A Simplified Design Approach - Simplified Design of Reinforced Concrete Buildings - page 12

1.8 PRELIMINARY DESIGN

Preliminary member sizes are usually required to perform the initial frame analysis and/or to obtain initial
quantities of concrete and reinforcing steel for cost estimating. Practical initial member sizes are necessary
even when a computer analysis is used to determine the load effects on a structure. The guidelines for
preliminary design given in the following sections are applicable to regular buildings of moderate size and
height. These guidelines were used to obtain the preliminary sizes listed in Sections 1.7.1 and 1.7.2 for the two
example buildings. Chapters 8 and 9 list additional guidelines to achieve overall economy.

1.8.1 Floor Systems

Various factors must be considered when choosing a floor system. The magnitude of the superimposed loads
and the bay size (largest span length) are usually the most important variables to consider in the selection
process. Fire resistance is also very important (see Section 1.8.5). Before specifying the final choice for the
floor system, it is important to ensure that it has at least the minimum fire resistance rating prescribed in
the governing building code.

In general, different floor systems have different economical span length ranges for a given total factored load.
Also, each system has inherent advantages and disadvantages, which must be considered for a particular
project. Since the floor system (including its forming) accounts for a major portion of the overall cost of a
structure, the type of system to be utilized must be judiciously chosen in every situation.
Figures 1-5 through 1-7 can be used as a guide in selecting a preliminary floor system with › = 4000 psi. 1.3
A relative cost index and an economical square bay size range are presented for each of the floor systems listed.
In general, an exact cost comparison should be performed to determine the most economical system for a
given building.

Once a particular floor system has been chosen, preliminary sizes must be determined for the members in the
system. For one-way joists and beams, deflection will usually govern. Therefore, ACI Table 9.5(a) should be
used to obtain the preliminary depth of members that are not supporting or attached to partitions and other
construction likely to be damaged by deflection. The width of the member can then be determined by the
appropriate simplified equation given in Chapter 3. Whenever possible, available standard sizes should
be specified; this size should be repeated throughout the entire structure as often as possible. For overall
economy in a standard joist system, the joists and the supporting beams must have the same depth. This also
provides an optimum ceiling cavity to a uniform bottom of floor elevation with maximum clearance for building
mechanical/electrical/plumbing (M/E/P) systems.

For flat plate floor systems, the thickness of the slab will almost always be governed by two-way (punching)
shear. Figures 1-8 through 1-10 can be used to obtain a preliminary slab thickness based on two-way shear
at an interior square column and › = 4000 psi. For a total factored load q u (psf) and the ratio of the floor
tributary area, A, to the column area c 1 2 , a value of d/c 1 , can be obtained from the figure. Note that d is the dis-
tance from the compression face of the slab to the centroid of the reinforcing steel. The preliminary thickness
of the slab h can be increased by adding 1.25 in. to the value of d (see Chapter 4).

It is important to note that the magnitude of the unbalanced moment at an interior column is usually small.
However, at an edge column, the shear stress produced by the unbalanced moment can be as large as or larger than
the shear stress produced by the direct shear forces. Consequently, refined calculations to account for the effect
of the unbalanced moment should be done according to Chapter 4.