provisions of ACI 318 to monolithic footings you will discover several things, some of which i have commented on previously:

http://www.boaf.net/phpBB2/viewtopic.ph ... highlight=

1. the classic prescriptive 16x16 footing with (2) no.5 longitudinal bars does not meet the minimum reinforcement ratio required by ACI 318 10.5.
2. there are no shear stirrups in prescriptive footings.
3. there are no top bars in prescriptive footings.
4. there are no transverse bars in prescriptive footings.

Basically, the steel in prescriptive footings is not structural in that it is not sized based on any calculated load or loading configuration. In order to avoid providing items (1) thru (3) the prescriptive code assumes (a) that building loads are transmitted more-or-less uniformly to the footing and (b) that footings are uniformly supported by soil. In order to avoid providing item (4) the building loads and footings widths are kept small so that the cracking moment and shear capacity of the concrete is not exceeded.

If the soil conditions cannot be expected to provide uniform support then a grade beam or structural mat foundation would be in order. This type of design would consider stress reversals in the footing due to loss of soil support at critical locations and would most likely include top steel, stirrups, and minimum reinforcement per ACI 318.

If the structure does not load the monolithic footing uniformly as at a wall opening or if wall footings are wide due to soft soils or high building loads then the designer has to decide if the prescriptive assumptions have been substantially exceeded so that a wider footing or spread footing with transverse reinforcement is called for. At this point it would be wise to consider ACI 318 in the design although one might still make the case that it doesn't technically apply to monolithic footings.
_________________
Christopher Banbury, PE
President
Ark Engineering, Incorporated
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