Page 255 - Design of Reinforced Masonry Structures

4.118 CHAPTER FOUR

2006 IBC (Section 2108.2) specifies that the development length given by Eq. (4.135)
need not exceed 72d (no such upper limit in MSJC-08).
b
Development lengths for epoxy-coated bars are required to be 50 percent longer than
required by Eq. (4.135). Bars spliced by noncontact lap splices can be farther apart no more
than one-fifth the required length of lap or 8 in.
Perusal of Eq. (4.135) shows that the development length of a reinforcing bar is pro-
portional to the square of its diameter. Therefore, the larger the bar diameter, the longer
would be the required development length. Also, the reinforcement size factor, g , increases
as the bar diameters increase (Table 4.11). For these reasons, it is advisable to use smaller
diameter bars; the use of larger diameter bars would require relatively longer development
lengths that may not be practical. Factor K has a significant influence on the required devel-
opment length for a bar depending on whether there is only one bar or two closely spaced
bars in a grouted cell. See Example 4.31.
According to the MSJC-08 Commentary, the moment diagrams customarily used
in design are approximate as some shifting of the point of maximum moment may
occur due to changes in loading, settlement of supports, lateral loads, or other causes.
A diagonal tension crack in a flexural member without stirrups may shift the location
of calculated tensile stress approximately a distance d toward a point of zero moment.
When stirrups are provided, this effect is less severe, but still present. To provide for this
uncertainty, the MSJC Code requires the extension of reinforcement a distance equal
to a distance d or 12d beyond the point at which it is theoretically no longer required
b
to resist flexure.
The longitudinal reinforcing bars may be spliced as required. The above requirement
applies to contact splices of standard deformed bars as well. Normally, these bars would
extend from the column footing, which may require splicing as construction progresses.
Splice lengths should be sufficient to transfer the loads in reinforcing bars by proper devel-
opment length (MSJC-08 Section 3.3.3.3).

Example 4.31 Development lengths.
Calculate the developments for No. 5 Grade 60 bar placed in a standard 8 × 8 × 16 CMU
when (a) only one bar is placed in each cell, (b) when two bars are placed in each cell. Use
′ f = 1500 psi.
m
Solution
Figure E4.31 shows cross sections of standard 8 × 8 × 16 CMUs with No. 5 bars. The
development length for a reinforcing bar is given by Eq. (4.135):

2
.
013 df γ
l = b y
d
Kf ′ m
a. One No. 5 Grade 60 bar in each cell.
d = 0.625 in., f = 60,000 psi, ′ f = 1500 psi, g = 1.0 (Table 5.1). K is the smallest of
b
y
m
1. Masonry cover = ½(7.625) – ½(0.625) = 3.5 in.
2. Spacing between the adjacent reinforcement = ½(15.625) – 0.625 = 7.19 in.
3. 5 times d = 5(0.625) = 3.125 in. ← governs
b
2
.
013 df γ ( . )( . 2 60 000 1.0)
,
0 13 0 625) (
0
)(
l = b y = = 25 17 in .
.
d
Kf ′ m (. 1500)
3 125)(