This chapter shall apply to the design of plain concrete members, including (a) and (b):
(a) Members in building structures
This chapter shall not govern the design of cast-inplace concrete piles and piers embedded in ground.
Plain concrete shall be permitted only in cases (a) through (d):
(a) Members that are continuously supported by soil or supported by other structural members capable of providing continuous vertical support
(b) Members for which arch action provides compression under all conditions of loading
(c) Walls
(d) Pedestals
Structures assigned to Seismic Design Category C, D, E or F shall not have elements of structural plain concrete, except as follows:
- Isolated footings of plain concrete supporting pedestals or columns are permitted, provided the projection of the footing beyond the face of the supported member does not exceed the footing thickness.
- Plain concrete footings supporting walls are permitted, provided the footings have at least two continuous longitudinal reinforcing bars. Bars shall not be smaller than No. 4 and shall have a total area of not less than 0.002 times the gross cross-sectional area of the footing. For footings that exceed 8 inches (203 mm) in thickness, a minimum of one bar shall be provided at the top and bottom of the footing. Continuity of reinforcement shall be provided at corners and intersections.
Exceptions:
- For foundation systems consisting of a plain concrete footing and a plain concrete stemwall, a minimum of one bar shall be provided at the top of the stemwall and at the bottom of the footing.
- Where a slab on ground is cast monolithically with the footing, one No. 5 bar is permitted to be located at either the top of the slab or bottom of the footing.
Plain concrete shall not be permitted for columns and pile caps.
Design properties for concrete shall be selected to be in accordance with Chapter 19.
Steel reinforcement, if required, shall be selected to be in accordance with Chapter 20.
Materials, design, and detailing requirements for embedments in concrete shall be in accordance with 20.7.
Tension shall not be transmitted through outside edges, construction joints, contraction joints, or isolation joints of an individual plain concrete element.
Design of precast members shall consider all loading conditions from initial fabrication to completion of the structure, including form removal, storage, transportation, and erection.
Precast members shall be connected to transfer lateral forces into a structural system capable of resisting such forces.
Footing thickness shall be at least 8 in.
Base area of footing shall be determined from unfactored forces and moments transmitted by footing to soil and permissible soil pressure selected through principles of soil mechanics.
Ratio of unsupported height to average least lateral dimension shall not exceed 3.
Contraction or isolation joints shall be provided to divide structural plain concrete members into flexurally discontinuous elements. The size of each element shall be selected to limit stress caused by restraint to movements from creep, shrinkage, and temperature effects.
Upcodes Diagrams
The number and location of contraction or isolation joints shall be determined considering (a) through (f):
(a) Influence of climatic conditions
(b) Selection and proportioning of materials
(c) Mixing, placing, and curing of concrete
(d) Degree of restraint to movement
(e) Stresses due to loads to which an element is subjected
(f) Construction techniques
Required strength shall be calculated in accordance with the factored load combinations defined in Chapter 5.
Required strength shall be calculated in accordance with the analysis procedures in Chapter 6.
No flexural continuity due to tension shall be assumed between adjacent structural plain concrete elements.
The critical section for Mu shall be located in accordance with Table 14.4.3.2.1.
Table 14.4.3.2.1—Location of critical section for Mu
| Supported member | Location of critical section |
|---|---|
| Column or pedestal | Face of column or pedestal |
| Column with steel base plate | Halfway between face of column and edge of steel base plate |
| Concrete wall | Face of wall |
| Masonry wall | Halfway between center and face of masonry wall |
For one-way shear, critical sections shall be located h from (a) and (b), where h is the footing thickness.
(a) Location defined in Table 14.4.3.2.1
(b) Face of concentrated loads or reaction areas
Sections between (a) or (b) of 14.4.3.3.1 and the critical section for shear shall be permitted to be designed for Vu at the critical section for shear.
For two-way shear, critical sections shall be located so that the perimeter bo is a minimum but need not be closer than h/2 to (a) through (c):
(a) Location defined in Table 14.4.3.2.1
(b) Face of concentrated loads or reaction areas
(c) Changes in footing thickness
For each applicable factored load combination, design strength at all sections shall satisfy ϕSn ≥ U, including (a) through (d). Interaction between load effects shall be considered.
(a) ϕMn ≥ Mu
(b) ϕPn ≥ Pu
(c) ϕVn ≥ Vu
(d) ϕBn ≥ Bu
Flexure and axial strength calculations shall be based on a linear stress-strain relationship in both tension and compression.
λ for lightweight concrete shall be in accordance with 19.2.4.
No strength shall be assigned to steel reinforcement.
Mn shall be the lesser of Eq. (14.5.2.1a) calculated at the tension face and Eq. (14.5.2.1b) calculated at the compression face:
 |
(14.5.2.1a) |
| Mn = 0.85f'cSm | (14.5.2.1b) |
where Sm is the corresponding elastic section modulus.
Pn shall be calculated by:
 | (14.5.3.1) |
Unless permitted by 14.5.4.2, member dimensions shall be proportioned to be in accordance with Table 14.5.4.1, where Mn is calculated in accordance with 14.5.2.1(b) and Pn is calculated in accordance with 14.5.3.1.
Table 14.5.4.1—Combined flexure and axial compression
| Location | Interaction equation | |
|---|---|---|
| Tension face |  | (a) |
| Compression face |  | (b) |
[1]
Bn shall be calculated in accordance with Table 14.5.6.1.
Table 14.5.6.1—Nominal bearing strength
| Relative geometric conditions | Bn | ||
|---|---|---|---|
| Supporting surface is wider on all sides than the loaded area | Lesser of: |  |
(a) |
| 2(0.85f'cA1) | (b) | ||
| Other | 0.85f'cA1 | (c) | |
At least two No. 5 bars shall be provided around all window and door openings. Such bars shall extend at least 24 in. beyond the corners of openings.
Upcodes Diagrams
Detailed plain concrete structural walls are walls conforming to the requirements of ordinary structural plain concrete walls and 14.6.2.2.
Reinforcement shall be provided as follows:
- Vertical reinforcement of at least 0.20 square inch (129 mm2) in cross-sectional area shall be provided continuously from support to support at each corner, at each side of each opening and at the ends of walls. The continuous vertical bar required beside an opening is permitted to substitute for one of the two No. 5 bars required by 14.6.1.
-
Horizontal reinforcement at least 0.20 square inch (129 mm2) in cross-sectional area shall be provided:
- Continuously at structurally connected roof and floor levels and at the top of walls;
- At the bottom of load-bearing walls or in the top of foundations where doweled to the wall; and
- At a maximum spacing of 120 inches (3048 mm).