Q16: What are typical bar spacings in a fully reinforced wall?
A:
The two-cell CMU illustrated by Figures 1 through 8 would allow minimum spacings of 8 in.
vertically and horizontally. However, it is more economical to place fewer bars at wider
spacings up to the code maximum of 4 feet. The design requirements will dictate the area
of steel, the spacing can be 8, 16, 24, 32, 40, or 48 in. on center. For instance: #5 bars
at 32 in. provide 0.115 in.2 (per ft); #6 bars at 40 in. provide 0.13 in.2 ; and #7 bars at 48 in.
provide 0.15 in.2.
Q17: What are the
rebar splice requirements?
A: They depend on the design criteria and the local building code, but in general,
a 40-bar or 48-bar diameter lap splice is specified. If the wall is designed as a
shearwall, the end vertical reinforcement may be large diameter (#8 through #11) bars
placed within the block cells. In that case, mechanical connections or welding may be
specified not only to eliminate long splice lengths but also because the inside dimensions
of the cells may not provide enough area to accommodate a lap splice and the grout.
Q18: Would not the
longer splices of larger size vertical bars complicate the laying up of the CMU?
A: Not necessarily. For a multistory wall, the use of open-end block (Figures 3
through 7) and high-lift grouting procedures would allow the vertical rebar to be spliced
at floor levels. See Figure 13.
Q19: Can splices be
avoided?
A: Not generally. Vertical bar splices are located at levels governed by the floor
height, the mason's method of laying up the block, the use of open or closed end units,
and the specified grouting procedure. The number of splices in horizontal rebar will
depend on the length of the wall and the maximum length of an individual rebar that the
mason is willing to install. In general, 30-foot lengths are acceptable, but in certain
areas 20-foot lengths are the norm. The length limit is generally established by the
estimator and included in the supply contract quotation as a stipulation.
Q20: Should masonry
rebar extend into adjacent concrete construction?
A: If the masonry wall is non-load bearing and not part of the lateral load
resisting frame, the answer is no (see Question 22). But if the masonry and concrete walls
are designed to act together as part of the load resisting frame, then the answer is yes.
Extending or doweling the reinforcing bars will provide the continuity necessary for the
two materials to act in unison. In some instances, non-load bearing masonry walls may be
tied to the supporting concrete with dowels matching the vertical rebars, but the ends and
top of the wall should be free from the balance of the structure.
Q21: How are the
rebars held in proper position?
A: Wire positioners in a cradle configuration can be used to secure both vertical
and horizontal rebar against displacement.
Q22: Recent
earthquakes in California, Mexico, and the Philippines demonstrate that masonry walls are
prone to failure. Can you explain this?
A: Properly designed and constructed, fully-reinforced masonry walls
can resist seismic lateral loads. Most damage to masonry during the Coalinga and Loma
Prieta (California) earthquakes occurred mainly to unreinforced bearing, in-fill and
partition walls. Partition or in-fill walls, if non-load bearing, should be isolated from
the structural load carrying frame with a flexible joint separation material in order not
to influence the structural frame's response to lateral loads.
