Masonry Magazine October 2001 Page. 8
By John Chrysler, P.E.
Over the past several years, the model building codes have struggled with reinforcement in masonry design and construction. There are many important issues that are unresolved between designers and contractors. Most recently, designers try to pack more steel into smaller walls, yet maintain clearances and expect grout to flow adequately and fill all the intended cells or cavities.
This is not meant to criticize the designer, model codes or contractor, but rather widen the scope of understanding so that we can appreciate the needs of everyone involved. Those involved in the development of model building codes submit to the recommendations of research, testing and historic performance of buildings to subsequently adopt code changes. These recommended code changes may involve introduction of structural reinforcement in masonry where it did not exist before, or additional reinforcement in masonry. Changes may also take the form of eliminating lap splicing of reinforcing steel in critical wall sections.
It is significant to understand that the building codes are living, ever changing documents. Unfortunately, this message is often not conveyed to the contractor, the one carrying the marching orders. Simply stated, yesterday's design or construction method may not be appropriate for the current code.
In the past, many parts of the country did not consider deformed reinforcing steel as an integral and frequent component of masonry. That has changed. Federal, state and local governments, as well as the insurance industry, have paid billions of dollars after natural disasters to reconstruct damaged buildings. Most associate these disasters with earthquakes, but they also include hurricanes, tornados and floods. As a result, there was an enormous amount of pressure on the industry to unify and improve building codes to minimize building damage associated with natural disasters. Welcome to the International Building Code and the increased masonry reinforcement requirements.
Rather than belabor the right or wrong of the reinforcement requirements, attention will be given to the manner in which reinforcement works in harmony with masonry. Masonry is an outstanding compression element, and has almost no tension capacity. Conversely, reinforcing steel, including joint reinforcement, has excellent tension capabilities, but does not work well in compression. Join the two for the best of both worlds. This is what reinforced masonry does best.
STEEL IN THE CENTER
When considering a masonry building without external forces, such as wind and seismic, it is fair to assume that the walls are loaded in pure compression. Loads contributing to this pure compression include the weight of the building, objects and people in the building and external loads, such as snow. In the real world, external lateral forces do try to push the building over, thereby putting some of the masonry into tension.
If the external forces always pushed the building in one direction, the reinforcing steel could be located in the highest tension region and be most effective. This is depicted in Figure 1. However, when the direction of the wind changes, so does the tension region. It may move to the opposite face of the masonry wall as shown in Figure 2. For maximum efficiency, the reinforcement should then be on the opposite side of the wall.
REINFORCING STEEL:
Location, Location, Location