David P. Gustafson

Beams and Girders

Although the terms “beam” and “girder” are frequently used in all codes, and, specifically, in the 1995 ACI Code, it was considered unnecessary to provide definitions for these terms in the Code. The common usage of these terms was evidently considered sufficiently definitive. For the purposes of this book, beams and girders are considered to consist of flexural members, generally horizontal, which act as a primary part of the structural framing system. A beam is herein considered to carry distributed loads such as parallel walls or tributary areas of the floor or roof systems; a girder, although it too may support these distributed loads, supports major concentrated loads such as columns or beam reactions. Most of the special provisions of the ACI Code will be applicable to both beams and girders under the authors’ definitions of common usage, even where the Code refers merely to beams. Most of the requirements of the general codes in effect, such as fire protection and reduction of live loads, will also apply equally to both. For convenience in this chapter, all references to beams will apply equally to girders; provisions peculiar to girders will be identified.

Structural Plain Concrete

When the title of the 1989 Code (Revised 1992) was changed from “Building Code Requirements for Reinforced Concrete” to Building Code Requirements for Structural Concrete (ACI 318–95), Chapter 22 on structural plain concrete was added. Structural plain concrete is defined as either unreinforced or with less reinforcement than specified in the Code for reinforced concrete (Section 2.1). Chapter 22 provides minimum requirements for design and construction of structural plain concrete members (cast-in-place or precast).

Structural Lightweight Aggregate Concrete

Unless special exceptions are made, the entire Code is applicable to lightweight aggregate concrete. The principal exceptions and special provisions for lightweight aggregate concrete are discussed and illustrated in the following section.

Two-Way Solid Flat Plate Design

Traditionally, the term “flat slab” has represented a solid uniform depth slab with thicker drop panels supported upon “mushroom head” conical capitals with round columns or various prismatic brackets with square columns. Early codes distinguished between four-way reinforcement and two-way reinforcement systems. The four-way system, intricate by today’s standards, was soon abandoned because of the excessive field labor required, particularly with the truss bar systems popular then. Also, effective depth was sacrificed when four layers of the reinforcement intersected at the columns. As forming costs increased, most flat slab design eliminated either the drop panels or the capitals and brackets. Today, the flat plate form is preferred for light loads and short spans; the flat slab with capitals and brackets only, in heavy duty designs where few partitions are used; and the flat slab with drop panels only, for heavy duty design with or without partitions. Edge beams can be used with all three forms of the solid flat slab, but they are usually avoided where not structurally required to avoid the added forming expense. The waffle flat slab (Chapter 7) extends both the span and load limits without excessive dead weight.

One-Way Reinforced Concrete Slabs

The simplest routine structural element for illustration of design provisions in the Code is the one-way slab. A one-way slab is defined for purposes of this book as a flexural member with depth small relative to other dimensions, supporting (gravity) loads applied normal to and directly over its surface, spanning in one direction between parallel supports, and reinforced for flexure in that direction only. For purposes of analysis, one-way slabs may be restrained to any degree at the supports or may be unrestrained. A number of Code provisions refer to “flexural members,” which include one- and two-way slabs, beams, girders, footings, and, where bending exists with the axial load, walls and columns. In general, when the Code provision is intended to apply to one-way slabs, the term will be used in the sense of the definition herein. For example, the Code requires stirrups in most beams, but specifically exempts slabs and footings from the requirement (Section 11.5.5.1). Where the Code term “structural slabs” is used, as in the provision that the minimum reinforcement in the direction of span shall be that required for temperature and shrinkage (Sections 7.12 and 10.5.4), the requirement applies to both one- and two-way slabs as herein defined. In this book, one-way slabs and beams are discussed separately. Beams are defined herein as one-way flexural members designed to support concentrated line loads such as slabs, girders as one-way flexural members designed to support concentrated loads such as beams. Either beams or girders or two-way slabs may be utilized as primary members of a frame. For design of beams, see Chapter 9. For design of two-way slabs, see Chapters, 5, 6, and 7.

Structural Analysis and Design-General

The Code is based upon design in two stages, strength and serviceability. Design for strength (safety) (Section 9.1.1) involves use of load magnification factors to obtain the required strength (Section 9.2) and strength-reduction (O) factors (Section 9.3.1) to determine the design strength. Strength-reduction factors allow for various possible understrength effects and to reflect the required ductility, reliability, and relative importance of the member. The second stage of design, serviceability (Section 9.1.2), consists of controls on the computed deflection and crack widths under service loads (Sections 9.5 and 10.6). This design method is called “Strength Design.” The design philosophy is referred to as “Load Resistance and Factor Design” by building codes for other construction materials.

Field Inspection and Construction

The ACI Building Code is a document intended to become part of a law regulating all construction for the benefit of the public. Thus, there are frequent references outlining obligations (which could be undertaken by the Engineer, or Architect, representing the Owner or the Contractor) for securing approval of the Building Official. The Code covers the entire subject of field inspection with a few short minimum requirements (Section 1.3). It states that inspection of concrete construction shall be performed by an Engineer or Architect or competent representative thereof (Section 1.3.1). Minimum records which must be kept during construction and for 2 years thereafter by the inspecting Engineer or Architect (Section 1.3.4) are prescribed. Since the Code is intended to regulate concrete construction for small and large projects, no requirements for continuous inspection, frequency of intermittent inspection, special inspection or inspection procedures can be given.

Splices and Details of Reinforcement

Structurally sound details and proper arrangements of reinforcing bars are vital to the satisfactory performance of reinforced concrete members and structures. Concurrently, such details and bar arrangements should be practical and buildable, as well as cost-effective. Hence details, development, and splices of reinforcement—the topics of this chapter—are a crucial aspect of the design process. Of the ten more important items of information that must be included in the design drawings, typical details, or project specifications, two of the items cover development and splices of reinforcement (Sections 1.2.1-h and 1.2.1-i, and Commentary R1.2.1). These items are anchorage (embedment) length of reinforcement and location and length of lap splices; and the type and location of mechanical and welded splices. Code Chapter 12 also states: splices of reinforcement shall be made only as required or permitted on design drawings, or in project specifications, or as authorized by the engineer (Section 12.14.1).

Two-Way Slab-Beam Design

Dissatisfaction with results of previously required procedures of analysis and design for two-way slab-beam systems was the original reason for the massive research upon two-way slab systems begun about 1955. All available load test results indicated that the flat plate or flat slab was a most efficient design with an adequate safety factor. The two-way slab on beams, analyzed as separate elements, appeared overdesigned with unnecessarily high safety factors. Since 1971, all two-way reinforced systems, with or without beams, have been subject to the same integrated analysis and design procedures (Chapter 13). The requirements have been adjusted to provide nearly uniform strength design capacity for all two-way designs (Chapter 13).

One-Way Joist Systems

Concrete joist construction consists of a monolithic combination of regularly spaced ribs and a top slab. It may be constructed with permanent or removable fillers between ribs (joists) arranged to span in one direction or in two orthogonal directions (Section 8.11.1). Two-way joist construction which spans in two orthogonal directions, commonly referred to as waffle slab construction, must conform to the requirements of Chapter 13 of the 1995 Code for slab systems reinforced in flexure in more than one direction. Chapter 7 of this book gives a detailed explanation of these requirements. One-way joist construction with joists parallel to one another must conform to the requirements for analysis and design of the 1995 Code which will be discussed in this chapter.