Civil Engineering Reference
In-Depth Information
Figure 12.12
must be a minimum area of dowels furnished equal to no less than 0.005 times the gross
cross-sectional area of the column or pedestal.
The development length of the bars must be sufficient to transfer the compression to
the supporting member, as per the ACI Code (12.3). In no case may the area of the de-
signed reinforcement or dowels be less than the area specified for the case where the allow-
able bearing force was not exceeded. As a practical matter in placing dowels, it should be
noted that regardless of how small a distance they theoretically need to be extended down
into the footing, they are usually bent at their ends and set on the main footing reinforcing,
as shown in Figure 12.13. There the dowels can be tied firmly in place and not be in danger
of being pushed through the footing during construction, as might easily happen otherwise.
The bent part of the bar does not count as part of the compression development length
(ACI Code 12.5.5). The reader should again note that the bar details shown in this figure
are not satisfactory for seismic areas as the bars should be bent inward and not outward.
An alternative to the procedure described in the preceding paragraph is to place the
footing concrete without dowels and then to push straight dowels down into the concrete
while it is still in a plastic state. This practice is permitted by the Code in its Section
16.7.1 and is especially useful for plain concrete footings (to be discussed in Section
12.14 of this chapter). It is essential that the dowels be maintained in their correct position
as long as the concrete is plastic. Before the engineer approves the use of straight dowels
as described here, he or she must be satisfied that the dowels will be properly placed and
the concrete satisfactorily compacted around them.
The Code normally does not permit the use of lapped splices for #14 and #18 com-
pression bars because tests have shown that welded splices or other types of connections
are necessary. Nevertheless, based on years of successful use, the Code (15.8.2.3) states
that #14 and #18 bars may be lap-spliced with dowels (no larger than #11) to provide for
force transfer at the base of columns or walls or pedestals. These dowels must extend into
the supported member a distance of not less than the development length of #14 or #18
bars or the splice length of the dowels, whichever is greater; and into the footing a dis-
tance not less than the development length of the dowels.
Figure 12.13
