Masonry Magazine June 1999 Page. 21
Thick and Thicker?
Are we losing the benefits of thermal inertia or jeopardizing the safety of structures by getting so thin that they no longer are safe to use? The answer is, first they are economical. They have been tested and they have performed both for out of plane forces and in plane forces. They are capable of carrying vertical load as demonstrated by the strength of the materials required and lastly they present a tremendous advantage in cost savings in this competitive world of construction. The cost savings is the result of less masonry material required. Less reinforcing steel because the wall is thinner and lighter to resist earthquake forces. Smaller footings are required because the walls weight less. Lower seismic forces are used in their design because of less weight in the wall.
Construction time is reduced, as lighter units can be laid faster and there may be reduced interim financing because construction time is reduced and the buildings go up faster. These are the competitive advantages of using tall thin walls.
Another advantage is it is competitive to our major competitors of tilt up walls. By the time a slab is poured, the tilt up panels are poured, the perimeter masonry walls are already in place ready to receive the roof trusses. This advantage for the owner is that it gives him an opportunity of an enclosed space where he can start working on the inside rather than wait for the walls to be tilted up.
Because these walls are built using the strength design techniques, it is necessary to provide inspection while they are being built. This is to the benefit of the owner, the architect, the engineer and the contractor. With inspection as the wall is built, the contractor need not delay waiting for a job inspection from city hall. It provides assurance to the owner, architect and engineer that the steel is in the right place, is the right size, the masonry units are as specified, and the project is being built as designed. Use of inspection on strength design projects adds a dimension of security, it doesn't cost, it pays to use inspection when using tall slender walls.
Although there are new techniques of design using strength design for tall thin walls, there are many design aids and computer programs available to the designer making implementation of slender wall is relatively easy.
Below is an example of the design of a wall 24 feet between support points that has an h/t of nearly 38.
Determine the vertical reinforcing steel required for a wall that is 24 ft. between lateral supports and carries a ledger load of 500 pounds/ft.
The lateral load is 20.3 psf wind load or seismic load for earthquake zone IV is 23.4 psf. (See Reference 5)
Solution:
a) Height of wall 24'0" between lateral supports
b) Ledger load 500 plf
c) Masonry System
1. Concrete block
2. 8" thick, nominal
3. f'm = 1,500 psi
fy - 60,000 psi
4. Eccentricity, e =7.3
5. Special Inspection required
6. Steel is center of wall; d = 3.81"
7. Wall is solid grouted
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MASONRY-MAY/JUNE, 1999 21