About the author  ⁄ Mike Mota, Ph.D., P.E., SECB, F.ACI, F.ASCE, F.SEI

Mike Mota, Ph.D., P.E., F.ASCE, F.ACI, is the Vice President of Engineering for the Concrete Reinforcing Steel Institute (CRSI). Mike is an active member of several ACI and ASCE committees, Member of ACI 318 and 318 sub B and sub R, Chair of ACI Committee 314 on Simplified Design of Concrete Buildings, serves on the Board of Directors of the Concrete Industry Board of New York City/NYC ACI Chapter and is a past member of the editorial board of STRUCTURE magazine. He may be reached at mmota@crsi.org.

Part 5: Foundations

Spread Footings

Flexural Reinforcement

Requirements for the distribution of flexural reinforcement in two-way footings are given in Sections 13.3.2.2 and 13.3.3.3 of ACI 318-14, Building Code Requirements for Structural Concrete. For square footings, the reinforcement is to be distributed uniformly across the entire width of the footing in both directions. In the case of rectangular footings, the reinforcement must be distributed in accordance with the requirements in Section 13.3.3.3 of ACI 318-14, which are illustrated in Figure 1. Reinforcement in the long direction is uniformly distributed across the entire width. A portion of the reinforcement in the short direction is uniformly banded over the column with the remainder uniformly distributed outside of the band width.

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Part 1: Two-way Slabs

This article is the first in a series on recommended reinforcement details for cast-in-place concrete construction.

Two-way slabs are generally defined as suspended slabs where the ratio of the long to the short side of a slab panel is 2 or less. In two-way construction, load transfer is by bending in two directions. The main flexural reinforcement usually consists of two mats of reinforcing steel – a top mat and a bottom mat – that run predominately in the directions that are orthogonal and parallel to the rectangular grid of column lines. The bottom mat of reinforcement resists the positive bending moments at the critical sections in the span and is usually continuous over the entire slab area.

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In addition to supporting gravity loads, floor and roof systems in typical reinforced concrete buildings act as diaphragms which transfer the lateral forces to shear walls, frames, or other elements that make up the lateral force resisting system (LFRS). A three-dimensional analysis that considers the relative rigidities of the diaphragm and the elements of the LFRS provides the most accurate distribution of the forces in these components. A more straightforward analysis is possible when assumptions are made concerning the rigidity of a diaphragm.

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Design for SDC D, E, and F Buildings

This article presents an overview of the design requirements for shallow reinforced concrete foundations (spread footings and mat foundations) supporting buildings assigned to Seismic Design Category (SDC) D, E, or F. Also included is a proposed design method that goes beyond requirements in current codes and standards. Although the following discussion focuses exclusively on spread footings supporting members of the seismic-force-resisting system (SFRS), it is also applicable to mat foundations.

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Flat plate voided concrete slab systems have been used for many years in Europe and other parts of the world. These systems are becoming increasingly popular in the U.S. This is due to many inherent benefits which include reduced self-weight (resulting in smaller column sizes and foundations as well as smaller seismic forces); larger allowable superimposed loads for given span lengths; economical longer spans; reduced floor-to-floor heights; and accelerated construction schedules.

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STRUCTURE magazine