Q23: What recommendations can be made regarding details
that would simplify estimating and detailing the rebar required in masonry walls?
A: It is recommended that the project
drawings be specific as to the design requirements by showing not only typical details,
but specific sections detailing the rebar required. Estimators and detailers of
reinforcing bars are not qualified to interpret design or code requirements and they need
specific instructions as to what is to be furnished. The architectural and structural
drawings should clearly show the location, extent, opening sizes, and dimensions of all
masonry walls in a similar manner as for concrete walls.
Q24: In some seismically active
zones, legislation requires masonry structures built prior to a certain date to be
upgraded to resist lateral forces. Do you have any suggestions how this can be done?
A: Each structure to be retrofitted must be
analyzed to find a solution to fit its unique case. Generally, older unreinforced CMU
walls have empty cavities (without grout) and may have some horizontal reinforcement in
the top of the wall. Retrofit strengthening to resist seismic lateral loads may require
both vertical and horizontal reinforcement. Vertical reinforcing bars can be placed in the
empty cavities, however, in some cases core drilling may be necessary to assure sufficient
space for the grout. A clean-out at the bottom of the cell is necessary.
"High-lift" grouting procedures are used after placement of the vertical bar in
the empty cavity. Horizontal reinforcement of the wall presents a more difficult problem.
Drilling horizontal cores the length of the wall, inserting a reinforcing bar and using
pressure grout may be one method. Another procedure is to install one or two continuous
rods with threaded ends at floor lines (or other fixed spacing) from wall end to wall end.
These rods, which are located outside the confines of the wall, would have bearing plates
at each end. Tightening the nuts induces tension forces in the rods and in effect
"reinforces" the wall (see Figure 14). It should be noted that these general
procedures may not fit all conditions.
CONCLUSION
This report is an informational essay explaining in
part how the physical dimensions of CMU may affect the furnishing and placement of
reinforcing bars. It is not intended to be a definitive treatise on masonry design nor
application of code requirements. For more information, consult the references.
REFERENCES
American Concrete Institute, American Society of Civil Engineers and
The Masonry Society:
Building Code Requirements for Masonry Structures (ACI
530-921ASCE 5-921TMS 402-92)
Specifications for Masonry Structures (ACI 530.1-92/ASCE
6-92/TMS 602-92)
Uniform Building Code, 1991 Edition, International
Conference of Building Officials.
BOCA National Building Code, 12th Edition, Building
Officials & Code Administrators International, Inc., 1993.
Standard Building Code, 1991 Edition, Southern Building
Code Congress International, Inc.
Masonry Designers' Guide, The Masonry Society and American
Concrete Institute, 1993.
Concrete Masonry Design Tables, Publication No. TR 121,
National Concrete Masonry Association, 1993.
Recommended Practice for Engineered Brick Masonry, Brick
Institute of America, 1969.
Technical Notes on Brick Construction, Brick Institute of
America.
Reinforced Masonry Engineering Handbook, J.E. Amrheim, 4th
Edition, Masonry Institute of America, 1983.
Reinforcing Steel in Masonry, Masonry Institute of America.
TEK Bulletins, National Concrete Masonry Association.
Concrete Masonry Handbook for Architects, Engineers and Builders,
5th Edition, Portland Cement Association, 1991.
