Civil Engineering Reference
In-Depth Information
girder on one level will be equal and the interior columns will have no axial force,
since only lateral loads are considered.)
14.9
COMPUTER ANALYSIS OF BUILDING FRAMES
All structures are three-dimensional, but theoretical analyses of such structures by hand
calculation methods are so lengthy as to be impractical. As a result, such systems are nor-
mally assumed to consist of two-dimensional or planar systems, and they are analyzed in-
dependently of each other. The methods of analysis presented in this chapter were
handled in this manner.
Today's electronic computers have greatly changed the picture, and it is now possible
to analyze complete three-dimensional structures. As a result, more realistic analyses are
available and the necessity for high safety factors is reduced. The application of comput-
ers is not restricted merely to analysis; they are used in almost every phase of concrete
work from analysis to design, to detailing, to specification writing, to material takeoffs, to
cost estimating, and so on.
Another major advantage of computer analysis for building frames is that the de-
signer is able to consider many different loading patterns quickly. The results are some-
times rather surprising.
14.10
LATERAL BRACING FOR BUILDINGS
For the usual building, the designer will select relatively small columns. Although such a
procedure results in more floor space, it also results in buildings with small lateral stiff-
nesses or resistance to wind and earthquake loads. Such buildings may have detrimental
lateral deflections and vibrations during windstorms unless definite lateral stiffness or
bracing is otherwise provided in the structure.
To provide lateral stiffness it will be necessary for the roof and floor slabs to be at-
tached to rigid walls, stairwells, or elevator shafts. Sometimes structural walls, called
shear walls
are added to a structure to provide the necessary lateral resistance. (The de-
sign of shear walls is considered in Chapter 18 of this text.) If it is not possible to provide
such walls, stairwells or elevator shafts may be designed as large, box-shaped beams to
transmit lateral loads to the supporting foundations. These members will behave as large
cantilever beams. In designing such members, the designer should try to keep resistance
symmetrical so as to prevent uneven lateral twisting or torsion in the structure when lat-
eral loads are applied.
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14.11
DEVELOPMENT LENGTH REQUIREMENTS
FOR CONTINUOUS MEMBERS
In Chapter 7 a general introduction to the subject of development lengths for simple and
cantilever beams was presented. This section examines the ACI development length re-
quirements for continuous members for both positive and negative reinforcing. After
9
Leet, K., 1991,
Reinforced Concrete Design
, 2nd ed. (New York: McGraw-Hill), pp. 453-454.
