Design Considerations for Economical Formwork - Simplified Design of Reinforced Concrete Buildings - page 258

Table 9-1 Permitted Irregularities in Formed Surfaces Checked with a 5-ft Template 9.1

Class of surface

A B C D

1/8 in. 1/4 in. 1/2 in. 1 in.

Class A

Class B

Class C

Quality

Figure 9-1 Class of Surface Versus Cost

9.3 ECONOMICAL ASPECTS OF HORIZONTAL FRAMING

Floors and the required forming are usually the largest cost component of a concrete building structure. The
first step towards achieving maximum economy is selecting the most economical floor system for a given plan
layout and a given set of loads. This will be discussed in more detail below. The second step is to define a
regular, orderly progression of systematic shoring and reshoring. Timing the removal of the forms and
requiring a minimum amount of reshoring are two factors that must be seriously considered since they can have
a significant impact on the final cost.

Figures 1-5 and 1-6 show the relative costs of various floor systems as a function of bay size and superimposed
load. Both figures are based on a concrete strength › = 4000 psi. For a given set of loads, the slab system that
is optimal for short spans is not necessarily optimal for longer spans. Also, for a given span, the slab system
that is optimal for lighter superimposed loads is not necessarily optimal for heavier loads. Reference 9.3
provides material and cost estimating data for various floor systems. It is also very important to consider the
fire resistance of the floor system in the preliminary design stage (see Chapter 10). Required fire resistance
ratings can dictate the type of floor system to specify in a particular situation.

The relationship between span length, floor system, and cost may indicate one or more systems to be
economical for a given project. If the system choices are equally cost-effective, then other considerations
(architectural, aesthetic, etc.) may become the determining factor.

Beyond selection of the most economical system for load and span conditions, there are general techniques that
facilitate the most economical use of the chosen system.

9.3.1 Slab Systems

Whenever possible, avoid offsets and irregularities that cause a “stop and start” disruption of labor and require
additional cutting (and waste) of materials (for example, breaks in soffit elevation). Depressions for terrazzo,