Structural Detailing of Reinforcement for Economy - Simplified Design of Reinforced Concrete Buildings - page 244

8.6 DEVELOPMENT OF FLEXURAL REINFORCEMENT

8.6.1 Introduction

The requirements for development of flexural reinforcement are given in ACI 12.10, 12.11, and 12.12. These
sections include provisions for:

• Bar extensions beyond points where reinforcement is no longer required to resist flexure.

• Termination of flexural reinforcement in tension zones.

• Minimum amount and length of embedment of positive moment reinforcement into supports.

• Limits on bar sizes for positive moment reinforcement at simple supports and at points of inflection.

• Amount and length of embedment of negative moment reinforcement beyond points of inflection.

Many of the specific requirements are interdependent, resulting in increased design time when the provisions
are considered separately. To save design time and costs, recommended bar details should be used. As was
discussed earlier in this chapter, there is potential overall savings in fabrication, placing, and inspection costs
when recommended bar details are used.

8.6.2 Requirements for Structural Integrity

The preceding chapters covered structure design under code prescribed conventional loads. Structures may
suffer local damage from severe local abnormal loads, or events which are not considered in the design. Such
loads or events include explosions due to gas or industrial liquids, vehicle impact; impact of falling objects,
and local effects of very high winds such as tornadoes. The overall integrity of a reinforced concrete structure
to withstand such abnormal loads can be substantially enhanced by providing relatively minor changes in the
detailing of the reinforcement. ACI 7.13 provides detailing provisions intended to improve the redundancy and
ductility of structures. This is achieved by providing, as a minimum, some continuity reinforcement or tie
between horizontal framing members. In the event of damage to a major supporting element or an abnormal
loading event, the integrity reinforcement is intended to confine any resulting damage to a relatively small area,
thus improving overall stability. The provisions of ACI 7.13 are not an alternate to design for major abnormal
loads and events such as; blast load and progressive collapse. Design for these abnormal loads is beyond the
scope of this publication.

8.6.3 Recommended Bar Details

Recommended bar details (including provisions of ACI 7.13) for continuous beams, one-way slabs, one-way
joist construction, and two-way slabs (without beams) are given in Figs. 8-3 through 8-6. Similar details can
be found in References 8.1 and 8.6. The figures may be used to obtain bar lengths for members subjected to
uniformly distributed gravity loads only; adequate bar lengths must be determined by analysis for members
subjected to lateral loads. Additionally, Figs. 8-3 through 8-5 are valid for beams, one-way slabs, and
one-way joists that may be designed by the approximate method given in ACI 8.3.3.* Fig. 8-6 can be used to
determine the bar length s for two-way slabs without beams.**

* Under normal conditions, the bar lengths give in Figs. 8-3 through 8-5 will be satisfactory. However, for special conditions, a m o re
detailed analysis will be required. In any situation, it is the responsibility of the engineer to ensure that adequat e b ar l en gths are provided.
** To reduce placing and inspection time, all of the top bars in the column strip of a two-way slab system can have the same length at
a particular location (either 0.30 ˜ n for flat plates or 0.33 ˜ n for flat slabs), instead of the two different lengths shown in Figs. 8-6(a)
an d 8 -6 (c).