Masonry Magazine October 2001 Page. 9
As a compromise
As a compromise, structural reinforcement is most efficiently located at the center of the wall as shown in Figure 3. This location allows the masonry wall to use reinforcement efficiently no matter which direction the lateral force, wind or seismic, comes from.
Designers rely on the location of reinforcement, presumably at the middle of the wall, in calculating lateral load capacities of a structure. The building codes permit tolerances, based on the "d" distance, for reinforcement location. When "d" is less than 8 inches, the tolerance allowance is plus or minus 1/2 inch. The tolerance for longitudinal placement of reinforcement in a wall is within 2 inches of the plan location. See Figure 4.
RETAINING WALLS
When the lateral load is constant, such as with a retaining wall, the reinforcing steel can be effectively placed away from the center of the wall for maximum efficiency. The cantilevered, or free standing, retaining wall has a tension region close to the soil side of the wall, therefore, the reinforcement should be located toward the retained side (Figure 5), while maintaining proper clearance coverage, which is 1-1/2 or 2 inches, depending on the applicable code provision.
A different type of retaining wall is one that is fixed at the top. This would be typical of a basement retaining wall attached to a floor. This condition moves the tension region to the opposite side of the masonry wall, and subsequently, the reinforcing steel should be placed as far away from the soil as practical (Figure 6). This maximizes efficiency of the masonry wall system.
STEEL ON BOTH SIDES
Occasionally, the design calculations with assumed reinforcing steel in the center of the wall do not satisfy code design requirements. This problem often occurs in masonry walls of 10 and 12 inches in thickness. The designer may consider a thicker wall, but this approach is usually impractical. The designer may also consider an alternate material, which is always an unsatisfactory alternative. The flexibility of reinforced masonry allows the designer to use reinforcement placed on both sides of the cell or cavity, located away from the center-line of the wall as constructed in Photo 1. This configuration will use two larger "d" distances, and significantly increase the lateral load capacity of the wall.
JOINT REINFORCEMENT
Joint reinforcement is popular in many parts of the country to effectively tie the masonry together as a homogeneous system without using grout. This system works very well and circumvents the need to grout horizontal reinforcement, but joint reinforcement does have limitations.
In areas of higher seismic risk (Zones 3 and 4; Design Categories D and E), joint reinforcement placed in every mortar bed joint of an 8 inch high masonry unit will not satisfy the minimum steel requirements for seismic shear resistance. Also, a masonry wall system reinforced with joint reinforcement may not be able to withstand hurricane forces as shown in Photo 2, Hurricane Alicia of 1983. With extreme seismic or wind loading, it is wise for the designer and contractor to integrate structural deformed reinforcement into the masonry system.
LAP SPLICES
There are three popular methods of installing vertical reinforcing steel prior to the grouting process. The first, and probably easiest, method is dropping the reinforcing steel into the wall after the units are laid, and then grouting the system. This method simplifies the unit placement process, as the bricklayers can place the structural units without obstruction of the reinforcement.
The second method is to construct the masonry with the reinforcement in place. This is somewhat more difficult for the mason and requires the use of open-end units. These units are not yet available in many parts of the country. A significant advantage to the designer is that there are usually no reinforcement lap splices from floor to floor. The critical area to avoid lap-splicing reinforcement is the mid height of the wall, where the moment imposed on the wall due to lateral loads is typically the greatest.
Finally, installing and grouting shorter pieces of reinforcement as the wall is constructed is relatively easy for the contractor, but may generate a lap splice at the mid height of the wall. With a little planning, this undesirable condition can be avoided.
MOVING FORWARD
It is important that the contractor understands the needs of the designer, as well as the designer understanding the needs of the contractor. The designer is placing more reinforcement in masonry walls to satisfy the ever-changing code requirements. The ever-changing code is a response to the desire to make safer, more durable buildings and the contractor should cooperate with the progress in design and construction.
In return, the designer should detail the reinforcing steel in a manner which is constructible, leaving ample room for needed field tolerances and grout placement.
Such a cooperative effort will go a long way in advancing masonry as the first choice in a building system as it has been for thousands of years.
Complete reinforcement detailing information can be found in Reinforcing Steel in Masonry, published by the Masonry Institute of America.
John Chrysler, P.E. is Executive Director of the Masonry Institute of America, a registered Civil Engineer in California and Arizona and formerly a practicing contractor for over 25 years.