Civil Engineering Reference
In-Depth Information
moment equals 185.3 ft-k. The left reaction for this beam is 44.8 k, as shown in Figure 7.20(a).
Using this value, an expression is written for the moment at point C (185.3 ft-k) at a distance
x
from the left support. The resulting expression can be solved for
x
.
x
2
44.8
x
(2.8
x
)
185.3
x
4.88 ft
Say, 4 ft 10 in.
By the time we reach point D (3 ft 5 in. to the right of C and 8 ft 3 in. from the left support), the
required moment capacity is
8.25
2
M
u
(44.8)(8.25)
(2.8)(8.25)
274.3 ft- k
Earlier in this section reference was made to ACI Section 12.10.5, where shear at bar cutoff
points was considered. It is assumed that this beam will be properly designed for shear as described
in the next chapter and will meet the ACI shear requirements.
7.12
BAR SPLICES IN FLEXURAL MEMBERS
Field splices of reinforcing bars are often necessary because of the limited bar lengths
available, requirements at construction joints, and changes from larger bars to smaller
bars. Although steel fabricators normally stock reinforcing bars in 60-ft lengths, it is often
convenient to work in the field with bars of shorter lengths, thus necessitating the use of
rather frequent splices.
The reader should carefully note that the ACI Code, Sections 1.2.1(h) and 12.14.1
clearly states that the designer is responsible for specifying the types and locations for
splices for reinforcement.
The most common method of splicing #11 or smaller bars is simply to lap the bars one
over the other. Lapped bars may be either separated from each other or placed in contact,
with the contact splices being much preferred since the bars can be wired together. Such
bars also hold their positions better during the placing of the concrete. Although lapped
splices are easy to make, the complicated nature of the resulting stress transfer and the local
cracks that frequently occur in the vicinity of the bar ends are disadvantageous. Obviously,
bond stresses play an important part in transferring the forces from one bar to another. Thus
the required splice lengths are closely related to development lengths. It is necessary to un-
derstand that the minimum specified clear distances between bars also apply to the distances
between contact lap splices and adjacent splices or bars (ACI Section 7.6.4).
Lap splices are not very satisfactory for several situations. They include: (1) where they
would cause congestion, (2) where the laps would be very long, as they are for #9 to #11
Grade 60 bars, (3) where #14 or #18 bars are used because the Code (12.14.2) does not per-
mit them to be lap spliced except in a few special situations, and (4) where very long bar
lengths would be left protruding from existing concrete structures for purposes of future ex-
pansion. For such situations, other types of splices such as those made by welding or by me-
chanical devices may be used. Welded splices, from the view of stress transfer, are the best
splices, but they may be expensive and may cause metallurgical problems. The result may
be particularly disastrous in high seismic zones. The ACI Code (12.14.3.4) states that
welded splices must be accomplished by welding the bars together so that the connection
