Question:
How do you calculate correct rebar lap splicing to meet code?
Answer:
The IBC 2006 adopted MSJC 2005 by reference; however they
decided to keep the previous lap lengths of 48 bar diameters.
...that engineers may have the option of using MSJC 2005 for
their design lap lengths. Depending on the f’m
the lap lengths can be shorter particularly in the smaller
bar sizes. ...Footnotes (1) & (2) represent comments in
the IBC code. (1) instructs the engineer on additional lap
requirements in areas of high stress, (2) addresses the requirements
for epoxy coated rebar. Footnote (3) assumes the bars are
spaced at least 8” on center if the bars are closer
the table will need to be adjusted. All bars are centered
in the wall, if the bars are designed off center the table
will need to be adjusted. The f’m is a variable
that may be changed to meet your project requirements. The
IBC 2009 has adopted MSJC 2008 including the lap lengths.
These laps match the MSJC 2005 requirements so the MSJC laps
should be what we see in the future.
Below is a lap splice spreadsheet, provided by Kelly Walker
of the Masonry Institute of Michigan, which has been updated
for IBC 2006 as adopted in MBC 2006. Also below, are the MSJC
2005 laps.
Question:
What is the approximate spacing of Control Joints (CJ’s)
for reinforced CMU?
Answer:
Using the Empirical Method in NCMA TEK 10-2B, Table
1 calls for a length to height ratio of 1.5 not to exceed
25ft. This assumes you have wire every 16” o.c.
The engineering method in NCMA TEK 10-3 has different
ratios but still does not exceed 20 or 25 ft.
Kelly L.K. Walker
Architectural Services Director Masonry Institute of Michigan, Inc.
Supporting
Documents:
NCMA
TEK 10-2B
Control Joints For Concrete Masonry
Walls - Empirical Method
Question:
What is the best wire for Single Wythe CMU and for
Multi-Wthe CMU Cavity Wall?
Answer: Contrary to popular
belief, wire reinforcing today typically offers no
structural stability. In fact, the primary purpose
of wire reinforcement in modern masonry wall systems
is simply to help resist CMU shrinkage cracking. Typically,
vertical and bond beam rebar reinforcement serves
to resist wind loading.
Truss
wire is not recommended for use is reinforced CMU.
Diagonal cross rods make it impossible to meet modern
grouting standards referenced in the IBC and sub-referenced
in ACI Building Code Requirements for Masonry Structures.
Ladder wire with cross rods spaced 16 inches on center
allows code required centering of rebar, freer flow
of grout and promotes full embedment in mortar at
every intersection where side and cross rods meet
on the inner and outer CMU face-shell, resisting wall
shrinkage.
Supporting
Documents:
Climb
the Ladder To
Improve Wall Reinforcing Design Written
by Kyle Lochonic & Jeff Snyder
and is published in the Masonry Magazine.
Question:
Is wire reinforcement with 3/16 inch diameter side rods
or 3/16 inch diameter side and cross rods better than
standard 9 gauge side and cross rods?
Answer:
No, for the most part it is just the opposite. The primary
purpose for wire in todays reinforced walls is to reduce
CMU wall shrinkage. It is very difficult to surround
3/16 inch diameter wire with mortar when placed in 3/8
inch bed joints. This is especially evident when considering
allowed tolerances for masonry units and joint sizes.
Mortar better surrounds wire when utilizing 9 gauge
flush welded side and cross rods, increasing resistance
to shrinkage cracking. It is no surprise demand for
truss wire has fallen significantly in the last decade.
The market simply demands higher performance 9 gauge
16 inch core-clear ladder!
Supporting
Documents:
TECH
Talk
Daniel S. Zechmeister, P.E. Executive Director Masonry Institute of Michigan, Inc.
